Tornetta Rockwood Adults 9781975137298 FINAL VERSION

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SECTION ONE • General Principles

data transfer. 74,139 For load application in biomechanical tests, these loading data can be combined into a resultant load vec- tor that changes in magnitude and orientation throughout a specific loading task. Reproducing these complex physiologic load vectors in bench-top testing requires sophisticated joint simulators or the use of robotic equipment capable of inducing arbitrary motion and forces in space. Physiologic loading and joint motion may also be facilitated by muscle force simulation through tendons. For this purpose, software is freely available that can simulate movement of musculoskeletal structures to estimate joint loading and muscle activation patterns for a wide variety of movements. 70 These recent advances in joint and muscle load assessment based on instrumented prostheses and computational simulation are crucial to resolve the apparent lack of consensus on test configurations and loading conditions in biomechanical studies. However, since many biomechanics laboratories only have a standard uniaxial loading frame, joint-specific physiologic load application is often reduced to quasi-physiologic loading along a resultant joint load vector using a standard test frame. Speci- mens are positioned at a specific angle and offset relative to the vertical actuator of the test system to yield a specific resultant load vector and loading moment. Ultimately, the quality of a biomechanical test setup should be judged by the degree with which it can replicate a clinically observed failure mode, and not by the complexity of biofidelic load application. Hence, the challenge becomes the replication of the principal load- ing mechanism in sufficient detail to reproduce clinical failure modes, while avoiding unnecessary complexity. For example, traditional testing of lag screw cut-out in pertrochanteric frac- ture models simulated only the dynamic axial loading during gait along a fixed load vector, but did not account for the flex- ion/extension or abduction/adduction motion during normal gait. 141 Adding a torsional load component to better simulate loading at the hip during gait yielded drastically different lag screw migration patterns (Fig. 1-35). 80 Furthermore, the more

biofidelic loading enabled the replication of clinically observed lag screw migration and cut-out in surrogate and cadaveric specimens (Fig. 1-36). Specimen Constraints For any given loading mode, specimen constraints can highly influence the load imparted to a specimen. For example, if the specimen ends of a plating construct are rigidly clamped to the actuator and base of a test system, the fixation construct acts as a rigid column that is largely constrained from buckling or bending (Fig. 1-37A). Under these overly rigid constraints, the fixation construct will yield a high axial stiffness and will exhibit very little gap motion since a metal plate will not shorten appre- ciably in response to the axial force. If the specimen ends are suspended between ball joints aligned along the diaphyseal midline, axial loading will impart more realistic plate bending due to the plate offset from the diaphyseal axis (Fig. 1-37B). With ball joint constraints, the axial stiffness of the same con- struct can be up to one order in magnitude lower than with rigid constraints, since plate flexion allows some axial displace- ment of the actuator. 204 As a practical compromise, axial testing is often performed with one specimen end rigidly fixed, and the other loaded through a ball joint. This loading constraint example illustrates how the same axial loading mode can lead to different test results, depending on the specimen constraints. Therefore, both the loading mode and specimen constraints must be considered when comparing results between studies. Loading Patterns Test loads can be applied statically or dynamically. In static loading, specimen load is gradually increased up to a defined level to assess construct stiffness (Fig. 1-38A). The load may be increased further until failure occurs to determine the construct strength and the failure mode. However, clinical failure rarely occurs from a single loading event, but typically from dynamic

A, B

C

Figure 1-35.  A: Test setup for simulation of lag screw migration and cut-out under either axial or com- bined axial and torsional loading. B: Combined axial and torsional loading can replicate the pathway of the resultant joint load vector ( F ) at the hip during normal gait. C: Adding the torsional load component to better simulate loading at the hip during gait yields drastically different lag screw migration patterns. (From Ehmke LW, Fitzpatrick DC, Krieg JC, et al. Lag screws for hip fracture fixation: Evaluation of migration resistance under simulated walking. J Orthop Res . 2005;23(6):1329–1335. Copyright © 2005 American Association of Physicists in Medicine. Reprinted by permission of John Wiley & Sons, Inc.)

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