‘Can We Classify Implants By Risk? – Resurfacing’ - Derek McMinn, London September 2014

This talk was given by Professor Derek McMinn in September 2014 at The Royal College of Surgeons of England in London. The meeting was titled ‘Rationalising the Follow up of Patients with Metal on Metal Hip Replacements’ and Mr McMinn’s talk on Hip Resurfacing came under the heading of ‘Can we classify implants by risk?’ In the talk, Mr McMinn explores diagnoses, failures and taper corrosion.
 
Over the last seventeen years, Professor McMinn has performed 3636 Birmingham Hip Resurfacing (BHR) operations. This number continues to rise. The mean age in men is 54 and the mean age in women is 52. Diagnoses include Osteoarthritis (OA), Inflammatory Arthritis (IA), Perthes/SUFE, Post-Trauma, Developmental Dysplasia of the Hip (DDH) and Avascular Necrosis (AVN). OA predominates these diagnoses.
 
Failures are separated into femoral failures, bearing failures and infection. Survival analysis shows a 96.1% implant survival at seventeen years. When looking at the effect of gender and age; men over and under 60 years and women over 60 years are no problem. The problem area is women under the age of 60 years. Diagnoses for women under 60 years show 299 patients (31.2%) with DDH compared to women over 60 years showing 28 patients (13.1%) with DDH. For good measure, women under 60 years show 23 patients (3.1%) with AVN compared to women over 60 years showing 0 patients (0%) with AVN.
 
Looking at failure by diagnoses, the worse groups are Dysplasia and AVN. Femoral failure specifically accounts for almost all failures in the AVN group and 3% of failures in the Dysplasia group. Bearing failure accounts for the majority of Dysplasia failures. 
 
The most challenging activity levels are men and women with OA under the age of 50 years. Survival analysis of these two groups is good. There is one infection in the women group and three in the men. Aseptic failure in men shows a 99.2% implant survival at seventeen years and women a 100% implant survival at seventeen years.
 
The head sizes in men are large, whilst the head sizes in women are small. Statisticians have looked in detail for risk of failure in Professor McMinn’s patients and small femoral heads did not represent a significant risk for failure. AVN in men and Dysplasia in women were significant risk factors. MARS CT scans of the first 351 patients were classified to the Mathys scale. 4.8% had a small effusion. There were no solid Pseudotumours. To put that into perspective for arthritic hips, 2.5% of those had an effusion.
 
Professor McMinn’s BHR patients have only been revised on the basis of symptoms. No muscle necrosis has been seen in the revision patients. Follow up of asymptomatic BHR patients is not required. This opinion relates to well done BHRs – it does not relate to badly done BHRs and it does not relate to other resurfacing types. Mr McMinn turned briefly to large head, metal-on-metal Total Hip Replacements (THR) and these represent 10% of the BHR numbers in this series. The survival curve is fairly satisfactory and better than the new NICE guidelines.
 
There was a ‘cross-breed’ mix and match series consisting of a stainless steel Centerpulse MS-30 stem and an MMT modular head and cup. The taper of the MS-30 was measured and it was geometrically compatible with the Birmingham modular head. Survival analysis shows that this series has been no problem, with 100% implant survival at ten years – there was one late revision for stem loosening. Two types of ‘pure-breed’ hips with Smith & Nephew manufactured implants all round were a problem. They failed the new NICE guidelines. Two of these had edge wear of the cup. High combined anteversion in a woman with Dysplasia was another cause. The rest were all due to taper corrosion. Another case showed, on revision, a milky fluid and the head taper showed corrosion and metal loss where the male taper contacted it. A further case presented with a large swelling, mixed cystic and solid Pseudotumour, extensive necrosis of the abductor muscles and clear taper corrosion.
 
Severe muscle necrosis has been seen with a Smith & Nephew stem on and a Smith & Nephew head and cup. These patients should be followed up. Smith & Nephew should be required to fund this follow up and revision surgery if it is required.
 
The Birmingham Mid-Head Resection (BMHR) has a titanium alloy stem which shows good results, with only two early failures – one for fractured neck of femur after a fall on ice and the other for deep infection. There were no taper failures. The flared proximal end of the BMHR stem renders the neck taper complex stiff, which is fine from a corrosion viewpoint. The two Smith & Nephew stems have reduced tapers and necks which may reduce the flexural rigidity meaning the use of these stems is associated with taper corrosion.