The Distribution of Impingement Region in Cam-Type Femoroacetabular Impingement and Borderline Dysplasia of the Hip With or Without Cam Deformity: A Computer Simulation Study

Purpose To identify the distribution of the impingement region in cam-type femoroacetabular impingement (FAI) or patients with borderline developmental dysplasia of the hip (DDH) using computer simulation analysis. Methods A total of 51 painful hip joints from 42 consecutive cases diagnosed as cam-type FAI (center edge [CE] angle ≥ 25°, alpha angle ≥ 55°) or borderline DDH (CE angle ≥ 20° and < 25°) with or without a cam deformity (alpha angle ≥ 55° or < 55°) were enrolled. ZedHip (Lexi, Tokyo, Japan) 3-dimensional computer simulation was used to identify impingement points. Computed tomography data were used for 3-dimensional modeling and impingement simulation. The maximum flexion angle and maximum internal rotation angle at 90° were evaluated. The impingement point was identified at a position of maximum internal rotation and 90° of flexion. Six impingement regions were defined. Differences in the distribution of the impingement region were evaluated between groups. Results There were significant differences in range of motion at maximum flexion and internal rotation among the 3 groups ( P < .0001). There was no significant difference in the distribution of the impingement point in the cam-type FAI group ( P = .71); similarly, there was no significant difference in the borderline DDH with a cam deformity group ( P = .071). On the other hand, in terms of proximal or distal sites, there was a significant difference between the borderline DDH with and without a cam deformity group ( P < .001). Conclusions The impingement region in cases of cam-type FAI was variable. The coexistence of a cam deformity affected the distribution of the impingement region in cases of borderline DDH; the region tended to be distributed across proximal rather than distal regions. The site of cam osteochondroplasty should be based on the identified impingement point, particularly in cases of cam-type FAI and borderline DDH with a cam deformity. Level of Evidence Level IV, case control study.