Abstract 14

MEASUREMENT OF ACETABULAR MICROMOTION IN A CANINE MODEL OF TOTAL HIP ARTHOPLASTY

Edmonds-Wilson RH¹, Anderson GI², Hearn TC¹

¹ School of Informatics & Engineering, Flinders University, Adelaide

² Department of Orthopaedic Surgery, Repatriation General Hospital (Daw Park), Flinders University, Adelaide.

This study assessed peri-acetabular fixation in an in vivo low morbidity model of focal osteolysis. Seven dogs received unilateral uncemented total hip arthroplasties with 220µg of polyethylene particles added behind the cup.

The harvested acetabular specimens were potted in polymethylmethacrylate in a jig allowing four motion transducers to contact the bone 2mm sub-adjacent to the metal backing of the cup in test 1, and the metal of the cup in test 2. This allowed the relative micromotion of the cup and bone to be calculated after applying compressive loads of up to 1.5 bodyweights using a servo-hydraulic materials testing machine (Instron 8511).

In five specimens minimal micromotion (<20µm) was observed, well within the level thought to indicate bony fixation. The other two specimens displayed levels between 50 and 100µm.

The contact radiographs indicated good bony support (in the range of 25-30% bone apposition) for three components. For the remainder, there was no bony fixation. Two of the non-contact specimens were known to be grossly loose prior to testing, and corresponded to the components with higher micromotion measurements. The other two components with no bony fixation, but supposedly excellent micromotion levels, indicate that the method of micromotion assessment may have over-constrained the components. The micromotion data, in combination with the contact radiographs, suggest that the purely compressive loading had a stabilizing effect.

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