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212  Section III: Spinal Procedures

           offering another point of fixation in cases where good vertebral body
           bone stock is not available. Screw orientation must take into consid­
           eration the path of the transverse foramen.
            In the case of cervical vertebral luxation, fixation of the adjacent
           vertebrae is often sufficient. In the case of cervical vertebral frac­
           ture, fixation may need to span the affected vertebra.
            Prior to application of PMMA, local resection of part of the lon­
           gus colli musculature improves ease of cement application. If multi­
           ple vertebrae are spanned, Kirschner wires can be used to reinforce
           the PMMA by gently bending them around screws or connecting
           them with small‐gauge cerclage wire. Twenty grams of PMMA is
           often sufficient to create a uniform cement bar around protruding
           screw heads and proximal threads if fixation spans two vertebrae.
           The cement mantle should be 1–1.5 cm thick. Increasing the height
           of the PMMA bar beyond the level of the longus colli muscles is not   Figure  25.5  Two String of Pearl (SOP) plates are applied to the ventral
           recommended as protruding cement can apply pressure on esopha­  aspect of the cervical spine with monocortical screw fixation. Note how the
           gus or trachea and may lead to difficulty in swallowing and respira­  plates are not contoured to the vertebral bodies but are locked in place by the
           tory signs.                                       locking screw mechanism.
           Vertebral Body Plates                             the plate. As a general rule, SOP plates should be applied with the
           The main benefit of plate fixation of the vertebral column is the   least contouring possible. Multiple vertebral bodies can be spanned
           decrease in implant profile and ease of closure after instrumenta­  with the SOP plate and two plates can be placed next to one another
           tion. However, traditional plate fixation can be challenging in the   to increase freedom with screw placement and strengthen the fixa­
           cervical vertebral column. Implant stability of nonlocking plates   tion construct (Figure 25.5).
           relies on friction between the plate and bone; this is provided by   The ventral aspect of the affected cervical vertebral bodies is
           screw tension on a well‐contoured plate with excellent bone con­  exposed and the appropriate size and length of SOP plate is selected
           tact. Good contouring with appropriate bone contact is difficult to   (3.5 mm for large, 2.0–2.7 mm for small to medium‐sized dogs). Care
           achieve along the vertebral column due to the complex vertebral   must be taken to avoid inadvertent screw placement into an interver­
           anatomy. In addition, with regular plates bicortical screw purchase   tebral disc. If screw hole spacing does not fit as desired, a different size
           is preferred to prevent premature loosening of screws, which poses   SOP can be considered or a second SOP plate can be applied adjacent
           a problem in light of the high risk of possible injury with bicortical   to the first. Dedicated drill guides are required for proper screw place­
           implant placement. Monocortical screw placement can be per­  ment to engage the locking mechanism. This will place screws per­
           formed with regular plates; however, any loosening of screws would   pendicularly to the plate into bone. Monocortical screw purchase is
           lead to implant failure, as friction would be lost.  sufficient and length of screw is determined via depth gauge. During
            The development of locking plates offers a solution to using mono­  depth measurement, the SOP plate must be held securely, with some
           cortical screw placement in traditional plates that rely on friction   pressure and in proper position, since once engaged in the plate screws
           [2,4]. Different locking mechanisms rigidly couple screws to the   will not pull the plate toward the bone. Once a screw is placed cranially
           plate, thereby eliminating the potential for loosening of the plate–  and caudally, the plate is fixed in position and all subsequent screws
           screw interface. Failure of locking plates typically occurs by shearing   must follow the fixed trajectory to be properly locked.
           of the screws or by pull‐out of the plate–screw construct from the
           bone. Because of the locking mechanism, perfect contouring of the   LCP Application
           plate to bone is unnecessary (provided that the gap between bone and   The LCP has hybrid holes allowing for placement of traditional corti­
           plate is restricted to a few millimeters) allowing for application of a   cal screws as well as locking screws. For use along the vertebral col­
           straight plate over the uneven vertebral body surface.  umn, only locking screws should be used. The locking portion of the
            Two commonly used locking plate systems for vertebral fixation   LCP screw holes have threads into which the threaded head of locking
           in veterinary medicine are String of Pearls™ (SOP; Orthomed UK   screws lock. Special threaded drill guides are used to ensure proper
           Ltd, Halifax, West Yorkshire, UK) and Locking Compression Plate   position of the drill hole and subsequent screw and avoid cross‐
           (LCP; DePuy Synthes, West Chester, PA). Both plate types have a   threading. The locking screws specific to the LCP are self‐tapping and
           fixed screw trajectory to ensure proper engagement of the locking   have a larger core than regular cortical screws. It is therefore important
           mechanism; this trajectory places screws perpendicularly to the   to use the appropriate diameter drill bit for the specific size plate used.
           plate. This can pose technical challenges when gaining experience   Since locking screws are used, there is no need for a high degree of
           with such an implant type as plates are fixed in position as soon as   contact between plate and bone. The locking screws in the LCP are
           the first screw is engaged and locked, making subsequent screw tra­  fixed trajectory and are placed perpendicularly to the plate. Careful
           jectory changes difficult. Planning, positioning and exact screw   preoperative planning and choosing the appropriate implant size and
           placement are extremely important to avoid a malpositioned lock­  length are important to avoid malpositioned screws (Figure 25.6).
           ing plate with inappropriately placed screws.
                                                             Other Locking Plates
           SOP Plate Application                             Other veterinary locking plate systems can be applied abiding by
           Because of its round core, the SOP plate can be contoured in every   the same principles of application. Human spinal locking plates can
           direction, making this a very versatile implant. However, with every   be utilized in veterinary medicine but are unfortunately prohibitively
           contouring, screw trajectories change, especially toward each end of   expensive. The design of these plates allows for two screws to be
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