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ARTICLE
Year : 1966  |  Volume : 14  |  Issue : 1  |  Page : 6-12

Experience with the posner-inglima tonometer


K. B. Haji Bachooali Eye Hospital, Bombay, India

Date of Web Publication12-Jan-2008

Correspondence Address:
B D Telang
K. B. Haji Bachooali Eye Hospital, Bombay
India
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Source of Support: None, Conflict of Interest: None


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How to cite this article:
Telang B D, Badlani H G. Experience with the posner-inglima tonometer. Indian J Ophthalmol 1966;14:6-12

How to cite this URL:
Telang B D, Badlani H G. Experience with the posner-inglima tonometer. Indian J Ophthalmol [serial online] 1966 [cited 2019 Sep 16];14:6-12. Available from: http://www.ijo.in/text.asp?1966/14/1/6/38557

Table 5

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Table 5

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There have been indeed many instruments and techniques to record the pressure of the contents of the eye ball which for clinical purposes cannot be measured directly as such but indirectly through the state of tension of the cornea and sclera. There are in fact, so many factors with wide variations involved in this indirect deduction of the intra-ocular tension that Sir Stewart Duke­-Elder has written that the results of tonometry will always remain as ap­proximations and will always involve uncertainties. The pursuit of an ideal instrument therefore appears to be unending and the scales of to­nometer still lure the research work­er.

Indentation tonometer appeals to ophthalmologists because the mea­surement is accomplished quickly. However, there is no scientific foun­dation for interpreting its results. Ap­planation tonometer on the other hand, requires more time, but is more accurate and is founded on scientific principles expressed through mathe­matical formulations. The Maklakov tonometer is the simplest type of ap­planation tonometer and has been in use for many years in U.S.S.R. and Eastern Europe. It uses a known force to produce an area of corneal flattening "Applanation", as against the Goldman type where the force is varied and the area of applanation is kept constant. The Posner - In­glima Applanometer is a modified Maklakov applanation tonometer. The applanation method of tonometry is indeed the more reliable for the detection of early cases of glaucoma, associated with high myopia and also of glaucoma in young individuals. Because it produces a minimal vol­ume displacement, the applanation measurement is not significantly af­fected by variations in ocular rigi­dity and thus permits better differ­entiation between normal and glau­comatous eyes.

Posner-Inglima applanometer is a set of 4 tonometers and a magnetic handle. Two tonometers are identi­cal and weigh 5 gms. each, one weighs 7.5 gms. and one weighs 10 gins. All the tonometers are similar in size and shape; two truncated cones with their smaller ends facing each other and connected by a cylin­drical shaft. The body of the tono­meter is made of high grade non­magnetic stainless steel and contains a freely sliding ballast which gives the tonometer greater stability when it is placed on the cornea, and en­sures a low centre of gravity which­ever end is being used. The handle is made of stainless steel and is hol­lowed out to accommodate a per­manent magnet with a minimum lift­ing force of one pound. The handle may be attached to the tonometer instantly by bringing it into contact with the steel tab into which is fitted the stainless steel ring which loosely encircles the shaft of the tonometer. Two 5 gm. weights are provided in each set so that duplicate readings may he taken. (The 5 gm; tonometer is the one most frequently used, since it is to be preferred for normal and border line tensions.) With the tonometer in place and the instrument suspended above the eye, the verti­cal handle acts as a guide to perpen­dicularity. The end plates which are 9 mm. in diameter each, and which are polished to optical flatness are made of Pyroceram, an extremely hard, heat-resistant ceramic material. They are fully exposed in order to facilitate cleansing and sterilization. They are highly resistant to scratch­ing by abrasive materials. The end plates are fitted and cemented to the body of the tonometer by a heat re­sistant adhesive. Since, Pyroceram is a highly polarized material the end plates accept a uniform coating of the brown stain (mild silver protein) and the imprint made by the cornea re­tains its sharply outlined edge which permits accurate measurement.

The applanometer, thus, is a cons­tant force applanation tonometer and the area of corneal applantion which it produces varies inversely with the height of the intra-ocular pressure. The absence of moving parts elimi­nates any instrumental error due to friction. Rigid specifications regard­ing weight of instrument and degree of flatness of the end plates assure absolute uniformity and obviate the need for future re-calibration.

Application of the Tonometer

The patient is placed either in a supine or in a reclining position and any inadvertent pressure on the ves­sels in the neck e.g. by a tight collar must be removed. He is made to fix his gaze vertically upwards by use of a fixation target. An anesthetic is instilled into the eyes. The end plates should always be wiped clean with a pledge of cotton moistened with alcohol or water, and no finger print should be allowed to remain on the applanating surface. To avert danger of infection each end plate is heated in the flame of a spirit lamp for 4 to 5 seconds and later cooled before use. The mild silver protein should be applied to the end plates in a uniformly thin layer to produce a light tan colour. A cotton-­tipped applicator may be used for this purpose. [Figure - 1]

The magnetic handle is applied to the surface of the metal tab. The eye lids are gently retracted, being careful not to exert pressure on the eye ball. The tonometer is suspended above the eye and lowered until the full weight rests on the centre of the cornea. [Figure - 2] The ring of the metal tab should not move more than half way clown the tonometer shaft. The tonometer is then lifted off the eye in a vertical direction. The tono­meter should not rest on the cornea for more than one-half of a second.

The interaction between the stain and the pre-corneal film displaces the stain from that portion of the end plate which comes in contact with the cornea. The area of contact ap­pears as a white circular imprint on the end plate, which is sharply demarcated from the surrounding stained area which had not made contact with the cornea. The area of this imprint bears an inverse rela­tionship to the height of the intra­ocular pressure. The ideal imprint is a white area circular in outline and contains a central spot of stain. This spot of stain is an artefact and is without significance for the mea­surement. Any deviation from the circular shape indicates that the to­nometer has slipped while in contact with the cornea. Such slippage may be caused either by movement of the tonometer or by movement of the patient's eye. If the imprint is oval in shape, it is accepted provided that the shortest diameter is measur­ed. If the imprint is grossly distort­ed or has indistinct margins, the measurements are repeated. Inspec­tion of the imprint thus permits the operator to verify the correctness of his technique and to control the accuracy of his measurements. This feature is unique to this form of to­nometry.

Since the corneal applanation is measured not on the cornea but on the tonometer end plate it is neces­sary to establish a relationship bet­ween the applanated area of the cor­nea and the area of the imprint. The process of corneal applanation gives rise to the formation of 3 zones; a central zone, an intermediate zone and the outer zone. [Figure - 3] When the tonometer flattens the cornea, the layer of stain on the correspond­ing portion of the end plate becomes dissipated and mixes with the pre­corneal film. The dissipation of the stain in this early stage of applana­tion produces a homogenous coloured circular area which may be referred as the central zone. As the area of applanation increases in size, the out­ward displacement of a portion of the pre-corneal film results in the gra­dual formation of a dark coloured liquid ring, which constitutes the in­termediate zone. The outer margin of this ring appears sharply defined. The inner margin, which is less clear­ly defined, represents the outer border of the flattened area and thus is the true measure of the corneal applana­tion.

Surrounding the liquid ring, there is formed an outer clear zone which is relatively free of stain. While the tonometer is being removed from the cornea, the area of contact becomes smaller and the liquid ring moves to­wards the centre. The outer margin of the clear zone, however, remains fixed in position and forms a circle the diameter of which is regarded as a measure of the "corneal applana­tion.". [Figure - 3]

The diameter of the imprint may be measured with a transparent scale supplied with the applanometer or by using a 8 X measuring magnifier with the scale divisions of 0.1 mm. The measuring scale contains two pairs of covergent guide lines and four columns of numbers. The col­umn of number at the left of the scale gives the values for the intra­ocular pressure in mmHg. when the 5 gm. weight is used; the next two columns of numbers give the values when the 7.5 Gm. and 10 Gm. tono­meters are used. The column of number at the right of the scale gives the diameter of the imprint in mm. This is useful for recording the tension by the size of the diameter of the imprint rather than by the con­ventional method in mmHg. If a measuring magnifier is used instead of the transparent scale, the corresponding intra-ocular pressure in mmHg, is obtained by referring to a conversion table. The transparent measuring scale used for measuring the diameter of the imprint introduces an element of inaccuracy and is not recommended if maximum degree of precision is to be achieved. The sources of error derive partly from parallex produced by the overlying protective plastic film, partly from the fact that the naked eye does not discern the innermost edge of the imprint. The use of the measuring scale with the naked eye generally leads to an overestimation of the dia­meter and consequently, to an under­estimation of the intra-ocular pres­sure. It is important to bring the imprint into direct contact with the scale so as to avoid parallex. The imprint on the end plate should be measured promptly. It can however, be transferred to a fine grain absorbent paper supplied with the ins­trument. The edges of the imprint are generally sharper and more dis­tinct on the end plate than they are on the transfer paper. The advan­tage of the transfer paper, however, is the fact that it enables to keep a permanent record of the measure­ment. If the ocular tension is ex­pected to be within normal limits the 5 gm. tonometer should be used. If the resulting imprint has a dia­meter smaller than 4 mm., the mea­surement should be repeated with the 7.5 gm. tonometer. In cases of high myopia, congenital glaucoma, or any other condition in which the possibility of abnormal ocular rigi­dity exists then, the 10 gm. tonometer should be used in conjunction with the 5 gm. tonometer. If the differ­ence between the two measurements is greater than 3 mmHg., the eye does not have average ocular rigi­dity.

Ocular Rigidity - With the ap­planometer it is possible to estimate ocular rigidity by means of paired readings. This is a valuable diag­nostic aid in cases of borderline ten­sion. If the tension as measured with the 7.5 gm. or the 10 gm. to­nometer is the same as that obtain­ed with the 5 gm, tonometer, the eye is said to have a "normal ocular rigi­dity". If the tension readings are higher with the heavier weights, the ocular rigidity is high. If the ten­sion readings are lower with the heavier weight, the ocular rigidity is low. If the ocular rigidity is abnor­mal in either direction, the readings obtained with the 5 gm. tonometer give the closest approximation to the true intra-ocular pressure. There is no tonometer which mea­sures the true P o state of the eye.

However, we may assume that the measurements obtained with the Goldman Tonometer come close to the value of P,. The Applanometer at best expresses values similar to those obtained with the Goldman Tonometer.


  Study Report Top


The series under the present study consist of 152 normal and glacoma­tous eyes examined with the new ap­planometer and Schiotz tonometer. Selection of a particular weight of the tonometer was based, as in the case of Schiotz tonometer, on the tension of the eye ball. 5 gm. weight is used for pressures upto 35 mmHg, 7.5 gm. weight for pressure ranging between 35 - 49 mmHg and 10 gm. weight for pressures above 49 mmHg.

Each case was first examined with the Applanometer and then with the Schiotz tonometer. Generally cor­responding weights were used in the two instruments. Normal cases were selected from the refraction depart­ment who showed no evidence of any eye disease on examination ' or inter­rogation. Glaucoma cases were chosen from those attending the `glaucoma clinic' at the Haji Bachoo­alli Eye Hospital, Bombay, and in­cluded treated as well as untreated cases.

The report is as follows:[Table - 1]

Average normal intra-ocular pres­sure in this series according to Ap­planometer was found to be 15.1 mmHg. The range of intra-ocular pressure varied from 10 to 18 mmHg.

The average normal intra-ocular pressure for the same subjects exam­ined with the Schiotz Tonometer was found to be 16.7 mmHg and the range varied from 10 to 21 mmHg.

All the tonometric findings are charted on a graph for comparison. There appears to be a general con­sistency of the readings on the two instruments for the lower ranges of tension. However, it appears that for higher tension, there is sometimes a great difference between the read­ings obtained from the two instru­ments. [Figure - 4]

An average difference of 2.5 mmHg was noticed. However, in terms of percentage difference, there was a significant increase of 9.5% in the total figure value of the Schiotz read­ings as compared with the same of the Applanometer, the maximum dif­ference between the readings of the two instruments was noted as high as 26 mmHg.

From these figures it appears that the Applanometer registers slightly lower values than the Schiotz Tono­meter for eyes with normal and slightly raised tensions. However, in cases of high tension, this differ­ence becomes at times, very marked. These differences can be due to many factors including the factor of ocular rigidity.

Extensive series of cases have been examined by Posner-Inglima and others to substantiate the accuracy and utility of this instrument. Our own experience with this instrument, al­though small, has impressed us of the many advantages that it possesses.

Comparison with Schiotz Tonometer

1. - The Applanometer is claimed to be a permanent tonometer which does not need any re-testing. The Schiotz tonometer which is a most common instrument in use has the greatest drawback of be­coming defective. From our own experience as well as from the personal communications with other colleagues, we feel that the Schiotz Tonometer often suffers from mechanical defects causing great uncertainty of its readings. In our country specially, where replacing or repairing and re­testing of the instrument is not an easy proposition, an introduc­tion of a permanent reliable ins­trument would be naturally most welcome.

2. - The calibration of Applanometer like that of Goldman Tonometer is based on scientific laws of mathematics unlike that of Schiotz indentation tonometer where the calibration is done on empirical basis which undergoes repeated changes.

3. - The instrument is non-traumatic. The mirror polish of the end plates ensures that no cor­neal injury takes place. In our work, we did not notice a single case of corneal abrasion.

4. - The method is more sensitive than Schiotz Tonometry.

5. - The inaccuracies caused by fric­tions in the instrument are en­tirely eliminated.

6. - Errors caused by abnormal ocular rigidity are reduced. This is particularly important for the diagnosis of glaucoma with the presence of High Myopia etc.

Advantages over the Goldman Tonometer:

1. -It is a simple portable instru­ment and does not need the use of a Slit Lamp.

2. -It is easy to handle and causes minimum trauma to the eye and hence can be conveniently used even in children and nervous patients.

3. - It can be sterilized and hence may be used in the Operation Theatre.

Other features: 1. A permanent re­cord of the actual measurements may be obtained by transferring the imprint from the end plate to the paper.

2. The reliability of the readings can be checked by examining the regularity of the outline of the imprint.

3. It may be used in conjunction with Tonography for the direct measurements of P o value.

4. The precision readings can be obtained specially for research work by utilising the special magnification system for measur­ing the diameter of the imprint.


  Summary Top


A new type of applanometer is described in which intra-ocular pres­sure is determined by measuring the area of flattening of cornea caused by resting on it a known weight with a perfectly flat surface.

The results in 46 normal and 106 glaucomatous eyes are compared with those obtained by a Schiotz tono­meter.

The advantages of this simple por­table device over a Schiotz Tono­meter or a Goldmann's Applanometer are stated.


    Figures

  [Figure - 1], [Figure - 2], [Figure - 3], [Figure - 4]
 
 
    Tables

  [Table - 1]



 

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