Objective: To explore weight-bearing stability of Pilon fracture fixed by external fixator.
Methods: Six ankle bone models (right side) and 4 pairs (8 ankle cadaver specimens) were selected. Pilon fracture model was prepared by using the preset osteotomy line based on Ruedi Allgower Pilon fracture type. Pilon fracture model was built by using a minimally invasive osteotomy. After ankle bone model and cadaver specimen model were fixed with external fixator, axial load was carried out on mechanical loading machine. After ankle bone model and cadaver specimen model were fixed with external fixator, axial load was carried out on mechanical loading machine. Axial loads of 150, 300 and 450 N were applied to ankle bone model, and displacements of fibula fracture blocks, lateral tibia fracture blocks and medial tibia fracture blocks in three-dimensional space (X, Y and Z axes) were recorded by dynamic capture instrument. Axial loads of 300, 600 and 900 N were applied to ankle cadaver model fixed by external fixator. X-ray films of Pilon fracture cadaver model fixed by external fixator under different loading conditions were taken. The anterior tibial angle, tibial malleolar point angle, talus shift value, talus tilt angle, lateral malleolar shift value, lateral malleolar shift value, medial malleolar separation shift value and articular surface step displacement value were measured under different loads by digimizer software.
Results: After 150, 300 and 450 N axial loads were applied to Pilon fracture models fixed by external fixator, no loosening or fracture of external fixator was observed, and no loosening, fracture or irreversible plastic deformation of Kirschner needle were observed. The displacement values of fibular fracture pieces on X-axis(around) were 0.032 (-0.022, 0.269), 0.061 (-0.002, 0.427), 0.212(-0.016, 1.223) mm, and the displacement values on Y-axis(above and below) were 0.002(-0.031, 0.103), 0.051(-1.133, 0.376), 0.128 (-1.394, 0.516) mm, and displacement values on Z-axis (front and rear) were -0.003 (-0.130, 0.171), 0.137 (-0.076, 0.433), 0.030(-0.487, 0.478) mm;the displacement values of lateral tibial fractures on X-axis were 0.000(-0.108, 0.027), 0.083(-0.364, 0.050), -0.121(-0.289, 0.165) mm, and displacement values on Y-axis were -0.009(-0.200, 0.025), -0.179(-0.710, 0.084), -0.257(-0.799, 0.027) mm, and displacement values on Z-axis were 0.112(-0.024, 0.256), 0.157(-0.068, 0.293), -0.210(-0.035, 0.430) mm;the displacement values of medial tibial fracture block on X-axis were -0.010(-0.060, 0.013), -0.165(-0.289, 0.056), -0.181(-0.395, 0.013) mm, and the displacement values on Y-axis were -0.036(-0.156, 0.007), -0.104(-0.269, 0.178), -0.245(-0.380, -0.011) mm, and displacement values on Z-axis were -0.005(-0.372, 0.189), -0.012 (-1.774, 0.380), 0.200 (-1.963, -0.540) mm. After 300, 600 and 900 N axial loads were applied to Pilon fracture cadaverous models fixed with external fixators, there were no significant difference in anterior tibial angles, angles of malleolar points of tibia, oblique angles of talus, fracture steps, shift values of talus, lateral shift values of lateral malleolus, lateral shift values of medial malleolus, lateral shift values of medial malleolus between under different loading conditions and those without loading (P>0.05). No loosening or fracture of external fixator as a whole, loosening, fracture or irreversible deformation of Kirschner needle at the local fixed fracture end occurred.
Conclusion: The early weight-bearing external fixator could maintain stability of fracture end and ankle joint, and the maximum weight is not more than 300 N. In clinical practical application, material characteristics of the implant and type of fracture should be selected.
Keywords: External fixator; Biomechanics; Load-bearing stability; Pilon fracture.