A PILOT
STUDY TO INVESTIGATE THE VALIDITY OF THE "RULE OF THREES" OF THE
THORACIC SPINE.
Michael A. Geelhoed* 1; Patricia
A Brewer1
1. Physical Therapy, University
of Texas Health Science Center at San Antonio, San Antonio, TX
PURPOSE: The purpose of this study is to examine the validity of using
the "rule of threes" to locate the transverse processes (TP's) of the thoracic spine relative to the spinous processes (SP's).
BACKGROUNDS/SIGNIFICANCE: Palpation of the TP's
of the thoracic spine is challenging because of their depth relative to the
more superficial structures of the spine. This can present a difficult
situation for clinicians, as many manual techniques for the thoracic spine
require hand or finger placement over the TP's. Most
clinicians use the more superficial and easily palpable SP's of the thoracic
spine to orient themselves for palpation of the deeper structures. In order for
clinicians to be specific and effective in their evaluation and treatment
techniques, it is crucial to be able to accurately locate and identify the
level of the TP's of the thoracic spine. Therefore, a
model is needed to predict the location of the TP's
relative to the easily palpable SP's. In 1979, Dr. FL Mitchell first described
a "rule of threes" for locating the TP's of
the thoracic spine. The rule of threes divides the thoracic spine into sets of
three vertebrae: a) T1, T2 and T3, b) T4, T5 and T6, c) T7, T8 and T9, and d)
T10, T11 and T12. Each set of vertebrae has a rule for locating the TP's depending on the location of the SP. The rule of
threes states: 1. The upper three thoracic vertebrae (T1-T3) have SP's that
project directly posterior and therefore the tip of the SP is in the same plane
as the TP's of that same vertebra. 2. The next three
vertebrae (T4-T6) have SP's that project slightly downward and therefore the
tip of the SP is in a plane that is halfway between its own TP's
and that of the TP's of the vertebra below it. 3. The
next three vertebrae (T7-T9) have SP's that project moderately downward and
therefore the tip of the SP is in a plane with the TP's
of the vertebra below it. 4. The last three vertebrae (T10-T12) have SP's that
project from a position similar to T9 and rapidly regress until T12 is like T1;
i.e., T10 SP is in the plane of the TP's of the
vertebra below it; T11 SP is halfway between its own TP's
and that of the vertebra below it; and T12 SP is in the plane of its own TP's. The rule of threes has been widely accepted in
orthopedic and manual therapy texts, however, scientific evidence to validate
it is lacking. This poster reports the results of a pilot study to determine if
the rule of threes is an accurate predictor of the location of thoracic TP's relative to their corresponding SP's.
SUBJECTS: 5 cadavers were dissected to clearly visualize and measure the
TP's and SP's of the thoracic spine.
METHODS AND MATERIALS: Based on observations of spinal anatomical
models, the hypothesis was that the rule of threes is not an accurate predictor
of the location of the TP's relative to the SP's. The
cadavers were dissected to clearly visualize and palpate the SP's and TP's of the thoracic spine. Pins were placed at the
midpoint of the SP and TP's of each thoracic
vertebra. The vertical distance between the plane of the TP's
of one vertebra and the pin inserted in the SP of the superior vertebra was
measured using a metric ruler. The midpoint was chosen for measurement because
it is typically the most prominent and easily palpable structure of the SP and
TP. All measurements were taken with the cadavers in the prone position, as
this duplicates a common position for manual therapy evaluation and treatment
techniques, and allowed for the easiest and most accurate measurement.
ANALYSES: The vertical distance between the TP's
of each thoracic vertebra and the SP of the vertebra above were averaged for
all 12 thoracic vertebra. The standard deviation was calculated to assess the
variability of the data.
RESULTS: All measurements showed a mean distance of 4 millimeters or
less, with all levels except the first and last having a mean distance of under
2 millimeters. Standard deviations were all less than 5 millimeters, with the
greatest variability seen in the superior thoracic spine, and variability
decreasing at the more caudal levels (see table).
CONCLUSIONS: This pilot study investigated the location of the TP's of the thoracic spine in relation to their
corresponding SP's. Based on the results obtained, the rule of threes was
determined not to be an accurate predictor of the location of the transverse
processes.
FUNDING SOURCE: None
KEYWORDS: manual therapy, palpation, spine
Measurements (in millimeters) of the vertical distance between the SP of one
thoracic vertebra and the TP's of the caudal vertebra
*negative values indicate that the SP is inferior to the plane of the TP's; positive values indicate that the SP is superior to
the plane of the TP's
