The biomechanical significance of washer use with screw fixation

Washers can be used with lag screws during fracture fixation to optimize compression and minimize the risk of unintentional intrusion of the screw head through cortical bone during screw insertion. The concept of using washers to optimize screw fixation is particularly applicable to iliosacral screw fixation and screw fixation of the femoral neck, distal femur, as well as the proximal and distal tibia. However, there is a paucity of literature on this topic. The purpose of this study was to detail the biomechanical consequences of washer use and screw intrusion.

Partially threaded 7.0-mm cannulated screws with and without washers were placed through synthetic bone blocks fabricated to simulate cortical and cancellous bone. A load cell was used to measure the compressive force before and after screw intrusion. Screws were tested with a washer (n = 8), without a washer (n = 8), and with a washer after initially being intruded (n = 8).

Screws inserted with washers generated significantly more compressive force than screws inserted without washers before screw intrusion. After intrusion, compressive force decreased significantly under all conditions, but screws inserted with washers maintained greater compressive force than screws inserted without washers. Screws with washers reinserted after intrusion without a washer, produced almost as much compressive force as screws inserted with washers primarily.

Screw intrusion during fracture fixation results in a loss of compressive force that may compromise fixation quality. Washers are advantageous in that they allow for more compression to be generated before intrusion occurs and can be used to salvage compressive force of intruded screws.