Biomedical Engineering Reference
In-Depth Information
FIGURE 10.6
Intramedullary devices: (a) Gross-Kempf (slotted), (b) Uniflex (Ti alloy, slotted), (c) Kuntscher,
(d) Samson, (e) Harris, (f) Brooker-Wills distal locking pin, and (g) Enders pins.
stress. Two different concepts are used to develop shear stress: (1) a three-point, high-pressure contact,
achieved with the insertion of curved pins and (2) a positive interlocking between the nail and intra-
medullary canal, to produce a unified structure. Positive interlocking can be enhanced by reaming the
intramedullary canal. Reaming permits a larger, longer, nail-bone contact area and allows the use of a
larger nail, with increased rigidity and strength (Kessler et al. 1986; Mazzocca et al. 2009).
The addition of screws through the bone and nail, proximal and distal to the fracture, known as
interlocking
, increases torsional stability and prevents shortening of the bone, especially in unstable
fractures (Perren 1989). The IM nail, which has not been interlocked, allows interfragmentary com-
pressive force, due to its low resistance to axial load. Another advantage of the IM nails is that they do
not require opening the fracture site, since they can be inserted through a small skin incision, typically
located in one extreme of the bone. The insertion of an IM nail, especially those that require reaming
of the medullary canal, destroys the intramedullary vessels which supply two-thirds of the cortex.
However, this is not of clinical significance because revascularization occurs rapidly (Kessler et al.
1986; O'Sullivan et al. 1989).
10.2 Joint Replacements
Our ability to replace damaged joints with prosthetic implants has brought relief to millions of patients,
who would otherwise have been severely limited in their most basic activities and doomed to a life in
pain. It is estimated that about 16 million people in the United States are affected by osteoarthritis,
one of the various conditions that may cause joint degeneration and may lead a patient to a total joint
replacement.
Joint degeneration is the end stage of a process of destruction of the articular cartilage, which results
in severe pain, loss of motion, and, occasionally, in angular deformity of the extremity (Buckwalter et al.
1993). Unlike bone, cartilage has a very limited capacity for repair (Safran and Seiber 2010). Therefore,
when exposed to a severe mechanical, chemical, or metabolic injury, the damage is permanent and often
progressive.
Under normal conditions, the functions of cartilage are to provide a congruent articulation between
bones, to transmit load across the joint, and to allow low-friction movements between opposing joint
surfaces. The sophisticated manner in which these functions are performed become evident from some
