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 degrees. 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. (c) 2015 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 34:924-931, 2016.