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Biomechanics of equine long bone fracture repair
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The principle objective of repairing an equine long bone fracture is immediate return to load bearing. This demands precise anatomical realignment and stabilization of the fracture fragments so that the damaged bone can re-assume load bearing responsibilities. Unlike the situation in small animals and humans where load on the repair can be controlled by imposed cessation of activity and ready diversion of load bearing away from the injury, horses need to immediately resume load bearing in the injured limb, lest they encounter complications such as laminitis and pain-associated gastrointestinal ulcers. Further, unlike smaller animals where defects in the reassembled bone can be successfully bridged by the implants who can temporarily assume a direct load bearing responsibility because of their relative strength, the horse’s size and strength generally precludes this option because it quickly risks implant failure due to overload.
Whether or not we successfully achieve precise anatomical realignment and stabilization of the fracture fragments is largely dictated by the surgeon’s understanding of: 1) the concept of biomechanics as it relates to implant selection and placement; 2) familiarity with the essential techniques of anatomic reconstruction and load sharing; and 3) familiarity with the different types of plate systems and their role in the fracture repair.
A starting point for the surgeon’s successful repair of long bone fractures is an appreciation for the normal load bearing forces on intact bone, illustrated to the left (Figure 1). These forces have to be successfully overcome by the correct selection and placement of the implants. Furthermore, a clear appreciation for associated soft-tissue trauma is a necessity since even the best mechanical bone repair will be compromised by poor tissue handling, ischemia and/or poor wound closure.
A detailed description of all of the biomechanical considerations needed for a successful repair of equine long bone fractures is beyond the scope of these notes but essential considerations include the following:
1. Minimize soft tissue disruption: The importance of maintaining the local biological environment around fractures in well recognized but in horses the need to accurately reconstruct the bone, place comparatively large implants, and debride contaminated or necrotic tissue secondary to high energy injury frequently precludes this opportunity.
Figure 1. Biomechanical force vectors experienced in a long bone
2. Use more than one bone plate: With only a few exceptions, the use of two (and occasionally three) plates allows us to position them relative to each other (~90º) such that when one is at the greatest risk of mechanical failure, the other is at least risk.
3. Stagger the position of the plates on the bone: When using more than one plate, it is mechanically advantageous to avoid a stress riser by positioning the plates in a staggered fashion in a proximo-distal axis.
4. Use plates that span the full length of the bone: It is mechanically advantageous to place at least one plate along the full length of the bone. This reduces the potential for stress riser formation and increases the stability of the fracture repair by maximizing screw placement and bone-plate interface.
5. Maximize the use of bone screws to affix the plate(s): The interfragmentary stability of the repair will be optimized by maximizing the number of screws used to secure the plate(s). Plate holes should generally be filled unless prohibited by the presence of an underlying vital structure or fracture plane.
6. Use at least 3 screws above and below the fracture plane:
7. Locking compression plates (LCPs): The use of LCPs lend additional stability to most fracture repairs by virtue of having the locking screw head firmly thread and embed into the bone plate. This greatly reduces the opportunity for implant pullout and builds overall interfragmentary stability.
8. Maximize the LCP-bone interface: In equine long bone fracture repair, we generally aim to maximize LCP-bone interface through accurate contouring of the plate to the bone topography and use of cortical bone screws to draw the plate down onto the bone.
9. Plate the tension side of bones whenever possible: Metal generally sustains tensile load more robustly than compressive load. Hence, wherever possible, identify the tension side of the bone during load bearing for plate application. This often allows us to convert unwanted displacing forces into beneficial compressive forces at the facture plane.
10. Plate over the leading edge of oblique fractures: Wherever possible, it is mechanically advantageous to secure a plate with screws below the fracture plane over the leading edge of oblique fractures. Axial loading then prompts the proximal fragment to move distal and be buttressed against the plate, preventing it from sliding down the shear plane.
11. Managing short proximal or distal fragments: Stabilizing short distal or proximal fragments requires special implants such as the angle blade plate or cobra head plate that provide sufficient purchase into a small volume of bone.
12. Use lag screw fixation: Use of lag screw fixation techniques outside of or through the bone plate(s) enhances inter-fragmentary compression.
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- Get unlimited access to books, proceedings and journals.
- Get access to a global catalogue of meetings, on-site and online courses, webinars and educational videos.
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[1] Kirker-Head CA: Novel Biological Agents to Enhance Fracture Healing. In Nixon A (ed). Equine Fracture Repair. Philadelphia, WB Saunders 93-103, 1995.
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