Modulation of fixation stiffness from flexible to stiff in a rat model of bone healing

Abstract

Background and purposeConstant fixator stiffness for the duration of healing may not provide suitable mechanical conditions for all stages of bone repair. We therefore investigated the influence of stiffening fixation on callus stiffness and morphology in a rat diaphyseal osteotomy model to determine whether healing time was shortened and callus stiffness increased through modulation of fixation from flexible to stiff.Material and methodsAn external unilateral fixator was applied to the osteotomized femur and stiffened by decreasing the offset of the inner fixator bar at 3, 7, 14, and 21 days after operation. After 5 weeks, the rats were killed and healing was evaluated with mechanical, histological, and microcomputed tomography methods. Constant fixation stiffness control groups with either stiff or flexible fixation were included for comparison.ResultsThe callus stiffness of the stiff group and all 4 experimental groups was greater than in the flexible group. The callus of the flexible group was larger but contained a higher proportion of unmineralized tissue and cartilage. The stiff and modulated groups (3, 7, 14, and 21 days) all showed bony bridging at 5 weeks, as well as signs of callus remodeling. Stiffening fixation at 7 and 14 days after osteotomy produced the highest degree of callus bridging. Bone mineral density in the fracture gap was highest in animals in which the fixation was stiffened after 14 days.InterpretationThe predicted benefit of a large robust callus formed through early flexible fixation could not be shown, but the benefits of stabilizing a flexible construct to achieve timely healing were demonstrated at all time points.

A Concert between Biology and Biomechanics: The Influence of the Mechanical Environment on Bone Healing

“…In this study we showed that when reverse dynamization, from flexible to rigid fixation, was applied at 3 weeks the healing outcome at the end of the treatment period was the same as the dynamization regimen (Claes et al, 2011; Bartnikowski et al, 2016). However, when reverse dynamization was applied at 1 week the healing outcome was exceedingly superior to that of the dynamized group (Table 1), constant rigid and flexible fixation groups (Bartnikowski et al, 2016). On the contrary, the dynamization regimen at 1 week was detrimental to bone healing when compared to any of the other groups tested (Claes et al, 2009).…”

“…For dynamization implemented at the later stages of healing, accelerated bone healing is more likely a consequence of bone adaptation following Wolff's law rather than fixator dynamization itself. In fact, this hypothesis was confirmed using the reverse dynamization regimen in the same 1 mm osteotomy rat model as was used by Claes' group (Bartnikowski et al, 2016). In this study we showed that when reverse dynamization, from flexible to rigid fixation, was applied at 3 weeks the healing outcome at the end of the treatment period was the same as the dynamization regimen (Claes et al, 2011; Bartnikowski et al, 2016).…”

“…In fact, this hypothesis was confirmed using the reverse dynamization regimen in the same 1 mm osteotomy rat model as was used by Claes' group (Bartnikowski et al, 2016). In this study we showed that when reverse dynamization, from flexible to rigid fixation, was applied at 3 weeks the healing outcome at the end of the treatment period was the same as the dynamization regimen (Claes et al, 2011; Bartnikowski et al, 2016). However, when reverse dynamization was applied at 1 week the healing outcome was exceedingly superior to that of the dynamized group (Table 1), constant rigid and flexible fixation groups (Bartnikowski et al, 2016).…”

Regenerative rehabilitation: The role of mechanotransduction in orthopaedic regenerative medicine

“…By 3 weeks the effect of reverse dynamization was lost and the outcome was equivalent to that occurring under standard, forward dynamization (Fig. ) . The modest gains produced by forward dynamization are not surprising considering our study demonstrating that any improvement noted when forward dynamization was implemented at the later stages of healing, was more likely a consequence of bone adaptation following Wolff's law, rather than fixator dynamization as such (Fig.…”

“…To study this we created 1 mm, mid‐diaphyseal osteotomies in the rat femur. All defects were fixed initially at low axial stiffness, and reverse dynamization (increased stiffness) applied at various times from 3 days to 3 weeks . The optimum time for increasing the stiffness of fixation occurred 7 days after surgery (Fig.…”