23.1 Introduction

Life is a kyphogenic process affected mostly by the invariable onset of degenerative disk disease. Severe spinal kyphotic deformity occurs exclusively or collectively from additional conditions, including inflammatory (for example, ankylosing spondylitis), metabolic (osteoporotic fractures), congenital (failure of adjacent anterior vertebral endplate segmentation), traumatic, postinfectious (collapsed spondylodiskitis), iatrogenic (post-cervicothoracic laminectomy), tumoral or neurodegenerative conditions expressed as “bent-spine syndrome,” or camptocormia (Parkinson’s disease, among many others). In the presence of severe kyphosis, the body may compensate through intraspinal or extraspinal measures to remain upright within its “cone of economy.” Alternatively, the body requires a support (crutch or walking frame) or the body may undergo corrective surgery to achieve this. Crucially, in addition to functional requirements, patients have other key objectives including pain relief and cosmesis. Surgery should be planned to the extent that it follows a well-thought-out plan to the point that its execution requires minimal intraoperative thinking. Predictably, following the same steps makes intraoperative teamwork more effective.

23.2 Imaging of Spinal Deformity

Whereas radiographs provide the assessment of alignment, it is worth taking a photograph of the patient pre- and postoperatively to document the changes in posture, particularly if more than one operation is justified. From this, two angles are relevant. The whole-body kyphosis tilt angle (Fig. 23‑1) is used, with the apex set at the posterior spine at the level of the umbilicus and the angle subtended from the external auditory meatus to the apex to the lateral femoral condyle. ¹ Second, decompensation of cervical sagittal balance can be represented as an increase in the chin-brow vertical angle (CBVA) (Fig. 23‑2), ² the angle between that of the face and the vertical, a reliable clinical measure of horizontal gaze. This angle reflects activities of daily living and quality of life.

Standard radiographs of the spine and pelvis form both diagnostic and planning information when considering surgery. When considering surgery, it is critical to know whether there are a normal number of lumbar vertebrae and ribs. Hyperextension views are useful to highlight any residual intradiskal mobility. They are best taken in the supine position with a cushion under the apex of the rigidity. If one were reliant on fluoroscopy intraoperatively, preoperative radiographs are helpful to identify the vertebral anatomy because pedicle outlines, for example, may be difficult to visualize on fluoroscopy. EOS imaging or erect full-spine radiographs are used to assess sagittal and coronal spinal balance. The position of the femurs must be included to add this to alignment calculations.

The traditional technique for planning osteotomy involves the printing of the radiograph on paper, drawing and cutting along the osteotomy lines, and manually adjusting the image to approximate the required correction. The digital manipulation of erect full-spine radiographs with software programs such as Spineview (Paris, France) or Surgimap (New York, NY) are conventionally used in planning surgical correction. Correction should be planned at the apex of the deformity as it best approximates the normal anatomy and avoids persistence of postoperative compensatory mechanisms. ³ The software programs are particularly helpful where multiple osteotomies are likely. For example, they can demonstrate whether more than one osteotomy is required and which type. Whichever method is employed should be clearly presented to the patient to ensure that they receive sufficient information.

Computed tomography (CT) invariably yields the best osseous analysis, which is helpful to identify bone quality, pedicle morphology, previous fusion attempts, and integrity of posterior elements. A pseudarthrosis from an old fracture, hypermobile degenerative disk, or previous instrumentation will not have traversing trabeculae but corticated appositional surfaces. Lytic margins are also evident around loose screws/interbody devices or implant breakage. Preoperative CT also provides insight and simulation for what to expect on intraoperative CT. It is helpful to use software (e.g., OsiriX) to reconstruct digitally an axial profile of each vertebra, which can then be displayed as a set of images/prints to be referenced during the case. This exercise mandates thinking about pedicle screw insertion beforehand and then uses this aide-memoire during the “easy” part of the case.

Magnetic resonance imaging is valuable for cord compression but may be difficult because of the deformity. Patients are often uncomfortable within the scanner and may move. The spine may be some distance from the coil thus inhibiting optimal visualization. Consider positioning in lateral decubitus where possible.

23.3 Perioperative Management

If hip arthritis is present, it is worth treating before any spinal deformity correction. As hip arthritis is commonly present with ankylosing spondylitis (AS), hip arthroplasty will aid secondary spinal compensation and help patient positioning if operating on the spine at a later point. The acetabulum is more vertical and anteverted, and reconstruction is best guided by pelvis positioning and not the operating table. A large femoral head may help reduce the risk of anterior dislocations after total hip arthroplasty. By contrast, degenerative knee flexion contractures with AS may help secondary compensation to maintain sagittal balance.

Multidisciplinary input is important regarding aligning the patient’s treatment preferences and coordinating cessation of medical therapy. Smokers should be strongly urged to quit. A full anesthetic work-up is required preoperatively to identify AS-associated comorbidities such as right bundle branch block, aortic valve insufficiency, anemia, respiratory compromise from decreased diaphragmatic excursion and restrictive lung disease, aortitis, and so on. Preoperative evaluation of the cervical spine is essential. As the atlanto-occipital joint is the last to fuse, it may be unstable, and thus planning for video-assisted intubation is safest.

Preoperative planning may be a routine process in spine centers where this surgery takes place regularly, but it is always worth ensuring that ancillary services are adequately briefed, including anesthetics, nursing, rehabilitation, and so forth. One has the luxury of time preoperatively so that unexpected intraoperative events are minimized. For example, it is not ideal to discover during the surgery that the L2 left pedicle is malformed or that some material is missing in the operating theater. Developing a predictable pattern makes it easier for the staff, particularly the theater nursing team and surgical assistants (who will benefit more from the learning experience).

Spinal cord monitoring is advisable for these cases, including transcranial motor-evoked potentials, somatosensory-evoked potentials, and free-running electromyography (EMG) of the lower extremities. Evoked EMGs with pedicle screw stimulation is an additional safety measure for checking the proximity of pedicle screws to nearby nerve roots.

Prone positioning the patient is required in most cases with the aid of four padded supports—one under each of the anterior superior iliac spines and one supporting either side of the upper chest. The lower supports must not inhibit blood return from the lower extremities and allow the abdomen to hang unopposed. Thus, they are best placed slightly more distally. The upper supports must be distal enough not to compress the brachial plexus. Given the deformities in AS, it is possible that the head will lie below the level of the chest, and slight head-up positioning or supplementary higher thoracic bolsters may help this.

23.4 Osteotomy

Meticulously ensure that the size and limits of the deformity are analyzed and measured preoperatively, then one can plan what osteotomy is required and at what level(s). The apex of the deformity is the most appropriate level to plan correction as this will best approximate the normal anatomy. The anatomic Schwab classification of spinal osteotomy provides a broad guide for the type of osteotomy needed in most spinal deformities, all of which have been reported in the treatment of AS deformity (Fig. 23‑2). Each grade has several described modifications, thus creating significant correctional overlap and, thus, a specific classification is not possible. However, the principle is to release the posterior column and/or the anterior column to allow correction of the deformity; then reconstruction is made by instrumentation with or without anterior column reconstruction. Globally, spinal osteotomies can be divided into posterior column osteotomies and three-column osteotomies (3COs).

Historically, the osteotomy for treatment of AS deformity was a Smith-Petersen osteotomy (SPO), with a resection of the posterior elements, including spinous process, laminae and facets, manual three-point pressure on the spine to complete an audible rupture of the anterior longitudinal ligament (ALL), and application of a molded plaster jacket. This was typically performed in the lumbar spine and could, in theory, achieve corrections of up to 90° but was associated with aortic and gastrointestinal rupture. ^{4 ,​ 5} Thus, SPO, in its original format, is not used currently and, when referred to, is understood to mean a chevron or Ponte osteotomy, or at multiple levels, a polysegmental wedge osteotomy.

Grade 1 osteotomy involves inferior facetectomy and joint capsulectomy and is performed with most simple lumbar fusions. It provides minimal correction (5°) at each level but is more effective when combined with the insertion of an anteriorly placed interbody cage to increase the anterior opening angle.

Grade 2 osteotomy requires resection of both inferior and superior facets of the articulation at a given spinal segment, with the ligamentum flavum and potentially the lamina, or the spinous processes. This is an opening wedge osteotomy, also known as the Ponte procedure, requiring anterior column mobility. As most disks are ossified in AS, anterior opening occurs with disruption of the ALL or osteoclasis of the endplate. Mobilizing a rigid anterior column can lead to vascular and neurological sequelae; thus, one should be cautious with older patients displaying atheromatous plaques on preoperative imaging. The resection of multiple facets, along with resection of the spinous processes, involves a substantial amount of bone and ligament resection to afford deformity correction. Given that these osteotomies do not violate the pedicles, pedicle screws can be inserted into the osteotomized levels. Cage insertion may correct the spine more than the same procedure performed at the same level on a flat spine because the number of degrees that will change the affected level from kyphotic to neutral will be added to the usual 8°–10° achieved with this technique. ⁶

Grade 3 osteotomy includes grades 1 and 2 resections along with a pedicle subtraction osteotomy (PSO), a closing wedge-shaped transpedicular resection extending into the posterior and middle portion of the vertebral body, achieving up to 40° correction. This posterior-only approach is the “workhorse” of correctional procedures for most severe deformities. Some metanalyses have shown that PSO is the osteotomy of choice for sagittal deformities in AS, ^{7 ,​ 8} but others have shown equivalence of correction for both Ponte osteotomy and PSO. ⁹ Ponte osteotomies are technically easier, less bleeding and shorter than PSO, but aortic rupture and related death, albeit rare, during correction are reported during Ponte osteotomies and should be taken into consideration for decision-making. If there is a solid anterior fusion and immobility, then this greatly inhibits correction. PSO is considered more appropriate for more acute kyphotic deformities. PSOs are usually performed at the L3 or L4 level to correct the lack of lumbar lordosis. An L3 osteotomy is technically easier to perform because of its location, and L3 osteotomies can provide a substantial correction. As two-thirds of lumbar lordosis occur between L4 and S1, L3 PSO thus moves the lordotic apex proximally, which may increase the incidence of proximal junctional kyphosis (PJK). L4 PSO achieves a more anatomical correction.

Grade 4 osteotomy includes a transpedicular bivertebrae wedge osteotomy and diskectomy so that the more oblique caudal aspect opposes the relatively transverse inferior endplate of the cephalad vertebra. A limited amount of posterior translation occurs as the cephalad vertebra approximates the inferior aspect of the caudal vertebra. Given the smaller vertebral height and access to the disk, this has greater application at the lumbosacral junction. Therefore, advantages include a greater correction up to 50° and a decreased risk of pseudarthrosis by eliminating the disk above and allowing direct bone-to-bone contact between the osteotomized vertebra and the vertebra above. The location of the pedicle in the thoracic spine is high relative to the vertebral body and very close to the disk. Thus, when the thoracic pedicle is removed, the disk above is immediately involved. ¹⁰

Grade 5 osteotomy is otherwise known as a vertebral column resection (VCR) or vertebral column decancellation (VCD), including the vertebral body and both adjacent disks. It requires anterior column support. Although VCR is the most potent form of single-level osteotomy, it carries a high risk, is very complex, and, in terms of deformity correction, is for a narrow set of indications. One should avoid excessive spinal shortening in the thoracic spine to avoid spinal cord kinking. This can be performed through a combined anterior/posterior or all-posterior approach. The sharper and more angular the kyphosis, the easier the VCR is through an all-posterior approach.

Grade 6 osteotomy involves the removal of more than one vertebra, indicated for short-wedged or flat vertebral deformities. In cases of severe rotation, anterior instrumentation is possible through a posterior approach.

Surgical management of the deformity may also include treatment of posttraumatic pseudarthrosis with AS. As mentioned, hyperextension radiographs will outline this. Between T11 and L1, 77% of pseudarthroses will occur, but they have been described from T9 to L3. ¹¹ Mild neurological dysfunction is common. Both grades 2 (Ponte osteotomy) and 3 (PSO) are described treatments for these, with corrections of 38° to 45°, respectively. ^{12 ,​ 13} After PSO, supplemental anterior fusion is sometimes necessary to support the anterior and middle column in a second stage if there is a bone defect at the osteotomy site. ^{9 ,​ 14} Chang et al ¹³ , with a series of 30 pseudarthrosis cases fixed with Ponte osteotomy from the posterior only, suggests that the superior fusion capacity of AS, the rigid fixation, and the improved biomechanical environment with decreased shearing and distraction forces, together contributed to achieving solid fusion. Consideration for anterior column support must include the diaphragmatic detachment associated with anterior thoracolumbar surgery, which may not be ideal in older or compromised patients. The osteoclasis required to perform a Ponte osteotomy is already present in an AS pseudarthrosis, thus lowering the potential for vascular injury. If a pseudarthrosis exists, it should be taken advantage of for deformity correction in addition to a PSO at a nonadjacent level.

Normally, the deformities are corrected from caudal (mostly lumbar), where they are more powerful, to cephalad (mostly cervicothoracic). Severe “chin-on-pubis” deformities have been described, and their severity mandated working in a staged fashion starting with bilateral staged hip resectional arthroplasty, then C6 PSO, T11T12 VCR, and L3 PSO, then bilateral total hip arthroplasty. ¹⁵

As most AS deformities include both lumbar-based and cervicothoracic-based deformities, it is, therefore, logical to perform multiple staged procedures to address this—either at single or multiple outings—depending on intraoperative tolerance and likely correction gained (Fig. 23‑3 , Fig. 23‑4). Even within the lumbar and thoracolumbar spine, multiple osteotomies may be indicated. Fig. 23‑4 demonstrates the placement of instrumentation from iliac crests to T9 with PSO at L4. The patient was assessed in anticipation of a second PSO, which was carried out at L1. It is possible that a second PSO may not have been necessary and by extending the instrumentation to T9 on the first surgery was still relevant—not adding significant additional morbidity to an already stiff spine. Two-level PSO is better than aiming to achieve greater than 60° with a one-level PSO. ¹⁶ The advantages of this include a more harmonious correction, better global balance, less focal cord manipulation, and less force on the pedicle screws. T12 and L3 are the apical vertebrae of global thoracolumbar kyphosis and the normal lumbar lordosis in AS, respectively. Staged L4 then L1 PSOs are demonstrated in Fig. 23‑3. Higher corrections (>45°) at one level may increase the risk of excessive dural buckling, curving, or even kinking at the apex of the deformity. Fragile patients with severe deformity should be considered for staged procedures if this decreases the complexity of the surgery and the risk for the patient including blood loss and medical complications.

Restoration of lumbar lordosis affects alignment of the cervical region. ¹⁷ The C7 slope becomes more horizontal after lumbar PSO. This affects intracervical adjustment if the neck is flexible, as proximal (C0-C2) and distal (C2-C7) lordosis reciprocate each other to optimize horizontal sight. ¹⁸ The CBVA will also improve after more caudal corrective surgery and, as mentioned, reliably demonstrates the requirement for a cervicothoracic osteotomy (Fig. 23‑5).