Model of Hyperopic Shift After DSEK

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Model of Hyperopic Shift After DSEK

• Dana Wallace MD, Richard Hwang BS, and Natalie
  Afshari MD
• Duke University Eye Center
• The authors have no financial interests to
  disclose.

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DSEK A refractive neutral procedure?

• Partial posterior lamellar transplant indicated
  in patients with endothelial dysfunction
• Sutureless
• No astigmatic effect
• Now known to induce a hyperopic shift
• Implications on triple procedures (CE/IOL/DSEK)

Covert DJ and Koenig SB. Ophthalmology 2007
1141272-1277. Koenig SB and Covert DJ.
Ophthalmology 2007114221-226. Jun et al. Cornea
2009 28 19-23.
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Purpose

• Changes in posterior curvature are responsible
  for the hyperopic shift that occurs in DSEK.
• The magnitude of this shift remains
  unpredictable.
• To derive a mathematical model that can be used
  preoperatively to predict the observed hyperopic
  shift after DSEK.

Gorovoy MS. Cornea 2006 25886-889. Koenig SB,
et al. Cornea 2007 26 670-674. Price FW Jr,
Price MO. J Refract Surg 2005 21339-345.
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Methods

• Based on the assumptions of the thick lens
  equation and the Gullstrand model eye, we
  developed a mathematical model to predict the
  hyperopic shift that occurs after DSEK based on
  preoperative pachymetry and endothelial graft
  thickness.

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Formula

• Dcornea Dant Dpost - (d/n)(Dant)(Dpost)
• Dant refractive power of anterior surface
• (1/r1)(nant-nair)
• r1 radius of curvature of anterior surface
• Dpost refractive power of posterior surface
• (1/r2)(nant-naq)
• r2 radius of curvature of posterior surface
• d corneal thickness
• n refractive index

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As applied to DSEK

• Preoperative corneal power
• Determined by preoperative pachymetry
• Postoperative corneal power
• Graft thickness of the graft was added to the
  corneal pachymetry
• Graft thickness of the graft was subtracted from
  the posterior radius of curvature
• The difference in these calculations reveals a
  hyperopic shift.

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As applied to patients
Patient Preop Mrx Preop Pachymetry (µm) Graft Thickness (µm) Predicted Hyperopic Shift Observed Hyperopic Shift
1 0.00-0.50x092 643 103 0.07 1.25
2 0.25-1.25x105 640 100 0.06 0.96
3 0.00-0.25x165 640 142 0.09 0.375
Calculations were only performed in pseudophakic
eyes.
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Interestingly, if we use the eye as only one
refractive surface, the model is more accurate.
Dcornea (naq-nair)/radius of curvature
Patient Preop Mrx Preop Pachymetry (µm) Graft Thickness (µm) Predicted Hyperopic Shift Observed Hyperopic Shift
1 0.00-0.50x092 643 103 0.58 1.25
2 0.25-1.25x105 640 100 0.56 0.96
3 0.00-0.25x165 640 142 0.80 0.375
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Other Factors to Consider

• Endothelial graft is thicker peripherally than
  centrally
• In the first few postoperative months, the graft
  edges thin faster than the vertex

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A more accurate r2

• Can current imaging modalities facilitate these
  measurements?
• Pentacam
• Anterior Segment OCT
• Can we use a model of an ellipse to more
  accurately predict r2?

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Conclusions

• The hyperopic shift induced by DSEK is the result
  of changes in corneal thickness and posterior
  radius of curvature
• Prediction of the hyperopic shift induced by DSEK
  is critical in the planning of refractive
  outcomes
• Advances in anterior segment imaging will aid in
  more accurate prediction of refractive outcomes