Monosegmental anterior column reconstruction using an expandable vertebral body replacement device in combined posterior–anterior stabilization of thoracolumbar burst fractures

The purpose of this study was to assess the feasibility and outcome of thoracoscopic monosegmental ACR using an expandable VBRD in combined posterior–anterior stabilization of thoracolumbar burst fractures (T11–L2) in neurologically intact patients. Our study indicates that monosegmental ACR is feasible and represents a viable option for anterior spinal reconstruction in thoracolumbar burst fractures provided that the inferior endplate of the fractured vertebra is intact or features only a simple split fracture line. We furthermore found that clinical and radiological outcome of monosegmental ACR was comparable to that of bisegmental ACR, except for frequency of VBRD subsidence (5 vs. 0, P = 0.02) and bisegmental loss of correction ACR [5.2 ± 3.7° vs. 2.6 ± 2.5°, (P = 0.022)].

To the best of our knowledge, this is the first report systematically evaluating the feasibility and outcome of monosegmental ACR using a VBRD in combined posterior–anterior stabilization of thoracolumbar burst fractures. The optimal treatment of thoracolumbar injuries involving a burst fracture of the vertebral body is still controversial and under debate [18‐21]. If combined posterior–anterior stabilization using a VBRD for ACR is considered, monosegmental ACR may be a viable alternative to conventional bisegmental ACR in selected cases. In a substantial portion of burst fractures with significant vertebral body comminution, the caudal endplate is intact (incomplete burst fractures) or features only a single simple split fracture line (burst-split fractures) without significant splaying. In these cases, the VBRD may be sufficiently anchored in the intact caudal part of the fractured vertebra (i.e., monosegmental ACR) to spare one spinal motion segment in this generally young patient population and to reduce the distance for osseous bridging to achieve bony fusion. Preserving as many motion segments as possible by limiting the number of fused segments minimizes alteration of spinal biomechanics and the risk of early degeneration of adjacent segments, and therefore, constitutes a fundamental principle of spinal surgery. Following this principle, several studies [22‐26] reported favorable outcome after monosegmental posterior fixation of selected thoracolumbar fractures. However, studies on monosegmental anterior column reconstruction are scarce. Spiegl et al. [27] assessed the clinical and radiological outcome of 14 patients with incomplete burst fractures after a mean of 74 months (range 66–84 months) after thoracoscopic monosegmental ACR using a tricortical iliac crest bone autograft and an additional ventral plate. Nine patients were treated by an anterior only approach and five patients were treated by a combined posterior–anterior approach. Complete (≥ 80%), partial (> 30%), insufficient (≤ 30%) and no fusion at all (0%) was observed in 9, 4, 0 and 1 patient(s), respectively. Mean loss of monosegmental correction accounted for 5.7 ± 2.7°, and ten patients (71%) still reported persistent moderate or intense donor site pain at the iliac crest. Besides donor site morbidity, loss of graft volume and consequent loss of correction is an issue when using bone autografts for ACR. Morrison et al. [28] analyzed the CT scans of 15 patients treated by combined posterior–anterior stabilization and either monosegmental (9 patients) or bisegmental (six patients) ACR using a tricortical iliac crest bone autograft. After a mean of 12.5 months post operation, they observed a mean loss of initial graft volume and length of about 40 and 24%, respectively. The mean postoperative loss of reduction was 12° and was significantly correlated with both loss of graft volume and loss of graft length. The use of a VBRD for ACR in combined posterior–anterior stabilization allows to overcome the shortcomings of bone autografts for ACR, such as loss of bone graft volume, graft fractures and donor-site morbidity. Similarly, postoperative loss of reduction has been reported to be smaller when using a VBRD instead of a bone autograft for ACR in posterior–anterior stabilization [3, 29]. In addition, using a VBRD may be technically less demanding because the height of the expandable VBRD can be easily adjusted to the partial corporectomy defect in situ to restore height and correct sagittal deformity. Knop et al. [2] reported a series of 29 combined posterior–anterior stabilizations of thoracolumbar fractures (T7–L3) using a VBRD. In six patients, the VBRD was implanted monosegmentally. The mean monosegmental surgical correction and monosegmental loss of correction was 18.7° and 1.5°, respectively. Bisegmental kyphosis angles and clinical outcome, however, were not specifically given for monosegmental ACR.

The clinical and radiological outcome observed in the present study is within the range and consistent with that reported in previous studies on combined posterior–anterior stabilization using a VBRD [1‐3, 8, 29]. Furthermore, radiological outcome did not significantly differ between monosegmental and bisegmental ACR except for bisegmental loss of correction which was 2.6° greater after monosegmental ACR. On the one hand, however, this difference did not reach significance after exclusion of cases with VBRD subsidence. On the other hand, the clinical relevance of this difference is questionable, as minor differences in surgical correction or loss of correction are typically not reflected by worse clinical outcome [30, 31]. Moreover, about half of the loss of bisegmental correction did not occur at the fused but rather at the intact caudal motion segment (Table 3) and might be the result of an initial overdistraction of this spared segment via posterior reduction and fixation. Similarly, clinical outcome did not significantly differ between the two groups. Patients with monosegmental ACR, however, tended to report a poorer VAS Spine Score and Oswestry Disability Index at final follow-up than those with bisegmental ACR, whereas Roland and Morris Disability Questionnaire and WHOQOL-BREF scores were similar. Although we do not have a definitive explanation for this finding at the moment, the discrepancy between different clinical outcome scores possibly reflects the well-known lack of an appropriately responsive, spinal trauma-specific outcome measurement instrument [32]. Nevertheless, the observed mean loss, for example, in VAS Spine Score after monosegmental ACR was similar in size to that reported by others for bisegmental (or predominantly bisegmental) ACR (16.8 points [3] and 19.7 points [2]).

VBRD subsidence may be a concern in monosegmental ACR since the VBRD is anchored in the caudal part of the fractured vertebra. In fact, five cases of VBRD subsidence were observed in this series after monosegmental but none after bisegmental ACR (P = 0.02). In retrospect, the choice of monosegmental ACR was poor in three cases probably because the involvement of the inferior endplate was underestimated. A meticulous analysis of preoperative CT images revealed that the inferior endplate involvement did not only consist of a simple split fracture but rather of multiple fracture lines. In this scenario, the VBRD is positioned onto an inferior endplate whose integrity is severely compromised and axial loading of the VBRD is likely to result in subsidence into the adjacent intervertebral disc (Figs. 2, 3). In the two other cases of VBRD subsidence, the VBRD was placed too far cranially to the inferior endplate of the fractured vertebra and anchored into the weak cancellous vertebral bone (Fig. 4). Consequently, axial loading in an upright position lead to subsidence into the cancellous bone until the inferior endplate was reached. These five cases clearly illustrate the pitfalls of monosegmental ACR: positioning the VBRD (1) onto the weak cancellous bone (i.e., not close enough to the inferior endplate) and (2) onto a sort of “free floating” inferior endplate fragment created by the presence of multiple fracture lines. Both pitfalls can be easily avoided by proper VBRD placement as well as meticulous analysis of preoperative CT images for even subtle fracture lines at the inferior endplate. Interestingly, the VBRD subsidence did not necessarily result in substantially greater loss of correction or worse clinical outcome compared to the mean values of monosegmental ACR without VBRD subsidence (Table 4). Of note, VBRD subsidence was not observed when the VBRD was anchored onto an inferior endplate featuring a single simple split fracture line without significant splaying. In this situation, the VBRD can bridge the single fracture line and can be sufficiently anchored in the adjacent intact parts of the inferior endplate; one single split fracture line, therefore, did not seem to increase the risk for VBRD subsidence and placement thus appears to be safe and reliable.

Posterior instrumentation removal after ACR is another issue that may be debated. In general, the need and potential benefits of implant removal after thoracolumbar fracture stabilization remains controversial and substantially varies between different countries [33‐37]. Posterior instrumentation removal after monosegmental ACR, however, is essential to restore mobility in the nonfused segment and to provide the patients with the potentially beneficial effects of sparing one motion segment. In contrast, implant removal after bisegmental ACR does not restore segmental mobility, its benefits are questionable and available evidence is insufficient. Usually it is, therefore, considered only optional, but may be indicated in case of foreign body sensation and persisting discomfort and pain at the posterior spine attributed to local irritation of the muscles by prominent pedicle screw heads. Although we thus routinely recommended implant removal to our patients only after monosegmental but not after bisegmental ACR, half of the patients in the monosegmental group refused removal while about one-third of patients in the bisegmental group strongly wished and demanded implant removal. Anyhow, the potential additional risks of implant removal procedures have to be taken into account already in the initial decision making.

The strengths of this study include the high follow-up rate of 94% (17 of 18 patients who underwent monosegmental ACR), the relatively long mean clinical follow-up period of more than five years and the strict inclusion criteria such as traumatic burst fractures at the thoracolumbar junction (T11–L2) and absence of a neurological deficit. Furthermore, it is the first study to systematically evaluate the feasibility and outcome of monosegmental ACR using a VBRD in combined posterior–anterior stabilization of thoracolumbar burst fractures. Finally, the outcome of monosegmental ACR was related to that of bisegmental ACR using the same thoracoscopic technique, VBRD and aftercare protocol.

Limitations of our study include the relatively small sample size and the retrospective study design. Moreover, fusion was not assessed due to the lack of follow-up CT imaging, and posterior instrumentation was only removed in half of the patients after monosegmental ACR to regain mobility of the spared segment [33]. Finally, when relating the clinical and radiological outcome of monosegmental ACR to that of bisegmental ACR, we were aware that the average degree of vertebral body injury had been greater in the bisegmental than in the monosegmental ACR group. To the best of our knowledge, there are no studies specifically investigating whether the subtype/subgroup of vertebral body burst fracture (subtype A3 vs. A4 according to the AOSpine Thoracolumbar Spine Injury Classification System and subgroup A3.1 vs. A3.2 vs. A3.3 according to the Magerl classification system) affects radiological and clinical outcome after combined posterior–anterior fusion of thoracolumbar burst fractures. Reinhold et al. [3], however, reported that neither the Magerl type of injury (type A vs. B vs. C) nor the group (A1/A2 vs. A3) did significantly affect the bisegmental kyphosis angle (BKA) at final follow-up of 536 operatively treated thoracolumbar fractures (238 combined posterior–anterior, 272 posterior only and 26 anterior only stabilizations). In contrast, preoperative BKA, patient age and level of fracture (thoracic vs. thoracolumbar junction vs. lumbar) were found to significantly affect BKA at final follow-up. As these parameters did not significantly differ between the monosegmental and bisegmental ACR group in our study, relating the outcome of one group to the other was deemed reasonable and valuable to evaluate the short- and mid-term outcome of monosegmental ACR. Nevertheless, future prospective randomized studies with follow-up periods of at least 10 years are needed to clarify whether sparing one spinal segment by monosegmental ACR provides a potential long-term clinical benefit over bisegmental ACR.