November 15, 2008
The first point I want to make is I have done hundreds or thousands of total hip replacements (THR) on all sorts of people at all sorts of ages and activity levels. Some of this experience pre-dates hip resurfacing, but I still often do THR as well. There were many very happy THR patients, but quite a few with a host of problems. My patients and colleagues' patients referred to me for a second, third or more opinion! Dislocation [sometimes occurring so often further surgery was required], infection, leg length differences and completely inexplicable pain. Another frequent problem was limping or muscle weakness as a result of using the direct lateral or Hardinge approach to the hip in an effort to avoid the higher dislocation rate associated with the posterior approach - which more rarely has muscle weakness or limp associated with it.

So DO NOT assume THR is problem free. One of the worst effects of THR, and one that we are lucky to so consistently get away with, is the invasion of the marrow of the femur [thigh bone] when the stem is placed deep into it. This generates enormous pressure which forces large fat globules/marrow contents into the bloodstream. The fat globules are filtered by the lungs, but can cause a subtle alteration or activation of the coagulation and immune systems that makes patients equally subtly feel and look unwell. You cannot scientifically -or at this stage I can't - put a finger on it, but this effect is avoided or greatly minimised in [Birmingham] hip resurfacing (BHR). The latter seem to 'bounce back' from surgery quicker than comparable THR patients. Designing and executing a study to prove this would be extremely complex so as to avoid bias, achieve case matching etc. And it would require large numbers of patients in each group. Lastly it would seem unethical to randomise patients to receive either a BHR or THR just to prove my point. So it is a study that may never be done. But the evidence re intra-op embolisation is intuitively relevant. Sadly, elderly patients who have emergency surgery for neck of femur fracture that consists of a hemi or total hip replacement not infrequently die in theatre shortly after the stem was inserted. This by the way, does not happen if the fracture is extra-capsular and therefore treatable by placing a screw or pin in the broken hip.

Preserving the femoral head and neck is also so intuitively 'correct' that several of the earliest pioneers of hip surgery tried again and again to devise prostheses that would work. Charnley, Amstutz, Wagner, Freeman and others all experimented with resurfacing way before McMinn. They all failed because the materials [metal on plastic or ceramic on plastic] were not up to the job, NOT because the concept is flawed.

Metal on metal (MOM) bearings have been around for a very long time. McKee and Farrar in Norwich had a MOM THR that is well known. Sometimes it lasted for decades, but too often there were early failures. All MOM bearings need several factors to be just right in order to work well. Not nearly right, nearly perfect in order to work. Metal on plastic bearings can have several technical factors quite wrong, but they are more forgiving of these flaws, and work well regardless. So Charnley, with his metal on slippery polyethylene, was able to outshine and eventually eclipse all the other bearing couples being used, so that by the 70's and 80's metal on plastic was the near universal basis of THR.

When we realised that tiny plastic particles cause aseptic loosening and failure of THR - especially in young active patients, there was a surge of interest in alternatives eg ceramic on ceramic or back to MOM.

The genius of McMinn was to take 2 unrelated historical 'failures' and use modern metallurgy and manufacturing techniques to create a prosthesis that would solve nearly all the historical problems. So he married the concept of resurfacing with a MOM bearing that had low - in fact negligble - wear rates. And was repeatably reliable in terms of metal quality and manufacturing tolerances. McMinn was INCREDIBLY lucky to stumble upon just the right metal alloy AND the right manufacturing tolerances. Change the metal alloy even a little bit, or change the tolerances, gaps thickess etc. and the result may be very different. Hence the reason I have personally avoided copies of the McMinn BHR like the plague.

But that is not all that must be right. The alignment of the devices in the body is critical. This is hard to get consistently right and x-rays can be very misleading in terms of assessing whether you have 'got it right'.

Continued...