‘Metal-on-Polyethyene in Hip Resurfacing’ - Derek McMinn, Ghent, May 2014
In this talk, given in May 2014 at the 6th Advanced Hip Resurfacing Course in Ghent, Mr Derek McMinn MD FRCS discusses the idea of Metal-on-Polyethylene Hip Resurfacing.
The best outer diameter/inner diameter (OD/ID) difference achieved with a Metal-on-Polyethylene Total Hip Replacement (THR) is 16mm; with 4mm thick metal and 4mm thick polyethylene. By comparison, a Birmingham Hip Resurfacing (BHR) has a 6mm OD/ID difference, therein lies the problem for a polyethylene resurfacing as there is simply not enough space.
A polyethylene cup with a 1mm thick additive porous shell leaves a 2mm thickness of polyethylene. The regulatory bodies want 3mm of polyethylene all round and 4mm in the wear zone. So what are the options to try and achieve those requirements? Displacement of the hip centre laterally gives an acceptable thickness of cup, however this makes the acetabular subtend angle 148.6 degrees – treading dangerously into ASR type levels and leading to edge loading and fracture of the edge of the component. If the subtend angle is brought back to an acceptable 160 degrees, then there isn’t enough thickness of polyethylene. Peripheral expansion to 58mm on a 56mm cup, with the added 1mm thick porous layer, that is an acceptable thickness of polyethylene and subtend angle. The problem is; what to call this cup? Is it a 58 cup – as it is 58mm at the periphery – or is it a 56 – as it is 56mm at the pole? These decisions are normally made in boardrooms.
The hip centre can be displaced sideways which has been done, but doesn’t really help. Lateral and inferior displacement of the hip centre produces a better result The Indiana Conservative Hip and other total hip replacements have an inferior cut-out for reasons of impingement. Applying an inferior cut-out to the polyethylene cup as well as lateral and inferior displacement gives an acceptable outcome. Adding a 1mm porous shell is fine however after adding the 3mm polyethylene line, everything apart from the inferior aspect of the cup has a minimum 3mm of polyethylene.
The thin porous metal shells are flexible. A 1mm thick cup shows 5mm of easy deformation, a spiked shell measuring 3mm at the pole and 1mm at the periphery shows 1.5mm of deformation. A polyethylene cup shows 0.3mm of deformation so the polyethylene cup is stiffer than the metal. In whatever proportion the metal and polyethylene are put together, it will produce a springy cup and that may have a poor press-fit. There may be a need for supplementary fixation such as spikes. This device will deform when press-fitted so a big clearance is needed to avoid the cup clamping on the head. Perhaps it would be wiser to have a harder head, such as modern resurfacing ceramic component (provided it was not pink).
The downside of conventional polyethylene is the larger the head, the more the wear volume. Cross-linked polyethylene does not have that disadvantage. There are other disadvantages however, as cross-linked polyethylene weakens with radiation cross-linking and there is further weakening with melting to remove the free radicals. One strategy would be to remove the re-melting step, adding anti-oxidant will consume the free radicals and minimise the weakening of cross-linking. Another strategy is to use a hybrid polyethylene, only having cross-linked polyethylene in the wear zone where it is required and have stronger, conventional polyethylene all around the edge.
Mr McMinn’s idea for what a new polyethylene resurfacing should look like incorporates a 0.1mm thick shell, one spike or many micro-spikes, polyethylene thickness at the superior edge of 4mm, 3mm around the inferior edge, with loops moulded into the edge of the polyethylene which will be cut off when the cup has been positioned.
Results of Mr McMinn’s first 1000 consecutive BHRs show no loss to follow up, two to one male to female ratio, all ages and all diagnoses – 4.2% failure rate at fifteen years. Will a new polyethylene resurfacing be better? Will any modern resurfacing be better?
The only resurfacing that produces as good results as the BHR... is the BHR!