Biomedical Engineering Reference
In-Depth Information
and Finnegan 1984; Claes 1989). The underlying goals of this philosophical change are to increase the
fracture healing rate, to decrease the loss of bone mass in the region shielded by the plate, and, conse-
quently, to decrease the incidence of refracture which occurs following plate removal.
The interaction between bone and the plate is extremely important, since the two are combined into
a composite structure. The stability of the plate-bone composite and the service life of the plate depend
on accurate fracture reduction. The plate is most resistant in tension; therefore, in fractures of long
bones, the plate is placed along the side of the bone which is typically loaded in tension. Having excellent
apposition of the bone fragments, as well as developing adequate compression between them, is critical
in maintaining the stability of the fixation and preventing the plate from repetitive bending and fatigue
failure. Interfragmentary compression also creates friction at the fracture surface, increasing resistance
to torsional loads (Tencer et al. 1993; Parren 2002). On the contrary, too much compression causes
microfractures and necrosis of contacting bone due to the collapse of vesicular canals. Good mechani-
cal stability of the plate fixation requires strong compression between bone and the plate. However, it
disturbs periosteal blood supply to the cortical bone. In order to minimize contact between bone and
the plate, low-contact bone plate designs have been used to promote cortical perfusion (Uhthoff et al.
2006; Mazzocca et al. 2009).
Compression between the fracture fragments can be achieved with a special type of plate called a
dynamic compression plate
(DCP). The DCP has elliptic-shaped screw holes with its long axis oriented
parallel to that of the plate. The screw hole has a sliding ramp to the long axis of the plate. Figure 10.4
explains the principle of the DCP. Bone plates are often contoured in the operating room, in order to
conform to an irregular bone shape, to achieve maximum contact of the fracture fragments and anatomic
reduction of bone fragments. However, excessive bending decreases the service life of the plate. The most
common failure modes of a bone plate-screw fixation are screw loosening and plate failure. The latter
typically occurs through a screw hole, due to fatigue and/or crevice corrosion (Uhthoff et al. 2006). The
locking compression plate is another type of plate system where the plate and screw can be locked onto
the plate. In locking compression plate (LCP), screw holes in a plate and head of screw are threaded for
interlocking (Figure 10.5h). Advantages of the locking compression plate are: no compression of the plate
onto the bone is required; anatomic reduction of bone fragments can be achieved easily during surgery;
and postoperative screw loosening can be reduced (Perren 2002; Wagner and Frigg 2009). Additionally,
fracture and plate stability can be achieved without direct contact of the plate to the bone, and in some
constructs, the plate-screw unit can behave mechanically as an internal fixator (Wagner 2003).
FIGURE 10.4
Principle of a DCP for fracture fixation. During tightening a screw, the screw head slides down on
a ramp in a plate screw hole result in pushing the plate away from a fracture end and compressing the bone frag-
ments together.
