Performance of mixed screw-plate interfaces from four manufacturers

Abstract

Objective: To analyze the mechanical strength and stability of constructs created by intermixing locking compression plates (LCP) and screws (LS) from four manufacturers (A-D). Design: In-vitro study Methods: Each construct consisted of one LCP ("A-D") and six LS ("a-d"). All LS within a construct were from the same manufacturer, while the LCP was either from the same or a different manufacturer. Constructs in which both LCP and LS originated from the same manufacturer served as controls. Two copies of each were assembled (#1 and #2). Compressive cyclic axial loading was applied to all #1 constructs. Subsequently, all constructs (#1 and #2) underwent micro computed tomography (micro-CT) to evaluate average thread engagement (ATE), thread engagement height (TEH), angle of insertion, and distance from screw head to plate. Following micro-CT imaging, the #2 constructs were subjected to cyclic loading. Three screws from each #1 and #2 construct were then tested for peak reverse torque (PRT) (n=6) and the three remaining screws were tested for push-out strength (n=6). Results: ATE and TEH were strongly correlated with each other. "A.a" had the highest mean ATE, TEH and PRT within the "A" plate cohort (A.a-A.d); "B.b" had the highest TEH and PRT, while "B.d" had the highest ATE within the "B" plate cohort (B.a-B.d); "C.b" had the highest TEH and ATE while "C.d" had the highest PRT within the "C" plate cohort (C.a-C.d); and "D.b" had the highest ATE and TEH while "D.d" had the highest PRT within the "D" plate cohort (D.a-D.d). Data analysis for screw push-out was limited due to small sample size. All plates with "a" screws (A.a, B.a, C.a, and D.a) had significantly different distance from all plates with screws "b-d" (p<0.05), while the distance of screws "b-d" in all plates did not differ from each other (p>0.05). The screws in the "A" plates showed a significantly lower angle (further from 90°) than "C" plates (p = 0.012) and "D" plates (p = 0.029). Conclusion: Under the conditions of these mechanical tests, the "B" system consistently showed the most secure fixation in the parameters measured, and was more accommodating of mixing B-system screws or plates with plates or screws from other manufacturers, with the exception of those from manufacturer A. The A system proved to be the least adaptable to mixing with screws or plates from other manufacturers. Further studies are warranted to determine the clinical relevance of intermixing orthopedic implant components, particularly with respect to fracture healing and patient outcomes.