Background Postoperative acromial stress fracture is a troublesome postoperative complication after reverse shoulder arthroplasty. Our study aims to utilize routinely performed preoperative computed tomography scans to identify differences in the material properties of the acromion in patients who did and did not develop a postoperative acromial stress fracture. Methods Treatment records and computed tomography scans for 99 reverse shoulder arthroplasties were collected. Scans were calibrated using a phantom and transferred for post-processing where the acromion, full scapula, and humeral head were isolated. The final segmented model was used to assess acromial volume and volumetric bone mineral density for each region of interest. Results There was no association between age and volumetric bone mineral density in any region of interest (all R2 ≤ 0.048, all p > 0.082). Patients who developed an acromial stress fracture were not significantly different from those who did not in terms of age, acromial volume, or acromial volumetric bone mineral density (all p > 0.559). Patients with known osteoporosis or osteopenia had slightly lower volumetric bone mineral density, but the differences were not significant (all p ≥ 0.072). Conclusion Postoperative acromial fractures following reverse shoulder arthroplasty cannot be predicted by computed tomography-derived volumetric bone mineral density or volume. These mechanical characteristics also do not predictably decrease with age or osteoporosis diagnosis.

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.

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Biomechanical testing of long bones can be subject to undesirable errors and uncertainty due to malalignment of specimens with respect to the mechanical axis of the test frame. To solve this problem, we designed a novel, customizable alignment and potting fixture for long bone testing. The fixture consisted of 3D-printed components modeled from specimen-specific CT scans to achieve a predetermined specimen alignment. We demonstrated the functionality of this fixture by comparing benchtop torsional test results to specimen-matched finite element models and found a strong and statistically significant correlation (R2 = 0.9536, p < 0.001). Additional computational models estimated the impact of malalignment on mechanical behavior in both torsion and axial compression. Results confirmed that torsion testing is relatively robust to alignment artifacts, with absolute percent errors less than 8% in all malalignment scenarios. In contrast, axial testing was highly sensitive to setup errors, experiencing absolute percent errors up to 40% with off-center malalignment and up to 130% with angular malalignment. This suggests that whenever appropriate, torsion tests should be used preferentially as a summary mechanical measure. When more challenging modes of loading are required, pre-test clinical-resolution CT scanning can be effectively used to create potting fixtures that allow for precise pre-planned specimen alignment. This may be particularly important for more sensitive biomechanical tests (e.g. axial compressive tests) that may be needed for industrial applications, such as orthopaedic implant design.

Since the 1970s, the 2%−10% rule has been used to describe the range of interfragmentary gap closure strains that are conducive for secondary bone healing.Interpreting the available evidence for the association between strain and bone healing remains challenging because interfragmentary strain is impossible to directly measure in vivo. The question of how much strain occurs within and around the fracture gap is also difficult to resolve using bench tests with osteotomy models because these do not reflect the complexity of injury patterns seen in the clinic. To account for these challenges, we used finite element modeling to assess the three-dimensional interfragmentary strain in a case series of naturally occurring distal femur fractures treated with lateral plating under load conditions representative of the early postoperative period. Preoperative computed tomography scans were used to construct patient-specific finite element models and plate fixation constructs to match the operative management of each patient. The simulations showed that gap strains were within 2%−10% only for the lowest load application level, 20% static body weight (BW).Moderate loading of 60% static BW and above caused gap strains that far exceeded 10%, but in all cases, strains in the periosteal region external to the fracture line remained low. Comparing these findings with postoperative radiographs suggests that in vivo secondary healing of distal femur fractures may be robust to early gap strains much greater than 10% because formation of new bone is initiated outside the gap where strains are lower, followed by later consolidation within the gap.