Thoracolumbar decompression techniques in canine and feline neurosurgery

Thoracolumbar spinal decompression procedures in veterinary neurosurgery are performed to alleviate pressure on the spinal cord in the mid-to-lower back region, often caused by intervertebral disc herniation, but also by tumors, fractures, or foreign bodies. The objective is to facilitate neurological recovery and improve patient outcome by removing the compressive material. Various surgical approaches are employed based on the specific location and nature of the lesion, each designed to maximize visualization and access while minimizing iatrogenic trauma and postoperative instability.

Key Thoracolumbar Decompression Techniques

1. Hemilaminectomy Hemilaminectomy is a conventional surgical technique for thoracolumbar spinal decompression.

• Approach: This procedure typically involves a dorsolateral approach, where a skin incision is made 1–2 cm lateral to the dorsal midline, extending one to two vertebrae cranial and caudal to the lesion. The epaxial musculature, including the multifidus muscle, is carefully dissected and reflected laterally from the spinous processes.
• Bone Removal: A high-speed surgical drill is utilized to create a precisely defined bone window, removing the outer cortical layer and cancellous bone of the lamina to expose the thin inner cortical bone. This opening is then enlarged using rongeurs (e.g., Lempert or Kerrison rongeurs). The hemilaminectomy defect should extend at least one vertebral body length cranial and caudal to the affected intervertebral disc (IVD) and ventrally to the floor of the spinal canal to ensure complete visualization and removal of extruded disc material.
• Disc Material Removal: Extruded IVD material, often accompanied by necrotic epidural fat and hemorrhage, is carefully removed from the hemilaminectomy site using blunt probes, curettes, or spatulas. Extreme caution is exercised to avoid spinal cord damage. Hemorrhage, particularly from the ventral venous sinuses, can be managed with absorbable gelatin sponges or direct pressure.
• Limitations: Dorsal laminectomy (which differs from hemilaminectomy) is generally disfavored for thoracolumbar IVD herniation because it does not allow for direct removal of ventrally located extruded disc material without significant spinal cord manipulation. In some cases, residual disc material (up to 100%) may be present post-hemilaminectomy, though it is often not clinically significant.

2. Pediculectomy / Mini-Hemilaminectomy These terms are often used interchangeably and represent less invasive approaches compared to a full hemilaminectomy.

• Indications: This technique provides adequate visualization of the ventrolateral aspect of the vertebral canal, offering excellent access for retrieving ventrally or laterally extruded disc material with limited spinal cord manipulation. It is effective for decompression from T10 to L6.
• Approach: A dorsolateral or lateral approach is typically used. The modified dorsolateral approach involves incising through the longissimus muscle fibers directly over the intervertebral foramen, minimizing overall muscle dissection and providing direct access for ventral drilling and IVD fenestration.
• Bone Removal: The pedicle bone is thinned using a high-speed drill, noting the transition from cortical (white) to cancellous (red) bone. The accessory process overlying the dorsal aspect of the foramen is often removed to form the dorsal extent of the mini-hemilaminectomy. The window is enlarged over approximately half to two-thirds the length of each vertebra, extending ventrally as much as possible to the intervertebral foramen/spinal canal.
• Advantages: Pediculectomy is reported to be quicker, cause less tissue trauma, and result in less vertebral instability compared to hemilaminectomy, potentially leading to more rapid postoperative recovery. Preservation of the majority of articular processes reduces postoperative vertebral instability. It can also provide direct access for disc fenestration.
• Considerations: This window is created close to the vertebral venous plexus and foraminal structures, requiring care to prevent hemorrhage and nerve root damage. A partial pediculectomy creates an even smaller window limited to the pedicle bone, reducing hemorrhage risk, but may be too small for extensive lesions or complete disc removal, sometimes requiring extension to a mini-hemilaminectomy.

3. Thoracolumbar Lateral Corpectomy (TLLC) TLLC is a procedure primarily indicated for chronic, ventrally located disc protrusions or extrusions, where the material may be hardened or adhered, making access difficult via standard dorsal approaches.

• Approach: A ventrolateral surgical approach is used, aiming to access the disc material directly from the side and slightly towards the front of the vertebral body. This approach involves creating a "slot" perpendicular to the long axis of the spine through the lateral aspect of the vertebral body.
• Bone Removal: The slot typically extends one-quarter of the length of each vertebral body (craniocaocaudally), half the height of the vertebral body, and half to two-thirds of the vertebral canal diameter in width. A high-speed drill is used, with continuous saline irrigation and suction to manage bone dust and heat. The dorsal longitudinal ligament is exposed, and in cases of disc extrusion, it is excised to retrieve the disc material.
• Advantages: TLLC provides direct access to the ventral aspect of the spinal canal, allowing removal of disc material with minimal manipulation of the spinal cord. It is considered the technique of choice for chronic lateralized thoracolumbar disc disease and has shown good clinical outcomes with a low complication rate.
• Stability: TLLC can increase range of motion during lateral bending on the corpectomy side (up to 30%). If combined with mini-hemilaminectomy, instability is not significantly worsened, but it should be avoided with full hemilaminectomy without spinal stabilization. Multiple adjacent TLLC procedures can be performed safely.
• Complications: Potential complications include nerve root injury, which can lead to abdominal wall paralysis, though this typically resolves within 1–3 months. Hemorrhage from the venous sinus can occur, especially if drilling from the lateral aspect of the vertebra inward.

4. Dorsal Laminectomy (Thoracolumbar) This procedure involves removing the dorsal lamina of the vertebrae and is less commonly used for intervertebral disc herniation due to limited ventral access.

• Indications: Dorsal laminectomy is used for diseases causing dorsal, lateral, or ventral compression of the spinal canal, including intervertebral disc disease, fractures, neoplastic processes, and cysts.
• Approach: A dorsal midline skin incision is made, and epaxial musculature is reflected laterally. The spinous processes of the affected vertebrae are removed with bone rongeurs, and the laminectomy is performed using a high-speed surgical air drill.
• Classifications: Dorsal laminectomies are classified by the extent of bone removed:
• Funkquist A: Removal of the spinous process, cranial and caudal articular processes, dorsal laminae, and pedicle to the level of the spinal cord.
• Funkquist B: Removal of the spinous process and dorsal lamina, preserving both cranial and caudal articular processes, providing limited access.
• Modified Dorsal Laminectomy: The most common technique, removing an intermediate amount of lamina, preserving cranial articular processes but removing caudal ones.
• Deep Dorsal Laminectomy: Removal of articular processes, dorsal lamina, and pedicles to the ventral aspect of the vertebral canal; rarely performed due to instability.
• Considerations: Hemorrhage from bone (diploic hemorrhage) is common and controlled with bone wax. Maintaining hemostasis is crucial to prevent hematoma and seroma formation, which can lead to secondary spinal cord compression.

Ancillary Procedures

Intervertebral Disc Fenestration Disc fenestration involves the mechanical removal of the nucleus pulposus (NP) through a window created in the annulus fibrosus.

• Technique: It can be performed using an air drill (power-assisted fenestration) or a scalpel blade (blade fenestration). Power-assisted fenestration generally removes a higher percentage of the NP compared to blade fenestration. The lateral approach is considered to increase the efficiency of the procedure.
• Purpose: Fenestration is advocated to reduce the rate of early and late recurrence of disc herniation. Prophylactic fenestration of unaffected adjacent thoracolumbar discs has been shown to significantly reduce recurrence rates. For cervical disc herniation, fenestration of the affected disc space is always performed during a ventral slot procedure.
• Caution: Fenestration alone is not recommended for known herniated discs, as it can lead to neurological deterioration if disc material is forced into the spinal canal.

General Surgical Instrumentation and Principles

Neurosurgical procedures rely on specialized instrumentation and meticulous technique:

• Access and Visualization: Pneumatic or electric drills with various burr sizes are used to access the spinal canal. Surgical loupes or operating microscopes provide magnification for enhanced visualization. Self-retaining retractors (e.g., Gelpi, Weitlaner) are crucial for tissue retraction, though care must be taken to avoid neurovascular damage. Periosteal elevators are used to elevate soft tissues from bone.
• Hemorrhage Control: Monopolar and bipolar electrosurgical instruments are used to control bleeding. Bipolar cautery is preferred near delicate neural structures due to limited lateral thermal injury. Bone wax is used as a mechanical sealant for trabecular bone hemorrhage, but sparingly, as it can inhibit bone healing and promote infection. Gelatin sponges can also be used for hemostasis but should be removed where possible to prevent complications like granuloma formation or temporary neurological decline.
• Bone Work: Burring is continued to the level of the inner periosteum, confirmed by palpation with fine blunt-tipped probes (e.g., iris spatula). Once thin, the inner cortical bone/periosteum is incised with a bent hypodermic needle or scalpel blade to enter the spinal canal. Rongeurs and curettes are used to enlarge the defect and remove disc material.
• Implants and Stabilization: For spinal stabilization, orthopedic implants such as pins, screws, and bone plates are utilized, often in conjunction with polymethylmethacrylate (PMMA) bone cement.
• Pins and Screws: Pin-bone and screw-bone interfaces are critical for fixation stability. Proper insertion techniques, including low-speed drilling and irrigation, are essential to prevent thermal necrosis of bone. Positive-profile threaded pins and cortical screws generally offer better pull-out strength and stiffness. Precise measurement of bone engagement is necessary to avoid over-penetration and damage to vital structures. Notching pins or using specific screw designs can improve the bond with PMMA.
• Bone Cement (PMMA): PMMA is a strong, lightweight bone cement used to stabilize implants. It sets through an exothermic reaction, so preventing excessive heat near the spinal cord is paramount. PMMA provides stability by creating a mechanical interlock with implants rather than true adhesion.
• Locking Plates: Locking plate systems rigidly couple screws to the plate, providing angle-stable constructs. They do not require intimate bone contact for stability and are beneficial in poor-quality bone. The String of Pearls™ (SOP) plate is a versatile locking plate that accommodates regular cortical bone screws. Locking Compression Plates (LCP) also offer fixed-angle screw trajectories. While offering superior fixation, locking plates can fail catastrophically by cutting through bone.
• Dural Closure: After decompression, if the dura mater is incised, watertight closure is recommended using fascial or synthetic grafts; however, non-closure is sometimes acceptable if precautions against infection and pneumocephalus are taken. Fat grafts or absorbable gelatin sponges may be placed over the surgical site to deter scar tissue formation, though their efficacy and safety are debated.

Postoperative Care and Rehabilitation

Postoperative management is crucial for optimal recovery.

• Pain Management: Tailored pain relief, including systemic analgesics and potentially topical epidural analgesics, is provided.
• Supportive Care: Maintaining a clean, dry, and padded environment, regular repositioning, and meticulous management of bladder and bowel function (e.g., manual expression, indwelling urinary catheters) are essential, especially for non-ambulatory patients.
• Nutrition: Postoperative nutritional supplementation may be necessary if anorexia is prolonged.
• Monitoring and Complications: Close monitoring of neurological status is vital. Deterioration may warrant repeat imaging to rule out residual compression, hematoma, or ascending/descending myelomalacia, which has no known treatment and carries a grave prognosis. Surgical site complications like seroma formation or infection are managed with appropriate treatment, potentially including drainage or antibiotics.
• Physical Rehabilitation (Neurorehabilitation): This begins as soon as the patient is comfortable and plays a key role in functional recovery by leveraging neuroplasticity.
• Principles: Rehabilitation aims to "rewire" neural pathways to promote positive neuroplasticity and prevent maladaptive changes (e.g., dragging limbs, bunny-hopping, kyphosis).
• Exercises: Includes passive range of motion (PROM) exercises, assisted-active exercises, and active exercises, sometimes using specialized equipment like underwater treadmills. Exercises such as flexor stimulation, tapping to stimulate reflexes, brushing/scratching, sit-to-stand exercises, and weight shifting are employed.
• Confinement: Strict cage confinement, typically for 4 to 8 weeks, is enforced to allow for initial healing and bone fusion, especially after spinal fracture/luxation stabilization. Gradual return to activity is managed carefully.

Permanent Disabilities: For patients with permanent neurological disabilities, owners are educated on long-term care, environmental adaptations, and assistive devices like carts and harnesses to ensure quality of life.