Despite the success of total knee arthroplasty (TKA) restoration of normal function is often not achieved. Soft-tissue balance is a major factor leading to poor outcomes including malalignment, instability, excessive wear, and subluxation. Mechanical ligament balancers only measure the joint space in full extension and at 90° flexion. This study uses a novel electronic ligament balancer to measure the ligament balance in normal knees and in knees after TKA to determine the impact on passive and active kinematics. Fresh-frozen cadaver legs (N = 6) were obtained. A standard cruciate-retaining TKA was performed using measured resection approach and computer navigation (Stryker Navigation, Kalamazoo, MI). Ligament balance was measured using a novel electronic balancer (Fig 1, XO1, XpandOrtho, Inc, La Jolla, CA, USA). The XO1 balancer generates controlled femorotibial distraction of up to 120N. The balancer only requires a tibial cut and can be used before or after femoral cuts, or after trial implants have been mounted. The balancer monitors the distraction gap and the medial and lateral gaps in real time, and graphically displays gap measurements over the entire range of knee flexion. Gap measurements can be monitored during soft-tissue releases without removing the balancer. Knee kinematics were measured during active knee extension (Oxford knee rig) and during passive knee extension under varus and valgus external moment of 10Nm in a passive test rig. Sequence of testing and measurement:
Ligament balance was recorded with the XO1 balancer after the tibial cut, after measured resection of the femur, and after soft-tissue release and/or bone resection to balance flexion-extension and mediolateral gaps. Passive and active kinematics were measured in the normal knee before TKA, after measured resection TKA, and after soft-tissue release and/or bone resection to balance flexion-extension and mediolateral gaps.Background
Methods
Wear and fatigue damage to polyethylene components remain major factors leading to complications after total knee and unicompartmental arthroplasty. A number of wear simulations have been reported using mechanical test equipment as well as computer models. Computational models of knee wear have generally not replicated experimental wear under diverse conditions. This is partly because of the complexity of quantifying the effect of cross-shear at the articular interface and partly because the results of pin-on-disk experiments cannot be extrapolated to total knee arthroplasty wear. Our premise is that diverse experimental knee wear simulation studies are needed to generate validated computational models. We combined five experimental wear simulation studies to develop and validate a finite-element model that accurately predicted polyethylene wear in high and low crosslinked polyethylene, mobile and fixed bearing, and unicompartmental (UKA) and tricompartmental knee arthroplasty (TKA). Low crosslinked polyethylene (PE). A finite element analysis (FEA) of two different experimental wear simulations involving TKA components of low crosslinked polyethylene inserts, with two different loading patterns and knee kinematics conducted in an AMTI knee wear simulator: a low intensity and a high intensity. Wear coefficients incorporating contact pressure, sliding distance, and cross-shear were generated by inverse FEA using the experimentally measured volume of wear loss as the target outcome measure. The FE models and wear coefficients were validated by predicting wear in a mobile bearing UKA design. Highly crosslinked polyethylene (XLPE). Two FEA models were constructed involving TKA and UKA XLPE inserts with different loading patterns and knee kinematics conducted in an AMTI knee wear simulator. Wear coefficients were generated by inverse FEA.Background
Methods
Despite the success of total knee arthroplasty (TKA) restoration of normal function is often not achieved. Soft tissue balance is a major factor for poor outcomes including malalignment, instability, excessive wear, and subluxation. Computer navigation and robotic-assisted systems have increased the accuracy of prosthetic component placement. On the other hand, soft tissue balancing remains an art, relying on a qualitative feel for the balance of the knee, and is developed over years of practice Several instruments are available to assist surgeons in estimating soft tissue balance. However, mechanical devices only measure the joint space in full extension and at 90° flexion. Further, because of lack of comprehensive characterization of the ligament balance of healthy knees, surgeons do not have quantitative guidelines relating the stability of an implanted to that of the normal knee. This study measures the ligament balance of normal knees and tests the accuracy of two mechanical distraction instruments and an electronic distraction instrument. Cadaver specimens were mounted on a custom knee rig and on the AMTI VIVO which replicated passive kinematics. A six-axis load cell and an infrared tracking system was used to document the kinematics and the forces acting on the knee. Dynamic knee laxity was measured under 10Nm of varus/valgus moment, 10Nm of axial rotational moment, and 200N of AP shear. Measurements were repeated after transecting the anterior cruciate ligament, after TKA, and after transecting the posterior cruciate ligament. The accuracy and reproducibility of two mechanical and one electronic distraction device was measured.Background
Methods
Kinematic studies are used to evaluate function and efficacy of various implant designs. Given the large variation between subjects, matched pairs are ideal when comparing competing designs. It is logical to deduce that both limbs in a subject will behave identically during a given motion [1], barring unilateral underlying pathology, thus allowing for the most direct comparison of two designs. It is our goal to determine if this is a valid assumption by assessing whether or not there are significant differences present in the kinematics of left and right knees from the same subject. Gait studies have compared pre-and postoperative implantation kinematics for various pathologies like ACL rupture [2] and osteoarthritis [3, 4]. We designed a study to assess squatting in cadaver specimens. Sixteen matched pairs of fresh-frozen cadavers, (Eleven males, five females; aged 71 years [± 10 yrs]) were tested. Each knee, intact, was tested by mounting it on a dynamic, quadriceps-driven, closed-kinetic-chain Oxford knee rig (OKR), which simulated a deep knee bend from full extension to 120° flexion. We chose femoral rollback, tibiofemoral external rotation, tibial adduction, patellofemoral tilt and shift as our outcomes, which were recorded using an active infrared tracking system.Introduction:
Methods:
Telemetric knee implants have provided invaluable insight into the forces occurring in the knee during various activities. However, due to the high amount of cost involved only a few of them have been developed. Mathematical modeling of the knee provides an alternative that can be easily applied to study high number of patients. However, in order to ensure accuracy these models need to be validated with in vivo force data. Previously, mathematical models have been developed and validated to study only specific activities. Therefore, the objective of this study was compare the knee force predictions from the same model with that obtained using telemetry for multiple activities. Kinematics of a telemetric patient was collected using fluoroscopy and 2D to 3D image registration for gait, deep knee bend (DKB), chair rise, step up and step down activities. Along with telemetric forces obtained from the implant, synchronized ground reaction forces (GRF) were also collected from a force plate. The relevant kinematics and the GRF were input into an inverse dynamic model of the human leg starting from the foot and ending at the pelvis (Figure 1). All major ligaments and muscles affecting the knee joint were included in the model. The pelvis and the foot were incorporated into the system so as to provide realistic boundary conditions at the hip and the ankle and also to provide reference geometry for the attachment sites of relevant muscles. The muscle redundancy problem was solved using the pseudo-inverse technique which has been shown to automatically optimize muscle forces based on the Crowninshield-Brand cost function. The same model, without any additional changes, was applied for all activities and the predicted knee force results were compared with the data obtained from telemetry. Comparison of the model predictions for the tibiofemoral contact forces with the telemetric implant data revealed a high degree of correlation both in the nature of variation of forces and the magnitudes of the forces obtained. Interestingly, the model predicted forces with a high level of accuracy for activities in which the flexion of the knee do not vary monotonically (increases and decreases or vice-versa) with the activity cycle (gait, step up and step down). During these activities, the difference between the model predictions with the telemetric data was less than 5% (Figure 2). For activities where flexion varies monotonically (either increases or decreases) with activity (DKB and chair rise) the difference between the forces was less than 10% (Figure 3). The results from this study show that inverse dynamic computational models of the knee can be robust enough to predict forces occurring at the knee with a high amount of accuracy for multiple activities. While this study was conducted only on one patient with a telemetric implant, the required inputs to the model are generic enough so that it is applicable for any TKA patient with the mobility to conduct the desired activity. This allows kinetic data to be provided for the improvement of implant design and surgical techniques accessibly and relatively inexpensively.
Despite over 95% long-term survivorship of TKA, 14–39% of patients express dissatisfaction due to anterior knee pain, mid-flexion instability, reduction in range of flexion, and incomplete return of function. Changing demographics with higher expectations are leading to renewed interest in patient-specific designs with the goal of restoring of normal kinematics. Improved imaging and image-processing technology coupled with rapid prototyping allow manufacturing of patient-specific cutting guides with individualized femoral and tibial components with articulating surfaces that maximize bony coverage and more closely approximate the natural anatomy. We hypothesized that restoring the articular surface and maintaining medial and lateral condylar offset of the implanted knee to that of the joint before implantation would restore normal knee kinematics. To test this hypothesis we recorded kinematics of patient-specific prostheses implanted using patient-specific cutting guides. Preoperative CT scans were obtained from nine matched pairs of human cadaveric knees. One of each pair was randomly assigned to one of two groups: one group implanted with a standard off-the-shelf posterior cruciate-retaining design using standard cutting guides based on intramedullary alignment; the contralateral knee implanted with patient-specific implants using patient-specific cutting guides, both manufactured from the preoperative CT scans. Each knee was tested preoperatively as an intact, normal knee, by mounting the knee on a dynamic, quadriceps-driven, closed-kinetic-chain Oxford knee rig (OKR), simulating a deep knee bend from 0° to 120° flexion. Following implantation with either the standard or patient-specific implant, knees were mounted on the OKR and retested. Femoral rollback, tibiofemoral rotation, tibial adduction, patellofemoral tilt and shift were recorded using an active infrared tracking system.Introduction:
Methods:
Wear and polyethylene damage have been implicated in up to 22% of revision surgeries after unicompartmental knee replacement. Two major design rationales to reduce this rate involve either geometry and/or material strategies. Geometric options involve highly congruent mobile bearings with large contact areas; or moderately conforming fixed bearings to prevent bearing dislocation and reduce back-side wear, while material changes involve use of highly crosslinked polyethylene. This study was designed to determine if a highly crosslinked fixed-bearing design would increase wear resistance. Gravimetric wear rates were measured for two unicompartmental implant designs: Oxford unicompartmental (Biomet) and Triathlon X3 PKR (Stryker) on a knee wear simulator (AMTI) using the ISO-recommended standard. The Oxford design had a highly conforming mobile bearing of compression molded Polyethylene (Arcom). The Triathlon PKR had a moderately conforming fixed bearing of sequentially crosslinked Polyethylene (X3). A finite element model of the AMTI wear simulation was constructed to replicate experimental conditions and to compute wear. This approach was validated using experimental results from previous studies. The wear coefficient obtained previously for radiation-sterilized low crosslinked polyethylene was used to predict wear in Oxford components. The wear coefficient obtained for highly crosslinked polyethylene was used to predict wear in Triathlon X3 PKR components. To study the effect design and polyethylene crosslinking, wear rates were computed for each design using both wear coefficients.INTRODUCTION
METHODS
Knee contact force during activities after total knee arthroplasty (TKA) is very important, since it directly affects component wear and implant loosening. While several computational models have predicted knee contact force, the reports vary widely based on the type of modeling approach and the assumptions made in the model. The knee is a complex joint, with three compartments of which stability is governed primarily by soft tissues. Multiple muscles control knee motion with antagonistic co-contraction and redundant actions, which adds to the difficulty of accurate dynamic modeling. For accurate clinically relevant predictions a subject-specific approach is necessary to account for inter-patient variability. Data were collected from 3 patients who received custom TKA tibial prostheses instrumented with force transducers and a telemetry system. Knee contact forces were measured during squatting, which was performed up to a knee flexion angle that was possible without discomfort (range, 80–120°). Skin marker-based video motion analysis was used to record knee kinematics. Preoperative CT scans were reconstructed to extract tibiofemoral bone geometry using MIMICS (Materialise, Belgium). Subject-specific musculoskeletal models of dynamic squatting were generated in a commercial software program (LifeMOD, LifeModeler, USA). Contact was modeled between tibiofemoral and patellofemoral articular surfaces and between the quadriceps and trochlear groove to simulate tendon wrapping. Knee ligaments were modeled with nonlinear springs: the attachments of these ligaments were adjusted to subject-specific anatomic landmarks and material properties were assigned from published reports.INTRODUCTION
METHODS
Aligning the tibial tray is a critical step in total knee arthroplasty (TKA). Malalignment, (especially in varus) has been associated with failure and revision surgery. While the link between varus malalignment and failure has been attributed to increased medial compartmental loading and generation of shear stress, quantitative biomechanical evidence to directly support this mechanism is incomplete. We therefore constructed and validated a finite element model of knee arthroplasty to test the hypothesis that varus malalignment of the tibial tray would increase the risk of tray subsidence.Introduction
Methods
While in vivo kinematics and forces in the knee have been studied extensively, these are typically measured during controlled activities conducted in an artificial laboratory environment and often do not reflect the natural day-to-day activities of typical patients. We have developed a novel algorithm that together with our electronic tibial component provide unsupervised simultaneous dynamic 3-D kinematics and forces in patients. An inverse finite element approach was used to compute knee kinematics from in vivo measured knee forces. In vitro pilot testing indicated that the accuracy of the algorithm was acceptable for all degrees of freedom except knee flexion angle. We therefore mounted an electrogoniometer on a knee sleeve to monitor knee flexion while simultaneously recording knee forces. A finite element model was constructed for each subject. The femur was flexed using the measured knee flexion angle and brought into contact with the fixed tibial insert using the three-component contact force vector applied as boundary conditions to the femoral component, which was free to translate in all directions. The relative femorotibial adduction-abduction and axial rotation were varied using an optimization program (iSIGHT, Simulia, Providence, RI) to minimize the difference between the resultant moments output by the model and the experimentally measured moments. Maximum absolute error was less than 1 mm in anteroposterior and mediolateral translation and was 1.2° for axial rotation and varus-valgus angulation. This accuracy is comparable to that reported for fluoroscopically measured kinematics. We miniaturized the external hardware and developed a wearable data acquisition system to monitor knee forces and kinematics outside the laboratory.Background
Methods
Total knee arthroplasty (TKA) provides relatively pain-free function for patients with end-stage arthritis. However, return to recreational and athletic activities is often restricted based on the potential for long-term wear and damage to the prosthetic components. Advice regarding safe and unsafe activities is typically based on the individual surgeon’s subjective bias. We measured knee forces in vivo during downhill skiing to develop a more scientific rationale for advice on post-TKA activities A TKA patient with the tibial tray instrumented to measure tibial forces was studied at two years postoperatively. Tibial forces were measured for the various phases of downhill skiing on slopes ranging in difficulty from green to black. Walking on skis to get to the ski lift generated peak forces of 2.1 ± 0.20 xBW (times body weight), cruising on gentle slopes 1.5 ± 0.22 xBW, skating on a flat slope 3.9 ± 0.50 xBW, snowplowing 1.7 ± 0.20 xBW, and coming to a stop 3 ± 0.12 xBW. Carving on steeper slopes generated substantially higher forces: blue slopes (range 6° to 10°), 4.4 ± 0.18 xBW; black slopes (range 15° to 20°), 4.9 ± 0.57 xBW. These forces were compared to peak forces generated by the same patient during level walking: 2.6 ± 0.4 xBW, stationary biking 1.3 ± 0.7 xBW, stair climbing 3.1 ± 0.31 xBW, and jogging 4.3 ± 0.8 xBW. The forces generated on the knee during recreational skiing vary with activity and level of difficulty. Snow-plowing and cruising on gentle slopes generated lower forces than level walking (comparable to stationary biking). Stopping and skating generated forces comparable to stair climbing. Carving on steeper slopes (blues and blacks) generated forces as high as those seen during jogging. This study provides quantitative results to assist the surgeon in advising the patient regarding postoperative exercise.
Dislocation remains a major early complication after total hip arthroplasty (THA), and range of motion (ROM) before impingement is important in joint stability. Factors contributing to dislocation include design specific factors such as head-neck ratio, surgeon-related factors such as component placement, and patient-related factors such as bony anatomy. To study the relative importance of these factors, we analysed the effects of patient anatomy, implant design, and component orientation on hip ROM. Femoral and acetabular geometry were extracted from CT scans of 20 hips. CAD models of four different THA component designs were virtually implanted in the 3D-CT reconstructed anatomic models. The major design differences were in head-neck ratio and neck-stem angle. A previously reported contact detection model (D’Lima, J Orthop Research 2008) was used to measure restriction in hip ROM due to prosthetic or bony impingement. The following patient parameters were measured on plain AP radiographs: acetabular inclination, acetabular depth ratio, the arc-length between the tip of greater trochanter and ilium, and the arc-length between lesser trochanter and ischium. Multiple linear regression was used to determine correlation between radiographic parameters and hip ROM in flexion, extension, adduction, abduction, and external rotation. Mean head size was 51 ± 2mm, mean anatomic acetabular inclination was 41° ± 2, and mean acetabular depth ratio was 460 ± 60. When the cup and stem were implanted for best fit to the anatomy, mean hip ROM was 125° ± 8 (flexion), 57° ± 17 (extension), 29° ± 13 (adduction), 69° ± 7 (abduction), and 42° ± 13 (external rotation). Implanting the cup in “optimal” surgical alignment of 45° abduction and 20° anteversion reduced mean hip flexion, extension and abduction and increased adduction. Subject-to-subject variation was substantially greater than variation between CAD designs (differences in head-neck ratio) or component orientation (between ideal and anatomic). Hip flexion correlated moderately with acetabular abduction angle and the angle of the flare of the iliac wing (R2 = 0.59, p = 0.03). Hip abduction correlated moderately with the angle of the flare of the iliac wing and the length of the arc from the tip of the greater trochanter to the ilium (R2 = 0.50, p = 0.05). A universal cup position that permits optimal range of motion in all patients may not be valid. Since patient-related factors overshadowed implant design, cup position should be tailored to the individual patient. Preoperative radiographs can help predict postoperative hip ROM although not as accurately as 3D-CT reconstructions. These results may lead to enhancements in surgical navigation techniques.
Patellofemoral complications are among the important reasons for revision knee arthroplasty. Femoral component malposition has been implicated in patellofemoral maltracking, which is associated with anterior knee pain, subluxation, fracture, wear, and aseptic loosening. Rotating-platform mobile bearings compensate for malrotation between the tibial and femoral components. It has been suggested that rotating bearings may also reduce the patellofemoral maltracking resulting from femoral component malposition. We constructed a dynamic musculoskeletal model of weight-bearing knee flexion in a knee implanted with posterior cruciate-retaining arthroplasty components (LifeMOD/KneeSIM, LifeModeler Inc). The model was validated using tibiofemoral and patellofemoral kinematics and forces measured in cadaver knees on an Oxford knee rig. Knee kinematics and patellofemoral forces were measured after simulating axial malrotation of the femoral component (±3° of the transepicondylar reference line). Differences in patellofemoral kinematics and forces between the fixed- and rotating-bearing conditions were analysed. Rotational malalignment of the femoral component affected tibial rotation near full extension and tibial adduction at higher flexion angles. In the fixed-bearing conditions, external rotation of the femoral component increased patellofemoral lateral tilt, patellofemoral lateral shift, and patellofemoral lateral shear forces. Up to 6° of bearing rotation relative to the tibia was noted in the rotating-bearing condition. However, the rotating bearing had minimal effect in reducing the patellofemoral maltracking or shear induced by femoral component rotation. The rotating bearing does not appear to be forgiving of malalignment of the extensor mechanism resulting from femoral component malrotation. The rotating bearing may correct tibiofemoral axial malrotation near full extension but not at higher knee flexion angles. These results support the value of improving existing methodologies for accurate femoral component alignment in knee arthroplasty.
This phase III, multicenter, double-blind placebo controlled study evaluated safety and efficacy of aprotinin in reducing blood transfusion in subjects undergoing THA. Subjects were stratified by preoperative autologous blood donation and randomized to receive aprotinin (1 mL test dose; load, 2 million KIU and 0.5 million KIU/hour) or placebo. Subjects were assessed at baseline, postoperative days 1, 2, 3, 7 (or discharge) and 6±2 weeks. Primary efficacy variable was percentage of subjects requiring blood transfusion through day 7 or discharge. Safety was based on adverse event (AE). Of 359 randomized subjects, 175 in each group completed the study. Demographics of the groups were similar. Aprotinin reduced by 46% the requirement for any transfusion (17% vs 32% of subjects, p=0.0009). Aprotinin reduced allogeneic blood transfusion in subjects regardless of predonation status (11% vs 22%, p=0.0063), who made no predonation (13% vs 24%, p=0.0216), and who predonated (32% vs 62%, nd). The aprotinin group had a reduction of the number of any (48 vs 109 units; p=0.0003) and allogeneic (30 vs 72 units; p=0.0041) units transfused and total fluid loss (709 vs 957 ml; p=0.0002) compared with placebo. One patient died in the placebo group. AEs were reported in 83% of aprotinin-treated and 86% of placebo subjects, with 10% and 11%, respectively, described as serious AEs. No clinically important differences between aprotinin and placebo AEs were observed. Hypersensitivity to aprotinin was not reported. In this study, full-dose aprotinin was safe and effective in decreasing blood transfusion in subjects undergoing THA.
The knee is a complex joint that is difficult to model accurately. Although significant advances have been made in mathematical modeling, these have yet to be validated successfully in vivo. Direct measurement of knee forces should lead to a better understanding of the stresses seen in total knee arthroplasty. An instrumented knee prosthesis was developed to measure forces in vivo after total knee arthroplasty. An instrumented tibial prosthesis was implanted in an 80-year-old male weighing 66 kg. The prosthesis measured forces at the four corners of the tibial tray. The patient walked approximately 1.6million steps per year before surgery (ankle accelerometer measurements). Knee forces were measured postoperatively during passive and active knee flexion, rehabilitation, rising from a chair, standing, walking, and climbing stairs. The patient was walking with the help of a walker by postoperative day 3. Peak tibial forces were 1.2 times body weight (BW). By the sixth postoperative day the tibial forces during gait were 1.7 times BW. At six weeks the peak tibial forces during walking had risen to 2.4time BW. Stair climbing increased from 1.9 times BW on day 6 to 3.3 times BW at six weeks. This represents the first direct in vivo measurement of tibial forces. In vivo tibiofemoral force data will be used to develop better biomechanical knee models and in vitro wear tests and will be used to evaluate the effect of improvements in implant design and bearing surfaces, rehabilitation protocols, and orthotics. This should lead to refining surgical techniques and to enhancing prosthetic designs that will improve function, quality of life, and longevity of total knee arthroplasty. This information is vital given the current trend in the increase of older population groups that are at higher risk for chronic musculoskeletal disorders.
Highly cross linked polyethylenes have been shown to be substantially wear resistant. Typically, crosslinking is achieved by radiation in a low oxygen environment. While the early wear-simulation data is encouraging, concerns remain about the potential for aging and oxidative damage on exposure to oxygen during storage or in the body. This study measured wear rates in highly crosslinked liners that had been exposed to room air for up to 4 years. Polyethylene liners were divided into four groups: two groups of highly crosslinked liners, XL (freshly opened) and XL-Aged (aged); and two groups of nominally crosslinked liners, N (freshly opened) and N-Aged (aged). The highly crosslinked liners were crosslinked with 9.5 Mrad of warm electron-beam irradiation, treated to a post-cross linking heat treatment to quench free radicals (WIAM), followed by ethylene oxide sterilization. The nominally cross linked liners were sterilized with 2.5 Mrad. The aged liners (XL-Aged and N-Aged) were stored in saline (at 37°C) exposed to room air for 4 years. Three liners from each group were tested in a hip-wear simulator (90% bovine serum) for 5 million cycles. Gravimetric wear measurements were made at 500,000 cycle intervals. The N and N-Aged groups wore at rates of 14.76 ±3.1 and 15.58 ±1.21 mg/million cycles, respectively. The wear in both XL and XL-Aged groups was not measurable, resulting in weight gains of 2.73±0.5 and 2.17 ±1.1 mg/million cycles, respectively. WIAM cross linked polyethylene has been reported to generate the least free radicals and has the least potential for oxidative damage. There have been concerns regarding the validity of artificial aging by the high-temperature oxidation. Aging in saline at body temperature while exposed to room air is more representative of in vivo aging. This data supports the results of artificial aging and the long-term durability of WIAM polyethylene.
