Biomechanical comparison of distal femoral fracture fixation: Analysis of non‐locked, locked, and far‐cortical locked constructs

Abstract

To assess whether far‐cortical locking (FCL) screws alter the fracture site strain environment and allow shorter bridge plate constructs for supracondylar femoral fractures, we tested the fracture site displacement under force of synthetic left femora with a 5‐cm metaphyseal fracture gap, modeling comminution. Five models of nine constructs were tested (three types of diaphyseal screws [nonlocking, locking, and FCL] and two plate lengths [13 holes and 5 holes]). Long plate models using three or four diaphyseal screws (working length 13.5 or 7.5 cm, respectively) were compared with short plates with three diaphyseal screws (working length 7.5 cm). Models were loaded axially and torsionally; 100 cycles in random order. Primary outcome measures were axial and torsional fracture site stiffness. FCL screws decreased rotational stiffness 19% (P < .01) compared with baseline nonlocking screws in the same plate and working length construct, mirroring the effect (20% decrease in stiffness, P < .01) of nearly doubling the nonlocking construct working length (7.5‐13.5 cm). Similarly, FCL screws decreased axial stiffness 23% (P < .01) in the same baseline comparison. Fracture site displacement under loading comparable to a long working length nonlocked plate construct was achieved using a shorter FCL plate construct. By closely replicating the biomechanical properties of a long plate construct, a fracture site strain environment considered favorable in promoting fracture healing might still be achievable using a shorter plate length. Clinical Significance: It might be possible to optimize fracture site strain environment and displacement under loading using shorter FCL plate constructs. Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 00:00–00, 2020.

In vitro analysis and in vivo assessment of fracture complications associated with use of locking plate constructs for stabilization of caprine tibial segmental defects

“…The current study also evaluated varying working lengths of fixation, due to the custom nature of the locking plates utilized. Similar to previous mechanical studies, no significant effect of working length on construct stability was noted during compressive testing to failure [27,33]. However, one study noted differences in yield load, specifically, between constructs of short and long working lengths, and shorter working lengths were significantly associated with greater maximum load but not overall construct stiffness during cyclic compressive testing followed immediately by compressive load to failure [6].…”

“…As noted above, no significant differences in compressive maximal load or total strain were observed during compressive mechanical testing of two LP constructs of markedly different plate lengths. This differs from several in vitro models that noted a direct relationship between LP construct rigidity and plate length [5, 27, 33]. In a mechanical study utilizing simulated human supracondylar femoral fractures, locking plate length was significantly associated with construct stiffness, and longer plates generated significantly stiffer constructs regardless of number of screws and working length [33].…”

“…In a human FEA model utilizing medial placement of an LCP at the tibial diaphysis, construct rigidity increased directly with plate length, and the authors noted that clinically, the use of relatively longer plates could reduce the risk for fixation failure [5]. Biomechanical testing has corroborated this claim with multiple studies reporting a direct association between locking plate length and construct stiffness [27, 33]. The literature is more divided in regard to working length, the distance between the proximal and distal screws in the closest proximity to the fracture gap, with studies reporting that an increase in LCP working length could be associated with increased, decreased, or no change in construct stiffness in axial compressive testing [6, 10, 21, 33].…”

Rethinking the 10% strain rule in fracture healing: A distal femur fracture case series

“…Gap strain [mm/mm] was calculated by dividing the interfragmentary movement [mm] by the predeformed fracture gap width [mm]. Interfragmentary movement was calculated by probing the displacement of the proximal and distal fragment in the region opposite the lateral plate (i.e., at the medial cortex) 29‐31 …”

“…Interfragmentary movement was calculated by probing the displacement of the proximal and distal fragment in the region opposite the lateral plate (i.e., at the medial cortex). [29][30][31] 2.5 | Definitions of gap strain and continuum strain Two definitions of strain were used to evaluate the interfragmentary mechanical environment in this study: Perren's gap strain, which focuses on the axial movement at the fracture gap, and continuum strain, which considers the 3D deformation state of the tissues within the gap. To help visualize the differences between these two definitions, a series of idealized models of increasing complexity were also developed and are shown in Figure 2.…”