Biomechanical Evaluation of Interfragmentary Compression of Lag Screw Versus Positional Screw at Different Angles of Fixation

Objectives To compare the compressive force achieved and retained with the lag versus positional screw technique at various angles of screw application. Methods Sixty humeral sawbones were stratified into 6 groups based on the technique (lag or positional) and fixation angle (30, 60, or 90 degrees relative to the fracture plane). A sensor was placed between fragments to record compressive force. Absolute screw force is the final screw force. Normalized force is the final screw force minus force generated by reduction forceps. Retained force is the quotient of absolute force relative to reduction forceps force. Results Lag screws attained higher force than positional at 60 degrees (absolute force 41% higher, P = 0.041; normalized force 1300% higher, P = 0.008; retained force 60% higher, P = 0.008) and 90 degrees (absolute force 86% higher, P = 0.006; normalized force 730% higher, P = 0.005; retained force 70% higher, P = 0.011), but not at 30 degrees. For lag screws, compressive force was similar at 60 and 90 degrees (absolute force P = 0.174, normalized force P = 0.364, and retained force P = 0.496), but not 30 degrees. For positional screws, no difference was found between the 3 angles of fixation for absolute force (P = 0.059). Normalized force and retained force were similar at 60 and 90 degrees (P = 0.944 and P = 0.725, respectively), but not 30 degrees. Conclusions Lag screw technique compressive force was superior to positional screw technique at 60 and 90 degrees. Comparison of force at angles of 60 and 90 degrees showed no significant difference for both techniques. Indicating 30 degrees deviation from perfect technique is tolerated without significant decrease in compressive force.