Characteristic imaging findings of tendinosis of the direct head of rectus femoris and their interpretation

Tendinosis at the origin of the direct head of rectus femoris causing anterior hip pain is termed AIISpinitis, but no study has investigated its imaging findings. The aim of the present study was to determine the characteristic imaging findings of AIISpinitis and clarify their pathological significance. We reviewed the preoperative imaging findings of 62 hips in 58 patients who had undergone endoscopy with a diagnosis of AIISpinitis. The origin of the direct head of rectus femoris was evaluated by ultrasound (US) and magnetic resonance imaging (MRI), and the positive rate of abnormal findings and their agreement with endoscopy regarding injury of the direct head of rectus femoris were measured. Signal changes in the fat pad around the anterior inferior iliac spine (AIIS fat pad) in MRI were compared with the pathological findings of that harvested endoscopically. Hypoechoic regions in US (53/62, 85%) and signal change in MRI (55/62, 89%) were observed with high frequency and corresponded with injury of the direct head of rectus femoris observed by endoscopy (58/62, 94%) (kappa coefficient, 0.43 [moderate agreement], 0.69 [good agreement] respectively). Hypoechoic regions had high sensitivity (85%) and specificity (86%) for AIISpinitis. Regarding the AIIS fat pad, punctate and completely hypo-intense change relative to normal fat corresponded to fibrosis and scar formation, respectively (weighted kappa coefficient, 0.51 [moderate agreement]). Hypoechoic regions on US which had high sensitivity and specificity; and signal change at the origin of the direct head of rectus femoris and hypo-intensity of the AIIS fat pad on MRI were characteristic findings of tendinosis of the direct head of rectus femoris. These findings correspond pathologically to injury of the direct head of rectus femoris and fibrosis or scar formation in the AIIS fat pad, respectively.