Radiological and mid- to long-term patient-reported outcome after stabilization of traumatic thoraco-lumbar spinal fractures using an expandable vertebral body replacement implant

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 posterior- or anterior-only approach [3‐5]. Additional to the posterior stabilization with an internal fixator, different methods for the restoration of the anterior column have been established, including the use of bone grafts and metal-mesh cages. Not least because tricortical bone grafts are associated with donor site morbidity [6, 7], vertebral body replacement implants (VBR), that can be implanted through a minimal-invasive surgical approach have been developed over the last decades, providing an individual anatomical adaptation [8]. Expandable implants have been proven to be an alternative to conventional cages and bone blocks [9‐12]. Although their effectiveness in providing primary stability has been shown, data on restoring the bi-segmental kyphotic endplate angle (BKA) is still sparse. Noteworthy, the loss of ventral support and cage subsidence over time has been reported [13, 14]. It must be considered that a potential resulting kyphotic malalignment can impair spinal function and may even result in secondary neurological symptoms and disorders [15, 16]. Since several years the role of patient-reported outcome measure (PROM) has been emerging in trauma surgery, but the mid- to long-term outcomes after VBR implantation remain to be described more closely.

This study aimed to analyze the radiological and mid- to long-term PROM results after dorso-ventral stabilization of traumatic thoracic and lumbar spinal fractures using an expandable VBR (Obelisc™, Ulrich Medical, Ulm, Germany) for the reconstruction of the anterior column in a cohort of 96 patients.

After the initial screening of the data base, n = 96 (25 female, 71 male) patients met the inclusion criteria for radiological evaluation (Fig. 2). The mean age was 46.1 ± 12.8 (19–69) years. The mean time to the first follow-up was 21.7 ± 19.1 (6–69) months and 49.8 ± 46.0 (24–213) to the second radiological follow-up.

Fifty-one patients (11 female, 40 male) with complete radiological follow-up completed the PROM questionnaires and therefore were defined as “respondent group”. All patients without a complete PROM follow-up were defined as non-responders, resulting in a loss to (PROM-) follow-up of 46.9%. The mean age of the patients in the respondent group was 48.9 ± 12.4 years, the mean BMI was 25.6 ± 4.1 kg/m². The mean age of the patients that were lost to follow-up was 43.3 ± 12.5 years and the mean BMI was 24.5 ± 3.6 kg/m². Patients lost to follow-up were significantly younger than patients in the respondent group (p < 0.05), but there was no difference in the BMI of the compared groups. There was no statistically significant difference in the fracture localization, the morphology of the vertebral body fracture or the presence of an additional C-classified injury between the respondent and non-respondent group (all p > 0.05) In the respondent group the first radiological follow-up was conducted 11.1 ± 5.1 (6-25) months after index surgery, the second after 42.1 ± 38.8 (24–181) months. The mean time between index surgery and completion of the PROM questionnaire was 106.4 ± 44.3 (26–179) months.

The use of an expandable vertebral body replacement implant showed to be a feasible procedure with only few implant-associated adverse events of the dorsal instrumentation and a revision rate of 4.2% in the current study population. A high bony fusion of 97.9% was seen, in concordance with the existing literature [10, 26]. All 96 patients underwent surgery with insertion of a single type of expandable vertebral body replacement implant and VBR implant dislocation was seen in no patient. Loosening or misplacement of pedicle screw was observed in only two patients. Unlike other studies, that included patients with heterogenous indications and examined different implants, the current homogenous cohort contains only trauma patients with a minimum radiological follow-up of 2 years and a mean PROM follow-up of 106 months.

The presence of additional injuries and the trauma severity (ISS ≥ 16) must be discussed as a possible confounder of the PROM result. Many patients (76.5%) in the respondent group suffered from additional injuries of one or more body regions and in three cases complete neurological deficit (ASIA A) persisted postoperatively. Only nine (17.6%) patients in the respondent group were classified as polytraumatized patients with an ISS ≥ 16. Patients with postoperative complete neurological deficits or an ISS ≥ 16 showed significant worse outcomes in the EQ-5D index and a trend to worse outcomes in the SF-35 and EQ-5D scores. 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 [15].

The results of this study showed, that health-related QoL in terms of the SF-36 could not return to the values of an age-matched reference population, regardless the trauma severity [24]. Even if patients without residual neurological deficits (ASIA E) were analyzed separately, the level of a healthy German reference group could not be reached in terms of all eight main SF-36 items. Neither severely injured patients (ISS ≥ 16) nor less severe injured patients (ISS < 16) regained normative population values (PCS 33.4 and 38.8 vs. 52.4; MCS 30.8 and 38.9 vs. 48.8). Smits et al. conducted a study on the anterior stabilization of thoraco-lumbar fractures and compared SF-36 scores at least 1 year after surgery with a cohort that underwent posterior fixation only in the same hospital and with values of the normative healthy population. In line with our results, they found that health-related QoL in their cohort did not return to that of the normative population, but was comparable to that of patients with less severe fractures [27]. They reported higher SF-36 component scores as well for severely injured patients (PCS = 40.0; MCS = 49.0) as for less severely injured patients (PCS = 47.0; MCS = 46.0) compared to our mid- to long-term health-related QoL results [27]. However, regardless the high number of additional injuries and neurological deficits, the overall functional outcomes in our study were found to be satisfactory in terms of the EQ-5D and ODI scores. The EQ-5D subdimensions show 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. Consistent with our findings of an average EQ-5D index of 0.7, Deml et al. reported the same mean EQ-5D index of 0.7 at a 1.7-year follow-up after anterior column reconstruction of the thoraco-lumbar spine with a new modular PEEK vertebral body replacement device in 48 cases [28]. The mean ODI score reported in the current cohort was 28.4%, indicating moderate impairment (defined as 21.0–40.0%). Similar to this finding, Kumar et al. reported a mean ODI score of 14 points (=28.0%), 18 and 30 months after open surgery procedure for thoraco-lumbar burst fractures [29]. There was no evidence of neurological compromise in their collective and no additional injuries were reported, therefore representing a different collective with less severe injured patients. Schnake et al. reported an average ODI score of 12 points (=24.0%) at a five-year follow-up after posterior-anterior stabilization of traumatic thoraco-lumbar fractures using expandable titanium cages, which is also consistent with our results, nearly 9 years after surgery in average [10]. In line with our findings, Kreinest et al. reported most patients to be satisfied, 3 years after implantation of a hydraulic expandable VBR following traumatic thoracic and lumbar vertebral fractures [11]. Different to our methods they chose the VAS Spine score as an outcome parameter and found a mean rating of 65.2 ± 23.1 in their study population, 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]. Comparable with our findings, they observed a significant reduction of functional outcome scores in their study cohort when compared to healthy subjects, but most of the patients (85.1%) were “generally/very satisfied” with their outcome [11]. Other study-groups like Spiegl et al. achieved better VAS Scores with a thoracoscopic procedure [30], but in the current study we could not identify a significant influence of the surgical approach or other surgery-related parameters on the health-related QoL.

In our respondent group all patients were treated with a combined anterior and posterior strategy and in only a few cases (15.7%) the dorsal metalwork was removed after 13 months at average. No statistical difference in all evaluated PROM parameters was seen depending on removal of the metalwork. 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, but combined surgery is usually recommend for the treatment of instable thoraco-lumbar burst fractures due to an instable anterior column [4, 30, 31]. In a prospective cohort study, examining the six-year outcome of thoracoscopic ventral spondylodesis after unstable incomplete cranial burst fractures of the thoraco-lumbar transition Spiegl et al. compared ventral only and dorso-ventral treatment strategies and found a higher operative correction in dorso-ventrally treated patients [30]. They reported that a high percentage of the operative correction was lost after 6 years. Contrary, we did not see a clinically relevant loss of operative correction after a mean radiological follow-up of 42 months. Several authors claim an association between kyphotic malalignment and functional outcome [32, 33]. In contrast, other studies did not find a significant correlation between radiological and functional outcomes [4, 34]. In the current study n = 11 (21.6%) patients with a potential clinically significant loss of correction, which we defined as ±3.0° in the second follow-up were seen. Those patients did not report on worse health-related QoL results compare to patients with a stable correction. Noteworthy, the QoL outcome was assessed 106.4 ± 44.3 months after surgery, whereas the last radiological follow-up (t2) of the respondent group was conducted 42.1 ± 38.8 months after surgery. Therefore, the current results do not allow to evaluate a correlation between radiological and PROM results unconditionally.

The bony fusion rate in our cohort was 97.9% and in only two patients an a-symptomatic non-union was detected. No VBR implant had to be revised. The overall revision rate was 4.2%.

We showed a reduction of the BKA in n = 96 patients of 10.3° at the thoracic and of 10.7° at the lumbar spine (Fig. 3A & B) and similar values were found for the respondent group. At the thoraco-lumbar transition the BKA was significantly corrected by 9.6° in the respondent group. This is above 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 [35]. For the reconstruction of the ventral column different methods were performed and an intraoperative correction of the BKA by 11.5° could be achieved. 5.8° loss of reduction was seen after 6 weeks. At the one-year follow-up a loss of reduction of 4.9° after dorsoventral stabilization was reported [35]. The current study reveals a loss of reduction of 2.7° at the lumbar spine (Fig. 3B) after approximately 3.5 years, which we estimate to be clinically irrelevant. At the thoracic spine, a statistically significant but clinical irrelevant loss of correction of 1.7° at the second follow-up was seen. In the respondent group a significant loss of reduction was seen during the follow-up, at the thoracic and the lumbar spine and at the thoraco-lumbar junction, which again was not estimated to be clinically relevant. It is worth noting, that the comparison between the pre- and postoperative angle includes a possible influence of patients positioning, as the pre- and postoperative CT scan was assessed in a supine position and follow-up X-rays were taken with the patient standing up-right [36]. This should be kept in mind not to overestimate the loss of reduction between the immediate postoperative BKA in supine position and the follow-up in up-right position.

Schnake et al. reported an average postoperative loss of correction of 2.4° due to minimal subsidence of the cages in a five-year clinical and radiological follow-up of 54 patients that had received combined anteroposterior stabilization of thoraco-lumbar fractures which is similar to the findings in our cohort [10]. Cage subsidence has been reported and was interpreted as a “settling down” of the anteroposterior construction, that appeared mainly in the first postoperative year [10]. Although not examined in detail, cage subsidence may have also played a role in the loss of reduction in our study. In 2020 Deml et al., who retrospectively reviewed 48 patients that underwent a corpectomy at heights between T5 and L5 due to trauma or tumor and were stabilized with the new PEEK vertebral body replacement showed, that the BKA was corrected by 12.1° at average [28]. At the end of 1.7 years follow-up, they observed a mean loss of correction of 1.6° and the fusion rate was 92.1% [28], which resembles our results.

Summarized, similar results regarding the current findings for intraoperative reduction (10.5°, all fractures), loss of reduction (2.4°, all fractures) and fusion rate (97.9%) can be found in the literature. Health related QoL oucomes in the current cohort approximately 9 years after VBR surgery regarding EQ-5D index (0.7) and ODI scoring (28.0%) were comparable to literature reported short- to mid-term results. Mid- to long-term SF-36 scores did not return to normative population values and are below literature reported short-to mid-term outcomes. Patients with persistent neurological impairment, additional trauma and an ISS ≥ 16 showed significantly lower EQ-5D indices but other PROM parameters were not significantly influenced.

The reconstruction and stabilization of traumatic unstable thoraco-lumbar spinal fractures with an expandable VBR implant has been shown to be feasible in the current study population. Further prospective studies will have to be conducted to prove the safety and efficiency of this procedure. A significant operative correction of the BKA was provided at both the thoracic and lumbar spine. No clinically relevant loss of correction was observed during the follow-up and a high bony consolidation rate of 97.9% was achieved. No revision surgery due to VBR dislocation was indicated. Satisfactory PROM results were found in a large part of the respondent group. However, quality of life did not reach the normative population values, regardless of trauma severity. Postoperative persistent neurological symptoms, additional trauma, and an ISS ≥ 16 were factors associated with a trend towards worse QoL outcomes.