SLR - May 2014 - Rebecca Sundling
Reference: Bishop J, Behn A, Castillo T. The Biomechanical Significance of Washer Use with Screw Fixation. Journal of Orthopaedic Trauma. 28(2); 2014: 114-7.
Scientific Literature Review
Reviewed By: Rebecca Sundling, DPM
Residency Program: Grant Medical Center, Columbus, Ohio
Podiatric Relevance: Internal fixation of fractures is a common part of podiatric surgical practice. With this comes the importance of compression and maintaining correction. Washers can be used with lag screws to increase compressive force allowed by the construct by spreading the force over a larger surface area and preventing intrusion of the screw head. Previous research suggests washer use may optimize fracture fixation but there is no data quantifying this idea.
Methods: Synthetic bone blocks were fabricated to imitate bone with thin cortex and cancellous bone of normal density, made of polyurethane foam laminated with short fiber filled epoxy. A load cell was then used to measure compressive forces. Screw tightening was accomplished at 5 degrees per second by an actuator and was continued until intrusion of the screw head or washer was noted. Intrusion was defined as “a failure of the synthetic cortical bone at the screw head interface.” The synthetic block was drilled with a 4.5 mm drill and then 7.0 mm partially threaded cannulated screws were inserted. Three constructs were tested with eight trials in each group: screw alone, screw with a washer and screw with a washer after initially being intruded without a washer. This final construct was considered a salvage construct. Compressive forces were measured before and after intrusion.
Results: Partially cannulated screws inserted with a washer initially generated the highest compressive force at 2916 N prior to intrusion and 2669 N after. This was followed by the salvage construct, both before and after intrusion, measuring 2616 N and 2497 N respectively. The screw inserted without washer had a statistically significantly lower compressive force generated (1167 N prior to intrusion, 1032 N after). The difference between pre- and post-intrusion values was statistically significant for all groups, as well.
Conclusions: While the exact values of force demonstrated in this study is not directly applicable, as the study was completed on synthetic bone blocks, the trends verified by this study are. This study concludes that the use of a washer with screw insertion can increase compressive force generated by up to 2.5 times. It is also noted that, regardless of construct, compressive force and fixation quality is lost with screw intrusion. Washers can be beneficial in fracture fixation as they allow for significantly more compression prior to intrusion and they can be used to salvage intruded screws. Additional studies are warranted testing load-to-failure rates on these constructs with fracture fixation.