Morphological characteristics of humeri and ulnae relating to supratrochlear aperture expression
Date
2015-09-18
Authors
Ndou, Robert
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Abstract
The supratrochlear aperture (STA) is a perforation of the septum found
between the olecranon and coronoid fossae of the humerus. Its
prevalence is population specific and varies by sex. This study
investigated the prevalence, shape and population-level variability of the
STA, as well as the morphological attributes of dry humeri and ulnae
according to STA expression, in South African populations (White, Mixed
ethnicity and Black). The tissue found in the aperture was sampled from
cadavers and studied by histochemical techniques. The objective was to
identify the characteristics of STA bearing limbs in order to evaluate the
possible etiology and function of the aperture.
The overall prevalence of the STA among South Africans was found to be
32.5%. Significant population differences exist, with Whites having the
lowest rate of 16% and Sotho peoples expressing the highest prevalence
of 41% among the Black group. The STA was more prevalent in females
and on the left arm. In bilateral cases, females had twice the prevalence
rate.
Osteometric measures of bone size were used to test the propositions that
robust humeri are less prone to STA formation and that pressure from the
ulnar olecranon and coronoid processes may result in an STA.
Most measures of overall humeral size, such as epicondylar breadth,
humeral head circumference and the three shaft circumferences, were
significantly larger when the STA was absent in all three populations. This
supports the general proposition that gracile bones are prone to septal
perforation. However, an important result of this study is the finding from
the discriminant analyses that determinants of STA status are population
specific. For Blacks and the Mixed group, the olecranon process length
was the sole main contributor to STA status. Lower loadings for this
variable were found for the Mixed group. Therefore, this variable can more
reliably predict STA status in Blacks compared to the Mixed group. In
Whites, with a lower prevalence of the STA, more parameters are required
for STA prediction compared to the Mixed group or Blacks. These
additional parameters were proximal humeral shaft circumference (right)
and trochlear notch depth (left).
The presence of dense connective tissue found crossing the aperture
needs to be explained as it appears to be a feature unique to the distal
humerus. It is possible that the STA forms as a result of incomplete
remodeling in response to pressure from the olecranon process. The result
would be bone resorption and subsequent osteoblastic secretion of
collagen, but no incorporation of hydroxyapatite occurs – rendering the
aperture region pliant and able to withstand the continuing force of the
olecranon during joint motion. As to the function of the STA covering
tissue, it may be of considerable structural strength in light of its dense
regular connective tissue composition. It is strongly attached to the bony
septum in a manner comparable to the way tendons attach to bone,
suggesting a need for structural strength. One explanation for this
particular feature of the tissue may be that it functions to prevent locking
the olecranon process in the joint.
An additional aspect to be considered regarding function of the STA is that
the mean extension angle in the STA bearing arms was greater. This
implies that the olecranon process tip may penetrate the aperture (as
observed on many dry humeri, when articulated with ulnae and elbow
joints maximally extended), permitting a greater extension angle. It follows
that the elbow range of motion is greater in STA individuals.
Based on orthopedic surgical experience, bones with the STA are thought
to have a narrower medullary canal. Humeri with the STA in our study did
appear to have narrower medullary canal dimensions. However, this was
not the case after standardizing for bone size. The smaller medullary canal
width observed in STA humeri is due to the bone size differences rather
than the presence of the STA. We therefore propose that bone size be the
major factor to consider when making choices of a rod for intramedullary
fixation.
Description
A Thesis submitted to the Faculty of Health Sciences,
University of the Witwatersrand, Johannesburg,
in accomplishment of the requirements for the degree
of
DOCTOR OF PHILOSOPHY
25 May 2015