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

Abstract

OBJECTIVE: Determine compression generated by lag and neutral screws over 12&#x2009;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&#x2009;mm exterior shell with a 10&#x2009;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&#x2009;Nm torque and compression values recorded at 0, 0.5, 1, 2, 6, and 12&#x2009;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&#x2009;N) and lag (median 379&#x2009;N) bicortical screws compared to neutral unicortical screws (median 208&#x2009;N, p&#x2009;<&#x2009;.001); lag bicortical screws generated greater median compression than lag unicortical screws (median 265&#x2009;N, p&#xa0;=&#xa0;.012). For CSB, lag bicortical screws (median 293&#x2009;N) generated greater median compression than neutral unicortical screws (median 228&#x2009;N, p&#xa0;=&#xa0;.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.