The present study described the efficacy of FCL in conjunction with periarticular locking plates, where FCL screws are confined to diaphyseal fixation. Results support the hypothesis that FCL screws reduce the stiffness of a periarticular locking construct and induce nominally parallel interfragmentary motion while retaining construct strength.
A stiffness reduction of 81% was achieved by FCL fixation in the proximal segment of a periarticular locked plating construct. It approached the 88% stiffness reduction previously reported for diaphyseal plating constructs in which three FCL screws were applied on each side of a diaphyseal plating construct (9
). The comparably high stiffness reduction achieved by periarticular FCL constructs despite limiting FCL fixation to only one fracture side can largely be attributed to differences in screw length between the studies. The previous study evaluated diaphyseal fixation constructs with a constant screw length of 32 mm in synthetic bone cylinders. The average screw length required for cadaveric femurs in the present study was 40 mm (range: 38 – 50mm), whereby a 40 mm FCL screw has an approximately 95% higher bending flexibility than a 32 mm long FCL screw. The 81% stiffness reduction provided by FCL with MotionLoc screws in the present study was furthermore higher than the 16% stiffness reduction reported for dynamic locking screws (DLS) (6
The present study measured a far greater stiffness for the locked plating construct (5,919 N/mm) than previously reported for the same implant (168 N/mm (16
) to 1,137 N/mm (17
)). While stiffness is simply calculated by dividing an applied load by the resulting displacement, comparing stiffness reports between studies deserves further attention. Reports of construct stiffness can vary by over one order of magnitude for the same implant since they are highly influenced by the test setup, especially when stiffness is calculated from the displacement of the loading actuator. Actuator displacement represents deformation along the entire test specimen and can grossly overestimate the actual motion at the fracture site. Based on actuator displacement, construct stiffness of femoral locking plates has been reported in the range of 63 N/mm to 1,137 N/mm (8
). This stiffness range overlaps with that of external fixators (50–400 N/mm) (18
), suggesting that locked plating constructs would provide sufficient interfragmentary motion to promote fracture healing by callus formation. In the present study, construct stiffness based on actuator displacement was 1,100 N/mm (LP group) and 490 N/mm (FCL group). However, studies that measure the actual displacement at the fracture site with sensors applied to the far cortex report one order of magnitude greater stiffness (2,100 N/mm to 5,000 N/mm) for femoral bridge plating constructs (9
). Furthermore, flexion of a bridging plate causes asymmetric gap closure, whereby the near cortex adjacent the plate experiences less than half the interfragmentary motion seen by the far cortex (9
). The present study therefore captured interfragmentary motion at both the near and far cortex to assess asymmetric gap closure, and to calculate construct stiffness that accurately represents the effective interfragmentary motion experienced at the fracture gap.
Interfragmentary motion in LP constructs was asymmetric and was attenuated towards the near cortex adjacent to the plate. Under one body-weight postoperative weight-bearing, interfragmentary motion at the near cortex of LP constructs was less than 0.1 mm and remained below the 0.2–1.0 mm stimulus range of motion known to promote secondary bone healing (24
). This deficient interfragmentary motion at the near cortex corresponds to findings of a recent ovine in vivo
study, in which 50% of tibial osteotomies stabilized with a locking plate failed to heal at the near cortex (2
Interfragmentary motion of FCL constructs was nearly parallel, whereby one bodyweight loading yielded 0.57 mm and 0.63 mm motion at the near and far cortex, respectively. This interfragmentary motion corresponds to an 0.6 mm motion envelope of FCL constructs used in an ovine fracture healing model to assess the effect of FCL on fracture healing (2
). This in vivo
study demonstrated that parallel motion provided by FCL constructs yielded symmetric callus formation and consistent bridging of a gap osteotomy, with FCL specimens healing stronger and tolerating 154% more energy to failure than a control group stabilized with standard locking constructs.
Construct strength was assessed in terms of durability during dynamic loading, and residual strength after dynamic loading. Dynamic load simulation with walking forces of 1,870 N represented exaggerated post-operative loading and did not account for load sharing by progressive callus formation. In addition, 12 of the 22 specimens were either osteopenic or osteoporotic (T-score < −1). Despite this “worst case” combination of exaggerated loading in presence of poor bone quality, only four LP constructs and three FCL constructs failed during durability testing to 100,000 loading cycles. All of these failures were isolated to loss of distal fixation while proximal fixation remained fully intact. Similarly, Stoffel et al. reported that locked plating constructs applied to bridge distal femur fracture in cadaveric specimens consistently failed by loss of distal screw fixation and cut-out (23
). Subsequent evaluation of residual strength of the surviving specimens required on average a load of 5 kN in LP constructs and 5.3 kN in FCL constructs to induce failure. Despite these large failure loads, construct failure remained isolated to distal fixation in all but the strongest specimen pair (T-score: 2.5), which failed by diaphyseal fracture. Diaphyseal screws sustained neither bending nor fixation failure in any specimen. The finding that FCL constructs were as strong and durable as LP constructs despite being 81% less stiff may be attributed to two principal characteristics of FCL constructs: First, elastic cantilever bending of FCL screws provides even load distribution to all four FCL screws and thereby reduces stress risers (27
). Second, elastic flexion of FCL screws provides added support at the near cortex under elevated loading, enabling load transfer at the near and far cortices (9
Results of this study are limited to a particular physiologic loading mode representing the stance phase of level walking in a simplified and controlled manner (12
). Results are specific to a particular titanium locking plate system and a representative diaphyseal screw configuration. It is important to understand that the ability of FCL screws to reduce construct stiffness and to induce controlled interfragmentary motion relies on establishing a specific motion envelope in the near cortex. This is assured by placing the screw shaft concentrically in the near cortex drill hole. In absence of a concentric placement of the screw shaft, FCL functionality may be diminished or lost. FCL screws in the present study were inserted manually in a standard procedure, demonstrating that FCL functionality can be reliably achieved without the need for additional procedures to ensure concentric screw alignment. Most importantly, while results of the present study document the biomechanical benefits of diaphyseal FCL fixation, clinical studies will be required to document if FCL fixation using periarticular locking plates can improve fracture healing compared to standard locked plating.
In conclusion, the use of FCL screws in place of standard locking screws for diaphyseal fixation of periarticular locking plates significantly reduced construct stiffness and provided controlled interfragmentary motion for promotion of fracture healing by callus formation. FCL constructs delivered nominally parallel interfragmentary motion, whereby the magnitude of interfragmentary motion in response to one body-weight loading corresponded to the range of interfragmentary motion known to promote secondary bone healing (24
). Finally, FCL constructs were as durable and strong as standard locked constructs. Therefore, FCL screws provide a simple and effective approach to reduce the stiffness of periarticular locking constructs without requiring additional procedures. Future studies are required to determine the effect of periarticular FCL plating constructs on fracture healing.