Clinic for Small Animal Surgery, Department of Small Animals, Vetsuisse-Faculty, University of Zurich, Zurich, Switzerland
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 fixation 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 specific fixation 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 fixation. 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 fixation, new implants have been developed to fulfill the requirement of these new techniques. Internal fixators, locking plates or angular stable devices, consist of a plate-like implant and locking head screws. Internal fixators 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 fixator 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 fixator. 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 fixation construct does not depend on friction between plate and bone, the bonescrew threads are unlikely to become stripped during insertion. The fixed-angle connection between the screw and the plate clearly offers improved long-term stability. The fixed-angle connection neutralizes the tilting of the screw in the hole. Subsequently, it becomes very difficult for the p late 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 benefit 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 fixation of the fracture. Different locking devices are available, but they all share the concept of fixed-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 significantly 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 purchase can be obtained by engaging the sacrum through the ilium when applying a lateral bone plate in dogs, especially after pelvic osteotomy. Increased bone purchase might lead to less pelvic canal narrowing.
However, clinical studies evaluating the benefit of this increased screw purchase have yielded conflicting data. In the only study available involving cats, engaging the sacrum during lateral plating of the ilium did not significantly affect screw loosening or pelvic narrowing. In the presented study in only 1 of 3 cats with screw loosening in the cranial fragment, the screw purchasing the sacrum was loose. Screw loosening was not associated with concomitant contralateral or ipsilateral fractures however this might be underestimated due to the small number of cats in each group. Despite the limitations, this is the first study investigating locking plate for ileal fractures in cats. In conclusion, less screw loosening but no significant difference for narrowing of the pelvic canal was observed in lateral plating of feline ilial fractures with an LPS compared to lateral plating with DCP. Implant failure occurs due to screw loosening and not due to failure of the plate.
In some cases conventional plates, or combination plates that allow both locking and compression (LCP, ALP), are an appropriate choice. In most cases a simple fracture should be treated with the goal of absolute stability using a compression device. Both DCP and LCP are appropriate choices because they allow interfragmentary compression. Metaphyseal fractures can be fixed with anatomically pre-shaped plates with locking holes. However, if the plate is not pre-shaped and the locking holes are not placed according to the anatomical region, a conventional plate allows more precise placement of the screws.
The selection of the appropriate length of the plate is a very important step in the preoperative plan. The plate length depends on the fracture pattern and the function that the plate is intended to serve. To maximize implant stability in bridging plating, long plates with fewer screws should be used to decrease the lever arm and distribute the bending forces. The number and position of the screws is a major determinant of the stability and the mechanical properties of the construct. Two screws in each main fragment are the minimal number required to maintain the bone-implant construct from a purely mechanical point of view. However, loosening of one screw will cause overloading of the other screws, resulting in failure of the whole construct. By increasing the number of screws there is a decrease in the magnitude of load on each screw, and therefore a reduced risk of pull-out. However, more screws weaken the bone. Hence, it is important to find a balance with the appropriate number of screws. It is generally recommended to use three screws on either side of the fracture against bending, and four screws per fragment against torsion.
The position of the screws influences the stiffness of the bone-implant construct and stress distribution in the plate. Minimizing the distance between the nearest screws on either side of the bone fragments (also called working length) increases stiffness and reduces gap motion at the fracture. With a short working length the stress is concentrated and there is higher risk of implant failure from cyclic fatigue of the short segment with concentrated stress. When the innermost screws are further away from the fracture site, the working length of the plate is greater, allowing bone deformation, gap motion and better distribution of stress in the plate. To achieve a greater working length and a sufficient number of screws on either side of the fracture it is necessary to use long plates. Considering that on each side of the fracture there should be three screws and two empty holes, plates of 10 holes or more should be used most of the time.