Purpose: Unless exposed to stress, bone undergoes lysis. Osteoconduction is not observed in hydroxyapatite in contact with dead bone. We wanted to know whether bone blocks positioned on metal back cups in patients with total hip arthroplasty for acetabular dysplasia are destroyed lysis or “adhere” to the hydroxyapatite.

Material and methods: The series included 22 bone blocks in 21 patients with low weight-bearing displacement (n=10), high weight-bearing displacement (n=6), high non-weight-bearing displacement (n=6). Mean follow-up was six years (17 patients > 5 years).

Technique: all blocks were fashioned with autologous bone. The size of the cup, and thus the volume of the bone block, was determined by the largest anteroposterior diameter compatible with the desired position. The block was fashioned with a trial cup in place. The definitive cup was inserted separately then lag screwed to the bone block.

Implants: twenty-two titanium cups (3 mm) with hydroxyapatite surfacing, bone block width: approximately 28° from the centre of the prosthetic head.

Cup position: native acetabulum (n=11), neoacetabulum (n=11).

Results: Complications were : sciatic paresia (n=0), displacement (n=0), shift in cup position (n=1 at day 21). Partial lysis of the bone block was observed but there was no case of total lysis.

Titanium-hydroxyapatite-block adhesion: no lucent line was visible in 17 cases at more than five years. There were no cases of bony nonunion.

Discussion: Are grafts necessary with a metal back cup? The cup is slightly exposed in some primary degenerative hips but all goes well for ten years without a bone block. At what point would cup “exposure” require a bony support? As autograft material is available, it would appear inappropriate to not use it, especially since the lack of lysis suggests grafts are useful. All bony structures behave in an intelligent manner: an oversized bone block undergoes lysis but the part under stress above the cup becomes more dense.

Conclusion: Autologous bone blocks placed on metal back cups are not destroyed by lysis. The bone-hydroxyapatite couple adheres normally. Autologous bone blocks above metal back cups behave intelligently, like blocks inserted above cemented polyethylene cups.