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.

Introduction Non-invasive prediction of load bearing capacity is an important issue in the advanced clinical treatment of distraction osteogenesis in order to define the appropriate point of time for the removal of the external fixateur. Therefore, non-invasive stiffness measurements were recommended as a promising tool due to the high correlation between strength and various kinds of stiffness: Torsional, bending and compressive.

However, previous experiments only analysed the relationship between a single type of stiffness. This approach neglects the multi-dimensional characteristics of bone loading in compression, bending and torsion.

This study investigates how compressive, bending (ap and ml) and torsional stiffness are related to the torsional load bearing capacity of healing callus tissue using a common set of bone regenerate samples of sheep treated with distraction osteogenesis. In addition, this study compares the evolution of the various kinds of stiffness.

This study provides insight into how the various stiffness modes are suited to predict the load bearing capacity by in-vivo stiffness measurement.

Material and Methods Mid-diaphyseal osteotomies were performed in 26 right tibiae of mature, female domestic sheep. Tibiae were then stabilized using an external half-ring Ilizarov fixator. After a 4-day latency period the tibiae were distracted at a rate of 1.25 mm per day in two increments for 20 days. As a result of a parallel study, the callus was treated with different combinations of growth factors and carrier material resulting in four treatment groups plus a contralateral control group. The sheep were sacrificed and the tibiae were harvested on the 74th day.

The ends of the tibiae were embedded in PMMA and mounted to a sequence of special custom made jigs for compressive testing, 4-point-bending and torsion in a material testing machine.

Stiffness was calculated by regression of the initial linear part of the load-displacement curves.

In a final experiment, the specimens were loaded in torsion until failure to record the ultimate torsional moment.

Results Torsional stiffness exhibits the highest correlation with the ultimate torsional moment (r2 = 0.77), while the ones for compressive (r2 = 0.60) and bending (ap (r2 = 0.70); ml (r2 = 0.66)) are only slightly lower.

Discussion This ex-vivo study in sheep shows that torsional, bending (ap and ml) and compressive stiffness measurements are all suitable means to predict the load bearing capacity of healing callus tissue. Our results show that torsional stiffness measurements perform slightly better than compressive and bending stiffness measurements. However, further studies are necessary to underline the superior performance of torsional stiffness measurements, since the sheep-tibiae were failed by applying torsional stress.

Introduction: The long consolidation phase of patients undergoing distraction osteogenesis (DO) causes a high risk of side effects and contributes to high costs. Thus, the development and evaluation of treatments that accelerate the bone consolidation process is of great interest. Evidence suggests that recombinant human bone morphogenetic protein 2 (rhBMP-2) increases the mechanical integrity of the callus. However, the potential benefits of rhBMP-2 on trabecular microarchitecture during DO have not been investigated up to date. In this study the regenerate microarchitecture was assessed using 3D micro-computed tomography (CT).

Methods: Mid-diaphyseal osteotomies were created in the right limb of twenty-four skeletally mature sheep, which were stabilized with an external fixator. After a latency period of 4 days, the tibiae were distracted at a rate of 1.25 mm daily over a period of 20 days. The operated limbs were randomly assigned to three treatment groups and one control group: (A) triple injection of rhBMP-2/NaCl, (B) single injection of rhBMP-2/Hydroxylapatite, and (C) single injection of buffer/Hydroxylapatit, (D) no injection. Groups A and C were injected at day 27. Group B was injected on days 3, 10 and 17. The animals were sacrificed after 74 days. The tibiae were analyzed by CT and for bone volume/total volume (BV/TV), trabecular number (Tb.N), trabecular thickness (Tb.Th.), trabecular separation (Tb.Sp.) and Connectivity. The BV/TV was maesured for the total volume of the distraction zone (BV/TVtotal) respectively in a subvolume with emphasize on the cortical bone region (BV/TVcortical). All other microarchitecture parameters were measured in the cortical weighted subvolume.

Results: The stereologic evaluation revealed a significant higher BV/TVcortical, Tb.N and Connectivity in the triple rhBMP-2 injected group A than in the control (D). Furthermore, the Tb.Sp. in group A was significant lower than in group D. The single injections of rhBMP-2/carrier in group B showed a significant higher BV/TVcortical, Tb.N and Connectivity than the control (D). Although the BV/TVcortical was increased in group A and B, there was no significant difference in BV/TV total between the rhBMP-2 treated groups (A, B) and the control (D).

Discussion: In this DO model a triple injection of rhBMP-2 has been demonstrated to induce significant changes in trabecular microarchitecture. RhBMP-2 does not increase the total amount of newly formed bone, but it enhances the formation of the corticalis. The microstructural changes in the cortical volume: increase of Tb.N and Connectivity, decrease of Tb.Sp., are discussed to be biomechanically highly relevant. This study suggests that rhBMP-2 optimizes the trabecular microarchitecture, which might explain the advanced mechanical integrity of newly formed bone under rhBMP-2 treatment.