Posterior circumferential decompression and instrumented fusion was performed in all patients. Intraoperative neuromonitoring [transcranial motor evoked potential (TcMEP) and somatosensory evoked potential (SSEP)] were applied in all patients, but TcMEP was not obtainable in 2 patients (Case 1 and 3). All operations were performed via midline skin incision. First, pedicle screws were inserted 1-2 levels above and 1-2 levels below the index level. The lateral margin of lamina was cut at the junction of facet joint with the ultrasonic osteotome (BoneScalpel, Misonix, NY, USA) (Fig. 1). The lamina was gradually cut with ultrasonic osteotome by slight pushing down its blade against lamina with a vertical motion. When inner cortical bone was cut, loss of resistance was felt and the ostetome was not inserted beyond the depth (Fig. 2A) (Supplementary Video 1 in the online-only Data Supplement). After elevation of cut lamina with clamp, the cranial and caudal ligamentum flavum of elevated lamina was resected with Kerrison rongeur. After laminotomy, the remained facet joint, transverse process and pedicles were removed with conventional rongeur and punch. Spinal nerve roots at the index level were resected bilaterally after no change in intraoperative neuromonitoring (INM) upon 10 minutes of neurovascular bundle clipping. After dissecting the lateral margin of the OPLL mass from the dura, the superior and inferior margin of the beak-type OPLL mass was identified. The base of OPLL mass was removed initially. Next, the OPLL mass attached to dura was removed with the ultrasonic osteotome (Fig. 2B). Use of the ultrasomic osteotome allowed direct resection of the OPLL mass under gentle dural retraction with a suction tip until half of its width was resected. A similar technique was repeated on the contralateral side. After removal of OPLL mass, the removed lamina was replaced by connecting to cranial and caudal lamina with miniplate or translaminar screw (Fig. 3, 4)^22,29). Finally, rods were fixed to pedicle screws and pedicle screws were inserted at the index vertebra to enhance stability and reduce extent of surgery¹⁾. After grafting allobone chips on the decorticated lamina, the surgical wound was closed in a layer by layer fashion.

A 67-year-old female patient presented with 6 months of paraparesis. Preoperative MR imaging and CT scan showed a beak-type OPLL mass and spinal cord compression at T5-6 (Fig. 3A-D). Cervical laminoplasty was performed 3 years prior due to cervical OPLL, but paraparesis (motor power grade IV/V) developed 2.5 years after her previous surgery. Preoperative JOA score was 2/11 (1+0+0+1). Posterior circumferential decompression and instrumented fusion was performed with the use of the ultrasonic osteotome. With the help of tissue selectivity of ultrasonic osteotome, the OPLL mass could be directly resected from the dura. The final portion of OPLL was difficult to remove, once the mass lost connection with the base of the OPLL mass, due to ineffective transmission of ultrasonic energy. During the operation, there was no significant change in intraoperative SSEP monitoring. TcMEP was not detectable. Dural injury was occurred during removal of OPLL and the defect was repaired with adhesive dural sealant. Postoperative CT scan showed nearly complete removal of the OPLL mass (Fig. 3E-H), but a small fragment of OPLL mass was left (Fig. 2G). The lamina was replaced with mini-plates (Fig. 3I). There was no sequelae associated with the dural tear. The patient's preoperative paraparesis was improved and independent ambulation was possible 1 month after surgery. Ten months after the date of surgery, the JOA score was 8/11 (2+2+2+2); a recovery rate of 67% (Supplementary Video 2 in the online-only Data Supplement).

Different from Case 3, the base of OPLL mass was removed with ultrasonic osteotome and the OPLL mass was detached and removed from the dura. Because the energy was emitted from the end of osteotome, the T12 root could be safely retracted with the osteotome during resection of OPLL (Fig. 2B), and the T12 root was preserved. Preoperative paraparesis was improved at postoperative month 1 (a recovery rate of 43%) (Supplementary Video 3 in the online-only Data Supplement).

There are various kinds of surgical options for beak-type T-OPLL. Direct decompression could be obtained with anterior, posterior or a combined approach. Indirect decompression could be achieved with laminectomy, laminoplasty or a posterior instrumented correction of thoracic kyphosis^{5,811,1213,1415,1826,2728,30)}.

Matsumoto et al.¹³⁾ evaluated various surgical approaches for beak-type T-OPLL in 45 patients; 8 patients underwent laminectomy, 16 underwent laminoplasty, 12 underwent anterior decompression via anterior approach, 5 underwent anterior decompression via posterior approach, 2 underwent both anterior and posterior approach and 2 underwent anterior decompression via sternum splitting approach¹³⁾. Clinical outcome, surgical technical difficulty and safety are important factors in determining the desired approach. Regarding clinical outcome, removal of the T-OPLL mass seems to be associated with a better outcome; a mean recovery rate of JOA was about 60% with circumferential decompression and about 30-40% with only posterior decompression^13,28). An anterior approach may be preferable to remove T-OPLL, but the transthoracic/extraplerual approach or trans-sternal approach is technically challenging and is associated with a higher complication rate^4,1618). A recent review on this subject has shown that neurological deficit occurred in 2.7-18.8% of patients after an anterior approach^5,1718). Direct removal of the ossified mass with a posterior only approach may be an alternative, but it has been traditionally challenging secondary to associated kyphotic deformities, adhesion of T-OPLL to the dura and a narrow/blind surgical field^3,16). Direct removal of T-OPLL mass was associated with a high neurological complication rate; neurological deficit occurred when T-OPLL was excised in 7/48 (14.2%), thinned and floated in 7/25 (28%), and left untouched in 4/81 (4.9%)¹³⁾. The rate of neurologic deficit was especially high for patients with beak-type T-OPLL (30%)^11,1526,28). Therefore indirect decompression through instrumented correction of thoracic kyphosis after posterior decompression was suggested as a safe alternative to allow for floating back of the spinal cord^13,2830). However, postoperative magnetic resonance imaging (MRI) demonstrated that indirect decompression was not sufficient after correction of kyphosis alone in 13 out of 15 patients, and 11 patients underwent a secondary procedure for anterior decompression⁸⁾. Matsuyama et al.¹⁶⁾ showed that neurological deterioration occurred in 5/21 patients with beak-type T-OPLL after posterior indirect decompression because of persistent ventral compression. In addition, growth of beak-type T-OPLL was observed 3 years after laminectomy and corrected fusion of thoracic kyphosis²⁵⁾. Although indirect decompression through instrumented correction of kyphsosis was an alternative, considering those results together, direct removal of T-OPLL, especially for patients with beak-type T-OPLL, seems to be an appropriate surgical option.

Neurological deficits usually occurred immediately after surgery in cases of T-OPLL. Therefore, surgical insult to the neural tissue must not be disregarded^11,1226). During surgery, the T-OPLL mass is traditionally removed with a high-speed drill and curette. Neurologic injury may occur during removal of T-OPLL just beneath dura with high-speed drill¹¹⁾. When there are strong adhesions of T-OPLL mass to the dura, use of high-speed drill for removal of T-OPLL may be dangerous secondary to rotatory-motion of the drill⁶⁾. Ultrasonic osteotome has been used in various surgeries such as plastic surgery, but the usage was not widely spread in spine surgery^{2,67,2023,24)}. Ultrasonic osteotome utilize back and forth micro-motion rather than rotatory-motion, thus energy from the cutting edges of these devises is preferentially transmitted to hard structures, such as lamina^2,720,2324). The dura can bend, vibrate and deform to vibratory micromotion with minimal transmission of energy^2,720,2324). Moreover, the safety of surgical procedure may be enhanced, because movement of its tip is limited to a fine vibration without problem of bounce of rotatory drill⁶⁾. This features enable selective removal of beak type OPLL with minimal injury to the dura and spinal cord. The relative tissue selectivity allowed for brief contact with the dura, and thin shaving of T-OPLL and preservation of nerve root were possible (Fig. 2B)^2,720,2324). The ultrasonic osteotome may be preferable for removal of T-OPLL over a high-speed drill in this regards^2,720,2324). However, the laminotomy with ultrasonic osteotome is performed without direct visualization of dura, and spinal cord injury may occur by insertion of blade beyond the inner cortex of lamina, if penetration of lamina is not felt. Moreover, the pressure of irrigation fluid may injure spinal cord. These shortcomings should be noticed before utilizing ultrasonic osteotome.