Lars F.H. Theyse, PhD, DVM, DECVS
Associate Professor Orthopaedic-Neuro-Oral & Craniofacial Surgery, Department Clinical Sciences of Companion Animals, Faculty of Veterinary Medicine, Utrecht University, Utrecht, Netherlands
Fractures of the abaxial skeleton are a common finding after trauma in both dogs and cats. Surgical treatment of fractures in companion animals can be divided in external and internal skeletal fixation techniques. The external skeletal fixation (ESF) systems provide stability by using bone wires or bone pins which are connected on the outside of the limb by an external frame. Depending on the configuration, these systems are called unilateral or type 1, bilateral or type 2, multi-planar or type 3, and circular or type 4. Internal skeletal fixation includes the application of intramedullary bone wires and pins, cross pins, interlocking nails, compression and appositional screws, and plate and screw constructs. Plate and screw constructs were initially designed to treat long bone fractures but are now available for almost any fracture type. The standard plate and screw construct achieves stability by the friction between the plate and bone after screw tightening. This means that contact between plate and bone is essential to enable fracture healing. A disadvantage of extended plate-bone contact is poor vascularity at the plate-bone interface, which also has implications for fracture healing.
Early plate designs only aimed at neutralizing forces over the fracture zone. In order to allow compression over the fracture site and thus higher initial stability, the dynamic compression plate (DCP) was designed. This plate enables compression by using a sliding hole design in the plate in combination with eccentric drilling. In order to partially overcome the poor plate-bone interface properties, the limited contact DCP (LC-DCP) was designed. The LC-DCP uses cutbacks on the underside of the plate to allow ingrowth of blood vessels and increased bone vascularity. The stability and strength of plate osteosynthesis using standard DCP or LC-DCP is similar.1 In companion animals, the standard size of the DCP and LC-DCP correlates with the screw size and ranges from 2.0 mm, 2.7 mm, and 3.5 mm. Larger plates are available but are not frequently used. As plates were initially designed for use in human size, problems arose for the use in smaller companion animals. Veterinary cuttable plates (VCP) were introduced to accommodate fracture fixation in small dogs and cats.2 The VCP plates come in two sizes 1.5–2.0 mm and 2.0–2.7 mm respectively. The VCP screw holes can hold either 1.5 and 2.0 mm screws or 2.0 and 2.7 mm screws. The VCP differs from the small standard DCP and LC-DCP in the smaller distance between the screw holes. This is especially important in treating small bones, enabling a sufficient number of bone screws per segment. The VCP can be used as a stacking plate to improve stability.
As mentioned earlier, the major disadvantage of regular plates is that they rely on friction at the plate-bone interface. A major advantage is that they allow multidirectional screw placement, which is important when preventing screw interference with adjacent joints, the fracture zone and fissure lines. The regular plates also allow compression screws through the plate holes. To eliminate the need for plate-bone interface friction, the locking plate concept was introduced. Locking plates act as an internalized ESF. The screws are engaged in the bone similar to bone wires and pins and the screw heads are locked within the plate similar to the connection of the bone wires and pins to the external connecting frame in a regular ESF. An advantage is that the locking plate have very limited contact with the bone while providing excellent stability by the locking mechanism of the screw heads in the plate. Damage to the periosteum and vascularity of the bone during osteosynthesis is limited and bone healing is increased. The locking of the screw heads is accomplished in several different ways depending on the system used. Locking is achieved either by adding treads to the screw head and plate or by using a tapered screw head and plate interface.3,4 In both systems, the screw head is firmly attached in the plate. A disadvantage of these systems is that the direction of screw placement is now predetermined. To overcome this problem, the locking compression plate (LCP) has been introduced. The LCP has combined holes that allow regular cortical screw and locking screw placement in the same plate hole. The LCP system has slightly different screw sizes ranging from 2.0 mm, 2.4 mm, 2.7 mm and 3.5 mm. Several more recent locking plate systems do allow limited directional screw placement.5
As plate systems usually only offer unidirectional contouring, application to bones with three-dimensional curvatures can be highly demanding. The traditional reconstruction plate with side-cuts does enable three-dimensional contouring but still relies on plate-bone interface friction. The increased contouring ability of this plate has a limited stiffness and strength as a negative end result. More recent locking plate systems combine three-dimensional contouring of the plate with the advantages of locking screws.6-8
Although the plate systems can be very different, the main rules in plate and screw application remain universal. Screws can be divided in cortical screws with a small pitch for firm cortical bone and cancellous screws with a larger pitch for softer cancellous bone. Screws are placed after predrilling with a drill diameter close to the core diameter of the screw. A depth gauge is used to determine proper screw length. The screw windings should completely engage the cis- and trans-cortex in a bicortical screw or cis-cortex in a unicortical screw. The screw holes are either tapped before screw placement or self-tapping screws are used.
A minimal of three bicortical screws is preferred per bone segment. This means that a regular plate osteosynthesis is characterized by a total of 6 bicortical screws.9 Screws should not interfere with the adjacent joint and should not be placed close the fracture ends. A minimal distance of 1 cm away from the fracture line is advocated depending on the size of the patient. During fracture treatment of long bones, the complete length of the bone should be used to reduce stress on implants and on the fracture zone. To prevent additional surgical trauma during osteosynthesis, minimal invasive techniques can be used, but functional fracture alignment and stability remain the primary goals. Initial fracture treatment is assessed radiographically 4 to 6 weeks after surgery. Fracture healing will take several months and plate removal usually is not planned before 6 months after surgery. Whether plates and screws are removed depends on the location of the plate and whether the implants interfere with soft tissue function. In plates that are positioned with limited skin coverage lameness can occur in cold weather conditions. A regular radiographic check-up once a year is recommended when plates and screws are left in situ. Any lameness attributed to implants should be thoroughly assessed.
In conclusion, plate osteosynthesis is very effective in treating fractures in companion animals. The choice in different plate and screw systems is vast and each system has its benefits and drawbacks. In veterinary practice, the use of a regular plate system in combination with a locking screw system usually is sufficient to adequately treat the majority of companion animal fracture patients.
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