Biomedical Engineering Reference
In-Depth Information
FIGURE 10.5
Bone plates: (a) DCP, (b) hybrid compression plate (lower part has dynamic compression screw
holes), (c) reconstruction bone plate (easy contouring), (d) buttress bone plate, (e) L-shaped buttress plate, (f) nail
plate (for condylar fracture), (g) dynamic compression hip screw, and (h) locking plate and screw head (circle).
In the vicinity of the joints, where the diameter of long bones is wider, the cortex thinner, and cancel-
lous bone abundant, plates are often used as a buttress or retaining wall. A buttress plate applies force to
the bone perpendicular to the surface of the plate and prevents shearing or sliding at the fracture site.
Buttress plates are designed to fit specific anatomic locations and often incorporate other methods of
fixation besides cortical or cancellous screws, for example, a large lag screw or an I-beam. For the fusion
of vertebral bodies following diskectomy, spinal plates are used along with bone grafts. These plates are
secured to the vertebral bodies using screws. Similar approaches have been employed to restore stabil-
ity in the thoracolumbar and cervical spine region as well. Figure 10.5 illustrates various types of bone
plates.
10.1.5 Intramedullary Nails
Intramedullary devices (IM nails) are used as internal struts, to stabilize long bone fractures. IM nails
are also used for fixation of femoral neck or intertrochanteric bone fractures; however, this application
requires the addition of long screws. A gamut of designs are available, going from solid to cylindrical,
with shapes such as cloverleaf, diamond, and “C” (slotted cylinders). Figure 10.6 shows variety of intra-
medullary devices.
Compared to plates, IM nails are better positioned to resist multidirectional bending than a plate or
an external fixator, since they are located in the center of the bone. However, their torsional resistance
is less than that of the plate (Mazzocca et al. 2009). Therefore, when designing or selecting an IM nail, a
high polar moment of inertia is desirable to improve torsional rigidity and strength. The torsional rigid-
ity is proportional to the elastic modulus and the moment of inertia. For nails with a circular cross-sec-
tion, torsional stiffness is proportional to the fourth power of the radius of the nail. The wall thickness of
the nail also affects the stiffness. A slotted, open-section nail is more flexible in torsion and bending and
allows easy insertion into a curved medullary canal, for example, that of the femur (Tencer et al. 1993).
However, in bending, a slot is asymmetrical with respect to rigidity and strength. For example, a slotted
nail is strongest when bending is applied so that the slot is near the neutral plane; the nail is weakest
when oriented so that the slot is under tension.
In addition to the need to resist bending and torsion, it is vital for an IM nail to have a large contact
area with the internal cortex of the bone to permit torsional loads to be transmitted and resisted by shear
