NECK NARROWING IN RESURFACED HIPS WITH INFLAMMATORY PSEUDOTUMOUR

Abstract

Introduction: Metal on metal hip resurfacing arthroplasty (MoMHRA) has become an alternative option to THR in the treatment of young adults with OA. A recognised MoMHRA complication is the development of an inflammatory pseudotumour (IP). IPs can be cystic (predominantly posterio-laterally located), solid (mostly anteriorly located) or mixed in nature. Diagnosis is made with the aid of US and/or MRI. To-date, no radiographic aid in the diagnosis of IP has been identified. Neck thinning is a recognised phenomenon following MoMHRA, occurring in up to 90% of resurfaced hips, which has not been associated with any adverse clinical events. Its pathogenesis is considered multi-factorial secondary to stress shielding, impingement, pressure effect on cancellous femoral neck, bone necrosis secondary to femoral preparation and altered vascularity/AVN. Our aim was to establish whether neck thinning is associated with the presence of a pseudotumour.

Methods: Thirty-one hips (30 patients) with IP confirmed clinically, radiologically, intra-operatively and histologically were matched for sex, age, pre-operative diagnosis, component size and follow-up with an asymptomatic MoMHRA cohort without pseudotumour (Control n=60). Radiological and operative findings at the time of revision of all IP patients were reviewed regarding location of pseudotumour; 4 different locations were defined: anteriorly-extending, posteriorly-extending, anteriorly & posteriorly-extending and within joint only. For all patients, prosthesis-neck ratio (PNR) at follow-up was measured on plain AP pelvic radiographs as previously described and validated.

Results: All IP patients (6M:24F) and all (12M:48F) but two controls had a posterior approach at the time of MoMHRA. Mean femoral component size was 46 mm for both groups. At an average follow up of 3.5 years (0.7–8.3), IP patients (mean 1.26, 1.10–1.79) had a significantly higher (p< 0.0001) PNR in comparison to their controls (mean 1.14, 1.03–1.35). Greater neck thinning had occurred in both IP-males (p< 0.001) and IP-females (p=0.002) in comparison to their controls. Location of IP and hence nature did not appear to have an effect on the degree of neck thinning.

Discussion: This study shows that IP patients had significantly narrower femoral necks at follow-up. Processes, such as impingement and increased wear that are thought to contribute to the process of neck narrowing are also thought to be factors in IP development. Furthermore, the presence of an IP, could lead to altered vascularity via a mass effect and further contribute to neck narrowing. Interestingly, nature of IP did not have a significantly affect PNR. Although one cannot be certain whether neck narrowing is a consequence or a contributing factor for IP development, their association is significant. Surgeons should consider the possibility of pseudotumour in symptomatic MoMHRA patients with neck narrowing.