A rat model of hip joint contracture induced by mono-articular hip joint immobilization

To elucidate the formation process and therapeutic targets of hip flexion contracture, we developed a rat model of hip flexion contracture induced by hip mono-articular immobilization.

Kirschner wires inserted into the femur and hip bone were anchored at the hip in a flexed position in the immobilization groups and unanchored in the sham groups for up to four weeks. Age-matched untreated rats were used as controls. Hip extension range of motion (RoM) was measured at three different extension moments (7.5, 15, and 22.5 N•mm) in each successive myotomy step as follows: before myotomy, after sequential myotomy of the tensor fascia lata, quadriceps muscle, iliopsoas muscle, and after myotomy of all residual muscles (the gluteus medius and adductor muscles). Histological analysis of the hip joint was also performed.

After four weeks of immobilization, the RoM before myotomy at 22.5 N•mm was significantly decreased by 29° compared with controls, and this value was unaltered in the sham group. Analyses following serial myotomy suggested that the structures responsible for myogenic contracture were the tensor fascia lata, iliopsoas, gluteus medius, and adductor muscles because the RoMs were increased by these myotomies. Unexpectedly, arthrogenic contracture was not detected at moments other than at 7.5 N•mm, even after four weeks of immobilization. Histological analysis confirmed that pathological changes were not apparent in the anterior capsule of the hip joint.

The present findings suggest that myogenic contracture may be an important therapeutic target for immobilization-induced hip flexion contracture.