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Anterior-Posterior Surgeries
Introduction
There are many factors to consider in determining if a staged anterior-posterior (AP) lumbar procedure is the most appropriate for a given patient. These include the patient’s presenting symptoms (i.e., discogenic back pain, radiculopathy, myelopathy), bone quality, age, performance status, global alignment, coronal deformity, sagittal deformity, location, number of levels of the underlying pathology, and surgeon preference.
The benefits of anterior column release, curve flexibility, global alignment, sagittal/spinopelvic imbalance, magnitude of focal or multilevel kyphosis, and number of levels requiring correction are critical considerations in determining the roles for an osteotomy and the combined AP approach for lumbar deformity, degenerative, traumatic, and infectious pathologies, as well as revision procedures. This chapter outlines the indications, considerations, and operative steps for the combined AP lumbar approach.
Indications for Staged Anterior-Posterior Lumbar Spine Surgery
The anterior and middle columns of the lumbar spine provide 80% of the weight-bearing load of the spine and 90% of the osseous surface area, whereas the posterior column only provides 20% and 10%, respectively. Operative techniques and implants during the anterior approach may be placed to optimize surface area for osseous fusion, release the anterior column in fixed deformity, redistribute loads resulting from degenerative processes or deformity, address segmental lordosis, and decompress any neural element compression from the anterior column (i.e., vertebrectomy, discectomy). However, the posterior approach may be utilized for posterior element decompression, spinal canal decompression, stabilization across multiple levels, and osteotomies for fixed deformity.
There is a well-established association between sagittal malalignment, spinopelvic imbalance, and poor performing quality-of-life metrics. The Scoliosis Research Society (SRS) identified radiographic thresholds predictive of an Oswestry Disability Index (ODI) of at least 40 (moderate disability): pelvic incidence-lumbar lordosis (PI-LL) mismatch of 11 degrees or more, pelvic tilt (PT) of 22 degrees, and sagittal vertical axis (SVA) of 46 mm or more. In addition to pain and impaired quality of life, the presence of a PI-LL mismatch (>10 degrees) following lumbar fusion for degenerative pathologies has a 10-fold increased risk of requiring revision surgery, due largely to adjacent segment disease. As such, the combined AP approach to the lumbar spine has several indications depending on the underlying pathology, but is often used to address debilitating sagittal imbalance and spinopelvic imbalance, not possible by a single anterior or posterior approach. The benefits of anterior column release, segmental correction, and reconstruction is matched by the unparalleled stability provided by posterior instrumentation—which may be indicated for select patients.
Degenerative Conditions
Several degenerative conditions in the lumbar spine may be treated by a staged AP approach. Among them, spondylolisthesis, particularly of the lumbosacral junction, can be difficult to obtain a solid fusion. Risk factors such as multilevel spondylolisthesis, subluxation greater than 50%, prior failed fusion, diabetes, positive smoking status, and poor bone density are acceptable indications for an AP approach. However, the benefits of increased fusion rates with this approach must be weighed against the increased morbidity of the two-stage approach. In a study of the combined AP approach for lumbar degenerative disease, Moore et al. demonstrated that 95% achieved solid fusion, 86% had clinical improvement (defined as improved visual analog scale for pain, and functional questionnaire), and 85% were able to return to work by the follow-up period of 35 months. As such, the combined approach is an option for patients with multilevel disc disease who fail conservative management, require multilevel segmental lordosis, have discogenic back pain from a central disc herniation, and/or in whom achieving a solid fusion may be challenging.
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