Effect of the length of the superficial plate on bending stiffness, bending strength and strain distribution in stacked 2.0-2.7 veterinary cuttable plate constructs. An in vitro study.

Objectives: Use of stacked veterinary cuttable plates (VCP) increases the construct stiffness, but it also increases the stress protection and concentrates the stress at the extremities of the implants. We hypothesized that by shortening the superficial plate, it would not reduce the stiffness of the construct, but that it would reduce the stress concentration at the plate ends.

Methods: A 3 mm fracture gap model was created with copolymer acetal rods, stacked 2.0-2.7 VCP and 2.7 screws. The constructs consisted of an 11-hole VCP bottom plate and a 5-, 7-, 9- or 11-hole VCP superficial plate. Five of each construct were randomly tested for failure in four-point bending and axial loading. Stiffness, load at yield, and area under the curve until contact (AUC) were measured. Strains were recorded during elastic deformation for each configuration.

Results: During both testing methods, stiffness, load at yield and AUC progressively decreased when decreasing the length of the superficial plate. No statistically significant differences were obtained for load at yield in four-point bending and AUC in axial loading. The strain within the implant over the gap increased as the length of the superficial plate decreased.

Clinical Significance: Shortening the superficial plate reduces the stiffness and strength of the construct, and decreases stress concentration at the implants ends. As the cross section of the implant covering the gap remained constant, friction between the plates may play a role in the mechanical properties of stacked VCP.