Tornetta Rockwood Adults 9781975137298 FINAL VERSION

34

SECTION ONE • General Principles

A, B

C

Figure 1-38.  A: Static loading is used for construct stiffness assessment, but in absence of cyclic loading may not yield a clinically relevant failure model. B: Dynamic loading replicates cyclic loading in vivo, but specimens may or may not fail at a given load amplitude. C: Progressive dynamic loading ensures that all specimens will be loaded dynamically to failure within a predictable number of loading cycles.

histories in an accelerated manner (Fig. 1-38B). 151 Dynamic loading is increasingly being applied for testing of osteosyn- thesis constructs to simulate fixation failure, implant migra- tion, and fatigue. Failure may not occur consistently at a given dynamic load amplitude under a standard cyclic loading regime since the specimen may “run-out,” this being the term given to the completion of all cycles of dynamic fatigue testing with- out failure. Moreover, it is difficult to predict a dynamic load amplitude that yields a desirable number of dynamic loading cycles prior to failure. Therefore, several recent studies have applied progressive dynamic loading, where the amplitude of the dynamic load is increased in a stepwise manner. 88,95,149 This progressive dynamic loading ensures that all specimens will be subjected to a considerable number of loading cycles at the lower load steps, and that all specimens will fail within a rea- sonable number of cycles at the higher load steps (Fig. 1-38C). Static and dynamic loading patterns can be applied in displace- ment or load control. Displacement-controlled tests prescribe a defined linear or rotational displacement and assess the result- ing forces and moments. Load-controlled tests apply a force or moment, and measure the resulting deformation or motion of the loaded specimen. Most biomechanical tests are conducted in load control to simulate physiologic loading regimes. How- ever, displacement control remains an attractive alternative due to the relative simplicity of applying accurate cyclic displace- ments with cam-shaft or screw-type actuators. Important con- siderations regarding loading are as follows: • Construct testing should be conducted in all principal load- ing modes, or under more complex physiologic loading that resembles a clinically realistic combination of principal load- ing modes. Results obtained under one principal loading mode cannot be extrapolated to other loading modes. • Results of different studies can be compared only if they employed the same loading modes and the same specimen constraints. • Dynamic loading should be used to determine construct strength and durability to account for cyclic loading and

failure mechanisms in vivo. Static testing should be reserved for stiffness testing. • The best proof of an appropriate loading condition is repli- cation of a clinically relevant failure mode. OUTCOME PARAMETERS Standard outcome parameters such as construct stiffness, strength, and durability may seem deceptively simple, but their proper assessment and interpretation are of crucial importance when drawing conclusions from biomechanical test results. Stiffness is typically used to represent the elastic, recover- able deformation of a fixation construct in response to loading. A higher stiffness does not necessarily correlate with improved construct strength, durability, or performance. 95 In fact, many engineered structures, such as bridges, car struts, and airplane wings, rely on flexibility that reduces stress concentrations during cyclic loading to ensure their structural integrity and to prolong their service life. Construct stiffness can affect frac- ture fixation and fracture healing. For fracture fixation, exces- sive stiffness can increase the periprosthetic fracture risk due to increased stress risers at the implant–bone interface, especially in the presence of osteoporotic bone. 31 For example, the pro- nounced mismatch between the high stiffness of early rib plates and the low bending stiffness of ribs led to a high rate of screw pull-out and fixation failure. In response, elastic rib plates were introduced that better matched the inherent flexibility of ribs (Fig. 1-39A–C). 87 Similarly, because of the rigid angle-stable fix- ation in the plate, a locking screw at the end of the plate can increase the risk of a periprosthetic fracture relative to a non- locked screw (Fig. 1-39D). Substituting the locked end screw with a conventional screw can increase the construct strength by up to 40%. 31 Because periprosthetic fractures at the plate end are relatively rare, this relevant finding would unlikely be rec- ognized in any reasonably sized clinical study. A biomechanical bench-top study is however ideally suited to isolate and identify the problem and to quantify the effectiveness of simple solutions.

Copyright © 2020 Wolters Kluwer Health, Inc. Unauthorized reproduction of this content is prohibited.

LWBK1698-C01_p001-042.indd 34

05/12/18 8:39 PM