Microseparation has resulted in more than ten-fold increase in ceramic-on-ceramic and metal-on-metal bearing wear, and even fracture in a zirconia head [1–4]. However, despite the greater microseparation reported clinically for metal-on-polyethylene wear, less is known about its potential detrimental effects for this bearing couple. This study was therefore designed to simulate the effects of micromotion using finite element analysis and to validate computational predictions with experimental wear testing. Experimental wear rates for low and highly crosslinked polyethylene hip liners were obtained from a previously reported conventional hip wear simulator study [5]. A finite element model of the wear simulation for this design was constructed to replicate experimental conditions and to compute the wear coefficients that matched the experimental wear rates. We have previous described out this method of validation for knee wear simulation studies [6,7]. This wear coefficient was used to predict wear in a Dual-Mobility hip component (Fig 1). Dual mobility total hip arthroplasty components, Restoration ADM (Fig 1), with highly crosslinked acetabular liners were experimentally tested: the control group was subjected to wear testing using the ISO 14242-1 waveform on a hip wear simulator. The microseparation group was subjected to a nominal 0.8 mm lateral microseparation during the swing phase by engaging lateral force springs and reducing the swing phase vertical force.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.
Complications after total knee arthroplasty (TKR) such as malalignment, instability, subluxation, excessive wear, and loosening have been attributed to poor soft-tissue balance. Traditional approaches for soft-tissue balance involve static measurements in full extension and at 90° flexion. A trial prosthesis instrumented with force transducers was used to measure soft-tissue balance through the entire range of flexion. The trial prosthesis was instrumented with four force transducers, one at each corner of the tibial tray, and was implanted in four cadaver knees and four patients intra-operatively. Tibial forces were recorded during passive knee flexion after the tibial and femoral bone cuts were made and again after soft-tissue balance was achieved using standard techniques. In all eight knees measurable imbalance was initially recorded. The differences in forces were a mean of 18 N (range, 6 to 72) mediolateral and a mean of 26 N (range, 13 to 108) anteroposterior. After a routine procedure of soft-tissue balancing, the mean imbalance between the transducers was reduced by 62 % to 87 % (p <
0.05). However, even the knees that appeared perfectly balanced at 0° and 90° flexion, some imbalance occurred [mean 22 N (range, 2 to 34)] at flexion angles other than 0° and 90°. Soft-tissue balance in TKR remains a complex concept. Even after accurate static balancing was achieved in extension and 90° flexion, dynamic measurements revealed discrepancies in mid flexion, which may explain the wide variation in knee kinematics reported after TKR and in the reported incidences of mid-flexion knee instability. Computer-aided surgical navigation systems can increase the precision and accuracy of component alignment. However, these systems cannot directly address soft-tissue balance and knee tightness. An instrumented tibial prosthesis could be a useful adjunct to enhance the value of these navigation tools.
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.
