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The current study is aimed at investigating the differences in stability between short posterior fixation (SPF), hybrid posterior fixation (HPF), and long posterior fixation (LPF) with and without anterior column augmentation using calcium phosphate bone cement (CaP) for treating burst fractures (BFs).
The ideal treatment for thoracolumbar BF is controversial regarding the use of short or LPF constructs.
Seven human thoracolumbar spines (T9-L4) were tested on a six degree of freedom spine simulator in three physiologic planes, flexion-extension (FE), lateral bending (LB), and axial rotation (AR). Tested surgical constructs included the following: intact, injury (BF), SPF (T12-L2), HPF (T11-L2), LPF (T11-L3), SPF+CaP, HPF+CaP, LPF+CaP, and CaP alone (CaP). Range of motion (ROM) was recorded at T12-L2 in FE, LB, and AR.
The reduction in mean ROM trended as follows: LPF>HPF>SPF. Only LPF constructs and HPF with anterior column augmentation significantly reduced mean ROM in FE and LB compared to the intact state. All instrumented constructs (SPF, HPF, and LPF) significantly reduced ROM in FE and LB compared to the injured condition. Furthermore, the instrumented constructs did not provide significant rotational stability. Injecting CaP provided minimal additional stability.
For the injury created, LPF and HPF provided better stability than SPF with and without anterior column augmentation. Therefore, highly unstable fractures may require extended, long or hybrid fusion constructs for optimum stability.
The ideal treatment for thoracolumbar burst fractures (BFs) is controversial regarding the use of short or long posterior fixation constructs [
Previous investigators reporting the biomechanics of spinal instrumentation for thoracolumbar BFs have used different levels and species for testing the different methods of simulating BF or lacked a direct comparison of LPF, HPF and SPF with and without the addition of anterior column augmentation [
A total of 7 fresh frozen human cadaver spines (T10-L4) were used in the study. The specimens were obtained from Science Care (Phoenix, AZ, USA) tissue bank. The medical history of each donor was examined to exclude trauma, malignancy or metabolic disease that might otherwise compromise the biomechanical properties of the lumbar spine. The specimens were harvested from two female and five male cadavers (mean age of death, 58±9 years). Dual energy radiograph absorptiometry (QDRA-010, Hologic Discovery, Waltham, MA, USA) was used to quantify the bone mineral density (BMD) (mean BMD, 0.8±0.1 g/cm2). The specimens were radiographed in the anterioposterior and lateral planes in order to ensure the absence of fractures, deformities and any metastatic disease. The spines were dissected by carefully denuding the paravertebral musculature, avoiding the disruption of spinal ligaments, joints and disks. The spines were fixed at T10 proximally and L4 distally in a 2:1 mixture of Bondo auto body filler (Bondo Mar-Hyde Corp., Atlanta, GA, USA) and fiberglass resin (Home-Solution All Purpose, Bondo Mar-Hyde Corp.). Specimens were wrapped in saline-soaked (0.9% NaCl) gauze to assure moist conditions during testing [
Each of the specimens was tested using a custom designed 6 degree of freedom (DOF) spine simulator (
In order to create a BF L1, minimal osteotomies were created at the anterior and lateral cortex of the L1 vertebral body in order to ensure a reproducible fracture at L1. Then, each vertebral body was compressed at 50 mm/min in a materials testing machine (MTS Bionix, Eden Prairie, MN, USA) [
Stabilization of the fractured vertebral body was achieved using bilateral pedicle screw and rod constructs (REVERE Globus Medical Inc., Audubon, PA, USA) through LPF, HPF, and SPF. The L1 vertebral body was then reinforced with the calcium phosphate (CaP) bone cement (Kyphon Inc., Sunnyvale, CA, USA) through a balloon-assisted bilateral pedicular approach (
Each spine was tested for stability at T10-L4 in the intact condition in order to obtain the baseline values. Following intact testing, constructs were stabilized using SPF, HPF, and LPF for flexibility as follows: 1) BF and 2) augmentation using CaP. Lastly, the CaP alone construct without any instrumentation was tested for stability. The test constructs are shown in the order they were tested in
Statistical analysis was performed on raw data using a repeated measures analysis of variance for independent samples, followed by Tukey's post-hoc analysis for multiple comparison procedures. Significance was set at
All constructs were examined after testing, and none showed any visible signs of damage, loosening or breakage. The means and standard deviations for ROM in FE, LB, and AR are presented in
The ROM was significantly reduced for all instrumented constructs compared to intact, injured and CaP conditions with LPF providing better stability compared to HPF and SPF; however, it was significant only when compared to SPF.
As expected, after injury, the ROM significantly increased by 21% compared to that of the intact condition. The SPF, HPF, and LPF constructs significantly improved construct stability by 35%, 53%, and 65%, respectively, compared to the injured condition. There was no significant difference in ROM between HPF and LPF constructs.
Anterior column augmentation using CaP provided, on average, an additional stability of 6% (5% over SPF, 3% over HPF, and 9% over LPF) over uncemented SPF, HPF, and LPF constructs. All the instrumented constructs SPF, HPF, and LPF significantly enhanced construct stability by 39%, 51%, and 70% compared to the injured condition. CaP alone without posterior instrumentation did not provide significant stability compared to the intact and injured conditions.
Similar to FE, ROM reduced significantly upon instrumentation compared to the intact, injured and CaP conditions, and LPF provided improved stability over SPF and HPF. The injured and CaP constructs were the most unstable.
Following the BF creation, a 26% increase (
Reinforcement of SPF, HPF, and LPF using CaP enhanced construct stability by 6%, 3%, and 9%, respectively. After augmentation, using CaP, SPF, HPF, and LPF reduced motion by 41%, 56%, and 73%, respectively, compared to the injured condition. CaP augmentation alone did not restore stability back to the intact condition.
Unlike FE and LB, the instrumented constructs did not improve construct stability in AR. However, LPF provided better stability compared to SPF and HPF, similar to FE and LB.
Immediate to the BF, AR ROM significantly increased by 23% compared to the intact condition. All instrumented constructs reduced motion; yet, it was not significant compared to the intact condition. However, the reduction in motion (20% after HPF and 30% after LPF) was significant compared to the injured condition. There was no significant difference between LPF and HPF constructs.
Even after the addition of CaP, the motion did not significantly improve compared to the intact condition. Similar to the uncemented HPF and LPF constructs, the reduction in motion following augmentation using CaP was significant compared to the injured condition. However, CaP alone did not impart any stability.
The common surgical goal of treatment in thoracolumbar fractures is obtaining the most stable fixation with fusion of the fewest segments possible. Both anterior and/or posterior approaches can be used, and the efficacy of either approach has been shown to be the same [
At the thoracolumbar junction, previous authors have shown that the compressive forces act more anterior and therefore, require segmental transpedicular fixation two-levels above the fracture site in order to prevent progressive kyphosis and hardware failure [
In the present study, within the instrumented constructs stability offered trended as follows: SPF<HPF<LPF. LPF was the stiffest of the 3 instrumented constructs with and without anterior column augmentation using CaP in all loading conditions, but was significantly stiffer when compared to SPF only. The use of screws two above and two below (LPF) has shown to not only enhance the stability but also allow effective reduction of kyphotic deformity [
The BF segments demonstrated the most instability in AR, even in the instrumented segments. Anterior column augmentation also did not restore the stiffness to the intact level in AR. These results are consistent with those of Slosar et al. [
Previous studies have shown that the addition of CaP into the fractured vertebral body through a transpedicular approach to be a feasible technique [
The present study does not take into account the structural stability provided by muscle forces and soft tissue. For such factors, the primary objective of this study is simply a trend comparison of different reconstruction techniques immediately postoperative. Moreover, because the data was normalized to the intact conditions, the findings of the study may not be affected by these natural variables. Lastly, the present study, similar to other
In the burst fracture model, LPF and HPF provided better stability than SPF with and without anterior column augmentation. Therefore, highly unstable fractures may require extended, long fusion constructs for optimum stability. Hybrid posterior fixations may be considered as an alternative to LPF in order to avoid excessive load on the lower disc while increasing fixation.
The authors would like to acknowledge Kelly Baker for conducting critical revisions to the manuscript.
The authors acknowledge the funding for this project paid for by Globus Medical, Inc., which was conducted using machines and other equipment within their research department. Funding was provided in terms of staff salaries.
Spine simulator setup.
Reinforcement using calcium phosphate cement.
Surgical constructs. BF, burst fracture; SPF, short posterior fixation; HPF, hybrid posterior fixation; LPF, long posterior fixation; CaP, calcium phosphate cement.
Range of motion. BF, burst fracture; SPF, short posterior fixation; HPF, hybrid posterior fixation; LPF, long posterior fixation; CaP, calcium phosphate cement.
Mean range of motion and standard deviation (in degrees) at T10-L4 in flexion-extension, lateral bending, and axial rotation
No significant differences among the instrumented constructs were observed, with and without anterior column augmentation. Significance was set at
SPF, short posterior fixation; HPF, hybrid posterior fixation; LPF, long posterior fixation; CaP, calcium phosphate cement.
a)Significant compared to the intact condition; b)Sgnificant compared to the injured condition; c)Significant compared to CaP only construct.