The biomechanical cost of variable angle locking screws.
BACKGROUND
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°.


CONCLUSIONS
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.


CONCLUSIONS
Surgeons should weigh the risks and benefits of VA systems and attempt to minimize the off-axis angle magnitude when VA systems are selected.
UCSF Fresno, Department of Orthopaedics, 2823 Fresno Street, Fresno, CA 93721, USA.
John E
JE
Tidwell
Penn State College of Medicine and Hershey Medical Center, Department of Orthopaedics & Rehabilitation, 500 University Drive, Mail Code H089, Hershey, PA, 17033, USA.
Evan P
EP
Roush
Penn State College of Medicine and Hershey Medical Center, Department of Orthopaedics & Rehabilitation, 500 University Drive, Mail Code H089, Hershey, PA, 17033, USA.
Cassandra L
CL
Ondeck
Penn State College of Medicine and Hershey Medical Center, Department of Orthopaedics & Rehabilitation, 500 University Drive, Mail Code H089, Hershey, PA, 17033, USA.
Allen R
AR
Kunselman
Penn State College of Medicine and Hershey Medical Center, Department of Orthopaedics & Rehabilitation, 500 University Drive, Mail Code H089, Hershey, PA, 17033, USA.
J Spence
JS
Reid
Penn State College of Medicine and Hershey Medical Center, Department of Orthopaedics & Rehabilitation, 500 University Drive, Mail Code H089, Hershey, PA, 17033, USA. Electronic address: greglewis@psu.edu.
Gregory S
GS
Lewis
D001696:Biomechanical Phenomena; D001860:Bone Plates; D001863:Bone Screws; D004867:Equipment Design; D019544:Equipment Failure Analysis; D005593:Fracture Fixation, Internal / Q000295:instrumentation*; D006801:Humans; D008422:Materials Testing / Q000379:methods*; D066330:Printing, Three-Dimensional; D013314:Stress, Mechanical
Biomechanical testing; Distal femur fracture; Locking plates; Polyaxial; Variable angle