Tornetta Rockwood Adults 9781975137298 FINAL VERSION

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SECTION ONE • General Principles

A, B

C

Figure 1-41.  A: In static testing, a sudden drop in displacement or load indicates a simple, catastrophic failure and allows identification of strength in terms of peak load L MAX. B: If constructs fail gradually in a more complex failure mode, detection of the instant of failure for strength determination is uncertain. C: Under dynamic loading, specimens undergo elastic, recoverable deformation as well as subsidence or migration, which represents the accumulation of unrecoverable displacement. The number of loading cycles until subsidence reaches a predefined threshold is used as outcome measure for construct strength and durability.

Strength provides a clear indicator of implant performance that correlates clinically with the risk of implant failure. Strength of a fixation construct can only be determined by loading con- structs until failure occurs. The need for destructive testing nec- essarily increases the cost of implants and limits the number of variables that can be tested compared to a nondestructive study design that only evaluates construct stiffness. However, with- out destructive testing, no reliable claims or predictions of con- struct strength can be made. While strength simply represents the ultimate load at which a structure fails, strength assessment is highly dependent on the choice of the failure criterion. For a structure failing by sudden catastrophic fracture, the instance of failure is easily defined by the fracture event (Fig. 1-41A). However, when specimens fail gradually, the instance of failure may be more obscure (Fig. 1-41B). In these circumstances, a clearly defined and clinically relevant threshold for the onset of failure must be provided. Ideally, strength is assessed under dynamic loading, which allows the measurement of the onset and progression of failure in terms of subsidence or migration. Subsidence or migration represents the amount of unrecovered displacement after loading (Fig. 1-41C) due to gradual degra- dation of implant fixation or fatigue of the fixation construct. Stabilization of initial subsidence typically indicates success- ful settling of an implant into a stable position. Progression of subsidence reflects damage accumulation that eventually leads to failure. Progressive subsidence can coincide with a decay in construct stiffness, but diminished construct stiffness by itself is not necessarily indicative of failure. Decay in construct stiffness becomes important only if continued dynamic loading results in a clinically relevant failure mode. When strength is assessed under dynamic loading, it can also be expressed in terms of fatigue strength, which represents the number of load cycles of a given magnitude that can be sustained before failure occurs. The highest dynamic load amplitude at which no failure will

occur regardless of the number of loading cycles is called the endurance limit. Stability, unlike stiffness or strength, is not an engineering quantity. As such it cannot be used as an outcome parameter. It is frequently used to qualitatively describe the amount of displacement across a fracture or a bone–implant interface in response to a given loading event. Stress is assessed to illustrate load transfer mechanisms, to detect stress concentrations, and to predict failure induced by stress risers. Stress cannot be measured directly, but is typi- cally inferred from measurements of the strain produced by the applied stress. Stress depends on the induced strain as well as on the apparent stiffness of the deforming material, whereby a softer material will exhibit less stress than a stiffer material for a given deformation. Strain gauges have been used since 1938 and remain the most common strain sensors due to their high sensitivity and accuracy. They are glued onto a surface of inter- est and contain a conductive filament that changes electrical resistance when strained. Gluing a strain gage, however, onto a substrate is not trivial, and deficient bonding will invalidate strain readings. As an alternative, contemporary optical systems can capture continuous surface strain maps by correlation of digital images of a surface obtained before and after deforma- tion using pattern recognition or speckle interferometry. 42 The high sensitivity and noncontact approach of these optical strain sensors enables acquisition of strain distributions on hard and soft tissues, reflecting load transfer mechanisms and stress con- centrations. 37 However, optical strain sensors and strain gauges are confined to measurement of surface strain. Strain inside structures and at interfaces can only be estimated by numer- ical simulation, typically with finite element analysis (FEA), whereby experimental surface strain measurements are used for validation of the numerical simulation. Important facts to keep in mind about outcome parameters include the following:

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