© Schattauer 2013 Vet Comp Orthop Traumatol 3/2013 177 Original Research Ex vivo evaluation of the biomechanical effect of varying monocortical screw numbers on a plate-rod canine femoral gap model P. J. Delisser 1 ; G. P. McCombe 2 ; R. S. Trask 2 ; J. A. Etches 2 ; A. J. German 3 ; S. L. Holden 3 ; A. M. Wallace 1 ; N. J. Burton 1 1 School of Veterinary Science, University of Bristol, Langford House, Langford Bristol, UK; 2 Department of Engineering, University of Bristol, Advanced Composites Centre for Innovation in Science, Queen’s Building, Bristol, UK; 3 Depart- ment of Obesity and Endocrinology, The University of Liverpool, Leahurst Campus, Chester High Road, Neston, UK Keywords Biomechanics, stiffness, screw number, comminuted, fracture Summary Objective: To compare the biomechanical behaviour of plate-rod constructs with vary- ing numbers of monocortical screws applied to an ex vivo canine femoral-gap ostectomy model. Sample population: Twenty Greyhound dog cadaveric femurs. Methods: Bone mineral density (BMD) was assessed with dual x-ray absorptiometry. Bones were assigned to four groups. Bones had a 12-hole 3.5 mm locking compression plate with one bicortical non-locking cortical screw in the most proximal and distal plate holes and an intramedullary Steinmann pin applied across a 20 mm mid-diaphyseal os- tectomy. Additionally, one to four monocorti- cal non-locking cortical screws were then placed (Groups 1–4 respectively) in the proximal and distal fragments. Stiffness and axial collapse were determined before and after cyclic axial loading (6000 cycles at 20%, 40%, and 60% of mean bodyweight [total: 18000 cycles]). Constructs subse- quently underwent an additional 45000 cycles at 60% of bodyweight (total: 63000 cycles). Loading to failure was then perform- ed and ultimate load and mode of failure rec- orded. Results: The BMD did not differ significantly between groups. Construct stiffness for group 1 was significantly less than group 4 (p = 0.008). Stiffness showed a linear in- crease with an increasing number of mono- cortical screws (p = 0.001). All constructs sur- vived fatigue loading. Load-to-failure was not significantly different between groups. Mean load- to-failure of all groups was >1350N. Clinical relevance: Ex vivo canine large- breed femurs showed adequate stability bio- mechanically and gradually increasing stiff- ness with increasing monocortical screw numbers. Correspondence to: Dr. Peter J. Delisser University of Bristol School of Veterinary Science Small Animal Hospital Langford House Langford Bristol, Avon BS405DU United Kingdom Phone: +44 117 928 9420 Fax: +44 117 981 1277 E-mail: peter.delisser@bristol.ac.uk Vet Comp Orthop Traumatol 2013; 26: 177–185 doi:10.3415/VCOT-12-05-0061 Received: May 5, 2012 Accepted: December 23, 2012 Pre-published online: March 5, 2013 Introduction The biological osteosynthesis approach to fracture fixation is currently advocated as the treatment approach of choice for com- minuted, non-reconstructible diaphyseal fractures (1, 2). Goals of fracture repair when utilising this technique include maintenance of spatial alignment of the two main fracture fragments and maxi- mum preservation of the soft tissue en- velope surrounding the fracture in order to preserve extraosseous blood supply for healing. The use of a plate-rod construct for the repair of comminuted canine long-bone fractures has been reported for the hu- merus, femur, and tibia (1, 3, 4). The com- plementary load sharing achieved by the two implants provides a stronger and stiffer repair compared with the use of a bridging plate alone. In a fatigue loading biomech- anical study, inclusion of an intramedullary rod increased the fatigue life of a bone plate by as much as 10-fold (4). In this study, one bicortical screw and three monocortical screws were employed proximal and distal to the osteotomy (4). This number and configuration of screws has been fre- quently cited as a minimum guideline in the application of this technique to clinical cases, although, to the authors’ knowledge, no studies have assessed the optimum number or configuration of screws either biomechanically, or clinically (4). In a clini- cal case series describing the use of this technique, a greater number of screws was frequently employed with an average of 72% of screw holes filled, and the mean number of bicortical screws placed being For personal or educational use only. No other uses without permission. All rights reserved. Downloaded from www.vcot-online.com on 2013-07-29 | ID: 1000472242 | IP: 137.222.181.185