A method to measure cervical spine motion over extended periods of time

STUDY DESIGN

System validation study.

OBJECTIVE

To develop and validate a motion sensor system for measuring cervical spine motion over extended time periods.

SUMMARY OF BACKGROUND DATA

Many studies using different methodologies have tried to estimate cervical spine motion. These have mostly been carried out in a laboratory setting performing active/passive range of motion or activities of daily living. However, cervical spine performance over extended periods of time in natural environments remains unknown.

METHODS

A novel motion sensor system, Wisconsin Analysis of Spine Motion Performance (WASP), was validated using 2 benchmarks: a materials testing machine (MTS) and optical motion tracking laboratory. Parameters tested included drift, frequency response, accuracy, effect of sensor orientation, and coupled motions. Applied motions from the MTS and measured motions in subject volunteers under various conditions were compared with WASP using correlation coefficients. Intersubject and intrasubject variability analyses for WASP were also performed.

RESULTS

The average WASP slopes for accuracy (compared with MTS) in flexion-extension, lateral bending, and axial rotation were 0.89, 0.93, and 0.38, respectively. The correlation coefficient was 0.99 in all cases. Compared with optical motion tracking, the WASP regression slopes were 1.1, 1.02, and 0.4 and the correlation coefficients were 0.98, 0.92, and 0.93 in the 3 axes of motion. Coupled motion was noted during all subject motions. WASP peak detection algorithm had a 0% error discounting boundary conditions.

CONCLUSION

WASP was accurate in flexion-extension and lateral bending. In axial rotation, WASP was less accurate. However, the system was highly reliable with low intersubject and intrasubject variability. WASP can be used in estimating cervical spine motion with high reliability while keeping in mind the decreased accuracy in measuring axial rotation.

Assessment of injury to the thoracolumbar posterior ligamentous complex in the setting of normal-appearing plain radiography

BACKGROUND CONTEXT

The posterior ligamentous complex (PLC) is thought to contribute significantly to the stability of thoracolumbar spine. Obvious translation or dislocation of an interspace clearly denotes injury to the PLC. A recent survey of the Spine Trauma Study Group indicated that plain radiographic findings, if present, are most helpful in determining PLC injury. However, confusion exists when plain radiography shows injury to the anterior spinal column without significant kyphosis or widening of the posterior interspinous space.

PURPOSE

The objective of this study is to identify imaging parameters that may suggest a disruption of the posterior ligamentous complex of the thoracolumbar spine in the setting of normal-appearing plain radiographs. This study was performed, in part, as a pilot study to determine critical imaging parameters to be included in a future prospective, randomized, multicenter study.

STUDY DESIGN/SETTING

Survey analysis of the Spine Trauma Study Group.

PATIENT SAMPLE

None.

OUTCOME MEASURES

Compilation and statistical analysis of survey results.

METHODS

Based on a systematic review of the English literature from 1949 to present, we identified a series of traits not found on plain X-rays that were consistent with PLC injury. This included five imaging findings on either computed tomography (CT) scans or magnetic resonance imaging (MRI) and several physical examination features. These items were placed on a survey and sent to the members of the Spine Trauma Study Group. They were asked to rank the items from most important to least important in representing an injury to the PLC in the setting of normal-appearing plain radiographs.

RESULTS

Thirty-three of 47 surveys were returned for final analysis. Thirty-nine percent (13/33) of the members ranked "disrupted PLC components (i.e., interspinous ligament, supraspinous ligament, ligamentum flavum) on T1 sagittal MRI" as the most important factor in determining disruption of PLC. When analyzed with a point-weighted system, "diastasis of the facet joints on CT" received the most points, indicating that this category was ranked high by the majority of the members of the group. The members were also given freedom to add other criteria that they believed were important in determining PLC integrity in the setting of normal-appearing plain radiograph. Of the other criteria suggested, one included a physical finding and the other a variant of MR sequencing.

CONCLUSIONS

In a setting of normal-appearing plain radiographs, PLC injury as displayed on T1-weighted MRI and diastasis of the facet joints on CT scan seem to be the most popular determinants of probable PLC injury among members of the Spine Trauma Study Group. Between MRI and CT scan, most members feel that various characteristics on MRI studies were more helpful.

Cervical spine injury severity score. Assessment of reliability

BACKGROUND

Systems for classifying cervical spine injury most commonly use mechanistic or morphologic terms and do not quantify the degree of stability. Along with neurologic function, stability is a major determinant of treatment and prognosis. The goal of our study was to investigate the reliability of a method of quantifying the stability of subaxial (C3-C7) cervical spine injuries.

METHODS

A quantitative system was developed in which an analog score of 0 to 5 points is assigned, on the basis of fracture displacement and severity of ligamentous injury, to each of four spinal columns (anterior, posterior, right pillar, and left pillar). The total possible score thus ranges from 0 to 20 points. Fifteen examiners assigned scores after reviewing the plain radiographs and computed tomography images of thirty-four consecutive patients with cervical spine injuries. The scores were then evaluated for interobserver and intraobserver reliability with use of intraclass correlation coefficients.

RESULTS

The mean intraobserver and interobserver intraclass correlation coefficients for the fifteen reviewers were 0.977 and 0.883, respectively. Association between the scores and clinical data was also excellent, as all patients who had a score of > or =7 points had surgery. Similarly, eleven of the fourteen patients with a score of > or =7 points had a neurologic deficit compared with only three of the twenty with a score of <7 points.

CONCLUSIONS

The Cervical Spine Injury Severity Score had excellent intraobserver and interobserver reliability. We believe that quantifying stability on the basis of fracture morphology will allow surgeons to better characterize these injuries and ultimately lead to the development of treatment algorithms that can be tested in clinical trials.

A radiostereometric analysis of the Bryan Cervical Disc prosthesis

STUDY DESIGN

Prospective, radiostereometric study of the Bryan Cervical Disc prosthesis (Medtronic Sofamor Danek, Memphis, TN) for the treatment of a single-level disc disease of the cervical spine.

OBJECTIVE

To study the stability of the titanium endplates of the disc prosthesis at the interface between the bone and prosthesis.

SUMMARY OF BACKGROUND DATA

Cervical disc prosthesis is a motion-sparing technology in which the longevity is dependent on initial fixation as well as secondary fixation with bone ingrowth into the surface of the device. Little is known regarding the stability of this fixation with the currently studied prosthesis.

METHODS

Eleven patients with symptomatic cervical radiculopathy underwent implantation of a radiostereometry modified Bryan prosthesis after a standard anterior cervical discectomy. The adjacent vertebrae were perioperatively marked with tantalum markers. The patients were then frequently studied with radiostereometric radiographs and evaluated for pain, and neurologic and physical function for 2 years.

RESULTS

Both titanium endplates of the device were immediately stable in 5 patients. Small but measurable changes (micromotions) were recorded in 4 patients until the 3-month examination and in 2 patients at the 3-6-month time period. All prostheses continued to be stable after 6 months until the final assessment at 2 years after surgery. The recorded micromotions did not influence the clinical results.

CONCLUSIONS

The Bryan prosthesis is immediately stable in many patients and is securely fixed to the bone within 3-6 months in all patients. The result of this study suggests that there is sufficient bone ingrowth on the coated surface of the Bryan prosthesis endplates to stabilize securely the prosthesis.

Magnetic resonance imaging clarity of the Bryan, Prodisc-C, Prestige LP, and PCM cervical arthroplasty devices

STUDY DESIGN

Prospective, randomized, controlled and double-blinded study on imaging of artificial discs.

OBJECTIVE

The purpose of this study is to compare postoperative imaging characteristics of the 4 currently available cervical arthroplasty devices at the level of implantation and at adjacent levels.

SUMMARY OF BACKGROUND DATA

Cervical arthroplasty is being performed increasingly frequently for degenerative disc disease and, in most cases, with frank neural compression. Unlike lumbar arthroplasty, performed mainly for axial back pain, decompression of neural elements may need to be confirmed with postoperative imaging after cervical arthroplasty.

METHODS

Preoperative and postoperative magnetic resonance imaging scans of 20 patients who had undergone cervical arthroplasty were assessed for imaging quality. Five cases each of the Bryan (Medtronic Sofamor Danek, Memphis, TN), Prodisc-C (Synthes Spine, Paoli, PA), Prestige LP (Medtronic Sofamor Danek), and PCM devices (Cervitech, Rockaway, NJ) were analyzed. Six blinded spinal surgeons scored twice sagittal and axial T2-weighted images using the Jarvik 4-point scale. Statistical analysis was performed comparing quality before surgery and after disc implantation at the operated and adjacent levels and between implant types. RESULTS.: Moderate intraobserver and interobserver reliability was noted. Preoperative images of patients in all implant groups had high-quality images at operative and adjacent levels. The Bryan and Prestige LP devices allowed satisfactory visualization of the canal, exit foramina, cord, and adjacent levels after arthroplasty. Visualization was significantly impaired in all PCM and Prodisc-C cases at the operated level in both the spinal canal and neural foramina. At the adjacent levels, image quality was statistically poorer in the PCM and Prodisc-C than those of Prestige LP or Bryan. CONCLUSIONS.: Postoperative visualization of neural structures and adjacent levels after cervical arthroplasty is variable among current available devices. Devices containing nontitanium metals (cobalt-chrome-molybdenum alloys in the PCM and Prodisc-C) prevent accurate postoperative assessment with magnetic resonance imaging at the surgical and adjacent levels. Titanium devices, with or without polyethylene (Bryan disc or Prestige LP), allow for satisfactory monitoring of the adjacent and operated levels. This information is crucial for any surgeon who wishes to assess adequacy of neural decompression and where monitoring of adjacent levels is desired.

Contemporary posterior occipital fixation

Occipitocervical fixation is technically demanding but necessary in many clinical scenarios where junctional occiptocervical instability is present. The surgeon must have a thorough knowledge of the associated anatomy, biomechanics of spinal instrumentation, and familiarity with an ever-growing number of stabilization techniques and implants. The nature of the injury, the patient's anatomy, and the quality of the host bone will ultimately determine which form of fixation is optimal. Although the contemporary modular systems, at first glance, appear to add significant surgical complexity, in truth the designs actually simplify the process by allowing the surgeon to place occipital and spinal anchors in optimal anatomic locations.

Update on cervical artificial disk replacement

Cervical disk arthroplasty, one of the emerging motion-sparing technologies, is currently undergoing evaluation in the United States as an alternative to arthrodesis for the treatment of cervical radiculopathy and myelopathy. With both arthrodesis and arthroplasty, the primary surgical goal is thorough decompression of neurocompressive pathology--directly by removal of osteophyte and disk and indirectly by disk distraction. There is, however, one principal difference between arthrodesis and arthroplasty. With a solid fusion, resorption of osteophytes (in accordance with Wolff's law) further enhances decompression. In contrast, osteophyte resorption will not occur with motion-preserving arthroplasty. There are many challenges when deciding between arthrodesis and arthroplasty. Prosthetic performance demands exacting implantation techniques to ensure correct placement, thus placing increasing demands on special instrumentation and surgical skills. It is also important to understand the tribology (the study of prosthetic lubrication, wear, and biologic effects) of disk arthroplasty and to be familiar with currently available information regarding kinematics, basic science, testing, and early clinical results.

Intraoperative neurophysiologic monitoring: focus on cervical myelopathy and related issues

BACKGROUND CONTEXT

The use of neurophysiologic monitoring during surgical procedures for cervical spondylotic myelopathy (CSM) is controversial.

PURPOSE

The aim of this article is to review the literature regarding various monitoring techniques as applied to the patient with CSM.

STUDY DESIGN/METHODS

A systematic literature review.

CONCLUSIONS

Neurophysiologic monitoring is a diagnostic tool for assessment of neurologic function during cervical spine surgery. Recording of somatosensory evoked potentials (SSEPs), transcranial electrical motor evoked potentials (tceMEPs), and electromyograms (EMGs) may be useful as these monitoring modalities provide complementary information.

Work status as a predictor of surgical outcome of discogenic low back pain

STUDY DESIGN

Prospective observational study.

OBJECTIVES

To evaluate the role of work status as a predictor of outcome from anterior lumbar fusion.

SUMMARY OF BACKGROUND DATA

Many psychosocial factors have been identified as predictors of chronic disability and of outcomes of surgery. Workers' Compensation and job satisfaction are two of the strongest and most evaluated factors. Work status at the time of intervention may also be relevant but has rarely been studied independently in patients having lumbar fusion.

METHODS

A total of 106 patients with discogenic low back pain were treated by anterior lumbar interbody fusion. Patients were prospectively monitored by VAS, Roland Morris score, and work status. The influence of preoperative work status on outcome variables was assessed using odds ratios. A multivariate analysis was performed to assess influence of other confounding variables. Follow-up was a mean 29.7 months with 95% greater than 1 year.

RESULTS

Patients working at the time of surgery had a 10.5 times greater likelihood of working at follow-up. Overall, only 43% of nonworkers were working at follow-up compared with 90% of patients who were working before surgery. This association was independent of Workers' Compensation, number of levels treated, and other demographic variables. A greater degree of pain relief was seen in patients working before surgery but not in function as measured by the Roland Morris score.

CONCLUSION

These results show that patients with chronic low back pain should be encouraged to continue working up until surgery.

Assessment of injury to the posterior ligamentous complex in thoracolumbar spine trauma

BACKGROUND CONTEXT

Posterior ligamentous complex (PLC), consisting of supraspinous ligament (SSL), interspinous ligament (ISL), ligamentum flavum (LF), and the facet joint capsules is thought to contribute significantly to the stability of thoracolumbar spine. Currently, no consensus exists on radiographic imaging parameters that may indicate injury to the posterior ligamentous complex.

PURPOSE

To identify imaging parameters that may suggest a disruption of the PLC of the thoracolumbar spine.

STUDY DESIGN/SETTING

A survey analysis of members of the Spine Trauma Study Group.

PATIENT SAMPLE

None.

OUTCOMES MEASURES

Compilation of survey results.

METHODS

An extensive review of the literature from 1949 to the present was performed to identify key radiographic elements that have been suggested as indicators of PLC injury. Twelve items identified as such were placed on a survey and sent to the members of the Spine Trauma Study Group. They were asked to rank the items from most important to least important, and the results were compiled for analysis.

RESULTS

Twenty-eight surveys were returned for final analysis. Fifty-percent (14/28) of the members ranked "vertebral body translation" on plain radiographs as the most important factor in determining disruption of PLC. Plain radiographic signs were ranked higher than computed tomography or magnetic resonance imaging indicators, and history of the mechanism ranked lowest. The members were also given freedom to add other criteria that they felt were important in determining PLC integrity. "Interspinous spacing 7 mm greater than that of level above or below on antero posterior plain X-rays" was the only new category that was suggested.

CONCLUSION

Plain radiographic findings were felt to be most helpful in determining PLC injury by the members of the Spine Trauma Study Group. Physical examination findings and history of the mechanism of injury were ranked lower than imaging studies. Future analysis should focus on indicators of PLC injury when plain radiographic findings are either subtle or not present.

Biomechanical comparison of four C1 to C2 rigid fixative techniques: anterior transarticular, posterior transarticular, C1 to C2 pedicle, and C1 to C2 intralaminar screws

OBJECTIVE

We performed a biomechanical comparison of several C1 to C2 fixation techniques including crossed laminar (intralaminar) screw fixation, anterior C1 to C2 transarticular screw fixation, C1 to 2 pedicle screw fixation, and posterior C1 to C2 transarticular screw fixation.

METHODS

Eight cadaveric cervical spines were tested intact and after dens fracture. Four different C1 to C2 screw fixation techniques were tested. Posterior transarticular and pedicle screw constructs were tested twice, once with supplemental sublaminar cables and once without cables. The specimens were tested in three modes of loading: flexion-extension, lateral bending, and axial rotation. All tests were performed in load and torque control. Pure bending moments of 2 nm were applied in flexion-extension and lateral bending, whereas a 1 nm moment was applied in axial rotation. Linear displacements were recorded from extensometers rigidly affixed to the C1 and C2 vertebrae. Linear displacements were reduced to angular displacements using trigonometry.

RESULTS

Adding cable fixation results in a stiffer construct for posterior transarticular screws. The addition of cables did not affect the stiffness of C1 to C2 pedicle screw constructs. There were no significant differences in stiffness between anterior and posterior transarticular screw techniques, unless cable fixation was added to the posterior construct. All three posterior screw constructs with supplemental cable fixation provide equal stiffness with regard to flexion-extension and axial rotation. C1 lateral mass-C2 intralaminar screw fixation restored resistance to lateral bending but not to the same degree as the other screw fixation techniques.

CONCLUSION

All four screw fixation techniques limit motion at the C1 to 2 articulation. The addition of cable fixation improves resistance to flexion and extension for posterior transarticular screw fixation.

Biomechanical and clinical evaluation of a novel technique for surgical repair of spondylolysis in adolescents

STUDY DESIGN

A biomechanical comparison of a novel spondylolysis repair technique method to established spondylolysis repair constructs accompanied by a case series of 5 adolescent patients with spondylolysis treated with this technique.

OBJECTIVES

To provide retrospective data on these patients treated with an intralaminar link construct, as well as present comparative biomechanical data on this construct and that of an intact spine, lytic spine, and known methods of repair.

SUMMARY OF BACKGROUND DATA

Spondylolysis is a unilateral or bilateral defect in the pars interarticularis, and most commonly occurs at L5 and less frequently at L4. Surgical treatment may be indicated in patients with persistent pain and impairment despite conservative treatment. Unlike posterolateral fusion, direct pars repairs may benefit adolescents by sparing motion segments.

METHODS

Biomechanical testing of 5 cadaver lumbar spines was completed to provide comparative biomechanical data on this intralaminar link construct, and that of an intact spine, destabilized spondylolytic spine, Scott wiring technique, pedicle screw-cable system, and the pedicle screw-rod-hook constructs. There were 5 adolescent patients with spondylolysis treated with multiaxial pedicle screws, with a modular linkage that passes beneath the spinous process of the same segment.

RESULTS

Biomechanical evaluation of the intralaminar link construct showed the least displacement (across pars defect) followed by the pedicle screw-rod-hook system, Scott wiring, and, finally, the pedicle screw-cable system. Interbody flexion and extension stiffness was highest for pedicle screw-rod-hook followed by normal, intralaminar link construct, Scott wiring, pedicle screw-cable system, and, finally, the destabilized spine. Interbody torsional stiffness was highest for pedicle screw-rod-hook followed by intralaminar link construct, normal, pedicle screw-cable system, Scott wiring, and lytic spine. Retrospective follow-up (average 4.6 years) of patients showed complete radiographic healing or pain relief in all 5 patients. There was 1 patient with solid healing of L3 spondylolysis who later had onset pain despite evidence of solid fusion on computerized tomographic images. In this patient, pain resolved after implant removal.

CONCLUSIONS

Biomechanical evaluation of the intralaminar link construct showed excellent stability of a spondylolytic defect in comparison to established methods. Clinical follow-up of this method reflects the results of biomechanical testing with excellent clinical results.

The diagnostic and predictive value of hip anesthetic arthrograms in selected patients before total hip arthroplasty

Concurrent osteoarthritis of the hip and lumbar spine occurs frequently. Our study tests the hypothesis that hip anesthetic arthrograms can be used as predictive diagnostic tool before total hip arthroplasty when standard evaluation techniques fail to provide convincing evidence of the source of pain. Thirty-four consecutive hip anesthetic arthrograms were reviewed retrospectively. Quantified outcome measures included Visual Analog Pain Score, Harris Hip Score, and patient satisfaction. The pain relief after hip anesthetic arthrogram accurately predicted pain relief after hip arthroplasty (positive predictive value = 95.23%, negative predictive value = 87.5%). Our study supports the selected use of hip anesthetic arthrograms in the preoperative assessment of patients with concurrent hip and lumbar spine osteoarthritis associated with nondiagnostic history and physical examinations.

Treatment of neurogenic claudication by interspinous decompression: application of the X STOP device in patients with lumbar degenerative spondylolisthesis

OBJECT

Interspinous process decompression (IPD) theoretically relieves narrowing of the spinal canal and neural foramen in extension and thus reduces the symptoms of neurogenic intermittent claudication (NIC). The purpose of this study was to compare the efficacy of IPD with nonoperative treatment in patients with NIC secondary to degenerative spondylolisthesis.

METHODS

The authors conducted a randomized controlled study in patients with NIC; they compared the results obtained in patients treated with the X STOP IPD device with those acquired in patients treated nonoperatively. The X STOP implant is a titanium alloy device that is placed between the spinous processes to reduce the canal and foraminal narrowing that occurs in extension. In a cohort of 75 patients with degenerative spondylolisthesis, 42 underwent surgical treatment in which the X STOP IPD device was placed and 33 control individuals were treated nonoperatively. Patients underwent serial follow-up evaluations. The Zurich Claudication Questionnaire (ZCQ), 36-Item Short Form Health Survey (SF-36), and radiographic assessment were used to determine outcomes. Two-year follow-up data were obtained in 70 of 75 patients. Statistically significant improvement in ZCQ and SF-36 scores was seen in X STOP device-treated patients but not in the nonoperative control patients at all postoperative intervals. Overall clinical success occurred in 63.4% of X STOP device-treated patients and only 12.9% of controls. Spondylolisthesis and kyphosis were unaltered.

CONCLUSIONS

The X STOP device was more effective than nonoperative treatment in the management of NIC secondary to degenerative lumbar spondylolisthesis.

Classification of lower cervical spine injuries

STUDY DESIGN

Blinded assessment by multiple observers of consecutive case series.

OBJECTIVES

Measure the reliability of a new system of determining stability in subaxial cervical spine injuries.

SUMMARY OF BACKGROUND DATA

Classification is fundamental to allow communication, determine prognosis, and direct treatment. Current systems have many limitations, including difficultly of use, lack of proven reliability and validity, and no assessment of stability. A new system to assess instability is proposed.

METHODS

A literature review of the most commonly described classification systems is reported. The Cervical Spine Injury Severity Score was tested for reliability by 10 examiners who graded 35 consecutive cases of cervical trauma. Plain radiographs and CT were saved as read using Efilm Lite in random order. Each was scored and intraobserver and interobserver agreement was measured using intraclass correlation coefficients (ICC).

RESULTS

Intraobserver agreement was excellent with ICC ranging from 0.97 to 0.99. Interobserver agreement was also excellent with mean 0.80 ranging from 0.75 to 0.98.

CONCLUSION

A new cervical spine classification system of injury is paramount to treatment and outcomes. A new system may increase reliability and therefore allow more accurate determination of stability and dictate treatment.

Radiographic measurement techniques for sacral fractures consensus statement of the Spine Trauma Study Group

STUDY DESIGN

Literature review.

OBJECTIVES

Members of the Spine Trauma Study Group evaluated various imaging methods and compiled standardized approaches to making radiographic measurements for the assessment of sacral fractures.

SUMMARY OF BACKGROUND DATA

The diagnosis of sacral fractures is frequently missed or delayed, and the treatment is controversial, with significant variations in recommendations regarding nonoperative treatment, neural decompression, and internal fixation. A paucity of specific radiographic measurements and criteria exist to aid the clinician in making sound treatment decisions. This may obligate surgeons to base clinical decisions on nonstandardized, nonvalidated recommendations and measures and, in some cases, anecdotal evidence. We think that a critical first step toward valid and optimal clinical decision-making requires an accurate and clear definition of imaging measurement parameters used to evaluate sacral injuries and gather data for future studies.

METHODS

A review of the English-speaking literature was performed to assess cited radiographic measurement techniques of sacral fractures currently in use. This allowed the formulation of detailed radiographic assessments designed to more reliably describe sacral fracture morphometry.

RESULTS

These measurements include: anterior-posterior sacral fracture displacement (axial computed tomography [CT] of the pelvis), vertical sacral fracture displacement (coronal CT reconstruction), anterior-posterior translation and kyphotic angulation (sagittal CT reconstructions), and degree of central canal involvement and foraminal encroachment (axial CT with fine cuts and coronal and sagittal reconstructions). Other radiographic factors that should be considered that may impact treatment outcomes include the level and type of sacral fracture, lumbosacral junction and sacroiliac joint involvement, and associated pelvic ring injury.

CONCLUSIONS

With adoption of these radiographic guidelines, future studies will have a uniform method in which to describe sacral injuries and therefore allow study of the efficacy of various recommended treatment protocols.

Biomechanics of occipitocervical fixation

STUDY DESIGN

A human cadaveric biomechanical study comparing occipital fixation techniques.

OBJECTIVES

To compare ranges of motion between midline and lateral occipital fixation and between rigid and nonrigid occipital fixation of an unstable craniocervical spine.

SUMMARY OF BACKGROUND DATA

New fixation techniques using rods and screws increase surgical choice on where fixation is placed onto the occiput. Lateral fixation theoretically gives improved resistance to deformation because of its increased effective moment arm and bilateral purchase. Midline fixation allows significantly longer screw purchase. This study compares these two fixation location.

METHODS

Cadaveric occipital cervical spine specimens were tested biomechanically intact and under six different fixation techniques. Range of motion between the skull and C2 at 1.5 N-m and 2 N-m bending moments was measured in flexion-extension, lateral bending, and axial rotation. Mechanical testing of different rod diameters and a reconstruction plate was performed and compared with biomechanical testing. Results were compared between the intact condition and all fixations, between the medial and lateral fixations, and between the rigid and nonrigid fixations by analysis of variance.

RESULTS

The range of motion of all constructs was significantly reduced compared with intact. Significant differences between groups were only seen in lateral bending in fixation placed laterally. Mechanical testing demonstrated that construct stiffness was predicted by area moment of inertia of the rod and plate to a greater degree than variation in placement of occipital screws or locking of the implant.

CONCLUSION

The choice of location of occipital fixation should be based more on the ease of use and instability pattern. The decreased stiffness of the newer small rod systems should be considered.

Silver nitrate in silver zeolite A: three-dimensional incommensurate guest ordering in a zeolite framework

We report the results of a detailed examination of the occlusion of silver nitrate in silver zeolite A (AgA). The superlattice reported to occur in (AgNO3)9-AgA was found to melt at between 80 and 100 degrees C on heating and reappear when the sample was cooled down to 80 degrees C. Annealing in this temperature range and rigorous exclusion of water produced an enhancement of the superlattice peaks, which results from ordering of the contents of the zeolite cages. Peaks assigned to the superlattice were indexed with the tetragonal lattice parameters a = 17.440(5) and c = 12.398(4) A and proposed space group P4/nmm. The sharp peaks representing the lattice of the framework (a = 12.3711(5) A, Pm3m) remained largely unaffected by the guest in this compound, which was found to exhibit strong negative thermal expansion. The host and guest lattices are incommensurate with the tetragonal guest lattice being slightly larger than the cubic host in the c-direction and slightly smaller in the a- and b-directions.

The value of T2 relaxation times to characterize lumbar intervertebral disks: preliminary results

BACKGROUND AND PURPOSE

The present standard for staging intervertebral disk degeneration is a discrete scale, consisting usually of 5 stages. The purpose of this pilot study was to investigate the use of T2 measurements as a continuous measure of intervertebral disk degeneration.

METHODS

We obtained images in 5 volunteers with a 3D fast spin-echo sequence modified for the purpose of calculating T2 relaxation times from multiple echoes in the echo train. Disks were classified on the basis of conventional criteria into one of the 5 stages of disk degeneration. Average T2 values were calculated for stage II, III, and V disks, which were identified in the volunteers. Differences between the disk levels were analyzed with analysis of variance and differences between stages tested with a Student t test with significance set at the 0.01 level.

RESULTS

In the 5 volunteers, 20 stage II, 4 stage III, and a single stage V disk were found. Contour plots showed the highest T2 values in the nucleus pulposus near the vertebral endplates and lower T2 values in the intranuclear cleft region and peripheral annulus fibrosus. Average T2 values were significantly lower in the type III and V disks than in the normal disks.

CONCLUSIONS

The study suggests that intervertebral disks can be characterized and classified accurately by means of T2 values. More studies are warranted to determine the range of T2 values for normal disks.

Direct Observation of Defect Levels in InN by Soft X-ray Absorption Spectroscopy

We have used synchrotron-based near-edge X-ray absorption fine structure (NEXAFS) spectroscopy to study the electronic structure of nitrogen-related defects in InN(0001). Several defect levels within the band gap or the conduction band of InN were clearly resolved in NEXAFS spectra around the nitrogen K-edge. We attribute the level observed at 0.3 eV below the conduction band minimum (CBM) to interstitial nitrogen, the level at 1.7 eV above the CBM to antisite nitrogen, and a sharp resonance at 3.2 eV above the CBM to molecular nitrogen, in full agreement with theoretical simulations.

Synthesis and crystal structure of Li4BH4(NH2)3

The solid solution, (LiNH2)x(LiBH4)(1-x), formed through the reaction of the two potential hydrogen storage materials, LiNH2 and LiBH4, is dominated by a compound that has an ideal stoichiometry of Li4BN3H10 and forms a body-centred cubic structure with a lattice constant of ca. 10.66 A.