Indications

Fenestration of the herniated disk space at the time of decompressive surgery is recommended to prevent further extrusion of disk material through the ruptured annulus fibrosus (AF) in the early postoperative period [1–13]. Reports documenting early recurrent disk extrusion at a decompressed site [5, 10, 14] do not support the previous claims that recurrent herniated disk material would likely move spontaneously outside of the canal through the laminectomy site and be of no clinical consequence. It is, however, possible that the previously made laminectomy offers some relief from spinal cord or nerve root compression soon after surgery which could explain why some dogs documented as having recurrent herniated material do not develop neurological deficits [5].

Prophylactic fenestration of unaffected, adjacent thoracolumbar (TL) disks has been shown to significantly reduce confirmed late recurrence in dogs undergoing prophylactic disk fenestration in the TL region [9, 11, 12]. Prophylactic fenestration should be considered in dogs with mineralized disks [10, 11, 15, 16] and dogs of breeds predisposed to recurrence (Dachshunds and French Bulldogs) even when disks are not mineralized [7, 9, 13, 15, 17].

Intervertebral disk (IVD) fenestration can be performed using a drill and burr, known as power‐assisted fenestration, or with a scalpel blade, known as blade fenestration (Figure 7.1) [8, 18]. The simple creation of a window within the lateral AF does not result in a path for any remaining disk material to herniate, nor do postoperative chiropractic bending maneuvers result in disk material being expelled through the fenestration site [8, 10, 18, 19]. Furthermore, studies assessing the fate of the NP following surgical disk fenestration have failed to document a significant inflammatory reaction that would support the remaining disk material being subsequently dissolved preventing recurrence, nor has it confirmed that the window remains open to offer an alternate path for future disk extrusion, since fibrocartilage fills the void created by fenestration soon after surgery [19, 20]. Rather, the effectiveness of fenestration is thought to be governed by the amount of NP removed at the time of surgery [19] and directly related to the skill and experience of the operator [21]. Despite skill, complete removal of the remaining NP is not expected when performing fenestration [5, 8] and is likely to result in removal of more NP from the ipsilateral side and less from the contralateral side of the IVD being fenestrated [22] (Figure 7.2). This author believes that either fenestration technique can effectively remove large amounts of disk material as long as the surgeon is knowledgeable of local anatomy and is comfortable with the technique used.

Though the lateral approach may increase the effective removal of disk material compared with the dorsal or dorsolateral surgical approaches by providing a better angle and working depth for fenestration [23], the approach used for decompression will dictate the approach used for concurrent fenestration [13].

Technique – Surgical Approach

Right‐handed surgeons typically find that TL fenestration is more easily performed on the left side of the spine [24], but since fenestration is most commonly performed with concurrent decompression, the approach will depend on the side of the lesion and the preferred surgical approach for decompression. When positioning the patient for decompressive surgery, a towel roll is inserted under the site of herniation to open the disk spaces on the side of surgery and to facilitate fenestration. The surgeon should review the diagnostic images for anatomical anomalies including missing or unusually shaped ribs or transverse processes in the area of interest.

The IVD is located ventral to the intervertebral foramen, immediately cranial to the rib head or the base of the transverse process. Fenestration of the affected disk space does not require much additional exposure beyond that performed for mini‐hemilaminectomy or hemilaminectomy (Figure 7.3). The loose connective tissues containing the spinal nerves and vessels that overlie the disk space are retracted cranially to expose the glistening annulus for fenestration. If prophylactic fenestration of additional disks is to be performed, the skin and lumbar fascia incisions may need to be extended cranially and/or caudally accordingly. Using deep digital palpation between the fascicles of the iliocostalis musculature, the desired disk spaces are individually located and exposed by identifying the rib head or the tip of the transverse process caudal to the disk of interest. A Metzembaum scissor or Kelly forceps is used to split the iliocostalis thoracis and lumborum muscles in an oblique direction along the muscle fibers (dorsal to the tip of the transverse process or just cranial to the rib head) allowing the area of the disk space to be digitally palpated (Figure 7.4). In the lumbar region, a periosteal elevator is then used to elevate the loose layer of fascia that covers the lateral annulus from the edge of the transverse process. Dissection should proceed from the base of the transverse process in a cranial direction exposing the fibers of the AF. Retraction of the deep muscle is most easily maintained using small tipped, right‐angled Gelpi retractors (Figure 7.5). Additional retraction can be obtained using a Frasier suction tip, which has the added advantage of keeping the small field clear of blood. The exposure obtained is small but allows excellent visualization of the lateral annulus for fenestration.

Fenestration of thoracic disks is slightly more difficult and offers less visualization. After separating the fibers of the iliocostalis muscle, which attach to the 13th, 12th^, 11th, and 10th ribs, an index finger is used to follow the rib to the level where it articulates with the vertebral body. Alternatively, the iliocostalis muscles can be transected close to their insertion on the associated ribs [25]. The levator costae muscles originate on the transverse process of thoracic vertebrae 1–12 and insert on the anterior surface of the rib caudal to each process. This muscle is separated using a blade or periosteal elevator and is retracted ventrally. Retraction of the epaxial muscles dorsally and of the levator costae muscle ventrally is best achieved using a Gelpi retractor or handheld retractors. Care is taken to prevent pleural puncture while separating the levator costae muscle and while inserting retractors. With experience, one can “tunnel” down [26] or create a keyhole access [27] to each disk space by palpating the transverse process or rib head to avoid excessive tissue dissection and trauma.

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