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Strength of clear corneal incisions in cadaver eyes
Strength of clear corneal incisions in cadaver eyes Richard]. Mackool, MD, R. Scott Russell, MD ABSTRACT Purpose: To determine the variation in strength of clear corneal incisions, as demonstrated by degree of incision leakage when challenged by increased intraocular pressure, in relationship to the incision width and length. Setting: Mackool Eye Institute, Astoria, New York. Methods: Clear corneal incisions 3.0 or 3.5 mm in width and from 1.0 to 3.5 mm in length (0.5 mm increments) were studied in human cadaver whole globes. Pressure was applied at the corneal apex or 8.0 mm posterior to the external wound margin to determine the strength of the incision with the application of external force. Results: Clear corneal incisions of 3.0 or 3.5 mm in width and at least 2.0 mm in length demonstrated substantially greater resistance to incision failure than shorter incision lengths with both apical and posterior applied forces. Conclusion: Clear corneal incisions 2.0 mm or greater in length demonstrate resistance to leakage comparable to similarly constructed scleral tunnel incisions. J Cataract Refract Surg 1996; 22:721-725
T
he use of a clear corneal incision (eel) in cataract surgery has recently increased. With contemporary phacoemulsification techniques and foldable intraocular lenses (IOLs), a eel has the advantage of leaving the conjunctiva undisturbed in patients with pre-existing or planned filtering blebs, ocular cicatricial pemphigoid, Stevens-Johnson syndrome, or dry eye. A eel also leaves the sclera undisturbed in cases of scleritis and allows virtually bloodless surgery without the need for cautery in patients taking anticoagulants or those with a bleeding diathesis. Placing the eel at the steep meridian of the preoperative keratometric astigmatism can be used to reduce or eliminate pre-existing corneal astigmatism. Temporal corneal incision placement allows easier access in patients with prominent brows or deep-set eyes.
Reprint requests to Richard]. Mackool, MD, 31-2741 st Street, Astoria, New York 11103.
Temporal corneal incisions have been reported to be more stable than superior incisions. l However, the structural integrity of a eel has also been challenged; the stability of eels has been compared with that of scleral tunnel incisions? Important differences in eel architecture have been recognized; we are aware of at least five methods of incision construction: 1. The paracentesis incision, which is essentially parallel to the iris along its entire length. 2. The two-plane or grooved incision, in which the first cut is perpendicular to the corneal surface and the second plane is parallel to the iris. 3. A hinged incision, 3 which creates a deeper initial perpendicular corneal groove (approximately two-thirds corneal thickness) and a second plane that is parallel to the iris at one-half corneal thickness. 4. A three-plane incision, which is morphologically identical to a scleral tunnel incision and differs only by being entirely within the clear cornea. The length
J CATARACT REFRACT SURG-VOL 22, JULY/AUGUST 1996
721
CLEAR CORNEAL INCISION STRENGTH
of the "hinges" or corneal valves may vary, and the incisions may start at the limbus or 0.5 mm or more into clear cornea. 5. A two-plane incision created by a keratome, which initially enters the cornea at the limbus and advances at a slightly downward angle for a distance equal to the desired length of the incision. A short second plane is created by directing the keratome tip downward at an angle of approximately 45 degrees and advancing the tip into the anterior chamber (Figure O. The blade is then redirected to be roughly parallel to the cornea, as the entire blade enters the chamber. We used this technique in our study to ensure consistent incisions. We used two incision widths and varying incision lengths and measured their resistance to leakage during the application of force upon either the corneal apex or the sclera, posterior to the incision origin.
Materials and Methods Human cadaver whole globes that had not had prior surgery were obtained from the Lion's Eye Bank for Long Island, Manhasset, N ew York. Clear corneal incisions 3.0 or 3.5 mm in width and from 1.0 to 3.5 mm in length (0.5 mm increments) were made by one surgeon (R.S.R.) using an Alcon 3.0 or 3.5 mm steel keratome blade. Incision length was measured with a caliper that was checked for accuracy against a steel millimeter ruler. The incisions originated at the limbus (conjunctiva insertion) and were created as described above and shown in Figure 1. Table 1. Externally applied force (in pounds per square inch) producing either incision failure or severe globe deformation without incision failure. Incision Width
3.0mm Incision Length (mm)
1.0 1.5 2.0 2.5 3.0 3.5 722
Figure 1. (Mackool) The two-plane CCI. Dotted line represents the orientation of the blade after the tip enters the anterior chamber.
Balanced salt solution (BSS®) was injected through the optic nerve to establish an intraocular pressure (lOP) between 15 and 20 mm Hg. Intraocular pressure was measured with a Tono-Pen XL® (Oculab). The globes were then placed in a steel practice eye holder. External force was applied at the corneal apex or 8.0 mm posterior to the external incision margin using a flat round probe with a VB inch diameter. A Chatillon DFIS-lO digital force gauge with an accuracy of ±0.15% offull scale and a 10 lb maximum was used to measure and record externally applied forces. The calculated range of force was from 0 to 896 pounds per square inch (psi) ± 1.3. Forces were applied until incision failure or severe globe deformation occurred, making it technically impossible to apply greater force. Incision failure was defined as incision leakage, iris prolapse, or both. When severe globe deformation without incision failure occurred, the application of force was discontinued and the peak pressure was recorded. Measurements were repeated three times until incision failure or globe deformation occurred, and the mean pressure was calculated.
3.5mm
Apical Force (psi)
Posterior Force (psi)
Apical Force (psi)
Posterior Force (psi)
47 60 520 430 477 484
41 42 354 437 348 509
53 75 650 425 319 509
50 18 310 157 176 509
Results Clear corneal incisions 3.0 or 3.5 mm in width with a length of 2.0 mm or more did not leak with centrally applied external force of up to 650 psi. Identically wide incisions with lengths of 1.0 or 1.5 mm leaked or developed iris prolapse with centrally applied external forces at least eight times less (47 to 74 psi). Posteriorly applied external forces generally resulted in incision leakage at
J CATARACT REFRACT SURG-VOL 22, JULY/AUGUST 1996
CLEAR CORNEAL INCISION STRENGTH 3.0 mm INCISION WIDTH, APICAL FORCE
600
-----------••-
------------------- --
- - - - - - - - - - - - . ~ - - - - - - - - - - - - - - - - - - - - - - - --
Figure 2.
en
.:
(Mackool) Resistance to leakage of the 3.0 mm wide eel with force applied to the corneal apex.
300
200
100
---- - - - -------- ----- . ------
------------
I ------------- ----------
2.0
2.5
3.'
3.0
INCISION LENGTH, mm
lower pressures than centrally applied forces, with the exception of the 3.5 mm incision length when combined with a 3.0 or 3.5 mm incision width. No leakage was observed in these eyes with centrally or posteriorly applied forces of up to 509 psi. Incision lengths of2.0 mm or more were also at least eight times more resistant to leakage caused by posteriorly applied external forces than were incisions of 1.0 or 1.5 mm in length for both 3.0 and 3.5 mm incision widths. Incision lengths of 1.0 or 1.5 mm leaked with posteriorly applied external forces ranging from 18 to 24 psi. Incision lengths of
2.0 mm or more leaked with posteriorly applied external forces ranging from 157 to 436 psi. There was increased resistance to leakage with greater incision length and reduced incision width. Although there were variations within the two groups, the 3.0 and 3.5 mm incision widths displayed comparable resistance to leakage for all incision lengths studied. A rectangular incision 3.0 or 3.5 mm wide and 2.0 mm long, as well as a square incision of 3.5 X 3.5 mm, did not leak even with severe corneal and globe distortion that occurred between 509 and 650 psi (Table O. Fig-
3.0 mm INCISION WIDTH. POSTERIOR FORCE
-- - - - -- - --- •• •• -- -. - . ---------- - --
600
- - -- --- -- . - - - -
-..............................••••••••.. 400
~ '~"~-
~.........
. ............
T· . . ·
354
..
~
348
300
-- - - - - . - -- -- - - - - - - -------------- - ---
200
- __ - __ _ - ___
100
- --- - - -. - - - - - - - --- ----- --- --- ----- --- - - ---- -------- - - - -------- . . - -. ------------ - ------- -------- --
- __ - -
_______ - -
--- -~------
__ - ________ _ _____
-
----- - ----------------
-- -.----- - ------ ---- - -- --- --
Figure 3.
(Mackool) Resistance to leakage of the 3.0 mm wide eel with force applied 8.0 mm posterior to the eel.
0------- ------- - - -------- - - - - - ___ ___________ _
41 _ _ _ _--442
1.0
1.5
2.0
2.'
3.0
3.5
INCISION LENGTH, mm
J CATARACT REFRACT SURG-VOL 22, JULYIAUGUST 1996
723
CLEAR CORNEAL INCISION STRENGTH 3.5 mm INCISION WIDTH. APICAL FORCE
700
- - _ _ __ _ __ ___ ___ _ - _ • • _ ._. _ ___________ •
- - - - - - ------- ••
800
0----_0-.-.-._--._ ---- -- ------- --
---- ----- - -------.--------- ------- _ • • -- - . -- - - - - - - ---.- --
.0.
500
."
Figure 4.
(Mackool) Resistance to leakage of the 3.5 mm wide eel with force applied to the comeal apex.
31.
_. 0 __ 0_. _________ _
200
--------
100
_______ ••••••• - - ___ -
~
.3----- 75
~
.. ___ ._. 0 ____ ____ ____ ____ . ___ __ __ . ___ . _ . .____ _ .___ _.. ___ ____ ______ __ _ ~.
•• _ • • _. ________
I.'
1.0
.
0
-
-
_
.
-_ •
•
---- -- •
•
___ •
___ _
. _ . _ . _. ____ _
__________ ._." _
2.'
2.0
__ _
3.'
3.0
INCISION LENGTH , mm
ures 2 and 3 show the results obtained with the 3.0 mm wide incisions, and Figures 4 and 5 show results obtained with 3.5 mm wide incisions. An unpredicted relative decrease in wound strength was noted with the 3.5 mm incision width, central force, 3.0 mm incision length, and also with the 3.5 mm incision width, peripheral force, 2.5 mm and 3.0 mm incision lengths. These results may be attributable to inherent variations between tested globes and the small sample size. Figure 6 represents the data from all tested conditions. Although there are nonlinear variations from the
expected results, these data demonstrate a significant increase in corneal incision stability of incision lengths of 2.0 mm or more compared with incision lengths of 1.0 mm or 1.5 mm, for both 3.0 and 3.5 mm incision widths.
Discussion The strength of CCIs, particularly their ability to resist leakage, has been questioned, perhaps because of the variety of incisional architecture available. To date there has been no generally agreed-on standard or 3.5 mm INCISION WIDTH. POSTERIOR FORCE
600
- - - • • --- - ----- _ . - - - _._ . - - - _ • •---- -- -- - - - -- -- ------ -- --- ----- --. _ ___ • • _ . _________ _ __ _
600
----.-
400
• - ---
50.
- - - - - - -- -- ___ ____ • ____ ---- __________ _ ___ __ _ _______________ ___________________ __
_____ ____
____ _ _
Figure 5.
(Mackool) Resistance to leakage of the 3.5 mm wide eel with force applied 8.0 mm posterior to the eel.
iii
.:
310
300
•••••••••••••••••••.•••.•.• ~ .••••••••••••••••••••••••••.•....•• •••••••••• - -~157----- 176
200
100
- ---- - ---- --- ------ --- .-- -- -- - - - - - - - -- - --- ---- -_ . ________ - - --- __ _ _ _ _ __ _ _ • _ _____ . _____ _
.0
,.
oL-______________________________ 1.0
2.0
~
INCISION LENGTH. mm
724
J CATARACT REFRACT SURG-VOL 22, JULY/AUGUST 1996
____________________
2.'
3.0
,..
~
CLEAR CORNEAL INCISION STRENGTH 3.0 & 3 .6 mm INCISION WIOTHS. 700
600
APICAL ANO POSTERIOR FORCES
•••••••••••••••••••••• ••••••••••••••••• ••••
• ••••• .••••••••••••• ••••••••
• •••••••••••••••••
• ••••••••••••••••••••••••••••••••••
Figure 6. (Mackool) Resistance to leakage of the 3.0 and 3.5 mm wide eel with force applied to the comeal apex or posterior to the eel. [J 3.0 mm apical; • 3.0 mm posterior; D 3.5 mm apical; [j 3.5 mm posterior.
1.0
1 .•
2.0
2 ••
3.0
3 .•
INCISION LENGTH. mm
benchmark corneal incision by which to compare the strength of various incisional constructions. Relatively little comparative data concerning incision strength or other factors such as effects on corneal curvature have been reported in the current literature, making comparison of clinical results difficult. Scleral tunnel incisions have often been regarded as more stable and stronger than clear corneal incisions. A recent study of incision strength demonstrated that a square 3.2 X 3.2 mm scleral tunnel incision with a corneal lip resisted leakage with externally applied forces of at least 525 psi.2 Forces of this magnitude could be produced by direct ocular trauma. The site of origin of the corneal incision also contributes to incision strength and stability. An incision that begins at the limbus, which we define as the bluewhite junction, contributes independently to incision strength. A reported comparison of various CCI methods,4 including paracentesis and two-plane or hinged incisions between 2.5 and 5.0 mm wide and 1.75 mm and 2.0 mm long, demonstrated that those originating at the limbus were more resistant to leakage caused by externally applied forces when other parameters were constant. Square corneal incisions of 3.0 or 4.0 mm were the strongest constructions but were noted to be impractical either because of encroachment upon the
visual axis or because of the impediment they created to instrument use or 10L insertion. 4 We have shown that a two-plane, limbal-origin, clear corneal rectangular incision of 3.0 or 3.5 mm width and 2.0 mm or greater length is as resistant to leakage as a square scleral tunnel incision. Although the small sample size may have introduced variations in our data, the overall trend was clear. Desirable characteristics of a CCI include (1) ease of construction, (2) the ability to self-seal with a high resistance to leakage, (3) uncompromised access for phacoemulsification, and (4) a predictable effect on corneal curvature. We plan to address these criteria in future studies to expand our knowledge of the desirable morphology and application of CCIs.
References 1. Fine IH. Clear corneal incisions. 1m Ophthalmol Clin
1994; 34(2}:59-72 2. Ernest PH, Lavery KT, Kiessling LA. Relative strength of scleral corneal and clear corneal incisions constructed in cadaver eyes. J Cataract Refract Surg 1994; 20:626-629 3. Langerman DW. Architectural design of a self-sealing corneal tunnel, single-hinge incision. J Cataract Refract Surg 1994; 20:84-88 4. Ernest PH, Fenzl R, Lavery KT, Sensoli A. Relative stability of clear corneal incisions in a cadaver eye model. J Cataract Refract Surg 1995; 21:39-42
J CATARACT REFRACT SURG-VOL22. JULY/AUGUST 1996
725
T
he use of a clear corneal incision (eel) in cataract surgery has recently increased. With contemporary phacoemulsification techniques and foldable intraocular lenses (IOLs), a eel has the advantage of leaving the conjunctiva undisturbed in patients with pre-existing or planned filtering blebs, ocular cicatricial pemphigoid, Stevens-Johnson syndrome, or dry eye. A eel also leaves the sclera undisturbed in cases of scleritis and allows virtually bloodless surgery without the need for cautery in patients taking anticoagulants or those with a bleeding diathesis. Placing the eel at the steep meridian of the preoperative keratometric astigmatism can be used to reduce or eliminate pre-existing corneal astigmatism. Temporal corneal incision placement allows easier access in patients with prominent brows or deep-set eyes.
Reprint requests to Richard]. Mackool, MD, 31-2741 st Street, Astoria, New York 11103.
Temporal corneal incisions have been reported to be more stable than superior incisions. l However, the structural integrity of a eel has also been challenged; the stability of eels has been compared with that of scleral tunnel incisions? Important differences in eel architecture have been recognized; we are aware of at least five methods of incision construction: 1. The paracentesis incision, which is essentially parallel to the iris along its entire length. 2. The two-plane or grooved incision, in which the first cut is perpendicular to the corneal surface and the second plane is parallel to the iris. 3. A hinged incision, 3 which creates a deeper initial perpendicular corneal groove (approximately two-thirds corneal thickness) and a second plane that is parallel to the iris at one-half corneal thickness. 4. A three-plane incision, which is morphologically identical to a scleral tunnel incision and differs only by being entirely within the clear cornea. The length
J CATARACT REFRACT SURG-VOL 22, JULY/AUGUST 1996
721
CLEAR CORNEAL INCISION STRENGTH
of the "hinges" or corneal valves may vary, and the incisions may start at the limbus or 0.5 mm or more into clear cornea. 5. A two-plane incision created by a keratome, which initially enters the cornea at the limbus and advances at a slightly downward angle for a distance equal to the desired length of the incision. A short second plane is created by directing the keratome tip downward at an angle of approximately 45 degrees and advancing the tip into the anterior chamber (Figure O. The blade is then redirected to be roughly parallel to the cornea, as the entire blade enters the chamber. We used this technique in our study to ensure consistent incisions. We used two incision widths and varying incision lengths and measured their resistance to leakage during the application of force upon either the corneal apex or the sclera, posterior to the incision origin.
Materials and Methods Human cadaver whole globes that had not had prior surgery were obtained from the Lion's Eye Bank for Long Island, Manhasset, N ew York. Clear corneal incisions 3.0 or 3.5 mm in width and from 1.0 to 3.5 mm in length (0.5 mm increments) were made by one surgeon (R.S.R.) using an Alcon 3.0 or 3.5 mm steel keratome blade. Incision length was measured with a caliper that was checked for accuracy against a steel millimeter ruler. The incisions originated at the limbus (conjunctiva insertion) and were created as described above and shown in Figure 1. Table 1. Externally applied force (in pounds per square inch) producing either incision failure or severe globe deformation without incision failure. Incision Width
3.0mm Incision Length (mm)
1.0 1.5 2.0 2.5 3.0 3.5 722
Figure 1. (Mackool) The two-plane CCI. Dotted line represents the orientation of the blade after the tip enters the anterior chamber.
Balanced salt solution (BSS®) was injected through the optic nerve to establish an intraocular pressure (lOP) between 15 and 20 mm Hg. Intraocular pressure was measured with a Tono-Pen XL® (Oculab). The globes were then placed in a steel practice eye holder. External force was applied at the corneal apex or 8.0 mm posterior to the external incision margin using a flat round probe with a VB inch diameter. A Chatillon DFIS-lO digital force gauge with an accuracy of ±0.15% offull scale and a 10 lb maximum was used to measure and record externally applied forces. The calculated range of force was from 0 to 896 pounds per square inch (psi) ± 1.3. Forces were applied until incision failure or severe globe deformation occurred, making it technically impossible to apply greater force. Incision failure was defined as incision leakage, iris prolapse, or both. When severe globe deformation without incision failure occurred, the application of force was discontinued and the peak pressure was recorded. Measurements were repeated three times until incision failure or globe deformation occurred, and the mean pressure was calculated.
3.5mm
Apical Force (psi)
Posterior Force (psi)
Apical Force (psi)
Posterior Force (psi)
47 60 520 430 477 484
41 42 354 437 348 509
53 75 650 425 319 509
50 18 310 157 176 509
Results Clear corneal incisions 3.0 or 3.5 mm in width with a length of 2.0 mm or more did not leak with centrally applied external force of up to 650 psi. Identically wide incisions with lengths of 1.0 or 1.5 mm leaked or developed iris prolapse with centrally applied external forces at least eight times less (47 to 74 psi). Posteriorly applied external forces generally resulted in incision leakage at
J CATARACT REFRACT SURG-VOL 22, JULY/AUGUST 1996
CLEAR CORNEAL INCISION STRENGTH 3.0 mm INCISION WIDTH, APICAL FORCE
600
-----------••-
------------------- --
- - - - - - - - - - - - . ~ - - - - - - - - - - - - - - - - - - - - - - - --
Figure 2.
en
.:
(Mackool) Resistance to leakage of the 3.0 mm wide eel with force applied to the corneal apex.
300
200
100
---- - - - -------- ----- . ------
------------
I ------------- ----------
2.0
2.5
3.'
3.0
INCISION LENGTH, mm
lower pressures than centrally applied forces, with the exception of the 3.5 mm incision length when combined with a 3.0 or 3.5 mm incision width. No leakage was observed in these eyes with centrally or posteriorly applied forces of up to 509 psi. Incision lengths of2.0 mm or more were also at least eight times more resistant to leakage caused by posteriorly applied external forces than were incisions of 1.0 or 1.5 mm in length for both 3.0 and 3.5 mm incision widths. Incision lengths of 1.0 or 1.5 mm leaked with posteriorly applied external forces ranging from 18 to 24 psi. Incision lengths of
2.0 mm or more leaked with posteriorly applied external forces ranging from 157 to 436 psi. There was increased resistance to leakage with greater incision length and reduced incision width. Although there were variations within the two groups, the 3.0 and 3.5 mm incision widths displayed comparable resistance to leakage for all incision lengths studied. A rectangular incision 3.0 or 3.5 mm wide and 2.0 mm long, as well as a square incision of 3.5 X 3.5 mm, did not leak even with severe corneal and globe distortion that occurred between 509 and 650 psi (Table O. Fig-
3.0 mm INCISION WIDTH. POSTERIOR FORCE
-- - - - -- - --- •• •• -- -. - . ---------- - --
600
- - -- --- -- . - - - -
-..............................••••••••.. 400
~ '~"~-
~.........
. ............
T· . . ·
354
..
~
348
300
-- - - - - . - -- -- - - - - - - -------------- - ---
200
- __ - __ _ - ___
100
- --- - - -. - - - - - - - --- ----- --- --- ----- --- - - ---- -------- - - - -------- . . - -. ------------ - ------- -------- --
- __ - -
_______ - -
--- -~------
__ - ________ _ _____
-
----- - ----------------
-- -.----- - ------ ---- - -- --- --
Figure 3.
(Mackool) Resistance to leakage of the 3.0 mm wide eel with force applied 8.0 mm posterior to the eel.
0------- ------- - - -------- - - - - - ___ ___________ _
41 _ _ _ _--442
1.0
1.5
2.0
2.'
3.0
3.5
INCISION LENGTH, mm
J CATARACT REFRACT SURG-VOL 22, JULYIAUGUST 1996
723
CLEAR CORNEAL INCISION STRENGTH 3.5 mm INCISION WIDTH. APICAL FORCE
700
- - _ _ __ _ __ ___ ___ _ - _ • • _ ._. _ ___________ •
- - - - - - ------- ••
800
0----_0-.-.-._--._ ---- -- ------- --
---- ----- - -------.--------- ------- _ • • -- - . -- - - - - - - ---.- --
.0.
500
."
Figure 4.
(Mackool) Resistance to leakage of the 3.5 mm wide eel with force applied to the comeal apex.
31.
_. 0 __ 0_. _________ _
200
--------
100
_______ ••••••• - - ___ -
~
.3----- 75
~
.. ___ ._. 0 ____ ____ ____ ____ . ___ __ __ . ___ . _ . .____ _ .___ _.. ___ ____ ______ __ _ ~.
•• _ • • _. ________
I.'
1.0
.
0
-
-
_
.
-_ •
•
---- -- •
•
___ •
___ _
. _ . _ . _. ____ _
__________ ._." _
2.'
2.0
__ _
3.'
3.0
INCISION LENGTH , mm
ures 2 and 3 show the results obtained with the 3.0 mm wide incisions, and Figures 4 and 5 show results obtained with 3.5 mm wide incisions. An unpredicted relative decrease in wound strength was noted with the 3.5 mm incision width, central force, 3.0 mm incision length, and also with the 3.5 mm incision width, peripheral force, 2.5 mm and 3.0 mm incision lengths. These results may be attributable to inherent variations between tested globes and the small sample size. Figure 6 represents the data from all tested conditions. Although there are nonlinear variations from the
expected results, these data demonstrate a significant increase in corneal incision stability of incision lengths of 2.0 mm or more compared with incision lengths of 1.0 mm or 1.5 mm, for both 3.0 and 3.5 mm incision widths.
Discussion The strength of CCIs, particularly their ability to resist leakage, has been questioned, perhaps because of the variety of incisional architecture available. To date there has been no generally agreed-on standard or 3.5 mm INCISION WIDTH. POSTERIOR FORCE
600
- - - • • --- - ----- _ . - - - _._ . - - - _ • •---- -- -- - - - -- -- ------ -- --- ----- --. _ ___ • • _ . _________ _ __ _
600
----.-
400
• - ---
50.
- - - - - - -- -- ___ ____ • ____ ---- __________ _ ___ __ _ _______________ ___________________ __
_____ ____
____ _ _
Figure 5.
(Mackool) Resistance to leakage of the 3.5 mm wide eel with force applied 8.0 mm posterior to the eel.
iii
.:
310
300
•••••••••••••••••••.•••.•.• ~ .••••••••••••••••••••••••••.•....•• •••••••••• - -~157----- 176
200
100
- ---- - ---- --- ------ --- .-- -- -- - - - - - - - -- - --- ---- -_ . ________ - - --- __ _ _ _ _ __ _ _ • _ _____ . _____ _
.0
,.
oL-______________________________ 1.0
2.0
~
INCISION LENGTH. mm
724
J CATARACT REFRACT SURG-VOL 22, JULY/AUGUST 1996
____________________
2.'
3.0
,..
~
CLEAR CORNEAL INCISION STRENGTH 3.0 & 3 .6 mm INCISION WIOTHS. 700
600
APICAL ANO POSTERIOR FORCES
•••••••••••••••••••••• ••••••••••••••••• ••••
• ••••• .••••••••••••• ••••••••
• •••••••••••••••••
• ••••••••••••••••••••••••••••••••••
Figure 6. (Mackool) Resistance to leakage of the 3.0 and 3.5 mm wide eel with force applied to the comeal apex or posterior to the eel. [J 3.0 mm apical; • 3.0 mm posterior; D 3.5 mm apical; [j 3.5 mm posterior.
1.0
1 .•
2.0
2 ••
3.0
3 .•
INCISION LENGTH. mm
benchmark corneal incision by which to compare the strength of various incisional constructions. Relatively little comparative data concerning incision strength or other factors such as effects on corneal curvature have been reported in the current literature, making comparison of clinical results difficult. Scleral tunnel incisions have often been regarded as more stable and stronger than clear corneal incisions. A recent study of incision strength demonstrated that a square 3.2 X 3.2 mm scleral tunnel incision with a corneal lip resisted leakage with externally applied forces of at least 525 psi.2 Forces of this magnitude could be produced by direct ocular trauma. The site of origin of the corneal incision also contributes to incision strength and stability. An incision that begins at the limbus, which we define as the bluewhite junction, contributes independently to incision strength. A reported comparison of various CCI methods,4 including paracentesis and two-plane or hinged incisions between 2.5 and 5.0 mm wide and 1.75 mm and 2.0 mm long, demonstrated that those originating at the limbus were more resistant to leakage caused by externally applied forces when other parameters were constant. Square corneal incisions of 3.0 or 4.0 mm were the strongest constructions but were noted to be impractical either because of encroachment upon the
visual axis or because of the impediment they created to instrument use or 10L insertion. 4 We have shown that a two-plane, limbal-origin, clear corneal rectangular incision of 3.0 or 3.5 mm width and 2.0 mm or greater length is as resistant to leakage as a square scleral tunnel incision. Although the small sample size may have introduced variations in our data, the overall trend was clear. Desirable characteristics of a CCI include (1) ease of construction, (2) the ability to self-seal with a high resistance to leakage, (3) uncompromised access for phacoemulsification, and (4) a predictable effect on corneal curvature. We plan to address these criteria in future studies to expand our knowledge of the desirable morphology and application of CCIs.
References 1. Fine IH. Clear corneal incisions. 1m Ophthalmol Clin
1994; 34(2}:59-72 2. Ernest PH, Lavery KT, Kiessling LA. Relative strength of scleral corneal and clear corneal incisions constructed in cadaver eyes. J Cataract Refract Surg 1994; 20:626-629 3. Langerman DW. Architectural design of a self-sealing corneal tunnel, single-hinge incision. J Cataract Refract Surg 1994; 20:84-88 4. Ernest PH, Fenzl R, Lavery KT, Sensoli A. Relative stability of clear corneal incisions in a cadaver eye model. J Cataract Refract Surg 1995; 21:39-42
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