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.