Biomechanical study of screw and plate fixation of a posterior malleolar fracture for osteoporosis patients in a simulation of the normal gait cycle

Abstract

Objective Purpose of this research is to biomechanically compare the screw-fixed and plate-fixed posterior malleolus fracture in osteoporosis patients with fatigue loading system and space motion system in simulation gait cycle, and to give theory evidence on how to choose an internal fixation in osteoporosis patients with posterior malleolus fracture. Methods 72 osteoporotic cadaveric lower limbs were prepared to simulate Haraguchi I posterior malleolus fracture and randomized into 2 groups with 36 each. Screw group were fixed with 2 paralleled 4.0 mm titanium partial thread cancellous screws from posterior to anterior. Plate group were fixed with plate. Based on the ratio of fracture area on sagittal plane to distal tibial articular surface (S), the two groups were subdivided into three subgroups, named as screw group A, plate group A, screw group B, plate group B, screw group C, plate group C. S (screw group A, plate group A)=1/4; S (screw group B, plate group B)=1/3; S (screw group C, plate group C)=1/2. According to the height of fracture fragments, these 6 subgroups were further divided into subgroup a with height 19.3 mm and subgroup b with height 39 mm. Furthermore, 4 different conditions of gait cycle were simulated as follows: 1) specimen was loaded with 3.2 body weight (BW) at dorsiflexion angle of 12°; 2) Start-up phase: dorsiflexion angle of 5°, 3 BW; 3) neutral position, 3.6 BW; 4) maximum plantar flexion, angle of 12°, 4.5 BW. At the end of repeated loading, the displacement of fracture fragment was measured with space motion system. Results At dorsiflexion 12° and plantar flexion 15°, there was significant difference of displacement between screw and plate group with larger displacement in screw group, whatever S or H was. When ankle was at dorsiflexion of 5°, the displacement showed significant difference between screw group Aa and plate group Aa, screw group Ba and plate group Ba, screw group Bb and plate group Bb, screw group Cb and plate group Cb, with larger displacement in screw group. In neutral position, the differences were significant with larger displacement in screw group, except between screw group Bb and plate group Bb. Conclusion Plate can provide stronger fixation for osteoporosis patients with posterior malleolus fracture in most situations, but both of the two methods cannot provide stable fixation when posterior malleolus fragment was in too big area or height. Key words: Ankle injuries; Osteoporosis; Fracture fixation, internal; Biomechanics