The Effect of Scleral Buckling on Ocular Rigidity MARK W. JOHNSON, MD,t DENNIS P. HAN, MD/ KENNETH E. HOFFMAN, MS 2
Abstract: In a study of enucleated human eyes, the authors investigated the effect of scleral buckling on the ocular pressure-volume relationship. Intraocular pressure was recorded continuously during intravitreal infusion of saline solution before and after the application of encircling silicone elements. Scleral buckling produced a marked reduction in ocular rigidity, with reversibility of the effect on removal of the buckling elements. Similar results were obtained during incre mental intravitreal air injection. The authors propose that the greater extensibility of silicone compared with sclera and the induced alterations in ocular shape are the primary factors responsible for the observed change in ocular rigidity. The clinical implications of these findings for intravitreal gas injection are dis cussed. Ophthalmology 1990; 97:190-195
During the last 15 years, the value of intravitreal gas in treating retinal detachment has been firmly estab lished. 1- 7 A recent survey of vitreoretinal surgeons indi cated both widespread and frequent use of intravitreal injections ofair and long-acting gases in conjunction with scleral buckling and vitrectomy procedures. 8 Most indi cations for intravitreal gas require the injection of as large a volume as possible, with intraocular pressure {lOP) being a primary limiting factor. The change in lOP that results from a given change in intraocular volume is determined by the ocular pressure volume relationship, known as ocular rigidity (API AV). Although numerous investigators have attempted to de termine empirically and theoretically the mathematical expression that best describes the pressure-volume be havior of the eye, the results have been contradictory and the precise characterization remains incomplete. 9- 14 Con ceptually, ocular rigidity refers to the resistance offered by ocular tissues to the expansion of intraocular volume 9 ; Originally received: March 17, 1989. Revision accepted: September 25, 1989. 1
W.K. Kellogg Eye Center, Department of Ophthalmology, University of Michigan, Ann Arbor. 2 Department of Physiology, University of Michigan, Ann Arbor. Dr. Johnson is currently at Bascom Palmer Eye Institute, Miami. Dr. Han is currently with the Department of Ophthalmology, Medical College of Wisconsin, Milwaukee. Presented at the Vitreous Society Annual Meeting, Scottsdale, Arizona, December 1988. Reprint requests to Dennis P. Han, MD, Eye Institute, 8700 W. Wisconsin Ave, Milwaukee, WI 53226.
190
the greater the rigidity, the greater the pressure rise in response to a given volume change. Intuitively, therefore, it seems that a solid encircling element might restrict volumetric expansion of the globe, leading to greater rigidity in the buckled eye. However, previous investigations using Schi0tz and applanation to nometry to measure ocular rigidity suggest that a reduc tion in rigidity follows scleral buckling procedures. 15 - 17 The mechanism responsible for this change remains un clear. The purpose of this study is to investigate the change in the ocular pressure-volume relationship that accom panies scleral buckling and to further elucidate the caus ative mechanisms involved.
MATERIALS AND METHODS The authors studied ten enucleated human eyes from five donors ranging in age from 69 to 94 years. All eyes were studied within 96 hours of enucleation. Eight eyes were phakic, one was aphakic, and one was pseudophakic with an iris-supported intraocular lens (IOL). The exper imental apparatus is shown schematically in Figure 1. Each eye was warmed to room temperature and supported on moist cotton in a conical container. Throughout the experiment, the eye was kept moist by intermittent irri gation with normal saline solution. A 21-gauge needle was passed through the peripheral cornea into the central anterior chamber and suspended so as not to impinge on any .anterior segment structures. The needle was con nected by polyethylene tubing (Intramedic 7519 [Miami,
JOHNSON et al
•
SCLERAL BUCKLING AND OCULAR RIGIDITY
Pressure transducer
• • o • •
0
•
o
a
I
•
I
Infusion pump
Fig l. Schematic illustration of experimental apparatus used to generate pressure-volume curves.
FL]; internal diameter, 0.86 mm) to a Statham pressure transducer (Model P23AC, Hato Rey, PR). Intraocular pressures measured by the transducer were recorded by a Grass polygraph recorder (Model 790, Grass Instrument Company, Quincy, MA). A second 21-gauge needle, connected to a 5-ml syringe mounted on a continuous infusion pump (Model 975, Harvard Apparatus, Southnatick, MA), was introduced into the midvitreous cavity through the pars plana. The precision of the pump, determined by electronically weighing the volume delivered in each often consecutive test runs, was found to be within ±0.7%. The accuracy varied within 3.4% of the manufacturer's stated calibra tion. Before each experimental run, the pressure trans ducer-polygraph combination was calibrated from 0 to 100 mmHg using a mercury manometer positioned at the same horizontal level as the globe. After ensuring the absence of air in the needles and tubing, normal saline solution was infused continuously into the vitreous cavity of each of the ten eyes at a rate of 1.5 mlfminute. Intraocular pressure was recorded con tinuously on a strip chart throughout the infusion, which started at an lOP of 0 mmHg and was terminated when the lOP reached 80 mmHg. A pressure of 15 mmHg was used as a starting point for calculations of volume injected. Three similar runs were carried out on each eye and each data point for an eye represents the mean of the values from the three runs. Absence ofsignificant leakage or out flow was ensured by noting that the slope of the lOP decay curve recorded immediately after the termination of each
infusion was insignificant compared with the slope of the pressure-volume curve during the infusion. Each eye was then encircled with a solid silicone no. 276 tire and a no. 240 band (Mira, Inc, Waltham, MA) connected with a Watske sleeve. Six evenly spaced 5-0 Dacron mattress sutures, each with a 9.5-mm bed, were placed and tightened sufficiently to produce scleral im brication along the sides of the encircling tire. After the application of buckling elements, three continuous saline solution infusion runs were performed on each eye in the manner previously described. In each of five eyes, the buckling elements were re moved and one additional continuous saline solution in fusion was performed for comparison with the pressure volume curves obtained before buckling. Then, starting at an lOP of 5 mmHg, air was injected through a 30 gauge needle into the vitreous cavity in 0.1-ml increments until the lOP exceeded 80 mmHg. In the remaining five eyes, the buckle was left in place and air was injected in the same fashion . Linear regressions were generated for each pressure volume curve plotted for each eye, and statistical com parisons were made between mean regression coefficients using paired and unpaired t tests. An lnstron universal testing instrument (Model 1000 [Canton, MA]) was used to investigate the elasticity (re sistance to elongation) of silicone and sclera. Strips of sclera 2.5 mm wide were cut circumferentially from the equatorial region oftwo enucleated eyes (of a 72-year-old woman 24 hours after death). The strips were soaked in saline solution at room temperature. Force-elongation curves were then generated on four scleral strips and on a no. 240 silicone band (2.5 mm wide), each with an ef fective length of 23 mm. Each strip was subjected to con tinuous elongation at a rate of 20 mm/minute, and the tension generated was plotted as a function of the change in length. Results were expressed as the slope of the force elongation curve, or the ratio of tension divided by elon gation (g/mm).
RESULTS The mean lOP response of ten enucleated eyes to a continuous infusion of saline solution is shown in Figure 2. The relatively small variation between individual eyes is demonstrated by the narrow width of the standard error interval; the maximum standard error at 60 mmHg was 3.2 mmHg. A plot of the logarithm ofiOP against injected volume (Fig 3) is nonlinear, showing a gradual decrease in slope with increasing pressure. Figure 4 compares the average pressure-volume rela tionship during saline solution infusion before and after the application of encircling elements. Placement of the silicone buckling materials produced a marked flattening of the slope of the pressure-volume curve. Using linear regression to estimate slopes, the mean of the slopes of the pressure-volume curves for nonbuckled eyes was 0.78, compared with 0.17 after the application of encircling elements (mean difference ± standard deviation: 0.61 191
OPHTHALMOLOGY
•
FEBRUARY 1990
70
Ci J:
E
.§.
...
30
u 0
20
.5
10
Ul Ul Cll Q.
...
Ill
:;
...
Ill
NUMBER 2
100
E
... Cll
:I
0
Ul Ul
!!! Q.
...
Ill
:; u
0
I!
.5
Cl
....0 0
100
200
10
300
Fig 2. Pressure response to a continuous infusion of saline solution ( 1.5 mljminute). Each point represents mean value for ten non buckled eyes. Vertical bars indicate ± 1 standard error.
± 0.16, P < 0.0001, paired t test) (Table 1). That is, buck led eyes on average tolerated approximately four times the volume infusion as nonbuckled eyes before reaching a given lOP. In four eyes, pressure-volume curves were remeasured with saline solution infusion after removing the buckling elements. No statistically significant change in ocular ri gidity compared with pressure-volume data obtained be fore buckling (Fig 5 and Table 1) was found. The ocular pressure-volume relationship observed during intravitreal injection of air in 0.1-ml increments is illustrated in Figure 6. A marked reduction in ocular rigidity was induced by the application of encircling ele ments (P = 0.004, unpaired t test), the magnitude of which was similar to that observed during saline solution infusion (Table 1). As plotted by the Instron extensometer, the force-elon gation relationship of the silicone band was linear over the range of tension from 0 to 350 g. The curve for a scleral strip could be closely approximated by a straight line over the same tension range. Given this approxi mation, the average tension-elongation ratio (slope ofthe tension-elongation curve) for a scleral strip was 208 ±54 g/mm (mean± standard deviation, n = 4) compared with 5.5 g/mm (n = 1) for an equal length of silicone band. This indicates an extensibility of silicone approximately 40 times greater than that of an average scleral strip.
DISCUSSION The pressure-volume curves obtained during contin uous saline solution infusion into nonbuckled enucleated human eyes in our study (Fig 2) agree closely with those obtained similarly by Eisenlohr and co-workers. 11 Like other investigators, 11 •14• 18- 20 we found that the coefficient of ocular rigidity as defined by Friedenwald9 (slope of the
0
100
200
Injected volume
Injected volume (J.LI)
192
•
.§.
50 40
Cll
-a
VOLUME 97
J:
60
... :I
•
300
(~I)
Fig 3. Same data as in Figure 2, with lOP plotted on a logarithm scale.
Ci
J:
E
.§.
...CD
:I
Ul Ul CD Q.
... ...
Ill
:;
u
... 0
Ill
.5
30 20 10 0
0
100
200
300
Injected volume (~I)
Fig 4. Average pressure-volume relationship during saline solution in fusion in ten eyes before (filled squares) and after (open squares) appli cation of encircling elements. As estimated by linear regression, the dif ference between the slopes (mean regression coefficients) of the two curves is significant (P < 0.0001). Regression lines are not shown because the smoothly curved lines through mean data points are most representative of the curves achieved in individual eyes.
line generated by plotting logarithm of pressure against change in volume as in Figure 3) decreases with increasing ocular pressure over the pressure range studied. In our study eyes, scleral buckling with encircling sili cone elements produced a marked decrease in ocular ri gidity (LlP/ Ll V) (Figs 4 and 6). Buckled eyes could accom modate, on average, four times as great a volume change as nonbuckled eyes before reaching lOPs that would be considered dangerously high in a living eye. The magni tude of the effect was similar for saline solution infusion and air injection. Of particular interest is the observation that the marked lowering of ocular rigidity that accom panied scleral buckling was reversible on removing the
JOHNSON et al
•
SCLERAL BUCKLING AND OCULAR RIGIDITY
120
Table 1. Estimated Average Slopes of Pressure-volume Curves Mean Mean No. Slope± SO Difference ± SO Saline solution injection Non buckled 10 0.78 0.17 10 Buckled Saline solution injection Prebuckle 4 0.80 Postbuckle removal 4 0.81 Air injection Nonbuckled 5 0.47 ± 0.14 Buckled 5 0.12 ± 0.046
p
Abstract: In a study of enucleated human eyes, the authors investigated the effect of scleral buckling on the ocular pressure-volume relationship. Intraocular pressure was recorded continuously during intravitreal infusion of saline solution before and after the application of encircling silicone elements. Scleral buckling produced a marked reduction in ocular rigidity, with reversibility of the effect on removal of the buckling elements. Similar results were obtained during incre mental intravitreal air injection. The authors propose that the greater extensibility of silicone compared with sclera and the induced alterations in ocular shape are the primary factors responsible for the observed change in ocular rigidity. The clinical implications of these findings for intravitreal gas injection are dis cussed. Ophthalmology 1990; 97:190-195
During the last 15 years, the value of intravitreal gas in treating retinal detachment has been firmly estab lished. 1- 7 A recent survey of vitreoretinal surgeons indi cated both widespread and frequent use of intravitreal injections ofair and long-acting gases in conjunction with scleral buckling and vitrectomy procedures. 8 Most indi cations for intravitreal gas require the injection of as large a volume as possible, with intraocular pressure {lOP) being a primary limiting factor. The change in lOP that results from a given change in intraocular volume is determined by the ocular pressure volume relationship, known as ocular rigidity (API AV). Although numerous investigators have attempted to de termine empirically and theoretically the mathematical expression that best describes the pressure-volume be havior of the eye, the results have been contradictory and the precise characterization remains incomplete. 9- 14 Con ceptually, ocular rigidity refers to the resistance offered by ocular tissues to the expansion of intraocular volume 9 ; Originally received: March 17, 1989. Revision accepted: September 25, 1989. 1
W.K. Kellogg Eye Center, Department of Ophthalmology, University of Michigan, Ann Arbor. 2 Department of Physiology, University of Michigan, Ann Arbor. Dr. Johnson is currently at Bascom Palmer Eye Institute, Miami. Dr. Han is currently with the Department of Ophthalmology, Medical College of Wisconsin, Milwaukee. Presented at the Vitreous Society Annual Meeting, Scottsdale, Arizona, December 1988. Reprint requests to Dennis P. Han, MD, Eye Institute, 8700 W. Wisconsin Ave, Milwaukee, WI 53226.
190
the greater the rigidity, the greater the pressure rise in response to a given volume change. Intuitively, therefore, it seems that a solid encircling element might restrict volumetric expansion of the globe, leading to greater rigidity in the buckled eye. However, previous investigations using Schi0tz and applanation to nometry to measure ocular rigidity suggest that a reduc tion in rigidity follows scleral buckling procedures. 15 - 17 The mechanism responsible for this change remains un clear. The purpose of this study is to investigate the change in the ocular pressure-volume relationship that accom panies scleral buckling and to further elucidate the caus ative mechanisms involved.
MATERIALS AND METHODS The authors studied ten enucleated human eyes from five donors ranging in age from 69 to 94 years. All eyes were studied within 96 hours of enucleation. Eight eyes were phakic, one was aphakic, and one was pseudophakic with an iris-supported intraocular lens (IOL). The exper imental apparatus is shown schematically in Figure 1. Each eye was warmed to room temperature and supported on moist cotton in a conical container. Throughout the experiment, the eye was kept moist by intermittent irri gation with normal saline solution. A 21-gauge needle was passed through the peripheral cornea into the central anterior chamber and suspended so as not to impinge on any .anterior segment structures. The needle was con nected by polyethylene tubing (Intramedic 7519 [Miami,
JOHNSON et al
•
SCLERAL BUCKLING AND OCULAR RIGIDITY
Pressure transducer
• • o • •
0
•
o
a
I
•
I
Infusion pump
Fig l. Schematic illustration of experimental apparatus used to generate pressure-volume curves.
FL]; internal diameter, 0.86 mm) to a Statham pressure transducer (Model P23AC, Hato Rey, PR). Intraocular pressures measured by the transducer were recorded by a Grass polygraph recorder (Model 790, Grass Instrument Company, Quincy, MA). A second 21-gauge needle, connected to a 5-ml syringe mounted on a continuous infusion pump (Model 975, Harvard Apparatus, Southnatick, MA), was introduced into the midvitreous cavity through the pars plana. The precision of the pump, determined by electronically weighing the volume delivered in each often consecutive test runs, was found to be within ±0.7%. The accuracy varied within 3.4% of the manufacturer's stated calibra tion. Before each experimental run, the pressure trans ducer-polygraph combination was calibrated from 0 to 100 mmHg using a mercury manometer positioned at the same horizontal level as the globe. After ensuring the absence of air in the needles and tubing, normal saline solution was infused continuously into the vitreous cavity of each of the ten eyes at a rate of 1.5 mlfminute. Intraocular pressure was recorded con tinuously on a strip chart throughout the infusion, which started at an lOP of 0 mmHg and was terminated when the lOP reached 80 mmHg. A pressure of 15 mmHg was used as a starting point for calculations of volume injected. Three similar runs were carried out on each eye and each data point for an eye represents the mean of the values from the three runs. Absence ofsignificant leakage or out flow was ensured by noting that the slope of the lOP decay curve recorded immediately after the termination of each
infusion was insignificant compared with the slope of the pressure-volume curve during the infusion. Each eye was then encircled with a solid silicone no. 276 tire and a no. 240 band (Mira, Inc, Waltham, MA) connected with a Watske sleeve. Six evenly spaced 5-0 Dacron mattress sutures, each with a 9.5-mm bed, were placed and tightened sufficiently to produce scleral im brication along the sides of the encircling tire. After the application of buckling elements, three continuous saline solution infusion runs were performed on each eye in the manner previously described. In each of five eyes, the buckling elements were re moved and one additional continuous saline solution in fusion was performed for comparison with the pressure volume curves obtained before buckling. Then, starting at an lOP of 5 mmHg, air was injected through a 30 gauge needle into the vitreous cavity in 0.1-ml increments until the lOP exceeded 80 mmHg. In the remaining five eyes, the buckle was left in place and air was injected in the same fashion . Linear regressions were generated for each pressure volume curve plotted for each eye, and statistical com parisons were made between mean regression coefficients using paired and unpaired t tests. An lnstron universal testing instrument (Model 1000 [Canton, MA]) was used to investigate the elasticity (re sistance to elongation) of silicone and sclera. Strips of sclera 2.5 mm wide were cut circumferentially from the equatorial region oftwo enucleated eyes (of a 72-year-old woman 24 hours after death). The strips were soaked in saline solution at room temperature. Force-elongation curves were then generated on four scleral strips and on a no. 240 silicone band (2.5 mm wide), each with an ef fective length of 23 mm. Each strip was subjected to con tinuous elongation at a rate of 20 mm/minute, and the tension generated was plotted as a function of the change in length. Results were expressed as the slope of the force elongation curve, or the ratio of tension divided by elon gation (g/mm).
RESULTS The mean lOP response of ten enucleated eyes to a continuous infusion of saline solution is shown in Figure 2. The relatively small variation between individual eyes is demonstrated by the narrow width of the standard error interval; the maximum standard error at 60 mmHg was 3.2 mmHg. A plot of the logarithm ofiOP against injected volume (Fig 3) is nonlinear, showing a gradual decrease in slope with increasing pressure. Figure 4 compares the average pressure-volume rela tionship during saline solution infusion before and after the application of encircling elements. Placement of the silicone buckling materials produced a marked flattening of the slope of the pressure-volume curve. Using linear regression to estimate slopes, the mean of the slopes of the pressure-volume curves for nonbuckled eyes was 0.78, compared with 0.17 after the application of encircling elements (mean difference ± standard deviation: 0.61 191
OPHTHALMOLOGY
•
FEBRUARY 1990
70
Ci J:
E
.§.
...
30
u 0
20
.5
10
Ul Ul Cll Q.
...
Ill
:;
...
Ill
NUMBER 2
100
E
... Cll
:I
0
Ul Ul
!!! Q.
...
Ill
:; u
0
I!
.5
Cl
....0 0
100
200
10
300
Fig 2. Pressure response to a continuous infusion of saline solution ( 1.5 mljminute). Each point represents mean value for ten non buckled eyes. Vertical bars indicate ± 1 standard error.
± 0.16, P < 0.0001, paired t test) (Table 1). That is, buck led eyes on average tolerated approximately four times the volume infusion as nonbuckled eyes before reaching a given lOP. In four eyes, pressure-volume curves were remeasured with saline solution infusion after removing the buckling elements. No statistically significant change in ocular ri gidity compared with pressure-volume data obtained be fore buckling (Fig 5 and Table 1) was found. The ocular pressure-volume relationship observed during intravitreal injection of air in 0.1-ml increments is illustrated in Figure 6. A marked reduction in ocular rigidity was induced by the application of encircling ele ments (P = 0.004, unpaired t test), the magnitude of which was similar to that observed during saline solution infusion (Table 1). As plotted by the Instron extensometer, the force-elon gation relationship of the silicone band was linear over the range of tension from 0 to 350 g. The curve for a scleral strip could be closely approximated by a straight line over the same tension range. Given this approxi mation, the average tension-elongation ratio (slope ofthe tension-elongation curve) for a scleral strip was 208 ±54 g/mm (mean± standard deviation, n = 4) compared with 5.5 g/mm (n = 1) for an equal length of silicone band. This indicates an extensibility of silicone approximately 40 times greater than that of an average scleral strip.
DISCUSSION The pressure-volume curves obtained during contin uous saline solution infusion into nonbuckled enucleated human eyes in our study (Fig 2) agree closely with those obtained similarly by Eisenlohr and co-workers. 11 Like other investigators, 11 •14• 18- 20 we found that the coefficient of ocular rigidity as defined by Friedenwald9 (slope of the
0
100
200
Injected volume
Injected volume (J.LI)
192
•
.§.
50 40
Cll
-a
VOLUME 97
J:
60
... :I
•
300
(~I)
Fig 3. Same data as in Figure 2, with lOP plotted on a logarithm scale.
Ci
J:
E
.§.
...CD
:I
Ul Ul CD Q.
... ...
Ill
:;
u
... 0
Ill
.5
30 20 10 0
0
100
200
300
Injected volume (~I)
Fig 4. Average pressure-volume relationship during saline solution in fusion in ten eyes before (filled squares) and after (open squares) appli cation of encircling elements. As estimated by linear regression, the dif ference between the slopes (mean regression coefficients) of the two curves is significant (P < 0.0001). Regression lines are not shown because the smoothly curved lines through mean data points are most representative of the curves achieved in individual eyes.
line generated by plotting logarithm of pressure against change in volume as in Figure 3) decreases with increasing ocular pressure over the pressure range studied. In our study eyes, scleral buckling with encircling sili cone elements produced a marked decrease in ocular ri gidity (LlP/ Ll V) (Figs 4 and 6). Buckled eyes could accom modate, on average, four times as great a volume change as nonbuckled eyes before reaching lOPs that would be considered dangerously high in a living eye. The magni tude of the effect was similar for saline solution infusion and air injection. Of particular interest is the observation that the marked lowering of ocular rigidity that accom panied scleral buckling was reversible on removing the
JOHNSON et al
•
SCLERAL BUCKLING AND OCULAR RIGIDITY
120
Table 1. Estimated Average Slopes of Pressure-volume Curves Mean Mean No. Slope± SO Difference ± SO Saline solution injection Non buckled 10 0.78 0.17 10 Buckled Saline solution injection Prebuckle 4 0.80 Postbuckle removal 4 0.81 Air injection Nonbuckled 5 0.47 ± 0.14 Buckled 5 0.12 ± 0.046
p
