Tissue tearing caused by pulsed laser-induced ablation pressure

Joseph P. Cummings; Joseph T. Walsh

doi:10.1364/AO.32.000494

Applied Optics
Vol. 32,
Issue 4,
pp. 494-503
(1993)
•https://doi.org/10.1364/AO.32.000494

Tissue tearing caused by pulsed laser-induced ablation pressure

Joseph P. Cummings and Joseph T. Walsh

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Joseph P. Cummings and Joseph T. Walsh, "Tissue tearing caused by pulsed laser-induced ablation pressure," Appl. Opt. 32, 494-503 (1993)

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Abstract

Pressure induced by ablative pulses of laser radiation is shown to correlate with the mechanical disruption of tissue. The ablation pressure induced during Er:YSGG laser irradiation of skin, liver, and aorta was calculated from a ballistic pendulum-based measurement of recoil momentum. The ejected material and ablation crater were examined grossly and microscopically after ablation. A gas-dynamic model of laser-induced vaporization was used to understand the measured pressures. The results show that mechanical disruption of tissue occurs when the ablation pressure exceeds the strength of the irradiated tissue at sites of intrinsic weakness.

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Tissue	Expansion during $Pulse (\frac{D_{beam} - D_{creater}}{D_{beam}})$	Contents of Ejecta	Tissue Tearing
Tissue		Contents of Ejecta	Threshold	Manifestation
Skin	<10% at high fluences	Mostly vapor and 10–20-μm-diameter particles at high fluences	–	No tearing noted
Aorta	Longitudinal >75% at high fluences	Vapor and 10–50-μm diam particles at high fluences	20 J/cm²	Tearing through ground substance between elastin fibrils
	Transverse ≈25% at high fluences
Liver	≈100%	Vapor, strands, chunks of 50–500-μm length	10 J/cm²	Large intact chunks of matter in plume. Tears in nonablated tissue
Cornea	<10% at high fluences	Vapor, strands	10 J/cm²	Epithelial flaps
			50 J/cm²	Long radial tears

Tissue	Composition and Structure^b	Mechanical Properties^b
Tissue	Composition and Structure^b	U.T.S. (MPa)	Ult. Elong. (%)
Skin (dermis)	Water (69.3 wt.% total); collagen (75 wt. % dry); mucopolysaccharides (20 wt. % dry)	7.6	78
	Consists of randomly oriented collagen fibers
		Transverse
Aorta (media)	Water (79 wt. % total); elastin/elastin + collagen (≈65 wt. % dry)	1.1	77
		Longitudinal
	Elastic fibers in the media are oriented circumferentially with respect to the lumen	0.07	81
Liver (parenchyma)	Water (78.9 wt. % total); collagen (< 5 wt. % total)	≪0.04°	>100^a
	Extremely cellular structures grouped into cylindrical lobules

	Mechanical Properties
Components of Tissue	U.T.S. (MPa)	Ult. Elong. (%)	Mod. of Elasticity (MPa)
Collagen	3000	50	100
Elastic fibers	1	100	3

Symbols	Definitions
p	Pressure
ρ	Density
υ	Velocity
T	Temperature
D	Velocity of ablation front
R	Gas constant
I	Irradiance
∊	Specific internal energy
q	Laser power density
m	Mass loss
γ	Ratio of specific heat, 4/3
L	Effective heat of vaporization, 4.2 × 10⁶ J/kg
A	3.075 × 10¹⁰ Pa (curve fit to values for pure water²)
U/R	4688.2 K (curve fit to values for pure water²)

Tissue	Expansion during $Pulse (\frac{D_{beam} - D_{creater}}{D_{beam}})$	Contents of Ejecta	Tissue Tearing
Tissue		Contents of Ejecta	Threshold	Manifestation
Skin	<10% at high fluences	Mostly vapor and 10–20-μm-diameter particles at high fluences	–	No tearing noted
Aorta	Longitudinal >75% at high fluences	Vapor and 10–50-μm diam particles at high fluences	20 J/cm²	Tearing through ground substance between elastin fibrils
	Transverse ≈25% at high fluences
Liver	≈100%	Vapor, strands, chunks of 50–500-μm length	10 J/cm²	Large intact chunks of matter in plume. Tears in nonablated tissue
Cornea	<10% at high fluences	Vapor, strands	10 J/cm²	Epithelial flaps
			50 J/cm²	Long radial tears

Tissue	Composition and Structure^b	Mechanical Properties^b
Tissue	Composition and Structure^b	U.T.S. (MPa)	Ult. Elong. (%)
Skin (dermis)	Water (69.3 wt.% total); collagen (75 wt. % dry); mucopolysaccharides (20 wt. % dry)	7.6	78
	Consists of randomly oriented collagen fibers
		Transverse
Aorta (media)	Water (79 wt. % total); elastin/elastin + collagen (≈65 wt. % dry)	1.1	77
		Longitudinal
	Elastic fibers in the media are oriented circumferentially with respect to the lumen	0.07	81
Liver (parenchyma)	Water (78.9 wt. % total); collagen (< 5 wt. % total)	≪0.04°	>100^a
	Extremely cellular structures grouped into cylindrical lobules

	Mechanical Properties
Components of Tissue	U.T.S. (MPa)	Ult. Elong. (%)	Mod. of Elasticity (MPa)
Collagen	3000	50	100
Elastic fibers	1	100	3

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Applied Optics