Biomedical Engineering Reference
In-Depth Information
FIGURE 10.3
Bone screws: (a) a self-tapping V-threaded screw (has a cutting flute), (b) a nonself-tapping and
buttress threaded screw, and (c) a nonself-tapping cancellous screw (pedicle screw).
is firmly fixed, when the bone revascularizes, permanent secure fixation may be achieved. This is par-
ticularly true for titanium alloy screws or screws with a roughened thread surface, with which bone
ongrowth results in an increase in removal torque (Hutzschenreuter and Brümmer 1980). When the
screw is subject to micro- or macromovement, the contacting bone is replaced by a membrane of fibrous
tissue, the purchase is diminished, and the screw loosens.
The two principal applications of bone screws are: (1) as interfragmentary fixation devices to “lag”
or fasten bone fragments together and (2) to attach a metallic or plastic bone plate to bone (Mazzocca
et al. 2009). Interfragmentary fixation is used in most fractures involving cancellous bone, and in those
oblique fractures in cortical bone. In order to lag the fracture fragments, the head of the screw must
engage the cortex on the side of insertion without gripping the bone, while the threads engage cancel-
lous bone and/or the cortex on the opposing side. When screws are employed for bone plate fixation, the
bone screw threads must engage both cortices. In order to minimize bone damage due to trial and error
during drilling, when placing screws in a broad area of cancellous bone, preliminary pins may be placed
initially, and when optimal placement is obtained, cannulated drills and screws may be used to obtain
maximal purchase and a perfect location. Screws are also used for a fixation of spine fractures (for plate
fixation or compression of bone fragment) and for a spine fusion (Figure 10.1c).
10.1.4 Plates
Plates are available in a wide variety of shapes and are intended to facilitate fixation of bone fragments.
They range from the very rigid, intended to produce primary bone healing, to the relatively flexible,
intended to facilitate physiological loading of bone (Mazzocca et al. 2009).
The rigidity and strength of a plate in bending depends on the cross-sectional shape (mostly thick-
ness) and material from which it is made. Consequently, the weakest region in the plate is the screw hole,
especially if the screw hole is left empty, due to a reduction of the cross-sectional area in this region.
The effect of the material on the rigidity of the plate is defined by the elastic modulus of the material
for bending and by the shear modulus for twisting (Cochran 1982). Thus, given the same dimensions, a
titanium alloy plate will be less rigid than a stainless steel plate, since the elastic modulus of these alloys
is 110 and 200 GPa, respectively.
Stiff plates often shield the underlying bone from the physiological loads necessary for its healthful
existence (O'Sullivan et al. 1989; Perren 2002). Similarly, flat plates closely applied to the bone prevent
blood vessels from nourishing the outer layers of the bone (Perren 1989). For these reasons, the current
clinical trend is to use more flexible plates (titanium alloy) to allow micromotion, and low-contact plates
(only small surface of the plate contacts to bone), to allow restoration of vascularity to the bone (Uhthoff
