Creation of a simple distal femur morphology classification system.

The purpose of this study is to propose a binary classification system based on simple measurements that summarizes individual, race, and sex-specific differences in distal femur shape. Surface models of 165 distal femurs (79 female, 86 male; 85 African-American, 80 Caucasian, 28.8 ± 7.6 years) were created with a 3-dimensional laser scanner. Surface area, width, length, curvature, and angulation were measured. Knees were classified as either type A or B within five distinct categories: (i) aspect ratio, (ii) trochlear-intercondylar width ratio, (iii) trochlear tilt, (iv) medial-lateral trochlear width ratio, and (v) trochlear sulcus shape. Correlations between these measures and surface area were calculated, and receiver-operator curves were used to select cutoff values between type A and B knees to improve differentiation of femur shapes by sex or race. The cutoff values between type A and B knees for the five categories are as follows: Category I: 0.90, Category II: 0.51, Category III: 1.02, Category IV: 0.67, and Category V: 128.7°. Other than category IV (medial-lateral trochlear width ratio) (p = 0.004, R = 0.22), no categories were correlated with surface area (p > 0.25). Category I (aspect ratio, cutoff = 0.90) best differentiated femurs by sex (p < 0.001, AUC = 0.80), and Category V (sulcus shape) best differentiated femurs by race (p < 0.001, AUC = 0.73). This system uses simple measurements to summarize important individual, race, and sex-specific differences in distal femur shape. It can be used in a clinical setting to provide insight into the relationship between sex or race differences in knee shape and mechanically influenced knee disorders.