Brigitte A. Brisson University of Guelph, Guelph, Ontario, Canada Hemilaminectomy and mini‐hemilaminectomy are considered the standard of care for removal of extruded disk material in the thoracolumbar region [1]. Additional indications for mini‐hemilaminectomy and hemilaminectomy include decompression of inflammatory lesions (e.g. discospondylitis/osteomyelitis), decompression following spinal trauma (e.g. fracture, hematoma, protrusion of bony fragments), removal of laterally or ventrally located tumors to address appropriately located subarachnoid diverticula, and for removal of foreign material. Hemilaminectomy removes the articular processes of two adjacent vertebrae as well as a portion of the associated pedicles (Figure 6.1a). Mini‐hemilaminectomy, also termed pediculectomy, consists of removing a portion of the pedicle bone of two adjacent vertebrae to enlarge the existing intervertebral foramen while preserving the articular processes [2–7] (Figure 6.1b). Although the partial pediculectomy technique described by McCartney [8] spares the accessory process (Figure 6.1c), the pediculectomy procedure removes the accessory process to form the dorsal margin of the laminectomy [4–6]. Removal of the accessory process has been shown to result in mild to moderate invasion of the ventral aspect of the articular processes in most dogs [9, 10]. The window provided by the mini‐hemilaminectomy is adequate to visualize the ventrolateral aspect of the vertebral canal and provides excellent access to retrieve ventrally or laterally extruded disk material while limiting intraoperative spinal cord manipulation [2, 4, 9, 10]. Although direct access to the dorsal aspect of the canal may be limited compared to hemilaminectomy [9], this was not shown to have an impact on the ability to retrieve disk material in a series of clinical patients [10]. Mini‐hemilaminectomy offers good visualization of the dorsal nerve root and ganglia and of the venous plexus located on the floor of the spinal canal [4, 9]. Preservation of the majority of the articular processes reduces postoperative vertebral instability compared to hemilaminectomy [11]. Effective spinal cord decompression can be achieved from T10 to L6 using this procedure [4]. The dorsolateral and lateral approaches used for mini‐hemilaminectomy and hemilaminectomy also allow direct access to the IVD for fenestration [2, 4, 8]. As with hemilaminectomy, the mini‐hemilaminectomy window is created close to the vertebral venous plexus (sinus) and foraminal structures, requiring care to prevent hemorrhage and nerve root damage [4, 10]. A mini‐hemilaminectomy can easily be converted into a hemilaminectomy or be extended over several adjacent vertebrae if required [4]. This author has performed continuous mini‐hemilaminectomies over as many as five contiguous vertebrae without complication. The latter is also reportedly possible with hemilaminectomy [5, 12]. Because mini‐hemilaminectomy does not significantly invade the articular processes, it can also be performed bilaterally without causing vertebral instability. This is, however, dependent on a portion of the pedicle being left intact (cranial and/or caudal to the pediculectomy window) to prevent disconnecting the dorsal lamina from the vertebral body [13]. Similarly, mini‐hemilaminectomy can easily be extended into a corpectomy if desired. Figure 6.1 Illustrations depicting the approach and bony defect of (a) hemilaminectomy, (b) mini‐hemilaminectomy or pediculectomy or foraminotomy, and (c) partial pediculectomy. Mini‐hemilaminectomy can be performed through a lateral or dorsolateral approach. An area caudal to the scapula and cranial to the wing of the ilium is clipped bilaterally but more so on the side of the lesion and is prepared for aseptic surgery. The patient is positioned in lateral (limbs toward the surgeon) [4, 8, 14, 15] or oblique (midway between sternal and lateral recumbency with the spine rotated away from the surgeon and with the affected side facing up) recumbency for a lateral approach [16] and in sternal [14, 15] or oblique [9, 10, 17] recumbency for a dorsolateral approach [16]. This author positions the patient obliquely for mini‐hemilaminectomy and uses a dorsolateral approach as it appears to provide the best surgical field and access under the facet joints (Figure 6.2). Figure 6.2 Oblique patient positioning (midway between sternal and lateral) with the spine rotated away from the surgeon and with the affected side facing up. A sandbag is placed behind the patient and under the table cover to maintain the oblique position. A rolled towel of appropriate size is placed under the patient’s spine at the level of the lesion to open the disk space and facilitate fenestration of the affected disk. In some patients, as shown here, a small sandbag or towel can be tucked under the abdomen in order to straighten the spine and further stabilize the patient. With the lateral and oblique position, the front limbs are tied cranially and the hind limbs are tied caudally with tape or ties. A small, rolled towel is placed under the patient, perpendicular to the spine at the level of the lesion to elevate the spine toward the surgeon, open the disk space and facilitate fenestration of the affected disk space. A small sand bag or towel is tucked under the abdomen in order to straighten the spine cranio‐caudally and to further stabilize the patient. When performing surgery in a sternal position, the hind limbs are flexed cranially to maintain the normal curvature of the spine. Review of the diagnostic images prior to surgery is essential to ensure there are no missing ribs or unusually shaped transverse processes that could confuse lesion localization. The thoracolumbar area is palpated to identify the last rib and grossly localize the site of herniation. With the dorsolateral approach, a skin incision is made toward the affected side, approximately 1–2 cm lateral to dorsal midline over the area of interest and extending over 1–2 vertebrae cranial and caudal to the lesion as required. The incision is carried through the subcutaneous fat and fascia to identify the thick lumbo‐dorsal fascia. Although some descriptions recommend undermining the fat on either side of the proposed incision through the lumbodorsal fascia to facilitate closure [15, 17], this is not necessary; it creates dead space and increases the risk of postoperative seroma formation. An incision through the thoracolumbar fascia exposes a second layer of fat of variable thickness. In the caudal thoracic region, the caudal border of the spinalis and semispinalis thoracis muscles must also be incised. Focal finger palpation between the fascicles of the iliocostalis musculature allows the surgeon to palpate and count the ribs and transverse processes for orientation (Figure 6.3). The last rib and transverse process of L1 are landmarks used to localize the lesion site. Once the desired space is identified, the intermuscular plane between the multifidus and longissimus lumborum musculature is identified and bluntly dissected leaving the attachments of the multifidus muscle along the articular processes intact. Once the bone of the pedicle is identified, the longissimus muscle is elevated with a periosteal elevator to expose the pedicle and the attachment of the tendon of the longissimus muscle to the accessory process. This tendon is transected using a blade or Mayo scissors; the author typically uses bipolar cautery to cauterize and separate this attachment. Gelpi retractors are used to provide retraction of the multifidus muscle dorso‐medially and the longissimus muscle ventrolaterally. Figure 6.3 Focal finger palpation between the fascicles of the iliocostalis musculature in a large breed dog undergoing mini‐hemilaminectomy allows the surgeon to palpate and count the ribs and transverse processes for orientation. Bitetto and Thacher [2] described a modified lateral decompression technique that used a dorsal midline approach like that described for hemilaminectomy (see hemilminectomy approach below). This approach has since been used and reported on by others [5–7]. While the dorsal approach would allow easy conversion to a hemilaminectomy or dorsal laminectomy if this was required, it lengthens the procedure time and increases tissue dissection and trauma and is not considered the approach of choice for mini‐hemilaminectomy by this author. This author uses a modified dorsolateral approach that incises through the longissimus muscle fibers, directly over the area of the intervertebral foramen of interest. This previously described approach [13] was also described in a case series [9, 18]. As per the other approaches, focal finger palpation between the fascicles of the iliocostalis musculature allows the surgeon to palpate and count the ribs and transverse processes for orientation (Figure 6.3). After exposing the epaxial musculature through a dorsolateral approach made 1–2 cm lateral to the dorsal midline on the side of the lesion, a #15 blade is used to create a focal incision and dissection plane along and through the fibers of the m. longissimus thoracis and lumborum. The incision is made midway between the articular processes and the rib heads or transverse processes (Figure 6.4 and Video 6.2). Figure 6.4 The incision is made midway between the articular processes and the rib heads or transverse processes in a large breed dog undergoing mini‐hemilaminectomy. Palpation is repeated to confirm accurate surgical site. Figure 6.5 The pedicle bone is identified and the incision is extended as required cranially and caudally using a combination of sharp dissection, periosteal elevation, and muscle retraction (Gelpi). Figure 6.6 Short jawed (1‐in.), right‐angled Gelpi rectractors are preferred for mini‐hemilaminectomies in small dogs.
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Thoracolumbar Decompression: Hemilaminectomy and Mini‐Hemilaminectomy (Pediculectomy):
Indications
Procedures
Technique: Surgical Approach for Mini‐Hemilaminectomy (Video 6.1)
Dorsolateral Approach [14–17] (Video 6.2)
Variation
Recommended Variation
Through this small incision, pedicle bone is identified and the incision is extended as required cranially and caudally using a combination of sharp dissection, periosteal elevation and muscle retraction (Gelpi) (Figure 6.5). In small dogs, the author uses two 1‐in. right‐angled Gelpi (Figure 6.6) for their low profile. The attachment of the tendon of the longissimus muscle to the accessory process is cauterized and transected using a blade, scissors, the sharp edge of the periosteal elevator, or preferably bipolar cautery. Gelpi retractors are used at either end of the incision to provide retraction of the dorsal portion of the longissimus muscle dorsally and of the remainder of the longissimus and iliocostalis muscles ventrally. Sometimes, a third Gelpi or a weitlaner retractor is added midway through the incision to improve visualization. With this approach, only the desired adjacent pedicle bones and a small portion of the rib head/base of transverse process are exposed (Figure 6.7; Video 6.3). Although this approach traumatizes the fibers of the longissimus muscle focally, it reduces the overall amount of muscle dissection required for exposure (faster) and leads to a smaller mass of muscle that must be elevated and retracted either ventrally or dorsally compared to the other approaches. This results in an overall smaller incision and is especially helpful in the lumbar area of larger dogs where dorsal retraction of the bulky iliocostalis muscle can be challenging [3]. By providing direct access to the site of surgery, this modified dorsolateral approach also facilitates ventral drilling for mini‐hemilaminectomy or corpectomy and provides a direct access to the intervertebral disk for fenestration.
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