A Comprehensive System:

Re-design of the early hybrid resurfacing component was cleary required because of the introducer problem. The opportunity was therefore taken to markedly expand the range available (four cups increased to 23 cups) so that the extremes of patient sizes could be dealt with effectively.(Fig. 40)

A dysplasia system was also developed so that patients with severe acetabular dysplasia could be properly treated, a range of bridging cups was also introduced, resulting in a fully comprehensive system capable of dealing with the most difficult acetabular conditions whilst still mantaining an enhanced solid socket fixation.

We have not encountered any problem with socket fixation in the original Hybrid components except in severe dysplasia, but contemporary work indicated that hydroxyapatite on a smooth surface might not prove durable in the long term as hydroxyapatite does get resorbed.26 Hydroxyapatite on a porous surface has the advantage of encouraging early bone ingrowth. When the hydroxyapatite disappears, then enduring biological fixation continues with bone ingrowth in the porous network.

It was clearly important to maintain continuity and commonality as far as possible in all other aspects of this project. To this end the design team remained the same, Finsbury Ltd, the casting house, Centaur Precision, remained the same, the hydroxyapatite supplier, Plasma Biotal Ltd, remained the same as did the clinical development team, D.J.W. McMinn FRCS and R.B.C.Treacy FRCS. Only the finishing technologies changed significantly, to those developed by Finsbury Ltd, which were not available elsewhere but which allowed us much better dimensional control of the bearing surface geometry.

Development of porous ingrowth surface – PorocastTm:

It is known from clinical practice that conventional methods of producing a porous surface (plasma spray titanium and sintered beads) have a weak mechanical link to the substrate material (Fig. 41) and the coating not infrequently displaces. Particles can become lodged in the articulating parts and act as a third body. (Fig. 42) It was considered that this would be particularly detrimental to a metal on metal articulation, so a much more robust porous coating had to be developed.

Sintered beaded acetabular cup showing weak bead junctions. (Fig. 45)

1 year retrieval of polyethylene liner showing embedded plasma spray titanium particles displaced from uncemented cup shell. (Fig. 46, 47)

Porocast ™ is a cast-in porous surface and the beads are integral with the substrate metal. This was a joint development between Centaur Precision Castings (a division of Doncaster Industries Plc), Sheffield Hallam University, The Casting Development Centre (Sheffield) and Midland Medical Technologies Ltd.

BHR components during casting process. Ceramic coating of wax forms (Fig. 49)

Section through BHR cup showing Porocast Tm. x50

Metallurgy:

We wanted to faithfully reproduce the metallurgical microstructure and chem-istry of the McKee and Ring metal on metal hip arthroplasties which had given such good bearing durability. (Fig.51)

Ring bearing microstructure showing rich carbide content x100 (Fig. 51)

Carbide depletion following HIP + SHT x100 (Fig. 52)

Finishing Technologies:

It was clear that some of the early McKee/Farrar failures were due to poor manufacturing.In the modern era of metal on metal joints the highest possible technology is employed to achieve near perfect bearings.

In the case of the Birmingham Hip Resurfacing, roundness to within two microns is achieved, an order of magnitude improvement on the conventional THR. (Fig. 53-55) Surface roughness is well within the ISO standard for conventional THR.