Introduction and Objective

After anterior cruciate ligament reconstruction one of the risk factors for graft (re-)rupture is an increased posterior tibial slope (PTS). The current treatment for PTS is a high tibial osteotomy (HTO). This is a free-hand method, with 1 degree of tibial slope correction considered to be equal to 1 or even 1.67 mm of the anterior wedge resection. Error rates in the frontal plane reported in literature vary from 1 – 8.6 degrees, and in the sagittal plane outcomes in a range of 2 – 8 degrees are reported when planned on PTSs of 3 – 5 degrees. Therefore, the free-hand method is considered to have limited accuracy. It is expected that HTO becomes more accurate with patient specific saw guides (PSGs), with an accuracy margin reported in literature of 2 degrees. This proof of concept porcine cadaver case study aimed to investigate whether the use of PSGs improves the accuracy of HTO to less than 2 degrees. Secondly, the reproducibility of tibial slope measurement was evaluated.

Patients with cancer and bone metastases can have an increased risk of fracturing their femur. Treatment is based on the impending fracture risk: patients with a high fracture risk are considered for prophylactic surgery, whereas low fracture risk patients are treated conservatively with radiotherapy to decrease pain. Current clinical guidelines suggest to determine fracture risk based on axial cortical involvement of the lesion on conventional radiographs, but that appears to be difficult. Therefore, we developed a patient-specific finite element (FE) computer model that has shown to be able to predict fracture risk in an experimental setting and in patients. The goal of this study was to determine whether patient-specific finite element (FE) computer models are better at predicting fracture risk for femoral bone metastases compared to clinical assessments based on axial cortical involvement on conventional radiographs, as described in current clinical guidelines.

45 patients (50 affected femurs) affected with predominantly lytic bone metastases who were treated with palliative radiotherapy for pain were included. CT scans were made and patients were followed for six months to determine whether or not they fractured their femur. Non-linear isotropic FE models were created with the patient-specific geometry and bone density obtained from the CT scans. Subsequently, an axial load was simulated on the models mimicking stance. Failure loads normalized for bodyweight (BW) were calculated for each femur. High and low fracture risks were determined using a failure load of 7.5 × BW as a threshold. Experienced assessors measured axial cortical involvement on conventional radiographs. Following clinical guidelines, patients with lesions larger than 30 mm were identified as having a high fracture risk. FE predictions were compared to clinical assessments by means of diagnostic accuracy values (sensitivity, specificity and positive (PPV) and negative predictive values (NPV)).

Seven femurs (14%) fractured during follow-up. Median time to fracture was 8 weeks. FE models were better at predicting fracture risk in comparison to clinical assessments based on axial cortical involvement (sensitivity 100% vs. 86%, specificity 74% vs. 42%, PPV 39% vs. 19%, and NPV 100% vs. 95%, for the FE computer model vs. axial cortical involvement, respectively). We concluded that patient-specific FE computer models improve fracture risk predictions of femoral bone metastases in advanced cancer patients compared to clinical assessments based on axial cortical involvement, which is currently used in clinical guidelines. Therefore, we are initiating a pilot for clinical implementation of the FE model.

Patients with advanced cancer can develop bone metastases in the femur which are often painful and increase the risk of pathological fracture. Accurate segmentation of bone metastases is, amongst others, important to improve patient-specific computer models which calculate fracture risk, and for radiotherapy planning to determine exact radiation fields. Deep learning algorithms have shown to be promising to improve segmentation accuracy for metastatic lesions, but require reliable segmentations as training input. The aim of this study was to investigate the inter- and intra-operator reliability of manual segmentation of femoral metastatic lesions and to define a set of lesions which can serve as a training dataset for deep learning algorithms. F

CT-scans of 60 advanced cancer patients with a femur affected with bone metastases (20 osteolytic, 20 osteoblastic and 20 mixed) were used in this study. Two operators were trained by an experienced radiologist and then segmented the metastatic lesions in all femurs twice with a four-week time interval. 3D and 2D Dice coefficients (DCs) were calculated to quantify the inter- and intra-operator reliability of the segmentations. We defined a DC>0.7 as good reliability, in line with a statistical image segmentation study.

Mean first and second inter-operator 3D-DCs were 0.54 (±0.28) and 0.50 (±0.32), respectively. Mean intra-operator I and II 3D-DCs were 0.56 (±0.28) and 0.71 (±0.23), respectively. Larger lesions (>60 cm³) scored higher DCs in comparison with smaller lesions.

This study reveals that manual segmentation of metastatic lesions is challenging and that the current manual segmentation approach resulted in dissatisfying outcomes, particularly for lesions with small volumes. However, segmentation of larger lesions resulted in a good inter- and intra-operator reliability. In addition, we were able to select 521 slices with good segmentation reliability that can be used to create a training dataset for deep learning algorithms. By using deep learning algorithms, we aim for more accurate automated lesion segmentations which might be used in computer modelling and radiotherapy planning.

Introduction

The fixation of press-fit orthopaedic devices depends on the mechanical properties of the bone that is in contact with the implants. During the press-fit implantation, bone is compacted and permanently deformed, finally resulting in the mechanical interlock between implant and bone. For the development and design of new devices, it is imperative to understand these non-linear interactions. One way to investigate primary fixation is by using computational models based on Finite Element (FE) analysis. However, for a successful simulation, a proper material model is necessary that accurately captures the non-linear response of the bone. In the current study, we combined experimental testing with FE modeling to establish a Crushable Foam model (CFM) to represent the non-linear bone biomechanics that influences implant fixation.

Introduction

Cementless total knee arthroplasty (TKA) implants use an interference fit to achieve fixation, which depends on the difference between the inner dimensions of the implant and outer dimensions of the bone. However, the most optimal interference fit is still unclear. A higher interference fit could lead to a superior fixation, but it could also cause bone abrasion and permanent deformation during implantation. Therefore, this study aims to investigate the effect of increasing the interference fit from 350 µm to 700 µm on the primary stability of cementless tibial implants by measuring micromotions and gaps at the bone-implant interface when subjected to two loading conditions.

Introduction

Instability, loosening, and patellofemoral pain belong to the main causes for revision of total knee arthroplasty (TKA). Currently, the diagnostic pathway requires various diagnostic techniques such as x-rays, CT or SPECT-CT to reveal the original cause for the failed knee prosthesis, but increase radiation exposure and fail to show soft-tissue structures around TKA. There is a growing demand for a diagnostic tool that is able to simultaneously visualize soft tissue structures, bone, and TKA without radiation exposure. MRI is capable of visualising all the structures in the knee although it is still disturbed by susceptibility artefacts caused by the metal implant. Low-field MRI (0.25T) results in less metal artefacts and offers the ability to visualize the knee in weight-bearing condition. Therefore, the aim of this study is to investigate the possibilities of low field MRI to image, the patellofemoral joint and the prosthesis to evaluate the knee joint in patients with and without complaints after TKA.

Introduction

Polyetheretherketone (PEEK) has been proposed as an implant material for femoral total knee arthroplasty (TKA) components. Potential clinical advantages of PEEK over standard cobalt chrome alloys include modulus of elasticity and subsequently reduced stress shielding potentially eliminating osteolysis, thermal conduction properties allowing for a more natural soft tissue environment, and reduced weight enabling quicker quadriceps recovery. Manufacturing advantages include reduced manufacturing and sterilization time, lower cost, and improved quality control. Currently, no PEEK TKA implants exist on the market. Therefore, evaluation of mechanical properties in a pre-clinical phase is required to minimize patient risk.

The objectives of this study include evaluation of implant fixation and determination of the potential for reduced stress shielding using the PEEK femoral TKA component.

Introduction

Roentgen stereophotogrammetric analysis (RSA) is currently the gold standard to measure early prosthetic migration which can predict aseptic loosening. However, RSA has some limitations such as the need for perioperative placed markers and exposure to X-radiation during follow up. Therefore, this study evaluates if low field MRI could be an alternative for RSA. Low field MRI was chosen because it is less hampered by metal artifacts of the prosthesis than high field MRI.

Introduction

Fretting corrosion at the taper interface of modular connections can be studied using Finite Element (FE) analyses. However, the loading conditions in FE studies are often simplified, or based on generic activity patterns. Using musculoskeletal modeling, subject-specific muscle and joint forces can be calculated, which can then be applied to a FE model for wear predictions. The objective of the current study was to investigate the effect of incorporating more detailed activity patterns on fretting simulations of modular connections.

Introduction

Fifteen percent of the primary total knee arthroplasties (TKA) fails within 20 years. Among the main causes for revision surgery are instability and patellofemoral pain. Currently, the diagnostic pathway requires various diagnostic techniques to reveal the original cause for the failed knee prosthesis and is therefore time consuming and inefficient.

Accordingly, there is a growing demand for a diagnostic tool that is able to simultaneously visualize soft tissue structures, bone and TKA. Magnetic resonance imaging (MRI) is capable of visualising all the structures in the knee although a trade- off needs to be made between metal artefact reducing capacities and image quality. Low-field MRI (0.25T) results in less metal artefacts and a lower image quality compared with high-field MRI (1.5T). The aim of this study is to develop a MRI imaging guide to image the problematic TKA and to evaluate this guide by comparing low-field and high-field MRI on a case study.

Introduction

Although cementless press-fit femoral total knee arthroplasty (TKA) components are routinely used in clinical practice, the effect of the interference fit on primary stability is still not well understood. Intuitively, one would expect that a thicker coating and a higher surface roughness lead to a superior fixation. However, during implant insertion, a thicker coating can introduce more damage to the underlying bone, which could adversely influence the primary fixation. Therefore, in the current study, the effect of coating thickness and roughness on primary stability was investigated by measuring the micromotions at the bone-implant interface with experimental testing.

Introduction

Aseptic loosening is the main reason for total knee arthroplasty (TKA) failure, responsible for more than 25% of the revision procedures, with most of the problems occurring with the tibial component. While early loosening can be attributed to failure of primary fixation, late implant loosening is associated with loss of fixation secondary to bone resorption due to altered physiological load transfer to the tibial bone. Several attempts have been made to investigate these changes in bone load transfer in biomechanical simulations and bone remodeling analyses, which can be useful to provide information on the effect of patient, surgery, or design-related factors. On the other hand, these factors have also been investigated in clinical studies of radiographic changes of bone density following TKA. In this study we made an overview of the knowledge obtained from these clinical studies, which can be used to inform clinical decision making and implant design choices.

Objectives

In this prospective cohort study, we investigated whether patient-specific finite element (FE) models can identify patients at risk of a pathological femoral fracture resulting from metastatic bone disease, and compared these FE predictions with clinical assessments by experienced clinicians.

Introduction

Fretting corrosion of the modular taper junction in total hip arthroplasty has been studied in several finite element (FE) studies. Manufacturing tolerances can result in a mismatch between the femoral head and stem, which can influence the taper mechanics leading to possibly more wear. Using FE models the effect of these manufacturing tolerances on the amount of volumetric wear can be studied. The removal of material in the FE model was validated against experiments simulating the clinical fretting wear process, subsequently the mismatch and assembly force were varied to study the effect on the volumetric wear.

INTRODUCTION

Mechanical overloading of the knee can occur during activities of daily living such as stair climbing, jogging, etc. In this finite element study we aim to investigate which parameters could detrimentally influence peri-implant bone in the tibial reconstructed knee. Bone quality and patient variables are potential factors influencing knee overloading (Zimmerman 2016).

Improving the accuracy of measuring 6 degree of freedom tibiofemoral kinematics is a crucial step in gait analysis, but skin-marker estimated kinematics are subject to soft tissue artefacts. Fluoroscopic systems have been reported to achieve high accurate kinematics, but their induced irradiation, limited field of view, and high cost hampers routine usage on large patient cohorts. The aim of this study is to assess the feasibility of measuring tibiofemoral kinematics using multi-channel A-mode ultrasound system in cadaver experiment and to assess its achievable accuracy.

A full cadaver was placed with its back on a surgery table while its legs were overhanging the edge of the table. Upper body was fixated and right leg was moved by means of pulling a rope. Two bone pins with optical markers were mounted to the femur and tibia separately to measure the ground truth of motion. Six custom holders containing 30 A-mode ultrasound transducers and 18 optical markers were mounted to six anatomical regions. By measuring the bone to ultrasound transducer distance and using the spatial information of the optical markers on the holders, 30 bone surface points were determined. The corresponding bones (femur and tibia) were registered to these acquired points after which the tibiofemoral kinematics were determined.

This study presents a multi-channel A-mode ultrasound system and the first results have shown its feasibility of reconstructing tibiofemoral kinematics in cadaver experiment. Although the reconstructed tibiofemoral kinematics is less accurate than a fluoroscopic system, it outperforms a skin-mounted markers system. Thus, this A-mode Ultrasound approach could provide a non-invasive and non-radiative method for measuring tibiofemoral kinematics, which may be used in clinic gait analysis or even computer-aided orthopaedic surgery.

Background

Trabecular metal (TM) cones are designed to fill up major bone defects in total knee arthroplasty. Tibial components can be implanted in combination with a stem, but it is unclear if this is necessary after reconstruction with a TM cone. Implanting a stem may give extra stability, but may also have negative side-effects. Aim of this study was to investigate stability and strain distribution of a tibial plateau reconstruction with a TM cone while the tibal component is implanted with and without a stem, and whether prosthetic stability was influenced by bone mineral density (BMD).

Background

Fretting at modular junctions is thought to be a ‘mechanically assisted’ corrosion phenomenon, initiated by mechanical factors that lead to increased contact stresses and micromotions at the taper interface. We adopted a finite element approach to model the head-taper junction, to analyse the contact mechanics at the taper interface. We investigated the effect of assembly force and angle on contact pressures and micromotions, during loads commonly used to test hip implants, to demonstrate the importance of a good assembly during surgery.

The primary stability of an uncemented femoral total knee replacement component is provided by press-fit forces at the bone-implant interface. This press-fit is achieved by resecting the bone slightly larger than the inner dimensions of the implant, resulting in a so-called interference fit. Previous animal studies have shown that an adequate primary stability is required to minimize micromotions at the bone-implant interface to achieve bone-ingrowth, which provides the secondary (long-term) fixation. It is assumed that during implantation a combination of elastic and plastic deformation and abrasion of the bone will occur, but little is known about what happens at the bone-implant interface and how much interference fit eventually is achieved. Purpose of this study was therefore to assess the actual and effective interference fit and the amount of bone damage during implantation of an uncemented femoral knee component.

In this study, five cadaveric distal femora were prepared and femoral knee components were implanted by an experienced surgeon. Micro-CT scans and conventional CT-scans were obtained pre- and post-implantation for geometrical measurements and to measure bone mineral density. In addition, the position of the implant with respect to the bone was determined by optical scanning of the reconstructions (Figure.1). By measuring the differences in surface geometry, assessments were made of the cutting error, the actual interference fit, the amount of bone damage, and the effective interference fit.

Our analysis showed an average cutting error of 0.67± 0.17 mm, which pointed mostly towards bone under-resections. We found an average actual AP interference fit of 1.48± 0.27 mm, which was close to the nominal value of 1.5 mm. We observed combinations of bone damage and elastic deformation in all bone specimens (Figure. 2), which showed a trend to be related with bone density. Higher bone density tended to lead to lower bone damage and higher elastic deformation (Figure. 3). The results of the current study indicate different factors that interact while implanting an uncemented femoral knee component. This knowledge can be used to fine-tune design criteria of femoral components and obtain adequate primary stability for all patients in a more predictable way.

Introduction

Tibial slope was shown to majorly affect the outcomes of Total Knee Arthroplasty (TKA). More slope of the tibial component could help releasing a too tight flexion gap in cruciate-retaining (CR) TKA and is generally associated with a wider range of post-operative knee flexion. However, an excessive tibial slope could jeopardize the knee stability in flexion. The mechanism by which tibial slope affects the function of CR-TKA is not well understood. Moreover, it is not known whether the tibial bone resection should be performed by referencing the anterior cortex (AC) of the tibia or the center of the tibial plateau (CP) and whether the choice of either technique plays a role. The aim of this study was to investigate the effect of tibial slope on the position of tibiofemoral (TF) contact point, knee ligament forces, quadriceps muscle forces, and TF and patellofemoral (PF) joint contact forces during squat activity in CR-TKA.

Introduction

Fretting corrosion of the modular taper junction in total hip arthroplasty has been studied in several finite element (FE) investigations. In FE analyses, different parameters can be varied to study micromotions and contact pressures at the taper interface. However, to truly study taper wear, the simulation of micromotions and contact pressures in non-adaptive FE models is insufficient, as over time these can change due to interfacial changes caused by the wear process.

In this study we developed an FE approach in which material removal during the wear process was simulated by adaptations to the taper geometry. The removal of material was validated against experiments simulating the clinical fretting wear process.

Aims

Revision total elbow arthroplasty (TEA) is often challenging. The aim of this study was to report on the clinical and radiological results of revision arthroplasty of the elbow with the Latitude TEA.

To achieve a long-lasting fixation of uncemented femoral knee implants, an adequate primary stability is required. Several factors, including the applied load, bone quality, surgical preparation, and implant characteristics affect the primary fixation. Recently, novel Attune® cementless femoral component has been proposed by DePuy Synthes (Warsaw, IN, USA). We aimed to compare the primary stability of this novel high-flex design against the conventional LCS® under different loading conditions (gait, deep knee bend (DKB), and high-flex loading), while accounting for the effect of bone quality and cut accuracy.

Six pairs of femora were prepared following the normal surgical procedure. Calibrated CT-scans and 3D-optical scans of the bones were obtained to measure bone mineral density (BMD) and bone cut accuracy, respectively. After implantation of the appropriate size implants (Left legs: Attune; right: LCS), a black-and-white speckle pattern was applied to each specimen (Fig.1B). The micromotion measurement was repeated three times in nine regions of interest (ROIs): the medial and lateral condyles from the posterior view; anterior, distal, and posterior regions from the medial and lateral views; the proximal tip of the anterior flange. The reconstructions were subjected to a gait load and a portion (around 50%) of the peak force of a DKB to prevent fracture of the proximal femur (Fig. 1A and Table. 1). The loads were derived from the Orthoload database using implant-specific inverse dynamics [1]. In addition, a sequence of DIC-images synchronized with the applied load was captured to find the relationship between micromotion and load. Afterwards, implants were pushed-off simulating 150° of flexion, while force-displacement graph was recorded.

BMD and bone cut accuracy were not significantly different between the groups. Under both loading conditions, Attune had a significantly lower micromotion (Table. 1). Cut accuracy was not a significant factor, and BMD was only significant for the comparison under the gait loading (not under DKB conditions). High-flex push-off force was not significantly different. However, Attune required a significantly higher load to reach a micromotion of 50 or 150 µm during the push-off test. Different relations between micromotion and applied load, depending on the loading configuration and implant design, were found (Fig. 2).

Our study has shown a clearly lower range of micromotion for the novel implant. Potential factors to explain the higher micromotion of LCS are parallel anterior and posterior bone cuts in the LCS versus the tapered bone cuts of the Attune. In addition, LCS has a less surface area in contact with bone due to the presence of a rim at the borders of the implant, which may have resulted in lower pre-stresses at the bone-implant interface.

Taking to account, the promising clinical outcome of LCS and also the lower range of micromotion of Attune, we suggest that the Attune has a potential to be at least as successful as the LCS system from a bone fixation point of view. However, further clinical evaluation of the Attune is necessary to assess its performance on the longer term.

INTRODUCTION

Total hip arthroplasty (THA) is a very successful orthopaedic treatment with 15 years implant survival reaching 95%, but decreasing age and increasing life expectancy of THA patients ask for much longer lasting solutions. Shorter and more flexible cementless stems are of high interest as these allow to maintain maximum bone stock and reduce adverse long-term bone remodeling.1 However, decreasing stem length and reducing implant stiffness might compromise the initial stability by excessively increasing interfacial stresses. In general, a good balance between implant stability and reduced stress shielding must be provided to obtain durable THA reconstruction.2

This finite element (FE) study aimed to evaluate primary stability and bone remodeling of a new design of short hip implant with solid and U-shaped cross-section.

Introduction

Initial large-scale clinical studies of porous tantalum implants have been generally promising with well-fixed implants and few cases of loosening [1–3]. An initial retrieval study suggests increased bone ingrowth in a modular tibial tray design compared to the monoblock design [4]. Since micromotion at the bone-implant interface is known to influence bone ingrowth [5], the goal of this study was to determine the effect of implant design, bone quality and activity type on micromotion at the bone-implant interface, through FE modeling.

Introduction

Fretting corrosion at the taper interface has been implicated as a possible cause of implant failure. Using in-vitro testing, fretting wear observed at tapers of retrieved implants may be reproduced (Marriott, EORS-2014). In order to reduce time and cost associated with experimental testing, a validated finite element method (FE) can be employed to study the mechanics at the taper. In this study we compared experimental and representative FE simulations of an accelerated fretting test set-up. Comparison was made by between the FE wear score and volumetric material loss from the testing.

Introduction

A previous computational study on an all-polymer PEEK-on-UHMWPE total knee replacement implant showed improved periprosthetic bone loading, compared to a conventional implant [1]. That study used a simulated gait cycle to determine distal loading, but a patella was not included. Substantial distal decrease of bone remodeling stimulus was found, in accordance with previous reports [2], but it was not consistent with other clinical and post-mortem DEXA results, which found the largest loss of bone stock in the anterior region [3,4]. As patellofemoral forces are relatively low during gait compared to squatting, we simulated a deep squat, expecting that a high-demand activity would provide similar indications of bone loss as literature [3,4]. Consequently, we applied both high tibiofemoral and patellofemoral loads, to provide more insight in the potential benefits of a new PEEK-Optima® femoral component on periprosthetic bone stock.

Introduction

Special high-flexion prosthetic designs show a small increase in postoperative flexion compared to standard designs and some papers show increased anterior knee pain with these prosthesis. However, no randomised controlled trails have been published which investigate difference in postoperative complaints of anterior knee pain. To assess difference in passive and active postoperative flexion and anterior knee pain we performed a randomized clinical trial including the two extremes of knee arthroplasty designs, being a high flex posterior stabilized rotating platform prosthesis versus a traditional cruciate retaining fixed bearing prosthesis. We hypothesised that the HF-PS design would allow more flexion, due to increased femoral rollback with less anterior knee pain than the CR design. We specifically assessed the following hypotheses:

A durable biological fixation between implant and bone depends largely on the micro-motions [Pilliar et al., 1986]. Finite element analysis (FEA) is a numerical tool to calculate micro-motions during physiological loading. However, micromotions can be simulated and calculated in various ways. Generally, only a single peak force of an activity is applied, but it is also possible to apply discretized loads occurring during a continuous activity, offering the opportunity to analyze incremental micro-motions as well. Moreover, micro-motions are affected by the initial press-fit. We therefore aimed to evaluate the effect of different loading conditions and calculation methods on the micro-motions of an uncemented femoral knee component, while varying the interference-fit.

We created an FE model of a distal femur based on calibrated CT-scans. A Sigma® Cruciate-Retaining Porocoat® (DePuy Synthes, Leeds, UK) was placed following the surgical instructions. A range of interference-fits (0–100 µm) was applied, while other contact parameters were kept unchanged. Micro-motions were calculated by tracking the projection of implant nodes onto the bone surface. We defined three different micro-motions measures: micro-motions between consecutive increments of a full loading cycle (incremental), micro-motions for each increment relative to the initial position (reference), and the largest distance between projected displacements, occurring during a discretized full cycle (resulting) (Fig. 1A). Four consecutive cycles of normal gait and squat movements were applied, in different configurations. In the first configuration, incremental tibiofemoral and patellofemoral contact forces were applied, which were derived from Orthoload database using inverse dynamics [Fitzpatrick et al., 2012]. Secondly, we applied the same loads without the patellofemoral force, which is often used in experimental set-ups. Finally, only the peak tibiofemoral force was applied, as a single loading instance. We calculated the average of micro-motions of all nodes per increment to compare different calculation techniques. The percentage of area with resulting micro-motions less than 5 µm was also calculated.

The percentage of surface area was increased non-linearly when the interference fit changed from 0 to 100 µm particularly for squat movement. Tracking nodes over multiple cycles showed implant migration with interference-fits lower than 30µm (Fig. 1A). Loading configurations without the patellofemoral force, and with only the peak tibiofemoral force slightly overestimated and underestimated the resulting micro-motions of squat movement, respectively; although, the effect was less obvious for the gait simulation when no patella force was applied. Both incremental and reference micro-motions underestimated the resulting micro-motions (Fig. 1B). Interestingly, the reference micro-motions followed the pattern of the tibiofemoral contact force (Fig. 1B).

The calculation technique has a substantial effect on the micro-motions, which means there is a room for interpretation of micro-motions analyses. This furthermore stresses the importance of validation of the predicted micro-motions against experimental set-ups. In addition, the minor effect of loading configurations indicates that a simplified loading condition using only the peak tibiofemoral force is suitable for experimental studies. From a clinical perspective, the migration pattern of femoral components implanted with a low interference fit stresses the role of an adequate surgical technique, to obtain a good initial stability.

Femoral knee implants have promising outcomes, although some high-flex designs have shown rather high loosening rates (Han et al., 2007). In uncemented implants, it is vital to limit micromotions at the implant-bone interface, to facilitate secondary fixation through bone ingrowth (kienapfel et al., 1999). Hence, it is essential to investigate how micromotions of different uncemented implants are affected by various loading conditions when a range of bone qualities as a patient-related factor is applied.

Using finite element (FE) analysis, we simulated implant-bone interface micromotions during four consecutive cycles of normal gait and squat movements. An FE model of a distal femur was generated based on calibrated CT-scans, after which Sigma® and LCS® Cruciate-Retaining Porocoat® components (DePuy Synthes, Leeds, UK) were implanted. Using a frictional contact algorithm (µ=0.95), an initial press-fit fixation was simulated, which was previously validated against experimental data. The micromotions were calculated by tracking the projection of implant nodes on the bone surface excluding overhang area. The applied loading patterns were based on discretized simulations, providing incremental loads for each activity based on implant-specific kinematics, which was derived from Orthoload database using inverse dynamics (Fitzpatrick et al., 2012). This provided the opportunity to calculate incremental micromotions, but also the resulting micromotions for each single cycle, for both activities. In addition, the percentage of implant surface area with resulting micromotions less than a defined threshold was calculated.

Regardless of the type of loading, in all simulations, the predicted micromotions were highest in the first cycle, suggesting settling of the implant during initial cycle. The Sigma®implant displayed a 30% larger area with micromotions below the threshold of 5 microns, for both loading conditions (Fig. 1A). The highest micromotions occurred at the anterior flange, regardless of type of activity or design. Squatting had a more detrimental effect on the primary stability, with smaller areas of low micromotions as compared to the gait load (Fig. 1B). Bone stiffness had a minor effect, which was more apparent for squatting (Fig. 1B).

We found acceptable low ranges of micromotions in both implant designs, although demanding activities such as squatting generated higher motions. In addition, LCS® experienced higher micromotions, probably caused by the smaller contact area at bone-implant interface compared with Sigma®. Nevertheless, the predicted micromotions were all below the clinically relevant threshold for bone ingrowth (<40 microns) (kienapfel et al., 1999). Furthermore, our simulated settling behavior stresses the necessity for simulating multiple loading cycles, rather than just a single cycle. The effect of bone stiffness was evident, but only to a limited extent. The main current limitation of our study is the utilization of an elastic material model for the bone which is probably the reason to predict a low range of micromotions. We are planning to make the material model more realistic, by including plasticity and viscoelastic bone behavior.

Introduction

Conventional implant designs in total knee arthroplasty (TKA) are based on metal on UHMWPE bearing couples. Although this procedure is quite successful, early loosening is still a matter of concern. One of the causes for early failure is stress shielding, leading to loss of bone stock, periprosthetic bone fractures and eventually aseptic loosening of the component. The introduction of a polyetheretherketone (PEEK) on UHMWPE bearing couple could address this problem. With mechanical properties more similar to distal (cortical) bone it could allow stresses to be distributed more naturally in the distal femur. A potential adverse effect, however, is that the femoral component and the underlying cement mantle may be at risk of fracturing. Therefore, we analyzed the effect of a PEEK-Optima® femoral component on stress shielding and the integrity of the component and cement mantle, compared to a conventional Cobalt-Chromium (CoCr) alloy implant.

Introduction

Recent reports implicate fretting corrosion at the head-stem taper junction as a potential cause of failure of some large diameter metal-on-metal (MOM) devices. Fretting observed at modular junctions is thought to be a type of ‘mechanically assisted’ corrosion phenomenon, initiated by mechanical factors that lead to an increase in contact stresses and micromotions at the taper interface. These may include: intra-operative taper assembly, taper contamination by debris or body fluids, patient weight and ‘toggling’ of the head or increased frictional torque in a poorly functioning bearing.

We adopted a finite element approach to model the head-taper junction, to analyze the contact mechanics at the taper interface. We investigated the effect of assembly force and angle on contact pressures and micromotions, during loads commonly used to test hip implants.

INTRODUCTION

The burden of Musculoskeletal (M-S) diseases and prosthetic revision operations is huge and increasing rapidly with the aging population. For patients that require a major surgical intervention, procedures are unsafe, uncertain in outcome and have a high complication rate. The goal of this project is to create an ICT-based patient-specific surgical navigation system that helps the surgeon safely reaching the optimal functional result for the patient and is a user friendly training facility for the surgeons. The purpose of this paper is to demonstrate the advancements in personalized musculoskeletal modeling for patients who require severe reconstructive surgery of the lower extremity.

We present the results of 62 consecutive acetabular revisions using impaction bone grafting and a cemented polyethylene acetabular component in 58 patients (13 men and 45 women) after a mean follow-up of 27 years (25 to 30). All patients were prospectively followed. The mean age at revision was 59.2 years (23 to 82).

We performed Kaplan–Meier (KM) analysis and also a Competing Risk (CR) analysis because with long-term follow-up, the presence of a competing event (i.e. death) prevents the occurrence of the endpoint of re-revision.

A total of 48 patients (52 hips) had died or had been re-revised at final review in March 2011. None of the deaths were related to the surgery. The mean Harris hip score of the ten surviving hips in ten patients was 76 points (45 to 99).

The KM survivorship at 25 years for the endpoint ‘re-revision for any reason’ was 58.0% (95% confidence interval (CI) 38 to 73) and for ‘re-revision for aseptic loosening’ 72.1% (95% CI 51 to 85). With the CR analysis we calculated the KM analysis overestimates the failure rate with respectively 74% and 93% for these endpoints. The current study shows that acetabular impaction bone grafting revisions provide good clinical results at over 25 years.

Cite this article: Bone Joint J 2015;97-B:1338–44.

Unlinked, linked and convertible total elbow arthroplasties (TEAs) are currently available. This study is the first to report the clinical results of the convertible Latitude TEA. This was a retrospective study of a consecutive cohort of 63 patients (69 primary TEAs) with a mean age of 60 years (23 to 87). Between 2006 and 2008 a total of 19 men and 50 women underwent surgery. The mean follow-up was 43 months (8 to 84). The range of movement, function and pain all improved six months post-operatively and either continued to improve slightly or reached a plateau thereafter. The complication rate is similar to that reported for other TEA systems. No loosening was seen. Remarkable is the disengagement of the radial head component in 13 TEAs (31%) with a radial head component implanted.

Implantation of both the linked and the unlinked versions of the Latitude TEA results in improvement of function and decreased pain, and shows high patient satisfaction at mid-term follow-up.

Cite this article: Bone Joint J 2015; 97-B:681–8.

Summary

Our statistical shape analysis showed that size is the primary geometrical variation factor in the medial meniscus. Shape variations are primarily focused in the posterior horn, suggesting that these variations could influence cartilage contact pressures.

We determined the short-term clinical outcome and migration within the bone of the humeral cementless component of the Instrumented Bone Preserving (IBP) total elbow replacement in a series of 16 patients. There were four men and 12 women with a mean age at operation of 63 years (40 to 81). Migration was calculated using radiostereometric analysis. There were no intra-operative complications and no revisions. At two-year follow-up, all patients showed a significant reduction in pain and functional improvement of the elbow (both p < 0.001). Although ten components (63%) showed movement or micromovement during the first six weeks, 14 (88%) were stable at one year post-operatively. Translation was primarily found in the proximal direction (median 0.3 mm (interquartile range (IQR) -0.09 to 0.8); the major rotational movement was an anterior tilt (median 0.7° (IQR 0.4° to 1.6°)). One malaligned component continued to migrate during the second year, and one component could not be followed beyond three months because migration had caused the markers to break off the prosthesis.

This study shows promising early results for the cementless humeral component of the IBP total elbow replacement. All patients had a good clinical outcome, and most components stabilised within six months of the operation.

Cite this article: Bone Joint J 2014;96-B:229–36.

Pegs are often used in cementless total knee replacement (TKR) to improve fixation strength. Studies have demonstrated that interference fit, surface properties, bone mineral density (BMD) and viscoelasticity affect the performance of press-fit designs. These parameters also affect the insertion force and the bone damage occurring during insertion. We aimed to quantify the effect of the aforementioned parameters on the short-term fixation strength of cementless pegs.

6 mm holes were drilled in twenty-four human femora. BMD was measured using calibrated CT-scans, and randomly assigned to samples. Pegs were produced to investigate the effect of interference fit (diameters 6.5 and 7.6 mm), surface treatment (smooth and rough- porous-coating [friction coefficient: 1.4]) and bone relaxation (relaxation time 0 and 30 min) and interactions were studied using a DOE method. Two additional rough surfaced peg designs (diameters 6.2 and 7.3 mm) were included to scrutinize interference. Further, a peg based on the LCS Porocoat® (DePuy Synthes Joint Reconstruction, Leeds, UK) was added as a clinical baseline. In total seven designs were used (n = 10 for all groups). Pegs were inserted and extracted using an MTS machine (Figure 1), while recording force and displacement. Bone damage was defined as the difference between the cross-sectional hole area prior to and after the test.

BMD and interference fit were significant factors for insertion force. BMD had a significant positive correlation with pull-out force and subsequent analyses were therefore normalised for BMD.

Pull-out force increased significantly with interference for both surface coatings at time 0 (p < 0.05). However, after 30 minutes the effect remained significant for rough pegs only (p < 0.05-Figure 2A).

Additionally, the pull-out force increased with diameter in a non-linear manner for the rough pegs (Figure 2B). The two surface treatments were not significantly different to the clinical comparator. Interference fit was the only significant factor for bone damage.

BMD was significant for insertion and pull-out forces, reinforcing the need to account for this factor in biomechanical studies and clinical practice. This study also highlights the importance of time in studying bone interactions, with surface treatment and interference showing different interaction effects with relaxation time. Although smooth pegs initially have a higher pull-out force, this effect reduces over time whereas the pullout force for rough pegs is maintained. Smooth pegs also show time sensitivity in relation to interference and the benefit of increased interference reduces over time, whereas it is maintained in rough pegs. This may be explained by different damage (compressive and abrasive) mechanisms associated with different surface treatments.

In conclusion, BMD and interference fit are significant factors for initial fixation. Bone relaxation plays an important role as it reduces the initial differences between groups. Therefore, these findings should be strongly considered in the design development of cementless TKR.

The effect of an advanced porous surface morphology on the mechanical performance of an uncemented femoral knee prosthesis was investigated. Eighteen implants were inserted and then pushed-off from nine paired femurs (Left legs: advanced surface coating; right legs: Porocoat® surface coating as baseline). Bone mineral density (BMD) and anteroposterior dimension were measured, which both were not significantly different between groups. The insertion force was not significantly different, but push-off force was significantly higher in the advanced surface coating group (P = 0.007). BMD had direct relationship with the insertion force and push-off force (p < 0.001). The effect of surface morphology on implant alignment was very small. We suggest that the surface properties create a higher frictional resistance thereby providing a better inherent stability of implants featuring the advanced surface coating.

Introduction

Recent reports have implicated fretting corrosion at the head-stem taper junction as a potential cause of failure of some large diameter metal-on-metal (MOM) devices. While it has been suggested that larger MOM heads, may induce greater frictional torques at the taper connection, the exact mechanisms underlying fretting corrosion remain poorly understood.

It is likely that the onset of the corrosion process is caused by mechanical factors, such as contact stresses and micromotions occurring at the interface. These stresses and micromotions depend on the fixation of the head onto the stem and may be affected by blood, fat, bone debris or other contaminations. The fixation of the head is achieved intraoperatively through impaction.

To further study this phenomenon, we adopted a finite element approach in which we modeled the head-taper junction fixation mechanics. In this model, we analyzed the effect of impaction force on the micromotions occurring at the head-stem interface.

To achieve desirable outcomes in cementless total knee replacement (TKR), sufficient primary stability is essential. The primary stability inhibits excessive motions at the bone-implant interface, hence providing the necessary condition for osseointegration [1]. Primary stability for cementless TKR is provided by press-fit forces between the bone and implant. The press-fit forces depend on several factors including interference fit, friction between bone and implant surface, and the bone material properties. It is expected that bone mineral density (BMD) will affect the stability of cementless TKR [2]. However, the effect of BMD on the primary stability of cementless femoral knee component has not been investigated in vitro.

Phantom calibrated CT-scans of 9 distal femora were obtained after the surgical cuts were made by an experienced surgeon. Since the press-fit forces of the femoral component mainly occur in the Anteroposterior (AP) direction, the BMD was measured in the anterior and posterior faces for a depth of 5 mm; this depth was based on stress distributions from a Finite Element Analysis of the same implant design. In addition, four strain gauges were connected to different locations on the implant's outer surface and implant strain measured throughout as an indication of underlying bone strain. A cementless Sigma CR femoral component (DePuy Synthes Joint Reconstruction, Leeds, UK) was then implanted using an MTS machine. In order to simulate a ‘normal’ bone condition, the implanted bone was preconditioned for one hour at a cyclic load of 250–1500 N, and a rate of 1 Hz. Finally, the implants were pushed-off from the bone in a high-flex position. Forces and displacements were recorded both during insertion and push-off tests.

Strong correlations were found for insertion and push-off forces with BMD, R² = 0.88 and R² = 0.88, respectively (p < 0.001), so although implantation may be harder in patients with higher BMD, initial stability is also improved. A correlation was also found between final strain and push-off forces (R² = 0.89, p < 0.01) and BMD also showed a strong reverse correlation with total bone relaxation (R² = 0.76, p = 0.023). These results indicate that higher BMD induces higher bone strain, which can lead to improved fixation strength.

There is no consensus on the best fixation method for the TKR but some surgeons prefer a cementless design for young and active patients. The results of our study showed that the primary stability of a cementless femoral knee component is directly correlated with the bone mineral density. Therefore, patient selection based on bone quality may increase the likelihood of good osseointegration and adequate long-term fixation for cementless femoral knee components.

Introduction

Current clinical practice in total knee arthroplasty (TKA) is largely based on metal on polyethylene bearing couples. A potential adverse effect of the stiff metal femoral component is stress shielding, leading to loss of bone stock, periprosthetic bone fractures and eventually aseptic loosening of the component. The use of a polymer femoral component may address this problem. However, a more flexible material may also have consequences for the fixation of the femoral component. Concerns are raised about its expected potential to introduce local stress peaks on the interface.

The objective of this study was to analyze the effect of using a polyether-etherketone (PEEK-Optima®) femoral component on the cement-implant interface. We analyzed the interface stress distribution occurring during normal gait, and compared this to results of a standard CoCr component.

Introduction

Many finite element (FE) studies have been performed in the past to assess the biomechanical performance of TKA and THA components. The boundary conditions have often been simplified to a few peak loads. With the availability of personalized musculoskeletal (MS) models we becomes possible to estimate dynamic muscle and prosthetic forces in a patient specific manner. By combining this knowledge with FE models, truly patient specific failure analyses can be performed.

In this study we applied this combined technique to the femoral part of a cementless THR and calculated the cyclic micro-motions of the stem relative to the bone in order to assess the potential for bone ingrowth.

Introduction

A few follow-up studies of high flexion total knee arthoplasties report disturbingly high incidences of femoral loosening. Finite element analysis showed a high risk for early loosening at the cement-implant interface at the anterior flange. However, femoral implant fixation is depending on two interfaces: cement-implant interface and the cement-bone interface. Due to the geometry of the distal femur, a part of the cement-bone interface consists of cement-cortical bone interface. The strength of the cement-bone interface is lower than the strength of the cement-implant interface.

The research questions addressed in this study were: 1) which interface is more prone to loosening and 2) what is the effect of different surgical preparation techniques on the risk for early loosening.

INTRODUCTION

Electron beam melting is a promising technique to produce surface structures for cementless implants. Biomimetic apatite coatings can be used to enhance bone ingrowth. The goal of this study was to evaluate bone ingrowth of an E-beam produced structure with biomimetic coating and compare this to an uncoated structure and a conventionally made implant surface.

In the present study we describe the clinical results of the Scientific Hip Prosthesis® (SHP). With the goal of smoothening cement-bone interface stress peaks, the SHP was developed using shape optimization algorithms together with finite element modelling techniques. The resulting shape and cement stresses are seen in Figure 1. The introduction of the SHP prosthesis was performed in a stepwise fashion including a RSA study performed by Nivbrant et al¹. RSA studies for prosthetic types that are in long-term use are of great value in predicting the survivorship related to the migration rate and pattern for that specific type of prosthesis. If a stem in a patient shows a much higher migration rate than the typical one, the stem may be identified as at high-risk for early loosening. The study of Nivbrant et al¹ revealed unexpectedly high migration values and it was stated that the SHP stem was not the preferred stem to use despite the good Harris Hip Score and Pain score at two years follow-up.

In the present study the clinical results of a single surgeon study consisting of 171 hips with a follow-up of 5–12 years were evaluated. The mean follow-up was 8.2 years (5.0–12.0). The survival rate was 98.8% at ten years follow-up for aseptic loosening of the stem. The mean Harris Hip Score at 10 year follow-up was 89.2 ± 7.5. This study therefore indicates that a new prosthetic design may function clinically rather well, despite the relatively high migration rates which have been reported.

In case of a RSA study with a new prosthesis it may not be so evident what the expected “typical” migration rate or pattern is. So in order to predict early loosening the typical migration rate has to be known. Perhaps typical migration rates can be established using standardized cadaver migration experiments or computer simulation models techniques. Since these standardized tools are currently not available, the prediction of clinical survival of new prosthetic components remains a challenging task and the interpretation of migration rates with new designs should be considered with much caution.

In cemented total hip arthroplasty, the cement-bone interface can be considerably degraded in less than one year in-vivo service (Figure 1). This makes the interface much weaker relative to the direct post-operative situation. Retrieval studies show that patients do, to a certain extent, not suffer from the degraded cement-bone interface itself. It is, however, unknown whether the degraded cement-bone interface affects other failure mechanisms in the cemented hip reconstruction. A good understanding of the mechanics of the cement-bone interface is therefore essential. The aim of this study was to investigate the mechanics of the cement-bone interface in the direct post-operative and degraded situation by the utilization of finite element analysis (FEA) and laboratory experiments. It was subsequently analyzed how the mechanics of the cement-bone interface affect failure of the cement mantle in terms of crack formation.

In order to investigate the mechanical response of the cement-bone interface, laboratory prepared (direct post-operative state) and postmortem (degraded state) specimens were loaded in various directions in the laboratory and FEA environment. From all specimens, multiple interface morphology parameters were documented, which were related to the interfacial response and subsequently converted to a numerical cohesive model. As a validation, this cohesive model was implemented into two FEA models of transverse sections of cemented hip reconstructions with distinct mechanical characteristics (Figure 2). Finally, the differences in fatigue crack formation in a complete hip reconstruction were determined by varying the cement-bone interface compliance (Figure 3).

When loaded in multiple directions, the interface compliance could not be related to the cement interdigitation depth (r²=0.08). However, compliance did correlate to the gap thickness between the bone and cement (r²=0.81) and the amount of interfacial contact (r²=0.50). Surprisingly, for the same amount of contact, the interface was more compliant in degraded state than in the direct post-operative state. The mechanical response of the experimental and FEA cement-bone interface tests could, independent on the direct post-operative or degraded state, successfully be described by a cohesive model. The cohesive model was even more confirmed by the successful reproduction of the mechanics of the retrieved transverse sections. When the cohesive model was implemented in a complete reconstruction, we found that a compliant cement-bone interface resulted in considerably more fatigue cracks in the cement mantle than a very stiff interface.

This study showed that an increased compliancy of the cement-bone interface results in an increase of cement cracks in the cement mantle. It is therefore crucial to minimize the interfacial gaps and, as a result, increase the amount of contact between the bone and cement to generate a stiff cement-bone interface. It is, unfortunately, unknown how this well fixed interface can be maintained. We finally conclude that the derived cohesive model of the cement-bone interface can be used for multiple applications in orthopaedics, including pre-clinical of implants and patient specific studies of failed cemented reconstructions.

For amputated patients, direct attachment of upper leg prosthesis to the skeletal system by a percutaneous implant is an alternative solution to the traditional socket fixation. Currently available implants, the OPRA system (Integrum AB, Göteborg, Sweden) and the ISP Endo/Exo prosthesis (ESKA Implants AG, Lübeck, Germany) [1-2] allow overcoming common soft tissue problems of conventional socket fixation and provide better control of the prosthetic limb [3], higher mobility and comfort [2, 4]. However, restraining issues such as soft-tissue infections, peri-prosthetic bone fractures [3, 5–8] and considerable bone loss around the stem [9], which might lead to implant's loosening, are present. Finally, a long a residual limb is required for implant fitting.

In order to overcome the limiting biomechanical issues of the current designs, a new concept of the direct intramedullary fixation was developed. The aim was to restore the natural load transfer in the femur and allow implantations in short femur remnants (Figure 1). We hypothesize that the new design will reduce the peri-prosthetic bone failure risk and adverse bone remodeling.

Generic CT-based finite element models of an intact femoral bone and amputated bones implanted with 3 analyzed implants were created for the study. Models were loaded with two loading cases from a normal walking obtained from the experimental measurements with the OPRA device [10-11]. Periprosthetic bone failure risk was evaluated by considering the von Mises stress criterion [12-14]. Subsequently the strain adaptive bone remodeling theory was used to predict long-term changes in bone mineral density (BMD) around the implants. The bone mineral content (BMC) change was measured around implants and the results were visualized in the form of DXA scans.

The OPRA and the ISP implants induced the high stress concentration in the proximal region decreasing in the distal direction to values below physiological levels as compared with the intact bone. The stresses around the new design were more uniformly distributed along the cortex and resembled better the intact case. Consequently, the bone failure risk was reduced as compared to the OPRA and the ISP implants. The adaptive bone remodeling simulations showed high bone resorption around distal parts of the OPRA and the ISP implants in the distal end of the femur (on average −75% ISP to −78% OPRA after 60 months). The bone remodeling simulation did not reveal any bone loss around the new design, but more bone densification was seen (Figure 2). In terms of total bone mineral content (BMC) the OPRA and the ISP implants induced only a short-term bone densification in contrast to the new design, which provoked a steady increase in the BMC over the whole analyzed period (Figure 3).

In conclusion, we have seen that the new design offers much better bone maintenance and lower failure probability than the current osseointegrated trans-femoral prostheses. This positive outcome should encourage further developments of the presented concept, which in our opinion has a potential to considerably improve safety of the rehabilitation with the direct fixation implants and allow treatment of patients with short stumps.

INTRODUCTION

Patellofemoral joint (PFJ) replacement is a successful treatment option for isolated patellofemoral osteoarthritis. With this approach only the involved joint compartment is replaced and the femoro-tibial joint remains intact. Minimizing periprosthetic bone loss, which may occur due to the stress shielding effect of the femoral component, is important to insure long-term outcomes. The objective of this study was to investigate, using finite element analyses, the effects of patellofemoral replacement on the expected stress distribution of the distal femur eventually leading to changes in bone density.

Previously, we showed that case-specific non-linear finite element (FE) models are better at predicting the load to failure of metastatic femora than experienced clinicians. In this study we improved our FE modelling and increased the number of femora and characteristics of the lesions. We retested the robustness of the FE predictions and assessed why clinicians have difficulty in estimating the load to failure of metastatic femora. A total of 20 femora with and without artificial metastases were mechanically loaded until failure. These experiments were simulated using case-specific FE models. Six clinicians ranked the femora on load to failure and reported their ranking strategies. The experimental load to failure for intact and metastatic femora was well predicted by the FE models (R² = 0.90 and R² = 0.93, respectively). Ranking metastatic femora on load to failure was well performed by the FE models (τ = 0.87), but not by the clinicians (0.11 < τ < 0.42). Both the FE models and the clinicians allowed for the characteristics of the lesions, but only the FE models incorporated the initial bone strength, which is essential for accurately predicting the risk of fracture. Accurate prediction of the risk of fracture should be made possible for clinicians by further developing FE models.