Effects of Scleral Buckles on Ocular Geometry

• Scleral buckles reshape the eye depending on buckle material, suture placement, and circumferential tightening.
• This geometric alteration leads to secondary clinical effects, including:
  • Changes in axial length (elongation or shortening).
  • Induction of refractive errors (spherical equivalent shifts, astigmatism).
  • Altered ocular volume due to indentation.
  • Modified ocular rigidity/compliance, affecting intraocular pressure dynamics.

Axial Length Changes After Scleral Buckles

• Radial buckles (soft silicone sponges): → Minimal or negligible change in axial length.
• Segmental buckles: → Often cause a hyperopic shift due to localized shortening.
• Encircling buckles (hard silicone bands): → Typically increase axial length → myopic shift. → At very high buckle heights, paradoxically decrease axial length (dumbbell deformation). → Net effect depends on balance between circumferential shortening (elongation) vs scleral invagination (shortening).
• Clinical variability: → Some eyes show no net change if opposing forces balance. → Long‑term encircling buckles may contribute to progressive axial elongation unrelated to immediate mechanics.

(A) The normal spherical shape of the eye. (B) The spherical eye acquires the shape of a prolate spheroid after placement of a moderately high circumferential buckle. Horizontal and sagittal cross-sections of an eye with a broad circumferential buckle at moderate buckle heights show an ellipse. Coronal cross-section of an eye with a circumferential buckle shows a circle. The anteroposterior axial length of the eye increases at moderate buckle heights. (C) The eye acquires a dumbbell shape at very high circumferential buckle heights. Coronal cross-section of an eye with a very high circumferential buckle still shows a circle, but the sagittal and horizontal cross-sections show a dumbbell shape. The axial length of the eye decreases at very high buckle heights.

• Radial sponge → negligible axial length change.
• Segmental buckle → hyperopic shift.
• Encircling buckle → usually myopic shift (axial elongation).
• High encircling buckle → possible hyperopic shift (axial shortening).
• Balance of forces: circumferential shortening ↑ axial length vs scleral invagination ↓ axial length.
• Clinical relevance:
  • Post‑buckle biometry for IOL power can be inaccurate.
  • Long‑term encircling buckles may predispose to progressive myopia.

Scleral Chord vs. Scleral Arc Length

• Chord length:
  • The straight line between two points on the scleral surface (measured with calipers).
  • Always shorter than the arc length.
• Arc length:
  • The curved distance along the scleral surface between two points.
  • More anatomically accurate for large measurements.
• Discrepancy:
  • Small chord lengths → minimal difference from arc length.
  • Larger chord lengths → discrepancy increases nonlinearly.
  • Example:
    • 8 mm chord ≈ 8.16 mm arc (≈2% error).
    • 13 mm chord ≈ 13.74 mm arc (≈5.7% error).
• Clinical relevance:
  • Important when placing scleral sutures for buckles.
  • Misjudging arc vs chord length can alter buckle positioning and effectiveness.

🎯 High‑Yield Exam Points

• Chord = straight line; Arc = curved scleral surface.
• Chord < Arc (always shorter).
• Error increases with larger distances (nonlinear).
• Clinical impact: buckle placement accuracy depends on understanding this difference.

📝 Mnemonic

“Chord is short, Arc is actual.” → Remember: chord underestimates scleral distance, especially for larger spans.

Effects on Internal Geometry of the Eye

• Buckling effect:
  • Indentation of sclera closes retinal breaks and relieves vitreoretinal traction.
  • Geometry depends on buckle shape, composition (sponge vs hard silicone), suture placement, suture tension, and intraocular pressure.
• Scleral indentation variables:
  • Deeper suture bites → greater buckle height.
  • Loose sutures / too close or too far apart suture bites / high IOP → reduced indentation.
  • Orientation of buckle determines topography (radial vs circumferential).
• Radial buckles:
  • Useful for solitary horseshoe tears.
  • Provide localized support, minimize circumferential shortening.
• Circumferential encircling buckles:
  • Cause radial retinal folds due to reduced globe circumference.
  • Risk of fishmouth phenomenon (radial fold bisects tear, preventing closure).
  • Broad encircling buckle preferred in proliferative vitreoretinopathy (PVR) with circumferential traction.

The fishmouth phenomenon results when a radial fold bisects a retinal tear. The radial fold tends to keep the tear from settling on the buckle and may lead to persisting retinal detachment. The fishmouth phenomenon is most commonly produced by circumferential shortening of the globe from an encircling buckle, causing a radial fold to pass through a horseshoe-shaped retinal tear. Minimizing the circumferential shortening of the eye with a radial or wedge-shaped buckle decreases the likelihood of the fishmouth phenomenon.

Volume Changes in the Eye After Scleral Buckles

• Mechanism:
  • Buckle indentation displaces fluid from the vitreous cavity → reduces intraocular volume.
  • A sphere holds maximum volume for minimum surface area; indentation decreases volume.
• Magnitude of displacement depends on buckle type/geometry:
  • Radial sponge (5 mm): ~0.2 mL (~5% of vitreous cavity).
  • Encircling band (#240 style): ~0.5 mL (~12%).
  • Broad encircling buckle (#287 style): 1.3–1.7 mL (~33–43%).
  • MRI studies confirm significant volume reduction with encircling bands.
• Clinical relevance:
  • Important when:
    • Planning fluid drainage during buckle placement.
    • Injecting intravitreal drugs (dose/toxicity considerations).
    • Injecting expansile gases (risk of IOP rise).
  • Radial sponges rarely elevate IOP significantly; broad encircling buckles can cause substantial changes.

Scleral Buckles, Ocular Rigidity & Corneal Hysteresis

1. Ocular Rigidity

• Definition: Change in intraocular pressure (IOP) for a given change in intraocular volume.
• Effect of scleral buckle:
  • Encircling buckles decrease ocular rigidity → eye becomes more compliant.
  • Injection of fluid/gas causes less IOP rise compared to normal eyes.
  • Buckle stretches as eye becomes more spherical → reduces buckling effect.
• Clinical impact:
  • Tonometry methods relying on standard rigidity (Schiøtz, Tonopen, pneumotonometer, ORA) may underestimate IOP.
  • Eyes with buckles + gas bubbles show less IOP rise during air travel.
  • Larger fluid aspiration may be required to reduce IOP in buckle eyes.

2. Corneal Hysteresis

• Definition: Measure of corneal elasticity (difference between pressure at corneal indentation vs recovery).
• Effect of scleral buckle:
  • Buckle placement increases corneal hysteresis compared to eyes without buckle.
  • Alters accuracy of IOP measurements.
• Clinical impact:
  • Must interpret IOP readings cautiously in buckle patients.
  • Higher hysteresis may mask true IOP elevation.

🎯 High‑Yield Exam Points

• Encircling buckle → ↓ ocular rigidity → less IOP rise with fluid/gas injection.
• Tonometry may underestimate IOP in buckle eyes.
• Gas bubble + buckle → safer during air travel (less IOP rise).
• Scleral buckle → ↑ corneal hysteresis → affects IOP measurement accuracy.

📝 Mnemonic

“Buckle = Bendy eye” → ↓ Rigidity, ↑ Hysteresis, ↓ IOP rise (false readings).

Reference:

Thompson JT. The biomechanics of scleral buckles in the treatment of retinal detachment. In: Ryan SJ, Sadda SR, Hinton DR, Schachat AP, Wilkinson CP, eds. Ryan’s Retina. 7th ed. Philadelphia, PA: Elsevier; 2023: Vol 3, Chap 101.