Abstract
INTRODUCTION
Studies have suggested that there is a reduction in head-neck-ratio (HNR) associated with MoMHRA. A reduction in HNR at operation would decrease range of movement and increase impingement risk. Impingement could lead to 20 edge loading, increasing wear. Serum ion levels of Chromium (Cr) and Cobalt (Co) are surrogate markers of wear. Although acetabular component orientation has been shown to contribute to wear and PT development, the role of a decrease in HNR has only been highlighted in PT development. This study aimed to measure changes in HNR that occur at resurfacing and determine any gender- and component size-specific differences. In addition it aimed to determine whether changes in HNR could be associated with increased wear.
METHODS
84 patients (56M: 28F) with unilateral MoMHRA were included. The mean age at surgery was 57 years. The mean femoral component was 49mm. Components were considered small if <45mm, average if between 45–50mm and large if >50mm. Three designs were implanted; BHR, C+ and Recap. The average follow up was 4 years. All patients had Cr/Co levels measured at follow up. Patients were considered to have high ions if Cr and Co levels were 5.1ppb and 4.4ppb respectively.
Pre-operative HNR (HNRpre) and the post-operative HNR (HNRpost) were made from the respective pelvic radiographs. Assuming a 2mm thick cartilage layer, the HNR based on the diameter of the articular cartilage pre-operatively (HNRart) was calculated.
The immediate changes in HNR as a result of the operation were expressed relative to articular HNR pre-op:
HNRartpost=HNRpost–HNRart
RESULTS
The changes in HNR at operation were significantly negatively correlated with HNRpre, (p<0.001), (rho=−0.77). Females had greater ion levels (p=0.013) and smaller components (p<0.001). Females had bigger pre-operative HNRart and were downsized more (p<0.001). Similarly, patients with small components had higher ions (p=0.032). They had greater HNRart and were downsized more (p<0.001) Twelve patients comprised the high ion group. These patients had smaller components (p=0.004), greater HNRart and were down-sized significantly more at resurfacing (p<0.001).
DISCUSSION
This study highlights HNR changes that occur in resurfaced hips. Females, patients with small components and patients in the high ion group had higher pre-operative HNR and were downsized more at operation. Femoral downsize, would probably increase impingement risk, lead to secondary edge loading and contribute to greater incidence of wear related problems.
