Characterization of interfragmentary motion associated with common osteosynthesis devices for rat fracture healing studies

Abstract

Rat models are widely used in preclinical studies investigating fracture healing. The interfragmentary movement at a fracture site is critical to the course of healing and therefore demands definition in order to aptly interpret the experimental results. Estimation of this movement requires knowledge of the fixation stiffness and loading. The characteristic loading for the rat femur has been estimated, but the stiffness of fixation used in rat studies has yet to be fully described. This study aimed to determine the 6 degree of freedom stiffness of four commonly used implants, two external fixators (RatExFix and UlmExFix), a locking plate, and a locking intramedullary nail, in all degrees of freedom and estimate the interfragmentary movement under specific physiological loads. The external fixator systems allow the greatest movement. Mounted 45° anterolateral on the femur, the RatExFix allows an average of 0.88 mm of motion in each anatomic direction while the stiffer UlmExFix allows about 0.6 mm of motion. The nail is far stiffer than the other implants investigated while the plate allows movement of an intermediate magnitude. Both the nail and plate demonstrate higher axial than shear stiffness. The relatively large standard deviations in external fixator shear motion imply strong dependence on bone axis alignment across the gap and the precise orientation of the specimen relative to the loading. The smaller standard deviation associated with the nail and plate results from improved alignment and minimization of the influence of rotational positioning of the specimen due to the reduced implant eccentricity relative to the specimen axis. These results show that the interfragmentary movement is complex and varies significantly between fixation devices but establishes a baseline for the evaluation of the results of different studies.

Augmentation of fracture healing by hydroxyapatite/collagen paste and bone morphogenetic protein‐2 evaluated using a rat femur osteotomy model

“…Another possible reason was the excessive rigidity of the fixation. In contrast to the plate fixation system used in our study that achieved rigid fixation, external fixation permits certain levels of mobility at the osteotomy site, although the mobility depends on length of offset, diameter of the wires and materials . Given our result that 1‐mm bone defects fixed with the plate system did not heal spontaneously together with a previous report of fusion for all bone defects fixed using an external fixation system, we speculate that the high stability of the fixation in our study may have resulted in the lack of union.…”

Whole bone testing in small animals: systematic characterization of the mechanical properties of different rodent bones available for rat fracture models

“…Numerous fracture, critical size defect or osteosynthesis studies in rats have been published using the femur or the tibia [ 17 , 18 , 20 – 22 ]. Only in a few studies, the humerus has been subject to interventional experiments [ 23 , 24 ].…”

“…There is a risk of generating a concomitant fibular fracture which changes a stable into a rather unstable model and often implicates secondary infections in case of plate- or fixateur-application [ 26 ]. From a surgical standpoint, most authors prefer the femur in studies where interventions occur directly at the bone [ 20 , 27 ].…”

A methodology for the generation and non-destructive characterisation of transverse fractures in long bones

“…Interfragmentary movement (IFM) and mechanical stimuli have been found to have a strong influence in the fracture healing process and callus formation ( Wu et al, 1984 ; Goodship and Kenwright, 1985 ; Claes et al, 1995 ; Claes et al, 1997 ; Claes, 2011 ; Meyers et al, 2017 ). The tissue differentiation that leads to callus formation is believed to be controlled by the distribution of local strain, which is largely dependent on mechanical factors such as fracture topography and IFM ( Wang et al, 2017 ).…”