|Year : 1990 | Volume
| Issue : 3 | Page : 124-131
Keiki R Mehta
Breach Candy Hospital, Colaba Eye Hospital, Mumbai, India
Keiki R Mehta
Breach Candy Hospital, Colaba Eye Hospital, Mumbai
Source of Support: None, Conflict of Interest: None
Radial keratotomy has always produced ambivalent feelings in the Ophthalmologist. The severe complications produced by the Sato procedure has been an unforgettable episode. The work of Fyodorov and Durney (1979) and their American counterparts, Bores (1981) and Hoffer (1981) have done a great service to radial keratotomy in having it established as an acceptable procedure. The controversy really only abated with the work of the National Institute of Health Funded Prospective Evaluation of Radial Keratotomy (PERK) study which suggested that the procedure has a low initial risk and was effective in decreasing myopic correction. Over the last 3 years a series of cases where evaluated and followed up in detail in an effort to gain a perspective on the one problem which has bugged the radial keratotomy programme from the beginning-its predict-ability.
|How to cite this article:|
Mehta KR. Radial keratotomy. Indian J Ophthalmol 1990;38:124-31
| Material and methods|| |
The study is based on 475 eyes of 300 patients which were analysed. The instrumentation consisted of KOI and Storz Calibrated diamond blades, single cutting, with a Storz Ultrasound Pachymeter and an Apple Computer II E with a modified programme.
| Criteria for selection of patients|| |
Patients who had stable myopia between the powers of 0.50 to -13 with no sign of myopic degeneration in the posterior pole were selected. Cycloplegic refraction was carried out 45 minutes after cyclopentolate 1 % was instilled. Astigmatism of less than 2.0 dioptres was acceptable.
| Anesthesia|| |
Unless a patient specifically requested for general anesthesia, none was given. Local anesthesia with peribulbar block was utilised only if it was felt that the patient will not keep still during the time of surgery. In most patients (86.9%) radial keratotomy was done utilising deep surface topical anesthesia with a soft lens. A soft lens hydrated in 4% xylocaine of 15 mm diameter having a hydration factor of 60% was used. The lens is then placed on the cornea and 4% xylocaine drops placed over it every minute for 15 minutes. The anesthesia obtained is deep and the patient has never felt even accidental transgressions of the limbal zone.
| Surgical technique|| |
Followina topical anesthesia. the eve is washed out with1:10 Betadine (povidone iodine) solution. The central axis of the eye is marked by asking the patient to look at the light of an electric ophthalmoscope or into a red LED spot fixed at the base of the operation microscope coinciding with the right optic.
The operation light being turned low, a blunt IOL rotator hook is used to mark the exact spot of reflection. Subsequently the round and radial markers are utilised to delineate the areas for incision. All incisions were made from the centre to periphery. Incisions were terminated 1.0 mm inside the limbus. At the conclusion of the procedure, the incisions were irrigated with Ringer lactate solution to minimise the risk of epithelial inclusion cysts haemorrhage or particulate matter. Antibiotic drops were instilled and the eye was patched.
Subsequently Gentamycin sulphate drops were used for 1 week. On confirmation of healing with flourescein, the drops were changed to an antibiotic steroid combination for 4 weeks as a routine.
| Results and evalutions|| |
The entire concept of radial keratotomy is based on the ability to flatten the cornea; There are 3 major variablesoptical zone size, number of incisions and depth of the incisions. These variables are responsible for most of the power reduction. The three minor variables, keratotomy data, age and sex and intraocular pressure, do not matter much but are important as they serve to "fine tune" the procedure.
The most important variable of all is the optical zone size which works within a narrow range of 3-4.5 mm. Similarly the depth of the incision is critical - less than 85% leads to poor results and most problematical regression of the results; more than 100% leads to perforation (Though most diamond knives have a safe limit at 106%).
In an effort to judge predictability, it became imperative to reduce the variable to a minimum. Thus, the optical zone was kept at a fixed 3.0 mm. However, if the myopic power was less than -3.5 D then it was kept at 4 mm. The depth of the cuts was stabilised at a fixed 98% using the calibrated micrometer based diamond knives (KOI and Storz).
The concept of objective surgical success (OSS) by Rowsey and Hays (1984) is useful to evaluate the results. They defined Objective Surgical Accuracy as
Change in A dioptres obtained
O.S.S.= Change in A dioptres desired
Conversely, Objective Surgical Error = (i-O.S.S.)
Thus, a procedure which gives a perfect accuracy and a result of 1.0 would have a result eliminating the preoperative myopia.
[Table - 1] which gives the results of a 4 cut radial keratotomy with 2 constants, i.e. optical zone at 3 mm and depth at 98% shows in the third column, the quantum of myopia reduction is about 4.25 dioptres reduction (±0.75). Though a variety of powers were chosen for the table, most give the same quantum of results. The surgical accuracy will undoubtedly depend upon the choice of procedure and it will be inappropriate to consider it at this stage. Hoffer (1981) reported an average decrease of 3.4 dioptres x2.2 and in (1983) 3.80 x 1.13.
[Table - 2] displays similar results with 8 incisions having the optical zone at 3 mm and . depth at 98% content . Aaain from column 3 one can see the average reduction is fairly constant at 5.90 ±0.75 irrespective of preoperative power. Rowsey (1983) with idential parameters produced 5.38 ± 2.33 dioptres of refractive change in 1 year.
[Table - 3] use 16 incisions with the optical zone and depth at 3 mm and 98% respectively as constant. Again column three displays improvement of 8.15 ±1.0 irrespective of the preoperative power.
It is often mentioned, that 8 incisions are adequate and doing 8 more do not seem to improve the myopia reduction further. To evaluate that concept, [Table - 4], displays initial 8 incisions, with subsequently 8 more. It by Cowden (1982) where 1.18 dioptres change was noticed and by Rowsey & Hays (1984) where 2.24 dioptres average change was noticed. They further reported that the stage procedure enhanced the effect by 12% - a result which could not be validated in this study.
Thus to summarise, 4 cuts give almost 4.25 dioptres, 8 cuts give about 5.9 and 16 cuts give about 8 dioptres i.e. for every increase in incision the improvement is halved always assuming a 3 mm optical zone with 98% depth.
It is possible to get still further improvement in myopia reduction but it involves 2 techniques, First of all, and simplest is to reduce the optical zone to 2.5 mm and secondly to do peripheral redeepening. However with both, combined, the improvement does not exceed 2 further dioptres and that too only by minimising the margin of safety.
The other variables which can be applied are age/sex, keratometric data and intraocular pressure where: "modifiers" can be applied to "fine tune" the above results. Chart II gives the modifiers in summary as is used by the author.
[Table - 5] and [Table - 6] give all age/sex ratio with relation to myopia. As is expected, 82% of all patients are below 35 years of age and 78% of them being female. The percentage of high myopia is fairly high, with almost a third (34%) being above 6 dioptres in power. Thus simply considering a 4 incisions Radial. Keratotomy as a "be it all" of the R-K procedure is obviously incorrect.
[Table - 7] demonstrates the mean change in the spherical equivalent (i.e sph. = 1/2 Cyl.) in the radial keratotomy procedure is in direct relation to keratometric change. It demonstrates that for every dioptre of corneal keratometric (radius of curvature) flattening, the myopia reduction is 1.35 dioptres, i.e. 35% more myopia reduction. This is fairly within normal acceptable limits. [Table - 8] displays the improvement in visual acuity. It is commonly noticed that in radial keratotomy, the improvement in visual acuity is often more than the myopia reduction. Whether it can be explained on the pinhole effect of the reduced delimited optical zone is difficult to shy or confirm. [Table - 9] shows almost 65% achieve a 6/6 - 6/9 result while 92.1 % achieve a vision of 6/18 or better which has shown exceptional results.
There has always been a doubt regarding the stability of the procedure.
[Table - 10] and (J) attempt to answer some of these queries. [Table - 10] gives a stability reading based on the spherical equivalent while [Table - 11] gives for the keratometric data. Both show close similarities and will be considered together. In both the medium and high myopia which will have more cuts in the cornea, show instability with time displaying further progressive fall in myopia (hyperopic shift) which has recently been reported.
Though we have not yet had any traumatic cases to report on, cases with instability would also theoretically be more prone to damage and injury as basically, instability shows inadequate healing.
| Complications|| |
The complications of radial keratotomy are comparative few considering the quantum of incisional surgery done to the eye.
| Surgical complications|| |
The intra operative complications [Table - 12] are fairly infrequent thanks to the use of guarded knives and ultrasound pachymeter.
Microperforation occured in 14 cases. Surgery was not aborted, but continued again after a 5 minutes break decreasing the blade length by 0.06 mm. There was no macroperforation. Perhaps the safe method is to "seat" the knife in the cornea then wait for 5 seconds, prior to commencing the incision. Thus, a microperforation will be immediately apparent and will not be extended.
Displaced optical zone 1 mm was seen in the earlier days when the patient did not follow the instructions given and would look elsewhere. A new LED device has now been fitted on the microscope which leads to almost perfect centering.
In two cases, sudden movement of the patient under anesthesia led a traverse across the visual axis. Fortunately in both vision is good and well preserved as the knife moved eccentricaly. The mistake made is in assuming that sudden eye movements do not occur under anesthesia. Now-a-days full fixation with a Thornton ring is utilised.
Irregular sinuous incisions in 43% occured only if fixation of eye was improper and was a hall mark of hasty surgery. The Kremer 13 mm and Hofman 8 mm double forceps are ideal. Perhaps the best in the author's opinion is the Thornton ring which permits good stable fixation. In the earlier days, the author used a 3x2 fixation forcep which led to the sinusoidal incisions. Now they are rarely seen.
An incorrect axis in astigmatic correction is a surgical blunder and the fact that it occured in 4 case tells how easy it is to commit. In all these case, incorrect transposition led to the problem.
Limbal incisions are best avoided. Not only do they bleed and leave unsightly blotches of blood the next day, there is strong evidence (Rowsey 1982) that extending the incisions across the limbus decreases the overall optical effect. However, since there is a sudden curvature shift at the limbus, unless great care is exercised, limbal intrusions occur frequently (15.2% in this series).
Skip incisions (5.9%) with sinuous incisions (9%) are fertile cause for inadequate optical correction. Skip incisions occur in letting the cornea dry too much which leads to resistance build up and skip. It may also occur if inadequate pressure is applied to the knife and the incision is not done deliberately and slowly. Fortunately the percentage is low.
The post operative complications [Table - 13] (most frequent) are epithelial problems (both persistant epithelial defects (4.8%) and recurrent epithelial erosions 2.9%) which occur basically as the distance between incisions diminish towards the corneal apex leading to sloughing off, shift off or cut off of a tip of a pie shaped segment and show late healing, are typical of this problem. The use of ocular lubricants help in rapid healing but have to be used for a prolonged period of time. Blood in the incision line typically occurs only in a limbal incision. If noticed early it should be washed out and the bleeding stopped with a fine coaxial wetfield cautery. Incisional epithelial growths are typically seen with metal blades and rarely with diamond blades. Good wound toilet following surgery prevents this occurence.
Occasionally a small corneal iron line (2.5%) developed just interior to the optical zone; developing 6-12 months post operatively. Schachar has reported an incidence of over 10% while Sconders and Hofman report this incidence a high as 20-30%. It has no ill effects.
The presence of SPK like dots in the central zone (13.8%) seem characteristic of radial keratotomy and most are probably due to disruption of the epithelial tear fluid levels. The use of ocular lubricants rapidly help healing this condition.
Contact lenses over an inadequately done radial keratotomy are used for 2 reasons - most often for inadequate optical results, but occasionally for the induced astigmatism. Usually only soft contact lenses are comfortable enough after radial keratotomy and match the new plateau shape of the cornea. Usually these lenses are fitted much steeper than usual to stabilise them, leading to relative anoxia and vascular ingrowth typically along the radial lines and most frequently along the cuts which had transgressed the limbus (4.8%).
Optical problems of Radial Keratotomy [Table 14] are unfortunately quite common. The presence of fluctuating vision (13.6%) seem to be more frequent in those who have a higher myopia, have had 16 incisions, and have a flatter keratometric data with slightly bulging eyes. These fluctuations however occur mainly in the late afternoon or evening. Characteristically these patients also complain of irritable eyes and seem to rub their eyes more often.
Most of the irritable problems seem to be related to dry eye which radial keratotomy patients have to suffer from, probably the surface irregularities decrease proper surface coverage. However, the use of ocular lubricants seem to work wonders.
Star burst flare (9.6%) typically at night in patients who have a small optical zone seems normal. Surprisingly most patients do not complain of it after some months.
Some patient do complain of a change in optical image (6.1%) especially when high fall in power occur or with previously high astigmatic problems. It is of short duration and though disquietening these complaints fade away with time as mental reorientation occurs.
| Summary|| |
Radial Keratotomy is a safe procedure. Depending upon each case, properly applied, predictive values are fairly constant. It is however an intensely individual procedure with each surgeon obtaining his own level of improvement. Experience and good record keeping will help in "predicting" and improving the predicted results.
| CHART -1 MODIFIERS|| |
These modifiers can be applied to any RK Monogram preferred by the Surgeon.
Modifiers are factors which apply to the myopic power being corrected to compensate for other variables 1) Sex: For male - add 0 to the myopic power. For female add 5% to the myopic power.
2) Age: For every year below 30, add 2% to the myopic power till 20, then 1 % till 15. For every year above 30, subtract 2% from the myopic error to age 40 then 1 % till 50 then 1 /2 thereafter.
3) Intraocular Pressure: Based on applanation only for IOP 12 add 3% to the myopic error.
For IOP 19 subtract 3% to the myopic error.
4) Keratometry in mm:8.00-8.45 subtract 5% from myopic error.
7.70-7.95 substract 3% from myopic error 7.00-7.35 add 3% to myopic error 6.70-6.95 add 5% to myopic error
5) If corneal diameter > 13 add 13% to myopic error horizontal white to white. If < 11 subtract from myopic terror.
Modified from Thornton, Dietz, Arrowsmith and Fydorov/Boris RK Parameters.
| References|| |
Artfa RC: McDonald M.B.: Kaufman H.E. (1987) Current status of Refractive Surgery: Ophthalmology: Annual: Appleton, Century Crofts Connechcent.
Arrowsmith P.N. : Diet 2 M.R. : Marks R.G.: Sanders D.R. : Sewelson H (1984) Radial Keratotomy : ARK Study Group Slack Incorporated Thorafare N.J.
Cowden JW (1982) Radial keratotomy archieves of ophthalmology 100, 578-580.
Hoffer KJ.: Darin J.J. 91981) Ucla clinical trial of radial keratotomy Ophthalmology 88, 729-736.
Hoffer K.J. :Darin.J.J.: (1983) Ucla clinical trial of radial keratotomy Ophthalmology 90. 627-636.
Rowsey J. J. and Hays J.C. (1984) : Radial keratotomy : pp 63-81 : Recent advances in ophthalmology Churchil Luvingtha.
Rowesy J.J.: Balycat HD(1982) Preliminary results and complications of radial keratotomy A.J.Ophthal 93:473-455.
Sanders DR. Norman RF : Salz J.J (1986) Refractivecomeal surgery slack incorporated N.J.
Schachar R.R. 91983) Radial Keratotomy International Ophthalmology Clinics Vol 23 No.3 1983 Boston, Little Brown & Co. 1983 pp 112-128.
Smith S. R. 1986) Radial Keratotomy : Ophthalmology Annual : Appleton Century Crofts, Connecticut.
[Table - 1], [Table - 2], [Table - 3], [Table - 4], [Table - 5], [Table - 6], [Table - 7], [Table - 8], [Table - 9], [Table - 10], [Table - 11], [Table - 12], [Table - 13]