Neuroradiology (1996) 38:148-151 9 Springer-Verlag 1996

H. Inoue K. Ohmori T. Takatsu T. Teramoto Y. Ishida K. Suzuki

Received: 7 December 1994 Accepted: 2 February 1995

H. Inoue ( ~ ) 9K. Ohmori. T.Takatsu T.Teramoto. Y, Ishida. K. Suzuki Department of Orthopaedic and Spinal Surgery, Nagoya Daini Red Cross Hospital, 2-9 Myoken-cho,Showa-ku, Nagoya 466, Japan

DIAGNOSTIC

NEURORADIOLOGY

Morphological analysis of the cervical spinal canal, dural tube and spinal cord in normal individuals using CT myelography

Abstract To verify the conventional concept of "developmental stenosis of the cervical spinal canal", we performed a morphological analysis of the relations of the cervical spinal canal, dural tube and spinal cord in normal individuals. The sagittal diameter, area and circularity of the three structures, and the dispersion of each parameter, were examined on axial sections of CT myelograms of 36 normal subjects. The spinal canal was narrowest at C4, followed by C5, while the spinal cord was largest at C4/5. The area and circularity of the cervical spinal cord were not significantly correlated with any parameter of the spi-

Introduction It has long been recognised that spondylotic changes narrow the spinal canal, resulting in compression of the spinal cord [1, 2]. Payne and Spillane [3] first suggested that the development of myelopathy might be related to the initial size of the spinal canal, and Hinck and Sachdev [4] demonstrated that the sagittal diameter of the canal was the best guide to stenosis, advocating the concept of "developmental stenosis of the cervical spinal canal", retrospectively established by measuring the sagittal diameter of the spinal canal on plain radiographs of patients with cervical compression myelopathy. Di Chiro and Schellinger [5] first reported demonstration of the spinal cord using CT myelography (CTM), Thijssen et al. [6] measured the transverse and sagittal diameters of the spinal cord on CTM of 20 patients without cervical pathology; and Yu et al. [7] re-

nal canal nor with the sagittal diameter and area of the dural tube at any level examined, and the spinal cord showed less individual variation than the bony canal. Compression of the spinal cord might be expected whenever the sagittal diameter of the spinal canal is below the lower limit of normal, that is about 12 mm on plain radiographs. Thus, we concluded that the concept of "developmental stenosis of the cervical spinal canal" was reasonable and acceptable. Key words Developmental spinal canal stenosis - Myelopathy. CT myelography. Spinal cord

ported on the morphology and measurements of the spinal cord on CTM (with metrizamide) in 36 control subjects, providing a basis for qualitative and quantitative assessment of cord deformity. However, we question whether cord compression can really be expected whenever "spinal canal stenosis" exists. If the spinal cord within the narrow canal is relatively thin, the concept would be quite useless and unacceptable. It is, therefore, a matter of importance whether the size and shape of the spinal cord correspond to the bony canal by nature or vary less from person to person than the bony canal does. So far, there have been few systematic, fundamental studies of the configuration and dimensions of the spinal canal, dural tube and spinal cord in normal subjects. Our aim was to investigate the morphological relationships of these structures on CTM of normal individuals, and to confirm the validity of the conventional concept by morphological analysis.

149

Materials and methods We examined 36 consecutive patients, 10 women and 26 men, aged 13-69 years (mean 33.9 years), with lumbar disc disease but no cervicothoracic neurological symptoms and signs or evident spondylotic changes in the cervical spine on plain radiographs. The CT scanner had a slice thickness of 5 mm. Window width and window level were set at 1000 HU and 150 HU, respectively. The patients were examined prone, the neck in neutral position. CT was performed about I h after lumbar intrathecal injection of 10 ml iotran (240 mg/ml), and the gantry was perpendicular to the long axis of the cervical spinal canal. Sections were taken at the midvertebral level from C2 to C7, and at the intervertebral level from C4/5 to C6/7 in all patients. Sagittal diameters of the spinal canal, dural tube and spinal cord were measured on these sections. Areas and circularities, as defined by the formula (4 ~ area/perimeter2), were also respectively measured using a biological image analyser. In addition, dispersions, as defined by the formula (standard deviation/mean value), were also calculated. In the statistical analysis, the significance level was set up at 5 %. The significant differences by sex and in dispersions of the characteristics among the spinal canal, dural tube and spinal cord were evaluated by the Mann Whitney U test and the Wilcoxon signed rank test, respectively.

Results

Table 1 Sagittal diameter of the cervical spinal canal, dural tube

and spinal cord (mean • SD mm) Level

Spinal canal

Dural tube

Spinal cord

C2 C3 C4 C4/5 C5 C5/6 C6 C6/7 C7

17.1• 13.3• 12.9•

14.5• 11.9• 11.4• 11.7• 11.5• 11.0• 11.5• 11.5• 12.1•

7.8• 7.5• 7.2• 7.1• 7.0• 6.7• 6.7• 6.5• 6.5•

13.1• 13.2• 13.8•

* Significant correlation between spinal canal and dural tube (P < 0.05) Table 2 Area of the cervical spinal canal, dural tube and spinal

cord (mean _+SD cm2) Level

Spinal canal

Dural tube

Spinal cord

C2 C3 C4 C4/5 C5 C5/6 C6 C6/7 C7

3.12 • 0.67* 2.18 + 0.34* 2.12 + 0.35*

2.27 • 0.41" 1.80 _+0.28* 1.81 + 0.24* 1.88 -+0.23 1.81 + 0.25* 1.73 + 0.30 1.79 + 0.30* 1.65 -+0.29 1.79 + 0.29*'

0.65 • 0.09 0.66 + 0.09 0.69 + 0.08 0.70 +-0.08 0.69 + 0.08 0.66 + 0.09 0.65 + 0.11 0.56 +-0.10 0.52 + 0.10

2.15 + 0.32* 2.21 + 0.45* 2.17 + 0.40*

T h e m e a s u r e m e n t s o f s a g i t t a l d i a m e t e r , a r e a a n d circul a r i t y a r e s h o w n in T a b l e s 1-3.

* Significant correlation between spinal canal and dural tube (P < 0.05)

Sagittal diameter

Table 3 Circularity of the cervical spinal canal, dural tube and

T h e s a g i t t a l d i a m e t e r of t h e s p i n a l c a n a l was s m a l l e r f r o m C3 to C6; t h e m i n i m u m was 1 2 . 9 + 1 . 3 m m ( m e a n + S D ) at C4, f o l l o w e d b y 13.1 + 1.3 m m at C5. T h e s a g i t t a l d i a m e t e r o f t h e d u r a l t u b e t e n d e d to b e s m a l l e r f r o m C4 to C6/7, w i t h t h e m i n i m u m v a l u e at C5/ 6. T h e s a g i t t a l d i a m e t e r o f t h e s p i n a l c o r d d e c r e a s e d g r a d u a l l y in t h e c a u d a d d i r e c t i o n (Fig. 1). T h e s a g i t t a l diameter of the spinal canal correlated significantly with t h a t of t h e d u r a l t u b e at e v e r y level, b u t w i t h t h a t o f t h e s p i n a l c o r d at o n l y C4 a n d C5 ( P < 0.05).

Level

Spinal canal

Dural tube

Spinal cord

c2 C3 C4 C4/5 C5 c5/6 C6 C6/7 C7

0.77 + 0.04 0.71 + 0.06 0.67 _+0.05

0,80 • 0.08 0.77 + 0.04 0.75 + 0.03* 0.75 + 0.04* 0.74 + 0.04* 0.74 _+0.04* 0.75 _+0.03 0.76 + 0.05 0.78 • 0.04

0.81 + 0.04 0.79 • 0.04 0.75 _+0.04* 0.74 + 0.03* 0.74 + 0.03* 0.74 • 0.03* 0.75 _+0.03 0.74 _+0.04 0.76 +_0.04

spinal cord (mean + SD)

0.67 • 0.05 0.69 + 0.05 0.71 +_0.05

* Significant correlation between dural tube and spinal cord

(P < 0.05) Area T h e a r e a of t h e s p i n a l c a n a l was s m a l l e s t f r o m C4 to C5, w i t h a m i n i m u m o f 2.12 + 0.35 c m 2 at C4. T h e a r e a of t h e d u r a l t u b e was n a r r o w e r b e l o w C3 level. A s t a t i s t i c a l l y s i g n i f i c a n t c o r r e l a t i o n was o b s e r v e d b e t w e e n t h e s e t w o a r e a s at e v e r y l e v e l ( P < 0.05). O n t h e o t h e r h a n d , t h e a r e a of t h e s p i n a l c o r d a t t a i n e d a m a x i m u m o f 0.70 + 0.08 c m 2 at C4/5, a n d d i d n o t c o r r e l a t e signific a n t l y w i t h t h a t o f t h e s p i n a l c a n a l o r d u r a l t u b e at a n y l e v e l (Fig. 2). T h e s a g i t t a l d i a m e t e r o f t h e s p i n a l c a n a l c o r r e l a t e d s i g n i f i c a n t l y w i t h its a r e a at e v e r y level, a n d

w i t h t h e a r e a o f t h e d u r a l t u b e f r o m C3 to C7 ( P < 0.05). N o s t a t i s t i c a l l y significant c o r r e l a t i o n c o u l d b e f o u n d b e t w e e n a n y m e a s u r e of t h e s p i n a l c a n a l a n d t h e a r e a o f t h e s p i n a l c o r d at a n y level.

Circularity T h e c i r c u l a r i t y of t h e s p i n a l canal, d u r a l t u b e a n d s p i n a l c o r d was g r e a t e s t at C2. T h e c i r c u l a r i t y o f t h e s p i n a l can a l was l e a s t at C4 a n d C5. T h a t o f t h e d u r a l t u b e was

150

Age, sex and height

mm

mean value -4-2 S.D.

15

Only one measure, the area of the dural tube, correlated significantly with height, at C4 and C5 (P < 0.05). Except for that, however, the measures did not correlate significantly with age, sex or height at any level.

I0 Discussion

5

I

I

I

I

I

I

C2 C3 C4 C5 C6 C7 Fig.1 Sagittal diameter of the cervical spinal canal (O), dural tube ( [] ), and spinal cord ( 9 )

Cln 2

mean v a l u e +

2 S.D.

3-

2 1 i

~

i

i

i

i

C2

C3

C4

C5

C6

C7

Fig.2 Area of the cervical spinal canal (9 spinal cord ( 9

dural tube (n), and

least from C4 to C6, and correlated significantly with the sagittal diameter of the spinal canal from C3 to C6 (P < 0.05). The circularity of the spinal cord did not correlated with any characteristic of the spinal canal, or with the sagittal diameter or area of the dural tube at any level. However, it did correlate significantly with the circularity of the dural tube from C4 to C5/6 ( e < 0.05). Dispersion The dispersion of the three characteristics of the spinal canal, dural tube and spinal cord were greater in that order; there were statistically significant differences in the dispersion of the sagittal diameter, area and circularity between the spinal canal cord (P < 0.05), but none between those of the dural tube and spinal cord. Consequently, individual differences in any measure of the spinal canal were significantly greater than those of the spinal cord.

In discussing the aetiology of compressive cervical myelopathy, we should consider the anatomical constitution of the cervical spinal canal, and the static or dynamic relationships of the bony canal, dural tube and spinal cord in the normal cervical spine. In the presence of a compressive factor in the spinal canal, the spinal cord, following the dural tube, could be deformed and compressed. Yu et al. [8] demonstrated a close correlation between the degree of cord deformity and the severity of symptoms, and Fujiwara et al. [9] showed that the transverse area of the spinal cord at the level of maximum compression was significantly correlated inversely with the outcome of surgery for compressive cervical myelopathy. We therefore examined the configuration and dimensions of the cervical spinal cord and adjacent tissues on axial CTM. A statistically significant correlation was observed between the sagittal diameter and area of the spinal canal at every level examined, both being smallest at C4; the spinal canal was narrow whenever it sagittal diameter was small. The circularity of the dural tube correlated significantly with the sagittal diameter of the spinal canal from C3 to C6. Thus, the sagittal diameter of the spinal canal might be the best guide to the transverse configuration of the dural tube. The spinal canal was narrowest at C4, followed by C5, while the spinal cord was wider from C4 to C5, and widest at C4/5, the cervical expansion. Remarkably, even if not in the pathological state, there was not enough room for the spinal cord at the midcervical level, particularly from C4 to C5. According to "the spinal cord dynamics" analysed by Ishida et al. [10], the spinal cord shifts anteriorly in the subarachnoid space in flexion of the neck and posteriorly in extension, and the anteroposterior shift correlates significantly with the angle of the adjacent intervertebral flexibility at C4, C5 and C6, where the range of segmental motion of the spine is generally greatest [11]. The sagittal diameter of the cervical spinal canal decreases in extension of the neck, and the ligamentum flavum folds inwards, resulting in spinal canal stenosis at the intervertebral levels [12]. Hence, as commonly recognised, the spinal cord could be compressed from both anterior and posterior when the neck is extended. On the other hand, the spinal cord also shifts cranially in flexion of the neck [13]; the widest portion of the spinal cord, at C4/5, slides to-

151

ward the narrowest segment of the canal at C4. Consequently, the midcervical spinal cord could also be more easily compressed and damaged in flexion of the cervical spine when there is an anterior compressive factor. These facts should be appreciated anew and always taken into consideration in the treatment of compressive cervical myelopathy. It is worth noting that the area and circularity of the spinal cord did not correlate significantly with any measure of the spinal canal or dural tube except the circularity of the dural tube from C4 to C5/6. Suzuki and Shimamura [14] reported that the size and shape of the cervical spinal cord (measured on MRI) and the spinal canal (measured on CT) showed significantly positive correlations in normal controls. These findings are contradictory to ours, mainly because Suzuki and Shimamura measured the spinal canal at C4 only. A n o t h e r reason for the discrepancy might be that their normal subjects were older (range 33-89 years; mean 48.1 years) and were not examined radiographically to exclude spondylotic changes. Factors such as degenerative changes due to ageing of the spine should be eliminated in studying normal controls. Spondylotic changes are commonly observed at the midcervical level, where the spinal canal is narrow and the spinal cord is widest; the spinal cord or dural tube could more easily be compressed by the degenerate spine. We found no significant correlations between the area and circularity of the spinal canal and spinal cord at any level, but a significant correlation in the sagittal diameter between the spinal canal and spinal cord at only C4 and C5.

Morphological analysis of the spinal canal and spinal cord should be carried out not only at C4, at several segments, using the same imaging technique. The range of any measure of the spinal cord was significantly smaller than that of the spinal canal, the spinal cord showing fewer individual differences than the bony canal. Also, the cord parameters changed little with sex, age and height. It may, therefore, be stressed that the aetiology of compressive cervical myelopathy is closely related to the developmental size of the bony canal. The mean value -2 SD of the sagittal diameter of the spinal canal was 10.3 m m at C4 and 10.5 m m at C5 on CT. These would translate to 11.8mm at C4 and 12.0 mm at C5 if measured on plain radiographs with a focus-film distance of 200 cm, as used in our hospital. We consider that these values are the lower limits of the normal range of sagittal diameter of the cervical spinal canal. Hence, our results agreed with the traditional concept of "spinal canal stenosis" (i. e. that the sagittal diameter of spinal canal is less than about 12 m m on plain lateral radiographs). When the sagittal diameter of the spinal canal is less than the lower limit, most spinal cords, which vary little in size and shape, can be expected to be compressed and flattened. Therefore, the concept of "developmental stenosis of the cervical spinal canal", as established in a retrospective review of cases of compressive cervical myelopathy, was found to be reasonable and supported by morphological analysis of normal controls.

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