BioMed Central Page 1 of 10 (page number not for citation purposes) Journal of Orthopaedic Surgery and Research Open Access Research article Thoracic myelopathy caused by ossification of ligamentum flavum of which fluorosis as an etiology factor Wenbao Wang* 1,2 , Linghua Kong 3 , Heyuan Zhao 1 , Ronghua Dong 1 , Jing Zhou 1 and Yun Lu 3 Address: 1 Spine surgery department, Tianjin hospital, No. 406 Jiefangnan Road, Hexi District, Tianjin City, 300211, People's Republic of China, 2 106 Fort Washington Avenue, Room 3H, New York City, NY, 10032, USA and 3 Hand surgery department, Tianjin hospital, No. 406 Jiefangnan Road, Hexi District, Tianjin City, 300211, People's Republic of China Email: Wenbao Wang* - wangwwb@yahoo.com.cn; Linghua Kong - kongklh@yahoo.com.cn; Heyuan Zhao - zhaoheyuan99@yahoo.com; Ronghua Dong - dongdrh@yahoo.com; Jing Zhou - drwangwb@yahoo.com; Yun Lu - drwangwb@yahoo.com * Corresponding author Abstract Purpose: To evaluate the clinical feature, operative method and prognosis of thoracic ossification of ligamentum flavum caused by skeletal fluorosis. Methods: All the patients with thoracic OLF, who underwent surgical management in the authors' hospital from 1993–2003, were retrospectively studied. The diagnosis of skeletal fluorosis was made by the epidemic history, clinical symptoms, radiographic findings, and urinalysis. En bloc laminectomy decompression of the involved thoracic levels was performed in all cases. Cervical open door decompression or lumbar laminectomy decompression was performed if relevant stenosis existed. The neurological statuses were evaluated with the Japanese Orthopaedic Association (JOA) scoring system preoperatively and at the end point of follow up. Also, the recovery rate was calculated. Results: 23 cases have been enrolled in this study. Imaging study findings showed all the cases have ossification of ligamentum flavum together with ossification of many other ligaments and interosseous membranes, i.e. interosseous membranes of the forearm in 18 of 23 (78.3%), of the leg in 14 of 23 (60.1%) and of the ribs in 11 of 23 (47.8%). Urinalysis showed markedly increased urinary fluoride in 14 of 23 patients (60.9%). All the patients were followed up from 12 months to 9 years and 3 months, with an average of 4 years and 5 months. The JOA score increased significantly at the end of follow up (P = 0.0001). The recovery rate was 51.83 ± 32.36%. Multiple regression analysis revealed that the preoperative JOA score was an important predictor of surgical outcome (p = 0.0022, r = 0.60628). ANOVA analysis showed that patients with acute onset or too long duration had worse surgical result (P = 0.0003). Conclusion: Fluorosis can cause ossification of thoracic ligamentum flavum, as well as other ligaments. En bloc laminectomy decompression was an effective method. Preoperative JOA score was the most important predictor of surgical outcome. Patients with acute onset or too long duration had worse surgical outcome. Published: 02 November 2006 Journal of Orthopaedic Surgery and Research 2006, 1:10 doi:10.1186/1749-799X-1-10 Received: 06 January 2006 Accepted: 02 November 2006 This article is available from: http://www.josr-online.com/content/1/1/10 © 2006 Wang et al; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0 ), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Journal of Orthopaedic Surgery and Research 2006, 1:10 http://www.josr-online.com/content/1/1/10 Page 2 of 10 (page number not for citation purposes) Background Fluoride is an important element for bone mineralization. It causes an increase in bone mass by stimulation of the osteogenetic process [1]. However, over intake of fluoride may cause fluoride intoxication, so-called fluorosis [2- 10]. Its typical clinical features include dental fluorosis, diffuse densification of bone, calcifications of bony inser- tions of many ligaments, discs, and interosseous mem- branes, i.e. interosseous of the ribs, forearm, and leg, posterior longitudinal ligament, transverse atlantal liga- ment, ligamentum flavum, and membrana obturatoria [8- 10]. Thoracic spinal stenosis caused by ossification of lig- amentum flavum (OLF) is a rare disease [11-20]. How- ever, thoracic OLF caused by skeletal fluorosis is rather rare. Only 6 cases have been reported in the English liter- ature [3,13]. The authors' purpose is to evaluate the clini- cal feature, operative method, and prognosis of thoracic OLF caused by skeletal fluorosis. Materials and methods All the patients with thoracic OLF, who underwent surgi- cal management in the authors' hospital from 1993– 2003, were retrospectively studied. The cases accorded with the following criteria were included. Diagnostic cri- teria for fluorosis: epidemic history including a long his- tory living in a high fluorosis area; dental fluorosis; typical X-ray findings including diffuse densification of bone, cal- cifications of bony insertions of many ligaments, discs, and interosseous membranes, i.e. interosseous of ribs, forearm, and leg; urinalysis of fluoride may increase. Diagnostic criteria for thoracic ossification of ligamentum flavum: typical clinical symptoms and findings which included numbness in the lower limbs and below the rel- ative segment of trunk, motor weakness in the lower extremities and difficulty in walking; physical examina- tion showed increased lower limbs muscle tension, increased in deep tendon reflexes and appearance of path- ological reflex, i.e. Babinski sign. X ray, CT scan, and MRI were used to confirm the diagnosis. For each the patient, A-P view and lateral view X-ray of the thoracic spine were taken. Then thoracic MRI was taken to ensure the diagnosis and identify the involved segments. CT scan was performed for the involved segments. A-P view, lateral view X-ray of forearms and legs and A-P view of the chest were also taken. Indication of surgery: symptoms and signs of thoracic myelopathy; CT scan and MRI demonstrated significant thoracic canal stenosis; the symptoms and signs correla- tion with the imaging findings. En bloc decompression was performed on each patient. In one patient combined with cervical ossification of poste- rior longitudinal ligament, cervical open door decompres- sion was performed additionally. In one patient combined with lumbar stenosis, lumbar laminectomy decompression was performed additionally. Preoperative radiographic localization with a Kirschner wire was used to confirm the operative level on the morn- ing of operation day. After induction of general anesthe- sia, the patient was placed prone with an indwelling bladder catheter. The abdomen was decompressed to avoid excessive epidural bleeding. According to the X-ray localization result, a midline incision was made at the appropriate level and extended to the fascia. Subperiosteal dissection of the paraspinal muscles was performed using electrocautery cutting. The spinous processes were short- ened using rongeurs (not totally removed). The laminec- tomy was performed with high-speed drill. The width of the laminectomy was approximately one third the size of the inside of the facet. After the laminae were totally floated, it was taken off en bloc by holding the residues spinous processes (Fig 4). The ossified ligamentum flavum often adhered to the dura mater. So, much care should be paid to avoid rupture of the dura mater. Occasionally, the dura mater also ossi- fied. In those cases, we did not take away the ossified dura mater totally, just floated it. When coexistent lesions were present at noncontinuous thoracic levels, clinical symp- toms and neuro-imaging findings were examined. The level considered to be the likely cause of clinical symp- toms was then surgically treated. When coexistent lesions were present at the cervical or lumbar region, the depres- sion of the relevant region was performed. The neurological statuses were evaluated with the JOA scoring system of myelopathy preoperatively and at the end point of follow up (table 2). The recovery rate, described by Hirabayashi et al [21], indicating the degree of recovery of normal function postoperatively, was calcu- lated as follows: (postoperative JOA score – preoperative JOA score)/(11- preoperative JOA score) ×100. X-ray and CT scan were performed 3 days after the opera- tion to conform the decompression levels and decompres- sion area. X-ray was performed at the end of the follow up to identify whether there was spinal instability. Statistical analysis Paired t test was used to analyze the differences between the results before operation and at end of follow up. Mul- tiple linear regression was conducted to determine the quantitative variables best correlating to surgical out- come. ANOVA was used to analyse differences among the three groups according to the duration of preoperative symptom. When the results of ANOVA indicated < 0.05, further statistical analysis was followed to determine Journal of Orthopaedic Surgery and Research 2006, 1:10 http://www.josr-online.com/content/1/1/10 Page 3 of 10 (page number not for citation purposes) whether there was any significance difference between any two groups. The statistical results were analyzed using the Statistical Analysis System (SAS). Significance was accepted for P-values of < 0.05 in all of the above analyses. Results Clinical presentation 74 cases of thoracic OLF were surgically treated at the authors' institution between 1993 and 2003, 23 of which (16 male and 7 female) were caused by fluorosis. The 23 patients ranged in age from 42 to 72 years (mean 54.8 years). 6 cases had acute onset of clinical symptom, 4 of which had a traumatic history, 2 without markedly trau- matic history. The other 17 cases did not have a traumatic history and presented with progressive symptoms. Numb- ness in the lower limbs and below the relative segmental was the most common initial symptom in 17 of the 23 patients (73.9%). Motor weakness in the lower extremi- ties and difficulty in walking as initial symptoms were found in 6 patients. The details of the clinical findings are shown in table 1 and table 3. The mean duration of symp- toms between initial onset and operation was 37 months (range 1 day–11 years). All of the 23 patients had a long term, high fluoride area living history. Fluoride over intake was from water in 21 cases (91.3%) or from coal smoke in 2 cases (8.7%). 22 of 23 patients (95.7%) had different levels of dental fluorosis. Urinalysis showed markedly increased urinary fluoride in 14 of 23 patients (60.9%). Imaging study result The mean number of involved segments is 4.17, with a range from 1 to 9 (Fig 1). The ossified ligamentum flavum displayed obscuration the margin of the lamina on the A- P view X-ray in 8 patients (34.8%). On the lateral view X- ray, 12 of 23 patients (52.2%) showed high density pro- jection into the spinal canal at the level of compression. All the ossified ligamentum flavum displayed the density of cortical bone on CT scans and sometimes had a thin gap between the laminae (Fig. 3c). All the ossified liga- mentum flavum demonstrated triangular protrusion with a low-signal intensity resembling cortical bone on MR images (Fig 3a, 3b). X-ray of forearms, legs, and chest showed ossification of interosseous membranes of the forearm in 18 of 23 patients (78.3%) (Fig 2), of the leg in 14 of 23 patients (60.1%), and of the ribs in 11 of 23 patients (47.8%). Operation and the prognosis The operation time ranged from 2.5 hours to 4.3 hours, with a mean of 3.2 hours. The mean decompressed seg- ments number is 4.35 (ranged from 2 to 8, table 3). Blood loss ranged from 400 ml to 2800 ml, with a mean of 850 ml. Dura mater rupture occurred in 4 patients. Deep infec- tion occurred in one patient. No postoperative neurologi- cal deterioration occurred. All the patients were followed up from 12 months to 9 years and 3 months, with an aver- age of 4 years and 5 months. Paired t test showed that the JOA score increased significantly at the end of follow up (P = 0.0001, table 3). The recovery rate was 51.83 ± 32.36%. Multiple regression analysis revealed that the preoperative JOA score was an important predictor of sur- gical outcome (p = 0.0022, r = 0.60628, table 4). How- ever, the sex, age, preoperative symptom duration, and levels of OLF did not significantly influence the surgical outcome. ANOVA analysis showed that patients with acute onset (group 1) or too long duration (group 3) had worse surgical result (P = 0.0003) (table 3). Further t test showed that there was significant different between group Table 2: summary of the JOA scoring system for the assessment of myelopathy neurological status score lower-limb motor dysfunction unable to walk 0 Able to walk on flat floor with walking aid 1 Able to walk up/downstairs w/handrail 2 Lack of stability & smooth reciprocation of gait 3 no dysfunction 4 lower-limb sensory deficit severe sensory loss or pain 0 Mild sensory deficit 1 no deficit 2 trunk sensory deficit severe sensory loss or pain 0 Mild sensory deficit 1 no deficit 2 sphincter dysfunction unable to void 0 marked difficulty in micturition 1 minor difficulty in micturition 2 no dysfunction 3 Total score for a healthy patient is 11. Table 1: Summary of clinical features observed in 23 patients with OLF Symptoms number of cases numbness and sensory deficit 22 lower-limb weakness and gait disturbance 21 Low-back pain 8 "squeezing tight band" around chest or abdomen 6 neurological claudication 4 leg pain 12 fecal & urinary incontinence 18 knee and ankle hyperreflexia 18 positive patellar and ankle clonus 14 positive Babinski sign 15 Journal of Orthopaedic Surgery and Research 2006, 1:10 http://www.josr-online.com/content/1/1/10 Page 4 of 10 (page number not for citation purposes) Table 3: Data on Patients With Ossification of Ligamentum Fluvam case no. sex & age DPS & group JOA score levels & segment number of OLF levels & number of decompression recovery rate % pre-operation follow up LEMTSLESSDTotalLEMTSLESSDtotal 1 M, 42 15m(2) 2 2 0 1 5 4 2 1 2 9 T12–L1 (1) T12–L1 (2) 67 2 M,62 3d(1) 3 2 1 3 9 4 2 1 3 10 T12–L1 (1) T12–L1 (2) 50 3 F, 46 32m(2) 3 1 1 2 7 4 2 1 3 10 T12–L1 (1) T12–L1 (2) 75 4 M,54 12m(2) 4 1 1 3 9 4 2 2 3 11 C7–T1(1) C7–T1(2) 100 5 F,64 4y(2) 2 1 1 2 6 4 2 1 3 10 T1–4(3) T1–4(4) 80 6 M,51 5y(3) 1 1 0 2 4 3 2 1 3 9 T3–5(2) T3–5(3) 71 7 M,42 2d(1) 1 1 0 1 3 2 2 1 2 7 T7–12(5) T7–12(6) 50 8 M,65 7y(3) 1 1 1 1 4 3 1 1 1 6 T8–L1(5) T8–T12(5) 29 9 M,55 11y(3) 2 1 1 3 7 2 2 1 3 8 T7–L1(5) T8–L1(6) 25 10 F,56 15m(2) 2 1 1 2 6 3 2 1 2 8 T9–L1(4) T10–L1(4) 40 11 M,59 1d(1) 2 1 0 1 4 2 1 0 1 4 T9–L1(4) T9–L1(5) 0 12 M,45 6m(2) 4 2 1 3 10 4 2 2 3 11 T10–L1(3) T10–L1(4) 100 13 F,50 5y(3) 2 2 1 1 6 3 2 1 2 8 T10–L1(3) T10–L1(4) 40 14* M,62 1d(1) 0 0 0 0 0 0 0 0 0 0 T10–L1(3) T10–L1(4) 0 15 M,50 18m(2) 2 2 1 3 8 4 2 2 3 11 T10–L1(3) T10–L1(4) 100 16* F,48 5y(3) 2 1 1 2 6 2 1 1 2 6 T10–L1(3) T10–L1(4) 0 17 M,54 1d(1) 0 1 0 1 2 0 1 0 2 3 T3–7 11 T10–L1(4+3) T10–L1(4) 18 M,59 8y(3) 2 1 1 2 6 4 2 1 2 9 T1–T5 T1–T5(5) 60 T9–L1(4+4) 19 M,58 7y(3) 1 0 0 1 2 2 1 1 2 6 T1–6 44 T9–L1(5+4) T9–L1(5) 20 M,56 6d(1) 2 1 1 1 5 3 1 1 2 7 C7–T9(9) C7–T4(5) 33 21 F,52 7m(2) 2 1 1 2 6 4 2 2 3 11 T8–12(4) T8–12(5) 100 22 F,72 7y(3) 2 2 1 1 6 4 2 1 2 9 T9–11 T9–11 60 L3–S1(2+3) L3–S1(3+4) 23 M,58 4y(2) 1 1 0 2 4 3 2 1 2 8 C3–6 C3–6 57 T10–L1(4+3) T10–L1(4+4) DPS: duration of preoperative symptom. LEM: lower extremity motor; TS: trunk sensory; LES: lower extremity sensory; SD: sphincter dysfunction Paired t test showed that there is significant difference between the JOA score of pre-operation and followed up (P = 0.0001). The mean recover rate is 51.83%. ANOVA analysis of the three groups according to the DPS showed p = 0.0003. Further t test showed that there was significant different between group one and group tow (P = 0.0004). There was significant different between group two and group three (P = 0.003). However, there was no significant different between group one and group three (P = 0.197). Journal of Orthopaedic Surgery and Research 2006, 1:10 http://www.josr-online.com/content/1/1/10 Page 5 of 10 (page number not for citation purposes) A diagram of the OLF distribution of 23 patientsFigure 1 A diagram of the OLF distribution of 23 patients. Journal of Orthopaedic Surgery and Research 2006, 1:10 http://www.josr-online.com/content/1/1/10 Page 6 of 10 (page number not for citation purposes) Anteroposterior view radiograph of both forearms showed significant calcifications of interosseous membranes of forearmFigure 2 Anteroposterior view radiograph of both forearms showed significant calcifications of interosseous membranes of forearm. Journal of Orthopaedic Surgery and Research 2006, 1:10 http://www.josr-online.com/content/1/1/10 Page 7 of 10 (page number not for citation purposes) The en bloc removed lamina, note the nodular ossified ligamentum flavumFigure 4 The en bloc removed lamina, note the nodular ossified ligamentum flavum. a, b. T1 and T2 weight MRI of thoracic spine showed continuous multi-level ossification of ligamentum flavum between T7–12Figure 3 a, b. T1 and T2 weight MRI of thoracic spine showed continuous multi-level ossification of ligamentum flavum between T7–12. c. CT scan showed ossified ligamentum flavum, note that there was a thin gap between the ossified ligament and the lamina. Journal of Orthopaedic Surgery and Research 2006, 1:10 http://www.josr-online.com/content/1/1/10 Page 8 of 10 (page number not for citation purposes) 1 and group 2 (P = 0.0004). There was significant different between group 2 and group 3 (P = 0.003). However, there was no significant different between group 1 and group 3 (P = 0.197) (table 3). No postoperative instability occurred. Discussion Etiology The thoracic OLF was first reported by Polgar [17] in 1920 with lateral radiographs. From then on, several clinical series and many case reports have been reported. How- ever, the etiology of OLF was unclear. As most of the reported OLF's were located between T9 and T12, Barnett et al. [11] suggested that the hyper mobility of the lower thoracic spine might promote degeneration and canal ste- nosis. Liao's study [22] showed a high prevalence of coex- isting anterior osteophytes and herniated intervertebral disc at the symptomatic OLF segments. So they concluded that OLF might be a degenerative response to the micro injury of the ligamentum flavum. The hypothesis was his- tologically supported by Okada and colleagues [15] who found that OLF formed in the hypertrophic ligamentum flavum with fibrocartilage proliferation, and this was thought to be a phenomenon of mechanical injury. There- fore, it was thought that the development of OLF might be secondary to the specific fiber reconstruction of the liga- mentum flavum in response to mechanical stress. How- ever, Muthukumar [13] reported two cases of OLF caused by fluorosis, recently. Wang et al. [8-10] reported fluorosis could cause ossification of a lot of ligaments. All these reports showed fluorosis might play a role in OLF. Fluoride is one of the necessary minor elements for humans, and the daily requirement is 0.05–0.07 mg/kg body weight/day [2,5]. The benefits of water fluoridation in controlling dental caries were well documented. Fluo- ride was first used in water for caries control in 1945 and 1946 in the United States [1] and Canada [4], respectively. However, over intake of fluoride will cause fluorosis [2- 10]. Fluorosis caused by fluoride intoxication was first reported by Feil in 1930, and skeletal fluorosis was reported by Short in 1937 [7]. Normally, there are two sources of fluoride over intake, water and coal smoke. In the high fluoride area, the density of fluoride in the water is more than 5–8 mg/L, and the people drink the water directly from the well without any management. This will cause dental fluorosis, skeletal fluorosis, or even systemic fluorosis. It was reported that neurological complications occurred in approximately 10% of patients with skeletal fluorosis, usually in the later stages of the disease [7]. To date, the myeloradiculopathy caused by skeletal fluorosis was thought to be a result of compression of the spinal cord by osteophytes and vertebral osteosclerosis [7,23]. However, myelopathy caused by OLF in patients with skeletal fluor- osis has been recognized recently [3,13]. So, we think fluorosis should be entertained as an etiology factor of OLF, especially in patients from endemic areas. The pathogenesis of ossification of the ligaments in this condition remains speculative. High expression of trans- forming growth factor beta-1 (TGF-β1) by fibroblasts was found in the ossified matrix within ossified ligaments and in chondrocytes within cartilaginous areas adjacent to the ossified ligaments [24]. TGF-β1 could have played a role in chondroid metaplasia and ectopic ossification in OLF. Recent experimental evidence points to the involvement of proto-oncogenes c-fos and c-jun in skeletal fluorosis. Zhang et al. [25] have demonstrated that exposure to excessive fluoride could stimulate the activation and pro- liferation of osteoblast-like cells with enhanced expres- sion of messenger ribonucleic acid and proteins of c-fos and c-jun. Clinical feature of thoracic ossification of ligamentum flavum Thoracic OLF is rare and usually asymptomatic. The dis- ease usually has an insidious onset and very slow progres- sion. Analysis of previously published epidemiological data reveals that thoracic OLF most commonly involves the vertebrae between T-9 and T-12(as in our serious in figure 1), where greater mobility and vulnerability (due to spinal motion) may result in frequent mechanical injury. In our series, numbness in the lower limbs and below the relative segmental was the most common initial symptom in 17 of the 23 patients (73.9%). Motor weakness in the lower extremities and difficulty in walking as initial symp- toms were found in 6 patients (26.1%). This finding is in agreement with the observations reported in previous studies [11-20]. When an extradural compressive lesion develops at the thoracic level, pressure to the spinotha- lamic tract, fasciculus gracilis and fasciculus cuneatus causes the numbness and lost of proprioceptive sensation in the lower limbs and below the relative segment. Upper Table 4: results of a multiple linear regression analysis of selected variables to predict surgical outcome recover rate at final follow up Variable coefficient p value Age -0.24755 0.2548 duration of preoperative symptom -0.10367 0.6378 Preoperative JOA score 0.60628 0.0022 levels of OLF -0.31515 0.1430 Journal of Orthopaedic Surgery and Research 2006, 1:10 http://www.josr-online.com/content/1/1/10 Page 9 of 10 (page number not for citation purposes) neuron injury might occur and be caused by pressure to the cerebrospinal tract. This results in increasing muscle tension of the lower extremity, increasing in both patellar and Achilles reflexes. However, if at a lower thoracic level, the lesions exist at neighboring sites of the conus medulla- ris, the patellar or Achilles reflex will occasionally dissoci- ate, or both decrease. Compressive coexistent lesions, such as cervical or/and lumbar stenosis, also influence the clinical features, as showed in the literature [26] and in our series. Although the neurological findings in our series are similar with other authors' findings, OLF caused by fluorosis has their own features. Firstly, all the patients had the character features of fluorosis. Secondly, the seg- ment number of involved LF is more than others (figure 1). Surgical procedures Non operative method is not effective for symptomatic patients. So, early diagnosis and operation interference were recommended for the symptomatic patients. As the thoracic OLF compressed the spinal cord posteriorly, sev- eral posterior decompression methods were developed. These operative techniques include open-door laminec- tomy, en bloc laminectomy, fenestration, total decom- pression et al. [14,15,18-20]. In our cases, all the patients performed en bloc decompression. The segments are shown in table 3. The blood loss was much more when compared with our non-fluorosis cases (non published data). This was partly because the fluorosis made the soft tissue easily prone to bleeding and partly because the decompression segments are more than others. The results shown in table 3 stated that the decompression was effective. In 4 cases of our patients, ossification of dura mater occurred. Some authors also reported ossification of the dura mater together with the ossification of thoracic liga- mentum flavum [14]. In those cases, severe adhesion between ossified ligamentum flavum and dura mater might occur. Much attention must be paid to avoid rup- ture of the dura mater. However, some times we did not remove the ossified ligament totally. We just floated it and abraded it as thinly as possible with a high speed drill. The results were satisfactory. Sometimes, rupture of the dura mater did occur. In those cases, the dura mater needed repair. Okada reported the en bloc method may induce postop- erative spinal instability and preferred an open-door method [15]. However many authors reported en bloc method is safe and effective, with no postoperative spinal instability [16,26]. All patients in the present study under- went posterior thoracic laminectomy to remove the intruding ossified lesion. Efforts were made to preserve the lateral two thirds of the facet joints as much as possi- ble to maintain the segmental stability. No postoperative instability was observed in our series. The key point is to preserve the lateral half of the facet. However, fluorosis makes the spine more rigid, decreases movement, and decreases the possibility of postoperative instability. Prognosis predictors Several authors reported some factors influenced the sur- gical outcome which included preoperative neurological status, duration of preoperative symptoms, level and pro- gression of ossification, and degree of thoracic kyphosis et al [27-30]. The result of our investigation confirmed that the preoperative JOA score is the most important predic- tor of the recovery rate. However, the duration of preoper- ative symptoms was not significantly correlated with the outcome. It might be because there were six patients who suffered acute onset of the symptom, just like acute spinal cord injury. The outcomes of these patients were not all good. To study this more, we divided all the patients into three groups according to preoperative symptom dura- tion. Group one is acute onset, the duration shorter than three days. In group two, the symptom duration is between three days and five years. In group three, the symptom duration is longer than five years. ANOVA anal- ysis of these three groups showed p = 0.0003. It showed that there was significant difference between the groups. Further t test showed that there was significant different between group one and group two (P = 0.0004). 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J Neuro- surg Spine 2005, 3:348-54. . 1 of 10 (page number not for citation purposes) Journal of Orthopaedic Surgery and Research Open Access Research article Thoracic myelopathy caused by ossification of ligamentum flavum of which. ligament, transverse atlantal liga- ment, ligamentum flavum, and membrana obturatoria [8- 10]. Thoracic spinal stenosis caused by ossification of lig- amentum flavum (OLF) is a rare disease [11-20] and 2003, 23 of which (16 male and 7 female) were caused by fluorosis. The 23 patients ranged in age from 42 to 72 years (mean 54.8 years). 6 cases had acute onset of clinical symptom, 4 of which