Long-segment versus short-segment xation through a posterior approach for tuberculous spondylodiscitis of the mid-thoracic spine in adults: a study of mid- to long-term ecacy

This retrospective study aimed to perform comparatively evaluate the mid- to long-term ecacy of long-segment and short-segment xations via the posterior approach as treatment for tuberculous spondylodiscitis in the mid-thoracic spine. A total of 95 patients with tuberculous spondylodiscitis in the mid-thoracic spine underwent surgery via the posterior approach including single-stage posterior debridement, interbody fusion, and pedicle screw xation. Long-segment xations were performed for 46 patients (group A), while short-segment xations were performed for the other 49 patients (group B). Clinical and radiological outcomes were assessed during mid- to long-term follow-up. showing vertebral destruction with a kyphosis angle 26°. (2c) Postoperative X-ray demonstrating the correction of the deformity (kyphosis angle 14°). (1d) CT showing satisfactory bone fusion was obtained at 12 months. (2e) X-ray images displaying good internal xation position and solid bone fusion, with no loss of correction after the follow-up period of 63 months.


Background
Tuberculous spondylodiscitis (Pott's disease), a common type of extrapulmonary tuberculosis (TB), is a main cause of kyphotic deformity and paraplegia [1]. Spinal TB is endemic in developing countries due to the poor health care systems and nonspeci c manifestations in the early stage [2]. With the emergence of multi-drug resistant TB strains and HIV-triggered immunode ciency diseases, the incidence of spinal TB in developed nations has shown an upward trend recently [3].
Young adults are commonly susceptible to tuberculous spondylodiscitis in the mid-thoracic (T5-T8) spine. High risks of paralysis are present due to the spread of TB into the spinal canal, resulting in compression-induced spinal cord ischemia [4]. Antituberculous drugs are employed in the management of TB. In addition, surgical intervention is required in severe cases to immediately correct kyphotic deformities and to restore the remaining biological function of the spinal cord [5,6].
Surgical solutions have focused on the debridement of infected tissues that exhibit compromise, nerve decompression, and restoration of vertebral column stability by correction of kyphosis. The anterior approach was rst reported in 1960 for successful removal of dead tissue and decompression of the spinal cord [7]. With this approach, a large incision in the thoracic tissues is needed to reach the infection site, increasing the surgical risks. Additionally, osteoporosis associated with infection results makes the vertebral structure fragile and may lead to instrument loosening or displacement [8,9]. A combined anterior and posterior approach can overcome the stability-related disadvantage of the anterior approach, but this procedure is associated with more trauma and additional morbidity [10].
The posterior approach involves the use of a spinal instrumentation structure with screws and rods to correct the kyphotic deformity and decompress the spinal cord, facilitating treatment of tuberculous spondylodiscitis [11,12]. Pedicle screw xation and a rod structure mechanically create a spinal curvature to prevent the progression of kyphotic deformity and spinal cord compression induced by wedging vertebrae. Two techniques for posterior screw xation, long-segment and short-segment, are used in the surgical treatment. Short-segment xation is applied on the vertebrae one-level apart from the lesion vertebra, while long-segment xation involves two levels near the infected vertebra. Short-segment xation requires a small incision but might have a high risk of failure due to high local stress on the xation site. Long-segment can mechanically x more vertebrae to develop a solid support for spinal column stabilization, but requires a large surgical incision. Studies comparing the two xation techniques are lacking. To date, a comparative study evaluating the therapeutic bene ts of these two xations technique for Pott's disease has not been reported. The aim of the present study was to comparatively assess the mid-to long-term e cacies of the two xation techniques for the treatment of tuberculous spondylodiscitis of the mid-thoracic spine.

Patient data
A total of 95 patients with tuberculous spondylodiscitis of the mid-thoracic spine were enrolled in this study from January 2010 to April 2015. The diagnosis of tuberculous spondylodiscitis was bases on clinical presentations, laboratory tests, and imaging examinations. Final con rmation was achieved by biopsy or tubercle bacillus culture. The indications for surgical treatment of TB lesions were as follows: involvement of less than two adjacent vertebrae, unpreventable progressive kyphosis, spinal cord compression, and weak e cacy observed for anti-TB medication. Patients with multi-segment tuberculous spondylodiscitis with severe destruction of vertebrae, severe osteoporosis, extensive cold abscess, history of congenital scoliosis, and ankylosing spondylitis were excluded. All patients received single-stage posterior debridement, interbody fusion, and long-segment xation for group A (n = 46) or short-segment for group B (n = 49). Long-segment posterior xation involved two adjacent vertebrae above and below the lesion vertebra, while short-segment xation covered one adjacent vertebra on one side of the infected vertebra and two adjacent vertebrae on the other side.
The clinical symptoms included back pain, low fever, weight loss, and fatigue. Neurological impairment was present in some patients as well. All patients suffered thoracic back pain and/or intercostal neuralgia, and the pain intensity was assessed by the Visual Analog Scale (VAS). The erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP) level exceeded the normal levels at the initial stage. Imaging examination approaches, including plain radiography, computed tomography (CT), and magnetic resonance imaging (MRI), were used to detect vertebral body collapse, bone destruction, kyphotic deformity, epidural and paravertebral abscess, and destructive intervertebral discs as well. The lesion location and preoperative characteristics of the patients are described in Fig. 1 and Table 1.

Surgical method
In group A, pedicle screws were mounted on two adjacent vertebrae on each side of the infected vertebra (long-segment), and short pedicle screws were also xed in the affected vertebrae if the pedicle screw channel was not destroyed. A temporary pre-bent rod was stabilized on the side where bone destruction or spinal neurological damage was relatively mild to maintain spinal stability during focal debridement. Two custom-t rods as per the spinal curvature of a patient were longitudinally run through the screw heads to mechanically generate spinal stabilization and correct kyphosis. Autograft or allograft transplantation was employed for the intervertebral-associated intertransverse fusion. Streptomycin (0.1 g) and isoniazid (0.3 g) were used locally in the focus area, and the incision was closed in layers after placement of a drainage tube.
Group B received a short-segment solution involving screws and rods xation one level apart from the infected vertebra on one side and two levels on the other side. All other procedures were the same as followed in group A.

Postoperative management
All patients received anti-TB regimens for 12-18 months after surgery. In addition, routine blood and hepatorenal function examinations as well as CRP level and ESR measurement were performed to evaluate e cacy. Rehabilitation training and physical therapy were performed as early as possible to prevent thrombus and improve neurological function. Patients were permitted to walk slowly with the assistance of a rigid thoracic-lumbar brace for 3 months. Regular re-examinations were required every 3 months in the rst year postoperatively and every 6 months thereafter. Follow-up over 5 years was carried out for each patient to collect clinical and radiological data.

Evaluating standards and statistical analysis
Operation duration, intraoperative blood loss, and fusion period were documented for the two groups. Xray and CT examinations were performed to evaluate the internal xation condition and bone grafting.
The following assessment data were collected from the preoperative preparation to the end of follow-up: (1) ESR and CRP level; (2) VAS score; (3)  SPSS 20.0 software was used for statistical analysis. The follow-up outcomes of the two groups were compared by using Student's t test. Deviation from a normal distribution was evaluated using the ranksum test. Differences were considered statistically signi cant if the P value was less than 0.05.

Clinical outcomes
The postoperative patient data are summarized in Table 2. The average follow-up periods for groups A and B were 75.5 ± 11.8 and 76.8 ± 11.6 months, respectively. The operation duration and intraoperative blood loss in group A were greater than those in group B (P < 0.05). The ESR and CRP values in the two groups returned to normal levels within 3 months postoperatively. No signi cant differences in the ESR and CRP values were found between the two groups at 3 months postoperatively or at nal follow-up.  Neurological impairment was evaluated during the entire study from surgery to follow-up. The ASIA impairment scale protocol quanti ed spinal cord function in both groups at two stages, preoperative preparation and the last follow-up, as shown in Table 4. Increases in neurological function by at least one level were observed in 91.2% of patients in group A and in 89.5% of patients in group B after surgery as per the ASIA impairment scale assessment.

Radiographic outcomes
In group A, the kyphosis angle was signi cantly corrected from 22.2 ± 2.8° preoperatively to 12.3 ± 1.4°i mmediately after surgery. The correction rate was 44.2 ± 4.9%, and the correction loss was 1.1 ± 0.8°. The kyphosis angle in group B was signi cant decreased from 23.0 ± 3.0° preoperatively to 12.7 ± 1.5°i mmediately after surgery. The correction rate was 44.5 ± 4.5%, and the correction loss was 1.1 ± 0.7°. No signi cant differences were recorded in the preoperative kyphosis angle, postoperative kyphosis angle, correction rate, and correction loss between the two groups (P > 0.05).
The average fusion periods were 10.8 ± 2.1 months and 11.0 ± 2.0 months in groups A and B, respectively, with no signi cant difference between the two groups (P > 0.05). Surgical or postsurgical failures, such as nonunion, pseudoarthrosis, and loosening or fracture of instruments, were not detected at the last visit ( Figs. 2 and 3).

Complications
No operative mortality or permanent neurological impairment occurred in either group. Five patients (three in group A and two in group B) experienced super cial wounds that were cured by treatment with antibiotics. Local abscess recurrence due to irregular administration of anti-TB drugs was reported in two patients (one in group A and one in group B), who were treated by catheter drainage through minimally invasive incision and regular chemotherapy. Seven patients (three in group A and four in group B) complained of postoperative intercostal neuralgia, which was managed using nonsteroidal antiin ammatory drugs.

Discussion
The mid-thoracic spine possesses excellent stability due to its restricted mobility and thorax support, and the majority of patients with mid-thoracic spinal TB can be cured with conservative treatment. Nevertheless, a longer duration of therapy with anti-TB drugs is required for patients with vertebral collapse, severe kyphotic deformity, or incurable cold abscess, which may bring about drug resistance [13]. Therefore, a therapeutic strategy based on standard pharmacotherapy combined with appropriate surgical treatment, which involves removal of the affected vertebrae and restoration of spinal stability, could accelerate the recovery of patients with tuberculous spondylodiscitis of the mid-thoracic spine [14].
Lesion removal, bone grafting, and internal xation are regarded as critical techniques for surgical treatment of spinal TB. The posterior-only approach offers unique advantages for the treatment of tuberculous spondylodiscitis of the mid-thoracic spine. First, through the removal of the posterior column structure, this approach enables the affected vertebrae to be operated on with a direct view of the dura mater, and allows for simultaneous completion of bone grafting and correction of kyphotic deformity without spinal cord injury. Additionally, pedicle screws provide xation of three columns, effectively restoring the normal physiological curvature of the spine, and thus, a better holding force that can reduce the risk of loosening and fracture of the grafts and provides formation a strong biological xation in the temporary post-operation period. Intervertebral-associated intertransverse bone grafting to obtain 360°f usion ensures long-term spinal stability. Furthermore, the posterior approach is less invasive and avoids damage to complex anatomical structures or vital anatomical organs. Therefore, many surgeons have adopted the posterior approach for treating mid-thoracic TB, which has achieved good curative effects [14][15][16][17]. The rationale for using the posterior approach is based on removal of the pathological tissues and the ossi ed bone from around the lesion that prevents the entry of anti-TB drugs, thus destroying the favorable living environment essential for the survival of Mycobacterium tuberculosis. The rest of the small amount of lesion and abscesses can be absorbed with postoperative standardized anti-TB chemotherapy [18]. In our series, the ESR and CRP level returned to normal within 3 months postoperation, and all patients had achieved solid bone fusion at the nal follow-up. The results indicate that satisfactory curative effects were achieved with the posterior approach for tuberculous spondylodiscitis of the mid-thoracic spine.
Currently, there is no consensus regarding the best option for the xed segment range of mid-thoracic spinal TB. TB patients who commonly have an impaired spinal column shape due to kyphotic deformity and a pinched spinal cord experience intense pain and mental suffering. A healthy spinal curvature rebuilt by a reasonable xation of segments can signi cantly relieve back pain and improve the patient's quality of life. The long-segment xation mechanically links more vertebrae (more than four) into the screw and rod structure to rmly support the upper trunk weight [19,20]. Body weight and extra mechanical loads are spread across more vertebrae by virtue of involvement of over four vertebrae [21][22][23]. An extra therapeutic bene t in the present study was the small correction angle loss over 5 years of follow-up with respect to an over 40% reduction of kyphotic deformity after surgery. However, the large incision needed for this xation technique resulted in relatively high intraoperative blood loss and long surgical time. It should be noted that long-segment xation created a rigid segment on thoracic spine and further caused a negative effect on adjacent joints degeneration. The rigid structure also restricted more spinal functional segments that were reported by patients during follow-up [20].
The clinical and imaging data from the mid-to long-term follow-up were also in favor of the speci c strength of a short-segment xation on the management of mid-thoracic spinal TB. The short-segment technique apparently retained more normal vertebrae with less trauma in surgery and preserved more spinal motion. Short-segment instrumentation was reported to have potential risks for inappropriately stabilizing the spine after removal of infected vertebral tissues, as well as for a high stress concentration in the xation regions [24]. In this study, a short-segment employed one more level of vertebral xation on one side of a lesion vertebra, with exactly two more pedicle screw xations. This screw and rod structure created more supports to eliminate high local stresses [25]. In addition, sclerotic bone surrounding the affected vertebra might be helpful for the screw xation thanks to its high bone density [26,27]. As a result, no failure caused by screw loosening was reported in the short-segment group.
The mid-to long-term follow-up conducted in this study demonstrated that both long-segment and shortsegment xations achieved the expected clinical goal: signi cantly reduced kyphotic deformities without failures of xations such as loosened pedicle screws and cracked bone grafting. The kyphosis angle correction rate were 44.2 ± 4.9% in group A and 44.5 ± 4.5% in group B. The correction losses were only 1.1 ± 0.8° and 1.1 ± 0.7° in groups A and B, respectively, after 5 years of follow-up, suggesting a stable mechanical effect was achieved by those xations. Moreover, the posterior approach-based surgical protocol promoted e cient intervertebral fusions, whereby stabilization of the spinal column was established. Those therapeutic bene ts helped to restore a healthy spinal curvature, which positively reduced the health risks to the cardiopulmonary, neurological, and musculoskeletal systems. The VAS score re ecting pain intensity dropped over 80% over the long-term follow-up after surgical treatment. Signi cant increases in the SF-36 scores suggested favorable overall health status during follow-up. These assessments demonstrated that the patients' quality of life was improved, as expected.
Neither xation technique was overwhelmingly superior in terms of long-term e cacy and health-related quality of life. Some published studies also have reported no signi cant difference between long-segment and short-segment xations regarding clinical and radiological outcomes [28]. The comparative assessments in this study supported this nding based on a quantitative analysis of the correction of kyphotic deformities, VAS score for back pain, and SF-36 score for health status, especially mental health.
The short-segment protocol was superior to the long-segment protocol with respect to less surgical blood loss and a shorter operation time. Nevertheless, the long-segment protocol was the only choice in speci c cases with the following scenarios: severe kyphotic deformity, osteotomy involved in surgery, and osteoporosis. Therefore, neither segment protocol was found to be the overall best option for the posterior surgical approach to treating tuberculous spondylitis of the mid-thoracic spine.
This study also has some limitations. As a retrospective and single-center study with a relatively small sample size, statistical bias may have occurred. A multicenter study along with a large sample size is needed in the future to con rm the ndings in this study and better characterize the bene ts of the two approaches.

Conclusions
No therapeutic differences were observed between long-segment and short-segment xations for surgical treatment of mid-thoracic Pott's disease over mid-to long-term follow-up. Kyphotic deformity and neurological impairment were signi cantly relieved via both posterior xation approaches, and thus, patients' well-being reached to favorable levels. The short-segment xation has strengths regarding surgical blood loss and operation duration compared with the long-segment approach, but was less suitable for the treatment of severe cases. In conclusion, neither segment technique was found to always be superior for the surgical treatment of TB.

Consent for publication
All patients signed informed consent forms to publish their personal details in this article.

Availability of data and materials
The datasets and materials generated or analyzed during the current study are available from the corresponding author on reasonable request.