Stiffness modulation of locking plate constructs using near cortical slotted holes: a preliminary study

Axial stiffness is a critical mechanical parameter in fracture plating. Standard locked plates allow minimal opportunities for stiffness alteration, and current methods are arbitrary and may lead to stiffness mismatch between the implant and bone. Milling the near cortex into a slot allows for an increase in translation of the screw shaft at the near cortex. The purpose of this proof of concept study was to determine the effects of slots on stiffness and their ability to maintain fixation of locking plates under cyclic loading.

Using segments of fourth-generation synthetic diaphyseal bone, a simulated fracture with a gap was created and locked plates were applied with 4 bicortical locked screws in each fragment. On one fragment, the 4 near cortex holes were sequentially milled to 5 x 6-mm slots. Axial and torsional stiffnesses were determined for constructs with 0 through 4 slots. Specimens with 4 slots then underwent axial cyclic loading to determine the change in stiffness and loss of fixation. Extraction torque was measured for all screws to assess for screw loosening with cycling.

In constructs with 4 slots, axial stiffness decreased by 73% (P < 0.05) relative to the 0-slot constructs. Torsional stiffness of the 3- and 4-slot specimens decreased by 20% (SD, 13%; P < 0.05) and 17% (SD, 13%; P < 0.05), respectively, compared with the 0-slot specimens. With cyclic loading, no failures occurred in any specimen. No change in stiffness had occurred by the end of cycling (106% of initial stiffness; SD, 4%; P = 0.96). No screw loosening occurred during cyclic loading.

Purposeful stiffness modulation in fracture fixation is critical to facilitate uneventful fracture healing. Converting near cortical holes to slots allowed selective axial stiffness adjustment without sacrificing fixation stability under cyclic loading. With further refinement, this simple modification of standard implant application may allow the surgeon to decrease the modulus mismatch between plating constructs and bone to decrease the risk of fixation failure.