From the small number of failures in the Birmingham Hip Resurfacing series we have been able to obtain histological data relating to cup fixation. We have seen ingrowth into the acetabular porous surface from three weeks and this becomes more solid with the passage of time. Professor Archie Malcolm in Newcastle has examined these specimens and carried out histology.
removal with substantial bone still attached to
the acetabular component, despite surgical
Femoral Component Fixation
The circumstances for perfect cement fixation with this femoral component occur because of an open cancellous network, low viscosity cement and a high injection pressure generated by advancing the component into position. On sectioning we have seen excellent micro-interlock of cement into the peripheral femoral head cancellous network.
The few samples that we have had the opportunity of examining histologically have shown excellent acetabular component fixation and excellent femoral component fixation.
We have attempted to measure fixation by assessment of migration on both the acetabular and the femoral sides of the resurfacing. On the acetabular sides, we have compared the migration pattern of the hydroxyapatite fixed resurfacing cup to the Harris Galante cup using the Nunn method.52 On the femoral side, we have compared the migration of the cement fixed femoral component of the resurfacing to the cemented Exeter total hip replacement stem using the Walker method.53 For the Harris-Galantecups and Exeter stems 559 radiographs were used in determining migration measurements with time. In the resurfacings, 389 radiographs were used for measurement.
This work was carried out by Dr. Christian DeCockM.D., Fellow in Joint Replacement Surgery and Dr. Paul Pynsent, P.h.D., Director of Research, Royal Orthopaedic Hospital, Birmingham.
The migration plots of the four types of component are shown below.
- Linear regression lines were fitted to the data with 95% confidence limits as shown.
- The migration for the Exeter cemented femoral components was O.O98mm/yr.
- The migration for the cemented resurfacing femoral components was O.O31mm/yr.
- This difference was significant p= O.O22 (students t-test).
- The migration for the Harris-Galante uncemented cups was O.O55mm/yr.
- The migration for the hydroxyapatite coated uncemented resurfacing cups was 0.0 15mm/yr.
- This difference was not significant p= 0.35 (students t-test).
These data show that the migration pattern of the uncemented hydroxyapatite coated resurfacing cup is no different to the migration pattern of the Harris-Galante uncemented cup, an implant with a proven 15-year track record. The cemented Exeter stem has a higher migration rate than the cemented resurfacing component. However, it is known from other work that the Exeter stem has a successful track record and is designed to migrate a little within the cement mantle.
It is acknowledged that measurements from plain x-rays have a certain degree of inaccuracy and we are currently engaged in a collaborative study with the Karolinska Institute, Huddinge University Hospital in Stockholm and the Department of Orthopaedics in Skovde Hospital, Sweden, who are performing RSA54 migration measurements on the Birmingham Hip Resurfacing.
Birmingham Hip Resurfacing with attached markers and beads in the pelvis and femur used for RSA migration measurements.
Femoral Head Viability
The traditional objection to the concept of hip resurfacing is that surgeons considered that avascularnecrosis and collapse of the femoral head would be an inevitable consequence of this procedure. This view was supported when the poor results of the Wagner metal on polyethylene resurfacing showed collapsed heads, but as has already been mentioned, the evidence is that collapse of the femoral heads in the Wagner resurfacing arthroplasty was a result of bone destruction from polyethylene debris associated osteolysis. Michael Freeman showed that in the arthritic hip the blood supply to the femoral head was quite different to the normal hip.55 In the arthritic hip the blood supply is substantially intra-osseous, thus enabling a surgical approach to be made to the hip for hip resurfacing without causing avascular necrosis of the femoral head.
In our pilot series of resurfacings, the cemented cups performed poorly and many went on to loosening and failure and required revision surgery.42 These patients however had intact and well fixed femoral components and when these components were converted to conventional total hip replacement this gave the opportunity for studying the femoral head viability.
This specimen shows Tetracycline uptake on the surface of the trabeculae in the femoral head under ultra-violet light confirming femoral head viability.
In 1,839 metal on metal resurfacings performed over a period of eight years nine months, one patient has developed avascular necrosis of his femoral head.
© 2007 Smith & Nephew