An In Vitro Evaluation of Fracture Reduction Achieved by Inflatable Bone Tamps Under Simulated Physiological Load.

STUDY DESIGN: An in vitro biomechanical study. OBJECTIVE: To determine the fracture reduction achieved by a novel inflatable bone tamp under simulated physiological load. SUMMARY OF BACKGROUND DATA: Previous biomechanical studies have showed that kyphoplasty allows near-total restoration of lost vertebral height in unloaded conditions and partial height restoration under simulated physiological loads. Clinically, loss of reduction has been observed after bone tamp deflation, before cement injection. The present study evaluated fracture reduction achieved by an inflatable bone tamp during kyphoplasty while maintaining physiological load. Comparison to commercially available inflatable bone tamp was also performed. MATERIALS AND METHODS: Eighteen osteoporotic vertebral bodies (T11-L4) were alternately assigned to one of the 2 treatment groups: group A-AFFIRM (Algea Thearpies, a division of Globus Medical Inc., Audubon, PA); and group B-KYPHON (Kyphon Inc., Sunnyvale, CA). The vertebral bodies were compressed axially on an MTS Bionix 858 machine at a rate of 5 mm/min until compressed to 40% of the initial anterior height. Load versus displacement was recorded. The fractured VBs then underwent kyphoplasty with cement augmentation. The augmented vertebral bodies were then recompressed and anterior vertebral body height (mm) and wedge angle (degrees) was measured initially, after mechanically creating an anterior wedge fracture, and after repairing the compression fracture. Each vertebral body was subjected to 111 N load to simulate in vivo physiological loading during inflation and cement augmentation. The vertebral height, wedge angle, cement volume, and inflation pressures were compared between the treatment groups using an unpaired t test (P<0.05). Failure loads were compared between intact and repaired VBs using a paired t test (P<0.05). RESULTS: Average lost height restored in group A was 29%, and 30% in group B compared to the compressed state. Similar trends were observed in the mean changes of vertebral body wedge angle in both the groups. No significant difference in mean inflation pressures (group A 182±33 psi; group B 175±37 psi) were found between the 2 groups. Average percentage increase in failure load was 218% and 241% for groups A and B, respectively. Mean injected cement volume was 6.65±0.65 and 6.73±0.41 mL for groups A and B, respectively. CONCLUSIONS: Some height restoration was observed using the 2 bone tamps in fractured vertebral bodies under simulated physiological load. The fracture reduction achieved by the 2 inflatable bone tamps was equivalent. No significant difference between mean inflation pressures and failure load was demonstrated between the 2 groups.

Additional sagittal correction can be obtained when using an expandable titanium interbody device in lumbar Smith-Peterson osteotomies: a biomechanical study.

BACKGROUND CONTEXT: Insertion of intervertebral fusion devices between consecutive Smith-Peterson osteotomies (SPOs) provides an anterior fulcrum during compression, which has been documented to improve achievable Cobb angle correction. Extension of these principles to an expandable device would theoretically provide greater surgical adjustment for flatback and scoliotic cases than a static cage. PURPOSE: To investigate whether an expandable titanium interbody device would produce greater sagittal correction than a static spacer when used during SPO procedures. STUDY DESIGN/SETTING: Cadaveric research was performed. PATIENT SAMPLE: Seven T10-S1 human specimens were used. OUTCOME MEASURES: Cobb angle changes and range of motion are the physiological measures. No self-report/functional measures were applicable. METHODS: Bilateral pedicle screws were placed (T11-L5) before Smith-Petersen osteotomy creation from L2 to L4. A transforaminal lumbar interbody fusion titanium expandable implant was placed in each disc space from L2-L3 to L4-L5, which is currently an off-label use of this implant. Initial placement simulated a static spacer, and then incremental device expansion was performed to obtain an intermediate and final height. Lateral fluoroscopic images were taken for Cobb angle evaluation between L2 and L5, and range of motion as observed during application of pure bending moments was captured using a six degree-of-freedom spine simulator. A one-way analysis of variance with Tukey post hoc analysis was performed to determine significant differences (p<.05) between surgical constructs (intact, SPO only, contracted, semiexpanded, and expanded). Study costs were allocated within the research budget of a medical device company, where some authors are salaried employees; another author has been a paid consultant elsewhere. These financial associations were not believed to bias the results. RESULTS: Change in Cobb angle from L2 to L5 was significantly greater with the interbody spacer compared with SPO alone. Despite an obvious increase in lordosis with expansion height, there were no significant differences between implant expansion states for the L2-L5 Cobb angle. All instrumented constructs were statistically equivalent in every mode of motion once rigid instrumentation was implemented, regardless of expansion state. CONCLUSIONS: The expandable interbody did have a slight effect on lordotic correction; each additional millimeter in height expansion yielded approximately 1° in correction across the three SPO levels. Even without significant differences between the states, an expandable device may allow the surgeon more control of lordotic correction within the operating room than a static spacer alone.

Kinematics of a selectively constrained radiolucent anterior lumbar disc: comparisons to hybrid and circumferential fusion.

BACKGROUND: Despite encouraging clinical outcomes of one-level total disc replacements reported in literature, there is no compelling evidence regarding the stability following two-level disc replacement and hybrid constructs. The current study is aimed at evaluating the multidirectional kinematics of a two-level disc arthroplasty and hybrid construct with disc replacement adjacent to rigid circumferential fusion, compared to two-level fusion using a novel selectively constrained radiolucent anterior lumbar disc. METHODS: Nine osteoligamentous lumbosacral spines (L1-S1) were tested in the following sequence: 1) Intact; 2) One-level disc replacement; 3) Hybrid; 4) Two-level disc replacement; and 5) Two-level fusion. Range of motion (at both implanted and adjacent level), and center of rotation in sagittal plane were recorded and calculated. FINDINGS: At the level of implantation, motion was restored when one-level disc replacement was used but tended to decrease with two-level disc arthroplasty. The findings also revealed that both one-level and two-level disc replacement and hybrid constructs did not significantly change adjacent level kinematics compared to the intact condition, whereas the two-level fusion construct demonstrated a significant increase in flexibility at the adjacent level. The location of center of rotation in the sagittal plane at L4-L5 for the one-level disc replacement construct was similar to that of the intact condition. INTERPRETATION: The one-level disc arthroplasty tended to mimic a motion profile similar to the intact spine. However, the two-level disc replacement construct tended to reduce motion and clinical stability of a two-level disc arthroplasty requires additional investigation. Hybrid constructs may be used as a surgical alternative for treating two-level lumbar degenerative disc disease.

Pedicle screw instrumentation of thoracolumbar burst fractures: Biomechanical evaluation of screw configuration with pedicle screws at the level of the fracture.

BACKGROUND: Posterior fixation alone may not be adequate to achieve and maintain burst fracture reduction. Adding screws in the fractured body may improve construct stiffness. This in vitro study evaluates the biomechanical effect of inserting pedicle screws in the fractured body compared with conventional short- and long-segment posterior fixation. METHODS: Stable and unstable L2 burst fractures were created in 8 calf spines (aged 18 weeks). Constructs were tested at 8 Nm in the intact state and then with instrumentation consisting of long- and short-segment posterior fixation with and without screws in the fractured L2 vertebral body after (1) stable burst fracture and (2) unstable burst fracture. Range of motion was recorded at L1-3 for flexion-extension, lateral bending, and axial rotation. Statistical analysis was performed with repeated-measures analysis of variance, with significance set at P < .05. The data were normalized to the intact state (100%). RESULTS: Both long- and short-segment constructs with screws in the fractured body significantly reduced motion compared with the stable and unstable burst fracture in flexion-extension and lateral bending. Fracture screws enhanced construct stability by 68% (on average) relative to conventional short-segment posterior fixation and were comparable to long-segment posterior fixation. CONCLUSIONS: Screws at the fracture level improve construct stiffness. Short-segment constructs may suffice for stable burst fractures. More severe injuries may benefit from fracture screws and can be considered as an alternative treatment to long-segment constructs.