Abstract 9

PERI-ACETABULAR MICROMOTION IN AN IN-VIVO ASEPTIC LOOSENING MODEL

RH Edmonds-Wilson, TC Hearn, T MacKenzie, GI Anderson.

Faculty of Science & Engineering and Department of Surgery, Flinders University, Adelaide.

We aimed to investigate the peri-acetabular micromotion in an in-vivo low morbidity model of focal osteolysis. Seven large dogs underwent uncemented total hip arthroplasty with a custom-made titanium grit-blasted femoral stem and cup. At surgery, a controlled defect was created in the acetabular bone bed using a 4.5mm drill and placing 800_Ls of submicron polyethylene particulates in agarose gel into this defect prior to placing the cup. Radiography and gait analysis were performed regularly to assess osseointegration and return to normal weight-bearing function over six months. The harvested acetabular specimens were potted in polymethyl-methacrylate in a jig allowing four linear variable differential transducers (LVDTs) to contact the bone 2mm sub-adjacent to the metal-backing of the cup, in test 1, and the metal of the cup itself in test 2. This allowed the relative motion of the cup and bone to be calculated after applying loads of either 0.6 or 1.5 bodyweights for 50 cycles at 1Hz through the polyethylene cup liner using a servo-hydraulic materials testing machine (Instron 8511). Intertest reliability was assessed and found to be less than 10%. Initial testing of relative micromotion between the cup and the bone in the three dorsal positions was 12.5±3.1µm, and 4.1±3.0.1µm ventrally. Further micromotion and histo-pathological analysis to quantify the extent of the lytic defect is ongoing. The small peri-acetabular micromotion in combination with return to normal weight-bearing strongly suggest that the area of focal osteolysis behind the cup did not result in morbidity in these dogs at 6 months.

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