Elsevier

Injury

Volume 47, Issue 8, August 2016, Pages 1624-1630
Injury

The biomechanical cost of variable angle locking screws

https://doi.org/10.1016/j.injury.2016.06.001Get rights and content

Abstract

Introduction

Variable angle (VA) locking plates in fracture fixation surgery allow screws to be fastened to the plate within a conical “locus of vectors” in order to avoid existing prostheses, joint surfaces, or poor quality bone. Clinical failures of VA constructs in which screws have rotated at the plate/screw interface have been reported raising the concern that there may be a biomechanical cost for the increased flexibility that VA provides. The objective of this study was to test the mechanical properties of one commonly used VA locking mechanism with screws placed in both nominal and off–axis trajectories and compare these against the standard locking mechanism.

Methods

VA locking screws were inserted into plates for distal femur fractures (VA Curved Condylar) at various angles (0° to 15° away from perpendicular). A control group of standard locking screws/plates was also tested. Maximum moment at the screw/plate interface and moment at two reference displacements were determined.

Results

VA screws locked perpendicular to the plate provided the greatest maximum moment and moment at the reference displacements when using the VA system, and demonstrated lower moments compared to standard locking screws/plates (p < 0.001). Based on linear regression, there was an average decrease of approximately 0.4 Nm screw-plate interface strength for every 1° increase in screw-plate angle (p < 0.001). Decreases (p<0.05) were discovered in both maximum moment and moment at the reference displacements for screws locked at 5° relative to those locked at 0°, 10° relative to 0°, and 15° relative to 10°.

Discussion

Standard locking systems provided greater resistance to rotational failure at the screw/plate interface than variable angle locking systems. Variable angle systems provided the greatest resistance to rotation when the screw was inserted perpendicular to the plate. As the off-axis angle increased, the resistance to rotation at the screw/plate interface decreased almost linearly. It is unknown if these differences are clinically significant in an actual fracture construct, but recent reported failures in the distal femur suggest that they might be.

Conclusion

Surgeons should weigh the risks and benefits of VA systems and attempt to minimize the off-axis angle magnitude when VA systems are selected.

Introduction

Locking plate/screw systems in which rotation of the screw head at the plate interface is mechanically prevented are used frequently in modern fracture fixation [1], [2]. The presence of osteoporotic bone, and/or the control of short metaphyseal segments are the most common indications for use [3], [4], [5]. The initial products of this type, called standard locking (SL), constrained the trajectory of the screw through the plate to one direction via plate design. A relatively newer locking technology, variable angle (VA) or polyaxial, has been introduced into fracture fixation surgery over the last decade allowing screws to be placed through the plate and bone in any direction within a conical “locus of vectors” up to 15° off-axis and still prevent rotation between plate and screw head [6]. This VA technology frees the surgeon from placing screws strictly dictated by plate design and allows more adaptability in creating fracture fixation constructs. This could improve purchase in higher bone density areas and help avoid missing or osteoporotic bony areas potentially increasing overall construct stability. Additionally, VA screw placement can be used to avoid implants previously placed such as in periprosthetic fractures. However, clinical failures of fixation constructs in which VA screws have rotated at the plate/screw interface have been observed [7] raising the concern that VA technology may not provide the same strength of rotational “lock” at the plate/screw interface compared with standard locking.

SL screws placed at increased insertion angles from their nominal positions creating cross-threading between the screw and plate threads, and have been shown to be significantly weaker in static mechanical testing than in the nominal position (0°) [8]. Previous studies on cadavers and sawbones [9], [10], [11], [12] comparing polyaxial system constructs with standard locking systems showed no significant differences in strength. Hebert-Davies et al. [13] showed in biomechanical tests of individual screw locking mechanisms that two manufacturers’ polyaxial systems displayed significant reductions in strength with greater than 10° off-axis placement relative to nominal (0°) within a polyaxial plate; however, comparisons were not made to the same manufacturers’ non-polyaxial screw-plate interface. Screws placed in non-polyaxial systems were detrimental to the locking mechanism’s strength.

The primary objective of this study was to test mechanical properties of a widely used VA locking mechanism with screws placed in various trajectories (from 0° to 15°) for both direct comparison against the standard locking mechanism and internal comparison within the VA system. A secondary objective included quantitatively describing the expected decrease in strength with each degree of off-axis placement. The final objective was to evaluate the strength effects associated with changing the circumferential angle β (β = 0° or 45°, Figs. 1 and 2) which changes the direction of applied force relative to the plate. We hypothesized that standard locking would be stronger than VA on-axis screws, that increases in screw angle off-axis would correlate to decreases in screw-plate locking strength, and that changing the circumferential angle to β = 45° would also decrease the strength. These data points are useful for guiding surgical decision-making in fracture treatment. The results should help guide surgeon utilization of variable angle locking systems to maximize their biomechanical effectiveness and minimize the risk of screw/plate rotation under load. Further, a novel method for clinically relevant and accurate insertion of VA screws into plates for mechanical testing is described.

Section snippets

Materials

A total of 56 variable angle (VA) locking screws (5.0 mm Variable Angle Screw, Depuy-Synthes, West Chester, PA) and 8 non-variable angle (standard) locking screws (5.0 mm Locking Screw, Depuy-Synthes, West Chester, PA) were mechanically evaluated with their corresponding distal femoral locking plates. Using the central 8 holes on each plate without retesting any holes or screws, a total of seven 14-hole VA-LCP Curved Condylar Plates (Depuy-Synthes, West Chester, PA) and one standard LCP Condylar

SL 0° vs VA 0°

Standard locking screws/plates demonstrated significantly greater maximum moment (15.3 ± 0.1 Nm vs. 10.5 ± 1.1 Nm, p < 0.001), moment at 1 mm displacement (6.6 ± 0.3 Nm vs. 5.2 ± 0.5 Nm, p < 0.001), and moment at 2 mm displacement (10.6 ± 2.0 Nm vs. 8.3 ± 0.6 Nm, p < 0.001) compared to VA screws locked perpendicular (α = 0°) to the plates.

VA α = 0° vs 5°, 10°, and 15°

Descriptive data are provided (Table 1). ANOVA analysis considered β = 0° and 45° together (Table 2). This analysis showed significant (p < 0.05, all ANOVA p-values Tukey-Kramer

Discussion

The present study is the first to quantify the relationships between screw-plate locking strength and fixation angle for individual screw/plate interfaces for this manufacturer’s commonly used VA plating system of the distal femur (Fig. 5, Tables 2 and 3). We have shown that the greatest strength in polyaxial systems is provided when the screw is inserted in the nominal ‘0°’ position (Table 2). We demonstrated a significant negative correlation between strength and increasing off-axis angles

Conclusion

Variable angle locking systems have given the fracture surgeon a new tool with which to address difficult clinical scenarios. The ability to independently target locking screws around other implants and a prosthetic stem to create a stable fracture fixation construct is an important advance in treatment. However, this study has shown that there is a biomechanical price to be paid for this flexibility. Standard locking systems provide the greatest resistance to rotational failure at the

Conflict of interest

The authors received implants for this study from J&J Depuy-Synthes. JSR is a consultant for Depuy-Synthes and is a speaker for product related instructional courses for Smith and Nephew.

Acknowledgements

Implants were granted by Depuy-Synthes. Two members from Depuy-Synthes provided helpful input regarding the study design. They were also given the opportunity to review and comment on the final manuscript.

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Work Performed at Penn State College of Medicine and Hershey Medical Center, Department of Orthopaedics & Rehabilitation.

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