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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