Far cortical locking enables flexible fixation with periarticular locking plates

Abstract

The high stiffness of periarticular locked plating constructs can suppress callus formation and fracture healing. Replacing standard locking screws with far cortical locking (FCL) screws can decrease construct stiffness and can improve fracture healing in diaphyseal plating constructs. However, FCL function has not been tested in conjunction with periarticular plating constructs in which FCL screws are confined to the diaphyseal segment. This biomechanical study evaluated if diaphyseal fixation of a periarticular locking plate with FCL screws reduces construct stiffness and induces parallel interfragmentary motion without decreasing construct strength. Periarticular locking plates were applied to stabilize distal femur fractures in 22 paired femurs using either a standard locked plating approach (LP group) or FCL for diaphyseal fixation (FCL group) using MotionLoc screws (Zimmer, Warsaw, IN). Each specimen was evaluated under quasiphysiological loading to assess construct stiffness, construct durability under dynamic loading, and residual strength after dynamic loading. FCL constructs had an 81% lower initial stiffness than LP constructs. They induced nearly five times more interfragmentary motion than LP constructs under one body weight loading (P < 0.001). FCL constructs generated parallel interfragmentary motion, whereas LP constructs exhibited 48% less motion at the near cortex than at the far cortex (P = 0.002). Seven LP constructs and eight FCL constructs survived 100,000 loading cycles. The residual strength of surviving constructs was 4.9 ± 1.6 kN (LP group) and 5.3 ± 1.1 kN (FCL group, P = 0.73). In summary, FCL screws reduce stiffness, generate parallel interfragmentary motion, and retain the strength of a periarticular locked plating construct. Therefore, FCL fixation may be advisable for stiffness reduction of periarticular plating constructs to promote fracture healing by callus formation.

Figure 1

A) In the LP group, periarticular femur plates were applied with standard locking screws; B) In the FCL group, plates were applied to the diaphysis using four FCL screws that provide a controlled motion enveloped in the near cortex.

Figure 2

A) Fixation constructs were loaded along the mechanical axis of the femur. B) Construct stiffness was assessed by measuring interfragmentary motion at the near and far cortex with digital calipers in response to femur loading. C) Construct durability was assessed under dynamic loading, whereby motion sensors continuously captured the gradual collapse d_c at the osteotomy gap.

Figure 3

A) Average interfragmentary motion in response to femur loading. FCL constructs exhibited a bi-phasic stiffness profile, whereby elevated loading induced near cortex support of FCL screws and yielded progressive construct stiffening. B) The initial stiffness of FCL constructs was 81% lower than the stiffness of LP constructs (p < 0.001). C) Due to asymmetric gap closure in the LP group, interfragmentary motion (IFM) under one body weight (800 N) loading was 48% smaller at the near cortex than at the far cortex. FCL constructs exhibited substantially parallel interfragmentary motion (p = 0.15), which was on average four times larger than in the LP group (p = 0.003).

Figure 4

Radiographs of a representative LP (A) and FCL constructs (B), obtained before and after loading to failure. In each group, 10 of the 11 specimens sustained metaphyseal fixation failure. None of the constructs sustained diaphyseal fixation failure. No FCL screw sustained permanent bending or fixation failure.