Compression generated by cortical screws in an artificial bone model of an equine medial femoral condylar cyst

Objective: Determine compression generated by lag and neutral screws over 12 h using two bone analogs. Study design: Experimental study. Sample population: Bone analogs were made of composite synthetic bone (CSB) or three-dimensional printed polylactic acid (PLA). Analogs had a 2 mm exterior shell with a 10 mm thick internal layer of open-cell material. Methods: Bone analogs were opposed, making a 4-sided box with open ends. A central channel contained the sensor and the screws passed through it to engage both paired analogs. Four screw/analog conditions were tested: neutral and lag screw with bicortical engagement, neutral and lag screw with unicortical engagement. All screws were tightened to 2 Nm torque and compression values recorded at 0, 0.5, 1, 2, 6, and 12 h (six trials per condition). Medians were compared across groups for statistical significance. Results: There was no difference in median compression between lag and neutral bicortical screws. For PLA, greater median compression was generated by neutral (median 437 N) and lag (median 379 N) bicortical screws compared to neutral unicortical screws (median 208 N, p <.001); lag bicortical screws generated greater median compression than lag unicortical screws (median 265 N, p =.012). For CSB, lag bicortical screws (median 293 N) generated greater median compression than neutral unicortical screws (median 228 N, p =.008). Conclusion: Lag and neutral screws generated similar compression. Bicortical screws had higher median compression than unicortical screws in bone analogs. Clinical significance: Neutral screws generate compression in cancellous bone analogs that can be increased with bicortical bone engagement.