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Breast skin strain during gravitational and dynamic loading

Student thesis: Doctoral Thesis

The breast contains no muscle or bone and its primary supporting structures are reported to be the skin and Cooper’s ligaments. It has been hypothesised that independent breast motion, which is already known to cause pain for many women, may also cause damage to the breast structure leading to breast ptosis (sag). Breast damage could be estimated by applying the published strain failure limits (60%) for human skin to measurements of breast skin strain. However, there have been few attempts to measure breast skin strain as gravitational deformation makes it difficult to identify the neutral breast position in which there is no external strain on the breast skin.

A gold-standard method for estimating the neutral breast position based on Archimedes’ principle was developed and implemented within this thesis. Fourteen female participants, with breast sizes 30 to 34 under band and B to E cup size, had semi-permanent henna markers applied to their torso (4 markers) and left breast (an array of 17 markers including the nipple). Participants were immersed up to their neck in two fluids with mass-densities above (water) and below (soybean oil) the reported range of breast mass-densities (919 kg.m-3 to 986 kg.m-3). The mid-point between the breast position in water and soybean oil provided the gold-standard neutral position estimate. Participants also performed nine alternative novel or previously published methods for estimating the neutral breast position. Alternative methods were assessed for accuracy and precision when estimating the gold-standard neutral nipple position. To investigate the effects of gravity and dynamic activity on the breast, participants had their breast displacement and breast skin strain assessed in the static gravity-loaded (bare-breasted) position and during an incremental-speed (bare-breasted) treadmill test (4 kph to 14 kph). Breast pain was recorded in each condition using an 11-point numerical rating scale.

The gold-standard method was implemented to obtain an accurate (measurement error ≤ 1.4 mm) and precise (TEM ≤ 1.2 mm, SD ≤ 3.7 mm) measurement of the neutral breast position. Evaluation of novel and previously published neutral position methods revealed that the buoyancy in water method achieved the most accurate estimation of the gold-standard neutral nipple position (absolute differences ≤ 5.6 mm, TEM ≤ 1.2 mm, SD ≤ 2.6 mm). Comparison of the gravity-loaded and neutral breast positions demonstrated that gravity caused the nipple to move posteriorly (mean change 15.6 mm), laterally (mean change 7.5 mm) and inferiorly (mean change 25.9 mm) relative to the torso, and induced potentially damaging skin strains up to 75%. During bare-breasted incremental-speed treadmill activity the nipple was displaced furthest from the neutral position in the inferior direction (mean of 45.5 mm); a result which could not be attained using conventional measurements of nipple range of motion (ROM). Strain analysis indicated potentially damaging skin strains during the incremental-speed treadmill activity (up to 114%), particularly in the upper-outer region of the breast. This finding supports the anticipated relationship between breast motion and breast damage. Breast pain was most strongly correlated to superior nipple displacement from the neutral position during treadmill activity (p < 0.001, r = 0.725), and a significant correlation was observed between breast pain and breast skin strain (p < 0.001, r = 0.361). Combination of breast displacement and skin strain data provided a comprehensive analysis of the effect of gravity- and motion-induced breast displacements on the breast skin.

Incorporation of the neutral breast position into future biomechanical research may lead to improved assessment of breast motion and breast damage, and a deeper understanding of motion-induced breast pain. Within clinical research, identification of the neutral breast position may enable the development of breast models that are better able to predict the gravitational deformation of the breast during surgery. Consideration of the neutral breast position, and subsequent breast strain, also has applications within the field of breast support design and evaluation. A breast support garment that positions the breast in the neutral position and restricts motion to within the reversible strain limits of the skin may protect the breast from skin damage and the associated breast pain.
Original languageEnglish
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Award dateApr 2015
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