INTRODUCTION: The presence of disc tissue extending beyond the edges of the ring apophyses, throughout the circumference of the disc, is called ''disc bulging'' and is not considered a form of herniation. However, the definition of ‘disc bulging’ is unclear and the radiological differences between disc bulging and herniation are not clearly identified. The purpose of this study was to investigate the morphometry of disc bulging by computed tomography (CT) and magnetic resonance imaging (MRI).

METHODS: 52 patients (43 men, 9 women, average age: 70 ± 8.0 years-old) who underwent posterior decompression surgery for lumbar canal stenosis from August 2018 to February 2021were included in this study. Patients were CT-scanned pre-operatively. An axial CT–multiplanar reconstruction (MPR) was performed at all the lumbar intervertebral levels. The cross-sectional area of endplates (endplate area) was determined at the superior endplate (Fig. 1A) of each disc level. The cross-sectional area of IVDs (disc area) was determined at the midline between the superior and interior endplates (Fig. 1B). 1. Extended area of disc (EAD) was calculated as the subtraction of the endplate area from the disc area (Fig. 1). %EAD was calculated as follows: (disc area-endplate area)/endplate area×100. 2. Disc bulging length (DBL), defined as the distance from the posterior end of disc bulging to the posterior wall of the lumbar vertebra was measured at the mid-sagittal MRI (T2-weighted image) (Fig. 2). Disc height was evaluated by the disc height index (DHI) on MRI, as previously reported with a modification [1]. The cross-sectional area of the thecal sac (CSA) (mm²) was measured by tracing the outline of the thecal sac at the mid-axial slice of each disc level [2]. The correlation between %EAD and each parameter was evaluated using the Pearson correlation coefficient test.

RESULTS: The %EAD was highest at L3/L4 (21.8 ± 11.5), followed by L4/L5 (21.6 ± 11.8), L2/L3 (19.0 ± 10.9), L1/L2 (12.6 ± 11.3) and L5/S1 (12.1 ± 10.0). The %EAD at L2/L3, L3/L4 and L4/L5 was significantly higher than those at L1/L2 and L5/S1 (Fig. 2A). The DBL was highest at L4/L5 (5.1 ± 1.6), followed by L3/L4 (4.5 ± 1.8), L5/S1 (4.0 ± 2.1), L2/L3 (3.8 ± 1.7) and L1/L2 (3.2 ± 1.8). The DBL at L4/L5 was significantly higher than those at L1/L2, L2/L3 (P<0.01) and L5/S1 (P<0.05). (Fig. 2B). There was a weak positive correlation between %EAD and DBL (r=0.26, P<0.01), and a weak negative correlation between %EAD and CSA (r=-0.33, P<0.01) and DHI (r=-0.22, P<0.01).

DISCUSSION: This study successfully evalated the disc bulging as represented by EAD (%EAD) from pre-operative axial CT–MPR and showed significant differences among the lumbar spinal levels. Our results showed %EAD was weakly correlated with DBL, CSA and DHI, suggesting that %EDA is not directly associated with disc height narrowing and lumbar canal stenosis.