白內障手術及矯正散光的人工水晶體---後角膜散光(posterior corneal astigmatism)

白內障手術及矯正散光的人工水晶體

                      ---後角膜散光

     在我們診所的門診中，當患者白內障嚴重到無法以眼鏡矯正，即可以考慮接受白內障手術治療，將已混濁的水晶體藉由超音波乳化將之移除，並植入人工的水晶體，如果只想要靠飲食或者是藥物治療，絕對是沒有效果的。但是單純的白內障可以藉由植入一般人工水晶體後即可恢復正常視力，但是對於白內障合併有散光度數的患者，過去作法就是在術後配戴散光眼鏡，或是進行準分子雷射LASIK手術矯正散光。近年來較新的技術及觀念是植入具有『矯正散光度數的人工水晶體』，將白內障、近視、遠視、散光，甚至老花眼也一次解決。

     通常專業的醫療院所會藉由水晶體超音波(A-scan)、角膜斷層雷射掃描儀(Pentacam)、眼科裂隙燈、間接性眼底鏡檢查…為患者的眼睛從眼球外到內詳細評估每一個部份功能，讓治療達到最精準的層次，以確保病患術後獲得良好視力。最近門診中一位屈先生即將要接受白內障手術，屈先生患有白內障合併有250度散光，在檢查當下我們即從電腦驗光值及角膜斷層雷射掃描儀(Pentacam)中發現前角膜(Cornea Front)以及後角膜(Cornea Back)的散光軸度及散光度數，讓原本的預測的散光矯正值，差距達100度，若是未經這種較高階的測量儀器，將很有可能讓屈先生即使置入了自費的『矯正散光的人工水晶體』，手術後仍然還有散光殘留，儘管有較清晰的視力，但手術後仍然需要眼鏡的輔助。

     因此，建議有意植入『矯正散光的人工水晶體』，術前需經醫師謹慎評估患者的散光度數及軸度，哪些是屬於角膜散光？哪些是水晶體散光？角膜性散光中又須考量前角膜及後角膜造成的散光度數、軸度及比例？再加上白內障手術的切口位置本身也會造成散光的度數？……等等，都必須一併考慮計算。透過角膜斷層雷射掃描儀(Pentacam)的詳細檢查數據及執刀醫師的臨床經驗整合下，更有把握可以找出最適合患者的矯正散光人工水晶體的度數及軸度，讓手術變得更可以預測，而不只是碰運氣而已。

     雖然國內健保給付的人工水晶體仍然有其治療效果，但是如果有深度散光卻只使用健保給付的一般人工水晶體，就好像有近視合併有散光的人，配眼鏡時只配了近視卻沒有配散光度數，如此一來，視力當然不夠清晰。同樣的，進行白內障手術後，卻還要依賴散光眼鏡，似乎也就喪失了重獲輕鬆好視力的目的了。

       

       

 

       

此數據為屈先生在尚未經過Pentacam(前房雷射斷層掃描儀)檢查時，右眼總散光值為125度，但經由Pentacam(前房雷射斷層掃描儀)精密掃描前、後角膜數據值後，醫師計算出散光總共應矯正值差別到約100度，並以此數據，值入適當的『矯正散光的人工水晶體』。而屈先生在白內障手術後的散光驗光值，果然達到幾乎是零度的完全矯正狀態，視力清晰程度更是達到1.0以上。

        

Pentacam(前房雷射斷層掃描儀)可以拍出Scheimpflug images(莎姆定律的影像)，即是拍攝眼睛平面、鏡頭平面和成像平面三者交於一線時，此時拍攝眼睛平面上的全部景物都會呈現清晰的狀態，這就是Scheimpflug images。再以電腦分析出前、後角膜表面弧度及散光度數、軸度，角膜厚度、角膜深度及前後角膜的非球面Q值。

參考資料:

Refractive editor's corner of the world
Posterior corneal astigmatism vital to calculating correct total astigmatism

                                                                          

by Erin L. Boyle EyeWorld Senior Staff Writer

 

Posterior corneal astigmatism

Baylor toric IOL nomogram Source (all): Douglas D. Koch, M.D., and Li Wang, M.D.

Not measuring the posterior corneal astigmatism could result in incorrect estimation of total corneal astigmatism, hindering toric IOL selection through overcorrection in with-the-rule astigmatism and undercorrection in against-the-rule astigmatism, researchers found.
Douglas D. Koch, M.D., professor and the Allen, Mosbacher, and Law Chair in ophthalmology, Cullen Eye Institute, Baylor College of Medicine, Houston, and Li Wang, M.D., associate professor, Cullen Eye Institute, Baylor College of Medicine, Houston, are researching the effect of posterior corneal astigmatism and toric IOL selection in cataract surgery cases. Dr. Wang said both posterior and anterior corneal astigmatism measurements are important to all cases undergoing cataract surgery.
"It would be best to measure posterior corneal astigmatism," she said. "The magnitude of posterior corneal astigmatism cannot be predicted based on the amount of anterior corneal astigmatism. If there is no access to a device that measures the posterior corneal astigmatism, the average value of the posterior corneal astigmatism may be used." Drs. Koch and Wang and colleagues published study results on the topic in the Journal of Cataract & Refractive Surgery. They evaluated 715 corneas of 435 consecutive patients, calculating total corneal astigmatism using ray tracing, corneal astigmatism from simulated keratometry, anterior corneal astigmatism, and posterior corneal astigmatism.
They found that toric IOL selection based on anterior corneal measurements only could lead to problems.
"Patients who have anterior with-the-rule astigmatism—in other words, the cornea is steep at 90 degrees anteriorly—tend to have, on average, 0.5 diopter (D) of steepness vertically along the posterior cornea, and because the posterior cornea is a minus lens, steepness vertically translates into power horizontally or against-the-rule effect refractive power at 180," Dr. Koch said. "So you might measure a patient who has 2 D on the anterior cornea. And when all is said and done, that patient may only have 1.3 or 1.4 D on the total corneal power because the posterior cornea throws in about 0.5 or 0.6 D in the other direction."

Measuring devices
Measuring posterior corneal astigmatism is a challenge, Dr. Koch said. Two devices on the market, the Galilei Dual Scheimpflug Analyzer (Ziemer, Port, Switzerland) and the Pentacam (Oculus, Lynnwood, Wash.), measure it "moderately accurately," he said.
"I think that our measurements could improve," Dr. Koch said. "We do find that even the Galilei, which has a wonderful dual Scheimpflug mechanism for measuring the back, does not always seem to capture all of the posterior corneal astigmatism, and especially in patients [who have] with-the-rule astigmatism, it still seems to underestimate the amount of posterior corneal astigmatism based on our actual refractive outcomes."
Dr. Koch has created a nomogram that incorporates: ¹) the mean posterior corneal astigmatism in eyes having either with-the-rule or against-the-rule astigmatism and ²) the effect of against-the-rule drift that occurs with age. He said that their data indicate that the new nomogram greatly improves accuracy with toric IOLs.
In addition, manufacturers are interested in providing clinicians with this information because they are finding similar results retrospectively in their data, he said. However, to disseminate a new nomogram themselves, they would have to validate it in a clinical trial with the U.S. FDA, which could slow the approval process.

Toric IOLs

Dr. Koch began examining posterior corneal astigmatism when he noticed that some patients had unexpected results with toric IOLs. Patients who had with-the-rule astigmatism were being overcorrected, while patients who had against-the-rule astigmatism were undercorrected.
"It has a huge impact on my decision making now in patients who are seeking astigmatic correction during cataract surgery," Dr. Koch said. "It's completely changed everything I'm doing with regard to both relaxing incisions and with regard to the selection of toric IOLs. "I have backed off on toric IOL power in patients who have with-the-rule astigmatism and conversely ramped it up for those patients with against-the-rule. For example, for someone who has 1 diopter with-the-rule astigmatism, I will not put a toric IOL in because I am likely to overcorrect him and leave him with against-the-rule astigmatism," he said. He cited a long-term study by K. Hayashi and colleagues that followed patients' astigmatic change after undergoing 3-mm clear corneal temporal incisions. The study also had a control group that did not undergo cataract surgery. Researchers found that both groups had a comparable change of against-the-rule shift after more than 10 years.

"You would think that a corneal incision temporarily might weaken the cornea such that the cornea would not steepen along the horizontal meridian over time. But in fact it does," Dr. Koch said. "So in planning for our patients, I believe that we need to leave our patients a little bit on the with-the-rule side in order to compensate for the fact that they're going to drift to against-the-rule over time. This will provide them with better uncorrected acuity over a much longer period of time and perhaps serve them well for 20 or more years."