Results
In all samples, a fracture resulted in the target vertebral body by performing a single compression cycle. Yield strengths recorded during the fracture tests are shown in Table 1 and ranged from 1.9 kN to 5.4 kN.
Evaluations via CT scan and macroscopic inspection of the specimens showed no signs of injury to the facets or posterior ligamentous complex (PLC) or rotational injury. An independent senior spine surgeon and a senior radiologist identified a superior incomplete burst fractures in all samples (Figure 3).
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Figure 3.
CT scan after fracture infliction, axial and sagittal plane. Incomplete burst fracture with involvement of anterior and middle column was produced in all used specimens.
All specimens presented a significant increase of the measured RoM (p < 0.005) for all directions by induction of the cranial burst fracture in comparison to intact kinematic behavior. The RoM was decreased significantly (p < 0.005) by applying the dorsal bisegmental instrumentation to levels similar to the intact values. The NZ presented effects similar to those observed for RoM. After fracture infliction, the NZ increased significantly (p < 0.01).
An adequate screw positioning after posterior instrumentation was checked and documented via fluoroscopy.
After dorsal bisegmental instrumentation, the NZ was again reduced significantly (p < 0.005). No statistically significant differences were seen when comparing the NZ of intact 4 segmental specimens and bisegmentally instrumented fractured 4 segmental specimens.
VP was successfully performed in all specimens and an adequate cement distribution was observed by fluoroscopy.
Kinematic Effects on Primary Stability by Stand-alone Vertebroplasty (VP)
Flexion In flexion, fracture significantly increased NZ of the intact specimen by 57% (p < 0.005). EZ was also increased significantly (p < 0.01) after fracture by 16%. VP reduced this effect for NZ but was still increased significantly by 16% of intact values (p < 0.05). EZ was also reduced by VP but remained 11% significantly (p < 0.05) above intact kinematic values (Figure 4, Table 2).
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Figure 4.
Comparison of RoM values for five specimen groups in extension and flexion in neutral zone (NZ) and elastic zone (EZ). Incomplete burst fracture creation (2) resulted in a significant increase in NZ and EZ, posterior bisegmental instrumentation resulted in a significant decrease of NZ and EZ (3) for flexion and extension. HA resulted in no additional primary stability (4), whereas isolated PMMA VP (5) increased fractured values, but remained significant less than prefractured values in flexion (NZ and EZ) as well as NZ in extension.
Fracture also significantly decreased NZ stiffness (p < 0.005) to 44% and EZ stiffness to 86% of the intact specimen (p < 0.005). VP significantly increased NZ stiffness (p < 0.05) to 51% and EZ stiffness (p < 0.01) to 94%, but stiffness parameters remained lower than prefracture values (Figure 5, Table 2).
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Figure 5.
Comparison of stiffness values for five specimen groups in extension and flexion in NZ (sNZ) and EZ (sEZ). Incomplete burst fracture creation (2) resulted in a significant decrease of sNZ and sEZ in flexion as well as sNZ in extension. Posterior bisegmental instrumentation (3) restored sNZ but was not able to restore prefractured sEZ values in flexion. Hybrid augmentation (HA) takes advantage of changes in stiffness parameters by instrumentation (4). In addition, a significant change compared to fractured values of sEZ even for flexion was observed following HA. Stand-alone VP (5) increased sNZ on flexion and remained significantly lower than pre-fracture values. VP significantly increased sEZ in flexion compared to the fractured (2) state to reach approximately pre-fractured values.
Extension In extension, values for NZ were identical with flexion, caused by the defined zero crossing. EZ was increased by the fracture by 17% and this increase was not clearly influenced by VP. Stiffness in neutral zone (sNZ) was significantly reduced by the fracture (p < 0.05), whereas no influence of stiffness in elastic zone (sEZ) was seen after fracture creation. No perceptible change was seen from VP for sNZ and sEZ compared to the fractured state (Figures 4 and 5, Table 2).
Lateral bending In lateral bending, NZ was increased significantly (p < 0.005) by more than twice the intact kinematics after fracture creation. Also EZ was increased significantly (p < 0.005) by 28%. VP reduced NZ increase but remained 79% significantly (p < 0.005) more than the intact values. The increase of EZ could not be influenced by VP.
Stiffness in NZ and EZ remained significantly reduced by approximately half the prefractured values after VP for NZ stiffness (p < 0.01) and three quarters for EZ stiffness (p < 0.05) (Figure 6, Table 3).
Axial rotation In axial rotation, NZ was increased significantly by 61% (p < 0.01), and EZ by 29% (p < 0.005) after fracture creation. There was no distinct change of this increase in NZ and EZ caused by VP. NZ stiffness was reduced significantly (p < 0.01) by 29%, and EZ stiffness significantly (p < 0.005) by 15%. After VP, values persisted approximately at the fracture value level (Figure 6, Table 3).
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Figure 6.
Comparison of kinematic values for five specimen groups in lateral bending and axial rotation. Incomplete burst fracture creation (2) resulted in a significant increase in RoM and decrease of stiffness in neutral zone (sNZ) and elastic zone (sEZ). Posterior bisegmental instrumentation (3) restored prefractured kinematic values in lateral bending and axial rotation. Hybrid augmentation (HA) resulted in no additional increase in stiffness or further reduction of RoM (4). Stand-alone VP (5) did not alter fractured values for lateral bending and axial rotation.
Kinematic Effects on Primary Stability by Hybrid Augmentation (HA)
Flexion Instrumentation reduced intact NZ to 61% in flexion. This effect was incremented by HA to 53% of intact values. Also, EZ was influenced by instrumentation to 88% of intact kinematics and to 82% by HA. The additional influence in HA was not significant (Figure 4, Table 2).
Instrumentation and HA also increased NZ stiffness above intact levels. No significant influence was seen after HA in comparison to instrumentation. In EZ, instrumentation was not able to restore intact stiffness. Here, a significant (p < 0.05) increase in sEZ was observed in HA compared to instrumentation. Intact stiffness values were approximately restored by a combination of instrumentation and cement augmentation (Figure 5, Table 2).
Extension In extension, instrumentation and HA reduced motion in NZ and EZ below the prefracture values. Also, stiffness parameters in NZ and EZ were increased above intact kinematics. No significant changes between instrumentation and HA were observed in extension (Figures 4 and 5, Table 2).
Lateral bending Movement in NZ and EZ was reduced by instrumentation and HA below the prefracture values. No differences were seen between these 2 groups in lateral bending. NZ stiffness was also increased in both groups, without any intergroup differences. Instrumentation and HA restored sEZ approximately back to values similar to the intact state (Figure 6, Table 3).
Axial rotation Instrumentation and HA approximately restored prefracture values for NZ and EZ and sEZ in axial rotation. No significant changes between the 2 groups were seen (Figure 6, Table 3).