Page 27 - Zoo Animal Learning and Training

18 Section I: Diagnostics and Planning

advantages of the locking plate is its ability to provide superior
fixation in poor‐quality bone [32,33,38]. This characteristic may be
of great benefit in neurosurgery where the vertebral cortices are
thin, the bone is mostly cancellous, and sufficient bone stock for
traditional fixation is frequently lacking.
In human long bone fractures, it has been suggested that, because
of the fixed angle, sufficient stability could be achieved with the
exclusive use of monocortical locking screws. In dogs and cats, even
diaphyseal bones have thinner cortices than those of humans and
the exclusive use of monocortical locking screws must be consid-
ered carefully. Additionally, results obtained in long bone research
may not be directly applicable to vertebral bone in which cancellous
bone predominates. In human and canine spinal fixation the use of
bicortical implants has been shown to provide superior stability
compared with monocortical implants [14,39]. In dogs, however,
accurate placement of bicortical implants in the spine can be chal-
lenging and carries a high risk of spinal canal penetration [40,41].
For this reason, the use of monocortical locking screws may be ben-
eficial and has recently been investigated and used effectively to
stabilize spinal segments in vivo and ex vivo [41–47]. When using
monocortical locking screws, the longest screw possible should be
used to maximize bone purchase into the cancellous bone [13,39].
Although locking plates have significant advantages over tradi-
tional plating in neurosurgery, there are also disadvantages that are
worth noting. When using locking plates, there is limited to no abil-
ity to orient the screws and therefore appropriate plate design and
contouring is necessary for appropriate fixation. Contouring of
small and complex plates could be a difficult task and may result in Figure 2.14 Lateral radiographs and CT reconstruction of String of Pearls
inappropriate screw placement. Because of the extreme variability (SOP) plate system used for stabilization of a vertebral fracture in a dog.
in body shape and size of our veterinary patients, custom‐designed Source: Courtesy of Dr. Laurent Guiot.
plates that would fit all breeds is not a realistic concept and plates
appropriate for one animal may not be appropriate for another.
Some plates of the variable‐angle design provide more freedom Summary
when it comes to screw orientation; however, the insertion angles The recent years have seen the development of new surgical
remain limited and the screw–plate interface has been shown to be implants and surgical techniques. Each implant comes with its own
significantly weaker than that of fixed‐angle plates [48]. Another intricacies and technical requirements that makes each of them
disadvantage is that although locking plates provide superior hold- unique. A sound understanding of each implant’s capabilities and
ing in poor‐quality bone, if they fail they generally do so cata- shortcomings will allow the surgeon to successfully adapt and
strophically by cutting through the bone or by fracturing a large design their fixation to fit each specific situation, even if the implant
segment of bone [32,49]. This could be highly problematic in neu- may not have been originally designed for that purpose. Great
rosurgery and may significantly complicate any eventual revision attention to detail is paramount in neurosurgery as the fixations are
surgery. often technically difficult due to the unique nature of the axial skel-
eton. Small technical errors in implant selection or their application
String of Pearls™ can easily jeopardize the final outcome. Many of these errors can go
String of Pearls™ (SOP; Orthomed UK Ltd, Halifax, West Yorkshire, unnoticed during surgery and are difficult to detect on postopera-
UK) is a specific type of locking plate composed of a series of spher- tive radiographs. Familiarity with each implant system and a sound
ical nodes, linked together by thick cylindrical internode sections. knowledge of their unique capabilities is essential to ensure consist-
What makes the SOP different from other locking plates is that the ent, positive results.
nodes accommodate and provide locking capabilities for regular
cortical bone screws as opposed to specifically designed locking
screws. The plate can be cut to length and can easily be contoured Video clips to accompany this book can be found on
in multiple planes at the level of the internodes, giving great free- the companion website at:
dom to orient the screws in multiple directions in order to match www.wiley.com/go/shores/neurosurgery
complex bone surfaces. Biomechanically, the SOP compares favora-
bly to several other locking systems [50,51]. The regular bone
screws, with their smaller core diameter than typical locking screws,
appear to be the weakest point of this system and may explain why References
the SOP does not outperform other locking systems [50,51] despite 1 Bennett RA, Egger EL, Histand M, Ellis AB. Comparison of the strength and holding
being a stronger implant in itself [52]. The SOP has been used suc- power of 4 pin designs for use with half pin (type I) external skeletal fixation. Vet
Surg 1987;16:207–211.
cessfully in neurosurgery for stabilization of vertebral fractures and 2 Palmer RH, Aron DN. Ellis pin complications in seven dogs. Vet Surg 1990;19:
spinal instability [53] (Figure 2.14). 440–445.