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Chapter 22: Intervertebral Disc Fenestration  193

               Cervical Disc Fenestration                         is made obliquely from the lateral aspect of the dorsal vertebral
               Patient preparation, patient positioning, surgical approach, and   spines in the thoracic region (T9) towards the ventral aspect of the
               surgical closure are identical to those described for the ventral slot   wing of the ilium, stopping at about L5 [1,51]. Alternatively, an inci-
               procedure (see Chapter 17). Identification of the disc space of inter-  sion that follows the transverse processes of the vertebrae of interest
               est is facilitated by palpation of the large and prominent transverse   is appropriate and should extend over one to two vertebrae cranial
               processes of C6 and the ventral process of C1. The C5–C6 disc   and caudal to the intended fenestration sites. When fenestrating
               space lies on the midline just cranial to the most cranial aspect of   only a few disc spaces on either side of the herniated disc, the surgi-
               the transverse processes of C6. Once exposed the origin of the   cal incision made for decompression can be enlarged accordingly.
               paired tendons of the longus colli muscles overlying the disc are   For fenestration, the skin incision extends through the subcuta-
               separated or transected with bipolar cautery, a #15 scalpel blade, or   neous fat layer and the lumbodorsal fascia allowing its retraction.
               Mayo scissors and the muscles are elevated using a periosteal eleva-  A deep layer of fat of variable thickness is encountered and is incised
               tor. Retraction is maintained using Gelpi retractors. Once exposed,   to reveal the epaxial musculature. Using deep digital palpation, the
               the ventral AF is fenestrated using a #11 scalpel blade. First, a rec-  appropriate disc spaces are located and exposed by identifying the
               tangular window of no more than 50% of the width of the vertebral   rib head or the tip of the transverse process caudal to the disc of
               body is created through the annulus [30]. The window is created by   interest. Metzembaum scissors or Kelly forceps are used to split the
               puncturing the disc with the blade on the side opposite the surgeon   iliocostalis thoracis and lumborum muscles in an oblique direction
               and advancing it through the AF from endplate to endplate. Then   along the muscle fibers (dorsal to the tip of the transverse process or
               the blade follows each of the cranial and caudal endplates over no   just cranial to the rib head) allowing the area of the disc space to be
               more than half the width of the endplate. Finally, the rectangle is   digitally palpated (Figure 22.3). In the lumbar region, a periosteal
               completed by advancing the blade from endplate to endplate at the   elevator is then used to elevate the loose layer of fascia that covers the
               opposite end of the rectangle. In small dogs, care is taken not to   lateral annulus from the edge of the transverse process. Dissection
               penetrate the spinal canal located deep to the dorsal annulus. The   should proceed from the base of the transverse process in a cranial
               excised annulus is then removed using small curved mosquito for-  direction exposing the fibers of the AF. Retraction of the deep
               ceps or rongeurs and the exposed NP is removed using curettes,   muscle is most easily maintained using small‐tipped, right‐angled
               spatulas, or dental scrapers. The instruments are carefully directed   Gelpi retractors (Figure 22.4). The exposure obtained is small but
               within the disc space in a dorsocranial direction following the ori-  allows excellent visualization of the lateral annulus for fenestration.
               entation of the disc space while being mindful of the location/depth   Fenestration of thoracic discs is slightly more challenging and
               of the spinal canal. If performing fenestration at a site of herniation   offers less visualization. After separating the fibers of the iliocostalis
               without concurrent decompression, care is taken that additional   lumborum muscle which attach to the 13th, 12th, 11th and 10th
               material is not forced dorsally into the spinal canal [49].  ribs, an index finger is used to follow the rib to the level where it
                                                                  articulates with the vertebral body. Alternatively, the iliocostalis
                                                                  lumborum muscles can be transected close to their insertion on the
               Thoracolumbar Disc Fenestration                    associated ribs [51]. The levator costae muscles originate on the
                                                                  transverse processes of T1–T12 and insert on the anterior surface of
               Preparation/Positioning                            the rib caudal to each process. This muscle is separated using a blade
               Right‐handed surgeons typically find that thoracolumbar fenestra-  or periosteal elevator and is retracted ventrally. Retraction of the
               tion is more easily performed on the left side of the spine [50] but   epaxial muscles dorsally and of the levator costae muscle ventrally
               since fenestration is most commonly performed with concurrent   is best achieved using a Gelpi retractor or hand‐held retractors.
               decompression, the approach will depend on the side of the lesion.
               An appropriate area of the thoracolumbar region relative to the
               location of the lesion and/or location of proposed fenestrations is
               clipped and prepared aseptically. The patient is positioned in ster-
               nal recumbency or oblique recumbency (sternal recumbency with
               45° rotation away from the surgeon using a sandbag or towel and
               tape) for the dorsal and dorsolateral approaches (see Figure 21.2).
               The lateral approach can be performed in lateral or oblique recum-
               bency with the surgical side facing up. The front limbs are tied cra-
               nially and the hind limbs tied caudally. A towel roll is frequently
               inserted under the thoracolumbar region to open the disc spaces on
               the side of surgery and to facilitate fenestration. The surgeon should
               review the anatomy to ensure there are no missing or unusually
               shaped ribs or transverse processes in the area of interest.

               Approach
               Lateral Approach [1,8,42,51]
               The animal is placed in lateral or oblique recumbency to visualize
               the left or right side of the spine. Placing a sandbag or rolled towel
               under the thoracolumbar area (perpendicular to the spine) opens
               the disc spaces on the side of surgery and allows a larger annular   Figure 22.3  Metzembaum scissors are used to split the iliocostalis lumbo-
               window to be created, facilitating curettage of the disc spaces. For   rum muscle in the direction of its muscle fibers (dorsal to the tip of the
               fenestration of all disc spaces between T11 and L4, the skin incision   transverse process of L1) to expose the annulus fibrosus.
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