Relationship between cup position and obturator externus muscle in total hip arthroplasty

BACKGROUND: It is often challenging to find the causes for postoperative pain syndromes after total hip replacement, since they can be very allotropic. One possible cause is the muscular impingement syndrome. The most commonly known impingement syndrome is the psoas impingement. Another recently described impingement syndrome is the obturator externus muscle impingement. The aim of this study is to analyze pathological conditions of the Obturator externus and to show possible causes. METHODS: 40 patients who had undergone a total hip replacement were subjected to clinical and MRI examinations 12 months after the surgery. The Harris Hip Score (HHS) was used to analyze pain and function. Additionally, a satisfaction score and a pain score (VAS) were determined. The MRI allowed for the assessment of the spatial relation between the obturator externus muscle and the acetabulum. Also measured were the acetabular inclination angle as well as the volume and cross-sectional area of the obturator externus muscle. RESULTS: The patients were assigned to 3 groups in accordance with their MRI results. Group 1 patients (n = 18) showed no contact between the obturator externus and the acetabulum. Group 2 (n = 13) showed contact, and group 3 (n = 9) an additional clear displacement of the muscle in its course. It was not possible to establish a connection between the imaging findings, the HHS, the VAS, and patient satisfaction. What was striking, however, was a significant difference between the median inclination angle in group 1 (40° ± 5.4°) and group 3 (49° ± 4.7°) (p < 0.05), and the corresponding image-morphological pathology. The average inclination angle in group 2 was 43.3° ± 3.8° CONCLUSION: Contact between the obturator externus muscle and the caudal acetabula border occurs frequently, but is only rarely accompanied by a painful muscular impingement. The position of the acetabula must be seen as one of the main risk factors for contact between the acetabula border and the obturator. The hip replacement process must provide for sufficient osseous coverage of the caudal acetabula border. Furthermore, the retention of the transverse ligament may serve as protective cover for the incisura acetabuli.