Anterior Fixation of Floating Facet Fractures in the Cervical Spine: A Prospective Case Series and Biomechanical Analysis.

BACKGROUND: Unilateral fractures involving complete separation of the lateral mass from the vertebra and lamina (floating lateral mass fractures) are a unique subset of cervical spine fractures. These injuries are at significant risk for displacement without operative fixation. Posterior fixation has proven to facilitate adequate fusion. However, there are few data supporting the clinical success of single-level anterior fixation. METHODS: Biomechanical evaluation of floating lateral mass fractures and a consecutive case series of patients with rotationally unstable floating lateral mass fractures treated with anterior fixation using an integrated cage-screw device with anterior plating (ICSD) was performed. The study comprised 7 fresh human cadaver cervical spines (C2-C7), and 11 patients with floating lateral mass fractures. Segmental flexibility testing evaluating axial rotation, flexion/extension, and lateral bending was performed in a cadaveric model after 2 types of single-level anterior fixation and 1 type of 2-level posterior fixation. Eleven patients with a floating lateral mass fracture of the cervical spine underwent anterior fixation with an ICSD. Radiographs and clinical outcomes were retrospectively reviewed. RESULTS: Compared with the intact condition, posterior instrumentation significantly (P < .05) reduced range of motion (ROM) in all 3 planes; anterior fixation with cervical plate and interbody spacer significantly reduced ROM in lateral bending only; and the ICSD significantly reduced ROM in flexion/extension and lateral bending. In the clinical arm, there were no long-term complications, subsidence >2 mm, failure of fixation, reoperation, pseudoarthrosis, or listhesis at final follow-up. CONCLUSIONS: The addition of 2 screws placed through a cervical cage can improve anterior fixation in a human cadaveric model of floating lateral mass fractures. Early clinical results demonstrate a low complication rate and a high rate of healing with single-level anterior fixation using this technique.

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.

Axial pullout strength comparison of different screw designs: fenestrated screw, dual outer diameter screw and standard pedicle screw.

BACKGROUND: The pullout strength of pedicle screws is influenced by many factors, including diameter of the screws, implant design, and augmentation with bone cement such as PMMA. In the present study, the pullout strength of an innovative fenestrated screw augmented with PMMA was investigated and was compared to unaugmented fenestrated, standard and dual outer diameter screw. METHODS: Twenty four thoracolumbar vertebrae (T10-L5, age 60 to 70 years) from three cadavers were implanted with the four different pedicle screws. Twelve screws of each type were instrumented into either left or right pedicle with standard screw paired with unaugmented and dual outer diameter screw paired with augmented fenestrated screw in any given vertebra. Axial pullout testing was conducted at a rate of 5 mm/min. Force to failure (Newtons) for each pedicle screw was recorded. RESULTS: The augmented fenestrated screws had the highest pullout strength, which represented an average increase of 149%, 141%, and 78% in comparison to unaugmented, standard, and dual outer diameter screws, respectively. Pullout strength of unaugmented screws was comparable to that of standard screws, however it was significantly lower than dual outer diameter screws. CONCLUSIONS: Fenestrated screws augmented with PMMA improve the fixation strength and result in significantly higher pullout strength compared to dual outer diameter, standard and unaugmented fenestrated screws. Screws with dual outer diameter provided enhanced bone-screw purchase and may be considered as an alternative technique to increase the bone-screw interface in cases where augmentation using bone cement is not feasible. Unaugmented screws can be left in the pedicle even without cement and provide similar pullout strength to standard screws.

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.

Pedicle Screw Configuration for Thoracolumbar Burst Fracture Treatment: Short versus Long Posterior Fixation Constructs with and without Anterior Column Augmentation.

STUDY DESIGN: An in-vitro study. PURPOSE: 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). OVERVIEW OF LITERATURE: The ideal treatment for thoracolumbar BF is controversial regarding the use of short or LPF constructs. METHODS: 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. RESULTS: 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. CONCLUSIONS: 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.

Comparison of fatigue strength of C2 pedicle screws, C2 pars screws, and a hybrid construct in C1-C2 fixation.

STUDY DESIGN: A biomechanical study comparing the fatigue strength of different types of C2 fixation in a C1-C2 construct. OBJECTIVE: To determine the pullout strength of a C2 pedicle screw and C2 pars screw after cyclical testing and differentiate differences in stiffness pre- and post-cyclical loading of 3 different C1-C2 fixations. SUMMARY OF BACKGROUND DATA: Some surgeons use a short C2 pars screw in a C1-C2 construct, because it is less technically demanding and/or when the vertebral artery is high riding. Difference in construct stiffness between use of bilateral C2 pedicle screws, bilateral C2 pars screws, or a hybrid construct is unknown. METHODS: Biomechanical testing was performed on 15 specimens. A bicortical C1 lateral mass screw was used in combination with 1 of 3 methods of C2 fixation: (1) bilateral long C2 pedicle screws (LL), (2) bilateral 14-mm C2 pars screws (SS), and (3) unilateral long C2 pedicle screw with a contralateral 14-mm C2 pars screw (LS). Each construct was subject to 16,000 cycles to simulate the immediate postoperative period. Changes in motion in flexion-extension, lateral bending, and axial rotation were calculated. This was followed by pullout testing. RESULTS: The ability to limit range of motion significantly decreased after cyclical testing in flexion-extension, lateral bending, and axial rotation for all 3 groups. After loading, the LL and LS groups had less percentage of increase in motion in flexion-extension and lateral bending than the SS group. Overall, the average pullout strength of a pedicle screw was 92% stronger than a pars screw. CONCLUSION: C2 pedicle screws have twice the pullout strength of C2 pars screws after cyclical loading. In cases in which the anatomy limits placement of bilateral C2 pedicle screws, a construct using a unilateral C2 pedicle screw with a contralateral short pars screw is a viable option and compares favorably with a bilateral C2 pedicle screw construct. LEVEL OF EVIDENCE: N/A.

In vitro biomechanical study to quantify range of motion, intradiscal pressure, and facet force of 3-level dynamic stabilization constructs with decreased stiffness.

STUDY DESIGN: An in vitro biomechanical study. OBJECTIVE: To perform in vitro biomechanical testing on a lumbar spine using a 6-degree-of-freedom machine. To compare the range of motion (ROM), intradiscal pressure, and facet force of different 3-level dynamic stabilization constructs with traditional rigid constructs. To determine the effect of decreasing the stiffness of the dynamic construct on the various parameters. SUMMARY OF BACKGROUND DATA: Dynamic stabilization systems are a surgical option that may minimize the development of adjacent segment disease. METHODS: Seven T12-S1 specimens were tested at ± 7.5 Nm in flexion-extension, lateral bending, and axial rotation. The testing sequence was (1) intact, (2) intact with facet sensors, (3) L3-S1 rigid (3R), (4) L3-L4 dynamic and L4-S1 rigid (1D-2R A), (5) L3-L5 dynamic and L5-S1 rigid (2D-1R A), and (6) L3-S1 dynamic (3D A). Constructs 1D-2R A, 2D-1R A, and 3D A were tested again with the specialized designs of B and C of decreased stiffness. ROM, intradiscal pressure, and facet force were measured. RESULTS: In all loading modes there was a trend of increasing motion with decreased stiffness. Significant differences were seen with more dynamic stabilization levels but no significance was seen with only decreasing the stiffness. 3R facet force at the caudal instrumented level significantly decreased compared with intact and dynamic stabilization constructs during axial rotation. CONCLUSION: Biomechanical testing resulted in a trend of increased ROM across instrumented levels as the stiffness was decreased. Dynamic stabilization increased the ROM across instrumented levels compared with rigid rods. These results suggest that decreasing the stiffness of the construct may lessen the probability of adjacent-level disease. Although the specialized devices are not commercially available, clinical data would be necessary for a clearer understanding of adjacent level effects and to confirm the in vitro biomechanical findings. LEVEL OF EVIDENCE: N/A.

Biomechanical evaluation of a novel posterior integrated clamp that attaches to an existing posterior instrumentation for use in thoracolumbar revision.

STUDY DESIGN: An in vitro biomechanical study. PURPOSE: To evaluate the biomechanics of a novel posterior integrated clamp (IC) that extends on an already implanted construct in comparison to single long continuous bilateral pedicle screw (BPS) and rod stabilization system. OVERVIEW OF LITERATURE: Revision surgery in the thoracolumbar spine often necessitates further instrumentation following a failed previous back surgery. Stability of these reconstructed constructs is not known. METHODS: Six osteoligamentous T12-L5 calf spines were tested on a spine motion simulator in the following configurations: intact, four level constructs (T13-L4), three level constructs (L1-L4), and two level constructs (L2-L4), by varying the ratio between BPS and IC. A load control protocol of 8 Nm moments was applied at a rate of 1°/sec to establish the range of motion value for each construct in flexion-extension, lateral bending, and axial rotation. Statistical analysis was performed on raw data using repeated measures analysis of variance and significance was set at p<0.05. RESULTS: On an average, the reduction in motion for the four level continuous pedicle screw and rod construct (67%) was similar to those extended with integrated clamps (64%). Furthermore, for three level and two level constructs, no significant difference was observed between continuous pedicle screw constructs and those revised with the integrated clamps (regardless of the ratio between BPS and IC). CONCLUSIONS: The novel posterior IC showed equivalent biomechanical rigidity to continuous pedicle screw rod constructs in revision scenarios. Clinical studies on posterior rod adjunct systems are necessary to confirm these results.

Biomechanical comparison of spinopelvic reconstruction techniques in the setting of total sacrectomy.

STUDY DESIGN: An in vitro biomechanical study. OBJECTIVE: To biomechanically test and evaluate 4 different methods of spinopelvic reconstruction techniques and determine the most biomechanically stable construct for stabilization of the spinopelvic junction after total sacrectomy. SUMMARY OF BACKGROUND DATA: Total sacrectomy is necessary to treat a sacral tumor when it involves the S1 vertebra. Instrumentation and reconstruction of the lumbar spine and pelvis are required after total sacrectomy and can be achieved by various reconstruction techniques. Currently, the preferred method of spinopelvic fixation is controversial. METHODS: Seven human cadaveric (L1-pelvis) specimens were evaluated in flexion-extension, lateral bending, and axial rotation in a total sacrectomy model. Test constructs included (1) intact; (2) double-rod, double iliac screw (DDS); (3) single-rod, single iliac screw (SSS); (4) double iliac screw (DIS) fixation; and (5) modified Galveston technique (MGT). A load control protocol with 7.0 Nm moments applied at a rate of 1.5°/s was used to establish range of motion values for each tested construct on a 6-df spine motion simulator. Data were analyzed and normalized to intact. RESULTS: All instrumented constructs offered significant stability in all loading conditions compared with the intact condition. Stability offered by different constructs in all loading conditions trended as follows: DDS>DIS>SSS>MGT. Overall, the DDS construct provided 55%, 43%, and 60% more stability than SSS, DIS, and MGT, respectively. This was significant in flexion-extension when compared with SSS and in all loading conditions when compared with MGT. CONCLUSION: In the setting of total sacrectomy, the double-rod double iliac screw method provided the most rigid fixation, followed by DIS fixation, single-rod single screw, and the MGT. In spinopelvic reconstruction, the use of double iliac screws is recommended compared with single iliac screw fixation techniques when treating unstable conditions caused by total sacrectomy.

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.

A comparative biomechanical study of a novel integrated plate spacer for stabilization of cervical spine: an in vitro human cadaveric model.

BACKGROUND: Integrated plate-spacer may provide adequate construct stability while potentially lowering operative time, decreasing complications, and providing less mechanical obstruction. The purpose of the current study was to compare the biomechanical stability of an anatomically profiled 2-screw integrated plate-spacer to a traditional spacer only and to a spacer and anterior cervical plate construct. In addition, the biomechanical stability of 2-screw integrated plate-spacer was compared to a commercially available 4-screw integrated plate-spacer. METHODS: Two groups, each of nine cervical cadaver spines (C2-C7), were tested under pure moments of 1.5Nm. Range of motion was recorded at C5-C6 in all loading conditions (flexion, extension, lateral bending, and axial rotation) for the following constructs: 1) Intact; 2) 2-screw or 4-screw integrated plate-spacer; 3) spacer and anterior cervical plate; and 4) spacer only. FINDINGS: All fusion constructs significantly reduced motion compared to the intact condition. Within the instrumented constructs, spacer and anterior cervical plate, 2-screw and 4-screw integrated plate-spacer resulted in reduced motion compared to the spacer only construct. No significant differences were found in motion between any of the instrumented conditions in any of the loading conditions. INTERPRETATION: The application of integrated plate-spacer for anterior cervical discectomy and fusion is based on several factors including surgical ease-of-use, biomechanical characteristics, and surgeon preference. The study suggests that integrated plate-spacer provide biomechanical stability comparable to traditional spacer and plate constructs in the cervical spine. Clinical studies on integrated plate spacer devices are necessary to understand the performance of these devices in vivo.

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.

Cyclic loads do not compromise functionality of the interspinous spacer or cause damage to the spinal segment: an in vitro analysis.

STUDY DESIGN: A biomechanical study to evaluate the effects of interspinous spacer under cyclic complex loading. OBJECTIVES: To determine the risk of device migration and to assess damage on the device and specimen under extreme coupled motion. Another objective was to evaluate the effect on vertebral foramen and canal dimensions after spacer implantation. SUMMARY OF BACKGROUND DATA: Interspinous spacers are a relatively new treatment option that are clinically exposed to complex loads. However, the biomechanical performance of these spacers has not been well characterized. METHODS: Six human cadaveric motion segments were used for this study. The interspinous spacer (SuperionTM, Vertiflex Inc, California) was tested for 5 degrees extension/10 degrees flexion coupled with an axial rotation of +/-3 degrees. CT images were taken for specimens in neutral, 5 degrees extension, and 10 degrees flexion before and after the implantation of the spacer. Vertebral foramen and canal dimensions were quantified. RESULTS: There was no device migration or subsidence. Specimens did not sustain any significant injury during testing. Canal area was minimally altered and foramen height, width, and area increased in extension and were statistically significant as compared to intact. CONCLUSION: Interspinous spacer effectively prevents the motion at the implanted level and does not change the anatomy significantly.