Abstract 38

INFLUENCE OF IMPLANT FIXATION ON PREDICTED BONE DENSITY CHANGES AROUND A COMPOSITE FEMORAL THA COMPONENT

Turner, A.W.L., Walsh, W.R.

Orthopaedic Research Laboratories, University of New South Wales, Prince of Wales Hospital, Sydney, Australia.

In this study, bone remodelling was simulated for a femur implanted with a composite, low bending modulus, fully porous coated stem. The influence of implant-bone ingrowth area on bony remodelling changes was investigated.

Finite element models of a femur and the implant were created from CT scans of a cadaveric human femur and a CAD file obtained from the manufacturer, respectively. Initial bone properties were assigned according to the CT density. A complete joint and muscle force set was used, representing 45% of the gait cycle.

The influence of bone ingrowth area on simulated bone density changes was investigated.

The following idealised conditions were examined:
   · Fully-bonded contact;
   · Proximally-bonded, sliding contact distally; and
   · Distally-bonded, sliding contact proximally.
The remodelling simulations were run until remodelling equilibrium. Simulated DEXA images were output and analysed at the seven Gruen zones.

Bone remodelling was simulated out to 6 postoperative years. Most density changes occurred within the first 2 to 3 years for all cases. There was very little difference in bone density changes between the fully- and proximally-bonded implants. For the distally-bonded case, more significant bone density changes, relative to the fully-bonded case, were evident at the medial and lateral zones. Medial bone loss was enhanced for the distally-bonded implant while apposition was increased laterally.

This study shows that distal fixation is unfavourable in terms of proximal-medial bone loss for a primary, uncemented, low modulus stem. For this implant, proximal or full fixation is preferable.

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