The transepicondylar distance is a reliable and easily measured parameter for estimating femoral cartilage surface area using MRI

Purpose: Different cartilage repair techniques are widely used to regenerate cartilage, such as autologous chondrocyte implantation (ACI), osteochondral autograft transfer, microfracturing and minced cartilage. Defect size is a key parameter for selecting the best procedure to repair cartilage. However, the defect's size is not related to the patient's total cartilage surface. This is because assessing the femoral cartilage surface area is time-consuming and therefore unsuitable in the daily clinical routine. It has been proposed that the femur's total cartilage area correlates positively with the transepicondylar distance (TEA). Methods: The knees of 40 subjects were analysed. Their average age was 30.1 ± 8.6 years. Twenty-four female and 16 male subjects were examined. Their mean body height was 176.2 ± 8.8 cm. MRI scans were performed via 3-Tesla MRI. These data were postprocessed and quantified using the browser-based, customizable SATORI platform (Fraunhofer MEVIS). This software computed the femoral cartilage surface area (FeCA), the patella cartilage surface area (PCA), the TEA and the patella length. Results: Body height reveals a good correlation (r = 0.722, p < 0.001) with the distal femur's cartilage area surface. However, regression analysis shows only moderate dependence (R²: 0.514). A very good correlation (r = 0.830, p < 0.001) was observed between the TEA distance and the total cartilage surface area of the distal femur. The regression analysis yields a good value (R²: 0.684). The cranio-caudal length of the patella was chosen as a suitably measurable two-dimensional parameter for correlation analysis with the patella's total cartilage surface area. Those results yield a poor correlation (r = 0.577, p < 0.001) between the two parameters, and regression analysis reveals a low value (R²: 0.384). Conclusion: The TEA is a reliable parameter for estimating the femur's cartilage area using MRI. A simple determination of this parameter allows the estimation of the femur's total cartilage area as well as the surface-corrected defect size (SCDS) in daily routine. Level of Evidence: Level II.