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.

Simple Test No significant difference in Co, Cr, Mo. Between high activity group 1 & low activity group 2

1 Year follow-up Data

	BHR	All THR	<55 yrs THR
Harris Hip Score	90.8	72.4	76.1
Pain	5.5	4.6	4.7
Walking	5.5	4.1	4.5
Range of movement	4.9	3.9	4.3
Satisfaction	3.9	3.4	3.5

2 Year follow-up Data

	BHR	All THR	55 yrs THR
Harris Hip Score	90.7	70.0	70.0
Pain	5.7	4.5	4.6
Walking	5.3	4.0	4.5
Range of movement	4.3	3.7	4.4
Satisfaction	3.9	3.5	3.4

These data show that the results of the Birmingham Hip Resurfacing carried out in centres throughout the world compare favourably with the outcomes of conventional THR in all age groups and in particular compare favourably with the outcomes of THR in young patients.

Acetabular component fixation:

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.

The specimen shown is a six month cup removal with substantial bone still attached to the acetabular component, despite surgical extraction. (Fig.56)
Detailed histological sectioning reveals excellent bone ingrowth. (Fig.57)

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. (Fig.58)

The few samples that we have had the opportunity of examining histologically have shown excellent acetabular component fixation and excellent femoral component fixation.

Component migration:

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

This work was carried out by Dr. Christian DeCock M.D, Fellow in Joint Replacement Surgery and Dr. Paul Pynsent P.h.D, Director of Research, Royal Orthopaedic Hospital, Birmingham. The migration for the Exeter cemented femoral components was 0.098mm/yr. The migration for the cemented resurfacing femoral components was 0.031mm/yr. This difference was significant p= 0.022(students t-test). The migration for the Harris-Galante uncemented cups was- 0.055mm/yr. The migration for the hydroxyapatite coated uncemented resurfacing cups was 0.015mm/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 migraion pattern of the Harris-Galante uncemented cup, an implant with a proven 15 yr 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 in Stockholm and the Department of Orthopaedics in Skovde Hospital, Sweden who are performing RSA 54 migration measurements on the Birmingham Hip Resurfacing. (Fig.59)

Birmingham Hip Resurfacing with attached markers & beads in the pelvis & femur used for RSA migration measurements.

Femoral head viability:

The traditional objection to the concept of hip resurfacing is that surgeons considered that avascular necrosis 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/polyethylene resurfacing showed collapsed heads, but as had 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.(Fig.58)

Histological examination of bone from these femoral heads showed normal haemopoetic marrow. (Fig.60)

Prior to revision surgery for cemented cup loosening we have also given patients Tetracycline 2 weeks before operation.

This specimen shows Tetracycline uptake on the surface of the trabeculae in the femoral head under ultra-violet light confirming femoral head viability.(Fig.61)

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.

Thrombo embolism:

Hoffman from Vienna has investigated intra-operatively the pressurisation in the femur during a conventional cemented stem total hip replacement and the pressure in the distal femur rises to 1400mmHg during insertion of the cemented femoral component.47 This is equivalent to car tyre pressure and this pressure drives fat and marrow from the femur in to the systemic circulation, right heart and pulmonary vasculature.(Fig.51) This fat and marrow displacement with total hip replacement rarely causes acute haemodynamic upset but the fat and marrow are rich in tissue thromboplastin and are potent activators of the coagulation system. It is probable that this fat and marrow displacement at conventional stemmed total hip replacement is responsible for the very high post-operative thrombo-embolic complication rate. It has been shown that whether an uncemented femoral component or a cemented femoral component is inserted, when the canal of the femur is instrumented then activation of the clotting cascade occurs.58,49

Trans-oesophageal echocardiography showing major fat embolisation into right heart following THR.(Fig.62)

During the procedure of hip resurfacing the canal of the femur is not instrumented. Furthermore it is easy to insert a cannula through the lesser trochanter into the upper femur thus minimising the femoral canal pressure during femoral component insertion.(Fig.52) Markedly reduced amounts of fat and marrow are seen on trans- oesophageal echocardiography after resurfacing (Fig.53) compared to conventional stem total hip replacement. Remarkably few thrombo-embolic complications are seen following hip resurfacing and review of patient records from the authors last 500 resurfacings has shown only one patient who developed a clinical calf vein thrombosis. No patients were readmitted with pulmonary embolism or venous thrombosis. This aspect of thrombo-embolic complications is being further investigated by comparing the coagulation changes following total hip replacement and hip resurfacing using thromboelastograph, prothrombin F1 and F2, thrombin anti-thrombin complex, D dimers and tissue factor assays.

Insertion femoral component BHR with suction vent through lesser trochanter. (Fig.63)

Trans-oesophageal echocardiography showing minor fat embolisation following BHR. (Fig.64)

Telephone:	(0121) 455 0411
Fax:	(0121) 455 0259
Address:	The McMinn Centre, 25 Highfield Road, Edgbaston, Birmingham, B15 3DP, England
Email:	enquiries@mcminncentre.co.uk