Abstract
Background
In large diameter hip arthroplasty, the femoral head size and shape have to be optimised to avoid neck on socket impingement if the head is too small, or psoas tendonopathy if the head is too large, overhanging the normal head neck junction in the sagittal plane. Currently there is no published guideline to help the surgeon select an optimal size femoral head. Instead, the novice surgeon may inadvertently oversize the femoral component through fear of notching the femoral neck—causing psoas impingement, especially in female patients. We sought to provide anatomically based advice for surgeons to optimise both the position of the femoral head and the head neck ratio.
Materials and Methods
100 hips were reviewed. Fifty radiographically normal hips in elderly patients with fractures of the contralateral side and 50 hips from patients whose contralateral side was arthritic secondary, either to Cam or pincer type impingement, or DDH. The head neck ratios were calculated using two methods: the plain AP radiographs were measured on PACS (Picture Archiving and Communication System) and CT scans obtained as part of the work up to hip surgery were measured in validation. The head neck ratio was calculated by dividing the diameter of the widest point across the femoral head by the narrowest part across the femoral neck. The HNR of 39 patients who attended a painful MOM clinic were also reviewed.
Results
The normal mean HNR is 1.40 (min 1.22, max 1.58). Hips contralateral to a CAM type deformity have smaller HNRs (mean 1.30, min 1.23, max 1.41), while those on the other side of a DDH are larger, (mean 1.47, min 1.40, max1.53).
In our painful MOM group only seven of these patients (18%) had a HNR within 1.45-1.5, and only 13 (33%) had a HNR between 1.4-1.5.
Conclusions
There appears to be an optimal head neck ratio in the normal hip that is sufficient for normal function as defined by the absence of any arthritic change in either hip by the seventh decade. We recommend that a HNR of 1.45-1.5 (taking into consideration cartilage thickness) should be used to calculate the optimal femoral head size. This algorithm can be applied when resurfacing hips or when using large diameter MOM arthroplasty. Although the causes for a painful big ball arthroplasty may be multifactorial, there does appear to be a correlation between a painful joint and inappropriate HNR.
