Hard Tissue Replacements - Biomaterials - page 230

to remove, since it prevents the wire from sliding when pulled. When a wire is used with other metallic
implants, the metal alloys should be matched to prevent galvanic corrosion (Park and Lakes 2007).

10.1.2 Pins
Straight wires are called Steinmann pins; however, if the pin diameter is <2.38 mm, then it is called
Kirschner wire. They are widely used, primarily, to hold fragments of bones together provisionally or
permanently and to guide large screws during insertion. To facilitate implantation, the pins have differ-
ent tip designs which have been optimized for different types of bone (Figure 10.2). The trochar tip is the
most efficient in cutting; hence it is often used for cortical bone.
The holding power of the pin comes from elastic deformation of surrounding bone. In order to
increase the holding power to bone, threaded pins are used. Most pins are made of 316L stainless steel;
however, recently, biodegradable pins made of PLA or PGA have been employed for the treatment of
minimally loaded fractures.
The pins can be used as part of elaborate frames designed for external fracture fixation (Figure 10.1a).
In this application, several pins are placed above and below the fracture, but away from it. After the frac-
ture fragments are manually approximated (reduced) to resemble the intact bone, the pins are attached
to various bars, which upon assembly will provide stability to the fracture.

10.1.3 Screws
Screws are the most widely used devices for fixation of bone fragments. There are two types of bone
screws: (1) cortical bone screws, which have small threads, and (2) cancellous screws, which have large
threads to get more thread-to-bone contact. They may have either V or buttress threads (Figure 10.3).
The cortical screws are subclassified further according to their ability to penetrate into self-tapping and
nonself-tapping (Figure 10.3). The self-tapping screws have cutting flutes which thread the pilot drill-hole
during insertion; in contrast, the nonself-tapping screws require a tapped pilot drill-hole for insertion.
The holding power of screws can be affected by the size of the pilot drill-hole, the depth of screw
engagement, the outside diameter of the screw, and quality of the bone (Cochran 1982; DeCoster et al.
1990; Ricci et al. 2010). Therefore, the selection of the screw type should be based on the assessment of
the quality of the bone at the time of insertion. Under identical conditions, self-tapping screws provide
a slightly greater holding power than nonself-tapping screws (Tencer et al. 1993).
Screw pullout strength varies with time after insertion in vivo , and it depends on the growth of bone
into the screw threads and/or resorption of the surrounding bone (Wermelin et al. 2008; Ricci et al.
2010). The bone immediately adjacent to the screw often undergoes necrosis initially, but if the screw

FIGURE 10.2 Types of metallic pin tip: (a) trocher end and (b) diamond end.