KEY NOTES CHAPTER 1: GENERAL PRINCIPLES (I) Lasers and Microsurgery Flashcards
What does LASER stand for?
Light Amplification by Stimulated Emission of Radiation
What wavelength is the visible part of the electromagnetic spectrum?
Visible part of electromagnetic spectrum = 400nm (blue) to 700nm (red).
Invisible wavelengths:
∘ Shorter wavelengths (higher energy) – UV, X-rays, γ-rays.
∘ Longer wavelengths (lower energy) – infrared, microwaves, radio waves.
What is stimulated emission?
- An atom in its resting state is composed of a nucleus and circulating electrons in their ground state.
- Adding energy to an atom causes electrons to shift into a higher energy, unstable orbit.
- The excited electron falls back to more stable ground state, and releases a photon of light of specific wavelength to that atom.
- If that photon collides with another excited electron, that electron returns to its ground state, releasing another photon.
- The original photon is not absorbed, so there are now two photons of the same frequency.
- These photons are in phase.
What is light amplification?
- As photons hit other excited electrons, more photons are released and the light energy increases.
- Population inversion occurs when majority of the molecules in the laser exist in an excited state.
What is the structure of a basic laser?
- An external power source, e.g. flash lamp, diode, radiofrequency emission.
- A lasing medium:
– Solid, e.g. ruby crystal, Neodymium:YAG, Erbium:YAG, KTP.
– Gas, e.g. CO2, argon.
– Liquid, in dye lasers. - Reflective mirrors at each end of a laser tube.
– Photons hit mirrors and are reflected back into lasing medium, number of photons that travel back and forth between mirrors increases, parallel to the tube.
– 1 mirror is only partially reflecting. Light is allowed to escape from the tube as a beam of laser (can be focused with a lens).
What is Q-switching?
- Q-switched laser uses two fully reflective mirrors.
- Stimulated emission of light cannot escape and high power is generated because of a large population inversion.
- Q-switch dumps entire contents of the chamber, producing a short pulse of high intensity.
∘ A normal laser releases its energy like water escapes from a bath through the plughole.
∘ A Q-switched laser releases its energy like if the bottom of the bath was suddenly removed, dumping all the water at once.
What are the properties of laser light?
- Collimated: minimal divergence
- Monochromatic: 1 wavelength.
- Coherent: peaks and troughs of laser light waves are in phase.
How does laser react with tissue?
• Causes thermal, chemical, or photoacoustic effects.
When laser light hits tissue it can be: ∘ Reflected ∘ Scattered ∘ Absorbed (& causes biological effects) ∘ Transmitted.
A specific wavelength of laser light will be preferentially absorbed by a target chromophore e.g. water, Hb, melanin.
What are the thermal effects of laser?
- Coagulation
– Light absorbed by a target chromophore is converted to heat.
– Coagulation occurs when tissue containing the chromophore reaches 60 ∘C. - Vaporisation
– Tissue heated to 100 ∘C will vaporise. - Selective photothermolysis
– Thermal damage is induced in a tissue target that absorbs light of a specific wavelength.
– Selectivity occurs when exposure time of tissue to laser light is shorter than thermal relaxation time (TRT).
– TRT = time taken by a specific volume of tissue to dissipate 51% of the energy absorbed.
– Heat energy dissipated to surrounding non-target tissues can cause collateral effects.
– Once the TRT has elapsed, another pulse can be delivered to the target without generating
thermal damage to surrounding non-target tissue.
What effect does cooling have on laser treatment?
- Cooling protects superficial non-target tissue, e.g. epidermis, from collateral thermal damage.
- Allows higher energy levels to be used.
• Four basic methods:
- Bulk pre-cooling – epidermis and dermis cooled prior to pulse delivery.
- Dynamic pre-cooling - epidermis cooled prior to pulse delivery.
- Parallel cooling – epidermis cooled during pulse delivery.
- Post-cooling – epidermis and dermis cooled after pulse delivery.
• e.g. cryogen spray, gliding window handpiece, cold handpiece, cold air, cooling gel.
What variables are used to control lasers?
- Wavelength
– Measured in nanometres (nm); specific to lasing medium. - Power
– Measured in Watts (W) or Joules per second (J/s). - Spot size
– Measured in cm2.
– Larger spot sizes show less scatter and penetrate deeper. - Duration of action (pulse width)
– Measured in fractions of a second.
Variables 2-4 used to calculate:
∘ Power density: energy delivered per unit area of incident tissue (W/cm2).
∘ Fluence: product of power density and exposure time (J/cm2).
What are the clinical applications of laser?
Vascular lesions (Oxyhaemoglobin) Pulsed Yellow Dye Laser 585 KTP 532 Nd:YAG 1064 IPL* N/A
Skin resurfacing (Water)
Er:YAG 2940
CO2 10,600
Pigmented lesions (Melanin)
Diode 800
Ruby 694
IPL* N/A
Hair removal (Melanin) Alexandrite 755 Diode 800 Nd:YAG 1064 Ruby 694 IPL* N/A
Tattoo removal
- Black/blue/green
Q-switched ruby 694
Q-switched alexandrite 755
- Black
Q-switched Nd:YAG 1064 - Red/orange/brown
Q-switched Nd:YAG 532
What are the key points in laser safety?
Control of Artificial Optical Radiation at Work Regulations 2010: medical lasers are class IV.
Risks: retinal and cutaneous burns.
∘ Risk awareness through risk assessment.
∘ Cover reflective surfaces.
∘ Eye protection.
∘ Laser key stored away from machine.
∘ Corneal eye shields if Rx around eyes.
∘ Laser-safe ET tube with CO2 laser.
∘ Plume evacuators for e.g. CO2 laser, to prevent inhalation of live virus particles.
How is collateral damage of surrounding tissue prevented?
Duration of exposure of the laser must be equal to or shorter than the thermal relaxation time of the chromophore target.
How do you classify lasers in terms of mechanism of action?
Ablative and non-ablative.
What are the 2 main types of ablative lasers and what are their effects on the skin?
CO2 laser
Erbium:YAG laser
The goal is to
- vaporize superficial epidermal tissues and
- cause thermal coagulation deeper tissues
- without causing scarring.
• Areas of irreversible and reversible damage are produced, both of which produce inflammation and wound healing.
How does CO2 laser work?
Wavelength: 10,600nm Chromophore: water Depth of ablation: depends on: - no. of passes - cooling time b/t passes. Clinical endpoint: pale yellow skin surface (midreticular dermis)
Risks • 3-6mths erythema • hypopigmentation • erythema • hyperpigmentation (reduced by adding hydroquinone, kojic acid and sunscreen postop) • Milia • Acne exacerbation • Superficial infection • Stimulation of herpes simplex flare-ups • Hypertrophic scarring • Ectropion
What are the absolute contraindications?
Active viral, bacterial, fungal infection.
Isotretinoin use within 12mths.
How does fractionated CO2 laser work?
- Combines fractional photothermolysis with ablation.
- Pulse delivered in evenly spaced “pixilated” pattern (microthermal zones - MTZ).
• Supplies dermal coagulative energy without confluent epidermal damage
• Rapid reepithelialization and dermal reconstruction occurs from unaffected cells within
punctate zones of injury.
Indications
- facial rhytids
- actinic damage
- scarring
Therapy is guided by proper selection of coverage density and energy settings, not clinical
endpoints as with CO2 laser.
How does the Erbium:YAG laser work?
Wavelength 2940nm
Chromophore: water
Less thermal diffusion than CO2
Energy absorbed 12-18x more efficiently than CO2
Provides 3-5 mm of ablation per pass
Generates 20-50 mm of thermal damage
Clinical endpoint: punctate bleeding, fragmented dermis appearance.
Indications: epidermal/dermal lesions, mild acne scars and AK, subtle dyspigmentation.
- less depth of penetration
- shorter recovery time
- 5.5 days to re-epithelialise
- 3-4wks erythema
What are the non-ablative lasers?
Fractional resurfacing
Nd: YAG
IPL
How do non-ablative lasers work?
Heat generated in dermis causes inflammation, collagen reorganization, and new collagen generation, thus tightening and rejuvenating skin
Does NOT vaporise epidermis.
What is fractional resurfacing?
Wavelength: 1.5 microm
Penetration: 300 microm
Chromophore: Blue dye is pixilated onto skin and this acts as target (MTZ).
About 13-17% of face is treated each time.
Requires 4-5 treatments
Less downtime, but less noticeable results than ablative.
Suitable for: dyschromia, fine wrinkles (neck, chest, hands, face).
What is Nd:YAG laser?
Neodymium:yttrium-aluminum-garnet laser
Wavelength 1064nm
Non-specific absorption and heating of tissue
Chromophores: Proteins (Blood vessels, RBC, collagen, and melanin) and water
Due to scattering of laser, area of greatest photon density is 1-2mm below skin (in dermis)
1320 nm Nd:YAG laser targets water.
- Requires cooling to prevent blistering
Overall collagen remodelling and neocollagenesis is less than ablative, but there is no crusting, oedema and prolonged erythema.
What post treatment care is required?
Warn patients about appearance. Moist wound healing: - liquid based ointments - acyclovir - antibiotics \+/- antifungals, steroids \+/- Vit C cream \+/- Hydroquinone, kojic acid, and sunscreen to prevent hyper pigmentation.
What is IPL?
- IPL is not laser
- It emits photons 500-1300 nm
- Chromophore: water and hemoglobin at 550-580 nm, superficial pigment at
550-570 nm, and deeper pigment at 590-755 nm (filters used to exclude other wavelengths). - Apply topical anaesthesia
- Downtime 24hrs
- Sunblock.
What are the indications and contraindications of IPL?
Indications
• Hypervascularity (flushing, telangiectasias, or rosacea) - 50-75% success rate.
• Hyperpigmentation (solar lentigines, melasma, or freckling) - 40-60% success rate.
• Improvement of skin texture
• Decrease pore size
Contraindications • Unrealistic expectations • Tanned skin • Hypersensitivity to sunlight • Photosensitizing medications • Active isotretinoin (Accutane) treatment • Therapeutic anticoagulation medications • Pregnancy • Active lupus • History of wound-healing problems • Skin cancer • Fitzpatrick VI skin type (Fitzpatrick V is a relative contraindication.)
What are the potential side effects of IPL?
prolonged redness, transient speckling of pigmentation, scabbing, edema, hair loss, purpura, hyperpigmentation, hypopigmentation, herpes eruption, infection, and scarring
