Biomechanical comparison of bioresorbable F‑u‑HA/PLLA screw and tension band wire fixation in medial malleolar fractures

The authors have recently reported that forged composites of raw particulate unsintered hydroxyapatite/poly‑L‑lactide (F‑u‑HA/PLLA) screws are useful for the treatment of medial malleolar fracture. However, the biomechanical properties of these screws have not yet been fully elucidated. The aim of the present study was to compare the biomechanical properties of F‑u‑HA/PLLA screws with those of tension band wiring (TBW) using medial malleolar fracture models obliquely oriented at 45˚ angles in synthetic tibial bones. Fractures were fixed using two 4.5‑mm‑diameter partial thread F‑u‑HA/PLLA screws, two 4.5‑mm‑diameter full thread F‑u‑HA/PLLA screws, or TBW. The fracture models were tested parallel to the distal joint surface and parallel to the fracture line. The compression load at 1, 2, 3, 4 and 5 mm of fracture displacement was assessed. The compression load parallel to the distal joint surface at 3, 4 and 5 mm of fracture displacement was significantly higher in the partial thread screw group than in the TBW group (P<0.01). Additionally, the compression load parallel to the distal joint surface at 4 and 5 mm was significantly higher in the full thread screw group than in the TBW group (P<0.001). However, the TBW group exhibited a higher compression load at 1 mm compared to the full thread group (P<0.05). As regards the compression load parallel to the fracture line, the compression load at 3, 4 and 5 mm of fracture displacement was markedly higher in the partial thread screw group than in the TBW group (P<0.01); likewise, the compression load at 4 and 5 mm of displacement was significantly higher in the full thread screw group than in the TBW group (P<0.001). On the whole, these results strongly suggest that F‑u‑HA/PLLA screw fixation, particularly partial thread screw fixation, has good biomechanical strength and provides adequate stability for the treatment of medial malleolar fractures.