Abstract
Background
Congenital thoracic stenosis (CTS) occurs when the bony anatomy of the canal is smaller than expected in the general population. The diagnosis currently is made based on the clinical impression from subjective radiographic studies, and the normal values for CTS have not been established.
Questions/purposes
We provided a statistical definition for CTS based on objective measurements of thoracic spine specimens and explored parameters that might predict CTS.
Methods
We selected 700 adult skeletal specimens from the Hamann-Todd Collection in the Cleveland Museum of Natural History (Cleveland, OH, USA). We used calipers to measure the sagittal canal diameter (SCD), interpedicle distance (IPD), and pedicle length (PL). At each level, canal area was calculated using a geometric formula, a standard distribution was created, and values two SDs below the mean were considered congenitally stenotic. Corresponding values of SCD and IPD of the stenotic specimens were studied. The values of SCD and IPD predicting CTS with highest sensitivity and specificity were tabulated.
Results
At each level, CTS was defined as: T1, 160 mm2; T2, 135 mm2; T3, 131 mm2; T4, 130 mm2, T5, 129 mm2, T6, 127 mm2; T7, 127 mm2; T8, 129 mm2; T9, 130 mm2; T10, 132 mm2; T11, 140 mm2; and T12, 173 mm2. A SCD less than 15 mm and an IPD less than 18.5 mm were predictive of CTS at each level with sensitivities and specificities of 80% to 100%.
Conclusions
We statistically defined CTS at each level. A SCD less than 15 mm or IPD less than 18.5 mm predicted the presence of CTS at all levels.
Clinical Relevance
In a symptomatic patient, on routine radiologic examination, a physician should suspect stenosis of the thoracic canal if the SCD and IPD are less than 15 and 18.5 mm respectively. As a spinal deformity surgeon, the canal area is especially relevant when considering a possible canal intrusion by implants.
Introduction
The spinal canal has been the interest of human research since the late 19th century [3, 5, 7, 10, 24, 35, 40]. Gowers [14] in 1892 described the changes in cervical spondylosis as vertebral exostoses but it was only in 1955 that the earliest detailed account of cervical vertebra measurements were provided by Francis [12]. In 1957, Payne and Spillane [32] described the relationship between developmental stenosis and canal size. Since then, numerous anatomic and radiographic studies of various cervical and lumbar canal parameters have been performed to establish a standard for defining spinal stenosis [1, 2, 6, 9, 16, 20, 21, 24–27, 33, 35].
Congenital stenosis occurs when the bony anatomy of the spinal canal is smaller than expected in the general population. Shrinkage and loss of disc space attributable to degeneration with advancing age further aggravate the disease process [18, 29], predisposing an individual to symptomatic neural compression causing a wide array of clinical symptoms ranging from asymptomatic or mild back pain to severe myelopathy causing paralysis. Investigative studies use differing eligibility standards as there are no widely accepted diagnostic or classification criteria for the diagnosis, which further limits the interpretation of reported findings [13]. The diagnosis currently is made based on the clinical impression from radiographic studies, which is subjective at best.
Approximately 250,000 to 500,000 individuals in the United States have symptoms of spinal stenosis, with less than 1% occurring in the thoracic spine. The prevalence is expected to increase during the next decade to 18 million as the US population ages [19]. Although the majority of studies related to the thoracic spine have focused mainly on pedicle diameters and their angulations, little importance has been given to the vertebral body, spinal canal diameters (SCD), and canal area [4, 8, 17, 23, 28, 30, 39]. Objective parameters based on morphoanatomic measurements of the thoracic spine are needed to define this condition, as are simple parameters that will accurately predict if congenital thoracic stenosis (CTS) is present.
We therefore provide a definition for CTS based on objective measurements of thoracic spine specimens and establish parameters that will accurately predict CTS.
Materials and Methods
The Hamann-Todd Osteological Collection in Cleveland, Ohio contains more than 3300 treated and dried specimens, of which 700 specimens were randomly chosen for examination in no particular order. The specimens in the collection are from individuals who died in Cleveland between 1893 and 1938. We included 568 male and 132 female specimens, ranging in age from 17 to 105 years. Sixty-three percent of the specimens in the study were between 35 to 64 years of age (Table 1). Of the 700 specimens, 275 were from black individuals and the remainder from white individuals. As there is limited knowledge of the differences between living or fresh specimens and the dimensions of the skeleton prepared using the preservation methods, only the specimens with better bone quality were examined to ensure uniformity of the sampled data.
Table 1.
Systematic breakdown of sampled specimens into various age groups and sexual and racial distribution
| Age (years) | Number of specimens | Males | Females | Black | White |
|---|---|---|---|---|---|
| 15–24 | 50 | 34 | 16 | 42 | 8 |
| 25–34 | 92 | 63 | 29 | 55 | 37 |
| 35–44 | 161 | 128 | 33 | 70 | 91 |
| 45–54 | 180 | 156 | 24 | 65 | 115 |
| 55–64 | 103 | 94 | 9 | 24 | 79 |
| 65–74 | 74 | 62 | 12 | 11 | 63 |
| 75–84 | 31 | 27 | 4 | 3 | 28 |
| > 85 | 9 | 4 | 5 | 5 | 4 |
| Total | 700 | 568 | 132 | 275 | 425 |
One examiner (BNS) subjectively measured the gross specimens by digital caliper with a precision of one-hundredth of a millimeter. The flat surface of the table edge was used to align each vertebra in the axial plane and all measurements were taken from the superior aspect of the vertebra. The interpedicle distance (IPD) was measured as the maximal distance from the medial surface of the pedicles on either side (Fig. 1). The SCD was measured as the maximum AP distance of the spinal canal of each vertebra (Fig. 2). Pedicle length (PL) was measured starting from the origin of pedicle from the body to the superior articular facet on either side (Fig. 3). The average of both sides was used as the PL. To minimize the error, each of the measurements of IPD, SCD, and PL were calculated twice.
Fig. 1.
Measurement of the IPD after proper alignment of the thoracic vertebra is shown.
Fig. 2.
Measurement of the SCD from the superior surface of the thoracic vertebra is shown.
Fig. 3.
PL is measured from the superior aspect of the thoracic vertebra. The average of both pedicles was used in the study. PL = pedicle length.
After the measurements were taken, the morphologic dimensions of the vertebrae (SCD, IPD, PL) were used to calculate the canal area at each level using a standardized geometric formula [15] (Fig. 4). The reliability and accuracy of these measurements to calculate canal area were verified by comparing these with ImageJ (National Institutes of Health, Bethesda, MD) measurements on a sample of 48 random thoracic vertebrae (Table 2). The accuracy of the geometric formula to calculate canal area was high as the calculated canal area measurements were comparable to ImageJ measurements for all 48 specimens. A standard distribution curve for area at each level was created, and values two SDs below the mean were arbitrarily considered congenitally stenotic.
Fig. 4.
The total area of the thoracic canal was calculated as the sum of the area of the rectangle (white) and the isosceles triangle (gray). These measurements were further verified using ImageJ software.
Table 2.
Calculation of canal area (in mm2) by Image J and comparison with actual measurements (measured by geometric formula)
| Thoracic level | Specimen 1 | Specimen 2 | Specimen 3 | Specimen 4 | ||||
|---|---|---|---|---|---|---|---|---|
| Image J | Actual | Image J | Actual | Image J | Actual | Image J | Actual | |
| Area T1 | 215.3 | 213.1 | 198.7 | 195.9 | 188.6 | 191.8 | 196.5 | 191.4 |
| Area T2 | 187.6 | 185.9 | 152.8 | 145.3 | 149.5 | 143.8 | 155.7 | 156.6 |
| Area T3 | 183.5 | 184.7 | 158.9 | 161.5 | 160.2 | 156.8 | 152.6 | 149.2 |
| Area T4 | 185.8 | 187.7 | 158 | 158.4 | 145.2 | 145.3 | 150.3 | 149.1 |
| Area T5 | 180.2 | 177 | 148.4 | 151 | 152.9 | 147.8 | 141.1 | 144.4 |
| Area T6 | 188.9 | 186.7 | 145.5 | 147.7 | 145.6 | 144.5 | 145.9 | 147.1 |
| Area T7 | 153.2 | 148.6 | 195.7 | 197.7 | 233.6 | 238.8 | 177.4 | 177.4 |
| Area T8 | 166.6 | 162.2 | 202.9 | 202.7 | 256.4 | 249.6 | 172.3 | 174.9 |
| Area T9 | 165.5 | 164.3 | 223.2 | 215.7 | 233.3 | 229.1 | 169.2 | 164.3 |
| Area T10 | 150.7 | 146.2 | 215.6 | 218.8 | 238.6 | 243.1 | 180.2 | 182.6 |
| Area T11 | 162.1 | 159.7 | 238.5 | 234.8 | 218.9 | 220.2 | 205.2 | 211.4 |
| Area T12 | 226.4 | 222.2 | 276.5 | 280.1 | 260.8 | 262.9 | 261.7 | 255 |
For each specimen, the stenosis was defined and age, sex, and race were recorded.
The SCD and IPD values of all the specimens considered congenitally stenotic were examined. The values of SCD and IPD that predicted stenotic canal area with highest sensitivity and specificity were tabulated.
Results
Thoracic stenosis was defined at each level as: T1, 160 mm2; T2, 135 mm2; T3, 131 mm2; T4, 130 mm2; T5, 129 mm2; T6, 127 mm2; T7, 127 mm2; T8, 129 mm2; T9, 130 mm2; T10, 132 mm2; T11, 140 mm2; and T12, 173 mm2.
The corresponding values of SCD and IPD were examined in the specimens defined as congenitally stenotic. At each thoracic level, the values of SCD and IPD that predicted CTS with highest sensitivity and specificity were tabulated. A constant decrease in the IPD was noted from T1 to T6, with little variation from T6 to T10 and followed by a gradual increase from T10 to T12 (Table 3). The SCD dimensions showed little variation from T1 to T12, with the minimum value at the T3, T5, T6, T7, and T9 levels and maximum value at T12 level (Table 4). The PL did not correlate well with canal area changes and did not predict CTS.
Table 3.
Value of IPD predicting CTS at each level with highest sensitivity and specificity
| Thoracic level | IPD (mm) | Sensitivity (%) | Specificity (%) |
|---|---|---|---|
| T1 | 18.5 | 95 | 77 |
| T2 | 16 | 93 | 91 |
| T3 | 16 | 89 | 96 |
| T4 | 15 | 88 | 89 |
| T5 | 15 | 100 | 97 |
| T6 | 14 | 95 | 96 |
| T7 | 14.5 | 89 | 96 |
| T8 | 14 | 100 | 90 |
| T9 | 14.5 | 100 | 97 |
| T10 | 14.5 | 90 | 93 |
| T11 | 15.5 | 84 | 93 |
| T12 | 17.5 | 89 | 94 |
IPD = interpedicle distance; CTS = congenital thoracic stenosis.
Table 4.
Value of SCD predicting CTS at each level with highest sensitivity and specificity
| Thoracic level | SCD (mm) | Sensitivity (%) | Specificity (%) |
|---|---|---|---|
| T1 | 14 | 95 | 98 |
| T2 | 13.5 | 86 | 94 |
| T3 | 13 | 95 | 81 |
| T4 | 13.5 | 88 | 89 |
| T5 | 13 | 84 | 85 |
| T6 | 13 | 84 | 96 |
| T7 | 13 | 89 | 91 |
| T8 | 13.5 | 95 | 98 |
| T9 | 13 | 95 | 96 |
| T10 | 13.5 | 100 | 96 |
| T11 | 14 | 90 | 96 |
| T12 | 15 | 100 | 96 |
SCD = sagittal canal diameter; CTS = congenital thoracic stenosis.
Discussion
As the US population ages, the burden of spinal stenosis will increase. As less than 1% of total cases relate to thoracic stenosis, few studies detailing the morphologic features of the thoracic canal have been conducted. Most studies examine only morphologic features of the pedicle [4, 8, 17, 23, 28, 30, 38, 39]. None of these studies describe the morphologic features of the thoracic spine in the American population, although normal values of thoracic vertebral canal anatomy are pertinent to diagnose thoracic stenosis. In this study, we have provided a statistical definition for CTS based on objective measurements of thoracic spine vertebral specimens and determined parameters that predict CTS with highest sensitivity and specificity.
This study has some inherent limitations. First, we arbitrarily established the statistical values of stenotic canal area based on values less than 2 SDs, but it is not known whether these values are associated with clinical symptoms. Second, in this retrospective cadaver study, the findings of CTS in skeletal specimens have not been correlated with the relevant history, physical examination, serial imaging, and autopsy analysis after death, which would have provided the most reliable measure of thoracic canal parameters to define thoracic stenosis. Third, although we cannot account for the soft tissue components such as the posterior longitudinal ligament and herniated nucleus pulposus, which play a major role in the clinical syndrome of CTS [18], these do not jeopardize the actual bony canal area measurement (< 2 SD) and thus, as far as the anatomic parameters are concerned, a population analysis of a large number of cadaveric specimens provides the most viable answers.
The cross-sectional area of the thoracic canal has been examined in various studies [31, 37]. Panjabi et al. [31] reported a large decrease in canal area from T1 to T2 with little variation until T10, and then an increase from T10 to T12. Tan et al. [37] showed a similar trend in their study, with the canal area being smallest at T4. In our study, we observed a gradual decline in canal area from T1 to the T6 and T7 levels, after which the canal area steadily increased to a maximum value at T12 (Table 5). It is important to define all morphologic parameters of the thoracic spine in relation to the canal area on a large sample size to correctly establish an accurate criterion for thoracic stenosis. The above-reported studies had variable sample sizes ranging from six to 100 cadaveric specimens. As the main focus of these studies was to provide baseline morphometric data concerning the thoracic vertebrae, the IPD and SCD dimensions were not correlated with the canal area in any of these studies, and as a result, these studies were unable to establish a set standard or a definition of thoracic spinal stenosis. We examined and correlated the anatomic parameters with thoracic canal area to establish the values of SCD and IPD that can accurately predict a decreased canal area, that is, canal stenosis. As a spinal deformity surgeon this is especially relevant when considering pedicle screw placement and possible canal intrusion by implants. The morphologic features of the vertebrae change from one population set to another [22], so the same criterion cannot be applied to different populations. Thus, it is pertinent to look at thoracic stenosis in an average American population, as the above anatomic studies have European or Asian subjects. In our study, we morphoanatomically compared a large sample of representative specimens from an American population ranging in age from adolescence to old age and including black and white individuals.
Table 5.
Comparison of canal area (in mm2) in various studies
| Thoracic level | Panjabi et al. [31] | Tan et al. [37] | Current study |
|---|---|---|---|
| T1 | 204.1 | 169.4 | 210 |
| T2 | 170.9 | 145.7 | 179.5 |
| T3 | 173.3 | 145.4 | 175.3 |
| T4 | 178 | 140.1 | 174.8 |
| T5 | 183.3 | 141.5 | 174 |
| T6 | 187.8 | 144.8 | 173.3 |
| T7 | 184 | 144.2 | 175.3 |
| T8 | 183.2 | 148.1 | 179.8 |
| T9 | 185.3 | 145.9 | 181.7 |
| T10 | 184.5 | 142.3 | 184.8 |
| T11 | 196.6 | 148.6 | 198.8 |
| T12 | 242.5 | 177.8 | 237.7 |
Cadaveric studies have evaluated the baseline morphometric IPD and SCD data of the thoracic canal in reference to the musculoskeletal anatomy and biomechanics of the thoracic spine. Epstein and Schwall [11] defined thoracic stenosis in their study as an AP diameter of the thoracic canal less than 10 cm and reported primary thoracic stenosis frequently is associated with lumbar stenosis. Singh et al. [36] studied 100 cadavers and concluded that the maximum IPD was at T1. The IPD gradually decreased from T1 to T5 and then increased from T6 to T12 (Table 6). The SCD had relatively stable values between T1 to T12, the minimum being observed at T2 (Table 7). Panjabi et al. [31] studied thoracic morphologic features in 12 cadaveric specimens and reported the IPD decreased initially from T1 to T4 and then gradually increased from T5 to T12 (Table 6). The SCD ranged from a minimum of 14.4 mm at T2 to a maximum of 16.3 mm at T12 (Table 7). Some other studies [34, 37, 38] also have reported the thoracic canal IPD to be the narrowest at the T4 and T5 levels, but none of the previous studies examined the relationship of thoracic stenosis with these parameters. We found the smallest IPD at the T6 to T8 levels and these values were further correlated with thoracic canal area to predict CTS with high sensitivity and specificity.
Table 6.
Comparison of interpedicle distance (in mm) in various studies
| Thoracic level | Singh et al. [36] | Panjabi et al. [31] | Tan et al. [37] | Current study |
|---|---|---|---|---|
| T1 | 19.7 | 21 | 17.7 | 18.5 |
| T2 | 17.1 | 18.5 | 15.2 | 16 |
| T3 | 16.2 | 17.4 | 14.2 | 16 |
| T4 | 15.8 | 16.3 | 13.5 | 15 |
| T5 | 15.5 | 16.4 | 13.6 | 15 |
| T6 | 15.5 | 16.6 | 13.8 | 14 |
| T7 | 15.6 | 16.6 | 13.9 | 14.5 |
| T8 | 15.8 | 17 | 14.1 | 14 |
| T9 | 15.9 | 17.1 | 14.2 | 14.5 |
| T10 | 15.9 | 17.3 | 14.2 | 14.5 |
| T11 | 16.9 | 18.5 | 15.3 | 15.5 |
| T12 | 18.9 | 21.4 | 17.9 | 17.5 |
Table 7.
Comparison of sagittal canal diameter (in mm) in various studies
| Thoracic level | Singh et al. [36] | Panjabi et al. [31] | Tan et al. [37] | Current study |
|---|---|---|---|---|
| T1 | 13.8 | 15 | 11.6 | 14 |
| T2 | 13.8 | 14.4 | 11.7 | 13.5 |
| T3 | 13.9 | 14.9 | 12 | 13 |
| T4 | 14 | 15 | 11.8 | 13.5 |
| T5 | 14.1 | 14.9 | 11.5 | 13 |
| T6 | 14.3 | 15 | 11.6 | 13 |
| T7 | 14.2 | 15 | 11.9 | 13 |
| T8 | 14.1 | 14.9 | 11.9 | 13.5 |
| T9 | 14 | 14.7 | 11.8 | 13 |
| T10 | 14 | 14.7 | 11.9 | 13.5 |
| T11 | 14.9 | 14.9 | 11.8 | 14 |
| T12 | 15.9 | 16.3 | 12.4 | 15 |
Based on our study of a large population of adult skeletal specimens, we have statistically defined CTS at each level. As the morphologic features of the thoracic spine vary considerably from one level to another, it is impossible to set one lower limit for all thoracic vertebrae. Thus, we defined lower limits of SCD and IPD at each level that predicted the presence of CTS. Our study encompassed a large population of adult American individuals and the changes occurring in the thoracic region regarding development of stenosis. We studied various aspects of thoracic stenosis, correlating the smaller cross-sectional area with the predictive parameters of bony anatomy, namely, the SCD and IPD, with high sensitivity and specificity.
Footnotes
Each author certifies that he or she, or a member of their immediate family, has no commercial associations (eg, consultancies, stock ownership, equity interest, patent/licensing arrangements, etc) that might pose a conflict of interest in connection with the submitted article.
All ICMJE Conflict of Interest Forms for authors and Clinical Orthopaedics and Related Research editors and board members are on file with the publication and can be viewed on request.
This work was performed at Case Western Reserve University, Cleveland, OH, USA.
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