Anterior Column Release in Deformity

Lordosis

Surgically, the ACR procedure is intended to exploit an open disc space to achieve the desired segmental lordosis. A thorough knowledge of the patient’s global spinal alignment, age-directed spinal parameter goals, and regional lordosis harmony are important for creating a preoperative plan and determining whether an ACR is needed. Although it has been mentioned that some of the most severe sagittal deformities may not be best treated with MIS techniques, it too should be noted that not all sagittal imbalances require as much lordosis as that provided by an ACR. This is particularly true in older patients, where overcorrection of lumbar lordosis can result in worse outcomes.

Recently, a minimally invasive surgical decision-making algorithm (MISDEF) was created by a consensus panel of deformity surgeons to help classify those deformities that would respond best to different MIS approaches. The first class of patients were those who met no major sagittal malalignment criteria and were felt to benefit from focal MIS decompression or fusion surgery. The second class was patients who presented with evidence of moderate sagittal deformity, with a sagittal vertical axis greater than 6 cm, a pelvic tilt greater than 25 degrees, lateral listhesis greater than 6 mm, a coronal Cobb angle greater than 20 degrees, or a pelvic incidence to lumbar lordosis (PI-LL) mismatch of greater than 10 degrees. This class of patients was felt to benefit from MIS deformity surgery, with interbodies placed across the deformity. Patients in the third, most severe class, were those who had any of those parameters of the second class, but also evidence of either a fixed deformity, PI-LL mismatch greater than 30 degrees, or thoracic kyphosis greater than 60 degrees. The consensus suggestion for this group was that open posterior surgery with osteotomies with fusion extension into the thoracic spine was required.

Overall, the MISDEF paints a generalized picture of when MIS deformity surgery may be beneficial. However, as the techniques have progressed, it has become clear that ACR surgery also allows for treating greater PI-LL mismatches. Recently, Uribe and colleagues described an MIS ACR osteotomy classification. The work proposed an anatomical realignment grading system that provides a consistent description of the various posterior column release and ACR/osteotomies. They described the following five levels: grade 1 ACR: ALL release with hyperlordotic cage and inferior facetectomy (Schwab grade 1); grade 2 ACR: ALL release with hyperlordotic cage and posterior column osteotomies (PCOs; Schwab grade 2); grades 3 and 4 ACR: ALL release with hyperlordotic cage and three-column osteotomy (Schwab grades 3 and 4) ; and grade 5 ACR: vertebrectomy with ALL release ( Fig. 147.1 ). In general, if more than 30 degrees of lordosis correction is required, a grade 3 or higher osteotomy should be performed. The utilization of ACR and PCOs alone (grade 2) in general adds on average 23 degrees of segmental lordosis with the use of a hyperlordotic implant. The addition of an ALIF hyperlordotic implant at the lumbosacral junction, when possible, also gives MIS deformity surgeons the flexibility to achieve a large amount of lumbar lordosis for correction of severe sagittal plane deformities ( Fig. 147.2 ).

As can be seen in the discussed osteotomy classification, the amount of segmental correction varies significantly for ACRs depending on how much bone is removed. Measuring the intradiscal angle (IDA) allows the surgeon to determine the lordosis at the disc space being treated. Appropriately matching this angle with the desired implant and local goals ensures that there is adequate end plate contact with the cage to prevent subsidence and increase fusion rates. Uribe et al. performed a finite element model of a L3‒L4 vertebral segment. Changes in IDA were measured with hyperlordotic 20-degree and 30-degree cages, with the ALL released, ALL released + facetectomy, or ALL released + PCO. Releasing the ALL alone increased lordosis by 1 to 10 degrees for all hyperlordotic cages. The maximum extent to which this occurred was limited by the facet joints maintaining posterior disc height. ALL release and facetectomy increased the lordosis with both the 20-degree and 30-degree cages by 21 to 27 degrees, with additional lordosis prevented by spinous process contact. Adding a PCO gave rise to 32 to 33 degrees of lordosis for 30-degree cages. These results show the extent to which posterior bone release impacts the desired goals and allows for more appropriate implant-to–end plate interfacing.

Procedural Contraindications

Contraindications to the ACR procedure include any prior surgery or inflammatory procedure that may make the approach or dissection of the great vessels high-risk. This includes prior infections, radiation treatment, peritonitis, or previous surgeries. Preoperative computed tomography (CT) should be obtained to evaluate and rule out fusion of the segment. The presence of a vacuum disc phenomenon is a positive indicator of an open disc space amenable to interbody fusion. Fusion of the posterior facets with an open disc space is not a contraindication, but requires staged posterior Smith-Peterson–type osteotomies before performance of the ACR to accomplish the desired lordosis correction.

Additionally, preoperative examination of the vasculature on magnetic resonance imaging and/or CT may help to discern how easy the prevertebral dissection will be, as there may be varying amounts of space in that corridor. A T1 axial sequence at the level of the ACR disc space may demonstrate fat between the vessels and the ventral aspect of the disc space, signaling a safe plane for dissection. Evaluation of the relative psoas position can also be helpful, as ventral displacement of the psoas relative to the disc space has been correlated to anterior lumbosacral plexus migration and posterolateral iliac location. Vascular anomalies are common in the retroperitoneum and should be evaluated for retrocaval renal veins, duplication, and transitional anatomy at the lumbosacral spine. In cases where the vascular anatomy is unfavorable but an L5‒S1 ALIF is planned, ALL sectioning can be performed at L4‒L5 under direct visualization at the time of the ALIF, with the ACR then being completed at the second stage when the L4‒L5 LLIF is performed.