P. 583

A. Pozzi1
1Clinic for Small Animal Surgery, Department of Small Animals, Vetsuisse-Faculty, University of Zurich
Fracture treatment present several challenges that should be considered when choosing the right implant. For example, in older animals, the biology of fracture healing is different and healing occurs more slowly. Thus, the implants and the type of fracture  xation have to withstand the forces at the fracture site longer than in the younger animals. In addition, the bone is often more brittle than in younger dogs, which may require speci c  xation strategies such as locking plates. These principles will be discussed in more detail.
Plates and screws are versatile implants for different methods and techniques of fracture  xation. It is important to understand the indications for the different types of plating techniques, and to be aware of the available types of plates. The locking plates may offer better security in weaker bone such as in immature or geriatric patients. In the last two decades a multitude
of plate types and concepts have been described and proposed, in an attempt to decrease the complications of bone plating. Concurrent to the change in emphasis in internal  xation, new implants have been developed to ful ll the requirement of these new techniques. Internal  xators, locking plates or angular stable devices, consist of a plate-like implant and locking head screws. Internal  xators have some major differences from conventional plates. Understanding the mechanical properties of the locking plates and the conventional plates is important for choosing the appropriate implant. Conventional non-locking plates achieve stability from the friction
that develops between plate and bone as the screw is tightened. Tightening the screw compresses the plate onto the surface of the bone producing friction. If the force exerted on the bone while the patient is walking exceeds the friction limit, relative shearing displacement will occur between the plate and the bone, causing
a loss of reduction between the bone fragments, or loosening of the screws, or both. Conventional plates, including dynamic compression plates (DCP) and limited contact dynamic compression plates (LC-DCP) allow compression of bone fragments utilizing the dynamic compression holes. When the compression screws are inserted eccentrically into the end of the oval hole (far from the fracture), the lower hemi-spherical part of the screw head meets the dynamic compression incline
of the compression hole. This interaction between the screw head and the compression incline results in
fracture compression during screw tightening. In the older animal the plate-screw interface may be at risk of loosening because of the poorer quality of the bone. For this reason locking plates should be strongly considered. An internal  xator or locking plate does not achieve stability by creating friction between the plate and the bone. The implant consists of a plate-like device and locking head screws which together act as an internal  xator. The locking of the head screw into the hole gives the screws axial and angular stability, relative to the plate. Because the stability of the  xation construct does not depend on friction between plate and bone, the bone- screw threads are unlikely to become stripped during insertion. The  xed-angle connection between the screw and the plate clearly offers improved long-term stability. The  xed-angle connection neutralizes the tilting of the screw in the hole. Subsequently, it becomes very dif cult for the plate to fail by “pull-out” because the screws cannot be sequentially loaded or pulled out.
Locking plates have both mechanical and biological advantages, which may bene t the geriatric patient. Because the compression between plate and bone is unnecessary, the periosteal blood supply under the plate remains intact. Preservation of the periosteal vessels may improve healing and decrease the risk of cortical bone necrosis and infection. Another advantage is that the plate does not need to be perfectly contoured, because the bone is not “pulled towards” the plate while tightening the screw. For this reason, locking plates are often used for minimally invasive plate osteosynthesis, which involves closed reduction and percutaneous  xation of the fracture. Different locking devices are available, but they all share the concept of  xed-angle device. Some plates may have combination holes that allow placing a conventional screw in compression, neutralization or a locking screw. Feline pelvic fractures are a good application for locking implants because
of their increased risk of screw pull out and plant
failure. At the University of Zurich we have performed
a retrospective clinical study and a mechanical test
to determine if the locking plates would be a good choice. The objective of this historical cohort study
was to compare complications associated with the use of LPS and DCP for the repair of ilial fractures in cats. Both single and double locking plate constructs were associated with signi cantly less screw loosening than were non-locking plates. Loose screws were detected in 50% of the fractures plated with DCP, which is similar to rates reported by others for non-locking devices. Screw loosening did not occur in the LPS groups, except for one screw that was placed in the fracture line. Although not statistically different, pelvic canal narrowing was more severe in the DCP group compared to the locking plates groups. It has been suggested that increased screw
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