Thoracic Myelopathy Caused by Ossification of the Yellow Ligament as a Distal Adjacent Segmental Disease after Posterior Cervical-Middle Thoracic Fusion Surgery

Although adjacent segmental disease after posterior thoracic fusion surgery is rare, thoracic myelopathy due to ossification of the yellow ligament in the lower thoracic spine could develop because of mechanical stress when the lower instrumented vertebra has been set to the middle thoracic spine during the initial surgery. We report an extremely rare case of distal adjacent segmental disease after posterior cervical-middle thoracic fusion surgery requiring reoperation after exhibiting thoracic myelopathy due to ossification of the yellow ligament in the lower thoracic spine. An obese 53-year-old man with diabetes had undergone C3-6 laminoplasty and C7-T8 posterior decompression plus fusion due to ossification of the posterior longitudinal ligament at C5-T5. Although the short-term clinical course after the initial surgery was good, symptoms of myelopathy reappeared because of the ossification of the yellow ligament that developed at T9-11 with local flexibility. Thus, reoperation with fusion extension surgery was needed 1 year and 6 months after the initial surgery. Altogether, we recommend careful monitoring of the postoperative clinical progression and, if necessary, reoperation at the earliest.


Introduction
Adjacent segmental disease after posterior thoracic fusion surgery is much rarer compared to that after surgery involving the cervical or lumbar spine due to the reduced mobility of the thoracic spine [1]. Mechanical stress, however, is reportedly the most important cause of increased ossification of the yellow ligament (OYL) [2]. We report an extremely rare case of myelopathy caused by OYL in the lower thoracic spine as a distal adjacent segmental disease after posterior cervical-middle thoracic fusion surgery.

Case Presentation
An obese diabetic 53-year-old man (body mass index, 36.3 kg/m 2 ) presented to our hospital with severe myelopathy due to C5-T5 ossification of the posterior longitudinal liga-ment (OPLL) (Figure 1(a)). He had undergone C3-C6 laminoplasty and C7-T8 posterior decompression plus fusion a week after the first visit (Figure 1(b)). Although symptoms improved and the patient exhibited smooth gait after surgery, he experienced lower limb numbness and discomfort while walking 10 months postsurgically. The patient's spastic gait became more evident 1 year after surgery, and he needed Lofstrand crutches to walk.
The patellar and Achilles tendon reflexes were enhanced in both sides; however, manual muscle testing was normal. The patient experienced numbness in both lower limbs without urinary disorders. His Japanese Orthopaedic Association (JOA) score for thoracic myelopathy was 7 points out of 11 (full marks).
Imaging studies showed bone fusion in the posterior fusion surgery area, with no new spinal cord compression lesions. The lateral dynamic X-ray showed the kyphosis anglein T9-11 to be 15 degrees in the standing position ( Figure 1    Case Reports in Orthopedics ligament ossification at T7-9 before the initial surgery (Figures 1(a) and 2(a)), we diagnosed that myelopathy developed due to lower thoracic OYL as a distal adjacent disease. The patient underwent reoperation 1 year and 6 months after the initial surgery. The posterior fusion extension surgery involved inserting pedicle screws into T11, T12, and L1, followed by laminectomy from T9-11 and resection of the OYL lesions (Figures 3(a) and 3(b)). The lesion at T9-10 showed dural ossification. Therefore, the entire ossified dura mater was carefully removed along the margins and repaired through patching with an artificial dura.
The symptoms of thoracic myelopathy gradually improved without complications following surgery, and the patient returned to work as a bus driver 6 months after reoperation. One year after reoperation, X-ray showed bone fusion and no loosening of the implant (Figure 3(c)). The patient displayed smooth gait without crutches, and the JOA score improved to 9.

Discussion
Long-range posterior decompression and fusion are often the first-choice surgical approaches for thoracic OPLL [3]. In this case, although OYL lesions were observed at T9-11 levels, there was no spinal compression at the same levels before the initial surgery. Therefore, we decided to consider T8 as the lower instrumented vertebra and to not perform prophylactic surgery on any lower vertebra. The short-term clinical course after initial surgery was good; thus, we believed that the initial fusion range was adequately selected.
Although myelopathy due to OYL in the lower thoracic spine as a proximal adjacent segmental disease after lumbar posterior fusion surgery has been reported [4], the same cannot be said for distal adjacent segmental disease.
Distal adjacent segmental disease is usually considered to occur at T8-9 when the lower instrumented vertebra is T8. However, in this case, bony fusion of the anterior longitudi-nal ligament was already observed at T7-9 before the initial surgery (Figures 1(a) and 2(a)). Therefore, the lower "fused" vertebra was T9 after the initial surgery. Apart from intervertebral degeneration due to mechanical stress [2], diabetes and obesity, which were seen in this case, can cause the development of OYL and can increase OYL [5]. Furthermore, the frequency of dural ossification increases with the severity of OYL [6]. According to the dynamic X-ray, which showed marked flexibility between T9 and T11 depending on the posture prior to reoperation (Figures 1(c) and 1(d)), we believed that there was high mechanical stress between vertebrae, extending caudally from the fusion range, especially at the T9-10 level. Thus, OYL had advanced to dural ossification within a short time period and ultimately led to thoracic myelopathy as a distal adjacent segmental disease.
Distal adjacent segmental disease can occur after posterior thoracic fusion surgery, and preexisting lower thoracic spine OYL can develop and present as thoracic myelopathy. Careful monitoring of the postoperative clinical progression and early reoperation, if necessary, are essential.