Hard Tissue Replacements - Biomaterials - page 228

Nature provides different types of mechanisms to repair fractures in order to be able to cope with
different mechanical environments about a fracture (Hulth 1989; Giannoudis et al. 2007; Einhorn et al.
2008; Kakar and Einhorn 2009). For example, incomplete fractures (fissures), which only allow micro-
motion between the fracture fragments, heal with a small amount of fracture-line callus , known as
primary healing . In contrast, complete fractures which are unstable and, therefore, generate macromo-
tion heal with a voluminous callus stemming from the sides of the bone, known as secondary healing
(Brighton 1984; Hulth 1989; Einhorn et al. 2008; Kakar and Einhorn 2009).
The goals of fracture treatment are to obtain rapid healing, to restore function, and to preserve ana-
tomic shape, without general or local complications. Implicit to the selection of the treatment method
is the need to avoid potentially deleterious conditions, for example, the presence of excessive motion
between bone fragments which may delay or prevent fracture healing (Brighton 1984; Brand and Rubin
1987; Einhorn et al. 2008).
Each fracture pattern and location results in a unique combination of characteristics (“fracture
personality”) that require specific treatment methods. The treatments can be nonsurgical or surgi-
cal. Examples of nonsurgical treatments are immobilization with casting (plaster or resin) and brac-
ing with plastic apparatus. The surgical treatments are divided into external fracture fixation, which
does not require opening the fracture site, and internal fracture fixation, which requires opening the
fracture.
With external fracture fixation, the bone fragments are held in alignment by pins placed through the
skin onto the skeleton, structurally supported by external bars. With internal fracture fixation, the bone
fragments are held by wires, screws, plates, and/or intramedullary devices (Figure 10.1). All the internal
fixation devices should meet the general requirement of biomaterials, that is, biocompatibility, sufficient
strength within dimensional constrains, and corrosion resistance. In addition, the device should also
provide a suitable mechanical environment for fracture healing. From this perspective, stainless steel,
cobalt-chrome alloys, and titanium alloys are most suitable for internal fixation. Detailed mechanical
properties of the metallic alloys are discussed in the chapter on metallic biomaterials. Most internal
fixation devices persist in the body after the fracture has healed, often causing discomfort and requiring
removal. Recently, biodegradable polymers, for example, polylactic acid (PLA) and polyglycolic acid
(PGA), have been used to treat minimally loaded fractures, thereby eliminating the need for a second
surgery for implant removal. A summary of the basic application of biomaterials in internal fixation is
presented in Table 10.1. A description of the principal failure modes of internal fixation devices is pre-
sented in Table 10.2.
FIGURE 10.1 Radiographs of (a) an internal and external fixation of the wrist shows the entire fixation apparatus;
(b) a total hip joint replacement in a patient who sustained a femoral fracture and was treated with double bone
plates, screws, and surgical wire (arrows); (c) application of screws (pedicle screw) and rods in spine fusion.