Analysis of sagittal profile and radiographic parameters in symptomatic thoracolumbar disc herniation patients

In normal spine alignment, the thoracolumbar spine is routinely defined as T10-L2 and recognized as the inflexion region between the kyphotic thoracic spine and lordotic lumbar spine [1]. Although few studies describe it as an isolated category, it is frequently classified as the lower thoracic spine or upper lumbar spine in many studies [2, 3]. Thus, less is known about the morphology and aetiology of thoracolumbar disc herniation (TLDH) compared to lumbar disc herniation (LDH) [1].

It is widely accepted that explicit descriptions of a sagittal spine alignment assist in the determination of aetiology and the orientation of the surgical plan, both of which are achieved using two incisive tools (spinopelvic parameters and curvatures) in both degenerative spine disease and spine deformity [4‐6]. However, these descriptions are overemphasized on spinopelvic parameters, and this process is sometimes tedious due to the lack of a direct view of spine curvatures. Furthermore, the sagittal profile may be affected by complex compensatory mechanisms, especially in patients with a spine deformity, which may not be indicated in radiographic parameters [6]. To address these deficiencies, Albelin-Genovois et al. described sagittal alignment with a direct view of spine curvatures and supplemented this with relevant spinopelvic parameters, which were validated in a further study [6, 7].

We agreed with the Albelin-Genovois strategy in that an ideal and practical description of spine alignment should be based on a direct view of spine curvatures and supplemented with radiographic parameters [6]. Guided by this “Curvature First” tactic, regional curvatures of the thoracic and lumbar spine were first recorded separately and then pieced together to depict a direct view of the whole thoracic and lumbar spine in this study. Here, we present some exclusive sagittal alignment descriptions of TLDH, investigate the association between TLDH and curvature variants, and interpret alignment descriptions with spinopelvic parameters in this relatively rare condition.

TLDH is a relatively rare condition, which has been defined in previous studies as thoracic disc herniation or upper lumbar disc herniation, and little is known about its regional alignment and aetiology. In this study, we found two typical subtypes of thoracic-lumbar alignment in TLDH patients, classified according to the apex of the thoracic curve. Regional deformity, but balanced alignment, was validated in both profiles, as interpreted from radiographic parameters.

In previous studies, many attempts have been made to depict spine alignment for a better understanding of spine disease [11‐13]. Bae et al. investigated the spine alignment in upper lumbar disc herniation, which was defined as symptomatic disc herniation at L1/2 and L2/3 [13]. One limitation of this study was the neglect of thoracolumbar region alignment. In addition, TLK angle and other parameters were not recorded. The TLK angle has been reported to be less than 10° in the normal population, and the severity of kyphotic deformity is defined as mild (10–25°), moderate (26–50°), and severe (more than 50°) [14]. In this study, a TLK angle higher than the normal population was observed in both subtypes, indicating a common sagittal profile with thoracolumbar kyphotic deformity. The large TLK angle was validated as a risk factor for upper LDH by Wang, and a relatively higher mean TLK angle (16.9 ± 0.4°) was also observed in the TLDH group compared to the LDH group (7.6 ± 5.2°) in a comparative study [3, 15]. Our conclusion was consistent with that of previous studies, and we speculated that disc degeneration may be accelerated by excessive mechanical stress generated by a high TLK angle, resulting in the occurrence of TLDH.

Some radiographic parameters are considered as important tools for evaluating the balance status of spine alignment. SVA is the most widely used, and a novel parameter, T1PA, has attracted more attention [8]. As there are no reports about spine balance status in patients with TLDH, we first attempted to evaluate the spine balance status using these values, and found that both could define a balanced spine. Furthermore, L1PA was also utilised as a practical tool to evaluate regional lumbar curvature alignment, and a similar conclusion was drawn—a balanced lumbar spine. A balanced spine was observed in both whole spine alignment and regional spine alignment, indicating the wide range of compensatory ability of the spine, even in mild to moderate regional deformities.

Although a regional kyphotic deformity, but balanced spine, was observed in both subtypes, some different characteristics between type I and type II were also observed. First, we found that patients classified as type II had a higher TLK angle and lower LL value compared to patients classified as type I. Secondly, TLDH-I was the major manifestation in type I patients, and TLDH-A was the major manifestation in type II patients.

We considered that TLDH was caused by its intrinsic curvatures in each sagittal profile. In type I patients, the sagittal profile was similar to that of the normal population, and the thoracolumbar region was identified as the lower arc of the thoracic kyphotic curve and inflexion point between the thoracic curve and lumbar curve. Although mechanical stress was still diverted by the aligned thoracic-lumbar curve in this type, we speculated that disc degeneration was accelerated by regional kyphosis in the thoracolumbar junction and eventually caused disc herniation, identified by TLDH-I. On the contrary, the middle thoracic spine was replaced by the thoracolumbar region as the apex region in type II patients, resulting in the reciprocal change in regional thoracolumbar curvature whereby the thoracolumbar region was altered to the top of the thoracic and lumbar alignment. In this profile, the aligned thoracic-lumbar curve was disrupted, resulting in that excessive mechanical stress was directly loaded at the top of the curve (thoracolumbar apex region) rather than being diverted by an arc as in a normal population or type I patients, presenting as TLDH-A.

The Roussouly classification was first established in a normal population, and its effectiveness in pathologic conditions remained ambiguous. Some authors have used it as a practical tool for lumbar degenerative disease treatment, and the shape of curvature and SS values are well-matched in these populations [4, 11, 12]. However, mismatch of shape and radiographic values was frequently observed in patients with spine deformities, and some authors have attempted to reconstruct spine alignment according to this classification [7, 16, 17]. We found mismatches between parameters and shape in both types, which were different to findings for degenerative lumbar disease in previous studies [4, 11, 12]. We speculated this was the result of regional deformities. As we described in this study, regional kyphosis should be taken into consideration for TLDH aetiology in this “pathogenic but compensatory” sagittal profile.

To compensate for the Roussouly classification, Sebaaly further added the “anteverted type” or “retroverted type” category using PT values [18], and some authors have also advocated that compensatory pelvic retroversion was the possible compensatory mechanism of thoracolumbar kyphotic deformity [19, 20]. We attempted to interpret thoracolumbar profiles using PT values and set a PT value of more than 25° as pelvic retroversion and less than 5° as pelvic anteversion. A neutral pelvic morphology was frequently observed in both subtypes, indicating that compensatory pelvic mechanisms were not certain in TLDH. However, further studies with large samples are needed to validate these corresponding compensatory mechanisms.

We presented two distinctive sagittal profiles in TLDH patients, and a regional kyphotic deformity with a balanced spine was validated in both subtypes. In type I patients, disc degeneration was accelerated by regional kyphosis in the thoracolumbar junction and eventually caused disc herniation. In type II patients, excessive mechanical stress was directly loaded at the top of the curve (thoracolumbar apex region) rather than being diverted by an arc as in a normal population or type I patients. Mismatch between shape and sacral slope value was observed, and better agreement was found in Type II patients.