Background: Despite being prone to degenerative disease and other pathology, relatively little is known of the morphology and morphometry of thoracic intervertebral discs in comparison to discs in the cervical and lumbar spines. The few studies of thoracic intervertebral disc morphometry that have been performed have included relatively small numbers of cadavers. No previous studies have attempted to investigate how thoracic intervertebral disc morphometry varies with sex and age.
Aims: This study aimed to describe the normal morphometry of adult thoracic intervertebral discs in subjects with no known spinal disease and investigate potential associations with sex, disc level and age.
Methods: Computed tomography (CT) scans on 128 recently deceased unembalmed cadavers (70 males, 58 females, aged 20-79 years, divided into 10-year age cohorts) with no known spinal pathology were obtained from the archives of the Victorian Institute of Forensic Medicine, Australia. Intervertebral disc height, anteroposterior and transverse disc dimensions, wedge-index and other morphometric parameters were measured at alternate intervertebral discs throughout the thoracic spine, beginning at T2-3. Data were analysed to determine associations between intervertebral disc parameters and sex, disc level and age (10-year age groups). Linear mixed models were fitted to anterior and posterior disc height and anteroposterior and transverse disc dimensions. Intra- and inter-rater variation were assessed by intraclass correlation coefficients (ICCs) performed on 25% and 18% of the whole sample, respectively. Measurement reliability was further assessed by investigating agreement between radiographic and equivalent measures performed directly on two cadaver thoracic spine specimens.
Results: Intervertebral disc heights and axial dimensions were all significantly larger in males than females except middle disc height (anterior disc height 4.0 ± 1.4mm vs 3.6 ± 1.3mm; posterior disc height 3.6 ± 0.90 vs 3.4 ± 0.93, both p<0.01). This may represent a scaling effect, as the mean differences were similar to differences in stature between sexes. Disc heights and axial dimensions at T4-5 were similar or smaller than at T2-3, but thereafter increased caudally in the thoracic spine (mean anterior disc height at T4-5 in males 2.7 ± 0.7mm, females 2.6 ± 0.8mm; at T10-11 in males 5.4 ± 1.2mm, females 5.1 ± 1.3mm, both p<0.05). Discs in the upper and lower thoracic spine were significantly more wedge-shaped and displayed greater convexity than those in the mid-thoracic spine (T6-7). Except at T2-3, anterior disc height decreased with advancing age while anteroposterior and transverse disc dimensions increased; posterior and middle disc heights and indices of disc shape showed no consistent statistically significant change. Most measured parameters showed substantial to almost perfect agreement for both intra-rater and inter-rater reliability while cadaver validation showed fair to almost perfect agreement between radiographic and anatomic measures.
Conclusions: Thoracic intervertebral disc morphometry varies significantly and consistently with sex, disc level and age. This study provides unique data on normal thoracic intervertebral disc morphometry of adults, which should be useful in the detection of pathological changes and for informing biomechanical and functional studies.
