Aims

The extensive variation in axial rotation of tibial components can lead to coronal plane malalignment. We analyzed the change in coronal alignment induced by tray malrotation.

The Omnifit-HA femoral component has shown excellent results in early and mid-term industry sponsored multi-center clinical trials. To validate these results, an independent cohort of patients was followed prospectively for an average of ten years.

The senior author performed 103 consecutiveuncemented primary total hip arthroplasties in 89 patients from July 1991 to December 1996. The components implanted were the Omnifit-HA femoral stem and the Omnifit PSL porous coated acetabular shell. The cohort, with a mean ageat the time of the index procedure of 52 ± 9 years, was comprised of 45females and 58 males. The mean follow up was 10.3 years (range 7.3 – 12.7years). Two independent observers who were not part of the surgical team performed clinical and radiographic evaluations.

The senior author performed 103 consecutiveuncemented primary total hip arthroplasties in 89 patients from July 1991 to December 1996. The components implanted were the Omnifit-HA femoral stem and the Omnifit PSL porous coated acetabular shell. The cohort, with a mean ageat the time of the index procedure of 52 ± 9 years, was comprised of 45females and 58 males. The mean follow up was 10.3 years (range 7.3 – 12.7years). Two independent observers who were not part of the surgical team performed clinical and radiographic evaluations.

The Omnifit-HA femoral component continues to show excellent clinical results as indicated by the multi-center trials. This is the first study to report 10-year follow up by an independent surgeon. Despite the younger mean age, relatively high polyethylene wear, and 10% rate of lysis in the acetabulum, the femoral stem had a 100%survivorship. This supports the theory that proximal circumferential bone in growth affords protection against the migration of wear debris along the femoral stem.

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.

Quadriceps moment arm is one of the factors determining quadriceps force. Total knee arthroplasty designs with larger quadriceps moment arms should generate less quadriceps and patellofemoral forces. A study was conducted to measure knee kinematics, quadriceps and patellofemoral forces in two knee designs with differing centers of rotation. In addition, the effect of a central dome-shaped versus a medialized patella component was determined. Six human cadaver knees were tested before implantation and after sequential implantation with two posterior cruciate retaining designs: Scorpio and Control. The quadriceps moment arm of the Scorpio design was 1 cm longer than that of the Control design. Knee kinematics was measured with an eletromagnetic tracking device while the knee was put through dynamic simulated stair climbing under peak flexion moments of 40 N-m. Quadriceps tension and patellofemoral compressive and shear forces were measured for both conditions and for the central and medialized patella components. The normal unimplanted condition showed increasing rollback with flexion while both implanted conditions displayed relatively less rollback. Overall, quadriceps tension was highest in the unimplanted condition and lowest in the Scorpio condition. The Scorpio design showed a 10-20% reduction in quadriceps tension at angles greater than 40° when compared to the Control design. Patellofemoral forces were also significantly reduced in the Scorpio design when compared to Control. There were no differences noted between the central and medialized patella component. The Scorpio design, with its more posterior center of rotation, reduced quadriceps tension and patellofemoral forces. Reduced quadriceps forces may facilitate postoperative rehabilitation and activities such as stair climbing. Reduction in patellofemoral forces could reduce patellar complications such as anterior knee pain, component wear and loosening. These results are currently undergoing validation with a prospective clinical study.

Polyethylene (PE) wear affects survivorship in the long term while dislocation remains a significant factor in the short term. Increasing head size can reduce impingement and dislocation. However, this increases wear rates and reduces the net thickness of the liner. Several reports have demonstrated significant reduction in wear in cross-linked PE. This study reports wear rates in crosslinked PE liners with increased head size. Four groups of PE liners were tested against cobalt-chrome heads in a hip wear simulator: highly crosslinked liners with head size 28mm (28XPE) and 32mm (32XPE), and minimally crosslinked liners with head size 28mm (28PE) and 32mm (32PE). Additional liners were used as load-soak controls to monitor weight gain due to fluid absorption. Gravimetric analysis was performed every 500,000 cycles for a total of 5,000,000 cycles. 28PE and 32PE liners had mean wear rates of 12.5(±1.0) and 17.45 (±2.6) mg/million cycles. Both highly crosslinked PE liners (28XPE and 32XPE) had significant less wear rates that regular polyethylene 1.49 (±0.72) and 2.55 (±0.19) mg/million cycles respectively. Increasing head size resulted in increased wear, which is consistent with previous reports. Highly crosslinked PE significantly reduced wear rates in both head sizes. Although there was a small increase in wear in the 32XPE group compared to the 28XPE group, wear was significantly less than both 32PE and 28PE groups. These encouraging results suggest that a dual benefit (reduced wear and reduced dislocation rate) might be achieved using 32XPE liners. Further studies that evaluate fatigue damage, crack propagation and impingement are necessary.