Does the length of the intermediate pedicle screw influence outcome in unstable thoracolumbar burst fractures? A prospective randomized study in India
Article information
Abstract
Study Design
A prospective randomized study.
Purpose
To investigate the impact of intermediate pedicle screw length on radiological and functional outcomes in unstable thoracolumbar burst fractures (TLF) treated with short-segment posterior fixation (SSPF).
Overview of Literature
Although intermediate screws confer biomechanical advantages, there is no consensus on the ideal intermediate screw length.
Methods
Sixty-six patients with unstable TLF (Load Sharing Classification score ≥7) and normal neurology requiring SSPF were randomized into two groups. Group 1 (long intermediate screw [LIS]) underwent SSPF with a long intermediate screw (occupying >50% of the vertebral body, length ≥40 mm), while group 2 (short intermediate screw [SIS]) received a short intermediate screw (occupying <50% of the vertebral body, length ≤35 mm). Radiological parameters (restoration of anterior body height [ABH], posterior body height [PBH], ABH/PBH ratio, local kyphosis angle [LKA], and regional kyphosis angle [RKA]) and functional parameters (Visual Analog Scale score and Oswestry Disability Index) were evaluated.
Results
Demographic variables (age, sex), mode of injury, and fracture pattern were comparable between groups. The LIS group showed a significant improvement in RKA correction in the immediate postoperative period (p=0.019), but this difference was not sustained at the final follow-up (p=0.713). Other radiological and functional parameters were comparable between the two groups at the 2-year follow-up.
Conclusions
Although long intermediate pedicle screw provided better correction of regional kyphosis in the immediate postoperative period for unstable TLFs with LSC ≥7, the outcomes were comparable between both long and short intermediate pedicle screws at long-term follow-up.
Introduction
Thoracolumbar burst fractures (TLF) are the most common spinal fractures, accounting for nearly 20% of all thoracolumbar fractures [1,2]. In recent years, short-segment posterior fixation (SSPF) with an intermediate screw has emerged as the preferred surgical technique for unstable TLFs with no associated neurological deficit [3–5]. The concept of the intermediate screw was first introduced by Dick et al. [6], as a means to enhance the stiffness and strength of the posterior fusion construct. Several biomechanical studies have demonstrated that intermediate screws at the fracture level significantly enhance spinal stability, provide more robust fixation, and reduce stress on pedicle screws in the nonfractured vertebrae by providing stable three-column fixation [7–10]. In a cadaveric biomechanical study by Mahar et al. [11], an intermediate screw at the fracture level was shown to provide improved biomechanical stability by indirectly supporting the anterior column, as well as aiding in fracture reduction and deformity correction. Similarly, Guven et al. [12], in their study of 72 patients with unstable TLFs, found that the additional intermediate screw helps maintain better correction of deformity through vertebral endplate augmentation, owing to its buttress effect (bending force).
Although clinical studies have demonstrated the biomechanical advantages conferred by intermediate screws, there is no consensus on the ideal intermediate screw length. A recent meta-analysis by Kapoen et al. [13], which included 28 studies on intermediate screws, highlighted the lack of literature on the optimal size and length of these screws. Various types and dimensions of pedicular screws were employed as intermediate screws in these studies [13]. Farrokhi et al. [14] used long index screws in patients with thoracolumbar fractures, achieving good kyphosis correction, fewer implant failures, and no additional complications. A retrospective study by Deng et al. [15], which compared the efficacy of short and long intermediate screws in 97 patients with lumbar fractures, concluded that short screws resulted in better long-term sagittal balance and function. Similar results have been reported by Guven et al. [12] and Chang et al. [16]. However, no prospective studies has analyzed the influence of intermediate screws in patients with unstable TLFs. Therefore, the aim of this study was to evaluate the efficacy of intermediate pedicle screw length on the radiological and functional outcomes of unstable TLFs treated with SSPF.
Materials and Methods
This was a prospective, randomized, controlled study conducted from January 2019 to March 2024. The study protocol was approved by the Institutional Review Board (IRB) of Ganga Medical Center and Hospital, Coimbatore (IRB approval no., 2019/09/10) and complied with the principles enshrined in the Declaration of Helsinki. Written informed consent was obtained from all participants prior to their enrolment.
Inclusion and exclusion criteria
Patients aged ≥18 years who sustained unstable TLFs with a Load Sharing Classification (LSC) score of ≥7 and no associated neurological deficit were included in the study [17]. Patients under 18 years of age, those with associated visceral or appendicular injuries, neurological deficits, prior spine surgery, and elderly patients with osteoporotic fractures, tumors, and infections were excluded. Among 70 patients during the study period, four patients were excluded based on the exclusion criteria, and the remaining 66 patients were randomly assigned to group 1 (long intermediate screw [LIS] group) and group 2 (short intermediate screw [SIS] group) using computer-generated numbers. The allocation details of the study are presented using the Consolidated Standards of Reporting Trials guidelines for reporting randomized trials (http://www.consort-statement.org/) (Fig. 1). Patients in group 1 underwent short-segment fixation with LIS whereas those in group 2 underwent short-segment fixation with SIS. In our study, an intermediate screw was classified as an SIS if it occupied <50 % of the anteroposterior (AP) vertebral body diameter, and as an LIS if it occupied >50% of the AP vertebral body diameter. Specifically, the length of the SIS in this study was ≤35 mm, whereas the length of the LIS was ≥40 mm (Fig. 2). All patients were followed up for a minimum period of 2 years.
CONSORT (Consolidated Standards of Reporting Trials) flow diagram. LIS, long intermediate screw; SIS, short intermediate screw.
The lateral radiograph shows long intermediate screw (occupying >50% of the vertebral body, length ≥40 mm) (A) and short intermediate screw (occupying <50% of the vertebral body, length ≤35 mm) (B).
Surgical procedure
All patients were positioned prone on radiolucent bolsters with adequate padding for all bony prominences. A standard posterior midline approach was utilized, and SSPF was performed with pedicle screw insertion one vertebra above and one below the fracture level using the freehand technique. Intermediate screws were inserted into both pedicles of the fractured vertebra. The diameter of the pedicle screws used at the nonfractured vertebra was 6.5 mm, whereas 5.5 mm screws were used at the fracture level. The intermediate screw heads were left slightly proud to act as a push point and also achieve a reduction of kyphosis. No additional effort was made to achieve a complete reduction of kyphosis or restore vertebral height through rod over-contouring, compression, or distraction. After confirming the pedicle screw position using fluoroscopic guidance, the wound was closed in layers. Two experienced spine surgeons performed all the surgeries. Postoperatively, patients were encouraged to start mobilization as tolerated with a thoracolumbar brace. All patients were advised to wear the brace for 1 month, after which it was discontinued. All patients underwent periodic follow-up with clinical and radiologic evaluations.
Clinical and radiological parameters
The patient’s demographics and injury characteristics (age, sex, mode of injury) were analyzed. Preoperative radiographs and computed tomography scans were used to classify the fracture morphology according to the AO Spine Thoracolumbar Injury Classification and the McCormack Load Sharing Classification.
Intraoperative parameters, such as operative time and blood loss, were recorded. Clinical outcomes, namely Visual Analog Scale (VAS) score and Oswestry Disability Index (ODI) were evaluated preoperatively, one month postoperatively, and at the final follow-up.
The following radiological parameters were evaluated preoperatively, immediately postoperatively, and at the final follow-up (Fig. 3): (1) Anterior body height (ABH) was measured from the anterosuperior to the anteroinferior corner of the fractured vertebra. (2) Posterior body height (PBH) was measured from the posterosuperior to the posteroinferior corner of the fractured vertebra. (3) Anterior vertebral body compression percentage was calculated using the ABH/PBH ratio, which measures the degree of vertebral height compression. (4) Local kyphosis angle (LKA) was measured as the angle between both endplates of the fractured vertebra. (5) Regional kyphosis angle (RKA) was measured as the angle formed between a line drawn parallel to the superior endplate of the cranial vertebra above the fracture and a line drawn parallel to the inferior endplate of the caudal vertebra below the fracture.
In the lateral radiographs. (A) The red line (left) shows the measurement of anterior vertebral height, and the yellow line (left) shows the measurement of posterior vertebral height. (B) The regional kyphosis angle was measured between the proximal end plate of the proximal vertebra and the distal end plate of the distal vertebra (yellow line: right). The local kyphosis angle was calculated between the proximal and distal end plates of the fractured vertebra (red line: right).
Fracture healing, implant failure, revision surgery, and surgical complications were recorded until the final follow-up. Implant failure was defined as the occurrence of any of the following events: screw breakage or pullout, rod breakage or displacement, screw or rod deformation, screw head dislodgement, implant loosening, or progressive deformity.
Statistical analysis
Data analyses were performed using IBM SPSS ver. 21.0 (IBM Corp., Armonk, NY, USA). Continuous variables were compared using the unpaired t-test, while categorical variables were analyzed using Fisher’s exact or the chi-square test. For comparing continuous variables between groups, the one-way analysis of variance test was used, followed by the least significant difference test for post hoc analysis. Statistical significance was set at a p-value less than 0.05.
Results
Basic demographics and injury characteristics
The study included 66 patients, with 33 patients in each group. The mean age of patients in the LIS and SIS groups was 38.4 years and 40.1 years, respectively. The LIS group consisted of 26 males and seven females, while the SIS group included 28 males and five females. The most common mode of injury in both groups was a fall from height (16 patients in the LIS group, 14 patients in the SIS group) followed by road traffic accidents (12 patients in the LIS group, eight patients in the SIS group) (Table 1).
Demographic details
Radiological outcomes
The analysis of radiological parameters is presented in Table 2.
Radiological parameters between two groups
Restoration of ABH
The average increase in anterior vertebral body height following surgery was 7.15 mm in the LIS group and 6.17 mm in the SIS group. There was no significant difference in the restoration of ABH between the two groups (p=0.35)
Restoration of PBH
Similarly, there was no significant difference in the increase in the PBH between the LIS group and the SIS group (2.42 mm vs. 1.72 mm, respectively; p=0.43).
Anterior vertebral body compression percentage (ABH/PBH)
The mean difference between the preoperative and postoperative ABH/PBH ratio was 0.2 in the LIS group and 0.17 in the SIS group. There was no significant between-group difference in this respect (p=0.68).
LKA correction
The mean correction of the LKA in the immediate postoperative period was 8.35° in the LIS group and 8.72° in the SIS group, with no significant difference between the two groups (p=0.839). Similarly, the mean loss of correction of local kyphosis at the final follow-up was 0.56° in the LIS group and 1.44° in the SIS group, which was not statistically significant (p=0.381) (Figs. 4, 5).
36-year-old female with load sharing score of 8. (A–D) Lateral radiograph and computed tomography scan showing T12 AO A4 injury with regional kyphosis angle (RKA) of 23.1° and local kyphosis angle (LKA) of 21.5°. (E) Shows immediate postop radiograph with short intermediate screw with RKA of 17.0° and LKA of 16.6°. (F) Final follow-up radiograph with RKA of 18.9° and LKA of 17.7°.
60-year-old male with load sharing score of 7. (A–C) Lateral radiograph and computed tomography scan showing L1 AO A4 injury with regional kyphosis angle (RKA) of 19.8° and local kyphosis angle (LKA) of 23.7°. (D, E) Shows immediate postop radiograph with long intermediate screw with RKA of 7.9° and LKA of 14.1°. (F) Final follow-up radiograph with RKA of 13.1° and LKA of 15.7°.
RKA correction
The mean RKA correction in the immediate postoperative period was significantly greater in the LIS group (8.8°) compared to the SIS group (4.48°, p=0.019). The mean loss of correction in regional kyphosis at the final follow-up was 2.89° in the LIS group and 1.87° in the SIS group, which was not significantly different (p=0.713).
Functional outcomes
VAS score
There was no significant between-group difference between the LIS and SIS groups with respect to the mean VAS score before surgery (7.82±1.01 vs. 7.85±1.04, respectively), at 1 month (5.06±1.09 vs. 5.06±1.12, respectively), and the final follow-up (2.67±0.82 vs. 2.61±0.71, respectively).
ODI score
Similarly, there was no significant difference between the LIS and SIS groups with respect to the mean ODI score before surgery (48.36±5.63 vs. 47.61±5.02, respectively), at 1 month (19.79±4.46 vs. 18.97±3.78, respectively), and the final follow-up (13.09±3.34 vs. 12.67±2.79, respectively) (Table 3).
Comparison of functional outcome between two groups
No screw-related or neurological complications were encountered in any patient in this study. Moreover, no implant-related complications were noted at the final follow-up.
Discussion
SSPF with an intermediate screw is the standard treatment method for unstable TLFs [18]. The intermediate screw at the fracture level serves as a fulcrum to reconstruct the vertebral fracture, effectively converting a two-plane fixation into a three-plane fixation. This, in turn, reduces the cantilever effect that causes kyphosis [19].
A meta-analysis of 10 studies by Li et al. [20] compared the outcomes of the conventional screw group with the combined screw group. The results showed that the use of intermediate screws resulted in better reduction of fractured vertebrae, less correction loss during follow-up, and a lower implant failure rate [20]. Additionally, a retrospective analysis by Kanna et al. [4] involving 32 patients with an unstable burst fracture (LSC ≥7) found that the use of intermediate screws helps obviate the need for vertebral body augmentation techniques and anterior vertebral reconstruction. The addition of bilateral intermediate screws to SSPF has been shown to increase the stability of the construct by 25%, with a greater impact on stability during flexion and lateral bending [7].
The use of pedicle screws in treating fractured vertebrae offers several advantages. These include providing a good three-point fixation to reduce the suspension effect of the internal fixation system, reducing the parallelogram effect to increase stability, and avoiding stretching of the normal intervertebral disc, which aids in fracture recovery. Moreover, it dispenses the stress of the pedicle screw connection. Therefore, conditional application of an intermediate screw at the fractured site can enhance the stability of the posterior short-segment internal fixation system for thoracolumbar fractures, facilitating kyphosis correction and maintaining the corrective effect [21].
A meta-analysis by Kapoen et al. [13] revealed a significant knowledge gap, as none of the 28 studies on intermediate screws investigated the optimal size and length of these screws. Öztürk et al. [22] retrospectively analyzed the effect of intermediate pedicle screw size on the radiological and functional outcomes of TLFs at a mean follow-up of 20 months. They preferred a 4.5 mm pedicle screw with a length <50 % of AP vertebral body diameter. The dimension of the intermediate screw in the other group was 5.5 mm with a length exceeding 70 % of the AP vertebral body diameter [22].
The present study compared the radiological and functional outcomes of long versus short intermediate screws in treating unstable TLFs with an LSC score of ≥7, managed using SSPF. We preferred a 5.5 mm diameter pedicle screw at the fracture level, and the length of the intermediate screw was <50% of the AP diameter of the vertebral body in the SIS group and >50% in the LIS group.
Restoration of vertebral body height
Shi et al. [23] found that the postoperative vertebral body height in patients with TLFs depends on the preoperative vertebral compression and the degree of fracture comminution, based on their study of 186 patients. The greater the compression of the fractured vertebral body, the more severe the damage to its structure and the greater the degree of expansion required to restore vertebral body height [24]. During internal fixation with pedicle screws, repositioning the fractured cortical bone is challenging, and the collapsed cancellous bone cannot be fully restored, increasing the risk of postoperative vertebral collapse [25,26]. Consequently, the degree of preoperative vertebral compression may be a predictor of postoperative vertebral body height. In the present study, both the LIS and SIS groups showed restoration of ABH and PBH, as well as improvement in the ABH/PBH ratio. This was likely attributable to the fulcrum effect of the intermediate screw. However, our analysis did not reveal any significant difference between the two groups, suggesting that postoperative vertebral body height restoration is independent of the length of the intermediate screw used. Instead, our findings indicate that vertebral body height restoration indirectly depends on the degree of fracture comminution. In contrast, Öztürk et al. [22] found that the use of thick and LISs resulted in significantly better preservation of PBH, likely due to the long anterior vector arm during the indirect reduction of fracture fragments.
Correction of local and regional kyphosis
The use of the intermediate screw led to improvements in LKA in both groups, with mean corrections of 8.35° in the LIS group and 8.72° in the SIS group in the immediate postoperative period. Notably, the LIS group showed a significant improvement in regional kyphosis (8.8°) compared to the SIS group (4.48°) in the immediate postoperative period. However, the correction was not sustained until the final follow-up. This suggests that long intermediate screw constructs provide better regional kyphosis correction in the short term. The late kyphosis recurrence in our patients may be attributed to vertebral body collapse and the presence of associated cranial (predominantly) and caudal intervertebral disc injuries.
In the present study, all patients had an LSC score of ≥7, indicating severe vertebral body comminution and complete loss of anterior column support. However, the use of a lengthy intermediate screw (>50% of vertebral body diameter) increased the immediate stability and stiffness of the injured spinal segments, effectively supporting the endplate and preventing collapse. This additionally provided anterior column support, facilitating sustained kyphosis correction, which was not achievable in the SIS group (<50% of vertebral body diameter). These results are consistent with a biomechanical study by Liu et al. [27], where the restoration of vertebral body height was maintained with the screw inserted into 60% of the vertebral body. In another similar study, increasing the screw depth from 50% to 80% of the vertebral body enhanced screw stability under flexural and torsional loads by 30% [28].
Studies have shown that TLFs are often accompanied by injuries to the upper and lower intervertebral disc [29]. This can lead to postoperative complications, including progressive kyphotic deformity due to intervertebral disc space collapse. Progressive disc space narrowing may be caused by the adaptation of the disc to morphologic changes in the osseous endplate or the disc creeping into the central depression of the endplate [30]. Similarly, Kim et al. [31], in a retrospective analysis involving 42 patients with TLFs, identified disruption of adjacent intervertebral discs as a major reason for postoperative morphologic change, leading to a gradual reduction in the anterior disc height. This may eventually lead to kyphosis recurrence without additional bony morphological changes in the fractured vertebra. Moreover, the anterior portion of the intervertebral disc adjacent to the fractured vertebra is particularly vulnerable to injury from flexion and compression forces, especially in cases with severe anterior body collapse [31].
Our study’s findings align with those of Mohammed et al. [32], who observed a loss of kyphosis correction during follow-up in 36 patients treated with SPFF using an intermediate screw. They attributed the loss of correction to the fracture configuration but also suggested that other clinical factors may contribute [32]. Formica et al. [33] investigated these factors and found that obese patients (body mass index >30 kg/m2) experienced a significant loss of kyphosis correction. Variables such as sex, smoking habit, and preoperative neurological status showed no significant association with the loss of correction [33].
Does the length of the intermediate screw matter?
Research indicates that the pedicle, rather than the vertebral body, contributes approximately 80% of the stiffness and 60% of the pullout strength [19,34]. Zindrick [35] reported no difference in pullout strength between similarly sized screws inserted halfway or completely into the anterior cortex. They noted that the more cortices penetrated, the stronger the fixation [35]. In the present study, longer screws (extending >50% of the vertebral body diameter) provided initial anterior column support to the fractured vertebra, but subsequent loss of kyphosis correction occurred, likely due to severe anterior vertebral body comminution and associated disc injury. Furthermore, in highly comminuted fractures (LSC score ≥7), using longer and larger diameter screws poses a risk of distracting the bone fragments, potentially impeding fracture healing and compromising the canal. Nonetheless, no neurological complications related to the use of longer intermediate screws were encountered in the present study.
Some limitations of this study should be acknowledged. This was a single-center study with a limited sample size. Moreover, the follow-up period was limited to 2 years, which is considered relatively short. A multicenter study with a large sample size and a relatively long follow-up period is required to obtain more robust evidence.
Conclusions
Our study demonstrates that both long and short intermediate pedicle screws provide stable fixation, effective kyphosis correction, and minimal correction loss in unstable thoracolumbar fractures with LSC score ≥7. The length of the pedicle screw, whether short (≤35 mm) or long (≥40 mm), was not found to impact the clinical and radiological outcomes in unstable TLFs. The stability of the short-segment fixation construct with an intermediate screw depends mainly on the three-point fixation and push-point effect produced by the intermediate screw, which reduces the cantilever effect responsible for kyphosis.
Key Points
Short-segment posterior fixation with an intermediate pedicle screw is the standard treatment for unstable thoracolumbar fractures.
In this study, long intermediate screws provided relatively better support and significant improvement in regional kyphosis angle in the immediate postoperative period, with a mean correction of 8.8°, compared to 4.48° with short intermediate screws.
However, both long and short intermediate pedicle screws yielded comparable clinical and radiological outcomes at long-term follow-up.
The length of the intermediate pedicle screw does not appear to influence long-term clinical and radiological outcomes in unstable thoracolumbar fractures.
Notes
Conflict of Interest
No potential conflict of interest relevant to this article was reported.
Author Contributions
Conceptualization: TA, KR, APS. Methodology: TA, KR, NR. Investigation: TA, KR, NR, APS. Data curation: TA, KR, NR. Formal analysis: TA, KR, APS. Writing–original draft: TA, KR. Visualization: KR, APS. Writing–review & editing: KR, APS, RMK, SR. Project administration: APS, SR. Supervision: APS, RMK, SR. Validation: none. Resources: SR, APS. Final approval of the manuscript: all authors.