Increased psoas tendon running angle in patients with Legg-Calve-Perthes disease: a new theory for the aetiology of children with avascular femoral head necrosis

The cause of Legg-Calve-Perthes disease (LCPD) remains unknown. We propose a new hypothesis that the iliopsoas muscle and/or tendon affects the progression of ischemic necrosis of the femoral head as an anatomical factor. The purpose of this study was to test this hypothesis by measuring the psoas major tendon angle (PMTA) and cross-sectional area (CSA) of the iliopsoas muscle on MRI. We selected three predetermined axial MRI scans at the level of the psoas major tendon origin, the femoral head, and the lesser trochanter. We calculated the proximal, distal, and combined PMTA and compared these angles between the LCPD group and the transient synovitis (TS) group as a control. Our results revealed that the proximal PMTAs of the LCPD-affected sides were significantly greater than in the TS controls (P < 0.05), while there were no significant differences in the proximal PMTA, combined PMTA, and CSA. This result indicates that the psoas major tendon of the patient with LCPD curves sharply on the anterior capsule of the hip joint more than in the control group patients. This sudden curve of the psoas major tendon may be involved in the development of LCPD. We measured PMTAs in patients with LCPD. Our findings suggested that the running curve of the psoas major tendon is an anatomical factor that influences the development of mechanically-induced ischemia in LCPD.