Tornetta Rockwood Adults 9781975137298 FINAL VERSION

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SECTION ONE • General Principles

joint motion. In case of bone fracture, a fixation construct must temporarily accommodate the mechanical function of the struc- turally deficient bone. The mechanical competence of bones and implants for fracture fixation depends mainly on two fac- tors: their material properties and their structural properties. This section first provides a basic description of material prop- erties and structural properties, followed by important concepts of load transmission through joints and fixation constructs. MATERIAL PROPERTIES Material properties characterize the deformation and failure of a material under loading, without considering the geometry of its structure. In the case of the proximal femur, material proper- ties can be assessed on a small cancellous bone cylinder that is harvested from the femoral neck without considering the anat- omy of the intertrochanteric region (Fig. 1-1). By controlled compression of the bone cylinder, its compressive stiffness can be measured. The height of the cylinder will decrease with increasing amounts of compressive loading. The ratio of applied load to the resulting compression of the cylinder represents the material stiffness in compression. For a given compressive load, stiffer materials undergo less compression than more elastic materials do. For example, if a load of 10 N is required to compress the bone cylinder by 0.1 mm, the compressive stiffness of the cyl- inder is 10 N/0.1 mm = 100 N/mm. However, this stiffness depends not only on the material property but also on the height and cross-sectional area of the cylinder. To define stiffness inde- pendent of the specimen size, loading is expressed in terms of stress ( σ ), which is calculated by dividing the load by the area the load is acting upon. Likewise, the resulting compression of the cube can be expressed in terms of strain ( ε ), which represents the amount of compression ( ∆ l ) divided by the original height ( L ) of the cylinder. Stiffness can now be expressed in terms of the Elastic or Young’s modulus ( E = σ / ε ), which is indepen- dent of the sample size (Table 1-1). Assuming that the cylinder is 10 mm tall ( L = 0.01 m) and has a loading surface of 1 cm 2

INTRODUCTION

Management of a fractured bone requires the combined con- sideration of biologic and mechanical aspects to create a bio- mechanically sound fixation construct. Biologically, the con- struct should not be more invasive than necessary and should provide a fracture environment that supports bone healing. 167,210 Mechanically, the construct should provide sufficient strength and durability for early mobilization. 174 Since fracture fixation is a race between bone healing and construct failure, biologic requirements to promote healing and mechanical requirements to ensure durable fixation must be considered equally. Unfortunately, these requirements can be mutually exclusive, and one has been favored over the other during the history of internal fixation. For example, traditional splinting techniques are noninvasive and provide relative sta- bility to a fracture with the expectation of natural bone healing by callus formation. However, deficient mechanical stability requires prolonged immobilization. The advent of compres- sion plating greatly improved the mechanical strength of fix- ation constructs at the cost of a more invasive procedure and an absolute stable fracture environment that suppresses natural bone healing. This dichotomy was properly termed the “para- dox of internal fixation.” 3 Rigid fixation is required to restore function, while flexibility is necessary to stimulate natural bone healing and to restore normal mechanical properties of bone after union. This chapter provides the biomechanical foundation to facil- itate biologically friendly and mechanically durable fixation with modern implants and fixation strategies. First, a founda- tion of pertinent engineering concepts, fracture etiology, and biomechanical requirements for fracture healing are summa- rized. Next, generally applicable strategies and principles for fracture fixation are described, followed by implant-specific recommendations for intramedullary nailing, external fixation, and plating. Finally, a primer on bench-top testing of fixation constructs is provided, which will reinforce the basic engineer- ing concepts and help the reader evaluate the clinical relevance and limitations of biomechanical studies. In the spirit of full disclosure, it must be stated that two of the authors (MB, DF) have translated their research into new implants for controlled axial dynamization. To address this potential conflict of interest, great emphasis was taken to support related teaching points with multiple references from different, nonassociated research groups. It is the hope of the authors that readers will perceive biomechanics of fracture fixation not as a complex science, but as a scientific resource for practical, logical concepts that provide clear clinical guidance to achieve durable fracture fixation without impeding the fracture healing process.

BASIC MECHANICAL CONCEPTS

Figure 1-1.  Compression of a cylindrical specimen of trabecular bone. To determine material stiffness, the specimen is compressed and the change in height is measured. The resulting compression can be expressed in terms of strain ( ε ), which represents the amount of com- pression ( ∆ l ) divided by the original height ( L ) of the cylinder.

Bone represents the primary structural elements of the muscu- loskeletal system. It must have sufficient stiffness, strength, and durability to transmit muscles forces, bear loads, and support

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