Effect of different orientations of screw fixation for radial head fractures: a biomechanical comparison
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Screw fixation is a common method used for the treatment of Mason type II radial head fractures. The purpose of our study was to evaluate the mechanical properties of three different screw orientations used for fixation of Mason type II radial head fractures.
We sawed 24 medium-frequency fourth-generation Synbone radial bones to simulate unstable radial head fractures, which we then fixed with three different screw orientations. Implants were tested under axial load by the tension-torsion composite test system. If the implant-radial constructs did not fail after the axial load test, an axial failure load was added to the remaining constructs.
The stiffness of the divergent group was the highest of the three orientations, and this group had statistically significant difference from the other two groups (p < 0.05). However, there was no statistically significant difference between the convergence group and the parallel group (p > 0.05). When the displacement reached 2 mm, the load of the divergent screw was still larger than the other two groups (p < 0.05).
The divergent screw orientation was the most stable and had the greatest control of Mason type II fractures of these three groups. Therefore, it can be better applied in clinical settings.
KeywordsRadial head fractures Screw Biomechanical comparison Different orientation
Mason type was proposed for the first time in 1954 as a definition of radial head fractures . Mason type II fractures were 2-part fractures of the radial head with displacement and then were modified by Broberg and Morrey  as having more than 2 mm of displacement and involving at least 30% of the radial head. The optimal treatment for Mason type II fractures of the radial head is still controversial [3, 4]. Radial head fractures are infrequently seen in adults, with a reported incidence of approximately 55.4 per 100,000 people . The mechanism of injury in radial head fractures is usually caused by the arm reaching in a fall, and in a few cases it is caused by direct violence [1, 6]. Fortunately, type II radial head fractures can be managed with conservative treatment when the displacement is less than 2 mm . Although the management of displaced radial head fractures in adults remains unsatisfactory due to problems with loss of reduction, and non-union [6, 7, 8, 9, 10, 11, 12], there has recently been a great interest in preserving the radial head using open reduction with internal fixation. Since the important role of the radial head in elbow stability has been recognized , special implants for the maintenance of radial head fractures have been developed to improve fixation stability.
In clinical practice, two headless compression screws placed parallel to each other are used . However, there is no biomechanical data regarding this common fixation method if the screws occur to be placed in a non-parallel orientation. The purpose of this study was to quantify and compare the stiffness of three different screw configurations used to stabilize a simulated Mason type II fracture.
One fellowship-trained orthopedic surgeon performed the fixation on all 24 specimens to minimize the variability of fixation parameters. The Synbone surfaces were smooth, and the initial reduction was only accepted if it was anatomical. The clamps and drills used were the same in all three experimental groups. The transversely cut end of the radial shaft was potted in a metal tube using polymethylmethacrylate. The average length of the proximal radius exposed outside of the potting material was 6.79 cm, and the average diameter of the radial head was 2.28 cm. As in clinical use, the length of the screws did not exceed the contralateral cortex.
After the axial load test, if the fracture models did not fail, they were performed to the failure load at the rate of 2 mm/min. In our text, all 24 fracture models did not fail. Failure was defined as a new fracture seen on the radial bone, an acute change in the load-displacement curve indicating a rapid change in displacement and loss of construct stability, or radial head displacement greater than 2 mm (the fracture model had an intra-articular fracture, and the failure of intra-articular fracture fixation is defined as a displacement of more than 2 mm). Axial stiffness and axial failure loads were recorded.
The stiffness was determined from the slope of the regression line fitted to the loading segment of the cyclic load-displacement curves. Data from each group are presented as the mean ± standard deviation. SPSS 21.0 (IBM Corporation, Armonk, NY, USA) was used for statistical analysis. Mechanical parameters were compared using an LDS-test. p < 0.05 was considered statistically significant.
Average stiffness on axial load test of parallel group, convergent group, and divergent group
123.5 ± 25.94
149.5 ± 23.32
213.9 ± 28.00
The results obtained via conservative treatment may be satisfactory if the fracture is not displaced or is minimally displaced but movement is not impeded . In recent years, it has been generally acknowledged that these fractures are best treated by open reduction with internal fixation if the fragment is displaced more than 2 mm or involves more than 30% of the radial head. In a study by Demiroglu et al. , 23 patients were treated operatively with screw fixations with a follow-up period of more than 11 months. Their study showed that anatomical reduction of type II radial head fractures through open surgery and fixed with screws can have favorable results. Similarly, Pearce et al.  used Herbert screws to treat radial head fractures with satisfactory results. Burkhart et al.  recommended surgery to avoid the development of post-traumatic arthritis. Van Glabbeek et al.  also reported satisfactory results with open reduction of Mason type II radial head fractures. Consistent with these findings, the common interest in preservation of the radial head is steadily increasing. New techniques and implants have been developed for the maintenance of radial head fractures. However, only a few biomechanical studies have examined Mason type II radial head fractures to date [17, 18, 19, 20, 21, 22]. Klaus et al.  evaluated the 3.0-mm headless compression screw and the standard 2.0-mm cortical screw used for fixation of radial head fractures. No significant differences concerning the stability achieved by the 3.0-mm headless compression screw, and the 2.0-mm cortical screw could be detected in the experimental setup presented. In a recent article, Christina et al.  compared the mechanical properties of crossed screw and plate fixation in the model of a radial neck fracture; the two strategies provided similar strength and stiffness for the fixation of transverse, non-comminuted radial neck fractures.
In a recent article, Jeffrey et al.  evaluated high-fidelity composite bone models used in the biomechanics study; Synbone brand models was well represented in the hand and upper extremity biomechanics research. In our biomechanical experiments, the divergent screw group showed the highest axial compression stiffness of the three groups; in contrast, the axial compression stiffness of the convergent group was the lowest of the three groups, which was consistent with our expected results. Amanatullah et al.  evaluated the mechanical properties of three different screw orientations used for the fixation of vertical shear fractures of the medial malleolus. The use of a divergent screw pattern resulted in a stiffer fixation construct when used to stabilize an osteotomy model of vertical shear medial malleolus fractures. In their study, it was noted that when the screws were not placed in parallel only the first screw caused compression, and any non-parallel screws did not add additional compression, but instead acted as a rotation, translation, and tensile force neutralizer. However, in a non-rigid system, each screw provides additional compression and acts as a rotation, translation, and tensile force neutralizer. Similar results can be drawn from our study and their study: divergent screw placement allows the screws to be farther apart in the fracture plane. This wider screw orientation encompasses a larger surface of bone in the fracture plane that is held in compression and resists translation in the axial plane and rotation in the sagittal plane as a result of increased interfragmentary friction.
According to our biomechanical results, the divergent screws showed the greatest axial stiffness in our standard Mason type II radial head fracture model. It is envisaged that in clinical applications, no matter what the direction of the two screws, the soft tissue detachment is similar and only a small range of stripping is needed. Therefore, if divergent screws are applied in the clinical setting, their effect may be better than the clinical use of parallel screws. Early postoperative exercise after radial head fractures is the basic factor of postoperative rehabilitation . Stronger stiffness can be better for fixing the fracture block and preventing displacement of the fracture block after surgery, to achieve earlier and better postoperative rehabilitation exercise and prevent postoperative complications. However, this is only our vision; the feasibility requires a large amount of clinical validation. We can only form a purely biomechanical point of view, and pathogenesis and other internal fixation mechanisms need to be taken into consideration for the treatment of radial head fractures.
Our study also had some limitations. First, the standard bone without muscle and other corresponding soft tissue attachment cannot simulate the force transmission and role of the real human elbow joint. Second, our sample size in the study was not big enough; axial load direction cannot completely simulate the real daily activities of the human body or the mechanical mechanism of the injury. What is more, the load application is not representative of how load is transferred through the elbow. Finally, the biomechanics of this study only included axial and failure loads; the observed index was only axial stiffness due to the lack of more biomechanical performance indicators.
Our findings demonstrated that the divergent screws had more biomechanical advantages over the other two screw orientations. However, our conclusion needs to be supported by additional studies with large sample sizes looking at biomechanical and clinical applications.
The authors are grateful for the assistance from Yiwu high-level personnel for scientific research projects and for technical support and equipment from the Tianjin Institute of Orthopedics.
This research was supported by the Yiwu high-level personnel for scientific research projects (No.201603). The funders had no role in the study design, data collection or analysis, decision to publish, or preparation of the manuscript.
Availability of data and materials
The data that support the findings of this study are available from the Wenzhou Medical University (Wenzhou, Zhejiang, China). Data are however available from the authors upon reasonable request and with permission of Wenzhou Medical University (Wenzhou, Zhejiang, China). Please contact author for data requests.
XCS, JP, and RC designed the study. JP, DYW, and XCS obtained the funding. NYC, BL, and RZ collected the data. BL, NYC, and CWZ analyzed the data. JP, XCS, and RC interpreted the data. TLP, DYW, and RZ composed the article. All authors read and approved the final manuscript.
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