Patients who present with radiographic adjacent segment degeneration (adjacent segment disease) do not always necessitate reoperation. Only those who present with symptomatology such as neurological findings or axial pain are considered to have symptomatic adjacent segment disease. Presenting symptoms are the same as found in other degenerative processes of the lumbar spine and can include: radiculopathy, neurogenic claudication, weakness, and mechanical back pain. A thorough history and physical examination is used to determine if the patient’s symptoms localize to the adjacent segments.

The initial work-up consists of plain radiographs: standing full spine and bending. These allow assessment of overall spinal balance and stability at adjacent segments. Given the increased forces exerted on adjacent segments, proximal junctional kyphosis can result from loss of disk height and progressive compression fractures of the vertebral body above or below the construct. A computed axial tomography (CT) scan can be obtained to assess the fusion mass at the previous operative site, as well as look for any hardware failure. Finally, a magnetic resonance image (MRI) is acquired to assess the neural elements. Occasionally, other studies are needed to enhance operative planning, such as electromyelography (EMG), bone densitometry (DEXA) scan, and diagnostic injections.

If the patient’s symptomatology has been correlated with the adjacent pathology, an operative approach is tailored to the individual patient. The risk profiles of the lateral approach versus the posterior approach are inherently very different. The benefits and limitations of the lateral approach are discussed below. When deciding between the lateral approach and the more traditional posterior approach for adjacent segment disease, it is important to weigh the risks of each approach with the patient’s overall goals and risk tolerance.

The minimally invasive lateral approach has allowed for a direct route to the intervertebral disk without significant muscle dissection or blood loss. Patients with multiple medical comorbidities and the elderly can most benefit from this less invasive approach as operative time and hospital stay is significantly shortened [26]. There is also the added benefit to both the patient and the surgeon from not having to dissect through thick scar tissue to obtain decompression. Of note, the lateral approach relies upon an indirect decompression, which has been demonstrated to increase the neuroforamina by up to 57 % and area of the central canal by up to 143 % [28, 29]. This indirect decompression reduces the risk of a cerebrospinal fluid leak (CSF) and also avoids disrupting the posterior elements possibly preventing further adjacent segment degeneration (Fig. 24.2).

As with any new technique, the lateral approach requires special training to become comfortable with the anatomy and nuances. Also, given the minimal opening, this technique is highly reliant upon fluoroscopy, increasing the patient and surgeon radiation exposure, especially during the early learning phase. Unfortunately, one of the more common locations for adjacent segment disease, L5/S1, is precluded from the lateral technique because of the iliac crest. Also, in some patients with a high crest, even L4/L5 is not accessible. One must evaluate the preoperative anterior-posterior (AP) imaging carefully to evaluate accessibility around the crest.

Previous abdominal surgery, while not an absolute contraindication to this technique, must be weighed carefully. Smaller abdominal surgeries can be avoided by entering on the contralateral side. However, larger abdominal or retroperitoneal surgeries with a high likelihood of scar tissue in the retroperitoneal space can significantly increase the risk of surgery especially if the vasculature is scarred down.

EMG monitoring has made this technique safer around the lumbar plexus. Occasionally, a patient will have a very anterior lumbar plexus as evidenced by low stimulation thresholds with the dilators. In these instances, the lumbar plexus is swept posteriorly. Rarely, the plexus cannot be avoided and the procedure must be aborted. A meta-analysis of lumbar plexopathies following the lateral approach found that reporting of plexus injuries in the literature has been inconsistent with the incidence of motor weakness ranging from 0.7 to 33.6 % [30]. There was a lack of consistency in the descriptions of lumbar plexopathies and a lack of diagnostic paradigms. The true incidence of plexus injury is unknown, and evolving techniques have likely diminished the incidence over time [31].

Dissecting through the psoas muscle can also lead to transient weakness in hip flexion and anterior thigh pain/numbness from tension on the genitofemoral nerve which lies on the surface of the psoas muscle [25]. Anatomic variants, which place the vasculature lateral relative to the vertebral body, should be excluded from the lateral approach if there is not enough room to safely perform the diskectomy and place a cage.

Finally, this technique relies upon an indirect decompression to relieve any neurological symptoms. Though rare, patients whose neurological symptoms are not relieved will still require a posterior decompression.

The lateral approach for adjacent segment pathology utilizes the same techniques described previously since the reoperation is across virgin tissue. Cages allowing for concomitant lateral screw placement must be carefully planned to avoid the adjacent screws. Supplementation with posterior instrumentation requires an open technique to attach on to the previous hardware. Wang et al. reported 21 consecutive cases of adjacent segment disease treated with a minimally invasive lateral interbody fusion without posterior instrumentation [27]. All of these patients had good fusion at last follow-up (Fig. 24.3).

Those patients with significant proximal junctional kyphosis require a more aggressively lordotic cage for restoring sagittal balance. By placing the retractor at the midpoint of the disk space, the soft tissue can be dissected anteriorly to expose the anterior longitudinal ligament (ALL). The ALL can then be opened which allows for placement of a hyperlordotic cage to correct positive sagittal balance [18, 19]. Then these patients are supplemented with posterior instrumentation and facetectomies; substantial sagittal restoration can be achieved.

A thorough understanding of the risks unique to this approach and the best methods of complication avoidance are critical. The lateral approach requires entrance into the retroperitoneal space. As such, injury to the bowel is avoided by guiding the instruments to the disk space utilizing the two-finger technique to sweep away the abdominal contents as the dilators are directed to the disk space. Bowel injury can also be avoided by utilizing a mini-open technique for direct visualization of the peritoneum as the dilators are inserted.

Vascular injury can be avoided by carefully examining the relation of the vasculature to the disk space and psoas muscle on preoperative imaging. If there is not enough space to place a cage with a width of 18 mm, then the lateral approach is not an option. Also, entering on the left side is preferred if possible since it avoids the inferior vena cava, which is more vulnerable to injury and harder to repair than the aorta. Recognizing segmental branches and ligating them prior to incising the disk is critical to avoiding excessive blood loss.

The lumbar plexus is more posterior in the rostral lumbar spine. Since reoperation for adjacent segment disease is typically performed at the more rostral lumbar levels, injury to the plexus is much less likely. Techniques to even further lower this risk include shallow docking and an oblique lateral approach, which are discussed further in other chapters. Shallow docking can also be utilized, but frequently the psoas muscle is very thin at the upper lumbar regions.

Patients are generally sent home from the hospital in 1–3 days to ensure they can independently ambulate, void, eat, and pass flatus. In general, patients are placed in an external orthosis for up to 12 weeks, especially when there is no posterior supplementation. Strenuous activity and heavy lifting is avoided until fusion is documented.

The first follow-up visit at 6 weeks is to ensure proper wound healing and to evaluate on plain radiography that the alignment is stable. At the 3-month follow-up, dynamic radiographs or a CAT scan are obtained to ensure proper fusion has occurred. After the 6-month follow-up, the patient is usually followed as needed if symptoms have resolved.

At the level of T12/L1 or L1/L2, the thoracic cavity can also be entered. This engenders the risk of a pneumothorax. If the parietal pleura is encountered, care and attention must be taken to avoid a tension pneumothorax. The pleura can be stitched and repaired or a small chest tube inserted and managed in a very conservative manner.