Introduction: Non-invasive prediction of load bearing capacity during consolidation of distraction osteogenesis and fracture healing would represent a significant advance in the treatment of patients by defining the appropriate point of time for the removal of the fixator externe. Thereby the risk of refracture, malunion and infection could be reduced. Several methods have been proposed in the past to predict the load bearing capacity: dual-energy x-ray absorptiometry (DEXA), stiffness measurements, quantitative computed tomography, quantitative radiography and ultrasound. In this ex-vivo study stiffness- and DEXA-measurements were compared regarding their suitability to predict the load bearing capacity of bone regenerate.

In addition this study analysed how compressive, bending and torsional stiffness as suitable tools were related to the torsional load bearing capacity using a common set of bone regenerate samples of 26 sheep treated with distraction osteogenesis.

Material and Methods: After osteotomy the sheep tibiae were stabilized using an external half-ring Ilizarov fixator. Followed by a 4-day latency period the tibiae were distracted at a rate of 1.25 mm per day in two increments for 20 days. On the 74th day the sheep were sacrificed and tibiae were harvested. The ends of the specimens were embedded in PMMA for further biomechanical testing. Therefore, the specimens were mounted to a sequence of special costume made jigs for compressive testing, 4-point bending and torsional testing in a material testing machine. Stiffness was calculated by regression of the linear part of the load-displacement curves. The maximum torsional moment of the specimens was determined in a final experiment. In addition the bone mineral density (BMD) of the distracted bone tissue was measured using DEXA. The correlation between the maximum torsional moment and the various types of stiffness respectively BMD was analysed to gain information about the suitability predicting the load bearing capacity.

Results: Torsional stiffness exhibits the highest correlation with the maximum torsional moment (r2 = 0.77) followed by bending (ap (r2 = 0.70); ml (r2 = 0.66)) and compressive stiffness (r2 = 0.60). The correlation for BMD with the maximum torsional moment was smallest (r2 = 0.39).

Discussion: This ex-vivo study revealed that the stiffness measurements seem to be a helpful tool to predict the load bearing capacity of bone regenerate. The results of this study showed stiffness measurements as a more suitable mean to determine the load bearing capacity. Within the various types of stiffness measurements torsional stiffness measurements perform slightly better than bending and compressive stiffness measurements. Nevertheless, further studies are necessary to support the results of this study since the specimens failed applying torsional stress.

The ability to predict load-bearing capacity during the consolidation phase in distraction osteogenesis by non-invasive means would represent a significant advance in the management of patients undergoing such treatment. Measurements of stiffness have been suggested as a promising tool for this purpose. Although the multidimensional characteristics of bone loading in compression, bending and torsion are apparent, most previous experiments have analysed only the relationship between maximum load-bearing capacity and a single type of stiffness. We have studied how compressive, bending and torsional stiffness are related to the torsional load-bearing capacity of healing callus using a common set of samples of bone regenerate from 26 sheep treated by tibial distraction osteogenesis.

Our findings showed that measurements of torsional, bending and compressive stiffness were all suitable as predictors of the load-bearing capacity of healing callus. Measurements of torsional stiffness performed slightly better than those of compressive and bending stiffness.