Biomechanical comparison of anterior cervical spine instrumentation techniques with and without supplemental posterior fusion after different corpectomy and discectomy combinations: Laboratory investigation.

OBJECT: The objective of this study was to compare the stiffness and range of motion (ROM) of 4 cervical spine constructs and the intact condition. The 4 constructs consisted of 3-level anterior cervical discectomy with anterior plating, 1-level discectomy and 1-level corpectomy with anterior plating, 2-level corpectomy with anterior plating, and 2-level corpectomy with anterior plating and posterior fixation. METHODS: Eight human cadaveric fresh-frozen cervical spines from C2-T2 were used. Three-dimensional motion analysis with an optical tracking device was used to determine motion following various reconstruction methods. The specimens were tested in the following conditions: 1) intact; 2) segmental construct with discectomies at C4-5, C5-6, and C6-7, with polyetheretherketone (PEEK) interbody cage and anterior plate; 3) segmental construct with discectomy at C6-7 and corpectomy of C-5, with PEEK interbody graft at the discectomy level and a titanium cage at the corpectomy level; 4) corpectomy at C-5 and C-6, with titanium cage and an anterior cervical plate; and 5) corpectomy at C-5 and C-6, with titanium cage and an anterior cervical plate, and posterior lateral mass screw-rod system from C-4 to C-7. All specimens underwent a pure moment application of 2 Nm with regards to flexion-extension, lateral bending, and axial rotation. RESULTS: In all tested motions the statistical comparison was significant between the intact condition and the 2-level corpectomy with anterior plating and posterior fixation construct. All other statistical comparisons between the instrumented constructs were not statistically significant except between the 3-level discectomy with anterior plating and the 2-level corpectomy with anterior plating in axial rotation. There were no statistically significant differences between the 1-level discectomy and 1-level corpectomy with anterior plating and the 2-level corpectomy with anterior plating in any tested motion. There was also no statistical significance between the 3-level discectomy with anterior plating and the 2-level corpectomy with anterior plating and posterior fixation. CONCLUSIONS: This study demonstrates that segmental plate fixation (3-level discectomy) affords the same stiffness and ROM as circumferential fusion in 2-level cervical spine corpectomy in the immediate postoperative setting. This obviates the need for staged circumferential procedures for multilevel cervical spondylotic myelopathy. Given that the posterior segmental instrumentation confers significant stability to a multilevel cervical corpectomy, the surgeon should strongly consider the placement of segmental posterior instrumentation to significantly improve the overall stability of the fusion construct after a 2-level cervical corpectomy.

Chronic cigarette smoking causes hypertension, increased oxidative stress, impaired NO bioavailability, endothelial dysfunction, and cardiac remodeling in mice.

Cigarette smoking is a major independent risk factor for cardiovascular disease. While the association between chronic smoking and cardiovascular disease is well established, the underlying mechanisms are incompletely understood, partly due to the lack of adequate in vivo animal models. Here, we report a mouse model of chronic smoking-induced cardiovascular pathology. Male C57BL/6J mice were exposed to whole body mainstream cigarette smoke (CS) using a SCIREQ "InExpose" smoking system (48 min/day, 5 days/wk) for 16 or 32 wk. Age-matched, air-exposed mice served as nonsmoking controls. Blood pressure was measured, and cardiac MRI was performed. In vitro vascular ring and isolated heart experiments were performed to measure vascular reactivity and cardiac function. Blood from control and smoking mice was studied for the nitric oxide (NO) decay rate and reactive oxygen species (ROS) generation. With 32 wk of CS exposure, mice had significantly less body weight gain and markedly higher blood pressure. At 32 wk of CS exposure, ACh-induced vasorelaxation was significantly shifted to the right and downward, left ventricular mass was significantly larger along with an increased heart-to-body weight ratio, in vitro cardiac function tended to be impaired with high afterload, white blood cells had significantly higher ROS generation, and the blood NO decay rate was significantly faster. Thus, smoking led to blunted weight gain, hypertension, endothelial dysfunction, leukocyte activation with ROS generation, decreased NO bioavailability, and mild cardiac hypertrophy in mice that were not otherwise predisposed to disease. This mouse model is a useful tool to enable further elucidation of the molecular and cellular mechanisms of smoking-induced cardiovascular diseases.

Craniocervical fixation with occipital condyle screws: biomechanical analysis of a novel technique.

STUDY DESIGN: A human cadaveric biomechanical study comparing craniocervical fixation techniques. OBJECTIVE: To quantitatively compare the biomechanical stability of a new technique for occipitocervical fixation using the occipital condyles with an established method for craniocervical spine fusion. SUMMARY OF BACKGROUND DATA: Stabilization of the occipitocervical junction remains a challenge. The occiput does not easily accommodate instrumentation because of access and spatial constraints. In fact, the area available for the implant fixation is limited and can be restricted further when a suboccipital craniectomy has been performed, posing a challenge to current fixation techniques. Occipital screws are also associated with the potential for intracranial complications. METHODS: Six fresh frozen cadaveric specimens occiput-C4 were tested intact, after destabilization and after fixation as follows: (1) occipital plate with C1 lateral mass screws and C2 pars screws and (2) occipital condyle screws with C1 lateral mass screws and C2 pars screws. Specimens were loaded in a custom spine testing apparatus and subjected to the following tests, all performed under 50-N unconstrained axial preload: flexion, extension, lateral bending, and axial rotation at 1.5 Nm. The constructs were statistically compared with a one-way analysis of variance and compared with the intact condition. RESULTS: Motions were reduced by approximately 80% compared with the intact condition for both configurations under all motions. There were no statistically significant differences in the range of motion (ROM) between the 2 instrumentation conditions. The mean values indicated decreased ROM with the novel occipital condyle screw construct in comparison with the standard occipital plate and rod system. CONCLUSION: Craniocervical stabilization using occipital condyle screws as the sole cephalad fixation point is biomechanically equivalent with regard to the modes tested (ROM and stiffness) to the standard occipital plate construct.

Refinement of interbody implant testing in goats: a surgical and morphometric rationale for selection of a cervical level. Laboratory investigation.

OBJECT: The authors provide a surgical description of the ventral approach to the cervical spine in a goat model and identify selection of the most appropriate level for testing interbody devices. These constructs are designed for implantation in humans during anterior cervical discectomy and fusion. Such description and guidelines for level selection have never been published in either the medical or veterinarian literature. METHODS: The study comprised three phases: surgical, anatomical, and morphometric. Six goats underwent ventral approaches and were later killed; their necks were dissected and the cervical spines were processed to obtain clean specimens of the vertebral bodies. Measurements were made at each level using a contact digitizer. RESULTS: The anterolateral bone spurs, called alar processes, and the increased thickness of the longus colli muscle are the surgically relevant characteristics in the goat. The morphometric analysis showed that C2-3 is the most suitable level for implantation of interbody devices. The vertebral endplates at the C2-3 level are relatively flat and parallel to each other, and are perpendicular to the spinal canal axis. More distally, the endplates adopt a more curved arrangement, and the endplate angle becomes significantly greater than 90 degrees. The authors describe anatomical landmarks that are important to safely and effectively perform a ventral cervical spinal approach in the goat. CONCLUSIONS: The authors' model identifies C2-3 as the most appropriate level for animal testing of cervical implants because of its similarity to human anatomy. Further study with rigorous biomechanical range of motion evaluation of each caprine cervical level is needed.

In vitro biomechanical comparison of an anterior and anterolateral lumbar plate with posterior fixation following single-level anterior lumbar interbody fusion.

OBJECT: Anterior lumbar interbody fusion (ALIF) is often supplemented with instrumentation to increase stability in the spine. If anterior plate fixation provided the same stability as posterior pedicle screw fixation (PSF), then a second approach and its associated morbidity could be avoided. METHODS: Seven human cadaveric L4-5 spinal segments were tested under three conditions: ALIF with an anterior plate, ALIF with an anterolateral plate, and ALIF supplemented by PSF. Range of motion (ROM) was calculated for flexion/extension, lateral bending, and axial torsion and compared among the three configurations. RESULTS: There were no significant differences in ROM during flexion/extension, lateral bending, or axial torsion among any of the three instrumentation configurations. CONCLUSIONS: The addition of an anterior plate or posterior PS/rod instrumentation following ALIF provides substantially equivalent biomechanical stability. Additionally, the position of the plate system, either anterior or anterolateral, does not significantly affect the stability gained.

Microindentation in bone: hardness variation with five independent variables.

Microindentation is an investigational tool often used to determine hardness and other derived material properties of the material bone. This study explored the variation of microindentation hardness results with five independent variables. The variables were: applied mass, dwell time, drying time, time between indentation and measurement, and distance between the center of an indentation and the edge of other indentations and pores. These variables were selected because they represented a reasonable range of specimen investigational steps. We also investigated the cross sections of typical indentation residual impressions to determine the degree of material pile-up at the edges of the impressions. We found that microindentation hardness varied with applied mass and with distance between the indentation and neighboring indentations and pores but not with the other variables. Our recommended minimum applied mass is 0.10 kg versus a previously published value of 0.05 kg. We also found no discernable material pile-up at the residual impression edges, in contrast to reports of others.

Torsion-induced pressure distribution changes in human intervertebral discs: an in vitro study.

STUDY DESIGN: Biomechanical testing of human cadaveric lumbar specimens was performed to evaluate the effects of torsional torque on intradiscal pressure and disc height. OBJECTIVE: Evaluate the effects of small torsion torques on intradiscal pressure and disc height in human lumbar specimens. SUMMARY OF BACKGROUND DATA: Nuclear depressurization in addition to an instantaneous disc height increase were found in previous porcine research when small (<2 degrees) axial vertebral rotations were applied. If applicable to human spines, this phenomenon may support spinal manipulation for the relief of low back pain. METHODS: Six human lumbar cadaveric functional spine units (FSU) were loaded in the neutral position with 600 N axial compression. Intranuclear pressure measurements were then obtained at 0, 0.5, 1.0, and 2.0 Nm of torsion. Posterior elements were removed and measurements were repeated for the disc body unit (DBU). RESULTS: There was no statistically significant difference in nuclear pressure or intervertebral disc height with different torsion torques among or between the FSUs and DBUs. However, a disc height increase ranging from 0.13 mm to 0.16 mm occurred with the insertion of a 1.85-mm diameter pressure probe cannula. CONCLUSIONS: Small torsion torques showed no significant difference in intradiscal pressures or disc heights. This is an unlikely mechanism for the perceived benefits of spinal manipulation.

Augmentation of an anterior lumbar interbody fusion with an anterior plate or pedicle screw fixation: a comparative biomechanical in vitro study.

OBJECT: Posterior pedicle screw (PS) instrumentation is often used to augment anterior lumbar interbody fusion (ALIF) but at the cost of an increase in the morbidity rate due to the second approach and screw placement. If anterior plates were found to be biomechanically equivalent to PS fixation (PSF) after ALIF, then this second approach could be avoided without decreasing vertebral stability. METHODS: Eight cadaveric L5-S1 spinal segments were tested under four conditions: intact, following anterior discectomy and interbody spacer placement, after placement of an anterior plate, and following PSF. The elastic zone and stiffness were calculated for axial compression, flexion/extension, lateral bending, and torsion. Neither anterior plate stabilization nor PSF showed significant intergroup differences in stiffness or the elastic zone. Both exhibited greater stiffness in flexion than the intact specimens (p < 0.001). Pedicle screw fixation was associated with a decreased elastic zone in lateral bending compared with the intact specimen (p < 0.04). CONCLUSIONS: Anterior plate fixation is biomechanically similar to PSF following ALIF. Surgeons may wish to use anterior plates in place of PSs to avoid the need for a posterior procedure. This may lead to a decrease in operative morbidity and improved overall outcomes.

Load sharing in Premier and Zephir anterior cervical plates.

STUDY DESIGN: An in vitro biomechanical study using a simulated anterior cervical discectomy and interbody fusion model to compare the load sharing properties of two semiconstrained cervical (Premier and Zephir) plates. OBJECTIVES: To determine the percent load transmission through these plates and grafts under simple axial compression. SUMMARY OF BACKGROUND DATA: No published data exist as to the load transmission through these semiconstrained plates. METHODS: Cadaveric calf spines were subjected to axial compression loading while instrumented with an interbody graft and with the graft plus one of the plates. Load transmission was computed through an analysis of the load-displacement data. RESULTS: A mean load transmission of 23% was shared by the Premier plate. The Zephir, a more constrained plate but still semiconstrained, shared a mean of 32% of the load. CONCLUSIONS: The semiconstrained plates tested allow more graft loading than some previously tested constrained plates. However, there are differences between the research methods used in these studies that provide a less than satisfactory comparison.

Interbody allograft in a skeletally immature spine model.

The objective of this cadaveric biomechanical study was to establish further bovine spines as models for evaluating lumbar interbody allografts and to provide guidance for their use in pediatric humans. It is unknown whether interbody allografts can be used in the pediatric spine without failure of the host vertebral bone. Allografts were placed in cow and calf spines and loaded in compression. The cow spines were much stronger and stiffer than the calf, but moderate in vivo activities were estimated to result in loads on the allograft constructs that would result in host bone failure. Bovine spines were established as suitable models for the compressive behavior of interbody allografts in the human spine, when bone density is considered. Interbody allografts should continue to be used with adjunctive instrumentation so as to preclude host bone failure.