Presacral (AxiaLIF) Approach to L5-S1

What it exposes. The presacral (AxiaLIF) approach is a minimally invasive, fluoroscopically guided, paracoccygeal route to the L5-S1 intervertebral disc. Through a small incision near the coccyx, blunt dissectors are passed into the presacral (retrorectal) avascular space and a guidewire is driven cranially along the anterior sacral cortex to dock at the S1-S2 junction, then advanced through S1, across the L5-S1 disc and into L5. Sequential dilators create a working channel for an axial discectomy and a threaded axial rod that spans and distracts L5-S1. Why this approach is chosen. The presacral axial approach reaches the L5-S1 disc without traversing the abdominal cavity (unlike an anterior lumbar interbody fusion, ALIF) and without stripping the posterior paraspinal muscles (unlike posterior or transforaminal lumbar interbody fusion, PLIF and TLIF). It exploits the embryologically avascular presacral space, so the corridor is bloodless if the correct plane is maintained and no muscle is divided. This preserves the paraspinal musculature and the dorsal musculoligamentous envelope, which is the theoretical advantage for a selected L5-S1 fusion. Indications. - Symptomatic L5-S1 degenerative disc disease with persistent mechanically provoked low back pain that has failed at least six months of structured non-operative care, in a carefully selected patient

• Low-grade isthmic or degenerative spondylolisthesis at L5-S1 (typically grade I and selected low-grade II) where reduction and interbody support are desired
• L5-S1 pseudoarthrosis or revision fusion where an anterior or posterior-only revision is unsuitable and an additional interbody option is needed
• Selected L5-S1 deformity correction where an axial interbody restore of disc height is an adjunct to posterior instrumentation
• Adjunct to a posterior fusion when an anterior column support at L5-S1 is wanted without an abdominal or extensive posterior exposure Contraindications. - Prior presacral, retroperitoneal or rectal surgery that has scarred or obliterated the avascular presacral plane
• Pelvic or perirectal sepsis, a perirectal abscess, a fistula-in-ano, or active inflammatory bowel disease involving the rectum
• Sacral anomalies such as sacral agenesis, a markedly kyphotic or curved sacrum, or severe sacral dysplasia that makes the transsacral trajectory impossible
• Transitional lumbosacral anatomy (sacralisation of L5 or lumbarisation of S1) that distorts the osseous landmarks
• Severe collapse of the L5-S1 disc that cannot be distracted by the axial rod
• Morbid obesity or bowel gas that prevents adequate biplanar fluoroscopy of the sacrum
• High-grade spondylolisthesis (relative) where reduction and anterior support cannot be safely achieved through a fixed axial trajectory
• Active perineal or sacral skin infection at the intended incision Pre-operative work-up. Weight-bearing anteroposterior and lateral lumbosacral radiographs (with flexion-extension views when instability or low-grade spondylolisthesis is suspected); a computed tomography scan to map the sacral anatomy, the sacral curvature, the anterior sacral foramina and the feasibility of a midline transsacral trajectory; magnetic resonance imaging to confirm L5-S1 disc pathology and to exclude a high-riding or abnormal rectum, a presacral mass, or presacral scarring. A pre-operative bowel preparation and, in many centres, a rectal tube or a water-soluble contrast-filled rectal marker balloon delineate the rectum on lateral fluoroscopy. Consent emphasises rectal injury, vascular injury, nerve injury and the need for supplementary posterior fixation.

Position & landmarks. Place the patient prone on a radiolucent table with chest and pelvic bolsters, padding all pressure points and protecting the face, eyes and brachial plexus. Flex the hips slightly using a hip roll or table hip-flexion to relax the gluteal mass and open the paracoccygeal corridor. Prep the perineum and natal cleft into the sterile field, because the incision lies in the paracoccygeal skin and access to the anal margin may be needed to place a rectal marker. Before draping, confirm a true anteroposterior and a true lateral view of the sacrum and lumbosacral junction with the C-arm, rotating until the sacrum is seen en face on AP and the posterior elements are superimposed on lateral. Palpable and radiographic landmarks are the tip of the coccyx and sacrococcygeal joint, the posterior superior iliac spines and sacral hiatus, the anterior sacral cortex (the smooth concave cortical line to hug on lateral fluoroscopy), the L5-S1 disc and L5 body (the target), and the paired sacral foramina (which the trajectory must run exactly between on AP).

Work cranially through the avascular presacral plane, hugging the anterior sacral cortex under continuous biplanar fluoroscopy, then drive the transsacral trajectory into L5-S1, prepare the disc, deploy the axial rod and supplement it with posterior pedicle screws.

Structures at risk, by layer

Extensibility - deliberately limited. Proximally, the transsacral trajectory can be extended from S1 across L5-S1 into L4-L5 to place a longer two-level axial rod spanning L4 to S1; this is the limit of the approach and suits only a minority with favourable anatomy. Distally, the approach ends at the paracoccygeal skin incision and cannot be extended beyond the perineum. The fixed axial geometry of the sacrum confines the approach to the lumbosacral junction - it is not a general lumbar exposure and cannot be redirected to higher lumbar levels. This limited extensibility is one of the principal drawbacks and a recognised examination point. Closure. Achieve haemostasis in the presacral space and at the incision with tamponade and haemostatic agents for any presacral venous ooze; the space is left to collapse around the tract. Close the small paracoccygeal incision in layers - a deep fascial stitch to the anococcygeal tissues, then a subcuticular skin closure or interrupted skin sutures. A drain is not usually required because the avascular plane and the small tract rarely accumulate a significant collection. Close the separate percutaneous pedicle screw stab incisions with single skin sutures or skin glue, and apply a sterile dressing with care for the moist environment of the natal cleft. Early mobilisation is possible because no muscle was stripped.

Fixation principles and the supplemented construct. The axial rod provides anterior column support and disc-height restoration but is not a standalone fixation device; it is always used as one limb of a construct. Bilateral percutaneous pedicle screws at L5 and S1 form the posterior limb, producing a triangulated construct that resists flexion, extension and rotation. Some surgeons add a facet screw or an iliac bolt when sacral fixation is tenuous or the construct is long. Bone graft is placed both in the prepared disc space and, where used, as a posterolateral onlay, to achieve a solid arthrodesis. Always describe the supplementary posterior percutaneous pedicle screw construct - axial fusion alone is insufficient.

Guidelines, Registries & Global Practice Lumbosacral interbody fusion is performed worldwide, and the principles converge across examination systems (advanced orthopaedic practice and advanced orthopaedic practice, DNB and MS, MRCS, SICOT). The presacral (AxiaLIF) approach is a recognised minimally invasive option that is particularly discussed in the context of L5-S1 fusion where avoidance of both the abdomen and posterior muscle stripping is desired. Weight-bearing radiographs, computed tomography mapping of the sacrum and the sacral foramina, and magnetic resonance imaging to exclude a presacral abnormality are near-universal before the approach is offered, and the emphasis on protecting the rectum, the presacral vessels and the sacral roots with biplanar fluoroscopy is shared by every group that uses it. Side-by-side principles (where guidance converges): | Body | Position on L5-S1 interbody fusion and the presacral approach |

|------|----------------------------------------------------------------| | AO Foundation and spine trauma and deformity consensus | An anterior column support at L5-S1 improves fusion mechanics; the chosen corridor (anterior, posterior or axial) is dictated by anatomy, previous surgery and the structures at risk | | NICE and national health-technology bodies | Minimally invasive axial fusion is an option for selected L5-S1 disease, with the explicit caveat that long-term comparative evidence is limited and that serious bowel and vascular complications, although uncommon, are well described | | AAOS and international spine societies | Interbody fusion at L5-S1 should be reserved for defined indications after conservative care has failed; the approach is chosen to minimise approach-related morbidity while respecting the dangers specific to that corridor | Population and outcome evidence: - Early clinical series of the presacral approach reported low intra-operative blood loss and short hospital stays, attributed to the muscle-sparing, avascular-plane corridor.

• Reported fusion rates vary widely across series, and concerns about pseudoarthrosis and about rare but serious bowel and vascular complications have tempered early enthusiasm.
• Comparative studies against transforaminal and anterior lumbar interbody fusion generally show similar medium-term fusion but differences in blood loss and length of stay, with the rectum being a danger unique to the presacral route. Global practice variation. In high-resource settings, intra-operative three-dimensional fluoroscopy or navigation is used to confirm the transsacral trajectory, and percutaneous pedicle screw supplementation is routine. In resource-limited settings the same L5-S1 pathology is more often managed with a posterior or transforaminal approach, because the presacral technique depends on specialised implants and reliable biplanar imaging that may not be available. Consent (globally applicable): discuss rectal injury (rare but catastrophic), injury to the middle sacral vessels and presacral venous plexus with bleeding, sacral nerve-root injury, retrograde ejaculation or sexual dysfunction, pseudoarthrosis and the need for revision surgery, adjacent-segment degeneration, and the obligatory addition of posterior pedicle screw fixation.