Are Hook Plates Advantageous Compared to Antiglide Plates for Vertical Shear Malleolar Fractures?
This study was designed to evaluate the biomechanical properties of a hook plate (HP) vs an antiglide (AG) plate for supination-adduction (SAD)–ankle fractures. Identical polyurethane tibial models were obtained and vertical fractures were created. The fractures were stabilized with 1 of the following: one-third tubular plate in an AG fashion with 2 screws proximal to the fracture; an AG plate with an additional screw perpendicular to the vertical shear fragment (MAG), or an HP.
Ten models were randomly assigned to each of the 3 groups. The constructs were tested in offset-axial loading and were evaluated for construct stiffness and load-to-failure. The MAG construct yielded better stiffness compared with the AG plate (P < .05) and the HP (P < .05). The plate stiffness of the HP construct compared with the AG was not significant (P = .350). In regards to load-to-failure, the difference between MAG and AG was 638 N, and MAG and HP was 530 N (both P < .05). The HP had a load-to-failure that was, on average, 108 N more than the AG but was not significant (P = .063). A one-third tubular plate in the MAG fashion provided a stable, strong construct for fixation of vertical shear medial malleolus fractures.
One-way analysis of variance with post hoc Tukey HSD data analysis was performed to determine if there were statistical differences among the different fixation constructs during load-to-failure. To prevent skewing of results by different values of model elasticity, pretest stiffness was accounted for by calculating a ratio of construct stiffness as a function of pretest model stiffness. Total force-to-failure was the recorded maximum force (in N) to cause failure. A P value of < .05 was set for significance. All data were analyzed using SPSS software (SPSS Version 15.0; SPSS Inc.).
Results
Analysis of pretest stiffness showed no significant difference among models (P = .490). All models failed by a gap of 2 mm at the distal fracture site except for 3 models in the MAG group. These 3 models failed at a much higher load than the remainder of the models and failed by fracture of the models.
The MAG group demonstrated significantly superior stiffness to the 2 other models tested (Figure 4). On average, this group required 753.5 N of force before failure. This was 530 N higher than the HP (P < .05) and 638 N higher than the AG constructs, respectively (P < .05). The HP and AG groups required forces of 223.2 N and 115.5 N for failure, respectively. These numbers were not significant (P= .063).
The absolute construct stiffness and construct stiffness as a function of pretest stiffness of the MAG group was the highest of all groups, 271.7 N/mm and 57.2%, respectively (Figure 5). These numbers showed significance when compared with the values of the HP group (P < .05 for both) and the AG group (P < .05 for both). The average stiffness of the HP group was 159.7 N/mm, which was 36.8% of pretest stiffness.
The AG group had the lowest construct stiffness and percent of pretest stiffness (128.1 N/mm and 29.6%). The HP and AG groups were not statistically different in these comparisons, P = .350 for construct stiffness and P = .395 for percent of pretest stiffness.
Discussion
These results support the use of a one-third tubular plate and lag-screw construct for fixation of vertical shear medial malleolus fractures. This is clinically important because one-third tubular plates with 3.5-mm screws are readily available and cost significantly less than a precountoured anatomic-specific type of fixation. These results are based on the biomechanical properties of the constructs tested in this study.
The previous 2 studies8,9showed conflicting results about the most biomechanically sound fixation for SAD medial malleolar fractures. The study by Toolan and colleagues9 reported that 2 screws placed perpendicular to the fracture demonstrated the strongest overall construct. This study compared 3 separate types of 2-screw–only fixations and 2 plate-and-screw fixations. One construct was similar to the AG group in our study, and the other construct had a lag screw at the apex of the fracture. This previous study,9 however, did not investigate a similar construct to the MAG group that was tested in our study.
According to Dumigan and associates,8 a construct that consisted of a 4-hole plate with 2 screws proximal to the fracture and 2 lag screws showed the strongest fixation. This study, however, did not include a group like our study’s AG group, which is the traditional AG form of fixation.
In our study, we examined the biomechanic properties of a traditional fixation (AG construct), a commonly used fixation (MAG construct), and a newer construct (HP construct). The HP group is unique to this study and, to our knowledge, there is no literature on its use as fixation for this fracture. We did not include a 2-screw–only group, which is a limitation, because this fixation type is not common for the SAD fracture. This study also did not include an HP construct with an additional lag screw, which is an available option as well.
The current investigation used synthetic bone models constructed for biomechanical testing. The models were thought to provide a consistent model for fixation as opposed to using potentially osteopenic cadaveric bone. Each model was the same size and laterality. The stiffness as determined by pretest stiffness was not significantly different among models. Because all models were similar in composition and size, this allowed for more consistent osteotomies and similarly sized malleolar fragments. Theoretically, this allowed a more uniform comparison of all specimens and constructs.
Using models, however, is a limit of this study. While the models were of similar biomechanical quality, it is possible that a model may not reproduce the biology of a cavaderic specimen or the physiology of a construct in vivo. Of the 2 studies that investigated SAD fractures, the Dumigan study8 used cadaveric specimens. The fact that these models were all mildly osteoporotic and were embalmed specimens were study limits. The Toolan study9 used synthetic models. Although these models were consistent, they were models of bones and not intended for biomechanical studies, thereby increasing the potential for skewed results.
