角膜塑型文獻庫

驗配角膜塑形鏡（Fitting Orthokeratology Contact Lenses）

發表於：2001年10月

Fitting Orthokeratology Contact Lenses

By Belinda M.W. Luk, OD, Edward S. Bennett, OD, MSEd, and Joseph T. Barr, OD, MS, FAAO
October 2001

This comprehensive look at fitting corneal reshaping lenses will help you become a more confident ortho-k practitioner.

In the late 1950s and early 1960s, many practitioners observed that patients were slightly less nearsighted upon removing their lenses when corneal contact lenses were being fit slightly flatter than K. In the early 1960s, several optometrists developed their own orthokeratology techniques by using large, flat lenses with large optical zones. Unfortunately, their only choice of material was PMMA. Rigid gas permeable (RGP) lens materials, developed in the late 1970s, greatly improved the safety and efficacy of orthokeratology. These materials achieved an overall improvement in performance, in part the result of the ability to use larger overall diameters, which greatly enhanced centration of the orthokeratology lenses.

The first reverse zone lens design for orthokeratology was developed in the 1980s by Dr. Richard Wlodyga and manufactured by Mr. Nick Stoyan. These first lenses consisted of three curves, including a secondary curve often two to three diopters steeper than the base curve radius to accelerate the process. Currently, there are numerous four curve/zone or similar designs to further accelerate this process.

Of interest today is overnight orthokeratology, in which orthokeratology lens designs are manufactured in high Dk materials, such that the experimental lenses are worn during sleep and removed upon awakening. Numerous laboratories and all major RGP button manufacturers are currently very active in orthokeratology. It represents one of the fastest developing areas of contact lenses, and it is imperative for every optometrist who is interested in orthokeratology to keep updated on the available information and the different orthokeratology lens systems present.

General Fitting of Reverse Curvature lenses

Most modern orthokeratology lenses are designed with secondary curves steeper than their base curves. The steeper secondary curves serve the following purposes: to provide space for the cornea to move as the central cornea is flattened; to help lens centration, thereby reducing induced astigmatism; and to create a reservoir for tear exchange. Although there are many unique designs of orthokeratology lenses, most are variations of this form of reverse zone lens.

The first reverse zone lenses produced were of the three-zone design. The general design of these lenses consists of an optical zone, a reverse curve (the steep secondary curve) and a peripheral curve. The optical zone is typically 6.0mm in diameter, and the initial base curve is fit 1.00D to 1.50D flatter than the flat K reading. As the fitting relationship of this lens changes as a result of the flattening of the central cornea, a new lens with a flatter base curve will have to be dispensed promptly to prevent distortion and/or physiological problems of the cornea. The initial change occurs rather quickly; therefore, many fitters will order a second pair of lenses at the time of the initial order. The width of the reverse curve can vary from 1.0mm to 1.4mm, and the initial reverse curve radius is best determined through diagnostic fitting. The ideal fluorescein pattern should show a ring of at least 270 degrees due to with-the-rule corneal toricity (to 360 degrees in nearly spherical corneas) of midperipheral touch with an area of clearance to either side. The peripheral curve is approximately 0.4mm wide, with a radius from 10.5 to 12.25 mm. Centration is very important to the fit of these lenses and can be improved by steepening the base curve or the reverse curve.

With the four-zone reverse zone lenses, the expected maximum amount of myopia reduction is 3.50D to 4.25D. This lens design is similar to the three-curve design, except for the addition of an intermediate curve between the reverse curve and the peripheral curve, often referred to as the alignment curve or zone. The alignment zone is fit such that it is in alignment with the peripheral cornea, and it plays a very important role in lens centration and movement.

Typically, with four-zone lenses, the initial pair of lenses is expected to be used for the entire course of the treatment, as well as for retainer wear. These lenses are large, from 10.0mm to 11.0mm in overall diameter, with a small optical zone, often 6.0mm in diameter. In several designs the initial base curve radius is fit flatter than K reading by an amount just greater than the refractive change desired, often equal to +0.50D. For example, if the amount of myopia is ­2.00D, then the base curve radius should be 2.50D to 2.75D flatter than K; and the lens power will be +0.50D or +0.75D. This power allows for regression of myopic refractive error during the day. Accurately perform keratometry and preferably corneal topography in addition to a manifest refraction, and regularly calibrate all instruments.

The second curve is the reverse or return zone which is approximately 0.6mm wide and steeper than the base curve radius by two to 2.6 times the amount the base curve is flatter than K, and can range from 6.00D to 12.00D steeper than the base curve radius. In our example, if the base curve radius is 2.50D flatter than K, then the reverse curve radius should be 5.00 to 6.50D steeper than the base curve radius.

The third zone in the lens is the alignment curve which is typically about 1.0mm in width and fit in alignment with the midperipheral cornea. Determine an alignment curve radius using the central keratometry reading, a temporal keratometry reading or information from topography. The alignment curve can be ordered as 0.25D flatter than the central flat K reading or as the temporal keratometry reading measured by asking the patient to fixate on the edge of the keratometer target. Using the eccentricity value obtained from topography, the alignment curve radius can be determined as follows:

• The alignment curve radius should be equal to the central flat K reading if the eccentricity value is between 0 and 0.30;
• The alignment curve radius should be 0.25D flatter than the central flat K reading if the eccentricity value is 0.31 to 0.55;
• The alignment curve radius should be 0.50D flatter than the central flat K reading if the eccentricity value is 0.56 to 0.70. The alignment zone specifications may also be determined by using trial lens sets. Adjustments will have to be made according to the fluorescein pattern and lens centration. The outer curve of the lens is the peripheral curve, which is very similar to the peripheral curve of the three-zone design; it is most commonly 0.4mm wide with a radius of curvature of 10.50mm to 12.50mm.

Figure 1. Fluorescein pattern of a four-zone orthokeratology lens.

As with the three zone designs, a well-centered fitting relationship with limited lens movement with the blink is important. The fluorescein pattern should exhibit central touch, paracentral clearance, midperipheral touch and minimal peripheral clearance (Figure 1). The most important component of the fit is lens centration, which is primarily influenced by the alignment curve. As lens wear occurs while the patient is sleeping, lid-lens interaction may not have as much effect on lens centration as with standard RGP lenses, and on-eye evaluation of the lens may not be as valuable in assessing the lens position. Evaluate lens position during sleep by reviewing corneal topography and evaluating the centration of the treatment area in relation to the pupil. If the lens is sitting superiorly, the lens may be loose, causing it to move to the flatter superior cornea. Steepen the alignment curve if it appears flat from the fluorescein pattern, or increase center thickness, decrease edge thickness or add a prism to increase the mass of the lens if the fluorescein pattern appears to be ideal. For an inferiorly-fitting lens with heavy touch in the alignment zone, flatten the alignment curve. However, if the fluorescein pattern shows alignment in the midperiphery, then decrease the center thickness or increase the edge thickness. Again, depending on the fluorescein pattern in the alignment zone, the solution to a laterally-decentered lens may be to flatten the alignment curve or increase the diameter of the lens.

	Figure 2. "Smiley face" corneal topography.

Problem-Solving

Most successful patients obtain 20/20 to 20/25 visual acuity that is maintained throughout the day. However, a major cause of poor unaided vision is an inadequately-fitted lens, which can often be diagnosed from topography. If the treatment area is decentered, the topography map will exhibit an arcuate area of steepening in the cornea within or very near the pupil margin. An excessively flat or apical touch fitting lens design will often result in a "smiley face" topography map (Figure 2). In this case, refit the lens to create a more centered treatment area. Central islands in the topography are small areas of incomplete treatment or corneal distortion at or near the visual axis, which can be caused by either poor centration or a steep reverse curve (Figure 3). Poor unaided vision may also be due to under-treatment or a too-steep base curve. However, if the lens fits well and the initial base curve radius calculation was performed correctly, then the best strategy is patience, as some patients may be slower to respond or respond to a lesser degree than expected. Re-evaluate the patient in two to three weeks. If the changes are still insufficient, try fitting the base curve radius 0.50D to 0.75D flatter to cause more corneal epithelial movement. Remember some patients may not respond to their satisfaction. Another common visual problem with orthokeratology lenses is flare and glare at night, which may be due to a decentered lens or large pupil size.

Figure 3. Central islands corneal topography.

Potential physiological problems include central corneal staining and persistent lens adhesion. Corneal staining is often the result of either build-up of debris on the back surface of the lens or mechanical irritation from an excessively flat lens. Solve this problem by instilling lubricants, polishing the back surface of the lens, re-educating the patient on the importance of lens cleaning and reordering the lens with a steeper base curve radius. Excessive lens adhesion, like corneal staining, could also be due to deposit build-up on the back surface. A steep alignment or peripheral curve causing an insufficient tear reservoir could also result in lens adhesion. Flattening and/or widening the alignment or peripheral curve will prevent future lens adhesion.

FDA Approval for Overnight Orthokeratology

Overnight orthokeratology is an off-label use of the reverse curve lens design and is not FDA approved. Many fitters prescribe these lenses to patients for orthokeratology with informed consent, though they may not advertise them. The FDA does prohibit RGP laboratories and manufacturers from promoting such a design to practitioners.

Orthokeratology Lens Systems

Each orthokeratology lens system has its own design and fitting philosophy. In some of these systems, lenses are ordered empirically from the patient's keratometry readings and spectacle refraction; in others, lenses are ordered only after the diagnostic fitting is complete. Whether a practitioner chooses to order lenses empirically or from diagnostic fitting depends upon the present comfort level in fitting and evaluating these lenses. Each practitioner should learn more about each system to decide which one most closely approximates his or her own fitting philosophy. You may need to fit several patients in each lens system to fully appreciate the differences and to find the system with which you are most comfortable. Remember that, although the following systems are not associated with specific RGP consulting companies, all laboratories will have consultants on staff to assist practitioners in fitting.

Orthokeratology Lens Design Tools

For the practitioner who would like to begin fitting orthokeratology lenses, some useful tools are available. EyeQuip developed the WAVE Contact Lens Design Software, which enables practitioners to design single vision, front or back surface multifocal or orthokeratology (RGP) lenses. The program utilizes the mathematics of wavelet theory from topographic data and creates a digital signal to describe the cornea, which is used to create the lens design with an emphasis to match the periphery of the lens to the peripheral cornea. This results in a lens with excellent centration, according to the manufacturer. The final lens design can be e-mailed directly to the Optiform lathe at Custom Craft contact lens laboratory. The WAVE program is already integrated into the Scout topographer software and is included with the Keratron topographer as a stand-alone software.

OrthoTool 2000, developed by EyeDeal Software & Design, is an RGP design, tear film modeling and manufacturing software. OrthoTool 2000 performs the optical calculations from keratometry readings and spectacle refraction to display complete lens parameters, manufacturing data, the cross section of the lens, thickness profile and the tear film thickness across the lens diameter. The contact lens practitioner can choose from 12 different contact lens designs such as standard spherical designs, thin, ultra thin, aspheric or bitoric lenses, as well as several reverse geometry lens designs.

Orthokeratology Lens Design Consulting

Tabb developed the Nightmove lenses, which are manufactured with Boston Equalens II material. The Nightmove lens is of a reverse geometry back surface construction with up to nine curves, including the base curve, the reverse curve, a variable number of alignment curves and the peripheral curve. Nightmove lenses can be ordered through Advanced Corneal Engineering, Inc..

Many More Systems

Reversible Corneal Therapy by ABBA Optical is a standard four-curve reverse curve lens manufactured from the Paragon HDS 100 material which is FDA approved for overnight wear. Base curve radius determination is accomplished by fitting the lens flatter than K by the amount of desired refractive change +0.75D. Lens diameter is 10.6mm; alignment curve radius is 0.25D flatter than central flat K reading. The reverse curve radius is calculated by a consultant when the lens order is placed.

Contex was the first company to receive FDA approval for its orthokeratology lens for daily wear. The Contex OK Lens is fit based upon the central K reading, manifest refraction and corneal eccentricity value obtained from topography. The lenses are labeled with the flat K value, the desired refractive change and a peripheral fit size. For example, for a patient with a central flat K reading of 43.00D and a refractive error of ­2.00D, the first diagnostic lens to try should be labeled 43.00/­2.00, which will actually have a base curve of 40.25D (the base curve is automatically adjusted for the desired refractive change). Peripheral fit size is determined based on the corneal eccentricity value from topography data: XXL (extra extra loose; e = 0.8), XL (extra loose; e = 0.7), L (loose; e = 0.6), S (standard; e = 0.5), T (tight; e = 0.4), XT (extra tight; e = 0.3), XXT (extra extra tight; e = 0.2). After empirically selecting the initial diagnostic lens, the final lens is determined from fluorescein patterns and lens centration.

Correctech, Inc., is currently conducting FDA investigational studies on its overnight accelerated orthokeratology lenses. For the study, patient data is forwarded to the laboratory and lenses are empirically designed for the patient. The Correctech, Inc., orthokeratology lenses are four-zone reverse geometry lens designs in Boston Equalens II material.

The DreimLens, designed by Dr. Thomas Reim, is also currently involved in orthokeratology overnight wear FDA studies. These lenses, in Boston XO material, are of the four-zone design. The base curve radius of the central zone is calculated from the flat central K reading and the amount of refraction to be corrected. The standard fitting zone, alignment zone and peripheral zone parameters have been clinically and theoretically determined to work together to provide the desired results in the majority of cases. Lenses can be ordered empirically with patient's keratometry values and manifest refraction. Fluorescein patterns should be used only for gross observations, as similar patterns can be present with clinically significant differences in parameters, which will yield different refractive results.

The Emerald and Jade designs are available from Euclid Systems Corporation. Both are manufactured with the EPT manufacturing system, which offers polish-free lens finish, helping to eliminate inconsistencies in posterior sagittal depth from polishing. The Emerald design is based on a four-curve reverse curve design. The Jade design is more advanced and uses a conic model of the cornea and information about corneal eccentricity and patient's refraction to calculate the proper reverse curve. The Euclid system includes the lens designs and the Euclid ET-800 Corneal Topographer. Overnight lenses are manufactured with the Boston Equalens II material. The company has completed the first clinical study for an overnight wear orthokeratology lens, and the data is currently under review by the FDA.

Gelflex Laboratories in Australia manufactures the EZM orthokeratology lenses. EZM lenses are made of the Boston XO material, if used for overnight orthokeratology, and are available in 10.6mm or 11.2 mm overall diameters depending upon the patient's intrapalpebral aperture size. They are fenestrated at 120-degree intervals to prevent lens adhesion during overnight wear. Gelflex developed a computer calculator program to aid practitioners in determining the initial trial EZM lenses by incorporating corneal topography data and the overall lens diameter requested. Once the initial trial lenses are determined, Gelflex recommends performing an overnight trial to determine whether the patient is a fast or slow responder and whether the initial trial lens choices were correct. If, on the following morning, the response seems to be poor and due to an inadequately fitted lens, the patient will have to return for another trial with different lenses.

Paragon Vision Sciences is currently conducting investigational studies and applying to the FDA for approval of its Corneal Refractive Therapy (CRT) lens system. These lenses are different from the traditional reverse curve lenses. In standard reverse curve lenses, the four zones consist of curves of different widths and radii of curvature; in the CRT lenses, the reverse zone and the alignment zone are different from other designs. The reverse zone, called the return zone in CRT, is a sigmoid; it is not a curve that can be defined by a radius of curvature. The width of the return zone is kept constant; it is the depth of the sigmoid that is varied to change the fit of the lens. The alignment zone, called the landing zone in CRT, is a flat section (a straight line) defined by the negative angle that it makes with a horizontal line. The landing zone in CRT, like the alignment zone in reverse curve, is meant to be fit in tangent or alignment with the midperipheral cornea and aids in lens centration. Paragon requires practitioners interested in fitting CRT lenses to attend a fitting seminar. Fitters can select from two different diagnostic systems: a 24-lens diagnostic set with a Palm Pilot calculator program or a 65-lens diagnostic set.

Precision Technology Services is the only RGP laboratory in North America to produce Dr. John Mountford's BE lens design for orthokeratology. BE lens fitting is based on the theory that the sagittal height of the contact lens must match the sagittal height of the cornea, allowing for the tear film layer. Sagittal height of the cornea is determined with an equation that requires the following pieces of information: apical radius of the cornea, elevation of the cornea and chord length (the total diameter of the lens to be fitted). Also, as positive pressure from the lens is applied to the central cornea, due to the flat base curve, negative pressure is exerted on the paracentral cornea, in the area of the tear reservoir; this is called the squeeze film force, which can be manipulated by altering the base curve and the depth of the tear reservoir. The squeeze film force is the factor that determines how much epithelial movement will occur; therefore, it determines the amount of refractive change that will occur. Mountford developed the BE computer program which simplifies the calculations. One needs to input only the apical radius of the cornea and the corneal elevation from topographical data, the lens diameter desired and the desired amount of refractive change; the program will calculate the initial trial lens. Then, the patient should try the lenses overnight. On the following day, by inputting the resultant topographical and refractive information, the computer program will calculate the final lens order.

R & R Lens Design creators Drs. James Reeves and John Rinehart conduct training seminars for interested practitioners. By participating in such a training session, fitters learn about the lens designing process, the purpose of each curve and how the curves affect lens fit and lens performance. With this information, the practitioner will be able to design his or her own lenses and troubleshoot when complications arise. Practitioner may order lenses from their preferred RGP contact lens laboratory in their preferred material. Lenses are fit using a 14-lens diagnostic kit.

See Table 1 for a listing of the designs and design tools previously described. Another option for practitioners interested in learning more about orthokeratology is attending the Global Orthokeratology Symposium, sponsored by Contact Lens Spectrum, to be held August 2002 in Toronto. Visit www.healthcareconferencegroup.com for more information.

Orthokeratology is an exciting new frontier which provides a potentially valuable and reversible alternative for low myopic refractive error individuals who are not interested in or are poor candidates for refractive surgery. The advances in lens design and corneal topography instrumentation complemented by an overnight wearing schedule have resulted in a much shorter time frame for myopia reduction to occur as well as patient satisfaction.

References are available upon request to the editors of Contact Lens Spectrum. To receive references via fax, call (800) 239-4684 and request document #75. (Have a fax number ready.)

Dr. Luk is a former contact lens resident at the University of Missouri-St. Louis.

Dr. Bennett is an associate professor of optometry at the University of Missouri-St. Louis and executive director of the RGP Lens Institute.

Dr. Barr is editor of Contact Lens Spectrum and assistant dean of Clinical Affairs at The Ohio State University College of Optometry.

角膜塑型鏡片的設計及應用

	眼視光學雜誌 CHINESE JOURNAL OF OPTOMETRY & OPHTHALMOLOGY 2000 Vol.2 No.2 P.126-128

角膜塑型鏡片的設計及應用 張主君 ［關鍵詞］　角膜塑型； 角膜地形圖； 螢光素 ［中圖分類號］　R778.3　　［文獻標識碼］　A　　［文章編號］　1008-1801(2000)02-0126-03 　　角膜塑型有45年歷史，是一門悠久的科學，為何一直在世界各地不能被接受，這是一個令人深思的問題，為此，我們特對角膜塑型鏡片(OK鏡)作一概述，以作探討。 1　第一代角膜塑型鏡片 　　實際上是用有機玻璃(PMMA)製成的普通硬性隱形眼鏡。由於一位視光學醫生的錯 誤，為病人訂製了一對比角膜曲率平的鏡片，但後來發覺病人的裸眼視力有輕微改善，因而出現了角膜塑型的雛形。由於當時的鏡片無透氧性，戴鏡後易導致角膜上 皮水腫及其他不適等，並且由於鏡片的設計只有一般的硬性隱形眼鏡，若鏡片基弧比角膜曲率平坦超過兩個屈光度，便有可能令鏡片不能居中，不但減低近視效果不 佳，還可能引起散光及其他視光學的副作用。雖然如此，這種第一代的OK鏡片，在美國一直用了三十多年，使用者亦僅限於數十位視光學醫生。 2　第二代角膜塑型鏡片 　　1979年，美國一位資深視光學醫生通過多年的OK鏡探索實踐，提出了新的理論，認 為若將鏡片多做一條弧度，即將鏡片的光學區縮小至6～7mm左右，再做出第二條弧度， 而第二條弧度要比光學區弧度彎，便可令鏡片居中，這樣的話，便可訂製比角膜曲率平2～3D的鏡片，這種鏡片便是逆轉幾何學(Reverse Geometry)鏡片的始祖。正常角膜或一般的硬性隱形眼鏡，都是中央較彎而外周較平坦，而逆轉幾何學鏡片則與這概念相反，鏡片是中央平坦而外周較陡。 第二代的鏡片，一般第一對鏡片產生約2D的壓力，若效果明顯，再更換一對多0.5D壓力的鏡片，所以一般要用3～5對鏡片，約3～6個月，可將近視減去 2D左右，但維持的時間不長，戴鏡八至十小時後，可減去2～3D近視，維持約4～6小時後效果便逐漸消失。 　　鏡片的設計數據是曲率在前，寬度在後。第一對鏡片的光學區的基弧是8.23mm(41.00D)，寬度是6.4mm，第二弧即逆轉弧比基底弧彎3D， 即7.67mm(44.00D)，第三弧與一般的硬性隱形眼鏡無區別，即12.00mm/0.4mm。這種第二代的鏡片，其逆轉弧可比基弧彎2～6D，簡 稱2D鏡、6D鏡等，一般是根據試戴鏡片作定位，最常用是3D鏡，若3D鏡片不能居中，可試用4D鏡，如此類推。若鏡片比角膜中央最平曲率平2D，為了使 病人能在戴鏡後有清晰視力，鏡片的光度亦減了2D近視。鏡片的各弧度的寬度，可因需要而改動，基弧的寬度一般是6.5mm，若小於6.0mm，可能當瞳孔 擴大時便看見重影；若基弧寬度大於7.0mm時，由於受力面積大而使壓強小，效果不佳。 　　第二代的角膜塑型鏡片，面世後的十年，先後出現了八種以上不同的設計，由最早期的壓力方法到後期的靜水壓(Hydralic Pressure)方法。各種方法中，尤其以靜水壓方法最具爭論性並且最複雜和難以理解，因為其是利用比角膜曲率彎的鏡片，而光學區外的弧度則切割成非球面，可將近視減輕2D左右，這方法，亦是現時的第三代鏡片設計基礎。 　　上述例子是第二代鏡片中最常用的壓力法設計，這種鏡片對於1D左右的近視效果佳，亦能對付2D左右的角膜散光，這是第三代鏡片所不能取代的。 3　第三代角膜塑型鏡片 　　是結合了第二代的壓力方法和靜水壓方法而設計的。若分別應用這兩種第二代鏡片，每種 可將近視降低2D，而兩種方法聯合應用得出的第三代鏡片，可減去約4～5D度近視。而最早的第三代鏡片，於1998年初面世，由於鏡片可於數天內將近視減 去2～3D，在亞洲地區引起了極大反應，而接下來不到一年的時間，在美國出現了多種品牌的第三代鏡片，設計原理基本大同小異，其設計如下：基底弧(壓力 弧)：4～5D為中央的最平曲率，寬度是 5.8～6.3 mm，一般設定為6.0mm，由於鏡片居中良好，可訂製光學區較小的鏡片。第二弧(增效弧)：根據鏡片的壓力不同而改變，一般可比壓力弧彎6～12D甚至 更多，寬度是 0.6～0.8mm，一般是 0.6mm。第三弧(穩定弧)：一般與角膜7～9mm弧區的彎度相近，亦可製成非球面，作用是令鏡片居中。最合理而有效的鏡片，其穩定弧應該根據角膜矢高 而切割，藉弧度向上下移位而調節鏡片的鬆緊。外周弧的一般數值在10.5～12.0mm，寬度是0.4mm，其作用與所有的透氣硬片一樣，促使淚液在鏡下 交通，但鏡片外周弧設計過鬆，亦可令鏡片難於居中；外周弧過緊，則令鏡片固定不移動，嚴重影響效果。所以一般的OK鏡設計，外周弧都較緊，一般是10.5 至11.0mm，若鏡片過緊，可通過簡單的人工打磨，將鏡片改成11.5或12.0mm。第三代鏡片的作用機理，是利用基弧比角膜中央曲率平4～5D的鏡 片，產生一股壓力，根據容量恆定原理，若角膜中央被壓平，則多出來的容量必定向四面找出口，而增效弧的彎度，便剛好在光學區外周形成一空間，產生一種負壓 拉力作用，這是將靜水壓方法用於第三代鏡片的依據。增效弧的彎度，不同鏡片的設計者亦有所不同，其設計原則是，若鏡片基底弧寬度是6.0mm，而鏡片基弧 比角膜中央平1mm(例：角膜曲率為7.80mm,鏡片基弧是8.80mm)，則角膜中央6.0mm區向後推1.00mm後，便將一定量X體積的組織推 開，而增效弧的曲率設計，便要令角膜6.0mm至6.6mm這一立體圓環(增效弧區)的容積等於X。若增效弧區太少，多餘的組織便要再往外找出口，令鏡片 易偏離中央；若增效弧區空間太多，鏡片亦會難於緊貼角膜而影響療效。而實際上，由於角膜表面並不是球體，同樣的壓力，其X值亦因角膜的基礎值不同而成為一 個變數，所以鏡片的設計者都要將設計的概念輸入電腦進行計算。鏡片的穩定弧曲率，是根據角膜中央最平曲率、角膜散光的度數、偏心率的大小、角膜曲率的基線 大小等綜合計算而得出，數值一般在角膜最平曲率的±0.55mm之間。第三代OK鏡片，由於比第二代OK鏡多了一條增效弧，限制了鏡片光學區壓力所推出的 組織繼續向外擴展，不會干擾穩定弧的穩效作用，因而第三代OK鏡片一般可配戴數小時，使近視下降4D左右，並維持約十二小時的效果。而穩定弧的設計，使內 弧緊貼角膜，利用了表面張力、大氣壓力、眼瞼壓力等克服了重力作用而使鏡片居中。而部分醫生，甚至通過加大鏡片壓力使近視下降6D左右，但這樣做，必須要 掌握好鏡片的作用原理，對鏡片的設計稍加修改才能成功。由此可見，第三代角膜塑型鏡片的效果，是根據鏡片的設計而起作用，並不是某公司的品牌比其他公司的 品牌優勝。最理想的鏡片，是經過配鏡醫生經驗或通過使用試戴鏡片，再結合角膜地形圖的表現作出綜合分析而設計的鏡片。 　　現時，中國的OK鏡片使用量，可能已達到世界之最，但由於缺乏鏡片設計者及生產廠商的支持，以致鏡片配戴的效果不盡如人意。因而，在此我將鏡片設計的理論公開，希望中國的醫生能通過實踐，今後可設計及生產出更合適的鏡片。 4　第四代角膜塑型鏡片 　　近日曾有醫生提出，第四代的角膜塑型鏡片即將面世，可利用藥物注射入角膜，使之暫時 軟化，再用鏡片矯正視力，效果可維持數年。這一技術引起醫學界及視光學界極大的關注。而實際上，關於這門技術，美國正在進行臨床研究，效果不錯。所用的藥 物，是一種非常便宜而且安全性很高的老藥，並不是新藥，但有一點要注意，採用這一種第四代的產品(Corneoplasty,角膜成形術)，已失去了角膜 塑型的無創傷性原意。另外，醫生在使用前，必需完全掌握第三代鏡的設計及難題解決方法。不然，用藥後因鏡片效果差而引起嚴重散光、重影等副作用，可能令求 醫者要忍受數年，或需要再接受另一次藥物注射後，重新採用第四代OK鏡技術矯正。現時在美國，第四代鏡片的發明者，現正研究將針藥改為外用藥水，用棉球沾 藥敷在角膜表面使之軟化後，再使用OK鏡將近視降低，這種技術現時正在申請專利，相信在一年後會向富有OK鏡設計經驗的醫生提供。 5　影響角膜塑型鏡片療效的因素 5.1　鏡片的生產技術　現時能製作OK鏡片的先進 數控車床不多，部分機器製造商更吹噓某品牌的數控車床，能精確切割出各種鏡片，不須拋光。此點十分值得商榷。現時所有鏡片切割車床，皆是用鑽石刀在高分子 聚合物的原料上切出不同弧度，切割口相對於幼嫩的眼角膜來說，皆是尖銳的，所以不能盲目相信某種品牌的電腦車床所切割出的鏡片，無須再拋光。現時有部分新 成立的鏡片生產商，已切割出令人眼刺痛的鏡片。因而，若醫生給病人試用鏡片十分鐘後，仍見戴鏡者流淚，結膜明顯充血，便應將鏡片除下，在裂隙燈下詳細檢查 鏡片的邊緣。無須拋光而能生產高質鏡片，相信要在日後出現了激光車床及耐高溫並具有高透氣的原料後才能實現。 5.2　鏡片的拋光技術　醫生根據驗光結果訂鏡，在驗光時可能出現誤差，鏡片製作過程中亦可能出現誤差，若是輕微的誤 差，對鏡片的效果影響不大，但可令戴鏡者得不到清晰視力。有人認為，鏡片是晚上配戴，病人根本不須看物，所以鏡片全部設計為平光，即沒加上屈光度，這是一 種徹底錯誤的概念。戴鏡時視物不清，亦失去了OK鏡片的意義。由於鏡片的基弧及屈光度計算，並不是影響鏡片設計的重要數據，所以鏡片設計者或生產商都會將 數據公開，醫生一定要根據這公開數據初步計算鏡片設計是否合理。例：-3.0DS， K=43.00/44.00,鏡片的基弧應是8.60mm (39.25D)，屈光度是+0.75D。或可以是基弧8.44mm(40.00D)，屈光度是平光。不可能偏離太遠。 　　若驗光數據與鏡片的數據吻合，但病人戴鏡後仍看不清，而鏡片居中，裸眼視力改善情況滿意，便需要作人工拋光鏡片，將鏡片的屈光度稍作修改，令病人的戴 鏡視力良好。另外，由於鏡片的設計是根據醫生的驗光數據而設計，鏡片戴在角膜上可能出現過緊情況而影響鏡片的作用，可以通過簡單的拋光技術改變鏡片的外周 弧、穩效弧甚至增效弧以作改善，人工拋光技術簡單易學，學會後可提高配鏡的成功率。 5.3　角膜地形圖儀的選擇　角膜地形圖並非價錢越高越好，應儘可能選用可測量角膜矢高的儀器，有助於日後的鏡片設 計。目前角膜地形圖儀的品牌很多，每部售價12萬至35萬元不等，每部機都聲稱可提供角膜矢高。但實際上，大部分的角膜地形圖儀，都通過計算機估計出角膜 矢高，並不是實際的數值。一般的角膜地形圖儀，都是利用牛頓環(Placido Ring)原理，拍攝角膜表面的圖形，將數值通過電腦計算而得出圖像。大部分的地形圖儀，其牛頓環數目為12～22圈，為了方便醫生的操作，其設計都是對 焦鏡頭遠離角膜，用一個鏡頭攝取數據，缺乏立體成像的作用，不可能準確量出角膜矢高。而真正要量出矢高的角膜地形圖儀，應該是對焦鏡頭較接近角膜，並利用 兩點以上的光源對焦或利用多次攝像的結果綜合計算，才能量出真正的矢高。若將牛頓環的直徑縮小，對焦的光線從角膜兩側發出，讓光線被角膜頂端阻斷後再接 通，便能測出真正的矢高，現時採用這樣設計的機種不多，我曾試用了一部國內自行設計的角膜地形圖儀，便是採用了這一原理製成，相信能在近月面世。 5.4　螢光素的分析　螢光素的分析是配合OK鏡的重要一環，要成為一位角膜塑型專家，一定要細心解析和理解螢光素的 表現。若單看鏡片是否居中，無須使用螢光素。在滴螢光素後，要用藍光看鏡片對角膜所形成的壓力、壓力是否在瞳孔區，若有懷疑，可交替快速用藍光和白光檢 查，便可證實鏡片的壓力區位置；除此以外，要注意鏡片的第二弧區(增效弧區)內淚液的飽滿程度、第三弧區(穩定弧區)的鬆緊程度、外周弧度近端的鬆緊…… 等等。各種表現均可直接影響鏡片的活動度、居中性和效果，而這種種表現，很多時可通過簡單的人工拋光去改善。所以，作為一個角膜塑型醫生，一定要細心分析 螢光素表現，找出解決困難的辦法。 5.5　影響配鏡效果的其他因素　無論是角膜塑型鏡片或普通的硬性隱形眼鏡，除了鏡片的設計外，還要考慮很多影響因 素。角膜地形圖的分析，這一點非常重要，不應將地形圖儀作為角膜曲率儀使用，角膜地形圖儀所量出的中央曲率，很多時不及一部廉價的角膜曲率儀準確。我們需 要分析地形圖的變化，找出解決難題的方法，而不是用地形圖儀去量角膜中心曲率。 　　正確掌握試片的使用方法也很重要，角膜塑型鏡片的成功，最重要一點是令鏡片居中，而試片的應用，不是觀察試戴鏡的效果和鏡片對角膜中央的壓力，這一點 在國外很多醫生亦產生誤解。無論任何品牌的鏡片，使用試片的目的，皆是找出一對能居中的鏡片，然後根據這對鏡片的穩定弧數據去訂做合適的角膜塑型鏡片。鏡 片的拋光技術，配戴任何設計的透氣硬片，失敗例子中，約有百分之二十是可通過人工拋光解決，因為最好的醫生，驗眼也有誤差；最好的地形圖儀，結果也有誤 差；最先進的工廠，製作鏡片時也有誤差；甚至一切都很準確，鏡片戴在角膜上還可能出現屈光不准的情況，很多時要用人工去解決；吸管可在特殊情況下使用，應 教會病人用手法戴除鏡片，不應依賴吸管，吸管使用率越高，鏡片的壽命越短；護理液選用亦是重要一環，現時較常用的鏡片護理液，多含有一些廉價的防腐劑，較 易引致過敏，如洗必泰(CHLORHEXIDINE)、硫柳汞(DIMERCAPAL)、雙胍類(BIGUAN- IDE)、EDTA等。容易引致眼睛不適，結膜充血及眼睛分泌物增多，應儘可能採用不含上述成份的護理液。與軟性隱形眼鏡及普通硬性隱形眼鏡不同，第三代 OK鏡片的增效弧存在，使清洗鏡片較困難，在美國、台灣、日本、香港以及新加坡、馬來西亞等地，只有極少數醫生選用多功能的護理液(即洗鏡片與泡浸鏡片用 同一瓶護理液)清洗OK鏡，因為曾有資料顯示，多功能的透氣硬片護理液，其清潔油漬的能力與水相近。很多時候，醫生將鏡片對著燈光看，會發現鏡片的增效弧 內側有很多粘液，這現象可影響鏡片的活動度和療效。所以，清洗第三代OK鏡片，應儘可能選用單一的清潔液配以鏡片浸洗液，避免採用多功能護理液。 　　除了上述的常用第三代鏡片外，亦有設計者利用角膜偏心率(e-value)去設計角膜塑型鏡片，此種鏡片為瞭解決第三代鏡片的增效弧(第二條弧度)過 緊情況，特意在增效弧和基弧之間鑽了1～3個小孔，希望能增加淚液的交通。可惜事與願違，在實踐中得知，三個孔的鏡片，不單解決不了鏡片過緊的情況，而且 還因戴鏡者難於清洗鏡孔的內側，以致粘液和蛋白沉積，使鏡片更易與角膜粘連，效果適得其反。另外，由於透氣孔的存在，不能利用吸棒作真空固定鏡片，若鏡片 不合適，便難以作人工拋光，只能將鏡片拋棄。角膜塑型技術，是普通透氣硬鏡發展的一個高峰，現時並無一本有價值的參考教材，要掌握這一門技術，除了要理解 鏡片設計、作用原理、學會如何修改訂鏡數據以及掌握人工打磨鏡片技術外，還應掌握好角膜地形圖的閱讀和分析方法、戴鏡、除鏡、吸管應用、護理液的選用、正 確鏡片清潔方法……等等。不能迷信於某一品牌的鏡片是最好的。 作者簡介：張主君(1959-)，男，香港人，鏡片設計師，主要設計RGP鏡片。 張主君（香港視朔光學有限公司，香港） 收稿日期：2000-02-14 修回日期：2000-03-22

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孩子，我真希望你玩得好

蔡詩萍／一個女兒的老爸

我應該一輩子會記住那畫面吧。雖然，我不一定有把握能像那位父親一樣，做了這麼大的決定。

那年我剛結婚，還沒小孩。

我到台南府城主持一項文化活動的頒獎典禮，一位得獎藝術家，領到獎牌後，聲音哽咽。他說，兩年前舉家遷到中部一座山城，離開台北是家庭重大決定，尤其陸續要上小學的兩個孩子，放棄台北整體較好的教育環境，無論對哪位家長都是艱困的抉擇。但他還是想給孩子多接近大自然的童年，終於搬遷到那座山城。

他哽咽的繼續說，搬家後，兩個孩子每天徜徉於天地之間，在綠草、白雲、山坡、小溪裡認真嬉戲，認真遊玩，這些場景每每觸發他對藝術創作更多的新鮮體悟，而孩子在這些自然環境裡，發乎內在的跑跳喊叫，驚訝於蟲鳴鳥叫的稚嫩歡呼，尤其讓他覺得這選擇，於孩子們，將是一輩子的「童年記憶」。而今，有多少孩子，能有「真正的童年」呢？

最後他依舊哽咽的說，也許有一天孩子終將回到城市上學，但他們會記得這一段玩耍的歲月。

我一直記得這段畫面。許是我自己從小到大的經驗吧。我是眷村小孩，但我童年的暑假，都戲耍於客家聚落的外婆家，成天瘋進瘋出地，老媽老爸管不著，外婆外公疼惜長外孫，每個暑假我都是在陽光下、田埂上、池塘裡、穀倉屋頂、竹林叢中，穿梭度過的。就像那位藝術家父親的感嘆，以後，我或許有了城市、大都會、乃至於國外的種種人生經驗，然而童年於遊戲中、於自然環境裡瘋進瘋出的歲月，卻是不復可得。若每個父母，都從這體悟去想，也許，我們就會想方設法，給孩子留一段徜徉自然，舞於天地之間的記憶吧。

我是從這層體會，發現了城市一隅的，雲門的「生活律動」。

而今，於多數人，能夠像我那樣，每年有一段客家聚落的寒暑假，不容易了。能像那位藝術家父親，舉家遷至有山有水有蟲有鳥有草地之小山城的果斷或機緣，亦不容易了。

我們相對容易的機會，或許就是如何於置身城市的命運裡，積極去為孩子們找尋一處，可以「類似自然處境」下的身體律動環境，看他們在「想像中的情境裡」，或奔、或跳、或舞。

我在雲門教室裡，看每個孩子揮灑的肢體遊戲，竟會有一股「遲來的感嘆」！我是個「很不會跳舞」的男人，儘管我極愛運動，但一面臨肢體語言的律動時，總是感到束手綁腳，渾身不對勁。偶而於旅遊中，在飯店韻律教室跟老師上一堂課，也多半是以運動出汗為目的。可是也往往會在那一小段時間裡，突然觸動到一點「韻律感」的愉悅，明顯感受到自己的身體，是在「享受」一種內在的和諧。

人，應該是有天生的律動感吧。不然，不會每個小孩子，一聽到音樂，總能舞手舞腳地，隨興跳出一段逗引父母哈哈笑的舞蹈。我相信我小時候應該也是這樣，差別只在，我少了一份有老師引導的機會而已。

我喜歡開著車，穿過街衢，走過人群，牽著我女兒，看她喜孜孜地，走進雲門教室「生活律動」上課，那彷彿是她的舞台，她的手舞足蹈全無禁忌的肢體世界。我在一旁陪著，遂也墜入了每個父親都曾有過，都該有過的夢想，給孩子、給自己一個「有韻律感」的人生。

即便，那只是城市生活裡的一小段時光；即便，那終將是孩子生命裡的一小段童年而已。