Upper Cervical Biomechanics

Upper Cervical Biomechanics

Roderic P. Rochester

The biomechanics of the cervical region are among the most complex motions within the human spine. The segmental movements have six degrees of freedom relative to a Cartesian coordinate system and are coupled between lateral flexion, rotation, and flexion or extension, depending on the segmental levels during normal global motion. This chapter will review global ranges of motion, segmental motion, coupled motion, and biomechanical concepts that relate to the Orthospinology procedure.

Global Range of Motion

The global ranges of motion (ROMs) in the cervical spine consist of flexion, extension, bilateral lateral flexion, and rotation. The number of degrees and the direction the head travels relative to the thorax describes these movements. There are many different ways to measure ROM: goniometer (manual or computer assisted), inclinometers (manual or computer assisted), stereophotogrammetry, X-rays, computed tomography (CT) scans, magnetic resonance imaging (MRI), and three-dimensional MRI. The measurements tend to vary slightly between each method. A large study done in Brussels using an electrogoniometer consisted of 250 volunteers (aged 14–70) and was reported in 2001.1 Motion patterns were measured between the first thoracic vertebra and head to establish a normal database for clinical reference (Table 16-1). Reductions of all ROM with age were obtained; sex had no influence on cervical ROM. Lateral flexion and rotation were coupled in this study, meaning that one did not occur without the other. ROM is considered a valid outcome measurement having published research supporting its Validity and Reliability.2 It has been demonstrated in a large, double-blind, randomized control trial that adjusting the atlas increases active ROM.3 Another reference of value is the American Medical Assocation’s Guides to the Evaluation of Permanent Impairment. This database requires the use of the dual inclinometer method to make comparisons (Table 16-2).4

Coupled Motion

Coupled motion attempts to describe the complex interaction between individual spinal segments relative to an
absolute spatial coordinate system and global effects during normal ROM. It is an example of how structural architecture dictates function. Normal coupled motion requires a coordinated muscle recruitment pattern and well-functioning neurological control or its equivalent. The cervical spine exhibits the most complex combinations of coupled motion. One problem of trying to measure coupled motion is the accuracy of the selected method of measurement. The nature of radiographic examination is two-dimensional; thus, some information is lost in the measurement process. Research is done using cadavers but often the muscles are dissected away, which affects the measurement outcomes. Differences in coupled motion findings that exist in the peer-reviewed literature are often due to different methods of measurements and testing.

TABLE 16-1 Global Ranges of Motion for the Cervical Spine

Cervical Spine Range of Motion Measurement Mean (in degrees) Standard Deviation
Flexion/extension 122 (±18)
Lateral bending (left/right) 88 (±16)
Rotation (left/right) 144 (±20)

TABLE 16-2 American Medical Association Guides for Impairment Rating Normal Range of Motion for the Cervical Spine

Cervical Spine Range of Motion Measurement (in degrees)
Flexion 50
Extension 60
Lateral flexion 45 (each side)
Rotation 80 (each side)

Two 2004 studies using three-dimensional MRI have enabled accurate in vivo three-dimensional intervertebral movements during neck rotation in Healthy adults.5,6 Coupled motion was measured for head rotation and is summarized below (Table 16-3). The most interesting feature of this research is that when the head is maximally rotated, lateral flexion between the occiput and atlas occurs on the opposite side with a measured mean of 4.1° (SD 1.4). This indicates that Atlas laterality as measured by the Orthospinology procedure changes with maximum head rotation in normal adults. Observation of the graphed data indicates that it requires between 30 and 45° of head rotation to demonstrate the average magnitude in degrees (approximately 3°) of lateral flexion between the occiput and atlas that corresponds to the average measured Atlas laterality in the neutral posture for subjects who demonstrate signs of atlas Subluxation. It is logical to deduce that cervical spine rotation ROM would be affected if Atlas laterality exists with the patient in a neutral position because of the atlas alignment relative to the occiput being maintained in a laterally flexed state. This may explain the observations of improved ROM in the cervical spine by Whittingham and Nilsson3 following an atlas Adjustment.

In normal coupled motion, the axis spinous rotates opposite the side of occiput/atlas lateral flexion or laterality. The chiropractic upper cervical Subluxation complex often exhibits abnormally coupled biomechanics. One example finds that the axis spinous rotated toward the side of Atlas laterality (an inferior spinous in Orthospinology terminology) 75% of the time.7 This is exactly opposite of normal coupled motion. It is possible that a neurological mechanism causing unbalanced, uncoordinated muscle recruitment patterns is responsible for maintaining these abnormal Upper cervical biomechanics. It is Hypothesized that normal coupled motion minimizes stress to the spinal cord during cervical spine ROM by more equally distributing tensile forces throughout the length of the spinal cord. A chiropractic Subluxation complex in the upper cervical spine and segmental joint dysfunction alters normal coupled motion reducing this natural buffering system, allowing increased tension to be transfered to the central nervous system.

Neutral Zone, Elastic Zone, and Range of Motion

The neutral zone (NZ) is defined as a region of little resistance to motion in the middle of an intervertebral joint’s ROM. The elastic zone (EZ) is the region where resistance to motion increases because of ligamentous and supportive tissue tension. The ROM of a joint is the total of the NZ and EZ. An increased NZ is linked to spinal instability. Panjabi et al. measured intersegmental ROM and NZs in the upper cervical spine by using cadaveric whole cervical spine specimens.8 The most interesting findings as related to Orthospinology are:

  • The NZ at occiput/atlas is 1.5° for lateral flexion.

  • The NZ is 1.6° for rotation at occiput/atlas.

  • The atlas/axis NZ for lateral flexion is 1.2°.

  • The total ROM for occiput/atlas in lateral flexion is 5.5°.

  • The ROM for atlas/axis lateral flexion is 6.7° (Table 16-4).

TABLE 16-3 Coupled Motion Measurements In Vivo for the Cervical Spine in Rotation via Three-Dimensional Magnetic Resonance Imaging, 2004

Cervical Spine Coupled Motion Measurements with Maximum Cervical Rotation
  Rotation Mean, in degrees (SD) Lateral Flexion Mean, in degrees (SD) Extension or Flexion Mean, in degrees (SD)
C0/C1 1.7 (±1.5) 4.1 (±1.4) (opposite rotation) Ext. 13.4 (±4.9)
C1/C2 36.3 (±4.5) 3.8 (±3.0) (opposite rotation) Ext. 6.8 (±3.0)
C2/C3 2.2 3.6 Ext. 1.4
C3/C4 4.5 5.4 Ext. 2.3
C4/C5 4.6 5.0 Ext. 1.5
C5/C6 4.0 5.3 Flex. 0.9
C6/C7 1.6 4.9 Flex. 2.4
C7/T1 1.5 1.2 Flex. 3.0
SD, standard deviation.      

TABLE 16-4 Neutral zones and Intersegmental Ranges of Motion for the Upper Cervical Spine

Motion Neutral Zone (in degrees) Range of Motion (in degrees)
Oc/C1 flexion/extension 1.1 3.5/21.0
Oc/C1 right/left lateral flexion 1.5 5.5
Oc/C1 right/left rotation 1.6 7.2
C1/C2 flexion/extension 3.2 11.5/10.9
C1/C2 right/left lateral flexion 1.2 6.7
C1/C2 right/left rotation 29.6 38.9

These findings are interesting because the mean Atlas laterality in studies of subjects that show signs of chiropractic upper cervical Subluxation is approximately 3° while in the neutral posture and greater than the NZ but less than the limits of ROM. This seems to violate normal excepted biomechanics. However, following chiropractic intervention, post–X-rays indicate most often that the Atlas laterality returns to within the NZ range. However, the NZ does not explain why Atlas laterality Listings seem very stable over time without chiropractic intervention. One might assume that random movement of the atlas relative to the occiput occurs within its NZ ± 1.5° during the neutral posture, but upper cervical practitioners have not observed this phenomenon in clinical practice. In fact, research has demonstrated only about ½° differences in measurements of Atlas laterality in the neutral posture without chiropractic intervention. This likely represents marking error as opposed to fluctuation of Atlas laterality within the neutral posture.9 When Atlas laterality exists and measures 5.5°, the atlas is possibly positioned in maximum lateral flexion relative to the skull, although the patient is in the neutral posture. It seems that if this were the case, all cervical intersegmental motion would be affected because of altered coupled motion patterns. This may change afferent neurological inputs into the neural axis and alter global motions. Misalignments of this magnitude would be outside of the normal NZ but within the EZ. Measurements outside of the normal ROM greater than 5.5° of Atlas laterality may represent medical luxation or congenital malformation in the absence of trauma.

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Jul 24, 2016 | Posted by in ORTHOPEDIC | Comments Off on Upper Cervical Biomechanics

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