Introduction

Edge loading is a common wear mechanism in Metal-on-Metal (MOM) hip resurfacing and is associated with higher wear rates and the incidence of pseudotumour. The purpose of this study was to develop a method to investigate the contributions of patient, surgical and implant design variables on the risk of edge loading.

Introduction

It has been speculated that impact deformation of thin 1-piece cups used for modern metal-on-metal hip replacement may contribute to early failure. The purpose of this study was to reproduce typical impact deformation and quantify the effect of this on the frictional torque generated at the hip.

Introduction

The new Knee Society Score has been developed and validated, in part, to characterize better the expectations, components of satisfaction, and the physical activities of the younger, more diverse modern population of TKA patients. This study aims to reveal patients' activity levels' post-TKA and to determine how it contributes to their subjective evaluation of the surgery.

Introduction

Knee prostheses retrieved at revision often show patterns and severity of damage neither seen nor predicted from standard wear simulator testing. We hypothesized that this is because these implants are exposed to combinations of loads and motions that are more damaging than the simple loading profiles utilized in laboratory testing. We examined the magnitude, direction, and combination of forces and moments acting on the knee during various activities in order to guide the future development and testing of high-performance knee replacements.

Introduction

Although the “learning curve” in surgical procedures is well recognized, little data exists documenting the accuracy of surgeons in performing individual steps of orthopedic procedures. In this study we have used a validated computer-based training system to measure variations instrument placement and alignment in TKA, specifically those relating to tibial preparation.

Introduction

Femoro-acetabular impingement (FAI) is a common source of impaired motion of the hip, often attributed to the presence of an aspherical femoral head. However, other types of femoral deformity, including posterior slip, retroversion, and neck enlargement, can also limit hip motion. This study was performed to establish whether the “cam” impinging femur has a single deformity of the head/neck junction or multiple abnormalities.

Introduction

Proper rotational alignment of the tibial component is a critical factor affecting the outcome of TKA. Traditionally, the tibial component is oriented with respect to fixed landmarks on the tibia without reference to the plane of knee motion. In this study, we examined differences between rotational axes based on anatomic landmarks and the true axis of knee motion during a functional activity.

Joint Registries are a valuable resource for defining the survivorship of prostheses and procedures undertaken for the treatment of joint disease. However, the use of this data as a basis for advocating specific implant designs is controversial because of the confounding effects of variations in patient selection, the training, skill and experience of surgeons, and the priorities of individual patients. Despite these challenges, the Australian Joint Registry has utilized its early survivorship data to identify specific designs that are expected to exhibit lower than average durability in the long term. The aim of this study was to assess the accuracy of this practice in identifying implants providing inferior long-term performance.

Over the period 2004–8, the Australian Registry identified 48 prosthetic components used in primary THA, HRA, TKA or UKA which exhibited a statistically significant increase in the early revision rate. For each of these components, we compared the rate of revisions per 100 “component-years” when it was first identified by the Registry, to its ultimate fiveyear cumulative survival in 2008. These survival parameters were also compared to average values based on procedure (eg.THR) and fixation method (i.e. cemented, cementless, hybrid).

Regression analysis was performed to determine the accuracy of initial relative revisions per 100 OCY as a predictive measure of eventual component revision rate.

Five year survival data was available on 30 of the 48 implants identified by the registry. There was a strong correlation (R2=0.9614) between initial revisions per 100 component-years and the 5-yr survival of the identified designs. 29 of 30 designs (97%) exhibited lower than average survivorship at 5 years. Six designs (20%) had failure rates within 2% of average values, and 7 (23%) had a 5–year failure rate less than 50% above average values. Although, when identified by the Registry, 80% of identified components exceeded the average rate of revision by 100%, only 60% displayed more than twice the cumulative revision rate at 5 years post-op.

These results demonstrate that early data collected by Joint Registries can form the basis of accurate identification of designs which ultimately prove to be clinically unsuccessful. Predictions made by the Australian Registry concerning inferior designs have an accuracy of approximately 80%. Further work is recommended to enhance the valuable potential of Registry data in predicting the outcome of both implants and procedures.

Several studies have suggested that, in TKR, gender specific-prostheses are needed to accommodate anatomic differences between males and females. This study was performed to examine whether gender is a factor contributing to the variability of the size, shape and orientation of the patellofemoral sulcus.

3D computer models of the femur were reconstructed from CT scans of 20 male and 20 female femora. The patellofemoral groove was quantified by measuring landmarks at 10 degree increments around the epicondylar axis. The orientation of the groove was defined by the tracking path generated by a sphere moving from the top of the groove to the intercondylar notch. To assess the influence of gender on the shape of the distal femur, all morphologic parameters were normalized for differences in bone size.

Overall, the distal femur was 15% larger in males compared to females. The male condyles were 4% wider than the female for constant AP depth (p=0.13). When normalized for bone size, there was no gender difference in most patello-femoral dimensions, including the length, width, angle or tilt of the sulcus. Female femora had a less prominent medial anterior ridge (p=0.07), and a larger normalized radius of curvature of the tracking path (p=0.03). In addition, the orientation of the sulcus differed by 1–2 degrees in both the coronal and axial planes. Overall, gender explained 4.7% of the anatomic variation of the parameters examined, varying from 0 to 15.9%.

The size, shape and orientation of the patello-femoral groove are highly variable.

While the patello-femoral morphology of male and female femora are very similar, some of the anatomic variability is related to gender, particularly the prominence of the medial ridge and the sulcus radius of curvature. The biomechanical and clinical significance of these differences after TKA have yet to be determined.

Introduction: Most surgeons agree on basic parameters defining a successful joint replacement procedure. However, the process of acquiring the skills to achieve this level of success on a reproducible basis is much less straightforward. In reality, it is generally not possible to impart surgical training without some level of risk to the patient, particularly if a particular trainee or procedure has a long learning curve. In an attempt to address these issues, we have developed a new computer-based training system to measure the technical results of hip and knee replacement surgery in both the operating room and the Bioskills Lab.

Description of the System: This system utilizes Surgical Navigation technology combined with data analysis and display routines to monitor the position and alignment of instruments and implants during the procedure in comparison with a preoperative plan. For bioskills training, the surgeon develops a preoperative plan on a computer workstation using accurate 3D computer models of the bones and appropriate implants. The surgeon then performs the entire procedure using the cadaver or sawbone model. During the procedure, the position and orientation of the bones, each surgical instrument, and the trial components are measured with a three-dimensional motion analysis system. Through analysis of this data, the surgeon is able to view each step of the surgical procedure, the placement of each instrument with respect to each bone, and the consequences of each surgical decision in terms of the final placement of the prosthetic components When errors are detected in the implementation of the preoperative plan, the surgeon is able to replay each step of the procedure to examine the precise placement of each instrument with respect to each bone and the consequences of each surgical decision in terms of leg length, alignment and range-of-motion.

Conclusions: This system allows us to measure the technical success of a surgical procedure in terms of quantifiable geometric, spatial, kinematic or kinetic parameters. It also provides postoperative feedback to the surgeon by demonstrating the specific contributions of each step of the surgical procedure to deviations in final alignment or soft tissue instability. This approach allows surgeons to be trained outside the operating room prior to patient exposure. Once these skills have been developed, the surgeon is able to operate freely in the operating room without the risks associated with traditional surgical training, or the expense associated with intraoperative Surgical Navigation. The value of this approach in the training and accreditation of orthopedic staff warrants further investigation.

Introduction: Femoro-acetabular impingement (FAI) is a common source of impaired motion of the hip, often attributed to the presence of an aspherical femoral head and reduced concavity of the anterior head/neck junction. However, other types of femoral deformity, including posterior slip, retroversion, and neck enlargement, can also limit hip motion. This study was performed to establish whether the “cam” impinging femur is a unique entity with a single deformity of the head/neck junction or is part of a multi-component continuum of femoral dysmorphia.

Materials and Methods: Computer models of 71 femora (28 normal and 43 “cam” impinging) were prepared from CT scans. Morphologic parameters describing the shape and dimensions of the head, neck, and medullary canal were calculated for each specimen. The anteversion angle, alpha angle of Notzli, beta angle of Beaulé, and normalized anterior heads offset were also calculated. Average dimensions were compared between the normal and impinging femora. A dimensionless model of the femoral neck was also generated to determine whether there is an inherent difference in the shape of the femoral neck in cam impinging and normal femora, independent of any differences in specimen size.

Results: Compared to the normal controls, the impinging femora had wider necks (AP: 15.2 vs 13.3 mm, p< 0.0001), larger heads (diameter: 48.3mm vs 46.0mm, p=0.032) and decreased head/neck ratios (1.60 vs 1.74, p=0.0002). However, there was no difference in neck/shaft angle (125.7° vs 126.5°, p=0.582) or anteversion angle (8.70 vs 8.44°, p=0.866). Most significantly, 53% of impinging femora also had a significant posterior slip (> 2mm), compared to only 14% of normal controls. Average head displacements for the two groups were: FAI: 1.93mm vs Normals: 0.78mm (p< 0.0001). Shape indices derived from individual dimensionless models showed slight AP widening of the abnormal femora (ap/ml ratio: 1.10 abnormal vs. 1.07 normal).

Conclusions: The CAM impinging femur has many abnormalities apart from the morphology of the head/neck junction. These femora have increased neck width and head/neck ratio, a smaller spherical bearing surface, and reduced neck offset from the medullary canal. Moreover, the presence of posterior head displacement and reduced anteversion should be appreciated when assessing treatment options, as surgical treatment limited to localized re-contouring of the head–neck profile may fail to address significant components of the underlying abnormality.

Introduction: The ten-year survivorship of Oxford Unicompartmental Knee Arthroplasty (OUKA) has ranged from 98% in the hands of the developers to only 82–90% in reports from independent centers and national registries. This study was performed to investigate the effects of surgeon training and correct patient selection on the expected outcome of this procedure.

Methods: We created a computer-simulated joint registry consisting of 20 surgeons who performed OUKA on 1,000 patients. Mathematical models of the patient and surgeon populations and corresponding hazard functions were formulated using data from the Swedish and Australian joint registries. The long-term survivorship of UKA was assumed to average 94% at 10 years and was modeled as the product of hazard functions quantifying risk factors under the surgeon’s control, risk factors presented by the patient, and the inherent revision risk of the procedure. We performed four simulations looking at the effect of surgeon training by pairing surgeons and patients based on surgeon experience and patient risk factors.

Results: When experienced surgeons (> 40 cases) performed OUKA on low risk patients (bottom quintile), the revision rate dropped from 6.0% to 4.5%. The same surgeons had a revision rate of 7.5% when assigned to the highest risk patient group (top quintile). Conversely, when the least experienced surgeons (< 10 cases) selected the least fit patients, the revision rate increased from 6% to 8.25%. However, when these surgeons were assigned to the lowest risk group, only 5.25% of patients were revised. Taken simultaneously, these results indicate that the overall revision rate of this procedure can vary between 4.5% to 8.25%, depending upon the experience of the surgeon and the patients selected.

Conclusions:

Mathematical models of patients and surgeons can be built using joint registry data. These models can then be used in a computer simulation yielding results comparable to what has been reported in the literature.

Polyethylene wear debris in TKA arises from several sources, including the tibiofemoral articulation and the interface between the backside surface of the tibial insert and the metal tibial tray. In this study we identify a new source of abrasive damage to the polyethylene bearing surface: impingement of resected bony surfaces, osteophytes and overhanging acrylic cement on the tibial bearing surface during joint motion.

One hundred forty-eight tibial components of 24 different designs in a retrieval collection were examined. A digital image of the articular surface of each insert was recorded. The presence, location and projected area of abrasive wear to the non-articulating edges of the insert were assessed using image analysis software.

Significant abrasive wear was observed in 24% of the retrievals with cemented femoral components and 9% from non-cemented components. Of the retrievals exhibiting this abrasive wear mode, 46% experienced multiple site damage. The average damage area for each individual abrasive scar was 78±11mm^2. Within the group of worn inserts, the abrasive scars were seen with a frequency of 69% on the extreme medial edge, 19% on the extreme lateral edge, 38% on the posterior-medial edge and 23% on the posterior-lateral edge. In posterior stabilized components with an open femoral box design, scarring of the superior surface of the tibial post was also observed. This proposed mode of damage was confirmed with several retrieved femoral components containing either fixed cement protruding from the posterior condyles, from the medial and lateral edges or osteophytes embedded in the posterior capsule. The corresponding inserts exhibited significant abrasive scarring at those locations.

We have observed a previously unrecognized source of polyethylene damage resulting in gouging, abrasion and severe localized damage in cemented and uncemented total knee replacement. Clearly, acrylic cement, in bulk or particulate form, often contributes to severe damage of the tibial surface and improvements to instruments and techniques for cementing are needed to prevent this wear mechanism.

Wear of the underside of modular tibial inserts (“backside wear”) has been reported by several authors. However, the actual volume of material lost through wear of the backside surface has not been quantified. This study reports the results of computerized measurements of tibial inserts of one design known to have a high incidence of backside wear in situ.

A series of retrieved TKA components of one design (AMK, Depuy) with evidence of severe backside wear and extrusions of the polyethylene insert were examined. The three-dimensional surface profile of the backside of each insert was digitized and reconstructed with CAD software (UniGraphics). The volume of material removed was calculated from the volume between the worn backside surface and an “initial” surface defined by unworn areas.

Computer reconstructions showed that in all retrievals, the unworn surface of the remaining pegs, the rim of material extruded over the medial edge and unworn surfaces on the anterior-lateral edge all lie in a single plane. This demonstrates that the “pegs” present on the backside of these inserts correspond to residual, unworn protrusions remaining on each retrieved component and do not represent cold flow extrusions through the base plate holes. The average volume of material lost due to backside wear was 608mm^3 ± 339mm^3 (range:80–1599 mm^3). This corresponds to an average loss of 569mg and an average linear wear rate of 103mg/year, based on the time in situ for each implant.

The volume of material removed due to backside wear is significant and is of a magnitude large enough to generate osteolysis. Our results indicate that the appearance of pegs on the underside of components with screw holes on the baseplate are not due to creep, but instead are due to severe wear of the insert. The mechanisms of material removed due to pitting and burnishing actually produce debris of a size more damaging in terms of osteolysis than wear at the articulating surface making it clear that significant improvements in implant design are needed to prevent backside wear and osteolysis.