How do Metal/Metal Bearings work:

Jin, Dowson & Fisher 50 showed that in both metal metal bearings and ceramic/ ceramic bearings thick film lubrication can occur. This is recent information and had this information been available at the time of Charnley’s hip replacement development it would surely have influenced his choice of bearing materials. Thick film lubrication of course means that the articulating surfaces are not in sliding contact, instead they are separated by a synovial fluid film and this has profound implications for wear and friction of the bearing.

Metal/Metal Surfaces x100,000

Boundary lubrication in M/M bearing. Synovial fluid does not separate articulating surfaces and wear will occur.

Metal/Metal Surfaces x100,000

If we consider a 28mm metal metal articulation and look at the graph of minimum fluid thickness versus clearance between the head and cup, then it can be seen that as the clearance decreases the fluid film thickness increases.(Fig.68) At the commonly manufactured clearance however of 100 microns the fluid film thickness that is generated is not enough to separate the articulating surfaces and asperity contact occurs and wear will result. In this 28mm metal metal articulation if the clearance is brought down to 25 microns then the fluid film generated is thick enough to completely separate the two articulating surfaces, however, achieving reproducibly a 25 micron clearance is a very difficult manufacturing task.

Wear Retrieval analysis of Birmingham Hip Resurfacing:

It has been possible to examine very carefully three pairs of Birmingham Hip Resurfacing bearings retrieved from patients 6 – 18 months after implantation. One patient died of unrelated causes and bequeathed his hip for research.
One bearing was retrieved at revision surgery for avascular necrosis of the femoral head and one bearing was retrieved at revision surgery for infection.

Mitutoyo Roundtest RA-300 (Fig.69)

Frictional Torque:

It is now well appreciated that peak load in the hip in an active person reaches six to nine times body weight, which means that a sportsman engaged in high level sport will generate a load across the hip joint in the region of 500kg.

We have built a pendulum apparatus to test the bearing of hip replacements under 500 kg of load.(Fig.72,74) When testing a bearing using this apparatus, the pendulum is started at a fixed point and the number of swings until standstill is counted. A number of runs are then performed on each bearing.

Results obtained using 500 kg pendulum:

When the metal on polyethylene bearings are considered it can be seen that the 22, 28 and 32mm bearings decrease the number of swings per run and then come to a plateau. The different sized metal metal bearings have been tested in serum and hyaluronic acid (substitute for synovial fluid) and blood. Of course these metal on metal bearings in patients are initially bathed in blood and later bathed in synovial fluid.

Unlike the results from Sir John Charnley’s pendulum comparator, it can clearly be seen that the frictional torque of these different head sized Birmingham Hip Resurfacing bearings are not very different to a range of metal polyethylene bearings in common clinical use. It can be concluded therefore that frictional torque with these metal on metal bearings is not the issue that Sir John Charnley thought it would be. This low frictional torque from the metal on metal bearings is entirely consistent with the clinical experience of historic metal on metal joints having lasted 30 years or more.

Metal Ion Levels:

Chromium, Cobalt and Molybdenum are of course normal dietary constituents and indeed essential dietary constituents. All of us therefore have a certain level of these metal ions measurable in our blood. We were interested to see if very active patients with metal on metal bearings in situ had any elevation of their blood metal ions compared with another group of patients with metal/metal bearings who had a low activity level. We selected two groups of patients, therefore, one group with a very high activity level who all played sport following a successful Birmingham Hip Resurfacing and a second group of patients who were elderly and inactive following a historical metal on metal total hip replacement performed more than 20 years before.

We worked with the trace metals laboratory at AEA Harwell on this project and we measured mineral digested whole blood Cobalt, Chromium and Molybdenum levels using high resolution induction coupled plasma mass-spectrometry. The results of these measurements are shown and when comparison was done using the t-test, no significant difference in Cobalt, Chromium or Molybdenum was seen between the high activity group and the low activity group. The conclusion from these results is that there is no correlation between the activity level and wear of a metal metal bearing and is strong supportive evidence that thick film lubrication occurs in these metal metal bearings. This is totally different to a metal polyethylene bearing where the wear of the bearing is related to the number of walking cycles. Therefore, high activity on a metal polyethylene articulation leads to high wear and osteolysis.