application of piezosurgery to treat thoracic myelopathy caused by

Int J Clin Exp Med 2016;9(9):17812-17821www.ijcem.com /ISSN:1940-5901/IJCEM0031160

Original Article Application of piezosurgery to treat thoracic myelopathy caused by ossification of the ligamentum flavum

Hongwei Gao1, Lan Liu2, Deguo Xing1, Jiachun Zheng1, Yangyang Zhao1, Zhonghao Liu1, Mingzhi Gong1

Departments of 1Trauma and Orthopedics, 2Gastroenterology, The Second Hospital of Shandong University, Shan-dong Province, People’s Republic of China

Received April 25, 2016; Accepted July 30, 2016; Epub September 15, 2016; Published September 30, 2016

Abstract: Objective: Piezosurgery is based on microvibrations generated by the piezoelectrical effect, which results in selective bone cutting with preservation of adjacent soft tissue. We aimed to study the applicability of piezosur-gery in treating thoracic myelopathy caused by ossification of the ligamentum flavum (OLF). Methods: Between June 2011 and October 2015, thoracic myelopathy caused by OLF was diagnosed in 22 patients using plain radiography, CT, and MRI, and diagnoses were confirmed by postoperative pathological examination. Piezosurgery was sup-portively used to treat thoracic myelopathy in 22 patients with either OLF or dural ossification. After laminectomy, OLF or ossified dural mass was removed by piezosurgery to decompress the spinal canal. The modified Japanese Orthopedic Association (mJOA) scoring system was used to assess neurological status. The degree of postopera-tive expansion of the spinal cord was calculated on axial CT images. Results: In all 22 cases, Ossified mass, even those adhered onto dura or ossified dura could be safely removed by piezosurgery. No patient experienced any new neurological deficit after surgery. No serious complications ensured, only one patient suffered dura tear. Neu-rological status improved during follow-up (mean, 22.4 months) from a preoperative mean mJOA score of 5.05 ± 1.84 to 8.68 ± 1.52 points (t = 7.14, P < 0.05). Neurological-function recovery rate ranged from 33.3% to 100%. Conclusions: Piezosurgery could be a useful and safe technique for selective bone cutting in OLF decompression and removal, especially DO.

Keywords: Ossification of the ligamentum flavum, piezosurgery, thoracic myelopathy, bone cutting, dural ossifica-tion

Introduction

Ossification of the ligamentum flavum (OLF) is an important cause of thoracic spinal stenosis (TSS) in Asian populations, especially in Japan and China [1]. The disease has a predilection for the lower thoracic spine and frequently affects adults aged between 40 and 60 years. Dural-membrane involvement in ossification makes surgery more difficult and significantly increases the risks of spinal cord damage and complications, such as cerebrospinal fluid (CSF) leakage and meningitis. Under these cir-cumstances, the choice of surgical techniques is important to avoid unnecessary complica-tions, and surgical manipulation for removing the ossified mass must be performed very cautiously.

Laminectomy can decompress the spinal cord, and high-speed drills and rongeurs are often

used to resect laminae and ossified ligaments. However, serious OLF results in severe spinal stenosis and maximum spinal cord compres-sion, so any procedure using instruments inside the spinal canal can cause irreversible spinal cord injury and CSF leakage [2].

The relatively new piezosurgery technique is based on microvibrations, which are generated by the so-called piezoelectrical effect and has thus selective bone-cutting properties with preservation of adjacent soft-tissue structures [3, 4]. In this article, our piezosurgical devices manufactured by Surgybone (Silfradent SRL, Italy) with angled inserts was used supportively for the conventional rotating bur in 22 patients to remove ossified ligamentum flavum, espe-cially ossified dura. In all cases, the applicabili-ty, advantages, and disadvantages of piezosur-gery were evaluated for this indication.

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Removal of OLF by piezosurgery

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Materials and methods

Patient data

Twenty-two consecutively admitted patients with thoracic OLF underwent posterior decom-pression using the piezosurgery device between June 2011 and October 2015. Their mean age was 59.4 years (range, 48-71 years) at diagno-sis. Follow-up duration was 22.4 months (range, 12-36 months). All patients were subjected to a neurological examination in which the clinical features, mode of onset, and inaugural symp-toms of the illness were noted in a standa- rdized manner. Magnetic resonance imaging (MRI) and three-dimensional CT were per-formed in every patient. The radiologic work-up provided the characteristics of OLF (i.e., loca-tion, number of affected segments, and unilat-eral or bilateral nature), possible spinal cord

involvement, and accompanying narrowing of the cervical or lumbar canal. Decompression extent was evaluated by performing preopera-tive CT. Changes in signal intensity on the MRI of the vertebrae and spinal cord at the corre-sponding vertebral level were also observed for reference. The modified Japanese Orthopedic Association (JOA) scoring system was used by two experienced spinal surgeons to evaluate the neurological status of patients before and after surgical decompression [5, 6]. The maxi-mum score of 11 indicates normal function. An improvement rate (IR) was calculated as fol-lows: IR = (post-operative JOA score - preopera-tive JOA score)/(11-preoperative JOA score) × 100%. IR was then used to define surgical out-come: excellent (IR ≥ 75%), good (75% > IR ≥ 50%), fair (50% > IR ≥ 25%) and poor (IR < 25%) [7].

Figure 1. Piezosurgical devices with handpiece, footswitch, and different inserts. The main unit contains a peristal-tic pump for cooling with a jet of physiological saline solution and a control panel with digital display to select the different modes and power levels.


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Piezosurgical device

The piezosurgical device, manufactured by Surgybone (Silfradent SRL, Italy), was used to remove the laminae and ossified ligamentum flavum and decompress the spinal canal.

Our piezosurgery unit consisted of two piezo-electric handpieces and a footswitch connect-ed to a main unit that supplied power and had holders for the handpieces and irrigation fluids. It contained a peristaltic pump for cooling with a jet of physiological solution (0.9% saline solu-

Figure 2. Axial CT scan of the thoracic spine show narrowing of the central spinal canal due to ossification of the ligamentum flavum (OLF). The OLF may be either unilateral or bilateral. Axial CT image (A) show a V-shape of OLF. Axial CT scan indicate a typical tram track sign due to ossification of the ligamentum flavum (B).

Figure 3. Sagittal T2-weighted MRI of the thoracic spine (A) show hyperintense signal in the spinal cord due to compression. The ossification of the ligamentum flavum displays a hypointense signal. Axial T2-weighted MRI of the ossified ligamentum flavum (B) show a nodular low signal intensity mass.


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Table 1. Demographics and clinical outcomes of 22 patients surgically treated for thoracic ossifica-tion of the ligamentum flavum by piezosurgery

Patient NO Age Sex Symptom duration (months)

Follow up (months)

mJOA scoreIR (%) Blood loss

(ml)Surgical outcomePreOp. Postop.

1 48 M 3 24 5 8 50 350 Good2 65 F 6 36 6 10 80 360 Excellent3 62 M 12 16 5 10 83.33 260 Excellent4 58 F 8 15 7 9 50 310 Good5 68 F 16 12 4 8 57.14 400 Good6 57 F 18 26 3 6 37.5 280 Fair7 56 M 3 14 6 9 60 320 Good8 49 M 2 30 7 11 100 410 Excellent9 59 F 6 36 2 5 33.33 270 Fair10 71 M 4 24 5 9 66.67 300 Good11 69 M 3 12 9 11 100 230 Excellent12 55 F 4 20 4 8 57.14 270 Good13 62 M 6 16 6 9 60 250 Good14 69 M 8 18 3 7 50 300 Good15 58 F 10 24 8 10 66.67 210 Good16 49 F 7 30 5 9 66.67 180 Good17 64 F 11 26 6 8 40 200 Fair18 60 M 5 24 3 8 62.5 430 Good19 57 M 8 18 5 9 66.67 340 Good20 58 F 9 30 4 10 85.71 170 Excellent21 52 M 12 24 6 10 80 190 Excellent22 61 F 5 18 2 7 55.56 230 GoodM = male, F = female, IR = improvement rate, JOA = Japanese Orthopedic Association, Preop. = preoperative, Postop. = postop-erative.

tion) that discharged from the inlet at an adjust-able flow rate of about 0-80 mL/min and that removed detritus from the cutting area. The settings for power and frequency modulation can be selected on a control panel with a digital display.

Ultrasonic vibrations were generated by piezo-electricity with variable frequencies from 25 kHz to 35 kHz, which resulted in variations in cutting energy. The amplitude of the working tip ranged from 40 µm to 200 µm. The piezosurgi-cal device was endowed with various inserts having different tips, angles, and diameters (Figure 1).

Surgical technique

Under general anesthesia, the patient was placed in the prone position and the abdomen was decompressed to avoid excessive epidural bleeding. The appropriate surgical level was confirmed by intraoperative radiography, and a

standard posterior approach was taken. After the necessary dissection, the spinous process-es and the superficial part of the laminae were removed using rongeurs. High-speed drills were used to treat the deeper parts of the laminae. Afterwards, the ossified ligamentum flavum was first subjected to piezosurgery using the SB P0700 insert with caution to cut through ossi-fied ligamentun flavum. In this procedure, par-ticular attention was paid to ensure that the ossified mass was excised without any violence because the ossified lesions differed in size, and the ossified ligamentum flavum varied between the laminae and the ossified lesion in front of the facet joint. Moreover, excessive mechanical force can make the sawtooth punc-ture the dura and lead to CSF leakage. When the ossified ligamentum flavum had been excised away to be thin, we used SB P0100 inserts to carefully excise the ossified ligamen-tun flavum on the dura surface. During this pro-cedure, a sharp dissector was used to peel off


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the underlying dura. However, if ossification of the dura mater or a severe adhesion existed between the dura mater and OLF, we excised the ossified ligamentum flavum together with the adherent or ossified dura mater, leaving the arachnoid intact. An artificial membrane was not used to repair the dural defect. The lami-nectomy included the medial half of the width of the facet joint. We always placed a subfas-cial drain for posterior wounds, and drains were removed on the second postoperative day. All patients commenced a back extensor exercise program after hospital discharge.

Statistical analysis

Statistical analyses were performed using SPSS 16.0 statistics software. A paired t-test was used to assess the difference between mJOA scores before and after the operation. Continuous data are presented as mean stan-dard deviation (SD), and categorical variables are presented as numbers. All statistical as- sessments were two sided, and evaluated at the 0.05 level of significance.

Result

Radiographs, CT scans, and MRI were used to confirm diagnosis in each patient [8]. We evalu-ated the extent of the ossified thoracic ligamen-tum flavum using three-dimensional CT scan [9]. CT scanning revealed that OLF appeared as radiodense bars along the line of the laminae, radiating inward from the facet joint capsules toward the midline, either unilaterally or bilater-ally (Figure 2). Sagittal MRI demonstrated med-ullary compression because of a V-shaped or sawtooth-shaped lesion in the posterior margin of the canal. These lesions were hypointense on T1- and T2-weighted MRI, as shown in Figure 3.

A previous report has indicated that the best method of diagnosing TSS associated with OLF is MRI combined with CT scan [10]. A clear diag-nosis can be obtained with a combination of medical history and thoracic radiography, CT scan, and MRI. For 22 patients, the MRI signal of the spinal cord was abnormal at the stenosis level. Demographics and clinical outcomes of

Figure 4. Preoperative axial CT scan (A), Sagittal reconstructed CT images (B) of the ossified ligamentum flavum show a nodular shaped mass anterior to the lamina and facet joints of the lesion segments at multiple levels. Pre-operative axial T2-weighted (C) and sagittal T2-weighted (D) MRI of the ossified ligamentum flavum show a nodular shaped (C) and sawtooth-shaped (D) lesions in the posterior margin of the spinal canal due to ossification of the ligamentum flavum. Intraoperative photographs (E) after laminectomy show laminae and ossified mass (F) were removed by piezosurgery device and the dural sac shifts toward the posterior side. Postoperative axial (G) and sagittal reconstructed (H) CT scan show that the range of decompression reached the medial border of the pedicle bilaterally, no ossified mass remain and the spinal cord is decompressed sufficiently.


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22 patients surgically treated for thoracic ossi-fication of the ligamentum flavum are shown in Table 1.

All patients were clearly diagnosed with TSS associated with OLF, one patient had an accom-panying cervical-canal narrowing, and another had an accompanying lumbar-canal narrowing. All patients underwent posterior decompres-sion with laminectomy, and an ossified mass was excised. The decompression extent was three to five laminae. Every patient underwent laminectomy without internal fixation to restore stability, and no thoracic kyphosis was observed after follow-up.

No serious complications were noted with this procedure. One dura tear occurred and repaired was by suture because we accidentally pricked the dura with a ragged tip. Then, we improved and used the nontraumatic tips to cut the ossi-fied ligamentum flavum, and no dura tear occurred. For one patient who had dural ossifi-cation (DO), we carefully cut the ossified mass by piezosurgery bone; when we removed the ossified mass, we found a small area of dural defect and no CSF leakage because of the arachnoid membrane was intact. No subarach-noid space formed, and no postoperative wound infection occurred. No patient experi-enced thoracic myelopathy because of OLF regrowth or compression fractures in the involved vertebrae.

The diameter of the compressed spinal cord returned to normal postoperatively, as shown in the pre- and postoperative CT scans (Figure 4). Generally, neurological status improved at fol-low-up from a preoperative mean JOA score of 5.05 ± 1.84 (range, 2-9) points to 8.68 ± 1.52 (range, 5-11) points (t = 7.14, P < 0.05). These results were considered significant, and the surgery for OLF was considered effective. The IR of neurological function ranged from 33.3% to 100% (mean ± SD, 64.04% ± 18.23%). Surgical outcome was excellent in six (27.3%) patients, good in thirteen (59.1%) patients, and fair in three (13.6%) patients (Table 1). No patient had worsened neurological symptoms.

In all 22 cases, the piezosurgical instruments selectively cut bone with no damage to the dura, spinal nerves, and spinal cord. The han-dling of the instrument was generally easy, and the cutting process was extremely safe. The cut

appeared to be precise and selective because the effect of bone drilling was due to microvi-brations instead of rotating power, and bone can thus be removed using shaping movements as with a curette. Ossified mass, even those adhered onto dura or ossified dura, could be safely removed. Supportive use of decompres-sion on the dura surface was also easily achieved without any complication. The inte-grated irrigation provides cooling and renders additional irrigation dispensable.

The handpieces cannot be used with mechani-cal force because the tips are not very strong; thus, the tip may break if the mechanical force outweighs its support. This phenomenon occurred at the beginning of our use. Another rea-son was the tips could prick the dura with mechanical force, especially ragged tips, so we must softly cut the bone and ossified mass with piezosurgery device. Therefore, the bone removal rate of the piezosurgery device was lim-ited, resulting in prolonged operation time.

Discussion

Ossification of the ligamentum flavum in the thoracic spine is being increasingly recognized as a cause of thoracic myelopathy in Asian pop-ulations [11]. This condition commonly affects the lower thoracic spine, especially T9-L1 [12, 13].

Posterior decompression with laminectomy is commonly used to treat thoracic OLF. Traditional laminectomy (pedicle-to-pedicle en bloc lami-nectomy) includes removal of laminae, ossified ligamentum flavum, and medial half of the facet joint [14, 15]. Previous studies have suggested various surgical techniques depending on the radiological type of OLF [16, 17]. All of our patients underwent en bloc resection by cut-ting the bony junction between the pedicle and upper facet through piezosurgery.

Performing en bloc laminectomy in the pres-ence of DO is not recommended because of the high probability of extensive dural lacerations. Wang and Chen [18] suggest leaving a floating fragment adherent onto the dura mater to avoid this complication, but this may not provide acceptable decompression. In our study of patients who were found to have an associa-tion between OLF and DO intraoperatively, we cannot resect the OLF from the ossified portion


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of the dura mater, and both DO and OLF were resected by piezosurgery. Arachnoid mem-brane was intact, and no CSF leakage was observed. The dural defect was not repaired, and meticulous closure of the paraspinous muscles, deep fascia, and skin was performed with application of continuous pressure to the wound. The IR of neurological function ranged from 33.3% to 100% (mean ± SD, 64.04% ± 18.23%). Surgical outcome was excellent in six (27.3%) patients, good in thirteen (59.1%) patients, and fair in three (13.6%) patients (Table 1). No patient had worsened neurologi-cal symptoms. Therefore, this method was effective.

In a retrospective study of 81 patients by Gu [19], 843 patients underwent decompression without instrumentation, and surgery was sup-plemented with posterior transpedicular screw fixation in 38 patients. These patients did not show a significant benefit compared with those who underwent nonfixation procedures with respect to the fact that all showed excellent to good outcomes. In a study of Jia [14], more than four laminae were successively removed without fixation in 13 patients, and no thoracic kyphosis was observed after a follow-up of more than 2 years. These studies show that posterior surgical methods do not affect tho-racic-spine stability in patients with OLF and that no additional internal fixation or bone grafting is needed, thereby avoiding the eco-nomic burden caused by the need for internal fixation. In our study, all patients who under-went posterior decompression did so without screw fixation to restore the stability of the involved segments. Results of pre- and postop-erative kyphosis of the involved vertebrae indi-cated that all patients who underwent posterior decompression without screw fixation main-tained spinal stability postoperatively.

Piezosurgery is a relatively new technique that allows the soft tissue to rest and has a tenden-cy for less bleeding by transmitting a special modular ultrasonic vibration frequency on the scalpel. This technique is clinically effective, and histological and histomorphometric evi-dence of wound healing and bone formation in experimental animal models show that tissue response is more favorable in piezosurgery than in conventional bone-cutting techniques, such as with diamond or carbide rotary instru-ments [20]. Voltage applied to a polarized

piezoceramic causes it to expand in the direc-tion of and contract perpendicular to polarity. A frequency of 25-35 kHz is used because the micromovements that are created within this frequency (40-200 µm) cut only mineralized tis-sue; neurovascular and other soft tissues are cut at frequencies exceeding 50 kHz [21]. These properties result in selective cutting of mineralized bone without or with minimal dam-age to soft tissue [22]. Its physical and mechan-ical properties have several clinical advantag-es: precise cutting, sparing of vital neurovascular bundles, and better visualization of the surgical field. Piezoelectric bone surgery seems to be more efficient in the first phases of bony heal-ing; it induces an earlier increase in bone mor-phogenetic proteins, controls the inflammatory process well, and stimulates remodeling of bone as early as 56 days after treatment [23].

Piezosurgery was first used in the field of dental and oral implant surgery [24]. Later, the tech-nique was also used for some indications in maxillofacial and otological surgery [4, 25, 26] and some have even used the device in neuro-surgery or hand surgery [27]. Thus far, only Grauvogel J [28] reported the use of piezosur-gery in spinal surgery. They believed piezosur-gery is a useful and safe technique for selective bone cutting in neuroforamina decompression and osteophyte removal with preservation of neuronal and soft tissues in ACDF. Moreover, different-angled tips appear to be effective in cutting bone spurs behind adjacent vertebrae. No data exists on the use of piezosurgery in OLF and DO. However, the supportive use of an ultrasonic bone curette (Sonopet), which works on a similar physical basis, has been described for different spinal procedures [29, 30]. These studies do not specify the spinal procedures the device is used for, and they only provide some case illustrations in their papers. Kim [30] reported injuries of the dura and the spi-nal cord because of vibrations of the Sonopet device.

Almost all studies using the technique of piezo-surgery, regardless of the operative discipline, reported a safe, precise, and selective bone cut without any injury of adjacent soft tissue [4, 22, 27]. The present study also confirmed that the cut was selective to bony osteophytes and did not injure spinal nerves, spinal cord, and dura, even if it was placed on the dura surface with-out mechanical strain. Mist airflow produced by


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ultrasonic vibration also lowered the local tem-perature, avoiding thermal damage to spinal cord and dura using high-speed drills. It could make the cutting line clear as well, thereby ensuring cutting accuracy. Therefore, the pro-cedure was safe and the device was generally easy to handle because it worked with microvi-brations instead of rotating power and bone can be easily removed by shaping movements as with using a curette.

In this study, operating room time was pro-longed because the device was used for the first time for this indication. Documentation of the intraoperative use for scientific reasons was also time consuming. Prolonged operation time was in accordance with the results of most studies using piezosurgery [25, 27].

Conclusions

Surgery for patients suffering from TSS because of OLF and DO is problematic. Resection of both OLF and DO is preferred because any attempt to preserve the dura mater introduces an unnecessary risk. Piezosurgery proved to be a useful and particularly safe technique for selective bone cutting in decompressing and removing OLF, especially DO. Thus, surgical techniques for OLF associated with DO in the tho-racic spine with piezosurgery were effective.

Acknowledgements

This work was supported by the Youth Fund of the Second Hospital of Shandong University (Y2014010027).

Disclosure of conflict of interest

None.

Address correspondence to: Dr. Mingzhi Gong, De- partment of Trauma and Orthopedics, The Second Hospital of Shandong University, 247 Beiyuan Street, Jinan 250033, Shandong Province, People’s Republic of China. E-mail: [email protected]

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application of piezosurgery to treat thoracic myelopathy caused by

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