Radiological and Long-Term Patient-Reported Outcome after Stabilization of Traumatic Thoraco- Lumbar Spinal Fractures using an Expandable Vertebral Body Replacement Implant

Siegmund Lang University Hospital Regensburg: Universitatsklinikum Regensburg Carsten Neumann University Hospital Regensburg: Universitatsklinikum Regensburg Christina Schwaiger University Hospital Regensburg: Universitatsklinikum Regensburg Andreas Voss University Hospital Regensburg: Universitatsklinikum Regensburg Volker Alt University Hospital Regensburg: Universitatsklinikum Regensburg Markus Loibl University Hospital Regensburg: Universitatsklinikum Regensburg Maximilian Kerschbaum (  maximilian.kerschbaum@ukr.de ) Universitatsklinikum Regensburg


Introduction
Spinal column injuries make up a relevant share in trauma patients. The thoracic and lumbar spine is frequently affected (1,2). A combined dorso-ventral stabilization has been shown to be a feasible procedure that can provide more stability for unstable traumatic vertebral fractures than a single posterior or anterior approach (3)(4)(5). Next to a posterior stabilization with an internal xator, different methods for restoration of the anterior column have been established. Not least because tricortical bone grafts are associated with harvest site donor morbidity (11,12), vertebral body replacement implants (VBR) have been developed over the last decades (13). Expandable implants have been proven to be an alternative to conventional cages and bone blocks (6)(7)(8)(9). Although their effectiveness in providing primary stability has been shown, data on restoring the bi-segmental kyphotic endplate angle (BKA) is sparse. Nevertheless, the loss of ventral support and cage subsidence over time has been reported (10,11). The resulting kyphotic malalignment can impair spinal function and may even result in secondary neurological complaints and disorders (12,13).
This study aimed to analyze the radiological and long-term patient-reported outcome measure (PROM) after dorso-ventral treatment of traumatic thoracic and lumbar spinal fractures using an expendable VBR (Obelisc™, Ulrich Medical, Ulm, Germany).

Inclusion and exclusion criteria
Patients with traumatic thoraco-lumbar spinal fractures treated with a VBR implant in our trauma department (level 1 trauma center) between 01/2001 and 01/2015 were identi ed for this study. Only patients with two radiological follow-ups were included, the rst conducted at least 6 months after initial surgery. Patients younger than 18 years, pathological fractures and osteoporosis were excluded. Besides patient related data (sex, age, BMI), injury mechanism, additional injuries, fracture type as well as treatment details (surgical regime; adverse events) and PROMs were collected prospectively.

Ethics statement
This study was carried out in accordance with the Declaration of Helsinki and approved by the ethics committee at the University of Regensburg in 03/2017 (Institutional Review Board Number 2017-0781-10). Written informed consent was obtained from all individual participants included in the study.

Fracture Classi cation and Surgical procedure
Fractures were classi ed based on plain X-rays and computer tomographic (CT) scans according the AOSpine thoracolumbar injury classi cation system (14). All patients were treated by one senior spine surgeon (CN) with a single type of expandable titanium cage for vertebral body replacement (Obelisc™, Ulrich Medical, Ulm, Germany) using thoracoscopic-assisted surgery (15), a ventral/retroperitoneal approach or a dorsal approach. During discectomy of the cranial and caudal intervertebral disc the adjacent endplates were kept intact. Prior to the vertebral body replacement, dorsal instrumentation, anatomic reduction, and stabilization with a minimal invasive Schanz Screw xateur interne were performed in prone position. The ventral stabilization was either achieved in the same session, or as part of a sequential two-step procedure.

Radiological assessment
All patients received pre-and postoperative CT scans and plain X-rays for surgery planning and veri cation of the correct implant position. Postoperative X-rays mark the time point t0. Additionally, plain X-rays were made at least 6 months (t1) and 3 years (t2) after index surgery. Plain, medio-lateral X-rays were used to measure the bi-segmental kyphotic endplate angle (BKA) (Fig. 1) (16). Negative values indicate kyphosis and positive values lordosis. All measurements were performed digitally using the software package OsiriX MD (Pixmeo, Bernex, Switzerland).

Quality of Life (QoL)-instruments
The German Short-Form 36 (SF-36) (17), Oswestry Disability Index (ODI) (18,19) and EuroQol in 5 Dimensions (EQ-5D) were collected prospectively. Patients were contacted by telephone and asked to participate in the study. If patients were not reachable by phone, forms were sent to the last known address. As a reference, normative data from Germany was used on the SF-36 (20). and EQ-5D (21).

Statistics
Statistical analysis was carried out using SPSS software version 24 (SPSS Inc., Chicago, Illinois). The Mann-Whitney U and Kruskal-Wallis tests were performed to compare categorical variables; the independent t-test was used to compare continuous variables after determining the distribution was appropriate for parametric testing. P-values < 0.05 were considered signi cant. Data are presented as mean ± standard deviation (SD) for continuous variables and as absolute and relative frequencies for categorical data.

Results
After the initial screening of the data base, n = 117 (31 females, 86 males) patients met the exclusion and inclusion criteria for radiological evaluation (Fig. 2). The mean age was 52.7 ± 16.5  years. The mean time to the rst radiological follow-up was 26.7 ± 20.3 (6-73) months and 61.0 ± 50.7  to the second radiological follow-up. N = 64 patients (15 females, 49 males) with complete radiological follow-up completed the PROM questionnaires and therefore were included in the study group (loss to follow-up 45.3%). The mean age of the study patients was 53.2 ± 14.8 years, the mean BMI was 25.9 ± 3.9 kg/m². In the study group the rst radiological follow-up was conducted 13.6 ± 3.0 (6-25) months after index surgery, the second after 38.5 ± 42.9 (21-181) months. The mean time between index surgery and completion of the PROM questionnaire was 109.4 ± 44.6 (26-181) months.
In the study group n = 26 (40.6%) patients suffered a tra c accident, n = 10 (15.6%) had a fall from more than 3 meters and n = 15 (23.4%) from less than 3 meters. In n = 13 (20.3%) cases a not classi ed trauma was reported. In the study group n = 47 patients (73.4%) had additional injuries including additional spine injuries (Table 1). Radiological Outcome
The mean ODI of the total cohort was 28.4 ± 17.6. N = 20 (31.3%) patients reported minimal, n = 28 (43.8%) moderate and n = 15 (23.4%) severe disability. One patient (1.6%) reported crippling back pain and no patient (0%) suffered exacerbated back pain or was bed-bound. There was no signi cant difference in the distribution of the scorings between thoracic and lumbar fractures (p = 0.82) (Fig. 5). Table 3 shows the results of the 8 main SF-36 items of the study group in comparison to a healthy reference population (20). The cohort showed signi cant lower values compared to the healthy population for each item (each p < 0.05). Comparing thoracic and lumbar location of the treated fracture, no signi cant difference between the single items was found (p > 0.05).  Table 3. The eight main SF-36 items in comparison to the normative data of the German population (20).
The reference values are presented as mean with 95% con dent interval. There remains statistically signi cant difference between the study group and a healthy German population in all main items.
To assess the in uence of loss of reduction on the PROM we de ned relevant loss of reduction as a change of the BKA between the postoperative X-ray and the second follow-up by ± 3°. In N = 12 (18.8%) cases a relevant loss of reduction of 8.3 ± 4.0° at mean was found. In all other cases the mean change in BKA was 0.9 ± 1.1°.
Between the two groups with and without a relevant loss of reduction no statistically signi cant differences in all SF-36 items, ODI and EQ-5D parameters were detected (all p > 0.05).

Discussion
The use of an expandable vertebral body replacement implant proves to be a safe procedure with only few implant-associated adverse events and a revision rate of 5.1% in the current study population. Bony consolidation was seen in 95.7% of patients, in concordance with the existing literature (7,22). All patients underwent surgery with insertion of a single type of expandable vertebral body replacement implant and implant dislocation, or loosening was observed only in n = 5 patients. Unlike other studies, that included heterogenous indications and used different implants, the current homogenous cohort contains only trauma patients with a 9-years PROM follow-up at average.
The presence of additional injuries must be discussed as a possible confounder of the PROM. Many patients (73.4%) in the study group suffered from additional injuries of one or more body regions. In this study no correlation between additional injuries and PROM was detected. Patients with initial motor de cits showed signi cant worse outcomes in the SF-36 dimension "physical function" and in the EQ-5D index. Similarly, McLain et al. documented in a 5-year follow-up after spinal injuries of 62 severely injured patients treated with segmental spinal instrumentation, that a neurological injury had a greater impact on functional outcome than other variables (12).
The results of this study show a lower patient-related life quality especially in the SF-36 compared to an age-matched reference population (20). However, regardless of the high number of additional injuries and neurological impairments the overall functional outcomes were satisfactory. The EQ-5D subdimensions reveal the distribution of cases with only a few "poor" results and mostly "moderate" or even "excellent" results. Regarding the ODI outcome only one patient reported crippling back pain and no patient suffered exacerbated back pain or was bed-bound. The mean ODI of the total cohort was 28.4 points indicating mild impairment. Contrary to this nding, Kumar et al. reported a mean ODI score of 14 points 18 months after an open surgery procedure for thoracolumbar burst fractures (23). However, in their collective there was no evidence of neurological compromise and no additional injuries, therefore representing a different collective with less severe injured patients. In line with our ndings, Kreinest et al. found most patients to be satis ed, 3 years after implantation of a hydraulic expandable VBR following traumatic thoracic and lumbar vertebral fractures (8). The mean rating of the VAS Spine score of their study population was 65.2 ± 23.1, which was found to be comparable to the rating of 58.4 from the German Spine Fracture Registry for operatively treated patients after spine fractures (4). Like our ndings, they saw a signi cant reduction of functional outcome scores in their study cohort when compared to healthy subjects (8). Indeed, other study-groups like Spiegl et al. achieved better VAS Scores with a thoracoscopic procedure (24), but in the current study we could not identify a signi cant in uence of the surgical approach or other surgeryrelated parameters on the PROM.
In our study group all patients were treated with a combined anterior and posterior strategy and in only a few cases (12.5%) the dorsal instrumentation was removed during the follow-up period. The data found in literature does not suggest any difference in PROM depending on ventral or dorsal or combined surgery strategies of traumatic fractures of the thoracic and lumbar spinal column (4,25). In a prospective cohort study, examining the six-year outcome of thoracoscopic ventral spondylodesis after unstable incomplete cranial burst fractures of the thoracolumbar junction Spiegl et al. compared ventral only and dorsoventral treatment strategies and found a higher operative correction in dorso-ventrally treated patients (24). They also reported that a high percentage of the operative correction was lost after six years, which is in line with our ndings. Several authors claim an association between kyphotic malalignment and functional outcome (26,27). In contrast, other studies did not nd a signi cant correlation between radiological and functional outcomes (4, 28).
We showed a reduction of the BKA in 117 patients of 7.3° at the thoracic and of 6.7° at the lumbar spine and similar values were found in the study group. This is in the range of the reduction potential of posttraumatic kyphotic deformity, reported by the Spine Study Group of the German Trauma Association of 5.7 and 5.1° for the bi-segmental wedge angle using various implants with an open approach in the thoracic and lumbar spine, respectively (4). In 2015 our study group conducted a clinical and radiological one-year follow-up assessment of 26 patient with fractures of the thoracic and lumbar spine treated with bi-segmental dorsal instrumentation with a minimal-invasive transpedicular Schanz Screw system (29). For additional ventral reconstruction different methods were performed and an intraoperative reduction of the BKA by 11.5° among all patients could be achieved. 5.8° loss of reduction were seen after six weeks.
At the one-year follow-up a loss of reduction of 4.9° after dorsoventral stabilization was reported. Similarly, the current study reveals a loss of reduction of 6.0° at the lumbar spine after approximately 3 years. At the thoracic spine, no signi cant loss of reduction was found in 117 patients. In the study group of 64 patients a signi cant loss of reduction was seen during the follow-up, at the thoracic and the lumbar spine and at fractures of the thoraco-lumbar junction. However, to difference in the PROMs of patients with a relevant loss of reduction of minimum 3.0° and patients without a relevant loss of reduction was observed. It is worth noting, that the comparison between the pre-and postoperative angel includes a possible in uence of patients positioning, as the preoperative CT scan was assessed in a supine position and postoperative and follow-up X-rays were taken with the patient standing up-right (30).
Schnake et al. reported an average postoperative loss of correction of 2.4° due to minimal subsidence of the cages in a ve-year clinical and radiological follow-up of 54 patients that had received combined anteroposterior stabilization of thoracolumbar fractures (7). The cage subsidence has been reported and was interpreted as a "settling down" of the anteroposterior construction, that appeared mainly in the rst postoperative year (7). Although not examined in detail, cage subsidence may have also played a role in the loss of reduction in our study.

Conclusion
Stabilization of traumatic thoraco-lumbar spinal fractures using an expandable VBR implant is an effective and safe procedure. A signi cant operative correction of the BKA could be achieved initially at both the thoracic and lumbar spine. Although the initial reconstruction of the BKA did not consist over time in the study group, satisfactory PROMs were found. Nevertheless, the level of the healthy reference population could not be reached.

Consent for publication
This manuscript does not contain any individual person's data in any form.

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

Competing interests
Financial and personal relationships: The Author Carsten Neumann is a consultant for Ulrich medical Germany.
Potential competing interests: None.

Funding
No funding has been received for this study.
Authors' contributions SL helped to design the study, conduct the study, analyze the data, and write the manuscript.
SL has seen the original study data, reviewed the analysis of the data, approved the nal manuscript, and is the author responsible for archiving the study les.
CN helped to conduct the study.
CN has seen the original study data, reviewed the analysis of the data, and approved the nal manuscript.
CS helped to conduct the study.
CS has seen the original study data, reviewed the analysis of the data, and approved the nal manuscript.
AV helped to conduct the study.
AV has seen the original study data, reviewed the analysis of the data, and approved the nal manuscript.
VA helped to conduct the study.
VA has seen the original study data, reviewed the analysis of the data, and approved the nal manuscript.
ML helped to conduct the study.
ML has seen the original study data, reviewed the analysis of the data, and approved the nal manuscript.
MK helped to design the study, analyze the data, and write the manuscript. He is the corresponding author of this manuscript.
MK has seen the original study data, reviewed the analysis of the data, and approved the nal manuscript.