Biomechanics of two external fixator devices used in rat femoral fractures

Abstract

The use of external fixators allows for the direct investigation of newly formed interfragmentary bone, and the radiographic evaluation of the fracture. We validated the results of a finite element (FE) model with the in vitro stiffness' of two widely used external fixator devices used for in vivo analysis of fracture healing in rat femoral fractures with differing construction (Ti alloy ExFix1 and PEEK ExFix2). Rat femoral fracture fixation was modeled using two external fixators. For both constructs an osteotomy of 2.75 mm was used, and offset maintained at 5 mm. Tufnol, served as standardized substitutes for rat femora. Constructs were loaded under axial compression and torsion. Overall axial and torsional stiffness were compared between the in vitro models and FE results. FE models were also used to compare the fracture movement and overall pattern of von Mises stress across the external fixators. In vitro axial stiffness of ExFix1 was 29.26 N/mm AE 3.83 compared to ExFix2 6.31 N/mm AE 0.67 (p à < 0.05). Torsional stiffness of ExFix1 was 47.5 Nmm/˚AE 2.71 compared to ExFix2 at 19.1 Nmm/˚AE1.18 (p à < 0.05). FE results predicted similar comparative ratios between the ExFix1 and 2 as the in vitro studies. FE results predicted considerably larger interfragmentary motion in the ExFix2 comparing to ExFix1. We demonstrated significant differences in the stiffness' of the two external fixators as one would expect from such variable designs; yet, importantly we validated the utility of an FE model for the analysis and prediction of changes in fracture mechanics dependent on fixator choice. ß

The influence of gap size on the development of fracture union with a micro external fixator

“…The fixator used in this study has been shown to be significantly stiffer at 4.7 times the axial stiffness of the commercially available AO fixator (Osagie-Clouard et al, 2019), and hence will have provided a relatively more rigid fixation. Interestingly, increasing the fracture gap, which increases the working length of the carbon fiber bars did not have any statistically significant effect on construct stiffness, indicative of the relatively rigid fixator design compared with the physiological forces it withstands.…”

“…The most common fixators in use for rodents are the thermoplastic polyether ether ketone (PEEK) Glatt fixator from the AO Research Institute Davos (Glatt and Matthys, 2014), which is commercially available and the titanium alloy ‘Harrison style’ fixator (Harrison et al, 2003; Ho et al, 2014; Lee et al, 2005; Smitham et al, 2014). The more rigid Harrison fixator (Osagie-Clouard et al, 2019), is a unilateral uniplanar fixator with a double carbon fiber connecting bar, which has the novel function of permitting variable gap size, by sliding the adjustable distal titanium block along the bar. This approach to varying gap size maintains the pin to osteotomy gap distance irrespective of gap size, whereas other micro fixators require an ostectomy of the desired gap distance to vary said gap.…”

“…The Harrison style fixator has previously showed consistent union with a 0.5 mm gap and consistent non-union with a 3 mm gap with a rat femoral osteotomy after 5 weeks (Harrison et al, 2003) and in female adult wistar rats (Lee et al, 2005; Smitham et al, 2014). The AO fixator is considerable less stiff (Osagie-Clouard et al, 2019) and although studies generally use controls, direct comparison of results on the biology of fracture fixation using different fixators is probably inappropriate due to the difference in their mechanics and hence differences in healing.…”

A Novel External Fixator to Facilitate Accurate and Efficient Fracture Treatment

“…The external skeletal xation technique is a minimally invasive operation in which nails are percutaneously implanted and connected with external frames. The stress transmitted between nails stimulates the regeneration and reconstruction of bone tissue [8]. Previous studies have justi ed the bene ts of an external xator in the treatment of fractures, as it can (1) maintain fracture stability by resisting rotation, shearing and varus-valgus forces [9,10]; (2) alleviate articular surface stress for articular fractures, thus protecting the articular surface [11]; and (3) supply a suitable environment for soft tissue and muscle recovery [12].…”