Characteristics and Clinical Relevance of the Osseous Spur in Patients with Congenital Scoliosis and Split Spinal Cord Malformation

Feng, Fan MD; Shen, Jianxiong MD; Zhang, Jianguo MD; Li, Shugang MD; Yu, Keyi MD; Tan, Haining MD

Journal of Bone & Joint Surgery - American Volume:
doi: 10.2106/JBJS.16.00414
Scientific Articles
Abstract

Background: The natural history of split spinal cord malformation (SCM) is still unclear. Knowledge of the characteristics of the osseous spur and its relationship with the spinal deformity may allow early identification of patients with a higher risk of a neurological deficit and enhance surgical decision-making.

Methods: Eighty-five consecutive patients with congenital scoliosis and type-I SCM who had undergone surgical treatment at our hospital from May 2000 to December 2013 were identified retrospectively. There were 22 male and 63 female patients with an average age of 13.9 years at the time of surgery. Preoperative clinical and radiographic data were collected to investigate the characteristics of the scoliosis and the osseous spur. Two groups were identified on the basis of whether the patients had intact neurological function (Group A) or a neurological deficit (Group B).

Results: There were 52 patients (61%) in Group A (intact neurological function) and 33 patients (39%) in Group B (neurological deficit). There were no significant differences in the demographic distribution, curve magnitude, or length and thickness of the osseous spur between the 2 groups. In Group A, the location of the osseous spur relative to the apex of the major curve was proximal in 13 patients (25%), distal in 28 (54%), and central in 11 (21%). In Group B, the osseous spur was proximal in 7 (21%), distal in 8 (24%), and central in 18 (55%). The 2 groups differed significantly with respect to the location of the osseous spur (chi square = 10.898, p = 0.004). Group-B patients had a higher proportion of patients with kyphotic deformity (42%) than Group A (10%). The ratio of the diameters of the hemicords (concave side divided by convex side) differed significantly between the 2 groups (0.98 for Group A versus 0.89 for Group B, p = 0.030).

Conclusions: The neurological status in patients with congenital scoliosis and type-I SCM appears to be closely related to the location of the osseous spur relative to the congenital scoliosis. An osseous spur at the apex of the scoliosis may be related to a higher risk of developing a neurological deficit, especially in patients with kyphotic deformity. Asymmetric splitting of the spinal cord may contribute to neurological deficits.

Level of Evidence: Prognostic Level III. See Instructions for Authors for a complete description of levels of evidence.

Author Information

1Department of Orthopedics, Peking Union Medical College Hospital, Chinese Academy of Medical Science, Peking Union Medical College, Beijing, People’s Republic of China

E-mail address for J. Shen: shenjianxiong@medmail.com.cn

Article Outline

Split spinal cord malformation (SCM), or diastematomyelia, is a rare form of closed neural tube defect and tethered spinal cord syndrome. The most widely accepted theory about the embryogenesis of SCM was originally proposed by Bremer1 and subsequently modified by Pang et al.2 as the “unified theory of embryogenesis.” According to this theory, there are 2 types of SCM depending on the type of the midline mesenchymal derivative and the dural investment of the hemicords. Type-I SCM is characterized by 2 hemicords, each contained within its own dural tube and separated by a rigid osseous spur. Type-II SCM consists of 2 hemicords housed in a single dural tube separated by a nonrigid, fibrous median septum2,3.

As a result of the close relationship in embryonic development of intraspinal anomalies and congenital spinal deformity, SCM is commonly associated with congenital scoliosis4-8. There is a high rate of neurological compromise after scoliosis correction and instrumentation because of tethering of the spinal cord, especially in patients with type-I SCM as the rigid osseous spur may tether the spinal cord during the corrective surgery. It has been accepted practice to remove osseous spurs associated with SCM prior to correction of spinal deformity9-11. However, there is limited literature describing the natural history of SCM associated with congenital scoliosis. Some experts believe that the risk of developing neurological deficits increases with age and thus all patients with SCM should undergo prophylactic surgery, even if they are asymptomatic12,13. We have observed that some patients with no or minor neurological deficits have remained neurologically stable as adults. The risk factors leading to a neurological deficit in patients with congenital scoliosis and coexisting SCM are still unclear.

We are not aware of any published studies that specifically focused on the characteristics and clinical relevance of the osseous spur in patients with SCM. The objective of this investigation was to identify differences in radiographic parameters between patients with an intact neurological status and those with neurological deficits. These findings can be used to improve strategies for early identification of patients who have a higher risk of developing neurological deficits and thus may require prophylactic resection of the osseous spur and those with a lower risk who may not require such resection.

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Materials and Methods

Subjects

After approval from our institutional review board, the medical records of patients with congenital scoliosis and type-I SCM who had undergone spinal correction surgery at our center from May 2000 to December 2013 were retrospectively reviewed. All patients underwent magnetic resonance imaging (MRI) and computed tomography (CT) of the entire spine to evaluate and classify the type of SCM. The inclusion criteria were (1) congenital scoliosis with type-I SCM and (2) complete medical records. Patients with previous spinal surgery or spinal fracture were excluded.

Eighty-five patients with congenital scoliosis and type-I SCM were enrolled in this study. There were 22 male and 63 female patients with an average age of 13.9 years (range, 4 to 47 years) at the time of surgery. Each patient received a detailed neurological examination, including evaluations of sensory deficits, extremity muscle strength, abdominal reflexes, and tendon reflexes. They were divided into 2 groups, with patients with an intact neurological status assigned to Group A and those with a neurological deficit assigned to Group B. Group-B patients were divided into 2 subgroups: B1 if the deficit was minor (a sensory deficit or abnormal reflex) and B2 if it was major (a motor deficit, claudication, or incomplete paralysis).

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Radiographic Measurement

Standing posteroanterior radiographs were reviewed to determine the Cobb angle and the apex of the major curve, which was characterized as thoracic (apex between T1 and T11), thoracolumbar (apex between T12 and L1), or lumbar (apex between L2 and L5). Kyphotic deformities seen on lateral radiographs and measured with the Cobb method were also recorded, and were defined as a sagittal plane deformity of >40° in the thoracic region (T5-T12) or >30° in the thoracolumbar region (T10-L2), kyphosis in the lumbar region (L1-L5), or angular kyphosis at any segments of the spine14.

The location of the osseous spur was evaluated with MRI and CT of the entire spine, and its relationship to the apex of the scoliosis was defined as proximal, central (the osseous spur located at or across the apex of the major curve), or distal (Fig. 1). The length of the osseous spur was measured according to the number of vertebral bodies that it spanned. The thickness of the osseous spur was measured on axial CT scans. The ratio between the thickness of the osseous spur and the diameter of the spinal canal was also calculated (Fig. 2). To determine the severity of spinal cord tethering, the level of the cerebellar tonsil against the basion-opisthion line was also measured on sagittal MRI images15. The value was defined as positive when it was above the basion-opisthion line and as negative when it was below it. All of the measurements were performed by an independent observer who was blinded to the neurological status of the patient.

To quantify the severity of asymmetric splitting of the spinal cord, the diameter of the hemicord on the concave side of the scoliosis and the diameter of the hemicord on the convex side were both measured on the axial MRI scans (Fig. 3), and the ratio between these diameters was calculated. The axial images used for the measurement were selected by referencing the sagittal image and viewing the cut that was located at the sagittal center of the osseous spur and perpendicular to the spinal canal.

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Statistical Analysis

The statistical analysis was performed using SPSS 17.0 software for Windows (SPSS). The differences between the 2 groups were analyzed using the independent sample t test and the chi-square test. Significance was set at p < 0.05.

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Results

On the basis of the details of the patient’s history and findings of the physical examination at presentation, 52 patients with an intact neurological status (Group A, 61% of the series) and 33 patients with a neurological deficit (Group B, 39%) were identified (Table I).

Group A included 39 female and 13 male patients with a mean age of 13.4 years. Group B was composed of 24 female and 9 male patients with a mean age of 14.8 years. The 2 groups did not differ significantly with respect to the distribution of sex or age (p > 0.05).

In both groups, the osseous spurs were most commonly located at the thoracolumbar region (Fig. 4). The average length of the osseous spur was 2.5 (range, 1 to 6) vertebral segments in Group A and 2.8 (range, 1 to 7) in Group B. The average thickness of the osseous spur was 3.37 mm (range, 0.66 to 7.90 mm) in Group A and 3.77 mm (range, 1.72 to 8.36 mm) in Group B. The average level of the cerebellar tonsil against the basion-opisthion line was 0.62 mm (range, −4.38 to 6.79 mm) in Group A and 0.59 mm (range, −4.02 to 7.40 mm) in Group B (Table II).

Although the Cobb angle of the major curve was lower in Group A than in Group B (69.3° versus 76.2°), there was no statistically significant difference between the groups (p = 0.056). The apex of the major curve was most commonly in the lower thoracic spine (Fig. 4). No significant difference was detected in the distribution of the apex of the major curve between the 2 groups (Table II). In Group A, the location of the osseous spur with respect to the apex of the scoliosis was proximal in 13 patients (25%), distal in 28 (54%), and central in 11 (21%) whereas, in Group B, it was proximal in 7 (21%), distal in 8 (24%), and central in 18 (55%). This difference was significant (chi square = 10.898, p = 0.004) (Table II). Group B had a higher proportion of patients with kyphotic deformity (42% compared with 10% in Group A, p < 0.001). In 2 patients with incomplete paralysis, the osseous spur was located at the apex of the scoliosis as well as the apex of the kyphotic deformity (Fig. 5).

In Group A, the average diameters of the hemicords on the 2 sides of the scoliosis were nearly equal (4.00 and 4.14 mm, p = 0.149). In Group B, the average diameter of the hemicord was significantly smaller on the concave side than on the convex side (3.95 and 4.48 mm, p = 0.002). The ratio of the diameters of the hemicords (concave/convex) differed significantly between the 2 groups (0.98 versus 0.89, p = 0.030) (Table III). In Group B, the hemicord on the concave side was smaller than that on the convex side in 26 (79%) of the patients, although the hemicord diameter was larger on the concave side in 4 (12%) (range of ratios, 1.08 to 1.61) and the diameters were equal in 3 (9%). However, regardless of whether they were on the concave or convex side, all of the smaller hemicords were on the side corresponding to the side of the poor neurological status.

In Group B, the neurological deficit was minor (Group B1) in the majority (24; 73%) of the 33 patients. Only 9 patients (27%) had a major (motor) neurological deficit (Group B2) (Table IV). When compared with Group-A patients, the patients in Group B2 had a higher prevalence of osseous spurs in the central location relative to the apex of the scoliosis (21% in Group A versus 67% in Group B2, p = 0.019) and kyphotic deformity (10% versus 56%, p = 0.004). Asymmetric splitting of the spinal cord was more severe in Group B2 than in Group A (average concave/convex ratio = 0.82 in Group B2 versus 0.98 in Group A, p = 0.033).

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Discussion

To our knowledge, our study is the first investigation in the literature that identifies the characteristics of the osseous spur and its relationship with neurological status in a large cohort of patients with congenital scoliosis and SCM. We found a preponderance of female patients in both of our groups (those with and those without a neurological deficit). The thoracic region was the most common location of the apex of the major scoliotic curve, followed by the thoracolumbar region and then by the lumbar region, whereas most of the osseous spurs were located in the thoracolumbar region (Fig. 4). Thus, the pattern of the scoliosis did not predictably match the pattern of the SCM. The locations of the osseous spur (proximal, distal, or central) relative to the apex of the spinal curve differed among the individual patients.

Some authors have suggested that, when the SCM is distal, the growth of the spine proximal to it is greater and the chances of tethering with growth are also greater16. On the other hand, if the SCM is proximal, then lengthening distal to it does not have much of a tendency to pull the cord against the tether17,18. However, the results of our study did not provide evidence for this theory. In Group B (patients with a neurological deficit), the majority of the osseous spurs occurred at the scoliosis apex, and the prevalence of this central location was much higher in Group B (especially in the 9 patients with a major deficit [Group B2]) than in Group A (patients without a neurological deficit).

There were no statistically significant differences between the 2 groups with regard to the length of the osseous spur or the Cobb angle of the major curve. Neurological deficits in patients with SCM result from the tethering of the spinal cord, due to a distant spur, anchoring the caudal end of the spinal cord, which prevents cephalad movement of the lumbosacral portion of the cord. This is followed by ischemia and tissue necrosis as well as unilateral spinal deformities at the site of the SCM19,20. In our patients, the spinal canal was relatively narrow at the apex of the curve, especially in those with a kyphotic deformity, and this combination led to spinal canal stenosis. A central location of the osseous spur results in the greatest tethering force, increasing the likelihood of presentation with a stretch-induced functional disorder of the spinal cord.

These findings may have important implications for the management of type-I SCM. Since occult spinal dysraphism can lead to irreversible neurological complications, early diagnosis and treatment are necessary21. When a child is diagnosed with congenital scoliosis and type-I SCM, it is imperative to know whether he or she has a high risk of neurological compromise. An osseous spur located at the apex region of the spinal deformity can serve as an “early warning sign” because, even if a patient is currently neurologically intact, he or she is more likely to develop neurological deficits with progression of the scoliosis16. We believe that these patients require early treatment of the osseous spur and spinal deformity. Early surgical intervention to remove the osseous spur is recommended for patients with progressive scoliosis and neurological symptoms11. For patients with an intact or stable neurological status, corrective and fusion surgery without prophylactic neurological intervention can safely and effectively prevent the progression of scoliosis. It is important to avoid aggressive distraction forces across the apex of the scoliosis during scoliosis correction, especially when the osseous spur is located at the apex of the major curve. Our practice is to apply the corrective force on the compensatory curve to achieve a satisfactory correction rate and coronal balance.

Some of our patients had asymmetric splitting of the spinal cord. In describing the unified theory of SCM, Pang3 reported that an oblique median septum divides the neural plate into a large, or “major,” hemicord and a small, or “minor,” hemicord. However, authors of previous studies3,16,22 with fewer cases reported asymmetric splitting to be rare compared with the symmetric splitting pattern. We found that the 2 hemicords were almost the same size in patients with no neurological deficits, whereas the hemicord on the concave side was significantly smaller than the hemicord on the convex side in patients with a neurological deficit. In studies of pathological specimens, Emery and Lendon23 and Rokos22 reported thin disorganized pathological entities in the smaller hemicord. The larger hemicord with robust neurons does not cross over to innervate the side of the minor hemicord3. Autopsy of a 1.5-year-old child showed disappearance of the gray matter within the spinal cord at the bifurcation of the dual tubes, with impaired development of neural cells and nerve fibers and dysplasia of motor cells24. In our series, the side on which the smaller hemicord was located, regardless whether it was the concave or convex side, always corresponded to the side of the poor neurological status. Consequently, the neurological deficits in patients with congenital scoliosis and SCM may result not only from tethering by the osseous spur but also from the asymmetric splitting of the spinal cord itself. This may also explain why some patients who had undergone excision of the spur and untethering of the cord still developed neurological deficits13,20.

Limitations of our study include its retrospective nature. Also, we did not assess the natural history of the osseous spur or its impact on patients’ neurological function at skeletal maturity. Neurological status was based on the results of a detailed physical examination as recorded in the medical records. A prospective study using an objective method to assess neurological status is needed in the future.

In conclusion, the current study revealed that neurological deficits in patients with congenital scoliosis and type-I SCM may have a close relationship with the location of the osseous spur relative to the apex of the spinal deformity. When the osseous spur is located at the apex of the major curve, it is more likely to lead to neurological deficits, especially in patients with kyphotic deformity. The neurological deficits in these patients may also be attributed to asymmetric splitting of the spinal cord. These findings may provide important clues for better understanding the clinical features and expected natural history of SCM associated with congenital scoliosis, thus improving the surgical strategy for these patients. Knowing the relationship between the osseous spur and the congenital scoliosis may help to identify patients with a higher risk of neurological compromise.

Investigation performed at the Department of Orthopedics, Peking Union Medical College Hospital, Chinese Academy of Medical Science, Peking Union Medical College, Beijing, People’s Republic of China

Disclosure: Support for the data collection and analysis in this work was provided by Grant 81330044 from the National Natural Science Foundation of the People’s Republic of China. The Disclosure of Potential Conflicts of Interest forms are provided with the online version of this article.

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