Posterior Approach to Lumbar Spine

What it exposes. The posterior midline approach is the workhorse exposure of the lumbar spine, giving direct access to the spinous processes, laminae, facet joints, ligamentum flavum, the spinal canal and cauda equina, and (by dissecting further laterally) the transverse processes and pedicles. Through this one corridor you can perform a laminectomy to decompress stenosis, a microdiscectomy for a herniated disc, and an instrumented fusion for instability, spondylolisthesis or deformity. L4-5 is the most commonly operated level (about 50 percent of cases). Why the midline. The midline is a relatively avascular plane, the subperiosteal dissection keeps you safely outside the canal until you choose to enter it, and the same exposure scales from a 3 cm discectomy window to a long deformity construct. The Wiltse muscle-splitting interval (between longissimus and multifidus, at the lateral edge of the facet) is a true internervous plane used for minimally invasive fusion and pars/transverse-process work; it spares the midline ligamentous and muscular complex.

Indication nuances worth knowing. - Spondylolisthesis is graded by Meyerding: grade 1 (0-25 percent slip), grade 2 (25-50 percent), grade 3 (50-75 percent), grade 4 (75-100 percent), grade 5 (spondyloptosis, complete). Isthmic slips (pars defect) occur at L5-S1 in about 80 percent; degenerative slips (facet arthropathy, no pars defect) occur at L4-5 in about 80 percent. Decompression alone for a grade 2 or more slip risks progression (30-50 percent progress within 2 years) — add a fusion. Reducing a high-grade slip is controversial; many fuse in situ to avoid nerve-stretch injury.

• Cauda equina syndrome (massive central disc) is the emergency indication: bilateral leg weakness, saddle anaesthesia, bladder dysfunction. Decompress within 48 hours of onset.
• Degenerative disc disease is operated only when there is objective instability (more than 3 mm translation or more than 10 degrees angulation on flexion-extension films, Modic changes, HIZ, more than 50 percent height loss) — pure discogenic back pain without instability is best managed non-operatively.
• Deformity (adult degenerative scoliosis more than 10 degrees, or sagittal imbalance with a trunk forward lean more than 5 cm) needs multilevel fusion, often T10 to pelvis, with osteotomies for rigid curves. Contraindications. Absolute: active infection (sepsis, osteomyelitis or discitis without cord compromise) — treat the infection first; severe medical comorbidity unable to tolerate prone surgery; and cauda equina with dense paraplegia beyond 48 hours (poor prognosis, a relative contraindication). Relative: obesity (BMI more than 35, infection 5-10 percent), active smoking (pseudarthrosis 5-10 percent rising to 20-30 percent), osteoporosis (T-score less than -2.5, poor screw purchase), untreated psychiatric comorbidity, workers' compensation or litigation (worse outcomes across spine surgery), and multilevel disease (4 levels or more — higher morbidity). Position, landmarks and setup. Prone on a Jackson table (adjustable, can break for lateral imaging) or Wilson frame (simpler, radiolucent). Both keep the abdomen free to reduce epidural venous engorgement and blood loss, and flex the hips and knees to flatten lumbar lordosis and open the interlaminar spaces. Induce supine on the trolley with large-bore access (and an arterial line for fusion), then log-roll with at least four people protecting the tube and lines. Support the head (ProneView or horseshoe — check the eyes every 30 minutes; Mayfield pins for long cases), pad the chest and pelvis, position the arms (swimmer's above the head with shoulders less than 90 degrees abducted, or tucked at sides) and flex the hips 45-60 degrees. Apply sequential compression devices. Stand on the left with the C-arm on the right; confirm you can get a true lateral and an AP before draping. Neuromonitoring. Somatosensory evoked potentials monitor the dorsal columns (greater than 50 percent amplitude drop or greater than 10 percent latency increase is concerning); motor evoked potentials monitor the ventral cord and are more sensitive; triggered EMG tests individual roots during pedicle-screw stimulation (a threshold less than 10 mA suggests a medial breach). Use monitoring for fusion and for decompression in myelopathy or severe stenosis. Guidelines, registries and global practice. Named-society indications concur across regions: NASS (North America), NICE/BOA (UK) and AOSpine/EFORT (Europe) all offer decompression for symptomatic stenosis or radiculopathy failing structured conservative care, and reserve instrumented fusion for demonstrable instability, spondylolisthesis or deformity — not for isolated stenosis or non-specific back pain. Return to work is a general guide, not jurisdiction-specific: discectomy 2-4 weeks sedentary / 6-12 weeks manual; laminectomy 4-6 weeks / 3-4 months; fusion 6-12 weeks / 6-9 months (await radiographic fusion). Document smoking-cessation counselling and consider deferring elective fusion until cessation is established (commonly 6-8 weeks); cotinine testing is used in selected cases.

The exposure is the point of the page: a midline, subperiosteal route that stays out of the canal until you deliberately enter it, then delivers the lamina, flavum, canal and pedicles in a controlled, teachable sequence. Master the anatomy first, then walk down the steps. Surface landmarks and level identification. The Tuffier line (joining the highest points of the iliac crests) crosses the L4-5 disc or L4 body in about 90 percent of patients — the most reliable surface landmark. The L5 spinous process is the last mobile process before the fixed sacrum. Lumbosacral transitional vertebrae (sacralised L5 or lumbarised S1, 10-15 percent; a large L5 transverse process pseudoarticulating with the sacrum is Bertolotti syndrome) defeat palpation alone, so always confirm the level with fluoroscopy before incision. The layers you cross. Thick lumbar skin and subcutaneous fat over the thoracolumbar fascia (a three-layer system; the tough posterior layer is your strong closure tissue and is incised in the midline raphe). Beneath it the paraspinal muscles: the multifidus (medial, segmental innervation from the medial branch of the dorsal rami — the primary muscle you elevate, and the muscle a subperiosteal dissection is designed to protect) and the erector spinae (iliocostalis laterally and longissimus centrally, dorsal rami). The Wiltse interval splits between longissimus and multifidus at the lateral facet edge — a true internervous plane. Bony anatomy of a lumbar vertebra. Large rectangular spinous processes; the lamina (roof of the canal, removed in laminectomy); the facet (superior and inferior articular processes — its capsule is nociceptive and a pain source; the lateral facet / transverse-process junction is the pedicle-screw entry point); the pedicle (a cylindrical tube, the strongest fixation point — L1-4 diameter 8-12 mm narrowing toward L1, L5 the largest at 12-18 mm, height 14-18 mm, axis 10-15 degrees medial and 10-15 degrees caudal); the transverse process (muscle insertion and a landmark for screw entry); and the pars interarticularis (the thinnest part of the arch, the site of spondylolysis). Neural anatomy that governs the whole operation. The conus ends at L1-2 (it is at L3 at birth and ascends); below that the canal holds the cauda equina, which is far more tolerant of gentle retraction than the cord. At each disc level two roots are at risk — the traversing root (crosses the disc medially; the L5 root at L4-5) and the exiting root (already in the foramen; the L4 root at L4-5). A typical posterolateral disc compresses the traversing root; a far-lateral disc compresses the exiting root. Far-out syndrome is different — bony compression of the L5 root in an L5-S1 foramen from a large L5 transverse process or high-grade slip, needing foraminotomy (not a discectomy). Dura, epidural space and ligamentum flavum. The dura is thin and adherent to the flavum in the elderly, stenosis and revision — the anatomical reason dural tears cluster there. The epidural space holds yellow fat (the visual cue that you have entered the canal) and Batson's valveless venous plexus (engorges with raised venous pressure — keep the abdomen free). The ligamentum flavum (yellow, elastic) connects adjacent laminae, hypertrophies with age (more than 4 mm contributes to stenosis), has a midline gap, and is removed to decompress the canal.

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Structures at risk, by layer.

The three safety-critical events.

Dural-tear repair in detail. Recognise the tear (CSF that keeps pooling; send fluid for beta-2 transferrin if you are unsure — CSF has it, irrigation fluid does not). For a small tear (less than 5 mm): extend the laminectomy to see the edges, primary repair with interrupted or running 4-0 Nurolon, fibrin glue or DuraSeal, then a Valsalva test. For a large tear (more than 5 mm) or an awkward site (ventral dura, root sleeve): attempt primary repair, otherwise bridge the defect with a dural patch (collagen matrix, AlloDerm or autologous fascia) sutured with 4-0 Nurolon, seal with fibrin glue, and overlay a paraspinal muscle flap. If a nerve root is extruded through the tear, reduce it back into the thecal sac; if it is tethered by scar, close the dura around it. After repair: flat bed rest 24-48 hours, no deep drain (or remove early at 6-12 hours), and watch for wound CSF leak, positional headache and pseudomeningocele. If a leak persists, place a lumbar drain (10-20 cc/hour for 3-5 days to drop CSF pressure and let the repair seal) or return for revision; an established pseudomeningocele can be observed if asymptomatic. Routine closure (no dural tear). Copious saline (or bacitracin) irrigation; haemostasis with bipolar, bone wax (sparingly) and haemostatic agents; verify a dry field before closing (residual bleeding forms a haematoma that can compress the cauda equina). A closed suction drain is controversial — many place a 10 mm Blake drain for multilevel fusion, extensive epidural bleeding, revision or planned anticoagulation, and remove it at 24-48 hours or when output is less than 50 cc per 8 hours; never drain a repaired dural tear. Close the thoracolumbar fascia with size-1 Vicryl or PDS (the strength layer; watertight if a tear was repaired), the subcutaneous layer with 2-0/3-0 Vicryl to obliterate dead space, and the skin with staples (most common), subcuticular Monocryl or nylon. Consider negative-pressure wound therapy for high-risk wounds (obesity, revision, diabetes). Extensile options. Extend proximally along the midline to reach higher lumbar levels or the thoracolumbar junction; extend distally to the sacrum for L5-S1 work or pelvic fixation. Go lateral through the Wiltse interval to reach the transverse processes, pars and disc for minimally invasive TLIF or far-lateral/far-out pathology, sparing the midline musculoligamentous complex. Postoperative care. Check motor and sensory function in recovery against the preoperative baseline — a new deficit means haematoma (progressive bilateral weakness and bladder dysfunction), nerve-root injury (unilateral) or a medial screw breach (focal radicular loss); image urgently (MRI for soft tissue, CT for hardware) and reoperate for a compressing haematoma. Use multimodal analgesia (paracetamol, short-course ketorolac, gabapentin/pregabalin for radicular pain, opioids only for breakthrough; wean by two weeks). Mobilise early — walking within 4-6 hours after discectomy, day 1 after laminectomy, day 1-2 cautiously after fusion. A lumbosacral orthosis for fusion is for comfort only (no evidence it improves fusion rates); no brace is needed after discectomy or laminectomy. Give DVT prophylaxis: mechanical SCDs plus early mobilisation; chemical prophylaxis (LMWH or unfractionated heparin) is balanced against epidural-haematoma risk (about 0.1-0.2 percent but catastrophic) — many surgeons wait 24 hours and withhold it while a drain is in. Activity restrictions and follow-up. After discectomy: no lifting more than 5 kg, no bending/lifting/twisting for 6 weeks; sedentary work 2-4 weeks, manual 6-12 weeks. After laminectomy: no lifting more than 5 kg for 6 weeks; sedentary 4-6 weeks, manual 3-4 months. After fusion: no lifting more than 5 kg and no BLT for 3 months; sedentary 6-12 weeks, manual 6-9 months awaiting radiographic fusion. Image fusion patients at 6 weeks, 3 months (AP, lateral and flexion-extension — bridging bone and less than 2 mm motion) and at 6-12 months; CT if fusion is uncertain. Fusion is confirmed by bridging bone across the disc and between transverse processes, less than 2 mm motion, and no hardware loosening or periscrew lucency. Complications and their management.

• Cauda equina decompression — the emergency operation for a massive central disc with bilateral weakness, saddle anaesthesia and bladder dysfunction (within 48 hours).
• Lumbar decompression for spinal stenosis — laminectomy with flavum and medial-facet removal for neurogenic claudication.
• Microscopic lumbar discectomy — sequestrectomy of a herniated fragment through the interlaminar window for radiculopathy.
• Posterior lumbar decompression and fusion (PLDF) — decompression plus TLIF/PLIF and pedicle-screw fixation for spondylolisthesis or instability.
• Posterior spinal fusion for adolescent idiopathic scoliosis — multilevel pedicle-screw construct for coronal deformity correction.
• Muscular-dystrophy scoliosis — posterior spinal fusion T2 to pelvis — long pelvic-foundation fusion for neuromuscular collapse.