Cup oversizing as a risk factor for postoperative groin pain in Total Hip Arthroplasty : a retrospective analysis of 437 hips.

The majority of patients are pain free after total hip replacement, but some experience anterior hip pain due to iliopsoas impingement. There is evidence that a prominent or malpositioned cup may cause iliopsoas tendonitis. The purpose of this study was to determine whether oversizing the cup is a risk factor for postoperative groin pain. We retrospectively investigated 437 total hip replacements in which the femoral head diameter had been measured for other research purposes. Data regarding the cup size and positioning was collected from implant identification labels and pelvis x-rays. Clinical data were recovered from the medical files. Native femoral head size, cup size, anteversion, inclination and DS (difference between native femoral head size and cup size) and type of pain (anterior hip pain or non-anterior hip pain) were analyzed and correlations were sought. There was a strong and significant correlation between native femoral head size and cup size. Mean DS was 5.5 mm in the no pain group, 6.9 mm in the anterior hip pain group and 5.9 mm in the non-anterior hip pain group. The difference in mean DS was significant (P=0.046) in patients experiencing anterior hip pain vs. those with no pain or non-anterior hip pain. As patients with anterior hip pain had a significant larger DS of 6.9 mm, it seems that a cup size of more than 6 mm above the native femoral head size should be avoided. We therefore recommend a systematic intraoperative head size measurement prior to definite cup choice.