PAPER 27: MEASURING THE STIFFNESS OF THE REGENERATE IN DISTRACTION OSTEOGENESIS USING RADIOSTEREOMETRIC ANALYSIS

Abstract

The healing index, external fixation time divided by length gain, is commonly used as an outcome measure in distraction osteogenesis. This can be imprecise, and experimentally, regenerate stiffness is the accepted measure, but this can be difficult to measure clinically. The aim of this study was to investigate whether radiostereometric analysis (RSA) may be used to determine stiffness of new bone.

Two Ilizarov frames of differing stiffness were constructed around Sawbones tibiae. Known loads were applied to the frames and RSA was used to analyse the movement that occurred at the distraction gap. The axial stiffness of the frames was calculated. The distraction gap was filled with materials with a range of stiffnesses, representative of regenerate at different stages of consolidation. Loads were applied and RSA was used to measure the stiffness of the construct. A simple load share model was then used to estimate stiffness of the materials, and these values compared with those obtained from a materials testing machine.

The measured stiffnesses of the frames were 94N/mm and 55N/mm. RSA tended to underestimate the material properties of the ‘regenerate’, and this effect became greater at higher stiffness. There was also a 30% difference in estimated stiffness of the ‘regenerate’ when comparing the two fixators.

RSA is a very precise non-invasive method for measuring regenerate displacement. However, simple models to estimate stiffness tend to underestimate the true value, and assumptions made in basic engineering models are not valid when the stiffness of the regenerate approaches that of surrounding bone.