**4. The Harrison normal spine model**

In the mid 1990s to the mid 2000s, the Harrison research team performed a series of spine modeling studies of the sagittal spinal curves (**Figure 7**) [17–24]. To

#### **Figure 7.**

*The Harrison normal sagittal spine model as the path of the posterior longitudinal ligament. The cervical, thoracic and lumbar curves are all portions of an elliptical curve having a unique minor-to-major axis ratio. The cervical curve is circular meaning the minor and major axes are equal (courtesy CBP seminars).*

#### *Complementary Therapies*

this day, this seminal work serves as the treatment outcome goal (i.e. gold standard) for providing structural rehabilitation by CBP methods (**Figure 8**). In a series of systematic studies, elliptical shape modeling of the path of the posterior longitudinal ligament was performed as it could be easily compared to the posterior vertebral body margins on X-rays, the same anatomical region used for measuring the sagittal spinal curves (i.e. Harrison posterior tangents (**Figure 9**) [25–28]).

Computer iterations of spine shape modeling were applied to determine the best-fit geometric spinal shapes by fitting ellipses of varying minor-to-major axis ratios to the digitized data points from the posterior vertebral body corners from X-ray samples for each of the three regions of the spine (cervical [17–19], thoracic [20, 21], and lumbar spine [22–24]). As shown in **Figure 7**, the Harrison normal spinal model features a circular cervical lordosis, an elliptical thoracic curve featuring greater curvature cephalad with a straightened thoraco-lumbar junction and an

#### **Figure 8.**

*Three patients demonstrating dramatically different spine alignment patterns. Left: excessive lumbar hyperlordosis, L4 anterolisthesis, and excessive anterior sagittal balance in a mid-aged female with disabling low back pain; middle: excessive thoracolumbar kyphosis and early degenerative changes in a mid-aged male; right: excessive thoracic hyperkyphosis in a young male with Scheuermann's disease. Red line is contiguous with posterior vertebral body margins; green line represents Harrison normal spinal model (courtesy PAO).*

*DOI: http://dx.doi.org/10.5772/intechopen.102686 An Introduction to Chiropractic BioPhysics® (CBP®) Technique: A Full Spine Rehabilitation…*

#### **Figure 9.**

*Harrison posterior tangent method involves lines drawn contiguous with the posterior vertebral body margins. Intersegmental as well as regional sagittal curves are easily quantified having a standard error of measurement within about 2° (courtesy CBP seminars).*

elliptical lumbar lordosis showing a greater distal lumbar curvature. The spine is assumed to be vertical in the front view.

Although some have attempted to criticize the Harrison normal spinal model, it is important to acknowledge that it has been validated in several ways. Simple analysis of alignment data on samples of normal, asymptomatic populations have been done [17–24]. Comparison studies between normal samples to symptomatic samples have been performed [17, 29]. Comparisons between normal samples to theoretical ideal models have been done [17, 18, 20, 23]. Statistical differentiation of asymptomatic subjects from symptomatic pain group patients based on alignment data has been performed [19, 24].

In subsequent biomechanical modeling studies, the Harrison group used a validated postural loading model to verify that sagittal spinal balance and the sagittal curves of the spine are critical biomechanical parameters for maintaining postural load balance in healthy subjects [30]. Keller et al. [30] stated "because the pattern of [intervertebral disc] IVD postural stresses mirrored the sagittal curvatures and sagittal displacement of the spine, a failure of the IVD's hydrostatic mechanism under these sustained loads could occur". In a similar biomechanical modeling study, Harrison et al. determined that anterior sagittal thoracic posture (anterior thorax translation relative to the pelvis) resulted in significant increases in disc loads and stresses for all vertebral levels below T9 and that the extensor muscle loads required to maintain static equilibrium in upright anterior posture increased almost five times that of normal [31]. In another study Keller et al. [32] determined that "postural forces are responsible for initiation of osteoporotic spinal deformity in elderly subjects".

The Harrison group also used an elliptical shell model to evaluate the loads and bending moments on the cervical vertebrae in varying cervical spine deformity

alignments [33, 34]. They found that in normal lordosis the anterior and posterior vertebral body stresses are nearly uniform and minimal, whereas, in cervical deformity configurations having kyphosis (S-shape kyphosis high or low, total kyphosis), the vertebral body stresses are 'very large' and opposite in direction compared to normal lordosis [33]. They concluded "This analysis provides the basis for the formation of osteophytes (Wolff's Law) on the anterior margins of vertebrae in kyphotic regions of the sagittal cervical curve. This indicates that any kyphosis is an undesirable configuration in the cervical spine" [33]. Anterior head translation and a 'military' neck also displayed significantly increased vertebral body stresses that are reverse in direction from C5-T1 and are also proven to be "undesirable configurations in the cervical spine" [34].
