A New Model to Study Healing of a Complex Femur Fracture with Concurrent Soft Tissue Injury in Sheep

Abstract

High energy bone fractures resulting from impact trauma are often accompanied by subcutaneous soft tissue injuries, even if the skin remains intact. There is evidence that such closed soft tissue injuries affect the healing of bone fractures, and vice versa. Despite this knowledge, most impact trauma studies in animals have focussed on bone fractures or soft tissue trauma in isolation. However, given the simultaneous impact on both tissues a better understanding of the interaction between these two injuries is necessary to optimise clinical treatment. The aim of this study was therefore to develop a new experimental model and characterise, for the first time, the healing of a complex fracture with concurrent closed soft tissue trauma in sheep. A pendulum impact device was designed to deliver a defined and standardised impact to the distal thigh of sheep, causing a reproducible contusion injury to the subcutaneous soft tissues. In a subsequent procedure, a reproducible femoral butterfly fracture (AO C3-type) was created at the sheep's femur, which was initially stabilised for 5 days by an external fixator construct to allow for soft tissue swelling to recede, and ultimately in a bridging construct using locking plates. The combined injuries were applied to twelve sheep and the healing observed for four or eight weeks (six animals per group) until sacrifice. The pendulum impact led to a moderate to severe circumferential soft tissue injury with significant bruising, haematomas and partial muscle disruptions. Posttraumatic measurements showed elevated intra-compartmental pressure and circulatory tissue breakdown markers, with recovery to normal, pre-injury values within four days. Clinically, no neurovascular deficiencies were observed. Bi-weekly radiological analysis of the healing fractures showed progressive callus healing over time, with the average number of callus bridges increasing from 0.4 at two weeks to 4.2 at eight weeks. Biomechanical testing after sacrifice showed increasing torsional stiffness between four and eight weeks healing time from 10% to 100%, and increasing ultimate torsional strength from 10% to 64% (relative to the contralateral control limb). Our results demonstrate the robust healing of a complex femur fracture in the presence of a severe soft tissue contusion injury in sheep and demonstrate the establishment of a clinically relevant experimental model, for research aimed at improving the treatment of bone fractures accompanied by closed soft tissue injuries.

Healing physiology following delayed surgery for femoral midshaft fracture caused by high-energy injury: an in vivo study in dogs

“…Four weeks after fracture, the fractures in Group A were scarring and the bone was slightly atrophic (Figure 8). Group B (3 weeks postoperatively) showed bone callus bridging (Figure 9(a)) and new callus containing a rich and energetic cutting cone (osteoclast population) and closing cone (osteoblast population) extending in different directions 14 ; the cutting cone was destroying the sequestrum, trailing the closing cone directly into the bone (Figure 9(b) and (c)). At 8 weeks after fracture, the original bone in Group A was thinner and had been replaced by new bone, which was blocked by scar tissue and unable to extend to the contralateral and surrounding regions, and atrophic nonunion had formed (Figure 10).…”

“…Certain studies have indicated that postfracture callus growth, which represents the bone healing capacity, is closely associated with damage to surrounding soft tissues 13 and that delayed union and nonunion caused by high-energy injury must be associated with even more severe soft tissue damage. 1,2,14 Severe soft tissue injury may lead to a greater local inflammatory response and poorer fracture healing. The physiological process of fracture healing is presently considered to involve benign inflammation caused by low-energy injury; [15][16][17] however, a model of bone fracture focusing on soft tissue damage and the inflammatory response has not been reported.…”