Boundary Conditions Matter—Impact of Test Setup on Inferred Construct Mechanics in Plated Distal Femur Osteotomies

Abstract

The mechanics of distal femur fracture fixation has been widely studied in bench tests that employ a variety of approaches for holding and constraining femurs to apply loads. No standard test methods have been adopted for these tests and the impact of test setup on inferred construct mechanics has not been reported. Accordingly, the purpose of this study was to use finite element models to compare the mechanical performance of a supracondylar osteotomy with lateral plating under conditions that replicate several common bench test methods. A literature review was used to define a parameterized virtual model of a plated distal femur osteotomy in axial compression loading with four boundary condition sets ranging from minimally to highly constrained. Axial stiffness, longitudinal motion, and shear motion at the fracture line were recorded for a range of applied loads and bridge spans. The results showed that construct mechanical performance was highly sensitive to boundary conditions imposed by the mechanical test fixtures. Increasing the degrees of constraint, for example by potting and rigidly clamping one or more ends of the specimen, caused up to a 25x increase in axial stiffness of the construct. Shear motion and longitudinal motion at the fracture line, which is an important driver of interfragmentary strain, was also largely influenced by the constraint test setup. These results suggest that caution should be used when comparing reported results between bench tests that use different fixtures and that standardization of testing methods is needed in this field.

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

“…Scans were imported into Mimics (v23.0, Materialize) for segmentation (Figure 1A). A lower threshold of 160 Hounsfield Units [HU] was used to isolate the distal femur fragments 14 . All voxels corresponding to an enclosed volume for each bone fragment were captured using a built‐in Mimics surface optimization tool (Figure 1B).…”

“…The proximal portion of the fractured femur that is typically missing from routine preoperative CT scans was virtually reconstructed using a CT scan of an intact femur from a 25‐year‐old male 14 . The virtual model of the intact femur was created using the same segmentation protocol as the fractured distal femur models.…”

“…Screw threads were omitted from the models to simplify finite‐element analysis 16 . A mesh convergence analysis was conducted to ensure solution convergence 14 . As a result, a quadratic tetrahedral (tet‐10) element type with a maximum edge length of 1.5 mm was applied to both the surfaces and volumes of every model (Figure 1F).…”

“…As described in a previous study, $${\rho }_{{BMD}}$$ was then converted to Young's modulus on an elementwise basis within each model (Figure 1G). 14 …”

“…A lower threshold of 160 Hounsfield Units [HU] was used to isolate the distal femur fragments. 14 All voxels corresponding to an enclosed volume for each bone fragment were captured using a built-in Mimics surface optimization tool (Figure 1B). If applicable, intra-articular fracture lines were virtually reduced and those fragments were fused, representing anatomic reconstruction of the articular block before lateral plate placement.…”