Ophthalmic Laser Procedures Flashcards
Ophthalmic lasers provide good examples of three fundamental laser applications based on
Total output energy
Output power
Photon energy
Spectral transmission
Nomenclature of laser is based on
Gain medium Wavelength Output power Repetition rate Pulse duration Patterns used to deliver the laser beam
Vitreolysis
“Breaking down glass”
Laser for floaters
What are floaters
- small pieces of debris that float in the vitreous
- common complaints
- the debris cases shadows on th retina
- not serious
- appear as floating spots, strings, or cobwebs across the field of vision
How do you get floaters
- age
- losing clarity and transparency of vitreous
- degernation of vitreous
- vitreous cortex separated from the retina
- causes forward light scattering
- some eye conditions predispose floaters (high axial myopia)
What is vitreolysis
- a non invasive procedure that can relieve the visual disturbance caused by symptomatic floaters via vaporization
- Nd:YAG laser light, nanosecond pulsed.
- evaporates the collagen and hyaluronin molecules that make up the floaters, thus fragmenting the vitreous opacities
- floater laser removal
Emitted light of the Nd:YAG laser
1064nm, in the infrared
Frequency doubled Nd:YAH
532 in the green
Laser used for vitreolysis
Nd:YAG
Photocoangulation
- tightly focused, spatially confined laser energy interacts wit hthe tissue and transfers energy to the tissue via absorption
- this slowly heats blood and tissue destabilizing proteins and other biomolecules
- as blood clot is formed, tissue coagulation of above 50 C but below 100 C is called photocoagulation
- a laser heating coagulation shrinks the tissue due to water removal; the tissue is burned and neutered
- this is a form of hemostatic laser surgery, which is a bloodless incision/excision
Energy density of photocoagulation
10^-2
Most serious floater
Weiss ring
- happens later in age
- cortex detaches from the retina
Vitrectomy
Surgical replacement of the vitreous (ncluding the symptomatic vitreous floaters) with an intern and translucent balance salt solution, through small openings in the pars plana
Cyclo photocoagulation (CPC)
Laser for glaucoma therapy
- cyclodestructive procedure that abates the ciliary body epithelium
- aim: to lower IOP by decreasing fluid production
- diode laser is preferred for CPC owing to its cost, efficiency, and probability
- origin: 1905
- detachments of the CB resulted in decreases in IOP
CPC variations
Transcleral cyclophotocoagulation (TCPC)
Endoscopic cyclo photocoagulation (ECPC)
Trans scleral cyclo photocoagulation
A laser probe is placed on the sclera to administer 15-25 laser treatment spots at th TM
Endoscopic cyclo photocoagulation
A small telescopic device is placed through a small incision at the edge of the cornea, much like for removal of a cataract. The telescope is used to directly visualize the CB, and laser energy is directed to this area
Excessive lowering of IOP and the CPC
Because the scleral is opaque, the surgeon cannot directly visualize the ciliary processes being destroyed. It is likely that other tissues in the eye are affected by the TCPC laser energy collateral damage, leading to unpredictable IOP lowering increases compaction rate
Nd:YAG in the resin l
Frequency doubled Nd:YAH 532nm is used for panretinal photocoagualtion in patients with diabetic retinopathy or retinal ischemia/Neo
Retinal photocoagulation
- PRP is indicated for the managment of retinal ischemia, retinal neo and non proliferative DR
- light from the laser is absorbed by the retinal pigment epithelium (RPE)
- absorption denatures tissue protein via thermal burns which lead to local lretinal cell death and coagualtive necrosis
- scarring usually welds the retina to underlying tissue
- in retail ishmia, the preocsdure leads to improved rental oxygenication. By destroying the largely unused, ischemic extramacular reinstating, PRP reduces the area of ischemic tissue, which in turn reduces total CEGF production in the eye and thereby reducing the impetus for NV
- in addition to the frequency double ND;YAG laser, the procedure utilizes xenon arc laser and argon laser
Retinal laser considerations
- anatomical effects and visual complications due to dispersion and the ram energy within the retina and choroid
- the multispot pattern laser has greatly mitigated but not eliminated these issues
- choroidal effusions, exudative retinal detachments, macular edema, VF deficits, etc
The femtosecond laser
- Infrared laser with ultra short pulse duration (Nd:YLF 1053)
- each 1053nm photon carries 1.17eV of energy. Because atoms typically must absorb > 10eV to ionize, need multiphoton absorption for ionization
- infrared can be focused anywhere within or behin the cornea (very low absorption from ocular tissue)
- causes photodisrutipon mainly by focal heating thermionic emission (from linearly absorbing chromophores)
Increased power density
Decreased wavelength
Decrease Focusing spot size
Decrease pulse duration
Decrease pulse energy
Femto second laser vs longer pulses
The femto second pulse gives two major advantages compared to nanosecond and longer pulses
- the reduction of the pulse energy necessary to induce ablation for fixed laser wavelength and focusing conditions and
- a significant reduction of heat affected zone and as consequence, the improvement of the contour sharpness for the laser processed structures
- this is a direct consequence of the pulse being much shorter than the heat diffuse time
Cataract surgery
Femtosecond laser assists in capsulorhexis being done in a very neat way
Optical principles of laser vision correction for myopia
We want
- less optical power
- decreased curvature (center)
We can
- flatten the cornea (ANTERIOR)
- remove central tissue
The optic principles of laser vision correction: hyperopia
We want
- more optical power
- increased curvature (center)
We can
- Steepen the cornea
- remove peripheral tissue
Spectacle vs laser correction
Glasses
- myopia + minus lens
- hyperopia + plus lens
Laser
- myopia-minus
- hyperopia-plus
Excimer laser
- all UV
- laser correction
- excited dimer
- Gas laser
- ABLATES
Photoablation by excimer laser
- ablate centrally=fix myopia
- ablate in the periphery=fix hyperopia
Energy in excimer
6.4eV
Amount of energy needed by a photon to damage tissue
3.5eV
Flap creation by microablation
Create bubbles in the cornea and then break them down
-allow more precision
LASEK
Similar to LASIK
Involves removal of the corneal epithelial layer only
Dry eye after LASIK
Yes
SMILE
Femtosecond
Takes a lenticular cut out of the stroma and takes it out without daageing the whole epithelium
-less dry eye
PTK
Spheroidal degneation Salesman nodular degeneration Calcium band keratoapthy RCE Bullouse keratophy Anterior cornea dystrophies Superficial scars Keratitis
Contraindications for PRK
Anterior corneal pathology only
Any viral activity within 6 months is a contraindications
Anterior stromal puncture
Anteiror cornea only
Photo mydriasis or photo miosis
It is procedures
Future of ophthalmic lasers
Blue femtosecond laser
- improves focus, same delivered power density with less energy
- tighter raster pattern for smaller incision size
Potential risks
-absorption by overlaying corneal structures
