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An Experimental Investigation of the Basic Phenomena of Retinopexy*
CHYMOTRYPSIN IN STRABISMUS SURGERY
113
REFERENCES
1. Miechowski, W. L., and Ercoli, N.: Studies on proteolytic enzymes: II. Trypsin and chymotrypsin in relation to inflammatory processes. Jour. Pharm. & Exper. Therapeutics, 116:43, 1956. 2. Hendley, C. D., et al: Studies on proteolytic enzymes: I. Toxicology of crystalline trypsin and chymotrypsin. Arch. Intern. Pharmacodynamie, 106:164,1956. 3. Suarez-Diaz, E. : Quimar in the treatment of asthma. Sinopsis Med. Internat, March, 1958, pp. 20-24. 4. Jenkins, B. H. : The use of intramuscular chymotrypsin in ocular conditions. J. M. A. Georgia, Octo ber 1956, pp. 431-433. 5. Parsons, D. J.: Asthma, bronchitis, rhinitis and sinusitis; Adjunctive treatment with intramuscular chymotrypsin. Clinical Med., 5:1491-1494 (Nov.) 1958. 6. Muftic, M. K.: Proteolytic enzymes in otolaryngology. Indian J. M. S e , 11:1015-1020 (Dec.) 1957. A N E X P E R I M E N T A L INVESTIGATION O F T H E BASIC PHENOMENA O F RETINOPEXY* III.
R E S I S T A N C E A N D CAPACITIVE REACTANCE O F T H E CAT'S CORNEA, SCLERA, CHOROID, AND VITREOUS H E N R Y A. K N O L L ^ P H . D .
Rochester, New York INTRODUCTION
Impedance values for ocular tissues were reported in an earlier paper in this series.1 It was shown that the impedance of all ocu lar tissues measured (sclera, choroid, and vitreous) varied inversely with frequency. An attempt was made to construct an elec trical analog which would produce the same type of relationship. The data could not be duplicated using fixed-value components in the analog; hence, it was concluded that resistance and the capacitive reactance of these tissues may vary with frequency. That this is true in lysed erythrocytes has been shown by Schwan and Carstensen.2 The present study was undertaken to test this assumption. APPARATUS
A special head and electrode holder was designed and constructed (fig. 1). The cat's head was held firmly in position by a pair of * From the Departments of Surgery (Ophthal mology) and Biophysics, University of California Medical Center, Los Angeles 24, California. This investigation was supported by a research grant (B-471) from the National Institute of Neurologi cal Diseases and Blindness, National Institutes of Health, United States Public Health Service, Bethesda, Maryland. t Present address: Ophthalmic Research and De velopment Section, Bausch & Lomb Optical Com pany, Rochester 2, New York.
ear bars and a mouth rod. Two electrode carriers were mounted orthogonally on a semicircular track whose center of curvature could be made to coincide with the center of curvature of either eye of the animal. The electrodes were then always perpendiculuar to the sclera or cornea. Electrical contact was made in one of two ways: through a pair of electrodes mounted on the electrode carriers, or by us ing one electrode and a ground return through the ear bars. Three different types of electrodes were used. They are described in Table 1. Note that the relative areas of the numbered elec trodes are indicated by the identifying num ber. These electrodes were used in various combinations and at separations of 4.0, 8.0 and 12 mm. The electrical measurements were made using three different techniques: 1. Bridge method. A General Radio TwinT Impedance Bridge, as described by Sin clair3 was used to balance to null approxi mately one volt developed across the eye by means of a Hewlett Packard Signal Gen erator. The signal was applied at frequen cies of 0.5, 1.5, 3.0, and 10.0 megacycles. 2. Phase-angle method. In this method, the same generator provided either onehalf or one volt r.m.s. signal across the eye
HENRY A. KNOLL
114
experiments, an electrode holder which pro vided radial contact of the electrode on the globe of the cat's eye, was used. RESULTS INITIAL EXPERIMENT
Fig. 1 (Knoll). Cat shown in head holder. Elec trodes are applied to the cornea.
at frequencies in the 100 cps to 1.0 mc, range. From the measurements of the voltage de veloped across the eye, the voltage developed across, a resistor placed in series with the eye, and the phase angle between these two voltages measured with a CRT delay sweep circuit, resistance and reactance values were obtained by calculation. 3. An approximate resistance method. This method is identical to the phase angle method except that it dispenses with the phase angle measurement. It is essentially the method described in the earlier paper.1 If a known resistor of large value is placed in series with the eye, approximations to the resistance of the eye may be calculated. PROCEDURE
Twenty-two adult cats were used as the experimental animals and Nembutal ad ministered intraperitoneally was used as the anesthetic. Except for certain corneal meas urements, the temporal portion of the sclera was exposed by means of a large canthotomy. During the course of measurements, the eye was occasionally treated with pontocaine, washed with saline, and turned in its socket to counteract the effects of drying. A single experiment took approximately four hours. The cats were killed after com pletion of each experiment. For the first experiment, and those made utilizing the bridge method of measurement, no electrode holder was used. For all other
Utilizing the phase angle method of meas uring impedance and electrode No. B on the cat's sclera, reactance and resistance values were obtained at frequencies in the 100 cps to 1.0 mc. range. The ear bars served as a ground return; no electrode holder was used. The stiffness of the connecting cable served to hold the electrode against the sclera. A slight, and no doubt variable, pres sure was exerted on the eye in this manner. Reasonable visual care was taken to place the electrode in the same position between readings. Approximately one volt rms was applied across the eye, as measured between the electrode and the ear bars. The results are presented in Figure 2. TABLE 1 DESCRIPTION OF ELECTRODES
Number A B 1 3 6 10 20 40 60 80 100
Description Right-angled, pointed, steel electrode of type used in retinopexy Flat, circular, steel electrode, 2.38 mm. in diameter, of the type used in cyclodiathermy Flat, circular brass electrode, insulated except for a flat end with an area equaling 0.0507 mm.» Flat, circular brass electrode, insulated except for a flat end with an area equaling 0.1S21 mm.1 Flat, circular brass electrode, insulated except for a flat end with an area equaling 0.3042 mm.* Flat, circular brass electrode, insulated except for a flat end with an area equaling 0.507 mm.* Flat, circular brass electrode, insulated except for a flat end with an area equaling 1.014 mm.1 Flat, circular brass electrode, insulated except for a flat end with an area equaling 2.028 mm.* Flat, circular brass electrode, insulated except for a flat end with an area equaling 3.042 mm." Flat, circular brass electrode, insulated except for a flat end with an area equaling 4.056 mm.s Flat, circular brass electrode, insulated except for a flat end with an area equaling 5.07 mm.*
115
BASIC PHENOMENA O F RETINOPEXY
Straight lines were drawn through the -experimental points and they can be given by the following power functions:
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hence, it can be seen that if tissues act at any time as a true capacitor, the slope of the line will be equal to minus unity. The highest . slope obtained in the present work was 0.56 ; therefore, our initial assumption that the capacitance changes with frequency is borne out.
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COMPARISON OF FLAT AND POINTED ELEC TRODES
Using the Twin-T Impedance Bridge Method, the impedance of the flat electrode (No. B) was compared with the impedance of the pointed electrode (No. A) on the sclera. No electrode holder was used. The
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Such an equation of the form "y when drawn on a log-log plot turns out to be a straight line where "a" equals the in tercept when "x" equals unity, and "b" is the slope of the line. The values of "a" above are given in ohms and "i" is the frequency in cps. All of the data seemed tofitthis type of relationship quite well. The reactance of a fixed value capacitor is given as follows : 1 x ==
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Fig. 3 (Knoll). Comparison of the flat (No. B) and the pointed (No. A) electrodes. Measurements made using the Twin-T Impedance Bridge Method. Each point represents an average of 10 readings. The conflicting data shown for Experiment D may be explained by excessive drying of the sclera.
HENRY A. KNOLL
116
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Figs. 4 and 5 (Knoll). (Fig. 4) Electrode area effect shown using approximate method of measurement. (Fig. S) Electrode area effects on choroid and sclera.
ear bars were used as the ground electrodes ure 5 shows the results of choroid and sclera except during one experiment when the with an electrode spacing of 4.0 mm. The ground electrode was placed in the conjunc- slopes of the lines for the various tissues tival sac. The results are shown in Figure 3. seem to vary. The resistance and the capacitive reactance values are lower for the larger flat electrode. POSTDIATHERMY EFFECTS One experiment (D) yielded conflicting data, Using the No. 1 electrodes, 4.0 mm. apart, which may be explained by excessive drying the impedances were measured before and of the sclera. after treatment with a Walker diathermy Each point in Figure 3 represents an aver unit. The results are shown in Figure 6. age of 10 readings. The scatter of points Note particularly the increased slope of the was taken to indicate the need for better capacitive reactance curve following treat electrode contact control. Hence, at this ment. This could be explained by the destruc point the electrode holder was built, and tion of cell membranes. the remaining data were taken with a much VARIOUS TISSUE EFFECTS more uniform electrode pressure control. In one of the previous reports,1 the effect AREA EFFECTS of cellular density upon impedance seemed Using the approximate method, the effects to be a direct proportion. Results, using the of the electrode area were determined. The phase angle method shown in Figure 7, electrode holder was used to place the elec bear this out for both resistance and capaci trodes on the eye at separations of 8.0 mm. tive reactance. and 4.0 mm. In Figure 4 are shown the re DISCUSSION sults as found on the corneas with an elec trode spacing of 8.0 mm. The slopes of the The amount of power absorbed in an elec lines are essentially the same, resistance be tric circuit is inversely proportional to the ing inversely proportional to the area. Fig resistance in the circuit, provided the volt-
117
BASIC PHENOMENA OF RETINOPEXY
age is kept constant. The data presented here would indicate that for a series of os cillators having the same voltage with dif ferent frequencies, the amount of heat pro duced in the tissues would be inversely pro portional to the frequency. At a given fre quency the amount of heat absorbed would be greatest in the vitreous, less in the choroid, and least in the cornea and sclera (fig. 7). It can be seen that in surface dia thermy it is fortunate that the flux density is higher in the outer coats of the eye and least in the choroid and vitreous. This leads to more intense heating in the outer layer of the eyes with relatively less heat dis sipated in the choroid and vitreous. These observations lead to the further conclusion that one must be very careful when punctur ing the coats of the eyeball, since the pro cedure leads to higher flux densities intern ally, which will lead to intense heating of the choroid, retina, and vitreous. A most difficult problem, which eludes solution at this time, is the measurement of the electrical parameters during the time of treatment. Observations during the vari ous experiments lead us to believe that the impedances drop during the treatment and may serve to heat all tissues to a much greater extent than would be indicated by the measurements recorded here.
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HENRY A. KNOLL
118
Enough data are now available to enable at least close approximations to the amount of heat absorbed by ocular tissues when treated by means of tube-generated currents. It is to be hoped that equipment will be made available using this type of generation, to replace the spark gap equipment now being so widely used.
2. The effect of electrode separation seems to be minor in relation to other fac tors such as electrode size, pressure, and dryness of the tissues. 3. The impedance is inversely propor tional to the area of the electrode. 4. The impedance varies inversely with the cellular density of the tissue.
SUMMARY
1. Using two methods of measuring the components of ocular tissue impedance, it has been established that both the resistance and capacitive reactance vary inversely with frequency.
ACKNOWLEDGM ENTS
I wish to acknowledge the skillful assistance of Mr. Harvey Kirk and Mr. Stanley Polsky who assisted ably in carrying out the experiments. With out their assistance the performance of the experi ments would not have been possible.
REFERENCES
1. Knoll, H. A.: An experimental investigation of the basic phenomena of retinopexy: I. Electrical impedance measurements. Am. J. Ophth., 45:546,1958. 2. Schwan, H. P., and Carstensen, E. L.: Dielectric properties of the membrane of lysed erythrocytes. Science, 125:985,1957. 3. Sinclair, D. B.: The Twin-T: A new type of null instrument for measuring impedance at fre quencies up to 30 megacycles. Proc. Inst. Radio Engineers, 28:310,1940.
THE USE OF ABSORBABLE SUTURES IN SURGERY OF THE CORNEA* SAMUEL D.
MCPHERSON,
JR.,
M.D.
Durham, North Carolina
The choice of the most satisfactory suture material for corneal surgery is a controver sial subject. The routine use of nonabsorbable sutures has been generally accepted, and the use of absorbable sutures has received * From the McPherson Hospital, Durham, North Carolina and the Division of Ophthalmology, Uni versity of North Carolina School of Medicine, Chapel Hill, North Carolina. The operations were performed with the assistance of the resident staffs of the McPherson Hospital and North Carolina Memorial Hospital. The experimental work was done in the Laboratory of Experimental Ocular Pathology of the University of North Carolina with the assistance of J. W. Draheim, M.D., and G. T, Kiffney, Jr., M.D., Histologie sections were prepared by Mrs. Vivian Sparks. The experimental studies were aided by a grant from the North Caro lina Association for the Blind, and the materials in the experimental portion of this paper were fur nished by the Ethicon Suture Company, Somerville, New Jersey. Candidate's thesis submitted for mem bership and published in the Tr. Am. Ophth. Soc, 1959, v. 57.
little attention. Absorbable suture materials offer a definite advantage in the closure of clear corneal wounds because they do not have to be removed. The removal of corneal sutures is frequently a hazardous procedure and may result in loss of the anterior cham ber, wound separation or disruption, or oc casionally dislocation of a corneal graft. In children the removal of nonabsorbable su tures usually necessitates a second general anesthetic. Most of these difficulties may be obviated by the use of sutures which are absorbable and do not require removal. In spite of the common use of catgut su tures in general surgery and in certain types of ocular surgery, the use of similar sutures for corneal surgery has not been widely ad vocated. The general impression seems to be that the use of absorbable sutures for the closure of corneal wounds may produce un-
113
REFERENCES
1. Miechowski, W. L., and Ercoli, N.: Studies on proteolytic enzymes: II. Trypsin and chymotrypsin in relation to inflammatory processes. Jour. Pharm. & Exper. Therapeutics, 116:43, 1956. 2. Hendley, C. D., et al: Studies on proteolytic enzymes: I. Toxicology of crystalline trypsin and chymotrypsin. Arch. Intern. Pharmacodynamie, 106:164,1956. 3. Suarez-Diaz, E. : Quimar in the treatment of asthma. Sinopsis Med. Internat, March, 1958, pp. 20-24. 4. Jenkins, B. H. : The use of intramuscular chymotrypsin in ocular conditions. J. M. A. Georgia, Octo ber 1956, pp. 431-433. 5. Parsons, D. J.: Asthma, bronchitis, rhinitis and sinusitis; Adjunctive treatment with intramuscular chymotrypsin. Clinical Med., 5:1491-1494 (Nov.) 1958. 6. Muftic, M. K.: Proteolytic enzymes in otolaryngology. Indian J. M. S e , 11:1015-1020 (Dec.) 1957. A N E X P E R I M E N T A L INVESTIGATION O F T H E BASIC PHENOMENA O F RETINOPEXY* III.
R E S I S T A N C E A N D CAPACITIVE REACTANCE O F T H E CAT'S CORNEA, SCLERA, CHOROID, AND VITREOUS H E N R Y A. K N O L L ^ P H . D .
Rochester, New York INTRODUCTION
Impedance values for ocular tissues were reported in an earlier paper in this series.1 It was shown that the impedance of all ocu lar tissues measured (sclera, choroid, and vitreous) varied inversely with frequency. An attempt was made to construct an elec trical analog which would produce the same type of relationship. The data could not be duplicated using fixed-value components in the analog; hence, it was concluded that resistance and the capacitive reactance of these tissues may vary with frequency. That this is true in lysed erythrocytes has been shown by Schwan and Carstensen.2 The present study was undertaken to test this assumption. APPARATUS
A special head and electrode holder was designed and constructed (fig. 1). The cat's head was held firmly in position by a pair of * From the Departments of Surgery (Ophthal mology) and Biophysics, University of California Medical Center, Los Angeles 24, California. This investigation was supported by a research grant (B-471) from the National Institute of Neurologi cal Diseases and Blindness, National Institutes of Health, United States Public Health Service, Bethesda, Maryland. t Present address: Ophthalmic Research and De velopment Section, Bausch & Lomb Optical Com pany, Rochester 2, New York.
ear bars and a mouth rod. Two electrode carriers were mounted orthogonally on a semicircular track whose center of curvature could be made to coincide with the center of curvature of either eye of the animal. The electrodes were then always perpendiculuar to the sclera or cornea. Electrical contact was made in one of two ways: through a pair of electrodes mounted on the electrode carriers, or by us ing one electrode and a ground return through the ear bars. Three different types of electrodes were used. They are described in Table 1. Note that the relative areas of the numbered elec trodes are indicated by the identifying num ber. These electrodes were used in various combinations and at separations of 4.0, 8.0 and 12 mm. The electrical measurements were made using three different techniques: 1. Bridge method. A General Radio TwinT Impedance Bridge, as described by Sin clair3 was used to balance to null approxi mately one volt developed across the eye by means of a Hewlett Packard Signal Gen erator. The signal was applied at frequen cies of 0.5, 1.5, 3.0, and 10.0 megacycles. 2. Phase-angle method. In this method, the same generator provided either onehalf or one volt r.m.s. signal across the eye
HENRY A. KNOLL
114
experiments, an electrode holder which pro vided radial contact of the electrode on the globe of the cat's eye, was used. RESULTS INITIAL EXPERIMENT
Fig. 1 (Knoll). Cat shown in head holder. Elec trodes are applied to the cornea.
at frequencies in the 100 cps to 1.0 mc, range. From the measurements of the voltage de veloped across the eye, the voltage developed across, a resistor placed in series with the eye, and the phase angle between these two voltages measured with a CRT delay sweep circuit, resistance and reactance values were obtained by calculation. 3. An approximate resistance method. This method is identical to the phase angle method except that it dispenses with the phase angle measurement. It is essentially the method described in the earlier paper.1 If a known resistor of large value is placed in series with the eye, approximations to the resistance of the eye may be calculated. PROCEDURE
Twenty-two adult cats were used as the experimental animals and Nembutal ad ministered intraperitoneally was used as the anesthetic. Except for certain corneal meas urements, the temporal portion of the sclera was exposed by means of a large canthotomy. During the course of measurements, the eye was occasionally treated with pontocaine, washed with saline, and turned in its socket to counteract the effects of drying. A single experiment took approximately four hours. The cats were killed after com pletion of each experiment. For the first experiment, and those made utilizing the bridge method of measurement, no electrode holder was used. For all other
Utilizing the phase angle method of meas uring impedance and electrode No. B on the cat's sclera, reactance and resistance values were obtained at frequencies in the 100 cps to 1.0 mc. range. The ear bars served as a ground return; no electrode holder was used. The stiffness of the connecting cable served to hold the electrode against the sclera. A slight, and no doubt variable, pres sure was exerted on the eye in this manner. Reasonable visual care was taken to place the electrode in the same position between readings. Approximately one volt rms was applied across the eye, as measured between the electrode and the ear bars. The results are presented in Figure 2. TABLE 1 DESCRIPTION OF ELECTRODES
Number A B 1 3 6 10 20 40 60 80 100
Description Right-angled, pointed, steel electrode of type used in retinopexy Flat, circular, steel electrode, 2.38 mm. in diameter, of the type used in cyclodiathermy Flat, circular brass electrode, insulated except for a flat end with an area equaling 0.0507 mm.» Flat, circular brass electrode, insulated except for a flat end with an area equaling 0.1S21 mm.1 Flat, circular brass electrode, insulated except for a flat end with an area equaling 0.3042 mm.* Flat, circular brass electrode, insulated except for a flat end with an area equaling 0.507 mm.* Flat, circular brass electrode, insulated except for a flat end with an area equaling 1.014 mm.1 Flat, circular brass electrode, insulated except for a flat end with an area equaling 2.028 mm.* Flat, circular brass electrode, insulated except for a flat end with an area equaling 3.042 mm." Flat, circular brass electrode, insulated except for a flat end with an area equaling 4.056 mm.s Flat, circular brass electrode, insulated except for a flat end with an area equaling 5.07 mm.*
115
BASIC PHENOMENA O F RETINOPEXY
Straight lines were drawn through the -experimental points and they can be given by the following power functions:
— _ -
27rfc 1
f-
2irc
hence, it can be seen that if tissues act at any time as a true capacitor, the slope of the line will be equal to minus unity. The highest . slope obtained in the present work was 0.56 ; therefore, our initial assumption that the capacitance changes with frequency is borne out.
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Fig. 2 (Knoll). Resistance and capacitive react ance of the sclera as a function of frequency, measured by the phase angle method. Electrode No. B was held against the sclera by a clamp (electrode holder not used). Ground obtained through ear bars. The lines represent the relation ships expressed as a power function.
COMPARISON OF FLAT AND POINTED ELEC TRODES
Using the Twin-T Impedance Bridge Method, the impedance of the flat electrode (No. B) was compared with the impedance of the pointed electrode (No. A) on the sclera. No electrode holder was used. The
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Fig. 3 (Knoll). Comparison of the flat (No. B) and the pointed (No. A) electrodes. Measurements made using the Twin-T Impedance Bridge Method. Each point represents an average of 10 readings. The conflicting data shown for Experiment D may be explained by excessive drying of the sclera.
HENRY A. KNOLL
116
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FREQUENCY
Figs. 4 and 5 (Knoll). (Fig. 4) Electrode area effect shown using approximate method of measurement. (Fig. S) Electrode area effects on choroid and sclera.
ear bars were used as the ground electrodes ure 5 shows the results of choroid and sclera except during one experiment when the with an electrode spacing of 4.0 mm. The ground electrode was placed in the conjunc- slopes of the lines for the various tissues tival sac. The results are shown in Figure 3. seem to vary. The resistance and the capacitive reactance values are lower for the larger flat electrode. POSTDIATHERMY EFFECTS One experiment (D) yielded conflicting data, Using the No. 1 electrodes, 4.0 mm. apart, which may be explained by excessive drying the impedances were measured before and of the sclera. after treatment with a Walker diathermy Each point in Figure 3 represents an aver unit. The results are shown in Figure 6. age of 10 readings. The scatter of points Note particularly the increased slope of the was taken to indicate the need for better capacitive reactance curve following treat electrode contact control. Hence, at this ment. This could be explained by the destruc point the electrode holder was built, and tion of cell membranes. the remaining data were taken with a much VARIOUS TISSUE EFFECTS more uniform electrode pressure control. In one of the previous reports,1 the effect AREA EFFECTS of cellular density upon impedance seemed Using the approximate method, the effects to be a direct proportion. Results, using the of the electrode area were determined. The phase angle method shown in Figure 7, electrode holder was used to place the elec bear this out for both resistance and capaci trodes on the eye at separations of 8.0 mm. tive reactance. and 4.0 mm. In Figure 4 are shown the re DISCUSSION sults as found on the corneas with an elec trode spacing of 8.0 mm. The slopes of the The amount of power absorbed in an elec lines are essentially the same, resistance be tric circuit is inversely proportional to the ing inversely proportional to the area. Fig resistance in the circuit, provided the volt-
117
BASIC PHENOMENA OF RETINOPEXY
age is kept constant. The data presented here would indicate that for a series of os cillators having the same voltage with dif ferent frequencies, the amount of heat pro duced in the tissues would be inversely pro portional to the frequency. At a given fre quency the amount of heat absorbed would be greatest in the vitreous, less in the choroid, and least in the cornea and sclera (fig. 7). It can be seen that in surface dia thermy it is fortunate that the flux density is higher in the outer coats of the eye and least in the choroid and vitreous. This leads to more intense heating in the outer layer of the eyes with relatively less heat dis sipated in the choroid and vitreous. These observations lead to the further conclusion that one must be very careful when punctur ing the coats of the eyeball, since the pro cedure leads to higher flux densities intern ally, which will lead to intense heating of the choroid, retina, and vitreous. A most difficult problem, which eludes solution at this time, is the measurement of the electrical parameters during the time of treatment. Observations during the vari ous experiments lead us to believe that the impedances drop during the treatment and may serve to heat all tissues to a much greater extent than would be indicated by the measurements recorded here.
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FREQUENCY
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FREQUENCY Experiment: V Sclera Electrode # 1 #4cm,
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4 cm. otter
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100 Kc
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FREQUENCY
Fig. 6 (Knoll). Postdiathermy effects. Phase-angle method.
Enperimentt; S,T Electrode #100 Dittane·:.4cm
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FREQUENCY
Fig. 7 (Knoll). The effects on various tissues.
HENRY A. KNOLL
118
Enough data are now available to enable at least close approximations to the amount of heat absorbed by ocular tissues when treated by means of tube-generated currents. It is to be hoped that equipment will be made available using this type of generation, to replace the spark gap equipment now being so widely used.
2. The effect of electrode separation seems to be minor in relation to other fac tors such as electrode size, pressure, and dryness of the tissues. 3. The impedance is inversely propor tional to the area of the electrode. 4. The impedance varies inversely with the cellular density of the tissue.
SUMMARY
1. Using two methods of measuring the components of ocular tissue impedance, it has been established that both the resistance and capacitive reactance vary inversely with frequency.
ACKNOWLEDGM ENTS
I wish to acknowledge the skillful assistance of Mr. Harvey Kirk and Mr. Stanley Polsky who assisted ably in carrying out the experiments. With out their assistance the performance of the experi ments would not have been possible.
REFERENCES
1. Knoll, H. A.: An experimental investigation of the basic phenomena of retinopexy: I. Electrical impedance measurements. Am. J. Ophth., 45:546,1958. 2. Schwan, H. P., and Carstensen, E. L.: Dielectric properties of the membrane of lysed erythrocytes. Science, 125:985,1957. 3. Sinclair, D. B.: The Twin-T: A new type of null instrument for measuring impedance at fre quencies up to 30 megacycles. Proc. Inst. Radio Engineers, 28:310,1940.
THE USE OF ABSORBABLE SUTURES IN SURGERY OF THE CORNEA* SAMUEL D.
MCPHERSON,
JR.,
M.D.
Durham, North Carolina
The choice of the most satisfactory suture material for corneal surgery is a controver sial subject. The routine use of nonabsorbable sutures has been generally accepted, and the use of absorbable sutures has received * From the McPherson Hospital, Durham, North Carolina and the Division of Ophthalmology, Uni versity of North Carolina School of Medicine, Chapel Hill, North Carolina. The operations were performed with the assistance of the resident staffs of the McPherson Hospital and North Carolina Memorial Hospital. The experimental work was done in the Laboratory of Experimental Ocular Pathology of the University of North Carolina with the assistance of J. W. Draheim, M.D., and G. T, Kiffney, Jr., M.D., Histologie sections were prepared by Mrs. Vivian Sparks. The experimental studies were aided by a grant from the North Caro lina Association for the Blind, and the materials in the experimental portion of this paper were fur nished by the Ethicon Suture Company, Somerville, New Jersey. Candidate's thesis submitted for mem bership and published in the Tr. Am. Ophth. Soc, 1959, v. 57.
little attention. Absorbable suture materials offer a definite advantage in the closure of clear corneal wounds because they do not have to be removed. The removal of corneal sutures is frequently a hazardous procedure and may result in loss of the anterior cham ber, wound separation or disruption, or oc casionally dislocation of a corneal graft. In children the removal of nonabsorbable su tures usually necessitates a second general anesthetic. Most of these difficulties may be obviated by the use of sutures which are absorbable and do not require removal. In spite of the common use of catgut su tures in general surgery and in certain types of ocular surgery, the use of similar sutures for corneal surgery has not been widely ad vocated. The general impression seems to be that the use of absorbable sutures for the closure of corneal wounds may produce un-