Effect of plate working length on plate stiffness and cyclic fatigue life in a cadaveric femoral fracture gap model stabilized with a 12-hole 2.4 mm locking compression plate

Abstract

Background: There are several factors that can affect the fatigue life of a bone plate, including the mechanical properties of the plate and the complexity of the fracture. The position of the screws can influence construct stiffness, plate strain and cyclic fatigue of the implants. Studies have not investigated these variables in implants utilized for long bone fracture fixation in dogs and cats. The purpose of the present study was to evaluate the effect of plate working length on construct stiffness, gap motion and resistance to cyclic fatigue of dog femora with a simulated fracture gap stabilized using a 12-hole 2.4 mm locking compression plates (LCP). Femora were plated with 12-hole 2.4 mm LCP using 2 screws per fracture segment (long working length group) or with 12-hole 2.4 mm LCP using 5 screws per fracture segment (a short working length group).

Results: Construct stiffness did not differ significantly between stabilization techniques. Implant failure did not occur in any of the plated femora during cycling. Mean ± SD yield load at failure in the short plate working length group was significantly higher than in the long plate working length group.

Conclusion: In a femoral fracture gap model stabilized with a 2.4 mm LCP applied in contact with the bone, plate working length had no effect on stiffness, gap motion and resistance to fatigue. The short plate working length constructs failed at higher loads; however, yield loads for both the short and long plate working length constructs were within physiologic range.

Figure 1

Testing constructs of long and short plate working length stabilization techniques. Long plate working length stabilization technique (left) and short plate working length stabilization technique (right). Each plate had a 2.4 mm locking screw (○) placed in the second hole from each end of the plate. The other screws were 2.4 mm cortical screws (●).

Figure 2

Testing results between long and short plate working length stabilization techniques. Comparison of mean ± SD values of construct axial stiffness (N/mm) loaded to body weight between the long plate working length stabilization technique (□) and the short plate working length stabilization technique (■). Measurements recorded at 1000, 2000, 5000, 10000, 20000, 50000, 100000, 120000, and 180000 cycles. No significant difference was found between the two stabilization techniques at any of the evaluated cycles.

Figure 3

Photographs and free body diagrams of a long (A) and short (B) plate working length construct before and after load to failure test. Photographs of a long (A) and short (B) plate working length construct before (left) and after (right) load to failure testing. The adjacent free body diagrams represent how bending moments were created. Bending moments (Force (F) x Distance (d)) were calculated based on measured scale distances obtained from photographs and the yield load of each construct. The off axis shift of the femoral head observed in the figure may be due to elastic displacement after unloading the specimen, as the pictures are taken without load.