Although conservative management is thought to be the optimal treatment for TL junction fracture without severe neurological impairment, it is often accompanied by discomfort and limited mobility. Surgical intervention is therefore preferred in patients with TL junction fracture, because it can maintain reduction, prevent further deformity and neurologic deterioration, and improve mobilization. Especially for young patients, surgical intervention may have advantageous effects for the recovery of spine sagittal alignment in the long run. The selection of the surgical approach in the management of TL junction fracture is dependent on many variables, such as bone intensity, kyphotic deformity, spinal canal encroachment etc. Either the isolated anterior/posterior approach or the combined approach can be applied for the stabilization of unstable spine. Studies have shown that the anterior instrumentation with bone graft can provide reliable internal fixation, but it is a more invasive approach that is associated with complications and prolonged postoperative recovery [8, 25]. Alternative intervention is considered prior to the anterior approach. LSIF via the posterior approach can also improve and maintain optimal stability of the spinal column, however, it might decrease spinal range of motion and increase the incidence of ASD. Therefore, other improved alternatives have been lately developed to minimize its adverse effects.
Superior biomechanical stability is found in SSIF with addition of pedicle screws at the fracture level without sacrificing benefits of SSIF. Studies have shown that SSIF with intermediate screws could significantly improve the biomechanical stability and construct stiffness as compared with SSIF [11, 26]. Moreover, clinical research has found that the restoration of the fractured vertebral height obtained in SSIF with intermediate screws was equivalent to that in LSIF [18]. Secondly, intermediate screws at the fracture level can optimize load on the instrumentation system and reduce the risk of broken screws or rods. Post-buckling of rod is more evident within the four-screw fixation construct than that within the six-screw fixation construct [27]. This is due to that the rod of the four-screw fixation construct spans a longer distance between two screws as compared with the six-screw fixation construct, tension strain at each level of the four-screw fixation construct is significantly increased compared with that at each level of the six-screw fixation construct [28, 29]. Although traditional SSIF with intermediate screws theoretically corrects kyphotic deformity, however, this instrumentation system is not able to provide adequate support to anterior column of the fractured vertebra for unstable TL fracture in practice.
We then developed a modified SSIF with inclined-angle intermediate polyaxial screws. There are some advantages as following: Firstly, this inclined-angle insertion can increase the length of pedicle screws, so it can increase the pullout force and provide greater construct stiffness. Denis type B fracture is a special categorized fracture, in which the superior endplate is mainly involved, while the inferior endplate and the lower portion of the injured vertebral body usually escapes from the injury site [30]. Therefore, the residual vertebral body and the caudal disc are preserved and they are able to tolerate anterior column reconstruction. The pedicle screws in the SSIF-IAP group were inserted into the lower residual portion of the injured vertebral body, which would contribute to the pullout strength. In addition, the “eggshell” deformity often occurred postoperatively and the fractured vertebra can’t provide enough construct stiffness during the healing process of fracture [31]. The potential reason for the “eggshell” effect is that the vertebral height is fully restored by the internal fixation device, but the compressed bone trabeculae are not restorable, which results in a defect in the injured vertebral body [32].To prevent this, several techniques have been developed to augment the anterior column in the unstable fractures, such as polymethylmethacrylate (PMMA) injection, however, injection of PMMA into the injured vertebral body might lead to cement extrusion, particularly when the posterior longitudinal ligament is torn [33]. Intermediate screws in the SSIF-SFM group are paralleled with the superior endplate, and the end portion of screws in the eggshell-like cavity can’t provide additional interface strength. Nevertheless, intermediate screws in the SSIF-IAP group can escape from this cavity, and contribute to the interface strength. It might minimize negative effects caused by the “eggshell” deformity, and promote fracture healing by increasing structural stability. However, no data are available to support this assumption that needed to be verified by further biomechanical study. During the follow-up period of over 2 years, none of patients in the SSIF-IAP group exhibited loosening or shifting of the intermediate screws at the fracture level. The main reason for this difference might be due to that screw-to-bone interface strength was improved by the increased angulation of screws, and the anterior and middle spinal columns were immediately strengthened by these inclined-angle polyaxial screws. It suggests that inclined-angle polyaxial screws at the fracture level can protect the fractured vertebra from anterior loads and improve construct stiffness.
This retrospective study evaluated radiological outcomes of 69 patients with TL fracture who were treated with three different internal fixations. SSIF with intermediate inclined-angle screws provided better postoperative correction and maintenance compared with using SSIF with intermediate straight-forward screws. Although there was no significant difference among the three groups with regard to SCA, however, significant changes of AVBH and VBI were observed postoperatively. The initial correction of AVBH and VBI in the SSIF-IAP group were better than those in the SSIF-SFM group. Moreover, the correction losses of AVBH and VBI in the SSIF-IAP group were also significantly decreased compared with those in the SSIF-SFM group at the 6-month and the latest follow-ups. Although AVBH, VBI and SCA are important radiological parameters for the evaluation of the fractured vertebra, however, they don’t go hand in hand sometimes [34]. We attributed minor changes of SCA to the fact that intermediate inclined-angle screws might restore the height of fractured vertebra more effectively as compared with the correction of kyphotic angle, which was similar to previous study [34]. Although the correction and maintenance of the fractured vertebral body was the best in the LSIF group, however, from a statistical point of view, the statistical difference for the correction losses between the SSIF-IAP and LSIF was not significant. In addition, we have also found that there is not a close relationship between SCE and neurological function recovery. It is due to that the postoperative neurological status is dependent on the severity of injury to the spinal cord at the moment of trauma [35]. We speculate that only patients with minor neurological impairment (Frankel grade C, D and E) were included in our study so that all of them gradually recovered thereafter. Our data supported that SSIF-IAP was comparable to LSIF, and it also can provide improved fixation and better correction than SSIF-SFM for the treatment of TL junction fracture.
Values of all considered parameters (incision length, blood loss, surgical duration and hospital stay) in the LSIF group were the highest among the three groups, however, no significant differences were observed between the SSIF-IAP group and SSIF-SFM group regarding these parameters. Moreover, significant improvements of functional outcomes (VAS back pain and ODI) were obtained in the SSIF-IAP group and SSIF-SFM group as compared with those in the LSIF group at the 6-month and the last follow-ups. Favorable surgical outcomes can be defined by 15% improvement in ODI score [36], and our data were consistent with this criteria. In addition, ODI score is associated with VAS and SF-36 [37]. The VAS changes might be explained by the corresponding ODI changes in our study. These results suggested that intermediate inclined-angle screws insertion at the fracture level did not significantly increase the surgical duration and the blood loss as compared with the traditional straight-forward screws insertion.
Alvine et al. [38] reported that 39% screw breakage was found and 23% reoperation was performed. McLain et al. [39] have shown that instrumentation failure incidence was more than 50%. In our series, instrumentation failure occurrence was decreased compared with that reported in these studies, one case of screw breakage in the SSIP-SFM group and one case of screw loosing in the LSIF group (instrumentation failure rate = 2.90%). One screw breakage above the fracture level was observed at the 6-month follow-up in a 28-years old man (instrumentation failure rate = 4.16%). We attributed the reason for this instrumentation failure to the increased stress on the pedicle screw. This man had a history of heavy work without brace protection postoperatively. One screw loosing occurred at the 1-year follow-up in a 72-years old woman (instrumentation failure rate = 5.26%). This patient was diagnosed as osteoporosis preoperatively, however, she didn’t follow the doctor’s advice and take medicine against osteoporosis regularly during the follow-up.
There are still several limitations to this study. First, underlying factors such as, the bone density, degree of disc degeneration and vertebral size are variable. These confounding factors were offset by investigating three internal fixation strategies in the same specimen. Again, this clinical observation was based on data from relatively healthy strong bones (average 34.5 years), and a different picture might emerge in osteoporotic bones. In addition, this study evaluated short-term and small-population clinical outcomes, and findings may be biased. A long-term and large-scale prospective study should be performed to accurately evaluate the feasibility of this technique. Lastly, the speculation of this study was based on clinical observation, future biomechanical research needed to be conducted to support this application.
In conclusion, SSIF-IAP can exert greater interface strength on the fractured vertebra and effectively maintain the height of the fractured vertebra compared with using SSIF-SF; SSIF-IAP can minimize the number of fused levels, and promote rapid relief of lumbar back pain and early rehabilitation compared with using LSIF. Taken together, SSIF-IAP is an effective and reliable operative technique for patients with Denis type B TL fracture.