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Percutaneous instrumentation with cement augmentation for traumatic hyperextension thoracic and lumbar fractures in ankylosing spondylitis: a single-institution experience

OBJECTIVE The typical traumatic thoracolumbar (TL) fracture in patients with ankylosing spondylitis (AS) is a hyperextension injury involving all three spinal columns, which is associated with unfavorable This cohort study was completed retrospectively. Back pain was assessed at preoperative, postoperative, and final follow-up visits using the visual analog scale (VAS). Patient-reported outcomes via the Oswestry Disability Index the new mobility score (NMS) were obtained to assess disability and mobility during follow-up. Radiological outcomes included the Cobb sagittal union rate, and implant failure. Intra- and postoperative complications were the ankylosed spine.

Thoracolumbar (TL) hyperextension fractures (type B3, AO Spine Thoracolumbar Spine Injury Classification System 6 ) are mainly restricted to patients affected with ankylosing spinal disorders and represent 20% to 40% of AS fractures. 7 Although nonoperative treatment has been reported, it is ineffective in most patients, with a high failure rate of almost 50% of ankylosed spine injuries. 8,9 Typically, the surgical treatment of TL fractures involves open posterior instrumentation. [10][11][12] However, it is related to high postoperative mortality and morbidity rates. 13,14 Recently, minimally invasive surgery (MIS) has been progressively accepted as a treatment option by ensuring the stability of the spine while limiting the risk of complications. [15][16][17] However, the surgical management of these highly unstable injuries remains to be clarified.
The aim of this study was to evaluate the clinical and radiological outcomes of percutaneous instrumentation with cement augmentation for traumatic hyperextension TL fractures in patients with AS at a single institution.

Study Design
This is a retrospective cohort study from a single institution. According to the study design and national and institutional guidelines, ethics committee approval was not required. At the time of hospitalization, all patients provided their written informed consent for surgery and data management for scientific purposes. This study complies with the World Medical Association Declaration of Helsinki.

Patient Population
Patients admitted at our tertiary academic referral center for traumatic TL fractures who underwent percutaneous instrumentation between December 2014 and December 2019 (5 years) were considered for eligibility. Inclusion criteria were a diagnosis of AS based on the modified New York criteria, 18 traumatic hyperextension TL fractures (B3 type) classified according to the AO Spine Thoracolumbar Spine Injury Classification System, and follow-up > 12 months. Exclusion criteria were incomplete clinical and radiological data, previous TL surgery, nontraumatic fractures, and < 12 months of follow-up.

Surgical Technique
The patient is positioned prone on a radiolucent table under general anesthesia. Anteroposterior and laterolateral views on radioscopic fluoroscopy are used for intraoperative guidance. The pedicles of the target vertebrae are first visualized, and the skin entry point is marked 1 cm lateral to the pedicle projection. Subsequently, Jamshidi needles, guide wires, and screws are inserted percutaneously using the standard technique. A convergent and straightforward pedicle screw trajectory is preferred. After placement of all screws, cement augmentation is performed in all patients. At the end of the procedure, contouring of the rods is performed, and these are inserted and locked in place.
Patients were treated with long-segment fixation (8 screws) using an instrumentation system involving two vertebrae above and two below the fractured vertebra. In addition, a posterior mini-open decompression was performed in 8 patients with neurological deficits. Screw sizes ranged in length and diameter from 30 mm to 55 mm and from 4.5 mm to 7.5 mm, respectively. Different percutaneous instrumentation systems were used over the years. Bracing was never prescribed after surgery in any case.

Clinical Outcomes
General and clinical conditions, as well as quality of life, were evaluated at admission (preoperative parameters), and postoperatively at 6-week, 1-year, and final follow-up visits, using a 10-point itemized visual analog scale (VAS) for back pain and the American Spinal Injury Association (ASIA) impairment scale for neurological examination. Patient-reported outcomes were evaluated at the 6-week, 1-year, and final follow-up visits to assess disability using the Oswestry Disability Index (ODI) score and to assess mobility using the new mobility score (NMS) as described by Parker and Palmer (a score of 0 indicates the impossibility of mobilizing independently, while a maximum score of 9 indicates a completely independent state of mobility). 19 Trauma types and mechanisms (low-or high-energy trauma) were recorded, and intra-and postoperative complications were collected.

Radiological Outcomes
Preoperative radiographs and CT scans were retrieved from the institutional PACS. Preoperative MRI was performed only in cases of neurological deficit. The following radiological parameters were evaluated on CT scans: the Cobb angle, as the angle between a line drawn parallel to the superior end plate of one vertebra above the fracture and a line drawn parallel to the inferior end plate of the vertebra one level below the fracture; 20 and the sagittal index (SI), as the measurement of segmental kyphosis at the level of a mobile segment (1 vertebra and 1 disc) adjusted for the baseline sagittal contour at that level. 21 Screw misplacement was evaluated on a postoperative CT scan and classified according to the grading system of Gertzbein and Robbins. 22 Bone formation on the CT scan was considered a criterion for bone union. The presence of screw breakage, screw pullout, peri-implant loosening, and rod breakage were considered as criteria for implant failure.

Statistical Analysis
Values are reported as mean ± standard deviation. The Student t-test was used to compare the quantitative continuous variables. The Fisher's exact test (2-sided) was used instead to compare the categorical variables. Statistical significance was predetermined at p < 0.05. SPSS Statistics version 14.0 (SPSS Inc.) was used for data analysis.

Patient and Operative Characteristics
A total of 22 patients with AS and traumatic hyperextension TL fractures (type B3) who underwent percutaneous instrumentation during the study period were included for data analysis. There were 7 (31.8%) females and 15 (68.2%) males. The mean age at the time of surgery was 74.2 ± 7.3 years (range 62-84 years). The mean follow-up was 39.2 ± 17.4 months (range 14-65 months). The most common comorbidity was cardiovascular disease (68.2%), followed by diabetes mellitus (45.4%), obesity (31.8%), and respiratory disease (27.3%). Five patients (22.7%) were smokers. The mean Charlson Comorbidity Index (CCI) score was 5.8 ± 1.7. Two patients (9.1%) were classified as American Society of Anesthesiologists (ASA) class II, 16 (72.7%) as class III, and 4 (18.2%) as class IV. All patients reported mechanical back pain unresponsive to medical treatment, and the ASIA grade was C in 2 patients (9.1%), D in 6 patients (27.3%), and E in 14 patients (63.6%).
The  Tables 1 and 2.

Clinical and Radiological Outcomes
Comparing the mean preoperative and last followup values, significative improvement was found in VAS scores (8.4 ± 1.1 to 2.8 ± 0.8, p < 0.001). The mean ODI score at 6 weeks (24.4 ± 6.1) improved to 18.7 ± 5.8 (p = 0.003) at the last follow-up, and the mean NMS (6.4 ± 1.4 to 7.5 ± 1.6, p = 0.02) showed good clinical outcomes with no disability and autonomous mobility during follow-up. The ASIA grade improved in 6 patients (75%) with neurological deficits and remained unchanged in 2 patients (25%) ( Table 3).
Bone union was observed in all patients. Three of the 176 (1.7%) implanted screws had a pedicle breach > 2 mm (≥ grade C); however, none of the screws were replaced. In 2 patients (9.1%), asymptomatic cement leakage was observed.

Complications and Reoperation Rate
No major complications were registered perioperatively or during follow-up. Two minor complications were observed; 2 patients (9.1%) had a superficial wound infection with complete resolution within 2 weeks after surgery.  One patient (4.5%) required reoperation for implant failure during follow-up; a 74-year-old female who underwent T10-L2 instrumentation for a T12 fracture and cement augmentation whose implant was not placed correctly because of leakage in T10. Due to recurrence of back pain, a lateral radiograph was obtained 2 months after surgery and showed a pullout of the right T10 screw and an extension of instrumentation to T9 where cement augmentation was performed.

Discussion
A consensus on the preferred management of TL fractures in patients with ankylosed spine disorders is still missing; this is due to high technical variability, surgeon personal experience, and institutional protocols. Although a few studies have reported on nonoperative management for treatment of these injuries, conservative treatment has led to poor outcomes with relevant spinal malalignment and instability resulting in intractable pain and neurological deficits.
Historically, hyperextension AS fractures have been treated using open posterior fusion techniques. However, newer instrumentation systems and surgical strategies, such as percutaneous instrumentation, have been providing similar clinical and radiological outcomes, eventually representing useful and effective alternatives. [15][16][17] Recent advances in MIS techniques and development of even newer instrumentation systems have been progressively revolutionizing spine surgery worldwide. [23][24][25][26][27] Furthermore, the frequency of the use of cement-augmented screws has recently increased, providing higher stability of the construct and reducing chances for implant failure. 28,29 Our results confirm data from the pertinent literature, in terms of good surgical, clinical, and radiological outcomes after percutaneous instrumentation for hyperextension TL fractures in the ankylosed spine. 16,[30][31][32][33] In the present series, surgery provided significative pain relief (with a mean VAS score from 8.4 ± 1.1 preoperatively to 2.8 ± 0.8 at the last follow-up, p < 0.001), along with lower disability grades and autonomous mobility. Furthermore, of the 8 patients with neurological deficits, ASIA grades improved in 6 (75%) and remained unchanged in 2 (25%).  Evaluation of radiological parameters demonstrated that the Cobb angle and SI were maintained over the follow-up with no loss of kyphosis, as a matter of proof of segmental stability. In our opinion, in neurologically intact patients, the goals of treatment are spine realignment and a long-term, stable construct providing bone union, while the role of segmental fusion should be critically evaluated. Additionally, the use of cement augmentation should be preferred, due to the high association of osteoporosis in patients with AS, since it decreases the risk of screw loosening and pullout. Moreover, in patients with neurological deficits, a mini-open decompression is associated with neurological improvement, decreased surgical duration, and less intraoperative bleeding.
In terms of complications, our data show that percutaneous instrumentation for AS fractures result in relatively low complication and reoperation rates. We registered 2 minor complications in 2 patients (9.1%) with superficial wound infections. The reoperation rate was 4.5% (1 patient had a screw pullout). Our results confirm data from the few pertinent studies in terms of the low complication rate associated with MIS techniques. 7,15,34 Surgery decreases the frequency of complications and reduces the mortality rate to 23% compared with 51% with nonoperative treatment. 7,32 Nevertheless, postoperative complications remain common. Westerveld et al. reported that more than 85% of patients with AS who underwent surgery experienced at least 1 pulmonary or infectious postoperative complication. 13 Moreover, in the literature, a 14% infection rate and a rate of screw loosening from 10% to 15% has been reported after open surgery. 7,11 Percutaneous surgery substantially decreases the risk of infection and cement augmentation reduces the risk of screw loosening.

Study Limitations
There are few limitations to be disclosed. The present investigation consists of a single-center, retrospective study, which must be considered for a proper data interpretation. Additionally, our study included a small cohort of patients without a control group, which may influence the relevance of our results, and several complications and implant failures could have been consequently unrecognized.

Conclusions
Percutaneous instrumentation showed good clinical and radiological outcomes, along with relatively low complication and reoperation rates, in the management of hyperextension TL fractures. Accordingly, MIS reduces morbidity and mortality rates and could be considered the preferred treatment for these injuries of the ankylosed spine.