Anita N. Vasavada

Influence of muscle morphometry and moment arms on the moment-generating capacity of human neck muscles

Study Design. The function of neck muscles was quantified by incorporating experimentally measured morphometric parameters into a three‐dimensional biomechanical model. Objective. To analyze how muscle morphometry and moment arms influence moment‐generating capacity of human neck muscles in physiologic ranges of motion. Summary of Background Data. Previous biomechanical analyses of the head‐neck system have used simplified representations of the musculoskeletal anatomy. The force‐ and moment‐generating properties of individual neck muscles have not been reported. Methods. A computer graphics model was developed that incorporates detailed neck muscle morphometric data into a model of cervical musculoskeletal anatomy and intervertebral kinematics. Moment arms and force‐generating capacity of neck muscles were calculated for a range of head positions. Results. With the head in the upright neutral position, the muscles with the largest moment arms and moment‐generating capacities are sternocleidomastoid in flexion and lateral bending, semispinalis capitis and splenius capitis in extension, and trapezius in axial rotation. The moment arms of certain neck muscles (e.g., rectus capitis posterior major in axial rotation) change considerably in the physiologic range of motion. Most neck muscles maintain at least 80% of their peak force‐generating capacity throughout the range of motion; however, the force‐generating capacities of muscles with large moment arms and/or short fascicles (e.g., splenius capitis) vary substantially with head posture. Conclusions. These results quantify the contributions of individual neck muscles to moment‐generating capacity and demonstrate that variations in force‐generating capacity and moment arm throughout the range of motion can alter muscle moment‐generating capacities.

Disc degeneration affects the multidirectional flexibility of the lumbar spine.

Study Design An in vitro biomechanical investigation using human lumbar cadaveric spine specimens was undertaken to determine any relationship between intervertebral disc degeneration and nonlinear multidirectional spinal flexibility. Summary of Background Data Previous clinical and biomechanical studies have not established conclusively such a relationship. Methods Forty-seven discs from 12 whole lumbar spina specimens were studied under the application of flexion-extension, axial rotation, and lateral bending pure moments. Three flexihility parameters were defined (neutral zone (NZ), range of motion (ROM), and neutral zone ratio (NZR = NZROM)) and correlated with the macroscopic and radiographic degeneration. Results and Conclusions In flexion-extension, the ROM decreased and NZR increased with degeneration. In axial rotation, NZ and NZR increased with degeneration. In lateral binding, the ROM significantly decreased and the NZR increased with degeneration. In all three loading directions, the NZR increased, indicating greater joint laxity with degeneration.

Mechanical Properties of the Human Cervical Spine as Shown by Three-dimensional Load–displacement Curves

Study Design. The mechanical properties of multilevel human cervical spines were investigated by applying pure rotational moments to each specimen and measuring multidirectional intervertebral motions. Objectives. To document intervertebral main and coupled motions of the cervical spine in the form of load–displacement curves. Summary of Background Data. Although a number of in vivo and in vitro studies have attempted to delineate normal movement patterns of the cervical spine, none has explored the complexity of the whole cervical spine as a three-dimensional structure. Methods. Sixteen human cadaveric specimens (C0–C7) were used for this study. Pure rotational moments of flexion–extension, bilateral axial torque, and bilateral lateral bending were applied using a specially designed loading fixture. The resulting intervertebral motions were recorded using stereophotogrammetry and depicted as a series of load–displacement curves. Results. The resulting load–displacement curves were found to be nonlinear, and both rotation and translation motions were coupled with main motions. With flexion–extension moment loading, the greatest degree of flexion occurred at C1–C2 (12.3°), whereas the greatest degree of extension was observed at C0–C1 (20.2°). With axial moment loading, rotation at C1–C2 was the largest recorded (56.7°). With lateral bending moments, the average range of motion for all vertebral levels was 7.9°. Conclusions. The findings of the present study are relevant to the clinical practice of examining motions of the cervical spine in three dimensions and to the understanding of spinal trauma and degenerative diseases.

On the understanding of clinical instability

Study Design Three-dimensional flexibility changes due to the application of an external fixator at C4-C5 were studied in cervical spine specimens. Objectives to evaluate the biomechanical effects of applying a cervical external fixator to a patient using an in vitro model. Summary of Background Data There is controversy regarding the relationship between the changes in spinal motion and clinical instability. Methods Using fresh cadaveric C4-C7 specimens, multidirectional flexibility was measured at all vertebral levels, before and after the fixator application at C4-C5, C5-C6, and C4-C6. Results The average ranges of motion for flexion, extension, lateral bending, and axial rotation were 8.3° 7.2°, 5.3°, and 5.6°, which descreased by 40%, 27%, 32%, and 58%, repectively, because of the fixator application. The corresponding neutral zones were 3.4°, 3.4°, 3.0°, and 2.0°, which decreased by 76%, 76%, 54%, and 69%, respectively. The decreases with the fixation at C4-C5 were similar to those for fixation at C5-C6. Conclusions This in vitro study documented that the application of an external fixator to the cervical spine decreases the intervertebral motion in general, and decreases flexion, extension and torsional neutral zones in particular. The findings help explain the clincal instability of the spine and support the hypothesis that the neutral zone is more closely associated with the clinical instability than is the range of motion.

Three-Dimensional Isometric Strength of Neck Muscles in Humans

Study Design. Three-dimensional moments were measured experimentally during maximum voluntary contractions of neck muscles in humans. Objectives. To characterize the maximum moments with attention paid to subject size and gender, to calculate moments at different locations in the neck, and to quantify the relative magnitudes of extension, flexion, lateral bending, and axial rotation moments. Summary of Background Data. Few studies of neck strength have measured moments in directions other than extension, and it is difficult to compare results among studies because moments often are resolved at different locations in the cervical spine. Further, it is not clear how subject size, gender, and neck geometry relate to variations in the moment-generating capacity of neck muscles. Methods. Maximum moments were measured in 11 men and 5 women with an average age of 31 years (range, 20–42 years). Anatomic landmarks were digitized to resolve moments at different locations in the cervical spine. Results. When moments were resolved about axes through the midpoint of the line between the C7 spinous process and the sternal notch, the maximum moments were as follows: extension (men, 52 ± 11 Nm; women, 21 ± 12 Nm), flexion (men, 30 ± 5 Nm; women, 15 ± 4 Nm), lateral bending (men, 36 ± 8 Nm; women, 16 ± 8 Nm), and axial rotation (men 15 ± 4; women, 6 ± 3) Nm). The magnitudes of extension, flexion, and lateral bending moments decreased linearly with vertical distance from the lower cervical spine to the mastoid process. Conclusions. Moments in three dimensions were quantified with regard to subject size and location along the cervical spine. These data are needed to characterize neck strength for biomechanical analysis of normal and pathologic conditions.

Morphology, architecture, and biomechanics of human cervical multifidus.

Study Design. Cadaveric dissections and biomechanical modeling were used to study the human cervical multifidus muscle. Objectives. To describe attachment patterns of the multifidus in the cervical region, to quantify the muscle’s architecture, and to use a biomechanical model to calculate the moment-generating capacity of the cervical multifidus. Summary of Background Data. Deep neck muscles such as the multifidus may play an important role in cervical spine stability and neck pain. However, there are limited data regarding the fascicular attachments or architecture parameters necessary to calculate force and moment. Methods. The multifidus spinae was studied by dissection of nine cadaveric specimens. Fascicles were grouped according to attachment, and architecture parameters (musculotendon length, fascicle length, and physiologic cross-sectional area) were quantified. The data were used in a biomechanical model to calculate moment arm, force-, and moment-generating capacity of the multifidus. Results. The multifidus originates from the facet capsules of lower cervical vertebrae and the transverse processes of upper thoracic vertebrae. The fascicles span 2 to 5 vertebral segments from origin to insertion, and they insert on the spinous processes and laminae of superior cervical vertebrae. For each fascicular subgroup, musculotendon lengths ranged from 2.0 to 6.9 cm, fascicle lengths ranged from 1.2 to 3.7 cm, and physiologic cross-sectional area ranged from 0.1 to 1.0 cm2. The total moment-generating capacity of the cervical multifidus in the neutral posture was predicted to be approximately 0.7 Nm for extension and lateral bending and 0.3 Nm for axial rotation. Conclusions. The fascicular attachment pattern of the multifidus spinae in the cervical region appears to be unique to that region. The direct attachment to cervical facet capsules supports a possible role in neck pain and injury. Characterizing the biomechanical function of the multifidus is important for the analysis of normal and pathologic conditions.

The Anatomy and Biomechanics of Acute and Chronic Whiplash Injury

Whiplash injury is the most common motor vehicle injury, yet it is also one of the most poorly understood. Here we examine the evidence supporting an organic basis for acute and chronic whiplash injuries and review the anatomical sites within the neck that are potentially injured during these collisions. For each proposed anatomical site—facet joints, spinal ligaments, intervertebral discs, vertebral arteries, dorsal root ganglia, and neck muscles—we present the clinical evidence supporting that injury site, its relevant anatomy, the mechanism of and tolerance to injury, and the future research needed to determine whether that site is responsible for some whiplash injuries. This article serves as a snapshot of the current state of whiplash biomechanics research and provides a roadmap for future research to better understand and ultimately prevent whiplash injuries.

Dynamic canal encroachment during thoracolumbar burst fractures.

In the burst fractures seen clinically, often poor correlation exists between the neurological deficit and the canal encroachment measured on post-trauma radiographic images. The purpose of the present study was to determine whether the dynamic canal encroachment during the trauma is greater than the static canal encroachment posttrauma. We successfully produced burst fractures in nine of 15 fresh human cadaveric thoracolumbar spine specimens (T11-L1). The specimens were incrementally impacted in a high-speed trauma apparatus until fracture occurred. During the trauma, dynamic canal encroachments were measured using three specially designed transducers placed in the canal at the levels of the superior end-plates of the T12 and L1 and the T12/L1 disk. After the trauma, residual static spinal canal encroachments were measured from the radiographs of the specimens that were prepared with 1.6-mm diameter steel balls lining the canal in the midsagittal plane. We found that the average canal diameter was 16.6 +/- 1.3 mm and the static canal encroachment was 18.0% of the canal diameter. The corresponding dynamic canal encroachment was 33.3%. Thus, the dynamic canal encroachment was 85% more than the static measurement. The clinical significance of this study lies in providing awareness to the clinician that the dynamic canal encroachment is significantly greater than the static canal encroachment seen on posttrauma radiographs or computed tomography scans. The finding may also explain the clinical observation of poor correlation between the canal encroachment measured radiographically and the neurological deficit.

Kinematics of the cervical spine canal: changes with sagittal plane loads.

Spondylotic myelopathy is a result of decreased spinal canal space due to degeneration. The space also may change with physiological movements. The knowledge of the normal physiological changes is necessary for a better understanding of the clinical symptoms. Using a novel technique, we measured the changes in disk bulge, ligamentum flavum bulge, and anteroposterior canal diameter in response to tension-compression forces (up to 40 N each) and combined loading: 2 Nm of flexion or extension moment combined with 20 N compression force in five human cadaveric lower cervical spine specimens (C4-C7). From tension to compression, the average disk bulge changed 1.13 mm or 10.1% of the original canal diameter. The ligamentum flavum bulge changed 0.73 mm or 6.5% of the canal diameter. From flexion to extension the average disk bulb changed 1.16 mm or 10.8% of the canal diameter, whereas the ligamentum flavum bulge changed 2.68 mm or 24.3% of the canal diameter. Most of the changes in the bulges occurred with a small load application around the neutral position of the spine. The results of this study demonstrate that ligamentum flavum bulge can contribute significantly to canal encroachment in extension and that a flexed posture increases the sagittal diameter of the spinal canal.

Musculotendon and fascicle strains in anterior and posterior neck muscles during whiplash injury

Study Design. A biomechanical neck model combined with subject-specific kinematic and electromyographic data were used to calculate neck muscle strains during whiplash. Objectives. To calculate the musculotendon and fascicle strains during whiplash and to compare these strains to published muscle injury thresholds. Summary of Background Data. Previous work has shown potentially injurious musculotendon strains in sternocleidomastoid (SCM) during whiplash, but neither the musculotendon strains in posterior cervical muscles nor the fascicle strains in either muscle group have been examined. Methods. Experimental human subject data from rear-end automobile impacts were integrated with a biomechanical model of the neck musculoskeletal system. Subject-specific head kinematic data were imposed on the model, and neck musculotendon and fascicle strains and strain rates were computed. Electromyographic data from the sternocleidomastoid and the posterior cervical muscles were compared with strain data to determine which muscles were being eccentrically contracted. Results. SCM experienced lengthening during the retraction phase of head/neck kinematics, whereas the posterior muscles (splenius capitis [SPL], semispinalis capitis [SEMI], and trapezius [TRAP]) lengthened during the rebound phase. Peak SCM fascicle lengthening strains averaged (±SD) 4% (±3%) for the subvolumes attached to the mastoid process and 7% (±5%) for the subvolume attached to the occiput. Posteriorly, peak fascicle strains were 21% (±14%) for SPL, 18% (±16%) for SEMI, and 5% (±4%) for TRAP, with SPL strains significantly greater than calculated in SCM or TRAP. Fascicle strains were, on average, 1.2 to 2.3 times greater than musculotendon strains. SCM and posterior muscle activity occurred during intervals of muscle fascicle lengthening. Conclusions. The cervical muscle strains induced during a rear-end impact exceed the previously-reported injury threshold for a single stretch of active muscle. Further, the larger strains experienced by extensor muscles are consistent with clinical reports of pain primarily in the posterior cervical region following rear-end impacts.