Design And Biomechanical Study Of Polyaxial Self-locking Anatomical Plate Of Distal Tibia

ObjectiveDistal tibial fractures account for certain proportion of overall fractures. Treatment for distal tibial fractures is difficult due to its irregular anatomical morphology, poor soft tissue envelope and complicated biomechanics. In resent years, application of lateral anatomical locking plate for the treatment of this kind of fractures is becoming more popular, which benefits minimally invasive plating osteosynthesis (MIPO). But the screw paths of first generation locking plate are predetermined by the manufacturers, and are not able to be regulated during assembly, which severely influents stability of fixation and even lead to failure of fixation. For this reason we have designed a polyaxial self-locking anatomical plate of distal tibia. The purpose of this study is to evaluate biomechanical properties of this polyaxial self-locking anatomical plate and offer scientific evidence for clinical application.MethodAccording to morphologic characteristics of distal tibiae of domestic people, a polyaxial self-locking anatomical plate for distal tibia was designed. The plate is spoon-liked with a flare distal part and a long- stem proximal part. The distal part of the plate twists anteriorly with the largest twisting angle of 80°and 12cm twisting segment. Distal part of the plate is 2.4x2.4cm2 in dimension. The proximal part of the plate is 1.7cm wide. Three polyaxial holes are located at distal part of the plate which are distributed triangularly. In proximal stem part of the plate polyaxial holes are distributed separately each other with common locking holes. Top view of the polyaxial hole is round shape. Cross section of the polyaxial hole is concave tympaniform with upper and lower diameter of 8mm and the largest middle diameter of 9.2mm. The polyaxial self-locking bushing is within the polyaxial hole, which is round shape in top view with a C-shaped defect. On the obverse surface of the bushing there are three triangularly distributed small concaves which are separated every 90°ach other together with the C-shaped defect. Cross section of the polyaxial self-locking bushing is bucket-shape with polish outer surface and threaded inner surface, which the inner axis is intersected with the outer axis at 10°. The polyaxial bushing is clasped by the upper and lower outlets of the polyaxial hole of the plate and is precisely fit with the inner concave surface of the polyaxial hole. The C-shaped defect of the polyaxial bushing and the three concaves on its obverse surface are precisely fit with the four dental processes on the tip of the polyaxial regulating sleeve which drives the polyaxial bushing rotating in the polyaxial hole to regulate the locking angle of the locking screw. Once the locking screw is tightened, the polyaxial self-locking bushing is expanded and snugly engaged with the concave surface of the polyaxial hole, so that strong friction is produced to fix locking angle of the screw. The polyaxial bushing can be maximally laterally rotated for 5°and the 10°eccentric angle of its inner thread axis can increase angular regulation amplitude for locking screw up to 30°. Paired cadaver tibiae were used to make fracture fixation models. The osteotomy levels were designed at the transition of segment 42 and 43 according to AO classification and 10mm above it, so that 10mm bone defect was made. All paired cadaver tibiae were divided into group A and B with 6 in each group. Left and right tibiae were randomly distributed into two groups. In group A common anatomical locking plates were assembled and in group B polyaxial self-locking anatomical plates were assembled. After assembly osteotomies were performed and a highly unstable type-A fracture was produced, with the implant alone transferring all loads. The biomechanical tests were performed using 858 Mini Bionix testing machine. In the first stage non-destructive tests were performed in both groups, including axial loading,4-point bending and torsional loading. In axial loading maximum 500N was loaded at a rate of 5N/S. In 4-point bending maximum 300N was loaded at a rate of 5N/S. In torsional loading maximum 5Nm was loaded at a rate of 0.1°/S. The constructs were preloaded to 10% of the maximum load before every test. In the second stage two pairs of constructs from two groups were selected for destructive test of axial loading,4-point bending, or torsional loading with the same loading rates. SPSS 13.0 software was used for statistical analysis, in which paired t test was performed to compare data from left and right tibial models of the same cadaver. All differences in comparisons were considered statistically significantly different at P<0.05.ResultsCompression stiffness of group A was 557.53±20.72 N/mm, and group B was 562.80±28.26 N/mm.4-point bending stiffness of group A was 268.02±36.77 N/mm, and group B was 265.76±27.21 N/mm. Torsional stiffness of group A was 0.28±0.01 Nm/deg, and group B was 0.29±0.02 Nm/deg. All differences of two groups in three tests had no statistical significance. ConclusionsSelf-designed polyaxial self-locking anatomical plate of distal tibia is better fit for the tibial morphology of domestic people. The design of polyaxial hole and polyaxial self-locking bushing can make it possible for locking screw to regulate its locking angle according to the fracture morphology, which can guarantee stability of internal fixation and make up for clinical limitations of common anatomical locking plate. Biomechanical tests have confirmed that biomechanical properties of polyaxial self-locking anatomical plate is equivalent to those of common anatomical locking plate.