test 1 Flashcards
What are the 2 types of lenses?
Scleral and cornea l
What is a scleral lens?
a lens that is extends beyond the diameter of the cornea
What are some examples of a scleral lens?
soft lenses and haptic lens
what is a corneal lens?
a lens that is the same diameter as the cornea
what is an example of a corneal lens?
rigid lenses
how are corneal lenses supported?
with fluid retention AND OR the lid
In what year and what concept did Leonardo Da Vinci come up with?
a very basic optical system where the cornea is neutralized through contact with an optical medium. 1508
What did Rene Discartes come up with and when?
sketched an elongated tube filled with water and at the end had a glass the same shape as a cornea. 1636
Thomas Young
understood how to neutralize cornea, but did not think of CLs. 1801
Sir John F. W. Herschel
1823, described the elements of a CL more precisely but did not put into practice
Adolf Fick
1888, A contact spectacle. Described the first contact lens with refractive power known to have actually been worn.
What was the very first contact lens?
a thin glass bowl invented by adolf fick in 1888. can wear it for 2 hrs without irritation but it had no power.
what was the second CL to be developed?
made with a glass blower. bi-curve lens of 14mm in diameter. This was used in irregular astigmats and they had visual improvement.
What were the first cosmetic lenses?
lenses painted with irises on them to avoid enucleating eye
Eugene Kalt
1888, made the first CL to tx keratoconus. and first cosmetic lens
August Muller
1889, structured lenses so that posterior surface could be held in place by cappillary attraction. anterior surface would correct refractive error. used cocaine as anesthesia. did fittings on himself to neutralize his -14.00 D myopia.
who created the lift off on the cornea?
August muller because he saw that tears were important to prevent corneal edema.
Karl Zeiss
- Developed the first trial lens made of glass..was very heavy and not successful.
spin casting
uses liquid polymer which is spun to create the correct curvature
cast molding
uses liquid polymer filled in a mold, most disposable lenses are cast in mold
lathe cutting
uses solid polymer that is cut and polished to the power required
Muller Co.
made lenses from blown glass on individual basis in the presence of the patient in an attempt to correct his ocular irregularity. had smooth edges and regular curvature.
ptosis crutch
ridges were upper eyelid can rest. caused DES bc patient had difficulty blinking.
joseph dallos.
emphasized fitting lens with slight difference bw. lens and cornea to allow tears to flow unnderneath. noticed central cornea spherical and peripheral was flatter.
william feimbloom
1936, the first to use plastic to capture the advantage of low specific gravity. hybrid lenses.
J. Teissler
1937, made corneo-scleral lenses shell from cellulose acetate plastic (first gas permeable lens)
Rhom and Haas Co.
1936, introduced to the US a transparant material known as PMMA
PMMA
dominated CL field for 4 decades.
John Mullen and Theodore Obrig
1938, developed techniques for making scleral lenses from PMMA (found CL could be made thinner but still had difficulty with oxygen delivery.
Kevin Tuohy
developed the first corneal CL that has became a practical device for correction. 1946, technician cut erroneously and created RGP. found these lenses eliminated the symptoms of halos.
George Butterfield
1950, – Recognized that by adding various peripheral curves to the posterior surface of the lens central curve could nearly match the radius of curvature of the cornea.
• This represented the first introduction of the modern term of fitting “on K” or contouring the cornea.
Otto Wichterle
1952, synthesized HEMA, produced hydrogel lenses of different degrees of water content. in 1961, created more casting machines.
C lenses
first lenses introduced by B&L was barely successful.
N lenses
2nd lenses introduced by B&L, was significantly more popular.
what did Vistakon do?
introduced the first disposable lenses. (Acuvue)
Cellulose Acetate Butyrate
was gas permeable but poor physical stability and warped easily
Norman Gaylord
1971, invented silicone methacrylate RGP material. permeability was similar to CAB and it was more stable.
how did intro of fluoride help?
increased Dk of lens. FS/A
function of tear film?
maintain hydration of soft lenses and cornea. anterior refractive surface.
what is the most commonly used CL?
Silicone hydrogel
high wetting angle
the higher the angle, the less tearsthat is spread over surface
components of tear
Mucous (basal layer)
Aqueous layer
Lipid layer
what glands produce mucus in tears
goblet cells, glands of moll and krause
glands that produce aqueous layer in tears
lacrymal gland, accessory gland of zeiss
glands that produce lipid layer
meibomian glands. this prevents evaporation
what does TBUT measure?
mucus layer, when that evaporates, water touches cornea and cornea is hydrophobic and the water breaks.
what is n of cornea?
1.376 and power of 45D
what is the thickness of the cornea
0.52 in the center and 0.97 in periphery
5 layers of the cornea
epithelium with basement membrane bowman's zone stroma descemet's membrane endothelium
epithelium of cornea
50 um in thickness, 10% of total corneal thickness. 5-6 layers basal, wings and surface cells
glycocalyx
helps mucin to adhere to corneal epithelial cells.
how fast are small corneal abrasions healed?
24 hours
what attaches epithelium to basement?
hemidesmosomes
how long does basement take to heal?
months
what makes up 90% of corneal thickness?
bowman’s zone (part of stroma)
descemet’s membrane
is elastic but consists of collagen
to avoid corneal edema how should the lens be fitted?
1.50D flatter than the flattest corneal meridian
what percent of the stroma is water?
78%
how much of tears is NaCL?
.91%
When the eye is open, tear osmolarity is ______________ due to evaporation into the atmosphere.
higher
When the eyes are closed the tear osmolarity is ___________ and hence a slight corneal thickness is produced
lower
__________ use when corneal oxygen supply is deficient.
anaerobic glycolysis
________ used 15% of the glucose in the cornea but produces three times the total corneal energy.
aerobic glycolysis
- During sleep, the oxygen supply to the cornea comes from the
palpebral conjunctiva
- _____________ is the primary source of oxygen in the endothelium
aqueous humor
- At 0% of oxygen level the cornea swells 8% in 3 hours.
this is maximum corneal swelling
- A contact lens worn overnight, under experimental extended wear conditions, show a corneal swelling of about
12- 13 %
- Low transmissibility lenses provide an additional ____________ oxygen from transmission, which in most patients reduces any corneal swelling to less than 1%.
3-6 %
- Medium transmissibility lenses provide oxygen levels of ____________ and eliminate clinical edema in daily wear patients
5-8%
- High transmissibility lenses provide levels of ______________.
8 - 16%
- A practical clinical description of the oxygen tension produced in the pre-corneal film by transmission through a contact lens
Equivalent oxygen percentage
- Which of the following material has the ability to transmit and increase oxygen level in the cornea?
silicone
- It’s termed the permeability of a contact lens material. It is an intrinsic characteristic of the material which is not related to its thickness.
Dk value
is termed the lens oxygen transmissibility. This measure is inversely proportional to lens thickness.
Dk/t
Low transmission material cause a corneal swelling of ______.
6- 8 %
Medium transmission material cause a corneal swelling of ______
2-4 %
High transmission material cause a corneal swelling of ______.
0-2%
- Developed with optical power with the help of a glass blower. Bi curve lens of 14mm in diameter. Was tried in 6 patients with irregular astigmatism due to scaring.
second lens
A thin, glass bowl, was worn for 2 hours, the shell stabilized well but had no
power.
first lens
previous tx of keratoconus was
d. Cauterization of cone with silver nitrate, instillation of myotics and application of pressure dressing.
Dk/t
D= diffusion
k= solubility
t= thickness
minimum is 24
F1
front surface power (n’-n)/ r1
F2
back surface power (n-n’)/ r2
Fv
front vertex power
[F2/1- (t/n)(f2)] + F1
Fv’
back vertex power
[F1/ 1-(t/n)(F1)] + F2
F1 with F2 and Fv’ given
(Fv’ - F2) / 1 + (t/n) (Fv’ - F2)
n of a keratometer
337.5
plus lenticular
for negative power lenses greater the 5.00 D it minimizes problems associated with thick edges.
minus lenticular
used to increase edge thickness in plus lenses and minus lenses up to -1.50 D
what effect does a spectacle corrected myope have?
Bi object appears further away
what effect does a spectacle corrected hyperope have?
bo, objects appear closer
who has more complaints with CLs at near?
exo myopes and eso hyperopes
Effective power
Ep= Fv’/1-d(Fv’)
Vergence F
(100/l) + Fv’
EpV effective power vergence
= VerF/ 1-d(VerF)
accommodation
EpV- Ep
how to calculate accommodation with spectacles?
1st calculate d
2nd: Ep
3: VeF
4: EpV
5: EpV - Ep
How to calculate accommodation in CLs
1st: now, lens is at cornea plane so distance of object is l+vd
Ex. L=40cm, VD=12mm so new distance is 41.2cm
2nd: calculate power of the CL at the corneal plane (Epcp) using just the vertex distance
3rd: calculate vergence power leaving CL
4th: calculate the effective power of that vergence at the principle plane
5th: calculate accommodation, where Epv-Ep (Ep, as in what was calculated for spectacles (at the principle plane) using d)
mag of spectacle wearing hyperopic aphakic patient
50-60%
mag of CL wearing hyperope aphakic patient
3%
