This study reports the mid-term results of a large bearing uncemented metal on metal total hip replacement (MOMHTHR) matched series using the Synergy stem and Birmingham modular head in 36 hips (mean follow up 61 months). All patients underwent clinical, metal ion and MRI assessment. Wear analysis was performed on retrieved heads using Redlux non-contact optical profilometry.

Seven patients (19%) have undergone revision surgery. All revisions had two or more of either symptoms, high metal ions or an MRI suggestive of an adverse reaction to metal debris (ARMD). There was no evidence of component malposition or impingement. Frank staining of tissues together with high volume dark brown fluid collections were found in all cases. All stems and cups were well fixed. In 4 cases pubic and ischial lysis (adjacent to the inferior fins) was observed. All 7 cases had radiological, intraoperative and histological evidence of ARMD (Figure 1). The failure cohort had significantly higher whole blood cobalt ion levels and OHS (p = 0.001), but no significant difference in cup size (p = 0.77), gender predominance, stem offset or cup position (p = 0.12). Sleeves had been used in all revision cases

Wear analysis (n = 4) demonstrated increased wear at the trunnion/sleeve interface in a distribution compatible with micromotion (Figure 2). There was normal wear at the articulating surface.

This series further demonstrates unacceptable failure rates in LHMOMTHR in a series where a compatible stem for the BHR modular head was used. Use of a CoCr sleeve within a CoCr head taper appears to contribute to abnormal wear and therefore potential ARMD and subsequent failure.

Introduction

Metal on metal hip arthroplasty continues to be controversial. Emerging evidence suggests that there are multiple modes of failure, and that the results of revision surgery are influenced by host and implant factors.

This study reports the mid-term results of a large bearing hybrid metal on metal total hip replacement (MOMHTHR) in 199 hips (185 patients) with mean follow up of 62 months. Clinical, radiological, metal ion and retrieval analysis were performed.

Seventeen patients (8.6%) had undergone revision, and a further fourteen are awaiting surgery (defined in combination as failures). Twenty one (68%) failures were females. All revisions and ten (71%) awaiting revision were symptomatic. Twenty four failures (86%) showed progressive radiological changes.

Fourteen revision cases showed evidence of adverse reactions to metal debris (ARMD). The failure cohort had significantly higher whole blood cobalt ion levels (p=0.001), but no significant difference in cup size (p=0.77), inclination (p=0.38) or cup version (p=0.12) compared to the non revised cohort. Female gender was associated with increased risk of failure (p=0.04). Multifactorial analysis demonstrated isolated raised Co levels in the absence of symptoms or XR changes were not predictive of failure (p=0.675). However the presence of pain (p<0.001) and XR changes (p<0.001) in isolation were significant predictors of failure. Wear analysis (n=5) demonstrated increased wear at the trunnion/head interface (mean out of roundness measurements 34.5 microns (normal range 8–10 microns) with normal wear levels at the articulating surfaces. Macroscopically corrosion was evident at the proximal and distal stem surfaces. Cumulative survival rate, with revision for any reason was 92.4% (95%CI: 87.4–95.4) at 5 years. Including those awaiting surgery, the revision rate would be 15.1% with 89.6% (95% CI: 83.9–93.4). Cumulative survival at 5 years.

This MOMHTHR series has demonstrated unacceptable high failure rates with evidence of high wear at the head/trunnion interface and passive corrosion to the stem surface. Female gender was an independent risk factor of failure. Metal ion levels remain a useful aspect of the investigation work up but in isolation are not predictive of failure.