Ankylosing Spondylitis - Deformity Management Continued

MANAGEMENT – continued

Transpedicular Screw Fixation

Targeting the lumbar region for correction has its advantages as were mentioned, but when desired curvature is ascertained and proper line of gravity is obtained the rest of the spine takes on a shape that is more structurally and functionally correct. Although plaster jackets and shells attempt to address such concerns, the nuisance of prolonged rehabilitation and risk of succumbing to previous malalignment does exist. Therefore, surgeons are constantly attempting to reduce postoperative discomfort and risks by internal stabilization of the spine in the desired posture. However, such instrumentation has been known to loosen or break, new advances are constantly being sought.

Since 1979, Puschel and Zielke implemented the Harrington compression system for posterior multilevel, four to six segments, osteotomy. (35) In 1982, Puschel and Zielke reported their experience of (14) AS individuals with such instrumentation. (35) Although promising, the Harrington compression system did not adequately provide harmonic relordosation and continued to pose a risk of sharp angled monosegmental extension osteotomy. (21, 30, 35) As a result, transpedicular screw –rod fixation was introduced which facilitated relordosation, decreased mortality, decreased neurologic compromise, and decreased the risk of sharp angled monosegmental extension osteotomy.(5, 23,30,34,41)

In 1990, Hehne, Zielke, and Bohm reported on the outcome of 177 AS patients who underwent polysegmental lumbar osteotomies with transpedicular screws. (20) Hehne et al performed a similar osteotomy as Smith–Petersen et al at T12 or L1 to L4. Following the osteotomy, transpedicular screws are placed between T9–S1. Hehne et al positioned the screws at the crossing of the axis of the transverse process and joint line. Screw direction was at 15 degrees to the midline and was not pushed through the anterior cortex of the vertebra. Closing of the osteotomy was accomplished through slow straightening of the table while tightening the nuts on the screws. Postoperatively, a plaster cast was worn for four months followed by wearing a Stagnara brace for an additional four months.

Complication rate was minimal with 77.4% reporting no complications. Seventy–seven percent presented no complications, complications were reversible in 18%, 7% required reoperation, and four patients died of causes not related to the procedure. The average correction was 44 degrees which translates to a gain of 9.5 degrees per segment and a 7 degree or 15 degree loss of correction at final examination. Back pain improved, no evidence of pseudoarthrosis was present, progressed kyphosis above the instrumentation did not occur, and all patients achieved solid bone fusion eight months postoperative. Hehne et al also found that patients exhibited a loss of lower thoracic correction after a period of three years. As a result, Hehne et al recommended application of internal instrumentation to be applied at T9 – S1.

Posterior Wedge Osteotomy with Vertebral Corpectomy

One of the first to propose a posterior wedge osteotomy approach with dorsal partial vertebral corpectomy was Thomasen in 1985. (42) Thomasen reported a similar procedure performed by Scudese and Calabro in 1963. (36) Thomasen noted the results of eleven patients who underwent his procedure and who achieved 12–50 degree correction. The procedure consisted of the patients in a prone position with endotracheal anesthesia. The vertebral segment of choice was L2. A midline incision was made at T11–L4 and subsequent spinous muscle retraction of L1–L4. A posterior wedge 5 cm broad osteotomy followed to remove the tip of the L2 spinous process and the superior portion of L3 spinous process. The osteotomy also consisted removal of L2's pedicles, laminae, transverse processes, superior articular processes, and the removal of the upper portion lamina of L3. As a result, the nerve roots and dural space were exposed and liberated. With caution, an osteotome was used to resect the posterior wall of the vertebra (Fig. 6). A ronguer was then implemented laterally to remove spongy vertebral bone which produced bleeding and necessary steps to establish hemostasis followed. Moreover, dura was stripped from adjacent laminae. By manual and operative table manipulation, the spine was extended and a compression fracture was created at the L2 vertebra and the osteotomy was closed (Fig. 7). A plaster jacket was worn for 3 months. Thomasen also applied plates and metallic wires posteriorly in some patients to facilitate stability and fusion. Patients reported a positive outcome. Thomasen's method has gained popularity because there is decreased stretch of the cauda equina, aorta, and intraabdominal muscles which could complicate the outcome.

Thomasen Pedicle Subtraction Method with Transpedicular Screw –Rod Fixation

Although many have contributed possible avenues for correction of lumbar fixed deformity in AS, the spine author's preference is a combined monosegmental Thomasen osteotomy and transpedicular screw – rod fixation. (23,44) The patient is positioned prone and a midline incision at the spinous process is performed between the levels of T12–L4. After necessary paraspinal and spinous muscles are reflected, transpedicular screws are inserted at T12, L1, L3, and L4 (Fig. 8). The screws positioned centrally, are inserted parallel to the superior end plate, converge towards the midline, and directed towards the anterior vertebral cortex without complete penetration. Afterwards, a posterior V–shaped wedge osteotomy, as Thomasen reported, is performed at the L2 vertebral segment and H frame rods are positioned over the pedicled screws (Fig. 9). The osteotomy is closed by table extension and manual leg elevation slowly while compressing the fixation device. The patient is then placed in a TLSO immobilizer for 12 weeks.

The spine author has found this method beneficial in correcting fixed lumbar deformity in AS. Lumbar correction is achieved providing the patient with a more erect posture, increased horizontal gaze, increased pulmonary function, subsidence of pain, and a more physically acceptable appearance (Fig. 10). Due to the nature of the osteotomy, the neural structures are visible which aid the physician to avoid peripheral neural compression when closing the osteotomy site. Since partial removal of the L2 vertebral body is accomplished, no elongation of the anterior column occurs which could pose a threat to proximal vascular and neural structures. The intact anterior longitudinal ligament and the anterior cortex of the vertebra act as hinges which provide sound fusion of the remaining rich vertebral cancellous bone and posterior segments. Furthermore, the use of transpedicular screw fixation prevents displacement, provides solid stability and sagittal correction, increases proper bone fusion, and decreases the wear of external orthosis (Fig. 11 A,B). (16,20,21,23,34,35,44)

Complications

With mortality rates as high as 10 %, (12,27,28,29,30) the search to decrease complications is priority. Although new innovations and technique refinement have decreased the risk of death, (16,20,23,44) the threat of neurological compromise and vascular and pulmonary complications should be omnipresent. In 1956 Lichtblau and Wilson first reported possible causes for aortic rupture in correction of AS. (32) Lichtblau and Wilson mentioned a case where a 35 year old male under general anesthesia underwent osteotomy at three levels. The aorta was not fully calcified and presumably when the adjacent calcified anterior longitudinal ligament ruptured, after the vertebra was separated at the intervertebral disc, the aorta did not give and the patient died. Further cases were also reported by Klems and Fried in 1971 (25) and Weatherley et al in 1988. (47) Weatherley et al reported two cases of aortic rupture due to lumbar extension osteotomy. In both cases, the patients developed a retroperitoneal hematoma and developed signs of cardiovascular collapse which led to death. Moreover, Camargo et al also reported a case where aortic rupture occurred on the ninth postoperative day. (12)

Further manifestations of AS could pose complications during surgery. An aspect of the inflammatory response in AS is the fibrosis of cardiac vessels and the aorta. As a result, the aorta weakens and is more prone to rupture from aneurysms . Cardiac conduction is also affected causing arrhythmia due to fibrosis invasion of the myocardium. Also, cardiomegaly is known to occur and angina (7) Furthermore, pulmonary fibrosis could occur in the upper lobe and could lead to edema.

Intraoperative methods of anesthetization could manifest complications. Due to the abnormal cervical kyphotic curvature that results in patients with progressed forms of AS, intubation for general anesthesia is difficult and could generate respiratory complications. (1,26,37) As a result of intubation, patients with a severely kyphotic cervical spine are unable to cough postoperatively and accumulate mucus in their respiratory tract. Moreover, progressed AS patients suffer from pulmonary restriction. Thus, positioning the patient on the operating table needs careful attention. (1,5,19,23,26,37,44)

Complications could also arise from elongation of the anterior column. Elongating the anterior column could stretch the superior mesenteric artery over the duodenum which leads to gastric dilation. (37) As a result, patients are prone to vomit and if intubated encounter further obstruction. Moreover, vascular vessel elasticity could develop thrombosis (28) not to mention aortic rupture. (12,25,32,47) Furthermore, neurologic compromise could result in the form of neuropraxia or paraplegia (5,20) from spinal cord compression as a result of insufficient bone removal, (44) subluxation, (22) and impingement. In addition, retrograde ejaculation is also considered a risk. (40,46)

Further complications also are present. Instrumentation failure could develop from improper application or failure to tighten properly for compression. (16,20,34,44,46) Furthermore, shock, (28) amyloidosis, (28) pneumonia, (22) pulmonary embolism (22,34) deep wound infection, (20,34) dural leak, (16,34,41) deep venous thrombosis, (5) nonunion (46) and over correction could also surface.