RYAN VOLUME 3 - The biomechanics of Escleral Buckle in the treatment of RD #101

Question 1

After a circumferential buckle, how is the horizontal and sagittal cross-section of the eye?

Answer 1

A prolate spheroid.

Question 2

How can the anteroposterior axial posterior decrease after a buckle?

Answer 2

If the buckle is too tight and high.

Question 3

How do the mattress sutures alter the axial length?

Answer 3

They shorten the AL.

Question 4

What else contributes to a shift towards myopia?

Answer 4

Shallowing of the anterior chamber associated with displacement of the lens anteriorly (It may become less pornouced over time). There is also a reduction in the trabecular iris angle.

Question 5

In which case does the buckle height decrease more: radial buckle or circumferential buckle?

Answer 5

Radial buckle.

Question 6

What tends to happen when placing a buckle in a child?

Answer 6

It may retard growth-related increases in the axial length of the eye, causing the eye ti develop less myopia than the fellow eye.

Question 7

Over time, the change in refractive error induced by the scleral buckle tends to normalize. Why?

Answer 7

1. Stress relaxation of the band over time.
2. The scleral invagination of the sutures decreases.

Question 8

Which is shorter:: the scleral chord or the scleral arc?

Answer 8

The scleral chord is always shorter than the scleral arc. The longest the scleral chord, the highest difference to the the scleral arch lenght.

Question 9

What is the lenght of the sutures that ensures maximum tension?

Answer 9

4 to 5 mm long (with deep bites).

Question 10

Major variables that determine the internal geometry of the indentation:

Answer 10

• Shape of the buckle
• Composition of the buckle (silicone, sponge, hard silicon)
• Suture placement
• Suture tension
• Distribution of the tension from the suture
• IOP

Question 11

Which buckle causes more fishmouth phenomenon: radial/wedge shaped buckle or encircling buckle?

Answer 11

Encircling buckle.

Question 12

Anterior vitreoretinal traction, funnel-shaped retinal detachments: which is the best buckle?

Answer 12

Circumferential buckle.

Question 13

How is the vitreous cavity volume after a bukcle?

Answer 13

It is reduced. A sphere contains the largest volume of fluid with the least surface area.

Question 14

A 2.5 mm wide silicone encircling band displaces about _________ of the vitreous cavity volume.

Answer 14

0.5 mL or 12%.

Question 15

Elastic bands width and height:
#40
#240
#41
#42

Answer 15

Elastic bands width and height:
#40 W 2.0 mm // H 0.75
#240 W 2.5 mm // H 0.60
#41 W 3.5 mm // H 0.76
#42 W 4.0 mm // H 1.25

Question 16

What is ocular rigidity?

Answer 16

It is the change in intraocular pressure for a given change in intraocular volume. It is a measure of the elasticity of the eye.

Question 17

IOP X scleral indentation.

Answer 17

Schiotz / Tonopen / Pneumotonometer / ORA are less accurate in eyes with scleral buckle.
Injection of gas or fluid in eyes with SB causes less elevation of IOP than normal eyes.

Question 18

Eyes with SB and small intraocular bubbles have LESS / MORE increase in IOP with air travel than eyes WITHOUT SB.

Answer 18

Eyes with SB and small intraocular bubbles have LESS increase in IOP with air travel than eyes WITHOUT SB. The gas is more compressible.

Question 18

After a SB, the corneal hysteresis INCREASES / DECREASES.

Answer 18

After a SB, the corneal hysteresis INCREASES.

Question 19

SB and ocular blood flow.

Answer 19

Encircling SB decreases ocular blood flow.

Question 20

Most vitreoretinal traction causing tears, tractional RD or RRD is ________.

Answer 20

Oblique (orthogonal vectors - 90º - or composed by radial and tangential traction).

Question 21

Wich eye movement appears to be especially importante to forcing fluid through a tear?

Answer 21

Rotational eye movement. RDD is more common if there is vitreous traction elevating the flap of the tear.

Question 22

ERM’s tend to form a _______ configuration.

Answer 22

Planar.

Question 23

Factors that promote adhesion between the retina and the RPE.

Answer 23

• Viscous mucopolysaccharide betwnn the villous processes of the RPE interdigitating with the photoreceptors.
• Oncotic pressure. Proteins in the choroid cannot easily pass through the RPE and Bruch’s membrane into the subretinal space.
• Hydraulic forces. The vitreous pushes the retina against the RPE. The hydraulic forces are produced by more rapid passage of fluid from the subretinal space through the sclera than from the vitreous through the retina.
• RPE pump.

Question 24

In vitro, paradoxically, the adhesive force between retina/RPE appears to be WEAKER with SMALL / BIG tractional forces.

Answer 24

In vitro, paradoxically, the adhesive force between retina/RPE appears to be WEAKER with SMALL tractional forces. Less traction, less adhesion, more likely to induce a tear.

Smaller tractional forces on the retina exerted over a longer period of time tend to produce larger traction RD’s than do larger tractional forces over a short period.

Question 25

Three thermal chorioretinal adhesions.

Answer 25

• Diathermy - electric current to external sclera. A burn including slcera, RPE and retina. RPE proliferates. Causes necrosis.
• Cryopexy - freezing of the slcera, RPE and retina. Minimal changes to the sclera.
• Laser - the energy is primarily absorbed by the pigmented RPE and choroid, leading to chorioretinal scar. No changes to the

All three have similar strenghts.
Laser creates more rapid chorioretinal adhesion (starts within 24h and increases within 3 days) than cryo or diathermy (takes several days to start and reach maximum strength 2 weeks later).

Question 26

Which are the two vectors of the traction of an EMR?

Answer 26

• Tangential to the retina.
• Radially inward toward the center of the eye (a result of a tangential traction on a curved surface).

The SB alters the shape of the eye wall from a normal concave contour to a convex contour over the indentation area. It changes the inward force to an outward force.

Question 27

ERMs in eyes with smaller radius of curvature exert more radial inward force on the retina.

Question 28

What is the volume of air required for covering 4 clock hours in the vitreous cavity?

Answer 28

0.635 ml of air.
This volume displaces 15% of the volume of the vitreous cavity with an arc of contact of 120 degrees (4 clock hours).

Question 29

SB alters the fluid flux through with effect?

Answer 29

Bernouilli effect.
Within a horizontal water pipe that changes diameter, regions where the water is moving fast will be under less pressure than regions where the water is moving slow