section epub:type=”chapter” role=”doc-chapter”> Bianca F. Hettlich Vertebral column trauma often results in unstable injuries such as fractures and/or luxations. The most commonly injured area requiring surgical stabilization is the thoracolumbar spine, particularly the thoracolumbar (TL) and lumbosacral (LS) junctions. Vertebral luxation without fracture usually occurs in a ventral direction, resulting in damage to intervertebral disks and ligaments. Luxation may also occur laterally with concurrent articular facet fractures. Hyperflexion coupled with compression often results in vertebral body and endplate fractures with cranioventral displacement of the caudal fracture fragment. Whether surgical stabilization is indicated depends on several factors such as patient age and weight, neurologic status, progression of neurologic deficits, pain caused by the injury, and degree of spinal instability. The latter can be difficult to discern radiographically. To provide a more objective evaluation, the surgeon can classify the injury according to the three‐compartment classification. This method divides the vertebra into dorsal, middle, and ventral compartments and proposes that significant instability is present if two or three compartments are affected (Figure 19.1). Figure 19.1 Illustration of the three‐compartment classification depicting the ventral, middle, and dorsal compartment of canine vertebrae. (Source: Created by Tim Vojt.) Rigid fixation methods described for the spine include a variety of techniques, including pins or screws and polymethylmethacrylate (PMMA), vertebral body plates, and external skeletal fixation. The most substantial anchorage of implants is achieved by placement into the vertebral bodies. For this, a dorsolateral to ventromedial trajectory is required unless implants can be placed directly laterally on the vertebral body. While fixation with pins and PMMA provides a high degree of versatility and freedom in placement of pins, this technique introduces a large amount of foreign material (PMMA) into the paraspinous musculature, making closure difficult. Once in place, adjustments to the construct can only be made by removing the PMMA. Furthermore, curing of PMMA causes an exothermic reaction that may damage surrounding tissues. By contrast, plate fixation eliminates these disadvantages and can provide strong, low‐profile stabilization. Compared to standard plates, locking plates do not require close bone contact, which makes perfect plate contouring unnecessary. Due to their angle‐stable screw mechanism, locking plates can be applied with monocortical screws without significant loss of stiffness, as demonstrated in long‐bone models. Both these qualities make locking plates very attractive for use in the vertebral column where contouring can be difficult and bicortical implants may pose risks to neurovascular structures. While locking plates can be applied to all parts of the spine, anatomic features and alignment can make their usage challenging. In these cases, despite the benefits of locking plates, alternative fixation methods (i.e. fixations with pins and PMMA, standard plates, polyaxial locking plates) should be considered, which allow easier adjustments in implant insertion location and angle. Examples include areas with physiologic changes in alignment such as the TL junction and malalignment due to deformities or nonreducible fractures. Scientific literature on the use of locking plates in the canine and feline TL vertebral column is scarce. Clinical application of various locking plates has been described mainly for cervical vertebral stabilization, while in vivo comparison studies between locking plates and other fixation methods in the TL spine have not been reported. Description of locking compression plate (LCP) and string of pearls (SOP) use for TL vertebral column injuries is limited to a few cases [1–3]. While clinical publications on the use of titanium alloy plate systems in the TL spine are lacking, plates such as Advanced Locking Plate System (ALPS), UniLock, Polyaxial (PAX) Advanced Locking System, titanium reconstruction locking plates, and the recently introduced titanium SOP plate would have the advantage of improved MRI compatibility as titanium produces less artifact when compared to stainless steel. Only one veterinary study has evaluated the in vitro biomechanical properties of a locking plates in comparison to pin‐PMMA fixation in the canine cadaveric lumbar spine [4]. Bilateral bicortical pin/PMMA fixation with a total of four pins was compared to unilateral monocortical LCP fixation with a total of four screws. Results of this cadaveric study showed that pin/PMMA fixation was significantly stiffer and stronger and that for most testing directions, unilateral monocortical LCP fixation was only as strong as the intact spine. The authors of this study therefore recommend that LCP is only used in this particular configuration for inherently stable spinal injuries. There are no studies evaluating the biomechanical properties of LCP fixation in other configurations (i.e. unilateral with more screws, bilateral, bicortical screws), nor is there published biomechanical testing of other types locking plates in the TL spine. Recently introduced PAX systems have been evaluated in vitro and in vivo for human and veterinary appendicular fractures. Unlike fixed‐angle locking implants, these plate systems allow insertion of screws at varying angles. Depending on the system, screws are locked via different mechanisms such as expanding bushings, locking caps, or by cutting of the screwhead into the plate‐hole metal. The ability to angle screws during spinal fracture repairs could offer considerable advantages, including greater flexibility of implant placement and an enhanced ability to avoid nervous tissues. However, their use has not been described for TL fractures in humans and there are no published reports on the use of these plates in veterinary TL surgery. Orthogonal radiographs are used to provide an overview of the injury. Since relying on radiographs alone can cause a surgeon to miss vertebral column injuries and compression within the spinal canal, CT or MRI are indicated for complete evaluation. Preoperative CT is also helpful in assessing patient specific anatomy and planning plate and screw location. Once the extent of the injury has been documented, degree of suspected instability is determined and the fixation construct chosen. Commonly used sizes for locking plates are 2.0 or 2.4 mm for small dogs and cats, 2.7 mm for medium, and 3.5 mm for large‐breed dogs. Preplanning screw location with the chosen locking plate type and size is extremely important, as the angle‐fixed screw position can easily interfere with the intervertebral disk spaces. Due to screw hole spacing within the plates, only two screws can usually be placed per vertebral body. The ideal implant stiffness required for appropriate vertebral column immobilization in cats and dogs is not known. The surgeon will have to make decisions on implant stiffness requirements based on each individual patient. Apart from size and shape of the affected vertebrae, a major factor will be degree of instability of the injury. With relatively stable injuries, unilateral plate fixation with two screws in adjacent vertebral bodies may be sufficient. With unstable injuries, unilateral locking plate fixation should span two vertebrae cranial and caudal to the lesion to assure sufficient points of fixation, or bilateral plate fixation should be used. This is expected to improve upon the reported biomechanical disadvantage of unilateral LCP fixation over bilateral pin/PMMA fixation in a cadaveric canine lumbar spine injury model [4]. It has been demonstrated that fixation with multiple monocortical locking screws has similar stiffness to bicortical fixation using nonlocking plates and cortical screws. While this is an important benefit of locking plates, it has only been assessed biomechanically in long bones, where cortical bone is more substantial compared to vertebral body bone, and multiple screws can be placed per segment to achieve the desired cortical purchase points. By contrast, vertebral cortices tend to be thin with soft medullary bone. Monocortical screw fixation is very attractive along the vertebral column, but it has not been sufficiently assessed biomechanically in the TL spine in cats or dogs. The surgeon will have to consider the possible implications of reduced stiffness with a monocortical fixation in addition to the planned fixation points cranial and caudal to the injury. If easily modified and achieved without violation of the vertebral canal or injury to adjacent ventrolateral perispinous vascular structures, bicortical screw purchase should be considered. Depending on the surgeon’s preference, the TL spine can be approached via a dorsal, dorsolateral, or lateral approach. The position of the animal is adjusted for the specific approach: ventral recumbency for dorsal, slightly lateralized for the other approaches. For unilateral plate application, the dorsolateral and lateral approach allows for improved access to the vertebral bodies and decreased soft issue interference during drilling and placement of screws. For bilateral plate fixation, a dorsal approach is chosen to provide access to both sides of the vertebral column. Vacuum bags or other holding devices and tape are used to maintain the desired position of the animal on the operating table. It is important to account for the change in vertebral position in case of oblique lateral patient position when applying implants at specific angles. With the animal in a straight ventral recumbent position, desired vertical or horizontal insertion angles can be more easily determined. During the approach to the affected TL segment, care must be taken to avoid iatrogenic injury by aggressive manipulation. The approach extends ventrally to expose the rib heads of the thoracic or the base of the transverse processes of the lumbar spine. Utilizing reduction forceps on spinous processes, careful traction and manipulation is performed to reduce vertebral subluxation and reestablish alignment. Comminuted vertebral body fractures or older injuries can be very difficult to manipulate, and emphasis may shift from realignment to establishing stability, which can impact the ability to apply plates for fixation. Temporary reduction can sometimes be maintained with transarticular K‐wires, which can be placed individually through each articular process or translaminar from one process through the dorsal lamina into the other. Kyphotic malalignment may not be reduced entirely this way, and further use of reduction forceps may be necessary.
19
Spinal Fractures and Luxations
19.1 Preoperative Planning
19.2 Approaches to the Vertebral Column
19.3 Reduction
19.4 Locking Plate Application
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