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.

Deformity

Adult degenerative scoliosis traditionally represented the vast majority of lumbar spinal deformity treated by a staged, combined AP operation. However, more recent studies reveal comparable sustained coronal and sagittal deformity correction with a posterior-only approach, with fewer complications than the combined AP approach.

There is no clear consensus in the literature on whether combined AP surgery is superior to posterior alone for lumbar degenerative scoliosis. In a study of 348 patients who underwent surgery for lumbar degenerative scoliosis and were followed for 8.5 years postoperatively, combined AP surgery resulted in a slightly greater Cobb correction (not significant) and significantly fewer levels fused compared with posterior alone, but was also associated with increased hospital stay and operative duration. Clinical, radiographic, and quality-of-life outcomes (SF-36) were comparable between the groups. Similarly, Pateder et al. found no significant difference in long-term postoperative sagittal or coronal curve correction when comparing patients with degenerative scoliosis who underwent posterior alone with those who had combined AP surgery. However, the combined group was notable for having an increased frequency of postoperative major complications (45% vs. 23%). Cho et al. identified preoperative sagittal imbalance and increased pelvic incidence to be significant risk factors for eventual sagittal decompensation (C7 plumb line >8 cm), among patients undergoing long-construct posterior fusion (mean 6.1 levels) alone for degenerative lumbar scoliosis. Pseudarthrosis at the lumbosacral junction and distal adjacent segment disease was more prevalent in the sagittal decompensation group. Interestingly, a study of patients with degenerative scoliosis compared minimally invasive posterior instrumentation with and without anterior column release, and found that anterior column release resulted in significantly improved coronal Cobb angle correction, lumbar lordosis correction, and SVA correction compared with the posterior instrumentation alone group. For those treated with anterior column release before instrumentation, the authors noted a 12-degree gain in segmental lordosis and 3.1-cm correction in SVA for each level released anteriorly. These findings are not generalizable to all degenerative scoliosis patients, but may suggest a potential role for anterior column release and segmental lordosis correction, particularly in patients requiring long instrument constructs to the lumbosacral junction, with preexisting sagittal imbalance, spinopelvic malalignment, rigid deformity, or fused disc spaces at the site of a planned open wedge osteotomy (i.e., Smith-Peterson osteotomy [SPO]).

In a biomechanical cadaveric model of lumbar sagittal deformity comparing anterior lumbar interbody fusion (ALIF) + posterior instrumentation with posterior instrumentation + pedicle subtraction osteotomy (PSO), Dahl et al. concluded that axial rotational stability is significantly improved in staged AP cases for lumbar sagittal deformity. Notably, anterior release and lordotic interbody strut placement followed by posterior stabilization provides a balanced correction to sagittal malalignment, compared with the abrupt angular correction offered by posterior osteotomies alone.

Posttraumatic deformity may develop throughout the spinal column following a traumatic spine fracture, but is most frequently a focal kyphotic deformity located at the thoracolumbar region following a flexion or flexion-distraction type injury. Patients with more than 30 degrees of focal kyphotic deformity are at increased risk for chronic pain in that region. Notably, disruption of the posterior ligamentous complex may lead to progressive deformity over time. Progression of posttraumatic deformity, new neurological deficit, pain, and cosmesis are considered acceptable indications for surgery. This may be treated with combined anterior column release and posterior stabilization or posterior alone. An anterior approach alone is insufficient to correct a sizable focal deformity, and the posterior approach alone may theoretically place excessive tension on the posterior instrumentation.

Trauma/Infection

AP staged procedures may be indicated in cases of thoracolumbar and lumbar trauma. Unstable burst fractures, Chance fractures (flexion-distraction) with posterior ligamentous complex involvement or neurological deficit, and fracture dislocations typically necessitate operative management. The Thoracolumbar Injury Classification and Severity (TLICS) scale is a convenient tool to readily identify patients requiring operative management for thoracolumbar fractures, based on morphology, posterior longitudinal complex integrity, and neurological status.

The decision to pursue a staged AP approach for traumatic thoracolumbar/lumbar fractures depends on the presence and site of neural compression, spinal instability, and the risk for late deformity. In a consecutive retrospective series assessing posterior versus combined AP approach for unstable thoracolumbar burst fractures, both demonstrated comparable clinical outcomes, neurological improvement, and fusion rates. However, loss of reduction (>5 degrees) and instrumentation failure was significantly greater in the posterior-only group. McCormack et al. devised a “load sharing classification” system for spine fractures to predict instrument failure with a posterior-only approach, thereby identifying patients requiring a combined AP surgery.

A specific AP approach, known as the Gaines procedure, is the treatment of choice for lumbosacral spondyloptosis, also known as grade V spondylolisthesis, whereby the dissociated body of L5 becomes entirely anterolisthesed, wedged anteriorly to the sacral promontory. The Gaines procedure consists of anterior vertebrectomy of L5, followed by posterior reduction and instrumentation of L4 on S1.

Surgical treatment of lumbar vertebral osteomyelitis is indicated in cases of failed medical management, epidural abscess formation, intractable pain, neurological deficit, spinal instability, and deformity. Vertebral osteomyelitis most often affects the anterior column, frequently necessitating a staged anterior approach for corpectomy and phlegmon debridement, followed by posterior stabilization. An additional advantage in the staged approach is extensive debridement and minimizing contamination of the posterior surgical field.

Revision Surgery

Pseudarthrosis, or nonunion, resulting in chronic pain refractory to conservative management is considered a potential indication for surgery. ALIF may be used as a salvage procedure for lumbar pseudarthrosis but the rates of fusion and mechanical stability are drastically increased with the presence of posterior instrumentation.

In addition to pseudarthrosis, iatrogenic destabilization (i.e., multilevel laminectomies) and lumbosacral distraction instrumentation may result in an eventual loss of lumbar lordosis or flatback syndrome. Flatback syndrome is characterized by a forward trunk, inability to stand erect without compensatory mechanisms, and pain. The loss of normal lumbar lordosis creates a PI-LL mismatch causing early compensatory mechanisms to maintain a neutral SVA—hyperlordosis in the adjacent lumbar segments, thoracic hypokyphosis, knee flexion, pelvic retroversion, and hip extension. However, paraspinal muscle fatigue and asymmetric disc degeneration eventually lead to sagittal imbalance with a positive SVA, and patients appear to be stooped forward. Initial treatment should focus on conservative management but operative treatment may be appropriate to address significant pain and quality of life. Combined AP surgeries are among the treatment options, accomplishing anterior column release, segmental stability, osteotomies for fixed deformity, and posterior stabilization. The goal of surgery is to restore normal lumbar lordosis within 10 degrees of the pelvic incidence and a balanced SVA with the knees fully extended. Osteotomies employed for flatback syndrome are most commonly SPO, PSO, and polysegmental osteotomies; however, these may narrow the neural foramina with a risk of a new postoperative radiculopathy. In a consecutive series of surgically managed flatback syndrome, consisting mostly of combined AP cases (85%), Booth et al. reported 79% had a decrease in pain and 50% had increased long-term function. Three-column sacral osteotomies have also been successfully described to address spinopelvic imbalance in flatback syndrome.

The “back-front-back” approach, as described by Shufflebarger and Clark for idiopathic scoliosis, is a reasonable option for cases of global malalignment with fixed rigid deformity in both sagittal and coronal planes, or a prior instrumented fusion. In the first stage, following pedicle screw placement/replacement, a posterior release is accomplished by partial removal of the posterior elements (i.e., spinous processes, ligamentum flavae, facet joints). An anterior release and reconstruction are subsequently performed. The final stage entails definitive deformity correction with posterior osteotomy and instrumentation.

Anterior Retroperitoneal Approach

To minimize risks of vascular, bowel, ureteral injury, and ileus, it is recommended that vascular or general surgery be involved in the operative approach to the anterior surface of the lumbar spine. Lateral and AP fluoroscopy are used to identify the level(s) of interest. Access is typically from the left side, owing to the location of the aorta and vena cava, using a paramedian or transverse skin incision. The external oblique fascia is identified and incised adjacent to the midline. The rectus muscle is mobilized laterally, revealing the posterior rectus sheath. The semilunar line of Douglas serves as a transition of the transversus oblique and posterior oblique, allowing entry into the retroperitoneal space. The peritoneum is mobilized from its attachment to the posterior rectus sheath and retracted, allowing for identification of the epigastric vessels, ureter, and iliac vessels. Notably, the iliolumbar vein overlies the lateral aspect of the L5 body, in close proximity to the L5 nerve root, and should be carefully identified. Other considerations include the sympathetic chain on the lateral aspect of the vertebral body, the genitofemoral nerve on the anterior surface of the psoas, and the ureter sandwiched between the psoas fascia and peritoneum. The middle sacral vessels may be carefully divided for added anterior exposure.

Disc spaces and vertebral bodies may be identified as peaks and valleys, respectively. Once the appropriate level has been identified, sharp dissection with a knife is used to release the anterior longitudinal ligament and annulus. Avoidance of monopolar cautery in this region decreases the incidence of postoperative sexual dysfunction. Subsequently, Kerrison punch, curette, and pituitary rongeurs are used for discectomy. A high-speed drill allows for end-plate preparation or corpectomy. Finally, anterior column reconstruction is performed with appropriately lordotic interbodies, strut graft, or cage placement.

Posterior Approach

With the patient positioned prone, with the hips extended to optimize lordosis and following localization of the correct level(s), a midline skin incision is made and monopolar electrocautery is used to advance soft tissue dissection between paraspinal muscles. Bipolar cautery should be used throughout to maintain meticulous hemostasis. Successively larger self-retraining retractors are employed as the dissection becomes deeper. A Cobb periosteal elevator assists in subperiosteal dissection, exposing posterior spinous processes, bilateral laminae, and the facet capsules from their ligamentous attachments. Rongeurs allow for removal of the supraspinous and intraspinous ligaments. The spinous processes may be removed with a rongeur and saved for bone graft. Ligamentum flavum is stripped from its insertion on the lamina using a curette. Troughs are drilled in the bilateral laminae to perform a laminectomy, and completed with the use of Kerrison punch; preoperative dynamic flexion-extension films are integral in determining the flexibility of deformity and need for posterior osteotomies. The Schwab Classification for posterior osteotomies consists of (I) partial facet joint; (II) complete facet joint; (III) pedicle/partial body; (IV) pedicle, disc, and partial body; (V) complete vertebra and discs; and (VI) multiple vertebrae and discs.