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214 Section III: Spinal Procedures

recommended for improved pull‐out resistance [6]. Pins with a pedicle feeler to probe the pin path within vertebral pedicle. The
blunt tip may reduce the risk of injury to adjacent soft tissues. angle of the probe should reflect the planned insertion angle as long
Fixation pins are preferred over screws in a PMMA construct as as presurgical planning for insertion landmark and angle were done
their larger core diameter affords them increased stiffness [7]. The appropriately. Because of the proximity of major vessels ventrolater­
pin–PMMA construct also offers more insertion freedom in areas ally to the thoracolumbar spine, the drill bit should not be advanced
of challenging anatomy that may not lend themselves well to further once the trans‐cortex has been penetrated. A drill stop can
implants with fixed insertion angle such as locking plates. be used to avoid over‐penetration. The length of the fixation pin is
Conversely, plate fixation has a much lower implant profile, elimi­ determined by measuring the length of the drill hole with a depth
nating the need for soft tissue resection to accommodate PMMA gauge (Figure 25.7). Depth gauge length should be similar to depth
and allowing normal closure of the surgical site. Also, removal of measure on preoperative CT images as long as pin location and
implants is significantly easier when PMMA is not used. insertion angles are similar. The depth gauge can also be used to
Depending on the type of injury (luxation versus fracture) and probe the walls of the drill hole. Intact bone should be felt in every
degree of instability, fixation may be limited to one adjacent verte­ direction when probing. Pins can be marked with sterile marker or
bra cranially and caudally (inherently stable fracture or luxation) or carefully notched to identify to which point they will be inserted.
include two on each side of the injury (unstable fracture or luxa­ Large dogs may require extended‐length positive‐profile pins to
tion). Fixation can be performed unilaterally or bilaterally depend­ allow threads to engage along the entire length of vertebral body
ing on desired degree of stiffness, anatomical considerations, or the bone. Slow‐power insertion of the pin is then performed to the pre­
selected implant type (i.e., the use of a locking plate with predeter­ determined depth. The entire trocar or blunt tip of the pin should
mined screw holes may require bilateral plate placement to engage be advanced to ensure full engagement of the threaded portion
a sufficient number of screws per segment). within the trans‐cortex (Figure 25.7).
The remaining pins are planned, predrilled, and placed in a simi­
Reduction lar manner. Each pin will have its individual insertion landmarks
Thoracolumbar fractures caused by hyperextension typically pre­ and angles, and insertion depth may vary. Orientation of pins can
sent with ventral and possibly cranial subluxation of the affected center around the injury (cranial pins inserted in a caudal to cranial
caudal vertebral segment. Reduction is aimed at reducing the sub­ direction, caudal pins inserted in a cranial to caudal direction) to
luxation and improving alignment. Point‐to‐point reduction for­ decrease the overall area of cement coverage. Pins do not need to be
ceps can be clamped on spinous processes to apply careful dorsal bent unless incorporation into bone cement is difficult due to the
and slight caudal traction of the caudal segment until alignment insertion angle. Bending pins can be challenging due to limited
appears normal. Manipulation must be performed with care so as space and has the potential for pin loosening and iatrogenic bone
not to further damage the spinal cord. Chronic injuries tend to be damage; it should be considered carefully prior to performing. Pins
challenging to reduce and one has to weigh the benefits of reduc­ are cut short, with 15–20 mm of pin protruding from the bone to be
tion against the potential for further spinal cord damage. In a incorporated into bone cement. Notching of the protruding pin
patient with mild subluxation on preoperative imaging, manipula­ ends is often not necessary as pins are generally placed in slightly
tion can aid in assessing overall stability of the injured interverte­ different angles, making cement loosening around the pins unlikely.
bral articulation. If articular processes are intact, a small K‐wire can Notching can otherwise be carefully performed with pin cutters. In
be placed transarticularly across the dorsal lamina into both facet cases where the threaded portion of the pin extends into the
joints to help maintain reduction and offer additional anchorage if PMMA, notching is not necessary. Muscles must be sufficiently
PMMA fixation is used. Landmarks for transarticular pin insertion reflected and Gelpi retractors adjusted to allow removal after
should be determined on preoperative CT to ensure that the K‐wire PMMA has been applied. At times, some muscle may have to be
is positioned in the dorsal lamina and does not violate the vertebral resected to allow room for PMMA. Reduction forceps can be used
canal. The ends of this K‐wire are gently bent to avoid migration. to maintain the unstable intervertebral articulation in proper posi­
If embedded into PMMA the ends should protrude far enough to tion while PMMA is applied and has hardened. If intact, tran­
allow incorporation into cement. If reduction is not easily achieved sarticular K‐wires can aid with reduction (Figure 25.8).
by gentle manipulation, a transarticular K‐wire may not suffice to Twenty grams of PMMA are mixed to a smooth, slightly runny
maintain reduction throughout the procedure; manual reduction liquid and poured into a 35‐mL catheter tip syringe to facilitate
with forceps may be required until implants have been applied. application around the pins. The PMMA is then applied around the
base of each pin, building upwards to cover the pin ends. Ideally the
Bicortical Pin and PMMA Fixation PMMA should be in the putty phase when applied around the pins
Based on preoperative CT planning, insertion points for each pin when it is still soft but will not spread and leak easily. During appli­
are confirmed using recognizable landmarks and other measure­ cation Freer elevators are used to keep the PMMA in the desired
ments. A goniometer is used to measure the predetermined angle of location. PMMA is applied in uniform thickness around and
pin insertion for that particular pin. Once location and angle are between all pins. Application has to be done efficiently to avoid
satisfactory, a drill bit of appropriate size is used to predrill for sub­ hardening of cement in the syringe; however, if PMMA is applied
sequent pin placement. Predrilling is essential to avoid thermal when too liquid, it is challenging to maintain cement around the
bone necrosis and premature pin loosening. During drilling atten­ pin ends appropriately. If a bilateral configuration is chosen, two
tion is paid to maintaining the desired angle of insertion, while both rows of PMMA are applied (depending on the size of the animal,
cortices are drilled (Figure 25.7). A drill bit with a sharp point at the 40 mg of PMMA may need to be utilized). Antimicrobials are not
tip (StickTite™; IMEX Vet Inc., Longview, TX) can be beneficial to routinely added to PMMA used for vertebral column fixation.
avoid drill bit slippage on the often steep outer cortical surface dur­ During curing of PMMA the surgical site is lavaged to decrease
ing initial drilling. In larger dogs, once the cis‐cortex has been thermal injury to soft tissues. Gelpi retractors are carefully removed.
drilled, a small K‐wire or other straight probe can be used as a The fascial layer is closed where apposition is possible. Typically,