Management of REM sleep behavior disorder: an American Academy of Sleep Medicine systematic review, meta-analysis, and GRADE assessment.

This systematic review provides supporting evidence for a clinical practice guideline for the management of rapid eye movement (REM) sleep behavior disorder in adults and children. The American Academy of Sleep Medicine commissioned a task force of 7 experts in sleep medicine. A systematic review was conducted to identify randomized controlled trials and observational studies that addressed interventions for the management of REM sleep behavior disorder in adults and children. Statistical analyses were performed to determine the clinical significance of critical and important outcomes. Finally, the Grading of Recommendations, Assessment, Development, and Evaluation (GRADE) process was used to assess the evidence for making recommendations. The literature search identified 4,690 studies; 148 studies provided data suitable for statistical analyses; evidence for 45 interventions is presented. The task force provided a detailed summary of the evidence assessing the certainty of evidence, the balance of benefits and harms, patient values and preferences, and resource use considerations. CITATION: Howell M, Avidan AY, Foldvary-Schaefer N, et al. Management of REM sleep behavior disorder: an American Academy of Sleep Medicine systematic review, meta-analysis, and GRADE assessment. J Clin Sleep Med. 2023;19(4):769-810.

Management of REM sleep behavior disorder: an American Academy of Sleep Medicine clinical practice guideline.

INTRODUCTION: This guideline establishes clinical practice recommendations for the management of rapid eye movement sleep behavior disorder (RBD) in adults. METHODS: The American Academy of Sleep Medicine (AASM) commissioned a task force of experts in sleep medicine to develop recommendations and assign strengths based on a systematic review of the literature and an assessment of the evidence using Grading of Recommendations, Assessment, Development and Evaluation methodology. The task force provided a summary of the relevant literature and the certainty of evidence, the balance of benefits and harms, patient values and preferences, and resource use considerations that support the recommendations. The AASM Board of Directors approved the final recommendations. GOOD PRACTICE STATEMENT: The following good practice statement is based on expert consensus, and its implementation is necessary for the appropriate and effective management of patients with RBD: It is critically important to help patients maintain a safe sleeping environment to prevent potentially injurious nocturnal behaviors. In particular, the removal of bedside weapons, or objects that could inflict injury if thrown or wielded against a bed partner, is of paramount importance. Sharp furniture like nightstands should be moved away or their edges and headboard should be padded. To reduce the risk of injurious falls, a soft carpet, rug, or mat should be placed next to the bed. Patients with severe, uncontrolled RBD should be recommended to sleep separately from their partners, or at the minimum, to place a pillow between themselves and their partners. RECOMMENDATIONS: The following recommendations, with medications listed in alphabetical order, are a guide for clinicians in choosing a specific treatment for RBD in adults. Each recommendation statement is assigned a strength ("strong" or "conditional"). A "strong" recommendation (ie, "We recommend…") is one that clinicians should follow under most circumstances. A "conditional" recommendation (ie, "We suggest…") is one that requires that the clinician use clinical knowledge and experience and strongly consider the patient's values and preferences to determine the best course of action.Adult patients with isolated RBD.1. The AASM suggests that clinicians use clonazepam (vs no treatment) for the treatment of isolated RBD in adults. (CONDITIONAL).2. * The AASM suggests that clinicians use immediate-release melatonin (vs no treatment) for the treatment of isolated RBD in adults. (CONDITIONAL).3. * The AASM suggests that clinicians use pramipexole (vs no treatment) for the treatment of isolated RBD in adults. (CONDITIONAL).4. The AASM suggests that clinicians use transdermal rivastigmine (vs no treatment) for the treatment of isolated RBD in adults with mild cognitive impairment. (CONDITIONAL).Adult patients with secondary RBD due to medical condition.5. * The AASM suggests that clinicians use clonazepam (vs no treatment) for the treatment of secondary RBD due to medical condition in adults. (CONDITIONAL).6. * The AASM suggests that clinicians use immediate-release melatonin (vs no treatment) for the treatment of secondary RBD due to medical condition in adults. (CONDITIONAL).7. The AASM suggests that clinicians use transdermal rivastigmine (vs no treatment) for the treatment of secondary RBD due to medical condition (Parkinson disease) in adults. (CONDITIONAL).8. * The AASM suggests that clinicians not use deep brain stimulation (DBS; vs no treatment) for the treatment of secondary RBD due to medical condition in adults. (CONDITIONAL).Adult patients with drug-induced RBD.9. * The AASM suggests that clinicians use drug discontinuation (vs drug continuation) for the treatment of drug-induced RBD in adults. (CONDITIONAL).* The Recommendations section of this paper includes remarks that provide additional context to guide clinicians with implementation of this recommendation. CITATION: Howell M, Avidan AY, Foldvary-Schaefer N, et al. Management of REM sleep behavior disorder: an American Academy of Sleep Medicine clinical practice guideline. J Clin Sleep Med. 2023;19(4):759-768.

Guiding principles for determining work shift duration and addressing the effects of work shift duration on performance, safety, and health: guidance from the American Academy of Sleep Medicine and the Sleep Research Society.

CITATION: Risks associated with fatigue that accumulates during work shifts have historically been managed through working time arrangements that specify fixed maximum durations of work shifts and minimum durations of time off. By themselves, such arrangements are not sufficient to curb risks to performance, safety, and health caused by misalignment between work schedules and the biological regulation of waking alertness and sleep. Science-based approaches for determining shift duration and mitigating associated risks, while addressing operational needs, require: (1) a recognition of the factors contributing to fatigue and fatigue-related risks; (2) an understanding of evidence-based countermeasures that may reduce fatigue and/or fatigue-related risks; and (3) an informed approach to selecting workplace-specific strategies for managing work hours. We propose a series of guiding principles to assist stakeholders with designing a shift duration decision-making process that effectively balances the need to meet operational demands with the need to manage fatigue-related risks.

Guiding principles for determining work shift duration and addressing the effects of work shift duration on performance, safety, and health: guidance from the American Academy of Sleep Medicine and the Sleep Research Society.

Risks associated with fatigue that accumulates during work shifts have historically been managed through working time arrangements that specify fixed maximum durations of work shifts and minimum durations of time off. By themselves, such arrangements are not sufficient to curb risks to performance, safety, and health caused by misalignment between work schedules and the biological regulation of waking alertness and sleep. Science-based approaches for determining shift duration and mitigating associated risks, while addressing operational needs, require: (1) a recognition of the factors contributing to fatigue and fatigue-related risks; (2) an understanding of evidence-based countermeasures that may reduce fatigue and/or fatigue-related risks; and (3) an informed approach to selecting workplace-specific strategies for managing work hours. We propose a series of guiding principles to assist stakeholders with designing a shift duration decision-making process that effectively balances the need to meet operational demands with the need to manage fatigue-related risks.

Motion response of a polycrystalline diamond adaptive axis of rotation cervical total disc arthroplasty.

BACKGROUND: Cervical fusion is associated with adjacent segment degeneration. Cervical disc arthroplasty is considered an alternative to reduce risk of adjacent segment disease. Kinematics after arthroplasty should closely replicate healthy in vivo kinematics to reduce adjacent segment stresses. The purpose of this study was to assess the kinematics of a polycrystalline diamond cervical disc prosthesis. METHODS: Nine cadaveric C3-T1 spines were tested intact and after one (C5-C6) and two level (C5-C7) arthroplasty (Triadyme-C, Dymicron Inc., Orem, UT, USA). Kinematics were evaluated in flexion-extension, lateral bending, and axial rotation. FINDINGS: Prosthesis placement at C5-C6 and C6-C7 was 0.5 mm anterior and 0.6 mm posterior to midline respectively. C5-C6 flexion-extension motion was 12.8° intact and 10.5° after arthroplasty. C6-C7 flexion-extension motion was 10.0 and 11.4° after arthroplasty. C5-C6 lateral bending reduced from 8.5 to 3.7° after arthroplasty and at C6-C7 from 7.5 to 5.1°. C5-C6 axial rotation decreased from 10.4 to 6.2° after arthroplasty and at C6-C7 from 7.8 to 5.3°. Segmental lordosis increased by 4.2°, and middle disc height by 1.4 mm after arthroplasty. Change in center of rotation from intact to arthroplasty averaged 0.9 mm posteriorly and 0.1 mm caudally at C5-C6, and 1.4 mm posteriorly and 0.3 mm cranially at C6-C7. INTERPRETATION: The cervical disc arthroplasty evaluated restored flexion-extension motion to intact levels and moderately increased segmental stiffness. Disc height increased by up to 1.5 mm and segmental lordosis by 4.2°. The unique prosthesis design allowed the axis of rotation after arthroplasty to closely mimic the native location.

Does Resection of the Posterior Longitudinal Ligament Affect the Stability of Cervical Disc Arthroplasty?

BACKGROUND: The need for posterior longitudinal ligament (PLL) resection during cervical total disc arthroplasty (TDA) has been debated. The purpose of this laboratory study was to investigate the effect of PLL resection on cervical kinematics after TDA. METHODS: Eight cadaveric cervical spine specimens were tested in flexion-extension (FE), lateral bending (LB), and axial rotation (AR) to moments of ±1.5 Nm. After testing the intact condition, anterior C5-C6 cervical discectomy was performed followed by PLL resection and implantation of a compressible, 6-degrees-of-freedom disc prosthesis (M6-C, Spinal Kinetics Inc, Sunnyvale, California). Next, a second prosthesis was implanted at C6-C7 with PLL intact. Finally, the C6-C7 PLL was resected while the disc prosthesis remained in place. Segmental range of motion (ROM) and stiffness in the high flexibility zone around the neutral posture were analyzed using repeated measures ANOVA. RESULTS: At C5-C6, following TDA and PLL resection, FE, LB, and AR ROMs decreased significantly. Anterior and posterior disc height, segmental lordosis, and flexion stiffness increased significantly. At C6-C7, TDA with the PLL intact resulted in a significant increase in anterior disc height and segmental lordosis with no change in posterior disc height. FE, LB, and AR ROMs all decreased significantly, while flexion stiffness increased significantly compared to intact. PLL resection at C6-C7 did not result in a notable change compared to TDA with PLL intact. At the same level, flexion stiffness decreased following PLL resection compared to TDA with a value closer to intact. Two-level TDA (C5-C7) with PLL resection did not result in a loss of segmental stability. CONCLUSION: PLL resection did not significantly affect motion segment kinematics following cervical TDA using a prosthesis with inherent stiffness. Motion segment stiffness loss after PLL resection can be compensated for by a TDA design that can provide resistance to angular motion.

Biomechanics of an Expandable Lumbar Interbody Fusion Cage Deployed Through Transforaminal Approach.

BACKGROUND: A novel expandable lumbar interbody fusion cage has been developed which allows for a broad endplate footprint similar to an anterior lumbar interbody fusion; however, it is deployed from a minimally invasive transforaminal unilateral approach. The perceived benefit is a stable circumferential fusion from a single approach that maintains the anterior tension band of the anterior longitudinal ligament. The purpose of this biomechanics laboratory study was to evaluate the biomechanical stability of an expandable lumbar interbody cage inserted using a transforaminal approach and deployed in situ compared to a traditional lumbar interbody cage inserted using an anterior approach (control device). METHODS: Twelve cadaveric spine specimens (L1-5) were tested intact and after implantation of both the control and experimental devices in 2 (L2-3 and L3-4) segments of each specimen; the assignments of the control and experimental devices to these segments were alternated. Effect of supplemental pedicle screw-rod stabilization was also assessed. Moments were applied to the specimens in flexionextension (FE), lateral bending (LB), and axial rotation (AR). The effect of physiologic preload on construct stability was evaluated in FE. Segmental motions were measured using an optoelectronic motion measurement system. RESULTS: The deployable expendable transforaminal lumbar interbody fusion (TLIF) cage and control devices significantly reduced FE motion with and without compressive preload when compared to the intact condition (P < .05). Segmental motions in LB and AR were also significantly reduced with both devices (P < .05). Under no preload, the deployable expendable TLIF cage construct resulted in significantly smaller FE motion compared to the control cage construct (P < .01). Under all other testing modes (FE under 400N preload, LB, and AR), the postoperative motions of the 2 constructs did not differ statistically (P > .05). Adding bilateral pedicle screws resulted in further reduction of range of motion for all loading modes compared to intact condition, with no statistical difference between the 2 constructs (P > .05). CONCLUSIONS: The ability of the deployable expendable interbody cage in reducing segmental motions was equivalent to the control cage when used as a standalone construct and also when supplemented with bilateral pedicle screw-rod instrumentation. The larger footprint of the fully deployed TLIF cage combined with preservation of the anterior soft-tissue tension band may provide a better biomechanical fusion environment by combining the advantages of the traditional anterior lumbar interbody fusion and TLIF approaches.

Kinematic assessment of an elastic-core cervical disc prosthesis in one and two-level constructs.

INTRODUCTION: Anterior cervical discectomy and fusion has been associated with the development of adjacent segment degeneration (ASD), with clinical incidence of approximately 3% per year. Cervical total disc arthroplasty (TDA) has been proposed as an alternative to prevent ASD. HYPOTHESES: TDA in optimal placement using an elastic-core cervical disc (RHINE, K2M Inc., Leesburg, Virginia) will replicate natural kinematics and will improve with optimal vs anterior placement. METHODS: Seven C3-T1 cervical cadaver spines were tested intact first, then after one-level TDA at C5-C6 anterior placement, after TDA at C5-C6 optimal placement, after two-level TDA at C5-C6 and C6-C7 optimal placement, and finally after two-level TDA at C5-C6 lateral placement and C6-C7 optimal placement. The specimens were subjected to: Flexion-Extension moments (+1.5 Nm) with compressive preloads of 0 N and 150 N, lateral bending (LB) and axial rotation (AR) (+1.5 Nm) without preload. RESULTS: C5-C6 TDA in optimal placement resulted in a non-significant increase in flexion-extension ROM compared to intact under 0 N and 150 N preload (P > 0.05). Both LB and AR ROM decreased with arthroplasty (P < 0.01). Optimal placement of C6-C7 TDA resulted in an increase in flexion-extension ROM with preload compared to intact (P < 0.05) while LB and AR ROM decreased with arthroplasty (P < 0.01). CONCLUSION: This six degree of freedom elastic-core disc arthroplasty effectively restored flexion-extension motion to intact levels. In LB the TDA maintained 42% ROM at C5-C6 and 60% at C6-C7. In AR 57% of the ROM was maintained at C5-C6 and 70% at C6-C7. These findings are supported by literature which shows cervical TDA results in restoration of approximately 50% ROM in LB and AR, which is a multifactorial phenomenon encompassing TDA design parameters and anatomical constraints. Anterior placement of this viscoelastic TDA device shows motion restoration similar to optimal placement suggesting its design may be less sensitive to suboptimal placement.

Biomechanical Stability Analysis of a Stand-alone Cage, Static and Rotational-dynamic Plate in a Two-level Cervical Fusion Construct.

OBJECTIVE: To test the following hypotheses: (i) anterior cervical discetomy and fusion (ACDF) using stand-alone interbody spacers will significantly reduce the range of motion from intact spine; and (ii) the use of a static or a rotational-dynamic plate will significantly augment the stability of stand-alone interbody spacers, with similar beneficial effect when compared to each other. METHODS: Eleven human cadaveric subaxial cervical spines (age: 48.2 ± 5.4 years) were tested under the following sequence: (i) intact spine; (ii) ACDF at C4 -C5 using a stand-alone interbody spacer; (iii) ACDF at C5 -C6 and insertion of an interbody spacer (two-level construct); and (iv) randomized placement of either a two-level locking static plate or a rotational-dynamic plate. RESULTS: Insertion of stand-alone cage at C4 -C5 and C5 -C6 caused a significant decrease in the range of motion compared to intact spine (P < 0.05). Placement of both the locking and the rotational dynamic plate further reduced the range of motion at C4 -C5 and C5 -C6 compared to stand-alone cage (P < 0.01). No significant differences in range of motion restriction at either C4 -C5 or C5 -C6 were found when the two plating systems were compared (P > 0.05). CONCLUSIONS: Cervical stand-alone interbody spacers caused significant restriction in the range of motion. Both plates significantly augmented the stability of stand-alone interbody spacers, with similar stabilizing effect.

Biomechanics of an Expandable Lumbar Interbody Fusion Cage Deployed Through Transforaminal Approach.

INTRODUCTION: A novel expandable lumbar interbody fusion cage has been developed which allows for a broad endplate footprint similar to an anterior lumbar interbody fusion (ALIF); however, it is deployed from a minimally invasive transforaminal unilateral approach. The perceived benefit is a stable circumferential fusion from a single approach that maintains the anterior tension band of the anterior longitudinal ligament.The purpose of this biomechanics laboratory study was to evaluate the biomechanical stability of an expandable lumbar interbody cage inserted using a transforaminal approach and deployed in situ compared to a traditional lumbar interbody cage inserted using an anterior approach (control device). METHODS: Twelve cadaveric spine specimens (L1-L5) were tested intact and after implantation of both the control and experimental devices in two (L2-L3 and L3-L4) segments of each specimen; the assignments of the control and experimental devices to these segments were alternated. Effect of supplemental pedicle screw-rod stabilization was also assessed. Moments were applied to the specimens in flexion-extension (FE), lateral bending (LB), and axial rotation (AR). The effect of physiologic preload on construct stability was evaluated in FE. Segmental motions were measured using an optoelectronic motion measurement system. RESULTS: The deployable expendable TLIF cage and control devices significantly reduced FE motion with and without compressive preload when compared to the intact condition (p<0.05). Segmental motions in LB and AR were also significantly reduced with both devices (p<0.05). Under no preload, the deployable expendable TLIF cage construct resulted in significantly smaller FE motion compared to the control cage construct (p<0.01). Under all other testing modes (FE under 400N preload, LB, and AR) the postoperative motions of the two constructs did not differ statistically (p>0.05). Adding bilateral pedicle screws resulted in further reduction of ROM for all loading modes compared to intact condition, with no statistical difference between the two constructs (p>0.05). CONCLUSIONS: The ability of the deployable expendable interbody cage in reducing segmental motions was equivalent to the control cage when used as a stand-alone construct and also when supplemented with bilateral pedicle screw-rod instrumentation. The larger footprint of the fully deployed TLIF cage combined with preservation of the anterior soft-tissue tension band may provide a better biomechanical fusion environment by combining the advantages of the traditional ALIF and TLIF approaches.

Biomechanical evaluation of DTRAX(®) posterior cervical cage stabilization with and without lateral mass fixation.

INTRODUCTION: Lateral mass screw (LMS) fixation with plates or rods is the current standard procedure for posterior cervical fusion. Recently, implants placed between the facet joints have become available as an alternative to LMS or transfacet screws for patients with cervical spondylotic radiculopathy. The purpose of this study was to evaluate the biomechanical stability of the DTRAX(®) cervical cage for single- and two-level fusion and compare this to the stability achieved with LMS fixation with rods in a two-level construct. METHODS: Six cadaveric cervical spine (C3-C7) specimens were tested in flexion-extension, lateral bending, and axial rotation to ±1.5 Nm moment without preload (0 N) in the following conditions: 1) intact (C3-C7), 2) LMS and rods at C4-C5 and C5-C6, 3) removal of all rods (LMS retained) and placement of bilateral posterior cages at C5-C6, 4) bilateral posterior cages at C4-C5 and C5-C6 (without LMS and rods), and 5) C4-C5 and C5-C6 bilateral posterior cages at C4-C5 and C5-C6 with rods reinserted. RESULTS: Bilateral posterior cervical cages significantly reduced range of motion in all tested directions in both single- and multilevel constructs (P<0.05). Similar stability was achieved with bilateral posterior cages and LMS in a two-level construct: 0.6°±0.3° vs 1.2°±0.4° in flexion-extension (P=0.001), (5.0°±2.6° vs 3.1°±1.3°) in lateral bending (P=0.053), (1.3°±1.0° vs 2.2°±0.9°) in axial rotation (P=0.091) for posterior cages and LMS, respectively. Posterior cages, when placed as an adjunct to LMS, further reduced range of motion in a multilevel construct (P<0.05). CONCLUSION: Bilateral posterior cages provide similar cervical segmental stability compared with a LMS and rod construct and may be an alternative surgical option for select patients. Furthermore, supplementation of a lateral mass construct with posterior cages increases cervical spine stability in single- and multilevel conditions.

Bilateral posterior cervical cages provide biomechanical stability: assessment of stand-alone and supplemental fixation for anterior cervical discectomy and fusion.

INTRODUCTION: Supplemental posterior instrumentation has been widely used to enhance stability and improve fusion rates in higher risk patients undergoing anterior cervical discectomy and fusion (ACDF). These typically involve posterior lateral mass or pedicle screw fixation with significant inherent risks and morbidities. More recently, cervical cages placed bilaterally between the facet joints (posterior cervical cages) have been used as a less disruptive alternative for posterior fixation. The purpose of this study was to compare the stability achieved by both posterior cages and ACDF at a single motion segment and determine the stability achieved with posterior cervical cages used as an adjunct to single- and multilevel ACDF. METHODS: Seven cadaveric cervical spine (C2-T1) specimens were tested in the following sequence: intact, C5-C6 bilateral posterior cages, C6-C7 plated ACDF with and without posterior cages, and C3-C5 plated ACDF with and without posterior cages. Range of motion in flexion-extension, lateral bending, and axial rotation was measured for each condition under moment loading up to ±1.5 Nm. RESULTS: All fusion constructs significantly reduced the range of motion compared to intact in flexion-extension, lateral bending, and axial rotation (P<0.05). Similar stability was achieved with bilateral posterior cages and plated ACDF at a single level. Posterior cages, when placed as an adjunct to ACDF, further reduced range of motion in both single- and multilevel constructs (P<0.05). CONCLUSION: The biomechanical effectiveness of bilateral posterior cages in limiting cervical segmental motion is comparable to single-level plated ACDF. Furthermore, supplementation of single- and multilevel ACDF with posterior cervical cages provided a significant increase in stability and therefore may be a potential, minimally disruptive option for supplemental fixation for improving ACDF fusion rates.

Are Collapsed Cervical Discs Amenable to Total Disc Arthroplasty?: Analysis of Prospective Clinical Data With 2-Year Follow Up.

STUDY DESIGN: Analysis of prospectively collected radiographic data. OBJECTIVE: To investigate the influence of preoperative index-level range of motion (ROM) and disc height on postoperative ROM after cervical total disc arthroplasty (TDA) using compressible disc prostheses. SUMMARY OF BACKGROUND DATA: Clinical studies demonstrate benefits of motion preservation over fusion; however, questions remain unanswered as to which preoperative factors have the ability to identify patients who are most likely to have good postoperative motion, which is the primary rationale for TDA. METHODS: We analyzed prospectively collected data from a single-arm, multicenter study with 2-year follow up of 30 patients with 48 implanted levels. All received compressible cervical disc prostheses of 6 mm-height (M6C, Spinal Kinetics, Sunnyvale, CA). The influence of index-level preoperative disc height and ROM (each with two levels: below-median and above-median) on postoperative ROM was analyzed using 2 x 2 ANOVA. We further analyzed the radiographic outcomes of a subset of discs with preoperative height less than 3 mm, the so-called "collapsed" discs. RESULTS: Shorter (3.0 ± 0.4 mm) discs were significantly less mobile preoperatively than taller (4.4 ± 0.5 mm) discs (6.7° vs. 10.5°, P = 0.01). The postoperative ROM did not differ between the shorter and taller discs (5.6° vs. 5.0°, P = 0.63). Tall discs that were less mobile preoperatively had significantly smaller postoperative ROM than short discs with above-median preoperative mobility (P < 0.05). The "collapsed discs" (n = 8) were less mobile preoperatively compared with all discs combined (5.1° vs. 8.6°, P < 0.01). These discs were distracted to more than two times the preoperative height, from 2.6 to 5.7 mm, and had significantly greater postoperative ROM than all discs combined (7.6° vs. 5.3°, P < 0.05). CONCLUSION: We observed a significant interaction between preoperative index-level disc height and ROM in influencing postoperative ROM. Although limited by small sample size, the results suggest discs with preoperative height less than 3 mm may be amenable to disc arthroplasty using compressible disc prostheses. LEVEL OF EVIDENCE: 2.

Effects of motion segment level, Pfirrmann intervertebral disc degeneration grade and gender on lumbar spine kinematics.

MRI allows non-invasive assessment of intervertebral disc degeneration with the added clinical benefit of using non-ionizing radiation. What has remained unclear is the relationship between assessed disc degeneration and lumbar spine kinematics. Kinematic outcomes of 54 multi-segment (L1-Sacrum) lumbar spine specimens were calculated to discover if such an underlying relationship exists with degeneration assessed using the Pfirrmann grading system. Further analyses were also conducted to determine if kinematic outcomes were affected by motion segment level, gender or applied compressive preload. Range of motion, hysteresis, high flexibility zone size and rotational stiffness in flexion-extension, lateral bending and axial rotation were the kinematic outcomes. Caudal intervertebral discs in our study sample were more degenerative than cranial discs. L5-S1 discs had the largest flexion-extension range of motion (p < 0.005) and L1-L2 discs the lowest flexion high flexibility zone size (p < 0.013). No other strict cranial-caudal differences in kinematic outcomes were found. Low flexibility zone rotational stiffness increased with disc degeneration grade in extension, lateral bending and axial rotation (p < 0.001). Trends towards higher hysteresis and lower range of motion with increased degeneration were observed in flexion-extension and lateral bending. Applied compressive preload increased flexion-extension hysteresis and augmented the effect of degeneration on hysteresis (p < 0.0005). Female specimens had about one degree larger range of motion in all rotational modes, and higher flexion extension hysteresis (p = 0.016). These results suggest that gender differences exist in lumbar spine kinematics. Additionally high disc loads, applied compressive preload or applied moment, are needed to kinematically distinguish discs with different levels of degeneration. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 34:1389-1398, 2016.

Is Cervical Sagittal Imbalance a Risk Factor for Adjacent Segment Pathomechanics After Multilevel Fusion?

STUDY DESIGN: A biomechanical study using human spine specimens. OBJECTIVE: The aim of this study was to assess whether the presence of cervical sagittal imbalance is an independent risk factor for increasing the mechanical burden on discs adjacent to cervical multilevel fusions. SUMMARY OF BACKGROUND DATA: The horizontal offset distance between the C2 plumbline and C7 vertebral body (C2-C7 Sagittal Vertical Axis (SVA)) or the angle made with vertical by a line connecting the C2 and C7 vertebral bodies (C2-C7 tilt angle) are used as radiographic measures to assess cervical sagittal balance. There is level III clinical evidence that sagittal imbalance caused by kyphotic fusions or global spinal sagittal malalignment may increase the risk of adjacent segment pathology. METHODS: Thirteen human cadaveric cervical spines (Occiput-T1; age: 50.6 years; range: 21-67) were tested first in the native intact state and then after instrumentation across C4-C6 to simulate in situ two-level fusion. Specimens were tested using a previously validated experimental model that allowed measurement of spinal response to prescribed imbalance. The effects of fusion on segmental angular alignments and intradiscal pressures in the C3-C4 and C6-C7 discs, above and below the fusion, were evaluated at different magnitudes of C2-C7 tilt angle (or C2-C7 SVA). RESULTS: When compared with the pre-fusion state, in situ fusion across C4-C6 segments required increased flexion angulation and resulted in increased intradiscal pressure at the C6-C7 disc below the fusion in order to accommodate the same increase in C2-C7 tilt angle or C2-C7 SVA (P < 0.05). The adjacent segment mechanical burden due to fusion became greater with increasing C2-C7 tilt angle or SVA. CONCLUSION: Cervical sagittal imbalance arising from regional and/or global spinal sagittal malalignment may play a role in exacerbating adjacent segment pathomechanics after multilevel fusion and should be considered during surgical planning. LEVEL OF EVIDENCE: N/A.

Postural Consequences of Cervical Sagittal Imbalance: A Novel Laboratory Model.

STUDY DESIGN: A biomechanical study using human spine specimens. OBJECTIVE: To study postural compensations in lordosis angles that are necessary to maintain horizontal gaze in the presence of forward head posture and increasing T1 sagittal tilt. SUMMARY OF BACKGROUND DATA: Forward head posture relative to the shoulders, assessed radiographically using the horizontal offset distance between the C2 and C7 vertebral bodies (C2-C7 [sagittal vertical alignment] SVA), is a measure of global cervical imbalance. This may result from kyphotic alignment of cervical segments, muscle imbalance, as well as malalignment of thoracolumbar spine. METHODS: Ten cadaveric cervical spines (occiput-T1) were tested. The T1 vertebra was anchored to a tilting and translating base. The occiput was free to move vertically but its angular orientation was constrained to ensure horizontal gaze regardless of sagittal imbalance. A 5-kg mass was attached to the occiput to mimic head weight. Forward head posture magnitude and T1 tilt were varied and motions of individual vertebrae were measured to calculate C2-C7 SVA and lordosis across C0-C2 and C2-C7. RESULTS: Increasing C2-C7 SVA caused flexion of lower cervical (C2-C7) segments and hyperextension of suboccipital (C0-C1-C2) segments to maintain horizontal gaze. Increasing kyphotic T1 tilt primarily increased lordosis across the C2-C7 segments. Regression models were developed to predict the compensatory C0-C2 and C2-C7 angulation needed to maintain horizontal gaze given values of C2-C7 SVA and T1 tilt. CONCLUSION: This study established predictive relationships between radiographical measures of forward head posture, T1 tilt, and postural compensations in the cervical lordosis angles needed to maintain horizontal gaze. The laboratory model predicted that normalization of C2-C7 SVA will reduce suboccipital (C0-C2) hyperextension, whereas T1 tilt reduction will reduce the hyperextension in the C2-C7 segments. The predictive relationships may help in planning corrective strategy in patients experiencing neck pain, which may be attributed to sagittal malalignment. LEVEL OF EVIDENCE: N/A.

Effect of physiological loads on cortical and traditional pedicle screw fixation.

STUDY DESIGN: Human cadaveric biomechanical study. OBJECTIVE: To determine the fixation strength of laterally directed, cortical pedicle screws under physiological loads. SUMMARY OF BACKGROUND DATA: Lateral trajectory cortical pedicle screws have been described as a means of obtaining improved fixation while minimizing soft-tissue dissection during lumbar instrumentation. Biomechanical data have demonstrated equivalent strength in a quasi-static model; however, no biomechanical information is available comparing the fixation of cortical with traditional pedicle screws under cyclic physiological loads. METHODS: Seventeen vertebral levels (T11-L5) underwent quantitative computed tomography. On 1 side, a laterally directed, cortical pedicle screw was inserted with a traditional, medially directed pedicle screw placed on the contralateral side. With the specimen constrained in a testing apparatus, each screw underwent cyclic craniocaudal toggling under incrementally increasing physiological loads until 2 mm of head displacement occurred. Next, uniaxial pullout of each toggled screw was performed. The number of craniocaudal toggle cycles and load (N) required to achieve pedicle screw movement as well as axial pullout resistance (N) were compared between the 2 techniques. RESULTS: The mean trabecular bone mineral density of the specimens was 202 K2HPO4 mg/cm. Cortical pedicle screws demonstrated significantly improved resistance to toggle testing, requiring 184 cycles to reach 2 mm of displacement compared with 102 cycles for the traditional pedicle screws (P=0.002). The force necessary to displace the screws was also significantly greater for the cortical versus the traditional screws (398 N vs. 300 N, P=0.004). There was no statistical difference in axial pullout strength between the previously toggled cortical and traditional pedicle screws (1722 N vs. 1741 N, P=0.837). CONCLUSION: Laterally directed cortical pedicle screws have superior resistance to craniocaudal toggling compared with traditional pedicle screws. LEVEL OF EVIDENCE: N/A.

The effect of posterior decompressive procedures on segmental range of motion after cervical total disc arthroplasty.

STUDY DESIGN: We quantified the segmental biomechanics of a cervical total disc replacement (TDR) before and after progressive posterior decompression. We hypothesized that posterior decompressive procedures would not significantly increase range of motion (ROM) at the index TDR level. OBJECTIVE: To quantify the kinematics of a cervical total disc replacement (TDR) before and after posterior cervical decompression. SUMMARY OF BACKGROUND DATA: A reported yet unaddressed issue is the potential for the development of same-segment disease after implantation of a cervical TDR and the implications of same-segment posterior decompression on TDR mechanics. METHODS: Eight human cadaveric cervical spines C3-C7 were tested in flexion-extension, lateral bending, and axial rotation while intact, after C5-C6 TDR, C5-C6 unilateral foraminotomy, C5-C6 bilateral foraminotomies, and after C5 laminectomy in combination with the bilateral foraminotomies. Moment versus angular motion curves were obtained for each testing step, and the load-displacement data were analyzed to determine the range of angular motion for each step. RESULTS: Unilateral foraminotomy did not result in a statistically significant increase in flexion-extension ROM, and did not increase the ROM to a degree greater than normal. Although bilateral foraminotomies did increase flexion-extension ROM, motion remained within a physiological range. A full laminectomy added to the bilateral foraminotomies significantly increased ROM and was also associated with distortion of the load-displacement curves. CONCLUSION: With respect to segmental biomechanics as demonstrated, we think that for same-segment disease, a unilateral foraminotomy can be performed safely. However, the impact of in vivo conditions was not accounted for in this model, and it is possible that cyclical loading and other physiological stresses on such a construct may affect the behavior and lifespan of the implant in a way that cannot be predicted by a biomechanical study. Bilateral foraminotomies would require close observation and additional clinical follow-up, whereas complete laminectomy combined with bilateral foraminotomies should be avoided after TDR given the significant changes in kinematics. In addition, future disc replacement designs may need to account for changes after posterior decompression for same-segment disease. LEVEL OF EVIDENCE: N/A.

Quantification of Shear Stresses Within a Transtibial Prosthetic Socket.

BACKGROUND: There is a paucity of objectively recorded data delineating the pattern of weightbearing distribution within the prosthetic socket of patients with transtibial amputation. Our current knowledge is based primarily on information obtained from finite element analysis computer models. METHODS: Four high-functioning transtibial amputees were fit with similar custom prosthetic sockets. Three load cells were incorporated into each socket at high stress contact areas predicted by computer modeling. Dynamic recording of prosthetic socket loading was accomplished during rising from a sitting position, stepping from a 2-leg stance to a 1-leg stance, and during the initiation of walking. By comparing the loads measured at each of the 3 critical locations, anterior/posterior shear, superior/inferior shear, and end weightbearing were recorded. RESULTS: The same load pattern in all 4 subjects was found during each of the 3 functional activities. The load transmission at the distal end of the amputation residual limbs was negligible. Consistent forces were observed in both the anterior/posterior and superior/inferior planes. Correlation coefficients were used to compare the loads measured in each of the 4 subjects, which ranged from a low of .82 to a high of .98, where a value approaching 1.0 implies a linear relationship amongst subjects. CONCLUSION: This experimental model appears to have accurately recorded loading within a transtibial prosthetic socket consistent with previously reported finite element analysis computer models. CLINICAL RELEVANCE: This clinical model will allow objective measurement of weightbearing within the prosthetic socket of transtibial amputees and allow objective comparison of weightbearing distribution within the prosthetic sockets of patients who have undergone creation of different versions of a transtibial amputation residual limb and prosthetic socket designs.

Biomechanical evaluation of a low-profile, anchored cervical interbody spacer device at the index level or adjacent to plated fusion.

STUDY DESIGN: In vitro biomechanical study. OBJECTIVE: To test the hypotheses: (1) an anchored spacer device would decrease motion similarly to a plate-spacer construct, and (2) the anchored spacer would achieve a similar reduction in motion when placed adjacent to a previously fused segment. SUMMARY OF BACKGROUND DATA: An anchored spacer device has been shown to perform similar to the plate-spacer construct in previous biomechanical evaluation. The prevalence of adjacent segment disease after fusion is well established in the literature.There is currently no evidence supporting the use of an anchored interbody spacer device adjacent to a previous fusion. METHODS: Eight human cervical spines (age: 45.1 ± 13.1 yr) were tested in moment control (±1.5 Nm) in flexion-extension, lateral bending, and axial rotation without preload. Flexion-extension was then retested under 150-N preload. Spines were tested intact and after anterior cervical discectomy and fusion (ACDF) at C4-C5 and C6-C7 with either a plate-spacer or anchored spacer construct (randomized). The specimens were tested finally with an ACDF at the floating C5-C6 segment using the anchored spacer device adjacent to the previous fusions. RESULTS: Both the plate-spacer and anchored spacer significantly reduced motion from the intact spine in flexion-extension, lateral bending, and axial rotation (P < 0.005). There was no statistically significant difference between the 2 fusion constructs in their abilities to reduce motions (P = 1.0). ACDF using the anchored spacer at the floating C5-C6 level (in between the plate-spacer and anchored spacer constructs) resulted in significant motion reductions in all modes of testing (P < 0.05). These motion reductions did not significantly differ from those of a single-level anchored-spacer construct or a single-level plated ACDF. CONCLUSION: The anchored spacer provided significant motion reductions, similar to a plated ACDF, when used as a single-level fusion construct or placed adjacent to a previously plated segment. LEVEL OF EVIDENCE: N/A.