Cataract Incision Fluid Ingress, an Engineering Analysis

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Cataract Incision Fluid Ingress, an Engineering
Analysis

• David S.C. Pao, M.D.
• Kristina Y. Pao, B.S.
• Erik A. Cheever, Ph.D.
• Cory Schroeder, B.S.

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Medicare data shows an increase in the incidence
of endophthalmitis from 1994 to 2001. The only
significant change in cataract surgery over this
period is the sutureless clear corneal incision.
Ingress of fluids through the incision is the
etiology. From various studies two mechanisms
are postulated 1. Prolonged hypotony results
in loss of wound architecture with wound gape.
2. Ingress of fluids due to pressure gradient
with normal and increased intraocular pressure
(IOP) without loss of wound architecture.
The second mechanism is described using
engineering analysis.
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.

• Figure 1. The eye is an elastic globe. When
  force is applied, there is no egress of fluid,
  since the wound construction will withstand high
  pressures produced by an outside force. The
  corneal wound serves as a one-way valve in which
  fluid can ingress, but fluid cannot egress. An
  outside force distorts the globe, but the volume
  remains the same. The syringe represents the
  elasticity of the globe accepting the displaced
  volume. When the force is no longer applied, the
  elasticity of the syringe refills the globe
  configuration. At this point there is an
  instantaneous pressure gradient that provides
  ingress of fluid through the wound.

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.a

• Figure 2. Mechanical analog. A force
  applied on the external eye at point a results in
  point a moving left causing an increase in IOP.
  This is indicated by compression of the spring.
  Note that positive displacement (x) is to the
  left. If the force is suddenly released, point a
  will begin to move to the right. When point a
  reaches its initial equilibrium position, the
  force on the spring is zero. If point a moves
  beyond the equilibrium position, the spring
  exerts a force to the left. This creates the
  pressure gradient (relative vacuum) with ingress
  of fluid through an incompetent wound. This
  gradient exists even without a wound. Assume the
  moving parts have a mass (M), and loss of force
  due to friction and damping is represented by a
  damper (B).

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A force balance equation yields the mathematical
formula and response curves that is represented
in the form of a standard second order system.
This is represented by the following
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Response CurvesUnder Damped, Critically Damped,
Over Damped
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• Goal ? 1
• 0 lt ? lt 1 system will oscillate with negative
  pressure gradient resulting in ingress of fluids
• k and M not controlled by surgical techniques
• k spring constant includes elastic modulus of
  sclera and cornea
• M mass of the eye variable (i.e. myopia,
  hyperopia)
• B damping coefficient
• Tissue damping of cornea and sclera
• Viscous damping of the vitreous
• Frictional damping is controlled by surgical
  technique
• Area of cornea incision (width and length) and
  IOP

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? must be greater than 1. From the formula the
spring constant (k), M, and much of B is not
under surgeon control. Only the wound friction
component and IOP is under surgeon control. Wound
construction is thus the main criteria for
surgical safety.
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The average IOP was 38 mmHg at the conclusion of
cataract surgery with the eye hyper inflated. 25
minutes later it was in the low 20s.Rhee, D.,
Deramo,V., Connolly, B., Blecher, M.,
Intraocular Pressure Trends After Supranormal
Pressurizaton to Aid Closure of Sutureless
Cataract Wounds J. Cataract Refractive Surgery,
Vol. 25, April, 1999.

• Clinical Observations
• Recording of IOP over 70 mmHg on squeezing
  lids tight for approximately 2 seconds. On
  command to open the lids the IOP dropped pass the
  average of 17 mmHg to less than 10 mmHg and
  rebounded back to 17 mmHg.
• Coleman, D. J., Trokel, S., Direct-Recorded
  Intraocular Pressure Variations in a Human
  Subject. Arch. Ophthal.,Vol. 82, Nov, 1969.

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Summary

• Formula
• Ma Bv kx 0
• Goal keep ? 1
• 0 lt ? lt 1
• Results in ingress of fluid
• Surgical control over
• Incision width
• Incision length
• IOP at end of procedure

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Conclusion

• The engineering analysis provides ranges of the
  parameters (i.e. IOP, incision size, shape,
  length) that confirms our clinical observations
  and adds to further understanding of parameter
  limits. This allows increase safety and reduces
  the risk of endophthalmitis.