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Effect of Buckling Material on Ocular Rigidity
Effect of Buckling Material on Ocular Rigidity Marc M. Whitacre, MD,1 Mark D. Emig, MD,1 Khatab Hassanein, PhD2
E
nucleated human eyes were banded with metal and silicone bands to produce reductions in their diameter of 2 mm and 4 mm. The ocular rigidity produced by each banding material at each diameter was measured in the pressure range of 10 mmHg to 40 mmHg. Metal bands produced mild reductions in ocular rigidity that were significantly (P < 0.05 to 0.01) lower than the control ocular rigidities in some pressure ranges. Silicone bands produced large reductions in ocular rigidity that were significantly (P < 0.01) lower than ocular rigidities observed in metal-banded or control conditions in all pressure ranges. The influence of the elastic silicone banding material on ocular rigidity was greater than the influence of altered shape and wall stress that occurred with metal banding. Ophthalmology 1992;99;498-502
Scleral buckling has been long known to reduce ocular rigidity. 1-3 The mechanism of this reduction in ocular rigidity has been a subject of speculation until recently, when Johnson et al4 observed in eye bank eyes that the reduction in ocular rigidity associated with scleral buckling was reversible with removal of the silicone buckle. Subsequently Friberg and Fourman5 found that a significant reduction in ocular rigidity occurred in eyes encircled with either steel or silicone bands, and concluded that the alterations in ocular rigidity that accompany buckling were primarily due to changes in scleral shape, and that the elasticity of the encircling band played a minor role. We studied the changes in ocular rigidity that accompanied banding with metal and silicone bands producing identical initial ocular indentations.
Materials and Methods Seven eye bank eyes were selected from donors not believed to have any ocular diseases. All eyes were used Originally received: September 27, 1991. Revision accepted: December 13, 1991. I Department of Ophthalmology, University of Kansas Medical Center, Kansas City. 2 Department of Biometry, University of Kansas Medical Center, Kansas City. Supported by an unrestricted grant from Research to Prevent Blindness, Inc, New York, New York, and the Kansas Lions Sight Foundation, Atwood, Kansas. Reprint requests to Marc M. Whitacre, MD, Department of Ophthalmology, Sudler Hall, University of Kansas Medical Center, 39th and Rainbow Blvd, Kansas City, KS 66103.
498
within 24 hours of death of the donor, and only 1 eye from each donor was used. The eyes were placed in a small metal cup lined with wet cotton. A short length of intravenous tubing connected a 22-gauge needle to a 3way stopcock. The needle was connected to a pressure transducer by the second arm of the stopcock. The third arm of the stopcock was connected to a second three-way stopcock. The second stopcock was connected to a bag of normal saline and a microliter syringe. A manometer was placed in line between the bag of normal saline and the second stopcock to allow calibration of the system. The rigidity of the system (with the eye removed from the circuit) was tested and found to be 1.0 JlI- 1 using Friedenwald's equation. 6 Before each experiment, the system was calibrated by closing the stopcock to the needle and opening the stopcock to the manometer and correlating the transducer reading to the manometer at 10, 20, 30, and 40 mmHg. Before connecting an eye to the apparatus, all bubbles present in the stopcocks, tubing, and needle were purged. The 22-gauge needle was then inserted into the anterior chamber at the comeoscleral limbus. After insertion of the 22-gauge needle, the intraocular pressure was increased to 40 mmHg by opening the stopcock to the normal saline. The insertion site was inspected and if any leak was noted it was sealed with a small amount of cyanoacrylate glue. Most eyes were found to have a very slow leak and required sealing of the insertion site. The intraocular pressure was adjusted to 10 mmHg and the stopcock to the manometer was closed. The diameter of the eye was measured with calipers to the nearest 0.25 mm at this pressure. Consecutive injections of 0.01 ml of normal saline were
Whitacre et al made into the system by means of the microliter syringe until the intraocular pressure reached at least 40 mmHg. After each injection, the intraocular pressure reported by the transducer had to remain stable for 3 to 5 seconds before the intraocular pressure reported by the transducer was recorded. After reaching 40 mmHg or higher, the intraocular pressure was reduced to 10 mmHg by withdrawing the plunger of the microliter syringe to its starting point. This was usually sufficient to restore the eye to the planned starting pressure of 10 mmHg, unless the eye had been manipulated to add or remove a band, in which case it was also necessary to open the stopcock connecting the system to the manometer and the attached bag of saline. This process was repeated for a total of three times and was considered the first control. A strip of sheet aluminum 0.4 mm thick and 2.5 mm wide was formed into a band with an inside diameter of 2 mm less than the original outside eye diameter by gluing the strip to itself with cyanoacrylate glue. The band was placed around the equator of the eye and was not secured to the globe by scleral sutures or any other means of mechanical fixation. Incremental injections of 0.01 ml of saline were administered until the intraocular pressure equaled or exceeded 40 mmHg on three occasions. The band was removed and a second control experiment was performed in the same manner as the first. While the intraocular pressure was left at 10 mmHg, a 240 silicone band (MIRA, Waltham, MA) was placed around the eye at the equator, and its ends were tied with a suture to produce a band that constricted the eye to produce an outside eye diameter of 2 mm less than the original eye diameter. The eye was again subjected to the identical procedure as described above. The silicone band was removed and a third control experiment was performed. The procedure was then repeated with the same silicone band adjusted to produce a 4-mm reduction in the outside eye diameter, followed by another control, followed in turn by a metal band producing a 4-mm indentation, and followed by a final control. The initial intraocular pressure of each experimental condition was 10 mmHg. It was sometimes necessary to aspirate liquid vitreous to place the smaller bands on the globe. Vitreous aspiration was performed through the pars plana with a 25-gauge needle, taking care to verify that the aspiration site was self-sealing. The same strip of aluminum was used to fashion all the bands in our experiment. Because even minor movements of the band on the eye produced substantial changes in the intraocular pressure, whenever a band moved spontaneously, that experiment was discarded. One eye was not included in our data because of persistent movement of the band. The cornea and globe were continuously irrigated while the experiments were performed. Each eye was subjected to the same procedure by the same personnel. Approximately 2 hours were required to complete all the procedures to each eye. The coefficient of ocular rigidity was calculated for each injection to each eye using Friedenwald's equation6 and assigned to the mean of the initial and final intraocular pressures used to calculate that coefficient of ocular rigidity. The coefficients of ocular rigidity were grouped to-
Ocular Rigidity gether at 5 mmHg intervals from 10 to 15 mmHg, 15 to 20 mmHg, 20 to 25 mmHg, 25 to 30 mmHg, 30 to 35 mmHg, and 35 to 40 mmHg. The mean ocular rigidity produced by each of the nine experimental conditions (five controls and four bands) for each of the six pressure ranges was calculated. When data were not present in a pressure range for a given experimental condition (because of a high coefficient of ocular rigidity that produced fewer points for analysis), the coefficient of ocular rigidity for that pressure range was estimated by linear interpolation. There were only 14 instances of 324 data points in which this was necessary. The mean rigidities for each pressure range were analyzed by analysis of variance (ANOVA) for repeated measurements. Newman-Keuls' multiple comparison test was used to locate the exact difference between group means.
Results The results for each eye are shown in Figure 1. Inspection of the behavior of the individual eyes shows that banding with silicone produced a marked reduction in the coefficient of rigidity of all the eyes studied. Reducing the diameter of the silicone band was associated with a further reduction in ocular rigidity in eyes A to D and little or no reduction in rigidity in eyes E and F. Banding the eyes with metal produced a less dramatic effect on ocular rigidity of the individual eyes. Banding with metal appeared to slightly reduce ocular rigidity compared with the initial control ocular rigidities in most eyes. Ocular rigidities produced by different diameters of metal bands frequently overlapped with control ocular rigidities. Compared with metal bands producing a 2-mm reduction in diameter, metal bands producing a 4-mm reduction in diameter were associated with an additional slight reduction in ocular rigidity in most eyes. The results of the averaged data of the studied eyes are shown in Figure 2. Although averaged ocular rigidities of the initial and final controls (the first and ninth experiments, respectively) show that the coefficient of ocular rigidity of the eyes decreased as the experiment progressed, no statistically significant differences were detected between any of the control experiments. Banding with silicone had a dramatic and easily detectable effect. A statistically highly significant (P < 0.01) difference was present between the coefficient of ocular rigidity produced by banding with silicone to produce a 2-mmor 4-mm reduction in diameter and the coefficients of ocular rigidity observed during control conditions or metal banding in all pressure ranges. Banding with silicone to produce a diameter reduction of 4 mm produced a significantly lower coefficient of ocular rigidity than silicone banding producing a 2 mm reduction in diameter in the 25 to 30 mmHg pressure range (P < 0.01) and in the 35 to 40 mmHg pressure range (P < 0.05). Banding with metal produced a mild reduction in the average coefficient of ocular rigidity to levels below those of the average final controls, except for the band producing
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Diameter: 25 rom
Diameter: 25 rom
0.02
Donor age: 67
0.02
Donor age: 75
0.015 Ocular rigidity
Ocular rigidity (in ul- 1)
(in urI) 0.01
0.01 0.005
O~r------.-------.--~--~
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10
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Figure 1. Coefficient of ocular rigidity for eyes A to F. Shown for each eye are the rigidities of the initial control (l» , final control (6), metal banding producing a 2-mm reduction in diameter (0) and a 4-mm reduction in diameter (0), and silicone banding producing a 2-mm reduction in diameter (0) and a 4 -m m reduction in diameter (0). The outside diameter of the unbuckled globe and the eye donor age are listed with the data for each eye.
a 2-mm reduction in diameter in the 30 to 40 mmHg pressure range. Bands producing 4-mm reductions in diameter were associated with a lower coefficient of ocular rigidity than bands producing a 2-mm reduction in di-
500
ameter. Statistical analysis showed that at 15 to 20 mmHg and 25 to 30 mmHg, a statistically significant (P < 0.05) to highly significant (P < 0.01) decrease in the coefficient of ocular rigidity was produced by metal bands that pro-
Whitacre et al . Ocular Rigidity 0.025
0.02
E Diameter: 24 mm Donor age: 91
0.D15
F
0.025
Diameter: 25 mm
0.02
Donor age: 90 GI,
0.015
Ocular rigidity
Ocular rigidity (in ul- 1)
(in urI) 0.01
0.01
0.005
0.005
10 Intraocular pressure (in mm Hg)
20
30
40
Intraocular pressure (in mm Hg)
Figure 1. (continued)
duced a reduction in diameter of 2 and 4 mm. At 30 to 35 mmHg, a statistically significant (P < 0.05) reduction in the coefficient of ocular rigidity was produced when the eyes were banded with metal to produce a 4-mm reduction in diameter.
Discussion Johnson et al4 studied the effect on ocular rigidity produced by buckling eyes with solid silicone tires and an encircling band sutured in place with Dacron sutures drawn tightly enough to produce scleral imbrication. The pressure-volume relationship of the studied eyes was determined by observing the pressure change that occurred during continuous infusion of air or saline from an intraocular pressure of 0 mmHg up to 80 mmHg. Four of the 10 experimental eyes underwent remeasurement of the pressure-volume relationship after removal of the buckling material, and no statistically significant difference between the initial controls was observed. Johnson et al4 also measured the extensibility of a 2.5-mm wide silicone band and found it to be approximately 40 times more extensible than sclera. Based on these results, they suggested that scleral buckling with silicone reduced ocular rigidity because of circumferential elongation of the encircling band, which also permitted flattening of the buckle height in regions of nonimbricated sclera. In addition, they speculated that changing the eye to a nonspherical shape might alter the pressure-volume relationship.
Means
0.02
0.015 Ocular rigidity (in ul- 1) 0.01
0.005
O~.------.-------,------~
10
20
30
40
Intraocular pressure (in mm Hg) Figure 2. The mean coefficients of ocular rigidity for all eyes. Shown are the values of the initial control (ll), final control (6), metal banding producing a 2-mm reduction in diameter (0) and a 4-mm reduction in diameter (0), and silicone banding producing a 2-mm reduction in diameter (0) and a 4-mm reduction in diameter (0).
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Subsequently Friberg and Fourman 5 tested the pressure-volume response of eyes encircled with two different diameter metal bands, a silicone band and an encircling silicone tire and band. All the bands and buckles were secured with sutures. The eyes were subjected to continuous infusion to a final intraocular pressure of 80 to 90 mmHg while the ensuing pressure changes were recorded. It was found that unbuckled eyes had significantly higher ocular rigidity values than eyes buckled with steel or silicone. Smaller diameter buckles caused a greater reduction in ocular rigidity than larger diameter buckles of similar composition. The authors observed that buckling of eyes with a steel band reduced ocular rigidity from 0.50 mmHgJ~1 to 0.28 mmHgJ~l, and buckling with silicone further reduced ocular rigidity to 0.22 mmHgj~l. The authors did not comment on whether this decline was statistically significant, but stated "the primary factors that alter ocular rigidity are the change in the overall shape of the globe and the change in scleral contour in the vicinity of the buckle." They added that the elasticity of the encircling band played a secondary role in the reduction of ocular rigidity. Our experiment was designed to compare the ocular rigidities produced under identical conditions of banding with a rigid and elastic material under physiologic intraocular pressures, using band heights that are encountered clinically. Because permanent alteration of the sclera has been reported in experiments in which the intraocular pressure has been raised,7-9 even when done by rapid injection, 10,1 1 we performed controls before and after every band was applied, thereby taking into consideration the possibility of an ongoing alteration of the mechanical characteristics of the sclera as each experiment was performed. It should be noted that the metal band producing a 2-mm reduction in diameter was the second of the nine experiments, and the metal band producing the 4-mm reduction in diameter was the eighth of nine experiments. The reduced rigidity observed with the smaller diameter metal band may be the result not only of the conformational change of the sclera, but also a progressive decline in ocular rigidity that occurred as each experiment progressed. Therefore, we did not consider the effect of banding on ocular rigidity to be significant unless a statistically significant difference existed between the ocular rigidity of the experimental condition and the preceding control. Had we not adopted this procedure, metal banding to produce a diameter reduction of 4 mm would have produced a statistically significant reduction in ocular rigidity at all intraocular pressures, instead of the pressure ranges we report. We did not suture the bands in place so as to avoid any weakening of the sclera and subsequent reduction in ocular rigidity that might ensue. Our results confirm the findings of Johnson et al4 that the elasticity of the buckling silicone element is the main
502
cause of a reduction in ocular rigidity in eyes buckled with silicone. Our results also demonstrate that banding with even an inelastic substance, such as metal, produces some reduction in the coefficient of ocular rigidity but not to the extent found by Friberg and Fourman. 5 This may be because we studied the behavior of our eyes in the intraocular pressure range of 10 to 40 mmHg, rather than the 10 to 15 mmHg to 80 to 90 mmHg range. Inspection of the figures reported by Friberg and Fourman5 shows that the pressure-volume curves of nonbuckled eyes and eyes buckled with steel did not noticeably diverge in their experiments until the intraocular pressure exceeded 40 mmHg. We cannot exclude the possibility that our experimental eyes might have behaved similarly, but we chose to limit the intraocular pressure range of our experiments to one that would cause less stress to the sclera. Our results also demonstrate that even in vitro individual eyes may respond differently to buckling with identical materials. This may be due to differences in scleral composition and thickness, ocular dimensions, or heretofore unknown factors, and suggests that there are limits to the predictive abilities of any model of ocular rigidity.
References I. Pemberton JW. Schi0tz-applanation disparity following
2. 3. 4. . 5. 6. 7. 8. 9. 10. 11.
retinal detachment surgery. Arch Ophthalmol 1969;81 : 534-7. Syrdalen P. Intraocular pressure and ocular rigidity in patients with retinal detachment. II. Postoperative study. Acta Ophthalmol 1970;48: 1036-44. Harbin TS Jr, Laikam SE, Lipsitt K, et al. ApplanationSchi0tz disparity after retinal detachment surgery utilizing cryopexy. Ophthalmology 1979;86:1609-12. Johnson MW, Han DP, Hoffman KE. The effect of scleral buckling on ocular rigidity. Ophthalmology 1990;97: 190-5. Friberg TR, Fourman SB. Scleral buckling and ocular rigidity: clinical ramifications. Arch Ophthalmol 1990; 108: 1622-7. Friedenwald JS. Contribution to the theory and practice of tonometry. Am J Ophthalmol 1937;20:985-1024. Greene PR, McMahon TA. Scleral creep vs temperature and pressure in vitro. Exp Eye Res 1979;29:527-37. Macri FJ, Wanko T, Grimes PA. The elastic properties of the human eye. Arch Ophthalmol 1958;60:1021-6. Perkins ES, Gloster J. Distensibility of the eye. Br J OphthalmoI1957;41:93-102. Lyon C, McEwen WK, Shepherd MD. Ocular rigidity and decay curves analyzed by two nonlinear systems. Invest Ophthalmol 1970;9:935-45. Gloster J, Perkins ES, Pommier M-L. Extensibility of strips of sclera and cornea. Br J Ophthalmol 1957;41 :103-10.
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nucleated human eyes were banded with metal and silicone bands to produce reductions in their diameter of 2 mm and 4 mm. The ocular rigidity produced by each banding material at each diameter was measured in the pressure range of 10 mmHg to 40 mmHg. Metal bands produced mild reductions in ocular rigidity that were significantly (P < 0.05 to 0.01) lower than the control ocular rigidities in some pressure ranges. Silicone bands produced large reductions in ocular rigidity that were significantly (P < 0.01) lower than ocular rigidities observed in metal-banded or control conditions in all pressure ranges. The influence of the elastic silicone banding material on ocular rigidity was greater than the influence of altered shape and wall stress that occurred with metal banding. Ophthalmology 1992;99;498-502
Scleral buckling has been long known to reduce ocular rigidity. 1-3 The mechanism of this reduction in ocular rigidity has been a subject of speculation until recently, when Johnson et al4 observed in eye bank eyes that the reduction in ocular rigidity associated with scleral buckling was reversible with removal of the silicone buckle. Subsequently Friberg and Fourman5 found that a significant reduction in ocular rigidity occurred in eyes encircled with either steel or silicone bands, and concluded that the alterations in ocular rigidity that accompany buckling were primarily due to changes in scleral shape, and that the elasticity of the encircling band played a minor role. We studied the changes in ocular rigidity that accompanied banding with metal and silicone bands producing identical initial ocular indentations.
Materials and Methods Seven eye bank eyes were selected from donors not believed to have any ocular diseases. All eyes were used Originally received: September 27, 1991. Revision accepted: December 13, 1991. I Department of Ophthalmology, University of Kansas Medical Center, Kansas City. 2 Department of Biometry, University of Kansas Medical Center, Kansas City. Supported by an unrestricted grant from Research to Prevent Blindness, Inc, New York, New York, and the Kansas Lions Sight Foundation, Atwood, Kansas. Reprint requests to Marc M. Whitacre, MD, Department of Ophthalmology, Sudler Hall, University of Kansas Medical Center, 39th and Rainbow Blvd, Kansas City, KS 66103.
498
within 24 hours of death of the donor, and only 1 eye from each donor was used. The eyes were placed in a small metal cup lined with wet cotton. A short length of intravenous tubing connected a 22-gauge needle to a 3way stopcock. The needle was connected to a pressure transducer by the second arm of the stopcock. The third arm of the stopcock was connected to a second three-way stopcock. The second stopcock was connected to a bag of normal saline and a microliter syringe. A manometer was placed in line between the bag of normal saline and the second stopcock to allow calibration of the system. The rigidity of the system (with the eye removed from the circuit) was tested and found to be 1.0 JlI- 1 using Friedenwald's equation. 6 Before each experiment, the system was calibrated by closing the stopcock to the needle and opening the stopcock to the manometer and correlating the transducer reading to the manometer at 10, 20, 30, and 40 mmHg. Before connecting an eye to the apparatus, all bubbles present in the stopcocks, tubing, and needle were purged. The 22-gauge needle was then inserted into the anterior chamber at the comeoscleral limbus. After insertion of the 22-gauge needle, the intraocular pressure was increased to 40 mmHg by opening the stopcock to the normal saline. The insertion site was inspected and if any leak was noted it was sealed with a small amount of cyanoacrylate glue. Most eyes were found to have a very slow leak and required sealing of the insertion site. The intraocular pressure was adjusted to 10 mmHg and the stopcock to the manometer was closed. The diameter of the eye was measured with calipers to the nearest 0.25 mm at this pressure. Consecutive injections of 0.01 ml of normal saline were
Whitacre et al made into the system by means of the microliter syringe until the intraocular pressure reached at least 40 mmHg. After each injection, the intraocular pressure reported by the transducer had to remain stable for 3 to 5 seconds before the intraocular pressure reported by the transducer was recorded. After reaching 40 mmHg or higher, the intraocular pressure was reduced to 10 mmHg by withdrawing the plunger of the microliter syringe to its starting point. This was usually sufficient to restore the eye to the planned starting pressure of 10 mmHg, unless the eye had been manipulated to add or remove a band, in which case it was also necessary to open the stopcock connecting the system to the manometer and the attached bag of saline. This process was repeated for a total of three times and was considered the first control. A strip of sheet aluminum 0.4 mm thick and 2.5 mm wide was formed into a band with an inside diameter of 2 mm less than the original outside eye diameter by gluing the strip to itself with cyanoacrylate glue. The band was placed around the equator of the eye and was not secured to the globe by scleral sutures or any other means of mechanical fixation. Incremental injections of 0.01 ml of saline were administered until the intraocular pressure equaled or exceeded 40 mmHg on three occasions. The band was removed and a second control experiment was performed in the same manner as the first. While the intraocular pressure was left at 10 mmHg, a 240 silicone band (MIRA, Waltham, MA) was placed around the eye at the equator, and its ends were tied with a suture to produce a band that constricted the eye to produce an outside eye diameter of 2 mm less than the original eye diameter. The eye was again subjected to the identical procedure as described above. The silicone band was removed and a third control experiment was performed. The procedure was then repeated with the same silicone band adjusted to produce a 4-mm reduction in the outside eye diameter, followed by another control, followed in turn by a metal band producing a 4-mm indentation, and followed by a final control. The initial intraocular pressure of each experimental condition was 10 mmHg. It was sometimes necessary to aspirate liquid vitreous to place the smaller bands on the globe. Vitreous aspiration was performed through the pars plana with a 25-gauge needle, taking care to verify that the aspiration site was self-sealing. The same strip of aluminum was used to fashion all the bands in our experiment. Because even minor movements of the band on the eye produced substantial changes in the intraocular pressure, whenever a band moved spontaneously, that experiment was discarded. One eye was not included in our data because of persistent movement of the band. The cornea and globe were continuously irrigated while the experiments were performed. Each eye was subjected to the same procedure by the same personnel. Approximately 2 hours were required to complete all the procedures to each eye. The coefficient of ocular rigidity was calculated for each injection to each eye using Friedenwald's equation6 and assigned to the mean of the initial and final intraocular pressures used to calculate that coefficient of ocular rigidity. The coefficients of ocular rigidity were grouped to-
Ocular Rigidity gether at 5 mmHg intervals from 10 to 15 mmHg, 15 to 20 mmHg, 20 to 25 mmHg, 25 to 30 mmHg, 30 to 35 mmHg, and 35 to 40 mmHg. The mean ocular rigidity produced by each of the nine experimental conditions (five controls and four bands) for each of the six pressure ranges was calculated. When data were not present in a pressure range for a given experimental condition (because of a high coefficient of ocular rigidity that produced fewer points for analysis), the coefficient of ocular rigidity for that pressure range was estimated by linear interpolation. There were only 14 instances of 324 data points in which this was necessary. The mean rigidities for each pressure range were analyzed by analysis of variance (ANOVA) for repeated measurements. Newman-Keuls' multiple comparison test was used to locate the exact difference between group means.
Results The results for each eye are shown in Figure 1. Inspection of the behavior of the individual eyes shows that banding with silicone produced a marked reduction in the coefficient of rigidity of all the eyes studied. Reducing the diameter of the silicone band was associated with a further reduction in ocular rigidity in eyes A to D and little or no reduction in rigidity in eyes E and F. Banding the eyes with metal produced a less dramatic effect on ocular rigidity of the individual eyes. Banding with metal appeared to slightly reduce ocular rigidity compared with the initial control ocular rigidities in most eyes. Ocular rigidities produced by different diameters of metal bands frequently overlapped with control ocular rigidities. Compared with metal bands producing a 2-mm reduction in diameter, metal bands producing a 4-mm reduction in diameter were associated with an additional slight reduction in ocular rigidity in most eyes. The results of the averaged data of the studied eyes are shown in Figure 2. Although averaged ocular rigidities of the initial and final controls (the first and ninth experiments, respectively) show that the coefficient of ocular rigidity of the eyes decreased as the experiment progressed, no statistically significant differences were detected between any of the control experiments. Banding with silicone had a dramatic and easily detectable effect. A statistically highly significant (P < 0.01) difference was present between the coefficient of ocular rigidity produced by banding with silicone to produce a 2-mmor 4-mm reduction in diameter and the coefficients of ocular rigidity observed during control conditions or metal banding in all pressure ranges. Banding with silicone to produce a diameter reduction of 4 mm produced a significantly lower coefficient of ocular rigidity than silicone banding producing a 2 mm reduction in diameter in the 25 to 30 mmHg pressure range (P < 0.01) and in the 35 to 40 mmHg pressure range (P < 0.05). Banding with metal produced a mild reduction in the average coefficient of ocular rigidity to levels below those of the average final controls, except for the band producing
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(in urI) 0.01
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O~r------.-------.--~--~
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Figure 1. Coefficient of ocular rigidity for eyes A to F. Shown for each eye are the rigidities of the initial control (l» , final control (6), metal banding producing a 2-mm reduction in diameter (0) and a 4-mm reduction in diameter (0), and silicone banding producing a 2-mm reduction in diameter (0) and a 4 -m m reduction in diameter (0). The outside diameter of the unbuckled globe and the eye donor age are listed with the data for each eye.
a 2-mm reduction in diameter in the 30 to 40 mmHg pressure range. Bands producing 4-mm reductions in diameter were associated with a lower coefficient of ocular rigidity than bands producing a 2-mm reduction in di-
500
ameter. Statistical analysis showed that at 15 to 20 mmHg and 25 to 30 mmHg, a statistically significant (P < 0.05) to highly significant (P < 0.01) decrease in the coefficient of ocular rigidity was produced by metal bands that pro-
Whitacre et al . Ocular Rigidity 0.025
0.02
E Diameter: 24 mm Donor age: 91
0.D15
F
0.025
Diameter: 25 mm
0.02
Donor age: 90 GI,
0.015
Ocular rigidity
Ocular rigidity (in ul- 1)
(in urI) 0.01
0.01
0.005
0.005
10 Intraocular pressure (in mm Hg)
20
30
40
Intraocular pressure (in mm Hg)
Figure 1. (continued)
duced a reduction in diameter of 2 and 4 mm. At 30 to 35 mmHg, a statistically significant (P < 0.05) reduction in the coefficient of ocular rigidity was produced when the eyes were banded with metal to produce a 4-mm reduction in diameter.
Discussion Johnson et al4 studied the effect on ocular rigidity produced by buckling eyes with solid silicone tires and an encircling band sutured in place with Dacron sutures drawn tightly enough to produce scleral imbrication. The pressure-volume relationship of the studied eyes was determined by observing the pressure change that occurred during continuous infusion of air or saline from an intraocular pressure of 0 mmHg up to 80 mmHg. Four of the 10 experimental eyes underwent remeasurement of the pressure-volume relationship after removal of the buckling material, and no statistically significant difference between the initial controls was observed. Johnson et al4 also measured the extensibility of a 2.5-mm wide silicone band and found it to be approximately 40 times more extensible than sclera. Based on these results, they suggested that scleral buckling with silicone reduced ocular rigidity because of circumferential elongation of the encircling band, which also permitted flattening of the buckle height in regions of nonimbricated sclera. In addition, they speculated that changing the eye to a nonspherical shape might alter the pressure-volume relationship.
Means
0.02
0.015 Ocular rigidity (in ul- 1) 0.01
0.005
O~.------.-------,------~
10
20
30
40
Intraocular pressure (in mm Hg) Figure 2. The mean coefficients of ocular rigidity for all eyes. Shown are the values of the initial control (ll), final control (6), metal banding producing a 2-mm reduction in diameter (0) and a 4-mm reduction in diameter (0), and silicone banding producing a 2-mm reduction in diameter (0) and a 4-mm reduction in diameter (0).
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Ophthalmology
Volume 99, Number 4, April 1992
Subsequently Friberg and Fourman 5 tested the pressure-volume response of eyes encircled with two different diameter metal bands, a silicone band and an encircling silicone tire and band. All the bands and buckles were secured with sutures. The eyes were subjected to continuous infusion to a final intraocular pressure of 80 to 90 mmHg while the ensuing pressure changes were recorded. It was found that unbuckled eyes had significantly higher ocular rigidity values than eyes buckled with steel or silicone. Smaller diameter buckles caused a greater reduction in ocular rigidity than larger diameter buckles of similar composition. The authors observed that buckling of eyes with a steel band reduced ocular rigidity from 0.50 mmHgJ~1 to 0.28 mmHgJ~l, and buckling with silicone further reduced ocular rigidity to 0.22 mmHgj~l. The authors did not comment on whether this decline was statistically significant, but stated "the primary factors that alter ocular rigidity are the change in the overall shape of the globe and the change in scleral contour in the vicinity of the buckle." They added that the elasticity of the encircling band played a secondary role in the reduction of ocular rigidity. Our experiment was designed to compare the ocular rigidities produced under identical conditions of banding with a rigid and elastic material under physiologic intraocular pressures, using band heights that are encountered clinically. Because permanent alteration of the sclera has been reported in experiments in which the intraocular pressure has been raised,7-9 even when done by rapid injection, 10,1 1 we performed controls before and after every band was applied, thereby taking into consideration the possibility of an ongoing alteration of the mechanical characteristics of the sclera as each experiment was performed. It should be noted that the metal band producing a 2-mm reduction in diameter was the second of the nine experiments, and the metal band producing the 4-mm reduction in diameter was the eighth of nine experiments. The reduced rigidity observed with the smaller diameter metal band may be the result not only of the conformational change of the sclera, but also a progressive decline in ocular rigidity that occurred as each experiment progressed. Therefore, we did not consider the effect of banding on ocular rigidity to be significant unless a statistically significant difference existed between the ocular rigidity of the experimental condition and the preceding control. Had we not adopted this procedure, metal banding to produce a diameter reduction of 4 mm would have produced a statistically significant reduction in ocular rigidity at all intraocular pressures, instead of the pressure ranges we report. We did not suture the bands in place so as to avoid any weakening of the sclera and subsequent reduction in ocular rigidity that might ensue. Our results confirm the findings of Johnson et al4 that the elasticity of the buckling silicone element is the main
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cause of a reduction in ocular rigidity in eyes buckled with silicone. Our results also demonstrate that banding with even an inelastic substance, such as metal, produces some reduction in the coefficient of ocular rigidity but not to the extent found by Friberg and Fourman. 5 This may be because we studied the behavior of our eyes in the intraocular pressure range of 10 to 40 mmHg, rather than the 10 to 15 mmHg to 80 to 90 mmHg range. Inspection of the figures reported by Friberg and Fourman5 shows that the pressure-volume curves of nonbuckled eyes and eyes buckled with steel did not noticeably diverge in their experiments until the intraocular pressure exceeded 40 mmHg. We cannot exclude the possibility that our experimental eyes might have behaved similarly, but we chose to limit the intraocular pressure range of our experiments to one that would cause less stress to the sclera. Our results also demonstrate that even in vitro individual eyes may respond differently to buckling with identical materials. This may be due to differences in scleral composition and thickness, ocular dimensions, or heretofore unknown factors, and suggests that there are limits to the predictive abilities of any model of ocular rigidity.
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