Optical Measurements of the Skin Surface to Infer Bilateral Distinctions in Myofascial Tissue Stiffness
Optical Measurements of the Skin Surface to Infer Bilateral Distinctions in Myofascial Tissue Stiffness
November 12, 2024
Anika R. Kao, Zack T. Landsman, Mary T. Loghmani, Gregory J. Gerling
About half the U.S. adult population suffers from chronic neuromusculoskeletal pain. While its evaluation and treatment are widely addressed by therapies using soft tissue manipulation (STM), their efficacy is based upon clinician judgment. Robust biomarkers are needed to quantify the effects of STM on patient outcomes. Among noninvasive methods to quantify the mechanics of myofascial tissue, most are limited to small (10 mm2), localized regions of interest. In contrast, we develop an approach to optically simultaneously measure a larger (~100 cm2) field of deformation at the skin surface. Biomarkers based on skin lateral mobility are derived to infer distinctions in myofascial tissue stiffness. In specific, three cameras track ink speckles whose fields of deformation and stretch are resolved with digital image correlation. Their ability to differentiate bilateral distinctions of the cervicothoracic region is evaluated with four participants, as a licensed clinician performs STM.
In this work, we develop mechanical biomarkers based on surface observations of skin lateral mobility, and evaluate their ability to infer myofascial tissue differences against stiffness measurements of a load cell. The development of sensitive biomarkers that do not impede direct skin contact of clinicians may enable greater precision in assessing myofascial pain.
Experimental setup and data collection using DIC
Two types of STM—manual compression and manual pull—were performed to assess the myofascial tissue of the cervicothoracic region (Fig. 1C-E). For comparison, baseline measurements were also collected in separate experiments using an instrumented force probe to capture force-displacement responses. The two approaches are compared to determine the ability of the skin mobility biomarker to capture bilateral differences in myofascial stiffness per participant with similar relative trends, i.e., greater stiffness per side of the body.
Equipment setup, participant positioning, and manual manipulation procedures. (A) The clinician assesses the myofascial tissue mobility of the left cervicothoracic region of a participant. An ink speckle pattern was applied to the left and right cervicothoracic regions for skin surface tracking with three overhead cameras. (B) An idealized abstraction of the experimental setup while the clinician applies compression to the left trapezius. (C) An abstraction of clinical administration of manual soft tissue compression with two hands, where the displaced speckles show inward movement toward the point of application. (D) Similar administration of compression using the instrumented force probe. (E) Administration of manual pull in the inferior direction, where the yellow arrow depicts the movement of speckles, and as such, skin displacement, in the direction of the pull.
Skin mobility biomarker
To assess skin mobility, we developed a biomarker representing the relationship between the minimum 1st principal stretch (compressive) and the maximum manual pull, per anatomical direction.
Magnitude and direction of skin surface stretch during the clinical application of manual pull assessment in four anatomical directions. (A) The 1st principal stretch is depicted by the colormap overlayed on the image from one camera for manual pull in the superior direction. A 1st principal stretch value of 1 (red) depicts no skin surface stretch between tracked points from the initial state.In contrast, a value of 0.8 (blue) depicts 20% compression in the tracked points from their initial state. Greater minimum stretch (i.e., a lower scale value) is measured near clinician finger contact and in the direction perpendicular to the manual pull. (B) The corresponding 3D quiver plot depicts skin surface movement as 3D vectors from their initial state, including rigid body motion. (C) Minimum stretch plotted against maximum pull, without rigid body motion. Clear separation between green (left) and purple (right) points indicates that at the same magnitude of manual pull, more compressive stretch is observed on the left side of the body.
Baseline tissue stiffness measure
To develop a baseline characterization for tissue stiffness, we develop force-displacement relations from the instrumented force probe and camera/DIC quantification of displacement. While maximum force levels are similar bilaterally (right: 9.93N, left: 9.34 N), despite the clinician not being given feedback on their employed force, displacement varies significantly. As this plot indicates, at similar levels of force, displacement levels are much higher on the right side of the body than on the left, indicating the left side offers greater resistance to compression and is stiffer.
Manual and instrumented force probe compression techniques. (A) Normal displacement of the skin surface in response to manual compression by clinician on the right trapezius. (B) Similar data for instrumented force probe compression for the same clinician, participant, and body side. (C) Cross-sections of 3D normal displacement taken before contact (black) and at maximum indentation for finger (blue) and instrumented force probe (purple) compression on the right side of the body. (D) Cross-sectional comparison of instrumented force probe compression on right (purple) and left (green) sides of the body (trial 1), indicating a bilateral distinction. The solid lines depict surface points tracked with DIC while the dotted lines depict a projected connection between the imaged skin surface and tracked probe. (E) Plots of the instrumented force probe tip’s normal displacement and recorded force (left and right sides of the body) show a steeper increase in probe normal displacement at a lower force on the right side, suggesting less stiff myofascial tissue on the right side as compared to the left, in agreement with the bilateral distinctions in the cross-sectional images in panel D.
Differentiating bilateral stiffness with both approaches
We bilaterally compare the results between the skin mobility biomarker, and force-displacement measurements, across the four participants (Fig. 5). The skin mobility biomarker captured clear bilateral differences, with a maximum pull of ~2 mm and a minimum stretch of 0.56 mm on the left side of the body. In contrast, on the right side, at similar pull levels, the minimum stretch was 0.93.
(A) Minimum stretch against maximum pull, in the superior pull direction. Clear separation is observed between points in green (left side) and purple (right side), suggesting distinct skin surface movement differences bilaterally. (B) For the same participant, measured force during instrumented (probe) compression against normal displacement also suggests distinct differences in myofascial stiffness bilaterally. This finding likely indicates that when myofascial mobility is lower, musculoskeletal regions may be more stiff and as such, the skin surface accounts for more of the mobility, seen as lower minimum stretch, or increased skin surface compression during manual pull.
Conclusion
This work develops mechanical biomarkers based upon observations of skin surface deformation, captured with digital image correlation, and evaluates their ability to infer myofascial differences in bilateral anatomy during soft tissue manipulation. The agreement of the skin mobility biomarker and force-displacement measurements in this case study suggests that the deformation of the skin surface can provide inference into underlying myofascial stiffness. The development of a skin surface biomarker able to differentiate stiffness differences felt by a clinician may enable greater precision in affective touch therapies used to assess and treat myofascial pain. Additionally, these biomarkers can inform design requirements for innovations in medical haptics such as clinical training simulations, tele-remote assessment and treatment, and knot detection via robotic massage. Moreover, the biomarkers described herein do not impede direct skin contact between clinicians and their patients.
References
A. R. Kao, Z. T. Landsman, G. J. Gerling and M. T. Loghmani, "Optical Measurements of the Skin Surface to Infer Bilateral Distinctions in Myofascial Tissue Stiffness," 2023IEEE World Haptics Conference (WHC), Delft, Netherlands, 2023, pp. 244-251, [DOI]
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