Fixation and Augmentation of Incomplete Burst Fractures
Fixation and Augmentation of Incomplete Burst Fractures
Nine spinal samples consisting of 5 vertebrae (4 functional spinal units, FSU) were harvested from postmortem donors in our anatomical institute and immediately frozen. All specimens were taken from the thoracolumbar junction. The median age of the specimens was 75 (Q1: 73; Q3: 87.5) years, with nearly equal sex distribution (male: female = 4:5). The ethics commission for the medical chamber of Westfalen-Lippe and the medical faculty of the University of Muenster approved the usage of post mortem samples of the local anatomical institution.
In all samples, bone mineral density (BMD) was measured using quantitative computed tomography (Q-CT). The median BMD was 103 (Q1: 70.9; Q3: 117.7) mg Ca-HA/mL and the median T-score of -2.7 (Q1: -3.57; Q3: -1.84) was calculated. Except for one sample, only osteoporotic or osteopenic spine samples were available (Table 1). Just before testing, all specimens were thawed slowly to room temperature. All soft tissue and muscles were dissected carefully to preserve the osseous and ligamentous structures.
The caudal and cranial vertebral body was rigidly fixed in a standardized manner into a piece of plastic pipe filled with 2-component resin embedding system (Technovit® 3040 Kulzer Heraeus GmbH, Germany). This setup could then be attached to customized tools to mount the samples into the servo hydraulic testing machine and the testing robot.
All samples were kept moist during the dissection and testing process.
The presented procedure took advantage of the published classical approaches to study burst fractures that utilized spine fragments mounted onto a fracture apparatus. All known methods were not able to specifically produce incomplete burst fractures, and thus needed to be refined for this study.
According to a validated testing setup, a standardized osteotomy was performed and temporarily transfixed. All specimens were mounted into a hydraulic material testing apparatus (Instron 8874, Instron Structural Testing Systems GmbH, Germany) in a 10° flexion angle. The specimens were then axially compressed under displacement control with a speed of 300 mm/s until the vertical distance was reduced to 20% of the original target vertebral body height. Detailed presentation of this technique is published elsewhere.
The posterior instrumentation was performed bisegmentally using the USS Fracture System (Synthes GmbH, Switzerland). USS Schanz screws with 6.2 mm dual core were used. No additional force by reduction due to fracture clamp was applied in consideration of the bone quality of the samples used.
VP was realized using PMMA (Vertecem, Synthes GmbH, Switzerland) according to the recommended surgical biportal transpedicular technique under fluoroscopic control (Figures 1 and 2). Cement was mixed according to the manufacturer's recommendations. All procedures were performed at room temperature. Before kinematic testing, the complete hardening of the cement at room temperature was ensured. The median volume of the injected cement was 5.7 mL (Q1: 4.3 mL; Q3: 8.2 mL).
(Enlarge Image)
Figure 1.
Experimental setup of in vitro vertebroplasty (VP) for hybrid augmentation (HA). After fracture creation, human thoracolumbar postmortem specimens were bisegmentally instrumented and additionally cement-augmented under fluoroscopic control.
(Enlarge Image)
Figure 2.
Fluoroscopic control after HA. The posterior instrumentation was performed bisegmentally and by biportal transpedicular VP.
The spine testing facility used in this study was based on a 6° of freedom robotic arm (KUKA/KR125, Kuka Augsburg, Germany) which enabled execution of complex motion patterns. The robot was used to manipulate the specimen via predefined load (7.5 Nm, pure moments) on the cranial vertebrae. A sensitive force/torque sensor (Theta SI-1500–240 from ATI Industrial Automation, USA) was mounted at the robot's end-effector which enabled simultaneous measurements of applied forces and torques during load-controlled robot movement according to a standardized and evaluated robotic-based setup.
All spine samples were subjected to moments in flexion-extension, lateral bending and axial rotation in the following 5 different states:
For each group of specimens, the range of motion (RoM), neutral zone (NZ) and elastic zone (EZ), and the stiffness of the NZ and EZ were determined. In extension–flexion, the combined motion was evaluated for extension and flexion separately. Therefore, zero-crossing was defined to be half of NZ.
Statistical analysis was performed using the Wilcoxon signed-rank test for paired samples at the significance level of p = 0.05. This nonparametric test is suitable for analyzing data that may not be normally distributed. In addition, a repeated measures analysis was performed.
The calculation of the statistics was performed with software programmed in C# (Visual C#, Microsoft Corporation, USA). This software is based on ALGLIB® (ALGLIB project, Russian Federation) and was validated using SPSS® (SPSS® Statistics, IBM®, USA).
Sample size was limited by the availability of human postmortem samples but was comparable to sample sizes used in similar studies and cited in published recommendations.
Methods
Specimens
Nine spinal samples consisting of 5 vertebrae (4 functional spinal units, FSU) were harvested from postmortem donors in our anatomical institute and immediately frozen. All specimens were taken from the thoracolumbar junction. The median age of the specimens was 75 (Q1: 73; Q3: 87.5) years, with nearly equal sex distribution (male: female = 4:5). The ethics commission for the medical chamber of Westfalen-Lippe and the medical faculty of the University of Muenster approved the usage of post mortem samples of the local anatomical institution.
In all samples, bone mineral density (BMD) was measured using quantitative computed tomography (Q-CT). The median BMD was 103 (Q1: 70.9; Q3: 117.7) mg Ca-HA/mL and the median T-score of -2.7 (Q1: -3.57; Q3: -1.84) was calculated. Except for one sample, only osteoporotic or osteopenic spine samples were available (Table 1). Just before testing, all specimens were thawed slowly to room temperature. All soft tissue and muscles were dissected carefully to preserve the osseous and ligamentous structures.
The caudal and cranial vertebral body was rigidly fixed in a standardized manner into a piece of plastic pipe filled with 2-component resin embedding system (Technovit® 3040 Kulzer Heraeus GmbH, Germany). This setup could then be attached to customized tools to mount the samples into the servo hydraulic testing machine and the testing robot.
All samples were kept moist during the dissection and testing process.
Fracture Creation
The presented procedure took advantage of the published classical approaches to study burst fractures that utilized spine fragments mounted onto a fracture apparatus. All known methods were not able to specifically produce incomplete burst fractures, and thus needed to be refined for this study.
According to a validated testing setup, a standardized osteotomy was performed and temporarily transfixed. All specimens were mounted into a hydraulic material testing apparatus (Instron 8874, Instron Structural Testing Systems GmbH, Germany) in a 10° flexion angle. The specimens were then axially compressed under displacement control with a speed of 300 mm/s until the vertical distance was reduced to 20% of the original target vertebral body height. Detailed presentation of this technique is published elsewhere.
Instrumentation and Augmentation
The posterior instrumentation was performed bisegmentally using the USS Fracture System (Synthes GmbH, Switzerland). USS Schanz screws with 6.2 mm dual core were used. No additional force by reduction due to fracture clamp was applied in consideration of the bone quality of the samples used.
VP was realized using PMMA (Vertecem, Synthes GmbH, Switzerland) according to the recommended surgical biportal transpedicular technique under fluoroscopic control (Figures 1 and 2). Cement was mixed according to the manufacturer's recommendations. All procedures were performed at room temperature. Before kinematic testing, the complete hardening of the cement at room temperature was ensured. The median volume of the injected cement was 5.7 mL (Q1: 4.3 mL; Q3: 8.2 mL).
(Enlarge Image)
Figure 1.
Experimental setup of in vitro vertebroplasty (VP) for hybrid augmentation (HA). After fracture creation, human thoracolumbar postmortem specimens were bisegmentally instrumented and additionally cement-augmented under fluoroscopic control.
(Enlarge Image)
Figure 2.
Fluoroscopic control after HA. The posterior instrumentation was performed bisegmentally and by biportal transpedicular VP.
Kinematic Testing
The spine testing facility used in this study was based on a 6° of freedom robotic arm (KUKA/KR125, Kuka Augsburg, Germany) which enabled execution of complex motion patterns. The robot was used to manipulate the specimen via predefined load (7.5 Nm, pure moments) on the cranial vertebrae. A sensitive force/torque sensor (Theta SI-1500–240 from ATI Industrial Automation, USA) was mounted at the robot's end-effector which enabled simultaneous measurements of applied forces and torques during load-controlled robot movement according to a standardized and evaluated robotic-based setup.
All spine samples were subjected to moments in flexion-extension, lateral bending and axial rotation in the following 5 different states:
intact specimen
after fracture induction
after bisegmental instrumentation
HA: combined bisegmental instrumentation and VP
stand-alone VP
For each group of specimens, the range of motion (RoM), neutral zone (NZ) and elastic zone (EZ), and the stiffness of the NZ and EZ were determined. In extension–flexion, the combined motion was evaluated for extension and flexion separately. Therefore, zero-crossing was defined to be half of NZ.
Statistics
Statistical analysis was performed using the Wilcoxon signed-rank test for paired samples at the significance level of p = 0.05. This nonparametric test is suitable for analyzing data that may not be normally distributed. In addition, a repeated measures analysis was performed.
The calculation of the statistics was performed with software programmed in C# (Visual C#, Microsoft Corporation, USA). This software is based on ALGLIB® (ALGLIB project, Russian Federation) and was validated using SPSS® (SPSS® Statistics, IBM®, USA).
Sample size was limited by the availability of human postmortem samples but was comparable to sample sizes used in similar studies and cited in published recommendations.
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