Volume of hip synovitis detected on contrast-enhanced magnetic resonance imaging is associated with disease severity after collapse in osteonecrosis of the femoral head

To evaluate the relationship between the volume of hip synovitis detected on contrast-enhanced magnetic resonance imaging (MRI) and the disease stage of osteonecrosis of the femoral head (ONFH). Sixty-three consecutive hips in 40 ONFH patients were reviewed using contrast-enhanced MRI. Ten unaffected hips in 10 patients with unilateral ONFH were used as controls. Based on the Japanese Investigation Committee system, these hips were classified according to stage and type. The volume and location of hip synovitis were semi-quantitatively measured on contrast-enhanced MRI. Clinicoradiological factors were statistically analyzed to determine the relationship with the volume of hip synovitis. The mean synovial volume was significantly larger in ONFH hips (8,020 ± 6,900 mm${}^3$) than in controls (910 ± 1,320 mm${}^3$; p = 0.001). The area of synovitis in the anterior portion of the hip joint was double (mean: 2.17 ± 1.77) that in the posterior portion. The volume of synovitis was small in pre-collapse-stage hips (stage 1: 680 ± 690 mm${}^3$, stage 2: 1,460 ± 1,200 mm${}^3$), but significantly larger in post-collapse-stage hips (stage 3A: 7,820 ± 4,490 mm${}^3$, stage 3B: 13,850 ± 7,110 mm${}^3$; p < 0.001). Multiple regression analysis showed that disease stage was the only factor related to hip synovitis. Our study suggests that hip synovitis in ONFH might occur after femoral head collapse and worsen with collapse progression, mainly in the anterior portion.