Comparison of two plates and screw osteosynthesis configurations in a rat model of critical sized femoral defects to reduce implant related failures

This study aimed to compare the failure rates of two different sizes of plates and screws to stabilize critical-sized (7 mm) femoral defects in male Sprague‒Dawley rats (aged 10 weeks). Femoral defects were stabilized with either a 4-hole plate (length 29 mm, thickness 1 mm, 10 rats, Group 1) and 4 cortical screws (diameter 2 mm) or with a 6-hole plate (length 30 mm, thickness 0.6 mm, 9 rats, Group 2) and 4 cortical screws (diameter 1.5 mm). A polymethylmethacrylate spacer was inserted into the defects to reproduce the first stage of the induced membrane technique. Radiographic evaluations, macroscopic and histologic assessments of the induced membranes were conducted at 1 week and 4 weeks. No implant failure occurred in Group 1 whereas in Group 2, 4/9 (44.4%) implant failures occurred during the follow-up (p = 0.03). On histomorphometry, cell density was higher in Group 1 (4996 ± 716 cells / mm²) than in Group 2 (3500 ± 728 cells/mm²) (p = 0.0195) but the membrane thickness in Group 1 (735 ± 44 μm) was non-significantly lower than in Group 2 (979 ± 165 μm) (p = 0.4). This study suggests that, in rat models of critical-sized femoral defects (7 mm) to study the induced membrane technique, fixation plates with a thickness of 1 mm and four screws (2 mm in diameter) provide stable fixation without implant failure. In contrast, thinner plates (< 1 mm) combined with screws of smaller diameter (1.5 mm) result in a high rate of implant failure.