Although the conservative management was thought to be the optimal treatment of TL junction fracture without serious 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 the 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 treatment of TL junction fracture is dependent on many variables, such as bone intensity, kyphotic deformity, spinal canal encroachment etc. Either anterior, posterior approach or combined approaches can be applied for the stabilization of the unstable spine. Studies have shown that anterior instrumentation with bone graft can achieve reliable internal fixation, but it requires a more invasive approach that is associated with complications and prolonged postoperative recovery [8, 25]. Alternative intervention was considered prior to this more invasive approach. Isolated posterior approach can also be applied to maintain the stability of the spinal column. LSIF via posterior approach can provide enough rigidity and stiffness of the spine, however, it might decrease spinal range of motion and increase the incidence of ASD. Therefore, other improved alternatives have been lately developed to take advantage of its benefits and minimize its adverse effects.
Superior biomechanical stability was found in SSIF with the addition of pedicle fixation at the fracture level without sacrificing the benefits of SSIF. Studies have shown that SSIF with intermediate screws insertion could significantly improve the biomechanical stability and construct stiffness as compared with SSIF [11, 26]. Moreover, clinical research has found that the preservation of the fractured vertebrae height obtained in SSIF with intermediate screws was equivalent to that in LSIF [18]. Secondly, the use of intermediate screws at the fracture level can optimize load on instrumentation and reduce the risk of broken screws or rods. Buckling distortion of rod was 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 six-screw fixation construct, tension strains at each level of the four-screw fixation construct were significantly higher than that at each level of the six-screw fixation construct [28, 29]. Although traditional SSIF with intermediate screws theoretically corrects kyphotic deformities, however, the present instrumentation is not able to provide adequate support to the injured anterior spine column for unstable TL fracture in practice.
We then developed a new strategy for SSIF with inclined-angle intermediate polyaxial screws. There are some advantages as following: Firstly, this inclined-angle screw placement increases the length of pedicle screws, so it can provide greater construct stiffness and increase the pullout force. 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 and the injured vertebrae 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 fractured vertebral body may often 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. However, Intermediate screws in the SSIF-IAP group can escape from this cavity, and contribute to the construct stiffness. It might minimize the 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. We believed that the main reason for this difference might be due to that screw-to-bone interface strength was improved by the increased angulation of the screws, and the anterior and middle spinal columns were immediately strengthened by these inclined-angle polyaxial screws. It suggests that inclined-angle screws at the fracture level can improve construct stiffness and protect the fractured vertebral body from anterior loads.
This retrospective study evaluated the radiological outcomes of 69 patients with TL fracture who were treated with three different internal fixations. As it was observed in the current study, SSIF with intermediate inclined-angle screws provided better postoperative correction and maintenance than SSIF with intermediate straight-forward screws. There was no significant difference among three groups with regard to SCA. However, changes of AVBH and VBI were not in agreement with SCA postoperatively. The initial correction of AVBH and VBI in the SSIF-IAP group were better than those in the SSIF-SF group. Moreover, the correction losses of the AVBH and VBI in the SSIF-IAP group were also significantly lower than those in the SSIF-SFM group at the 6 months and the latest follow-ups. Although AVBH, VBI and SCA are important parameters for the restoration evaluation of fractured vertebrae, however, sometimes they don’t go hand in hand[34]. We attributed minor changes of SCA to the fact that intermediate inclined-angle screws might restore the height of fractured vertebrae more effectively as compared with the correction of kyphotic angle, which was Similar to the previous study [34].No statistical difference of SCE among three groups is due to no significant correlation between canal narrowing and neurological encroachment, which is dependent on the injury of spinal cord occurs at the time of trauma[35]. Only patients with minor neurological impairment (Frankel grade C, D and E) were included in our study, and all of them gradually recovered thereafter. 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. Our data supported that the SSIF-IAP was comparable to LSIF, and it also can offer improved fixation and better correction than SSIF-SFM in the treatment of TL junction fractures.
Similar to the previous study, the values of all considered parameters (incision length, blood loss, surgical duration and hospital stay) in the LSIF group were the highest among three groups, however, no significant differences were observed for these parameters between the SSIF-IAP group and SSIF-SFM group. 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 that in the LSIF group at the 6-month and the last follow-up. Favorable surgical outcomes can be defined by 15% improvement in ODI scores[36], and our data were consistent with this criteria. In addition, it was found that ODI score is associated with VAS and SF-36[37]. The ODI changes might be explained by the corresponding VAS changes in our study. These results suggested that intermediate inclined-angle screws insertion at the fracture level didn’t increase the operation 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 percent. In our series, instrumentation failure occurrence was lower than that reported in these studies, one case of screw breakage in the SSIP-SFM group and one case of screw loosing in the LSIP 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 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, these clinical observations were 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 in this study was based on clinical observation, future bio-mechanical research needed to be conducted to support this application.
In conclusion, the SSIF-IAP can exert greater strength on the fractured vertebrae and effectively maintain the height of fractured vertebrae as compared with SSIF-SF; Compared with the LSIF, the SSIF-IAP can minimize the number of fused levels, and promote rapid relief of lumbar back pain and early rehabilitation. We recommended that SSIF with inclined-angle screws was effective and reliable operative technique for patients with Denis type B TL fracture.