The thoracic spine might be one of the most overlooked regions of human spine in health care practices. In my experience, there is a general lack of appreciation of both the need for radiographic views of the thoracic spine and the influence of the thoracic spine on whole body alignment and health potential. It is for these reasons that I offer this series of articles detailing the thoracic kyphosis to readership of the American Journal of Clinical Chiropractic. Also, this series of articles are pre-cursors to chapters in the new CBP® Structural Rehabilitation of the Thoracic Spine & Case Management textbook due out later this year.
Average Thoracic Kyphosis Angles
Prior to presenting normative alignment for the thoracic kyphosis, measurement methods must be developed and tested for reliability. Fortunately, for thoracic kyphosis, several groups including CBP® researchers have presented measurement methods and found these to have small standard errors of measurement and good to excellent intra- and inter-examiner reliability.1 Figure 1 shows the Harrison posterior tangent method for measurement of the thoracic kyphosis.

Several studies have reported “normal” values of thoracic kyphosis for people in a wide range of age groups.2-23 The density of the upper ribcage in the coronal plane in many people, causes an inability to accurately identify and measure the vertebral segments T1-T4. Thus, in the literature, various authors report different vertebral levels of kyphosis measurement. Table 1 provides an overview of different kyphosis measurement levels with their respective mean values from the literature. Taking two standard deviations from the reported mean values in Table 1 as a proposed type of statistical normal, a large range of values for thoracic kyphosis from 20° to 50° might be considered within normal limits.2-23

Problematically, some of this normal subject data is contaminated with subjects that should not be considered healthy. For example, Fon et al22 presented thoracic kyphosis measurement in 316 “normal subjects” aged 2-77 yrs. Their22 definition of normal was: “while the general status of some of these patients was not optimal, it was assumed that the patients were sufficiently fit to be ambulatory…” and if they could raise their arms above their shoulder!
In the past 6-7 years, CBP® researchers have proposed a more narrow distribution of thoracic kyphosis values as normal.2,17
For example, in Table 1, the reported values for Cobb T1-T12 and ratios of values from Cobb angles of T3-T11, T4-T11, T2-T12, T3-T12, T4-T12, and T5-T12, the mean thoracic kyphotic value would be between 40° and 50° for a Cobb angle from the superior endplate of T1 relative to the inferior endplate of T12.
Furthermore, Harrison et al24 identified that translated postures of the ribcage relative to the pelvis in the sagittal plane can have a strong influence on thoracic kyphosis. Specifically, a total change of 26° in thoracic kyphosis was found for maximum posterior translation to maximum anterior translation in normal subjects. See Figure 2. Regarding sagittal ribcage plane translation postures, biomechanical models have predicted large increases in extensor muscle loads and consequent increased compression and shear loads on the thoraco- lumbar spine discs.25,26 These high compressive and shear loads may produce pain and initiate or contribute to a degenerative remodeling response in the disc.

The issues associated with sagittal plane ribcage translation prompted Harrison et al17 to present thoracic kyphosis data from a group of 50 normal subjects whose sagittal translation was within one (1) standard deviation of the mean. This data is presented in Table 2.

Average and Ideal Thoracic Kyphosis Models
Looking at the data provided in Table 1 and more specifically two studies on thoracic kyphosis with large population groups, it is apparent that a near normal (bell shaped) distribution for thoracic kyphosis has been found.11,23 Statistically speaking then, there would be an average subject based ideal model for thoracic kyphosis at the top of the bell shaped curve (mean value).
Investigations have presented several types of thoracic kyphosis models in the literature.2,17,18-21,23 For example Harrison and colleagues2 presented average geometric models of the thoracic kyphosis (T1-T12, T2-T11, and T3-T10 segments were modelled) as a segment of an ellipse using pooled data from 80 normal subjects’ lateral thoracic radiographs. Figure 3 shows the average elliptical model of the segments T1-T12.

Harrison et al17 followed this paper with an optimized elliptical model of thoracic kyphosis based in part on data from 50 optimized normal subjects (sagittal translations within one (1) standard deviation from the mean). Since, the thoracic vertebral bodies increase in size considerably from T1 to T12, a uniformly increasing model was derived of disc and vertebral body sizes were derived from the anatomical literature. They found that the major axis of the ellipse (long axis of an oval) is parallel to the posterior body margin of T12, whereas the minor axis of the ellipse (short axis of the oval) passed through the superior endplate of T12. The minor axis to major axis ratio was computed to be 0.69.17 Figure 4 shows this optimized elliptical model in a template form that can be used for any height of a patient. Their17 modeling results were compared to mean values of 678 normal subjects and the data were found to compare closely with their proposed optimized elliptical model.

In the more recent literature, investigators have begun to develop individual subject optimized geometric sagittal plane curve models for thoracic kyphosis.18-21 There are certain anatomic variables that have been shown to have a striking influence on sagittal plane curvature. When these anatomic variables are outside of normal tolerances, a change in sagittal curvature can result. This information will be detailed in Part 2 of this series.
Summary
Using different types of kyphosis models as a normative starting positions of thoracic kyphosis, it becomes possible to characterize both normal thoracic kyphosis alignment and abnormal alignment. Chiropractic has a long history of identifying and trying to restore normal alignment to the spine; where abnormal alignment is specifically referred to as the mechanical component of vertebral subluxation. The key is to fully understand when the modelling and alignment data presented herein is useful in differentiating a subluxated thoracic spine from a normal spine and how to modify the data in specific patient situations.
Till next time.
References
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