Summary
Objective: To compare the biomechanical behaviour of plate-rod constructs with varying 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 ostectomy.
Additionally, one to four monocortical 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 subsequently underwent
an additional 45000 cycles at 60% of bodyweight (total: 63000 cycles). Loading to
failure was then performed and ultimate load and mode of failure recorded.
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 increase
with an increasing number of monocortical screws (p = 0.001). All constructs survived
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 biomechanically and gradually
increasing stiffness with increasing monocortical screw numbers.
Keywords
fracture - biomechanics - stiffness - comminuted - screw number
