New Anterior Chamber Device for Measurement of AngletoAngle Distance

1
New Anterior Chamber Device for Measurement of
Angle-to-Angle Distance

• W. Wonneberger1,3, M. Müller1, L. Werner1,2, M.
  R. Tetz1
• 1.Ophthalmic Research, Berlin Eye Research
  Institute (BERI), Berlin,Germany
• 2.Ophthalmology, John A. Moran Eye Center,
  University of Utah, Salt Lake City, UT, USA
• 3.Ophthalmology, Department of Ophthalmology,
  Sahlgrenska University Hospital,
  Gothenburg, Sweden
• The measuring device described in this poster
  is the issue
  of a patent filled by one of the
  authors (M. R. Tetz).
• The authors do not have any financial
  interest to disclose.

2
Background

• Exact measurement of the angle-to-angle
  distance (AAD) is crucial for correct sizing of
  anterior chamber intraocular lenses (AC-IOLs),
  thereby reducing common postoperative
  complications including pupil ovalization,
  dislocation and angle erosion1.
• Although widely used, measurement of
  white-to-white distance and adding or
  substracting a correction factor is inaccurate
  for predicting AAD2.
• Examining human cadaver eyes, Werner et al.
  found a positive correlation of the
  white-to-white distance and AAD only at the 6 to
  12 oclock but not the 3 to 9 oclock meridian3.

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Objective

• A new anterior chamber measuring device (ACMD)
  was designed at the BERI for intraoperative
  measurement of AAD through a small incision (3
  mm) before implantation of an angle-fixated
  AC-IOL.
• The aim of this study was to gain initial
  experience with the ACMD and to establish a
  simple, cost-effective and accurate preparation
  technique for the use of porcine eyes in the
  evaluation of the ACMD.

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Materials and Methods

• ACMD
• By rotating the lower part of the instrument,
  two stainless steal filaments are ejected in a 90
  degree position until they reach the angles on
  both sides.
• The rotating part is locked in position, the
  filaments retracted and the instrument removed
  from the AC for measurement reading (Figure 1).

Figure 1. ACMD (filaments partially ejected)
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Materials and Methods

• Eye model
• 20 eyes of piglets were evaluated within 12
  hours after enucleation. The eyes were stored
  under cooled conditions immersed in balanced salt
  solution (BSS).
• Preparation and measurement procedure
• 1. Each eye was mounted on a plastic holder.
  The 6 to 12 oclock meridian was marked
  with a tissue-dye and the white-to-white
  distance was measured in the same meridian three
  times per eye.

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Materials and Methods

• 2. After a 3.0 mm corneo-limbal incision, Healon
  5 (AMO) was injected into the anterior chamber to
  consistently keep the iris at a leveled plan,
  under microscopic control.
• 3. IOP was adjusted to 22 mmHg with a
  Schiötz-tonometer. AAD was then measured with the
  ACMD in the 6 to 12 oclock meridian three times
  per eye.

Figure 2. Measurement of the AAD with the ACMD,
in piglet eyes.
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Materials and Methods

• 4. After fixation in 10 Formalin solution for
  45 minutes vertical white-to-white was measured
  with a digital caliper two times per eye (Figure
  3).
• 5. The eyes were sagittally dissected in the
  meridian used for the internal measurement. The
  halves were positioned on a thin (lt1 mm)
  transparent plastic plate for stabilization (Fig.
  3).

Figure 3. White-to-white measurement after
fixation (top) halves on plastic plate after
fixation and dissection (bottom).
8

Materials and Methods

• 6. The plastic plate was turned upside down
  resting on the halves. AAD was measured directly
  in both halves with a digital caliper three times
  per half (Figure 5).
• Using the values for white-to-white
  measurement before and after fixation, a
  shrinkage factor was calculated.

Figure 5. Measuring AAD after fixation and
dissection.
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Results
Results are given as mean /- SD. Mean values
of the white-to-white, before and after fixation
were 10.99 /- 0.28 mm and 10.58 /- 0.27 mm,
respectively. Using these mean values the
shrinkage factor calculated was 3.88.
Table 1 shows the values for AAD and
white-to-white measured before and after fixation
and dissection.
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Results

• Mean value for measurement with the ACMD was
  13.16 /- 0.57 and for AAD directly measured in
  both halves after fixation and dissection was
  11.94 /- 0.4 mm.
• To estimate AAD before fixation and dissection
  mean AAD was corrected with the shrinkage
  factor mean AAD before fixation and dissection
  11.94 mm 11.94 mm x 0.0388 12.40 mm.
• On average the ACMD overestimated AAD by 0.76
  mm (6.1 ). The difference between the mean
  values for AAD obtained with the two methods was
  statistically significant (p lt 0.001 paired
  t-test).

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Discussion

• Several mechanical instruments have been
  proposed to measure AAD intraoperatively4-7. A
  new approach is made with the one-piece ACMD.
• Our study suggests that the ACMD slightly
  overestimates AAD. This is probably due to minute
  indentation of the angle structures with the
  filaments.
• The assumed linear character of this
  overestimation should, however, make it possible
  to find correction factors to adjust the scale on
  the instrument. This is the aim of another study
  currently being conducted at the BERI.

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Conclusion

• The ACMD after Tetz is a new alternative for
  intra-operative AAD measurement through a small
  incision. It was found to be easy to use in this
  preliminary study.
• A simple and reliable preparation technique for
  estimation of AAD in porcine eyes was
  established.
• Further evaluation of the ACMD using the method
  described and larger study groups will be
  necessary before clinical application of the
  ACMD.
• References
• 1.Lovisolo CF, Reinstein DZ et al (2005) Phakic
  intraocular lenses. Surv Ophthalm 50(6)  549
  587.
• 2.Heslin KB (1979) Is white-to-white right?. J
  Am Intraocul Implant Soc J 5(1) 50-1.
  
• 3.Werner L, Izak AM, Pandey SK, Apple DJ, Trivedi
  RH, Schmidbauer JM (2004) Correlation between
  different measurements within
• the eye relative to phakic intraocular lens
  implantation. J Cataract Refract Surgery 30(9)
  1982 1988.
  
  
• 4.Roberts JC (1981) A method for anterior
  chamber lens size determination. J Am Intraocul
  Implant Soc (2) 171.
• 5.Manchester PT (1983) A method for measuring
  the anterior chamber. J Am Intraocul Implant Soc
  9(3) 341-2.
• 6.Karickhoff JR (1983) Technique for sizing
  anterior chamber implants. J Am Intraocul Implant
  Soc 9(2) 206-8.
• 7.Tetz MR, Nimsgern, Apple DJ (2000) Sizing
  angle supported anterior chamber IOLs A new
  instrument for measuring anterior
• chamber angle dimensions. 98th Annual Meeting
  DOG 2000.