Biomechanical study of a unilocking T-plate system for prophylactic internal fixation of the radial osteocutaneous donor site using the sheep tibia model

Abstract

Prophylactic internal fixation (PIF), with a bone plate in either the anterior (over the section defect) or posterior (on intact cortex) position, has substantially reduced the incidence of fracture at the donor site of the radial osteocutaneous free flap. This study uses the sheep tibia model to compare the effectiveness of new T-shaped titanium plates utilising a unilocking screw system with a 3.5 mm steel plate and bicortical screw fixation system commonly applied for PIF. Forty matched pairs of adult sheep tibias were tested in torsion and 4-point bending. An osteotomised bone was significantly weaker (p<0.001) than an intact bone in both bending and torsion with a mean loss of 77% and 64% of strength respectively. The tibia withstood much greater bending loads. All of the constructs significantly strengthened an osteotomised bone by a factor of 1.73-2.43 times in bending and 1.54-2.63 in torsion. The 2.4 mm T-plate in an anterior position (section) was the baseline against which other plates in differing positions were compared. The 3.5 mm T-plate section, DCP section and DCP cortex constructs had 41%, 30% and 2% greater mean bending strengths respectively but only the 3.5 mm T-plate section result approached statistical significance (p=0.06). In torsion the DCP section, 3.5 mm T-plate section and DCP cortex constructs had 56% (p=0.01), 27% (p=0.06) and 25% greater mean strengths respectively. When compared to an intact bone the mean bending strength restored by the DCP section (84%) and 3.5 mm T-plate section (87%) constructs was greatest and effectively restored the strength to that of an intact bone (100%). In torsion the mean strength restored by the DCP section (62%), DCP cortex (44%), 3.5 mm T-plate section (40%) and 2.4 mm T-plate (36%) remained significantly less than an intact bone. All of the plate constructs significantly strengthened an osteotomised bone but overall the 3.5 mm T-plate section and DCP section were the strongest constructs and most suitable for PIF. The lighter 2.4 mm T-shaped titanium plate was least effective. The strongest reinforcement in bending and torsion was the 3.5 mm T-plate section and DCP section respectively. The 3.5 mm DCP section plate was significantly stronger (p=0.01) than the 3.5 mm T-plate in torsion and remains the most effective construct for resisting torsional stresses, which are probably the commonest cause of fracture in clinical practice.