Lumbosacral decompression and stabilization is a surgical approach aimed at treating Degenerative Lumbosacral Stenosis (DLSS), also known as cauda equina disease or syndrome and lumbosacral compression. The primary goals of this surgery are to decompress the cauda equina, release entrapped nerve roots, and in some cases, provide stabilization to prevent further collapse and degenerative disease.

Pathophysiology of DLSS DLSS is a common cause of pain and pelvic limb dysfunction in dogs, frequently observed in military working dogs. Clinical signs such as difficulty rising, reluctance to jump, and neurological deficiencies in the pelvic limb(s) are primarily due to compression of the cauda equina, the seventh lumbar nerve (L7), and their vascular supply. This condition arises from the high mobility of the lumbosacral joint, which is subjected to significant stress during dorsoventral, lateral flexion, extension, and rotation movements.

Contributing factors to DLSS include:

• Degeneration of the intervertebral disc (IVD): Chronic microtrauma and aging cause desiccation of the nucleus pulposus, leading to altered shock absorption and weakening of the annulus fibrosus. This can result in IVD protrusion (Hansen type II disc disease) into the spinal canal.

• Instability and Subluxation: The IVD changes lead to instability at the lumbosacral space, causing subluxation of the vertebral endplates and facetal joints between L7 and S1.

• Ligamentous Hypertrophy: As a response to instability, the dorsal longitudinal ligament and interarcuate ligaments may hypertrophy.

• New Bone Formation: Spondylosis deformans can form along the ventral aspect of the L7 and S1 vertebral endplates and facetal joints.

• Foraminal Stenosis: The intervertebral foramina can narrow due to facetal joint hypertrophy and spondylosis, further compressing nerve roots.

• Anatomical Malformations: Transitional and asymmetric vertebrae, hemivertebra at L7 or S1, and sacral osteochondrosis can also contribute to compression.

Diagnosis Diagnosis of DLSS involves a comprehensive approach:

• History and Clinical Signs: Pain is the most common sign, leading to various pelvic limb gait abnormalities. Neurological signs, such as proprioceptive deficits, mild paresis, loss of tail function, and incontinence, often progress with the condition. Specific physical examination findings include pain with pelvic limb extension, tail dorsiflexion ("tail jack"), lordosis test, lumbosacral spine rotation, and direct palpation of the lumbosacral spine.

• Radiography and Contrast Studies: Routine radiographs can show sacral subluxation, osteophyte formation, and spondylosis deformans, but do not always correlate with clinical signs. Myelography is of limited use for diagnosing DLSS due to variability in dural sac termination, and it doesn't adequately assess cauda equina compression. Flexion-extension myelography can reveal dynamic lesions.

• Advanced Imaging (Gold Standard):

◦ Computed Tomography (CT): Allows visualization of sacral and facetal joint subluxation, vertebral canal stenosis, and IVD/dorsal longitudinal ligament hypertrophy. It also visualizes the L7 nerve root lateral recess and the L7-S1 intervertebral foramen. CT is more readily available and provides excellent bony detail.

◦ Magnetic Resonance Imaging (MRI): Superior for visualizing soft tissues within and surrounding the spinal canal, including IVD degeneration, the cauda equina, and L7 nerve roots. Flexion and extension views on MRI are useful for dynamic lesions.

◦ It is crucial to correlate imaging findings with patient history and clinical signs to avoid over-diagnosis, as abnormalities can be present in asymptomatic animals.

Treatment Options

• Conservative and Medical Therapy: May include rest, nonsteroidal anti-inflammatory drugs (NSAIDs), corticosteroids, epidural methylprednisolone acetate injections, and physical therapy. However, these are often temporary solutions, especially for working dogs, and recurrence of clinical signs is common upon return to activity.

• Surgery: Indicated for dogs unresponsive to conservative management or those with neurological deficits.

Surgical Anatomy The spinal cord typically ends in the caudal half of L6 and cranial half of L7 in large-breed dogs, though it can extend more caudally in small breeds. The cauda equina, comprising L6, L7, S1-S3, and Cd1-Cd5 nerve roots, is bordered by the interarcuate ligament and laminae dorsally, dorsal longitudinal ligament, IVD, and vertebral bodies ventrally, and vertebral foramina and pedicles laterally. The L7 nerve root is particularly important, as it arises cranial to the intervertebral foramen and passes through a lateral recess, often requiring specific decompression beyond a standard dorsal laminectomy.

Surgical Techniques The most common decompressive procedure is dorsal laminectomy, often combined with disc fenestration or discectomy. Foraminotomy and facetectomy may also be performed for additional decompression.

1. Dorsal Laminectomy:

◦ Patient Positioning: Sternal recumbency with pelvic limbs drawn forward or in a neutral frog-leg position; sandbag support under the pubis can open the intervertebral space.

◦ Technique: A midline skin incision from L5 to the sacrum exposes deep gluteal and sacrocaudalis dorsal medialis epaxial muscles, which are elevated. The spinous processes of L7 and S1 are removed. A high-speed surgical drill removes bone from the caudal half of L7 and most of the sacrum. The laminectomy typically extends laterally to the caudal facets of L7 and cranial facets of S1. The ligamentum flavum is resected to expose epidural fat and the cauda equina.

◦ Disc Decompression: The IVD, often seen protruding, can be palpated under the cauda equina. The dorsal annulus and dorsal longitudinal ligament are excised to perform fenestration or partial discectomy. Protecting the cauda equina is paramount.

◦ Variations: A partial laminectomy (removal of ligamentum flavum and only S1 lamina) has been described to reduce instability.

2. Foraminotomy and Facetectomy:

◦ These procedures are performed to enlarge the L7-S1 foramen and decompress the L7 nerve root.

◦ Foraminotomy: Aims to preserve stability by undercutting rather than removing articular processes. The L7 nerve root should move a few millimeters with minimal traction; if not, further decompression is indicated.

◦ Facetectomy: Involves extending a standard dorsal laminectomy laterally by removing a portion of the articular facets. This procedure has been shown to cause significant instability.

◦ Surgical Approaches: Can be performed via a lateral approach (often combined with dorsal laminectomy), transiliac approach, or endoscope-assisted approach.

3. Distraction and Stabilization/Fusion:

◦ Often recommended following lumbosacral decompression, particularly if instability is present or a concern.

◦ Techniques:

▪ Screws through L7-S1 facetal joints with bone graft for fusion. Screws are typically placed in the center of the joint at a 30-45° angle from the sagittal plane.

▪ Pins or screws with polymethylmethacrylate (PMMA): Pins/screws are inserted into L7 and sacral wings, and embedded in PMMA as an internal fixator. Positive-profile pins are preferred for stiffness.

▪ Pedicle screw-rod constructs: Designed for human spines, these effectively stabilize the lumbosacral spine.

▪ Intervertebral spacer device: A threaded titanium intervertebral spacer can be placed into the L7-S1 space after discectomy, stabilized with a screw, and further reinforced with pins and PMMA or pedicle screws and rods.

◦ Fusion of joints is promoted by removing articular cartilage and placing autogenous cancellous or commercial bone graft.

Postoperative Management

• Rest: 4-8 weeks of restricted activity is recommended, followed by a gradual return over 4-6 weeks.

• Rehabilitation: Swimming and underwater treadmill exercises can aid recovery. A program designed by the surgeon and a qualified animal physiotherapist is essential.

Complications Reported complications include:

• Hemorrhage of the venous sinus: Most common, occurring in 25% of thoracolumbar lateral corpectomy cases, often due to adhesion to chronic disc protrusion.

• Wound infection.

• Nerve root injury.

• Incomplete spinal cord decompression: Can necessitate revision surgery.

• Postoperative spinal instability: May occur, especially if facetectomy is performed without stabilization.

• Transient worsening of neurological status: Reported in ~10% of patients immediately postoperatively.

• Seroma formation: Common after laminectomy procedures.

• Implant failure: Can occur due to chronic cycling and fatigue if fusion is delayed or absent.

Prognosis and Outcomes While long-term studies conclusively demonstrating an advantage of stabilization techniques over dorsal decompression and foraminotomy alone are lacking, surgical outcomes for DLSS are generally good, with many dogs improving or returning to normal function. However, recurrence of clinical signs months to years after surgery may be related to residual compression, instability, or laminectomy membrane formation.