Studies have shown that the normal patella tracks laterally with flexion of the knee joint, consistent with the findings of Eckhoff et al. that the femoral sulcus is lateral to the mid-plane between the 2 femoral condyles. Patellar pain and instability is a known complication of Total Knee Arthroplasty (TKA). To date, several studies have identified the effect of femoral and tibial components on complication after TKA. However, there is very little work on how the design of the implant affects patellar tracking. Our study compares lateralization of the patella in two different AP stabilized knee implants.

A modified caliper was used to measure the width and position of the patella relative to the femur at different degrees of knee flexion. The relationship of the patella midpoint to that of the femur was subsequently assessed. Group 1 consisted of 25 native knees. Group 2 consisted of 25 patients with antero-posterior stabilized knee implant with a spherical medial condyle and a deep lateralized patellar groove (MRK, Finsbury Orthopaedics, UK). And Group 3 consisted of 25 patients with traditional cam-and-post posterior cruciate-substituting implant with a symmetrical patellar groove (PFC-Sigma, DePuy, UK). The mean follow-up for the 50 TKAs was 28 months.

Lateral tracking corresponded well in all groups, but the mean lateral displacement of the patella in group 2 correlated more closely to that of group 1. At 90 degrees of flexion, the patella was displaced a mean of 7mm laterally in both groups 1 and 2, but a mean of 4mm in group 3. Two-tailed Mann-Whitney U test (95% confidence interval) showed that the difference in lateral patellar displacement between groups 1 and 3, and that between groups 2 and 3 were statistically significant (p< 0.05). However, the patellar displacement between groups 1 and 2 was not statistically different.

Our results indicate that lateral patellar displacement in group 2 is similar to that of native knees (group 1). The effect of the underlying lateralized deep patellar groove of the femoral component in group 2 is more able to mimic that of the native femoral sulcus. This intrinsic implant design accommodates the natural tracking of the patella.

Femoral component sizing can play a critical role in the clinical outcome and success of a TKR prosthesis. In particular, achieving the correct AP dimension for the femur is important to ensure correct balancing and to maintain flexion/extension spacing and the ML width dictates bone coverage which, if insufficient, can cause complications or affect long-term outcomes.

There has been some discussion in the literature about the optimal femoral component shape and size with reports of differences in anatomy between male and female patients or those of larger or smaller stature. The majority of these publications have been conducted on normal anatomy with un-cut bone, reporting on the epicondylar width of the femur which is difficult to relate back to the dimensions of a prosthesis. Some studies have measured resected bone, however, the prosthesis and instruments used to make the cuts dictate the amount of bone removed anteriorly and posteriorly which, in turn affect the footprint of exposed bone that is measured.

Data was gathered to assess whether a generic prosthesis with a standard AP/ML sizing ratio could be used to cover the range of femoral sizes dictated by a Caucasian population of 26 male and 26 female patients. MRI scans were obtained for these patients, all between 20 and 45 years of age and diagnosed with a meniscal tear. A theoretical size range for a prosthesis was determined from an analysis of literature data and a review of currently available devices. This consisted of 8 femoral sizes ranging from 50 – 74.5 mm in AP dimension with a constant AP/ML ratio of 0.9. Each MRI scan was viewed in the sagital plane and the maximum AP dimension was measured. This was sized to the closest available femoral component using the criteria of matching the existing articulating geometry as closely as possible. A ‘virtual’ distal condyle cut was made on the scan relating to the component size and the ML dimension of the resected bone was taken. The measured ML data was then compared to the implant dimension for each subject and component overhang/underhang was determined.

An appropriate femoral component match was found in all cases with a mean AP dimensional undersize of 1.71 mm across all patients (range: 0.16 – 3.77 mm). The mean ML femoral component overhang was 0.34 mm for the male population, 1.52 mm for the female population and 0.89 mm for all 52 patients. These values were all considered to be well within an acceptable range and not be significant in terms of clinical outcome. No patient was too large for the largest component, however no patient in the population that was assessed matched the smallest of the 8 components.

This simple dimensional assessment has shown that using a prosthesis with a standard AP/ML ratio, it is possible to accommodate a mixed gender population. The data reported here suggests that the anatomical differences between men and women femora is not hugely significant and can be covered with a common implant provided a sufficient size range is used.

Finsbury Orthopaedics would like to acknowledge Dr. Pinskerova for providing the MRI scans.