The range of atlanto-occipital joint motion in cetaceans reflects their feeding behavior.

The cetaceans display a wide variety of lifestyles, especially with regard to their feeding behavior. However, the evolutionary process of the feeding behavior in cetaceans is still poorly understood, in part because reconstructing the feeding behavior of extinct taxa remains difficult. In cetaceans, cranium mobility relative to the trunk largely depends on the range of motion permitted by the atlanto-occipital joint, given the lack of flexibility of the cervical series. In this study, we examined 56 extant cetacean skeletal specimens from 30 species in 25 genera and nine families in order to investigate the relationships between anatomical traits and feeding behavior. Our results suggest that the range of dorso-ventral motion allowed by the atlanto-occipital joint (ROM) depends on prey habitat and the feeding technique of cetaceans. Cetaceans feeding on benthic/demersal prey had a relatively large ROM compared with those feeding on pelagic prey. In addition, ROM was largest in raptorial feeders, intermediate in suction feeders, and smallest in ram-filter feeders. Among raptorial feeders, ROM tended to be larger in taxa that facultatively tear off the prey's flesh compared with taxa that swallow their prey whole. Therefore, we conclude that ROM is a powerful tool to reliably reconstruct the feeding behavior of extinct cetacean taxa.

Injury Threshold of Rectus Capitis Muscles at the Atlanto-occipital Joint.

OBJECTIVE: The objective of this study was to collect muscle stiffness data from the 4 rectus capitis (RC) muscles to better understand their role in stabilizing the atlanto-occipital joint. The passive load displacement properties of these muscles have not been previously reported. METHODS: Rectus capitis muscles were removed from 3 unembalmed head and neck specimens. Passive length-force (stiffness) data were collected by using a servo-controlled hydraulic test machine. Multivariate analysis of variance with Bonferroni correction was used to assess the significance of the differences among passive stiffness within the elastic region of each muscle and the load and strain at the yield points. RESULTS: Rectus capitis lateralis (RCL) muscles failed at significantly higher levels of load and strain compared with the other 3 pairs of muscles. Passive stiffness of both RCL and RC anterior muscles was significantly higher than the other 2 pairs of muscles. CONCLUSION: The anatomic location of the RCL muscles, along with their high levels of passive stiffness, would be expected to facilitate the maintenance of atlanto-occipital joint congruence during normal daily activities. The level at which the RC posterior minor muscles failed could put them at risk of a strain injury during a rear end motor vehicle accident. Diagnostic and treatment protocols that apply forces to the upper cervical spine should be tailored to consider the patient's age, gender, and history of previous injuries to avoid overstretching RC muscles.

Anatomy and biomechanics of the craniovertebral junction.

The craniovertebral junction (CVJ) has unique anatomical structures that separate it from the subaxial cervical spine. In addition to housing vital neural and vascular structures, the majority of cranial flexion, extension, and axial rotation is accomplished at the CVJ. A complex combination of osseous and ligamentous supports allow for stability despite a large degree of motion. An understanding of anatomy and biomechanics is essential to effectively evaluate and address the various pathological processes that may affect this region. Therefore, the authors present an up-to-date narrative review of CVJ anatomy, normal and pathological biomechanics, and fixation techniques.

Normative data on axial rotation of atlanto-occipital joint on 3 Tesla MRI using a simple and reliable method of calculation.

BACKGROUND: Various methods have been used to image and measure the normal range of axial rotation of the atlanto-occipital joint (AOJ), but a simple, precise, and reliable method is needed for everyday practice. PURPOSE: To generate normative ranges for AOJ rotation in various in-vivo positions and to investigate the reliability of a simple imaging method for measurement using routine high-field magnetic resonance imaging (MRI). MATERIAL AND METHODS: One hundred healthy volunteers were imaged on 3 T MRI with the AOJ in the center of the field of view. The scans were uniformly performed in seven different positions. The range of axial rotation was calculated by the angle between the craniofacial midline and the line linking the anterior and posterior tubercles of the atlas. The angle was defined as positive when it was angled right, and negative when it was angled left. The actual normative range of axial rotation was the difference between the angle in the supine neutral position and in the other positions. RESULTS: The normative axial rotation range of the AOJ in different positions was between -4.8° and +5.0°. The mean values of the actual rotation angles in the right supine position with maximum bending, the right supine position maximum rotation, and the right prostrate position maximum rotation were 0.1°, 1.70°, and 0.8°, respectively. The mean values of actual rotation angles in the left supine position with maximum bending, the left supine position with maximum rotation, and the left prostrate positive with maximum rotation were 0.1°, -1.7°, and -1.1°, respectively. The inter-observer reliability tested. CONCLUSION: A simple and reliable method of measurement on 3.0 T MRI demonstrated the normative axial rotation range of the AOJ in different positions to be between -4.8° and +5.0° and it was different from zero in neutral rotation. This method could be practically used to precisely diagnose AOJ rotary subluxation or dislocation.

A Comprehensive Review of the Superficial Anterior Atlanto-Occipital Ligament of the Craniocervical Junction.

This paper aimed to better describe the anatomy of the superficial anterior atlanto-occipital ligament of the craniocervical junction and discuss this ligament's potential function and clinical implications. A broad literature review on the anatomical features and findings of the superficial anterior atlanto-occipital ligament was performed. The superficial anterior atlanto-occipital ligament is located anterior to the anterior atlanto-occipital membrane. However, the physiological role of the superficial anterior atlanto-occipital ligament is still unclear due to a lack of anatomical and biomechanical studies although one study has suggested that this ligament is a secondary stabilizer of the craniocervical junction. Further studies are needed to clarify the function and anatomy of the superficial anterior atlanto-occipital ligament.

A specialized myodural bridge named occipital-dural muscle in the narrow-ridged finless porpoise (Neophocaena asiaeorientalis).

A dense bridge-like tissue named the myodural bridge (MDB) connecting the suboccipital muscles to the spinal dura mater was originally discovered in humans. However, recent animal studies have revealed that the MDB appears to be an evolutionarily conserved anatomic structure which may have significant physiological functions. Our previous investigations have confirmed the existence of the MDB in finless porpoises. The present authors conducted research to expound on the specificity of the MDB in the porpoise Neophocana asiaeorientalis (N.asiaeorientalis). Five carcasses of N.asiaeorientalis, with formalin fixation, were used for the present study. Two of the carcasses were used for head and neck CT scanning, three-dimensional reconstructions, and gross dissection of the suboccipital region. Another carcass was used for a P45 plastination study. Also, a carcass was used for a histological analysis of the suboccipital region and also one was used for a Scanning Electron Microscopy study. The results revealed that the MDB of the N.asiaeorientalis is actually an independent muscle originating from the caudal border of the occiput, passing through the posterior atlanto-occipital interspace, and then attaches to the cervical spinal dura mater. Thus the so called MDB of the N.asiaeorientalis is actually an independent and uniquely specialized muscle. Based on the origin and insertion of this muscle, the present authors name it the 'Occipital-Dural Muscle'. It appears that the direct pull of this muscle on the cervical spinal dura mater may affect the circulation of the cerebrospinal fluid by altering the volume of the subarachnoid space via a pumping action.

[Research on biomechanics properties of occipito-atlantoaxial complex by finite element method].

Based on the research history of the biomechanics of occipito-atlantoaxial complex, we have systematically summarized the use of finite element method for studying biomechanics of occipito-atlantoaxial complex. Then, combined with four basic principles of establishing an effective finite element model for mechanics, our comments are focused on the establishment of geometrical model, finite element model, finite element mechanics model, and on the method and implementation for validating the model. In addition, the developing trends, existing problems and future researching directions in this area are discussed.

Combined Procedure Allowing Atlantoaxial and Atlanto-Occipital Joint Motion for Complex Injuries in the Upper Cervical Spine.

This study was performed to explore an ideal limited fixation method for the treatment of unstable atlantoaxial fractures that can preserve the range of motion of the occipital and atlantoaxial joints and restore the stability of the upper cervical spine. A 64-year-old man was diagnosed with a complicated injury of the upper cervical spine. The anterior and posterior approach was used to reconstruct the stability of the upper cervical spine while preserving the range of motion of the occipital and atlantoaxial joints. Preoperative imaging and neurologic examinations were performed. Follow-up lasted 24 months. The patient clinically improved after undergoing this novel procedure. Seven days postoperatively, the patient's visual analog scale score was 3. Follow-up contrast computed tomography showed good reduction and fixation in the upper cervical spine. Two weeks postoperatively, the patient displayed good cervical vertebral activity, with no restriction during flexion, extension, rotation, or other movements. No intraoperative or postoperative complications occurred. This modified procedure for restoration of cervical stability may be an improvement over traditional posterior fusion because atlantoaxial motion is preserved. [Orthopedics. 2020;43(4):e329-e333.].

The demands of professional opera singing on cranio-cervical posture.

Difficulty with singing is a rare but important complication following cervical spine surgery but there is little objective information regarding the cervical and head postural changes taking place during singing. The aim of this study was to identify postural changes in the cranio-cervical region associated with the demands of voice production in professional opera singing. The two Roentgen-cephalograms, one of which are taken whilst performing a specified singing task were taken from 18 professional opera students, 12 females (mean age 20.86 +/- 3.07 years) and six males (18.66 +/- 1.36 years). A paired t test compared mean cranio-cervical postural and pharyngeal/hyoid variables between the two registrations (P = 0.05). The association between the cranio-cervical postural variables and the pharyngeal/hyoid region in each registration position was examined using Spearman's rank correlation coefficient. In singing, the position of the atlas with respect to the true vertical (P < 0.001), the axis (P < 0.001) and the C4 vertebra both with respect to the horizontal (P < 0.001), and the axis with respect to the cranium (P < 0.001), were all significantly different to those at rest. Of the cranio-cervical postural variables in the singing registration, the angles measuring positional change of the atlas and C4 relative to the true horizontal were shown be significantly related to an increased pharyngeal airway space at the C3 level (P < 0.01). An appreciation of the requirement for the cervical spine to undergo postural change during professional opera singing has relevance to the potential impact on voice quality in professional opera singers should they undergo cervical spine surgery.

Diagnosis of atlanto-occipital dissociation: Standardised measurements of normal craniocervical relationship in finless porpoises (genus Neophocaena) using postmortem computed tomography.

Due to the different craniocervical structures in humans and cetaceans, a standardised method assessing the normal craniocervical relationship in cetaceans is lacking, causing difficulties in defining the presence of atlanto-occipital dissociation (AOD) in cetaceans. The present study aimed to 1) describe a novel standardised method of determining the normal craniocervical relationships, and 2) define the 95% accuracy range of the normal craniocervical relationship in finless porpoises (genus Neophocaena), that allowed AOD diagnosis. Fifty-five out 83 stranded or by-caught finless porpoise carcasses were analyzed in term of their craniocervical relationship in dorsal-ventral and medial-lateral dimension, using postmortem computed tomography measurements. The normal craniocervical relationship in both dorsal-ventral (mean BD/OV: 0.87 ± 0.24 [2 SD]) and medial-lateral dimension (mean VR/VL: 0.98 ± 0.17 [2 SD]) was first defined. The 95% accuracy ranges of the normal craniocervical relationship in dorsal-ventral (0.63-1.11) and medial-lateral dimension (0.82-1.15) were proposed. The baseline ranges could facilitate AOD assessment, and provide an objective means of record for AOD related injury and death of cetaceans caused by anthropogenic trauma. The technique developed may be applied to live cetaceans with abnormal craniocervical relationship to aid diagnosis and guide corrective therapy.

Active range of motion as an indicator for ligament and membrane lesions in the upper cervical spine after a whiplash trauma.

In the present study, we examined whether active range of neck motion (AROM) differed between persons with and without a diagnosis of whiplash-associated disorder type 2 (WAD2) and explored whether magnetic resonance (MR)-verified lesions of specific ligaments or membranes at the craniovertebral junction was associated with increased or decreased motion in any particular direction among the WAD2 patients. A CROM goniometer was used for registration of flexion, extension, side bending (left and right) and rotation (left and right), respectively. The neck structures considered were the alar and the transverse ligaments, and the tectorial and the posterior atlanto-occipital membranes. Our study comprised 87 WAD2 patients and 29 control persons without any known neck injury. For comparing mean values of AROM between the groups, t-test and analysis of variance (ANOVA) were used. WAD patients had on average a shorter range of active motion for all movements compared with the control group. The difference was statistically significant for all measures considered, except side bending to the left. Among the WAD patients, increasing severity of lesions to the alar ligaments was associated with a decrease in maximal flexion and rotation. A similar pattern was seen for lesions to the transverse ligament, but the trend test was not significant. An abnormal posterior atlanto-occipital membrane was associated with shorter range of left rotation, with a significant trend test both in analyses with and without adjustment for lesions to other structures. No significant association was found in relation to lesions to the tectorial membrane, but very few persons had such lesions. These findings indicate that soft tissue lesions may affect neck motion as reflected by AROM. However, since lesions to different structures seem to affect the same movement, AROM alone is not a sufficient indicator for soft-tissue lesions to specific structure in the upper cervical spine.

Biomechanical evaluation of the craniovertebral junction after unilateral joint-sparing condylectomy: implications for the far lateral approach revisited.

OBJECTIVE The far lateral transcondylar approach to the ventral foramen magnum requires partial resection of the occipital condyle. Early biomechanical studies suggest that occipitocervical (OC) fusion should be considered if 50% of the condyle is resected. In clinical practice, however, a joint-sparing condylectomy has often been employed without the need for OC fusion. The biomechanics of the joint-sparing technique have not been reported. Authors of the present study hypothesized that the clinically relevant joint-sparing condylectomy would result in added stability of the craniovertebral junction as compared with earlier reports. METHODS Multidirectional in vitro flexibility tests were performed using a robotic spine-testing system on 7 fresh cadaveric spines to assess the effect of sequential unilateral joint-sparing condylectomy (25%, 50%, 75%, 100%) in comparison with the intact state by using cardinal direction and coupled moments combined with a simulated head weight "follower load." RESULTS The percent change in range of motion following sequential condylectomy as compared with the intact state was 5.2%, 8.1%, 12.0%, and 27.5% in flexion-extension (FE); 8.4%, 14.7%, 39.1%, and 80.2% in lateral bending (LB); and 24.4%, 31.5%, 49.9%, and 141.1% in axial rotation (AR). Only values at 100% condylectomy were statistically significant (p < 0.05). With coupled motions, however, -3.9%, 6.6%, 35.8%, and 142.4% increases in AR+F and 27.3%, 32.7%, 77.5%, and 175.5% increases in AR+E were found. Values for 75% and 100% condyle resection were statistically significant in AR+E. CONCLUSIONS When tested in the traditional cardinal directions, a 50% joint-sparing condylectomy did not significantly increase motion. However, removing 75% of the condyle may necessitate fusion, as a statistically significant increase in motion was found when E was coupled with AR. Clinical correlation is ultimately needed to determine the need for OC fusion.

Anatomy of Alar Ligament Part III: Biomechanical Study.

OBJECTIVE: Four layers of ligamentous stabilizers comprise the craniocervical junction, and the second layer is composed of apical and paired alar ligaments. The purpose of this study is to establish the tensile strength of the alar ligaments for better understanding the implications that can arise from trauma and other pathologies in the craniocervical region. METHODS: Nineteen sides from 10 fresh frozen adult cadaveric Caucasian heads were used in this study. The specimens were derived from 6 males and 4 females, and the age of the cadavers at death ranged from 67-90 years old. To measure the tensile strength, a tensile testing machine (M2-200, Mark-10 Corporation, Copiague, New York, USA) was used in this study. RESULTS: The force (N) necessary until failure for all alar ligaments ranged from 87-346 N with a mean of 186.9 ± 69.7 N. There was a significant difference when comparing tensile strength between males and females. CONCLUSIONS: Further studies will be necessary to determine their importance as secondary stabilizers and measure their ability to support similar forces when subject to rotation and lateral bending forces, as well as with flexion-extension.

Atlanto-occipital catheterization of young rats for long-term drug delivery into the lumbar subarachnoid space combined with in vivo testing and electrophysiology in situ.

BACKGROUND: Catheterization has been widely used in neuroscience and pain research for local drug delivery. Though different modifications were developed, the use of young animals for spinal catheterization remains limited because of a little success rate. A reliable technique is needed to catheterize young animals aimed for in vivo testing combined with spinal cord electrophysiology, often limited by animal age, to facilitate pain research. NEW METHODS: We describe intrathecal catheterization of young rats (3-week-old) through atlanto-occipical approach for long-lasting drug delivery into the lumbar subarachnoid space. The technique represents a surgical approach of minimized invasiveness that requires PE-10 catheter and few equipment of standard laboratory use. RESULTS: Behavioral assessments revealed that spinal catheterization does not change peripheral sensitivity of different modalities (thermal and mechanical) and gives no rise to locomotive deficit or anxiety-like behavior in young rats. The long-term administration of genetic material (oligodeoxynucleotides given up to 4days), examined both in vivo and in situ, produced no adverse effects on basal peripheral sensitivity, but changed the AMPA receptor-mediated currents in sensory interneurons of the spinal cord. COMPARISON WITH EXISTING METHODS: Dissimilar to already described methods, the method is designed for the use of young rats for behavioral testing in vivo and/or spinal cord electrophysiology in situ. CONCLUSIONS: A practical method for spinal catheterization of young animals designed for studies in vivo and in situ is proposed. The method is rapid and effective and should facilitate investigation of therapeutic effects on both systemic and subcellular levels, as an advantage over the existing methods.

Development and initial evaluation of a finite element model of the pediatric craniocervical junction.

OBJECT There is a significant deficiency in understanding the biomechanics of the pediatric craniocervical junction (CCJ) (occiput-C2), primarily because of a lack of human pediatric cadaveric tissue and the relatively small number of treated patients. To overcome this deficiency, a finite element model (FEM) of the pediatric CCJ was created using pediatric geometry and parameterized adult material properties. The model was evaluated under the physiological range of motion (ROM) for flexion-extension, axial rotation, and lateral bending and under tensile loading. METHODS This research utilizes the FEM method, which is a numerical solution technique for discretizing and analyzing systems. The FEM method has been widely used in the field of biomechanics. A CT scan of a 13-month-old female patient was used to create the 3D geometry and surfaces of the FEM model, and an open-source FEM software suite was used to apply the material properties and boundary and loading conditions and analyze the model. The published adult ligament properties were reduced to 50%, 25%, and 10% of the original stiffness in various iterations of the model, and the resulting ROMs for flexion-extension, axial rotation, and lateral bending were compared. The flexion-extension ROMs and tensile stiffness that were predicted by the model were evaluated using previously published experimental measurements from pediatric cadaveric tissues. RESULTS The model predicted a ROM within 1 standard deviation of the published pediatric ROM data for flexion-extension at 10% of adult ligament stiffness. The model's response in terms of axial tension also coincided well with published experimental tension characterization data. The model behaved relatively stiffer in extension than in flexion. The axial rotation and lateral bending results showed symmetric ROM, but there are currently no published pediatric experimental data available for comparison. The model predicts a relatively stiffer ROM in both axial rotation and lateral bending in comparison with flexion-extension. As expected, the flexion-extension, axial rotation, and lateral bending ROMs increased with the decrease in ligament stiffness. CONCLUSIONS An FEM of the pediatric CCJ was created that accurately predicts flexion-extension ROM and axial force displacement of occiput-C2 when the ligament material properties are reduced to 10% of the published adult ligament properties. This model gives a reasonable prediction of pediatric cervical spine ligament stiffness, the relationship between flexion-extension ROM, and ligament stiffness at the CCJ. The creation of this model using open-source software means that other researchers will be able to use the model as a starting point for research.

In vitro biomechanics of the craniocervical junction-a sequential sectioning of its stabilizing structures.

BACKGROUND CONTEXT: Occipitocervical dislocations involve translations of the craniocervical joints. The relative contributions of each ligament to overall stability and the effects of the occipitoatlantal joint capsules on the pathologic translation are unknown. Although incidences of occipitocervical dislocations are rare after blunt trauma, they are usually fatal. When patients do survive these dislocations, the proper diagnosis is difficult, which in turn may increase the fatality rate. A biomechanical model may provide a greater pathologic understanding of craniocervical subluxation. PURPOSE: The purpose of the study is to build an in vitro biomechanical model to determine which stabilizing ligament(s) of the craniocervical junction are most important in restraining rotation and translations during these rotations. This may guide clinical diagnosis, which could assist in treatment options. STUDY DESIGN/SETTING: The study design includes a biomechanical cadaveric test. METHODS: Seven cadaveric specimens were tested using a 6-degree-of-freedom spine simulator under the following conditions: intact, clivus/alar removal (CR), transverse ligament destruction (TLD), occipitoatlantal (OA) joint capsulotomyoccipitoatlantal (OA) joint capsulotomy (C0-C1 JC), and C1-C2 joint capsulotomy (C1-C2 JC). Flexion-extension (FE), lateral bending (LB), and axial rotation (AR) were applied (2.5 Nm) to a C0-C2 segment, whereas anterior-posterior (AP) and cranial-caudal (CC) translations were recorded. Average motions were normalized to intact (100%) for each joint. RESULTS: Increases in C0-C1 angular and translational motions occurred after TLD and C0-C1 JC. At the atlantoaxial joint, there were significant (p<.05) increases from intact in FE (TLD=154%, C0-C1 JC=174%) and in AR (TLD=178%, C0-C1 JC=224%). Anterior-posterior translation during applied LB increased significantly after TLD (248% intact). Cranial-caudal translation during applied FE increased significantly after TLD (188%) and C0-C1 JC (361%). Increases in C1-C2 angular motion occurred after TLD and C1-C2 JC and in translation after CR and TLD. At the C1-C2 joint, there were significant increases from intact in FE (TLD=172%, C1-C2 JC=160%) and in LB (TLD=286%, C1-C2 JC=332%); in AR, there were no statistical differences. Anterior-posterior translation increased significantly after CR (280%). Cranial-caudal translation also increased significantly after CR (205%) and TLD (298%) during LB. CONCLUSIONS: Transverse and alar ligaments appear to be the main stabilizers of the craniocervical junction. The vertical structures attached to the clivus and OA joint capsules function as secondary stabilizers. Craniocervical dislocations seem to affect FE and lateral bending the most, whereas increased translation seems to occur primarily in the AP and CC directions. Models of craniocervical trauma should section all three restraining structures for the future studies.

Dynamic response of the occipito-atlanto-axial (C0-C1-C2) complex in right axial rotation.

The torque-angular deformation in right axial rotation until failure of the ligamentous occipito-atlanto-axial complex subjected to variable loading rate (dynamic) axial torque was characterized using a biaxial MTS system. A special fixture and gear box that permitted right axial rotation of the specimen until failure without imposing any additional constraints were used to obtain the data. The specimens were divided into three groups and tested until failure at three different dynamic loading rates: 50, 100, and 400 degrees/s. A previous study by the authors provided data for quasi-static (4 degrees/s) loading conditions. The torque versus rotation curves can be divided into two straight regions and two transition zones. The plots clearly indicated that at loading rates higher than 4 degrees/s, the specimens became stiffer in the region of steadily increasing resistance prior to failure. The increase in stiffness was maximum at 100 degrees/s. The stiffness decreased somewhat at 400 degrees/s in comparison with 100 degrees/s, but this decrease was not significant. The resulting torque-right axial rotation curves were also examined to estimate the magnitude of maximum resistance (torque) and the corresponding angular rotation value. The average maximum resistance torque increased from 13.6 Nm at 4 degrees/s to 27.8 Nm at 100 degrees/s. The corresponding right angular rotation data (65-78 degrees), however, did not show any significant variation with loading rate. Posttest dissection of the specimens indicated that the type of injury observed was related to the rate of axial loading imposed on a specimen during testing.(ABSTRACT TRUNCATED AT 250 WORDS)

Biomechanics of the craniocervical region: the alar and transverse ligaments.

In the treatment of spine fractures and fracture-dislocations, stability of the spine is one of the major objectives. In the craniocervical joint, the alar and transverse ligaments provide much of the stability of the healthy spine. Because the anatomy appears well described, the contribution of each of these structures so far has received little attention. The alar ligament restrains rotation of the upper cervical spine, whereas the transverse ligament restricts flexion as well as anterior displacement of the atlas. A lesion in one or both structures can produce damage to the neural structures and/or cause pain. To investigate the possible role of each of these ligaments, a mechanical and histologic study of the upper cervical spine was made. The bone-ligament-bone complex of the alar and transverse ligaments was subjected to uniaxial mechanical testing in seven specimens. The alar ligaments had an in vitro strength of 200 N, and the transverse ligaments had an in vitro strength of 350 N. Histologic analysis revealed a mainly collagenous nature of these ligaments. Clinical evidence (broken odontoid processes) suggests that the transverse ligament is strong enough to withstand physiologic loads. The alar ligament, on the other hand, due to its lower strength and its axial direction of loading, might be prone to injury and therefore require stabilization of the appropriate vertebra more often than normally is assumed.

Co-ordinated mandibular and head-neck movements during rhythmic jaw activities in man.

Recent observations in man of concomitant mandibular and head movements during single maximal jaw-opening/-closing tasks suggest a close functional relationship between the mandibular and the head-neck motor systems. This study was aimed at further testing of the hypothesis of a functional integration between the human jaw and neck regions. Spatiotemporal characteristics of mandibular and associated head movements were evaluated for 3 different modes of rhythmic jaw activities: self-paced continuous maximal jaw-opening/-closing movements, paced continuous maximal jaw-opening/-closing movements at 50 cycles/minute, and unilateral chewing. Mandibular and head-neck movements were simultaneously recorded in 12 healthy young adults, by means of a wireless opto-electronic system for 3-D movement recordings, with retro-reflective markers attached to the lower (mandible) and upper (head) incisors. The results showed that rhythmic mandibular movements were paralleled by head movements. An initial change in head position (head extension) was seen at the start of the first jaw-movement cycle, and this adjusted head position was retained during the following cycles. In addition to this prevailing head extension, the maximal jaw-opening/-closing cycles were paralleled by head extension-flexion movements, and in general the start of these head movements preceded the start of the mandibular movements. The results support the idea of a functional relationship between the temporomandibular and the cranio-cervical neuromuscular systems. We therefore suggest a new concept for human jaw function, in which "functional jaw movements" are the result of activation of jaw as well as neck muscles, leading to simultaneous movements in the temporomandibular, atlanto-occipital, and cervical spine joints.