Biomechanics of far cortical locking

Abstract

The development of far cortical locking (FCL) was motivated by a conundrum: locked plating constructs provide inherently rigid stabilization, yet they should facilitate biologic fixation and secondary bone healing that relies on flexible fixation to stimulate callus formation. Recent studies have confirmed that the high stiffness of standard locked plating constructs can suppress interfragmentary motion to a level that is insufficient to reliably promote secondary fracture healing by callus formation. Furthermore, rigid locking screws cause an uneven stress distribution that may lead to stress fracture at the end screw and stress shielding under the plate. This review summarizes four key features of FCL constructs that have been shown to enhance fixation and healing of fractures: flexible fixation, load distribution, progressive stiffening, and parallel interfragmentary motion. Specifically, flexible fixation provided by FCL reduces the stiffness of a locked plating construct by 80% to 88% to actively promote callus proliferation similar to an external fixator. Load is evenly distributed between FCL screws to mitigate stress risers at the end screw. Progressive stiffening occurs by near cortex support of FCL screws and provides additional support under elevated loading. Finally, parallel interfragmentary motion by the S-shaped flexion of FCL screws promotes symmetric callus formation. In combination, these features of FCL constructs have been shown to induce more callus and to yield significantly stronger and more consistent healing compared with standard locked plating constructs. As such, FCL constructs function as true internal fixators by replicating the biomechanical behavior and biologic healing response of external fixators.

Figure 1

The conundrum: Locked plating constructs are comparably stiff than non-locked constructs that were designed to induce primary bone healing by rigid fixation (10, 21, 25, 26). However, locked bridge plating constructs rely on secondary bone healing with callus formation, which has traditionally been achieved with external fixation constructs that are considerably more flexible than locked plating constructs (, , –29).

Figure 2

FCL fixation: A) FCL screws are locked in the plate and in the far cortex, while retaining a controlled motion envelope Δd in the near cortex. B) Similar to the pins of an external fixator, flexible shafts of FCL screws provide a sufficient working length for flexible, fixed-angle connection of a locking plate to a diaphysis.

Figure 3. Flexible fixation

a) Stiffness comparison of bridge osteosynthesis with standard locking plate (LP), FCL, and external fixation. B) FCL reduced the stiffness of the standard locking constructs by 84%. The FCL and external fixator stiffness was within a range that permits 30% interfragmentary strain (IFS) known to promote fracture healing by callus formation, assuming a fracture gap in the range of 1–3 mm and partial load bearing of 400N.

Figure 4

Computational Finite Element Model (FEM) of an FCL bridge plating constructs for calculation of stress and strain distributions. B) Model validation in comparison to bench-test results of FCL constructs demonstrated close correlation between predicated and actual interfragmentary motion results.

Figure 5. Load distribution

A) Each FCL screws exhibits equal amounts of flexion, whereby strain is distributed over the entire working length of the FCL screw shaft. In contrast, standard locked plating screws exhibited focused strain adjacent to the near cortex, whereby the screw segment between the near and far cortex remained functionally latent. B) At 400 N loading, FCL screws provide even load distribution in the far cortex. At 1000 N, FCL screws furthermore provide load sharing between the far and near cortices.

Figure 6. Progressive stiffening

Under elevated loading, elastic flexion of FCL screw shafts provides additional support at the near cortex, which increases construct stiffness and protects the far cortex from excessive stress. Conversely, over-drilling the near cortex by 1 mm to mimic FCL function with standard locking screws can overload the far cortex due to deficient flexibility of the screw shaft.

Figure 7. Parallel interfragmentary motion

A) Standard locked constructs (LP) exhibit asymmetric gap closure, whereby motion at the near cortex is minimal. B) FCL constructs induce symmetric interfragmentary motion by cantilever bending of FCL screws. C) FCL and external fixation constructs delivered substantially parallel gap motion. The LP construct induced over five time less motion at the near cortex than at the far cortex. Interfragmentary motion in FCL and external fixator constructs in response to partial weight-bearing was within the 0.2–1 mm range known to stimulate callus formation.

Figure 8. Construct Strength

A) FCL constructs were comparable in strength to standard locked plating constructs (LP) and did not observe the characteristic weakness of unicortical fixation in torsion. B) In contrast to uni-cortical locking screws, the controlled motion envelope of FCL screws in the near cortex prevents excessive toggle.

Figure 9

Comparison of fracture healing between standard locked plating and FCL constructs in an ovine tibia gap osteotomy model.

Figure 10

Fracture healing: A) Deficient interfragmentary motion at the near cortex of standard locked plating (LP) constructs caused partial non-unions at the near cortex. Flexible fixation and parallel motion provided by FCL constructs yielded consistent and symmetric healing and increased bone mineral content by 44%. B) Healed tibiae of the FCL group tolerated 156% more energy to failure.