INTRODUCTION: Following Total Knee Arthroplasties, patellofemoral complications have shown to be responsible for approximately 50% of re-operations. Contemporary patellar designs employ both “onlay” and “inset” configurations. The latter promotes ease of placement, reduced bone removal and a heralded theoretic advantage of increased strength at the fixation interface. However, to date, no reports have compared the disassociation strengths of these two patellar component modes of fixation. The purpose of this study is to quantify the shear disassociation strength for both onlay and inset patellar fixation techniques.

METHODS: Two sets of synthetic solid foam patellae were prepared using standard milling techniques for symmetrical, three-peg onlay and inset polyethylene cylinders of identical dimension. The use of synthetic bones in mechanical testing was validated in the past. The cylinders were cemented to the synthetic patellae, using standard cementing techniques. The fixation resistance of both groups was measured using an Instron Testing Machine. A compressive joint force simulating chair rise was applied perpendicular to the anterior surface of the patellar component model. A shearing displacement was then applied to the composite until patellar component disassociation.

RESULTS: The mean shear strength of the onlay group was 2540 N SD 236 N, (n=7) and 3180 N SD 186 N, (n=6) for the inset group. The inset patellae was 25% (640 N) stronger than the onlay patellae, (p=0.0002, two-tailed student t-test).

DISCUSSION/CONCLUSION: The results of the study demonstrated a significantly higher resistance of inset patellar fixation to shear stress compared to onlay patellar fixation. Although further in-vivo studies are indicated, the data suggests that the use of inset patella in total knee replacements may offer stronger fixation and consequently decreasing morbidity associated with patella implant loosening.

Total knee replacement has proven to be a very successful procedure. However, problems have been encountered in fitting standard femoral implants to distal femurs using various popular total knee replacement systems.

Authors observed that difficulties matching femoral components with distal femurs most frequently occurred in female patients. In practice, as far as femoral sizing is concerned, women are just treated as small men.

Despite an extensive English literature search, only a limited number of studies addressing the gender differences in distal femurs proportions were identified.

In view of our experiences, we hypothesize that 1) Anterior-posterior (AP) dimensions do not increase in the same proportion to medial-lateral (ML) dimensions in men and women. 2) The AP/ML ratio is different in males and females. 3) Femoral implants AP/ ML ratios are more inline with men ratios then with women

Materials and Methods: In order to test the hypothesis, 50 male and 50 female patients consecutive knee MRI scans of the knee were studied. The patients were referred to UMC MRI Center with a variety of diagnoses. The knees involved with conditions distorting bony anatomy were excluded. The age distribution of the patients ranged from 17 to 77 years for the males and 17 to 85 years for the females.

Evaluation was focused on axial views of the distal femur. A cut with the maximum medial lateral dimensions of a studied femur was selected. The ML measurement was made along the epicondylar axis. The maximum AP dimension was obtained perpendicular to the epicondylar axis on the same cut. The ratios were then obtained. AP data for males and females was plotted against ML data. The data was found to approximate linear relation, permitting linear regression.

Inside AP and ML dimensions of eight popular TKR systems produced by six manufactures were obtained. The AP vs. ML plots were made as well as ratios were calculated for each system. The implants data were compared with male and female data.

A t-test was performed to demonstrate whether the AP/ML ratio was significantly different between males and females. In addition, ordinary least squares analyses were performed to establish whether the AP/ML ratios varied across different AP and ML sizes for both genders.

Conclusions: Our study demonstrated a significant difference in distal femur proportions between males and females. The variation in dimensions did not appear to be well covered by femoral implant sizes from studied seven commonly used TKA systems.

Dimensions propagation of femoral components for TKR fallowed significantly closer to males’ distal femur dimensions variation than to females’.