Dentistry by Laser Light

When dentist Richard Hansen goes after a cavity or cleans a root canal， his patients barely hear a thing. Instead of a whiny turbine drill， he uses a flash of laser light to blast away dentin and enamel. Hansen， director of the Center for Advanced Dentistry in Fullerton， California， is convinced that the laser is the dental tool of the future-precise， inherently sterile， less painful， and perhaps less damaging to teeth. Above all， he says， it puts the person in the chair at ease: \"What patients like most， of course， is that the laser is silent.\"

For now， this is still a rare luxury. Just 3.5 percent of the 7，553 dentists recently surveyed by the American Dental Association use lasers for surgical procedures. The steep cost and limited appli cations of early dental lasers have left many of them skeptical. But the tide is turning， as designs have improved and the Food and Drug Administration has certified lasers for more applications. In 1990 the agency allowed laser cutting of soft tissue such as gums. In 1997 the FDA approved lasers that remove tooth decay and earlier this year gave the thumbs up to laser treatments for root canals and other hard-tissue procedures. Meanwhile， engineers are striving to trans- form the laser into an all-purpose dental tool， useful for everything from treating gum disease to finding cavities before they surface.

A dental laser shines a powerful beam through a fiber-optic cable connected to a hand piece resembling a standard drill. The beam emerges from the tip， shining onto the tooth or gums. Water molecules there absorb the energy from the beam and convert it into heat that cuts or abrades. But the best way to impart energy to the water molecules depends on the type of tissue. The inflated beam from an erbium laser is absorbed efficiently by the tooth but poorly by the gums. The blue-green light of an argon laser reacts more efficiently with hemoglobin， so it cuts gums effectively and helps halt bleeding. Different jobs， therefore， call for different lasers. Keith Murray， a laser physicist at NASA's Langley Research Center in Hampton， Virginia， is working on a more elegant approach. He has designed an adjustable laser that can switch instantly between soft-tissue and hard-tissue wavelengths.

Other researchers are bypassing the difficulties of laser drilling and concentrating on soft-tissue applications， where lasers offer some significant benefits. Lasers cauterize incisions， reducing bleeding and infection， sealing off nerve endings， and minimizing postop- erative pain. Brian Wilson， a biophysicist at the University of Toron to， is expanding the repertoire of soft-tissue lasers to include applications for severe gum disease， which affects 5 to 20 percent of all Americans， especially teenagers and young adults.

Normally， dentists clean out infections between the teeth and gums by making an incision and inserting pellets of antibiotics. In- stead， Wilson proposes using lasers and other light sources for photodynamic therapy， originally developed in cancer treatments. The dentist would inject the infected parts of the gums with a dye molecule that remains inert until illuminated with a laser or other light source. Once energized， the dye forms singlet oxygen， a reactive molecule that kills bacteria. \"This process does away with complicat- ed， painful surgery. Also， the dye is not an antibiotic， so I don't believe bacteria will become resistant，\" Wilson says. In a recent lab test， photodynamic therapy killed 99.9 percent of the bacteria that cause gingivitis — in 10 minutes. Clinical trials are scheduled for later this year.

Lasers might also be useful for diagnoses. \"With fluoride treatments and other improvements， we rarely see big gaping holes in people's teeth，\" says Andreas Mandelis， an applied physicist working on lasers at the University of Toronto. In 2000， KaVo， a medical-imaging company in Germany， began selling a simple decay detector in the United States that supplements conventional X rays and the dentist's sharp explorer probe. Called Diagnodent， the $ 2，800 machine illuminates the tooth with a low-power red laser and measures whether the enamel glows， or luminesces， in response—a sign of bacterial degradation. About 5，500 units are in use in this country.

Diagnodent can distinguish between stains and incipient cavities， but it cannot locate decay more than about one-tenth of an inch into the tooth， because laser light is easily scattered. Mandelis and Toronto dentist Stephen Abrams are developing a more effective laser-luminescence cavity detector by looking at thermal effects as well.

Their technique uses a longer-wavelength infrared laser beam， which heats the tooth in addition to making it glow. Heat does not scatter as readily as light， so the thermal energy can penetrate through the enamel， exposing changes in composition. An infrared detector measures how the heat travels through the tooth while a light sensor picks up the fluorescent signal. The combined information allows a three-dimensional look at the tooth. \"It is more accurate than dental X rays， without the side effects，\" Abrams says. Mandelis's group has experimented on only extracted teeth， but he expects a commercial device to be available in about five years.

What will the world of tooth care look like then? Some dentists wonder how much will have changed. \"Lasers are not the panacea that has been presented in the media. It's a new and potentially promising technology that will need several years of research and im provements before I'll feel confident using it on my patients，\" says Jerry Gordon， a dentist in Pennsylvania. But Richard Hansen thinks laser precision will spark a revolution. \"What we've done in the last few years with lasers is to prevent the need for root canals，\" he says. \"Soon we may be able to eliminate most adult dentistry entirely. \"

注釋：

1．cavity n．洞，空穴，[解剖]腔

2．turbine n．渦輪

3．dentinn．[解]象牙質(zhì)，牙質(zhì)(的東西)

4．enameln．琺瑯

5．FDA=Food and Drug Administradon(美國)食品及藥物管理局

6．infrared beam 紅外光束

7．erbium n．[化]鉺

8. argon laser 氬激光器

9．hemoglobin n．血色素

10．gingivitis n．齒齦炎

11．fluoriden. 氟化物

12．luminescev. 發(fā)冷光，變明亮

13．incipient adj.初始的

14．fluorescent adj. 熒光的，瑩光的

當(dāng)牙醫(yī)理查德·漢森檢查齲洞或清洗牙槽的時候，他的病人只會聽到一點聲音。他用的是一道激光而不是煩人的渦輪鉆來清除象牙質(zhì)和牙齒的琺瑯質(zhì)。漢森是加利福尼亞福勒頓高級牙科中心主任。他確信激光是未來的醫(yī)牙工具——精確，無須消毒，痛苦小，可能對牙齒的傷害也小。他說，最重要的是激光醫(yī)牙使患者舒舒服服地坐在椅子里就可以了?！盎颊咦钕矚g的當(dāng)然是激光沒有任何噪音?！?/p>

就目前來說，這依然是一件非常奢侈的事。根據(jù)最近美國牙醫(yī)協(xié)會的調(diào)查，在7，553名牙醫(yī)中，只有3.5％在外科治療過程中使用激光。過高的費(fèi)用和早期激光醫(yī)牙的有限使用留下了很多疑問。但是潮流在逆轉(zhuǎn)，由于設(shè)計的提高，食品和藥品管理部門 (FDA)已經(jīng)鑒定了激光有更廣泛的用途。1990年，這個機(jī)構(gòu)允許使用激光切割像牙齦這樣的軟組織。1997年，F(xiàn)DA批準(zhǔn)激光用以切除牙齒腐爛部分。而今年早些時候，F(xiàn)DA又同意將激光用于治療牙槽和其它硬組織的治療過程中。同時，工程師們正努力地將激光改造成一種通用的牙科工具，使它從治療牙齦疾病到牙洞產(chǎn)生之前及時發(fā)現(xiàn)都極具用途。

醫(yī)牙激光通過連在手上類似一個標(biāo)準(zhǔn)牙鉆的東西上的光導(dǎo)纖維索而發(fā)射出強(qiáng)大的光束。光束從頂端射出，照在牙齒或牙齦上。水分子從那兒吸收來自光束的能量并將其轉(zhuǎn)換成可以切割或打磨的熱量。但是，將能量傳送給水分子的最佳途徑還在于組織的類型。來自鉺激光器的紅外光束能夠被牙齒有效地吸收，但是牙齦卻不行。氬激光器發(fā)出的藍(lán)綠色的光線對血紅蛋白能夠更有效地起作用，因此它可用以刺穿牙齦并有助于止血。所以，不同的工作需要不同的激光。在弗吉尼亞州漢普頓市的美國國家航空和宇宙航行局的蘭利研究中心就職的激光物理學(xué)家基思·默里正致力于研究出一種更適宜的方法。他已經(jīng)設(shè)計出一種可以調(diào)整的激光，它可以針對軟、硬組織迅速轉(zhuǎn)換波長進(jìn)行治療。

其他研究人員正設(shè)法回避激光鉆牙的困難，而集中精力于軟組織的治療上，因為在這方面激光可以提供一些重要的幫助。激光可以麻痹傷口，減少流血和感染，封閉神經(jīng)末梢，將術(shù)后痛苦降低至最小。多倫多大學(xué)的生物物理學(xué)家布賴恩·威爾遜將適用于軟組織的激光治療技術(shù)擴(kuò)展到嚴(yán)重的牙齦疾病的治療上，這種牙病影響著5％到20％的美國人，尤其是少年和青年人。

一般情況下，牙醫(yī)是通過切開一個小口，在里面敷上抗生素丸來清除牙齒和牙齦之間的感染的?？墒?，威爾遜卻建議用激光和其它光源進(jìn)行光力學(xué)治療，這種療法最開始是在癌癥治療中得以發(fā)展的。牙醫(yī)一般用染色分子對牙齦被感染的部分進(jìn)行注射，而這種染色分子在用激光和其它光源照亮之前一直都是遲鈍的。一旦活躍起來，這種染色分子就會形成純態(tài)氧，這是一種可以殺死細(xì)菌的活性氧。威爾遜說：“這個過程可以免除復(fù)雜、痛苦的外科手術(shù)。而且，這種染色體不是抗生素，所以我確信細(xì)菌不會再有抵抗能力?！痹谧罱囊淮螌嶒炇覝y試中，光力學(xué)療法在10分鐘之內(nèi)殺死了一種引起齒齦炎的病菌的99.9％．臨床實驗也定在今年的晚些時候進(jìn)行。

激光也還可以用于診斷。多倫多大學(xué)致力于激光研究的應(yīng)用物理學(xué)家安德里亞·曼德里斯說：“用氟化物療法和其它改進(jìn)了的療法，人們的牙齒之間就不會再有大豁口了?！痹?000年，德國的一家醫(yī)療成像公司開始在美國銷售一種蛀牙探測器用來補(bǔ)充傳統(tǒng)的X光和牙醫(yī)鋒利的探針。這種機(jī)器被稱作Diagnodent，價值￥2，800，它可以用低功率的紅外線照射牙齒，并且測出牙齒的琺瑯質(zhì)是否發(fā)熱或發(fā)亮，這是細(xì)菌性降解的相應(yīng)反應(yīng)。在美國，約有5，500套這種器械投入使用。

Diagnodent可以辨別牙斑和初期的牙洞，但是因為激光很容易分散，所以Diagnodent找不到牙齒里1/10英寸長的腐爛處。曼德里斯和多倫多的牙醫(yī)斯蒂芬·阿布拉姆斯通過考慮熱量效果來開發(fā)一種高效的冷光激光牙洞探測器。他們的技術(shù)是使用一種波長較長的紅外線激光器光束，除了使牙齒發(fā)亮之外，還可以加熱牙齒。熱量不像光那樣容易分散，所以熱能可以穿透琺瑯，暴露牙齒結(jié)構(gòu)中的變化。紅外線探測器測量熱量怎樣通過牙齒進(jìn)行傳播，而光敏感元件可以發(fā)現(xiàn)熒光信號。所以兩者相結(jié)合就可以對牙齒進(jìn)行三維檢查。阿布拉姆斯說：“這比牙科使用的X光更精確，而且沒有副作用。”曼德里斯小組僅僅在拔牙方面作了實驗，但是他希望五年后這種器械可以投入商業(yè)使用。

那么，到那時牙齒保健領(lǐng)域會是一派什么景象呢?有些牙醫(yī)懷疑究竟會有多少變化。賓夕法尼亞州的牙醫(yī)杰瑞·戈登說：“激光并不像媒體報道的那樣是一種萬能藥。這是一種嶄新的、有潛力的技術(shù)，但是在我很有信心地把它用在我的患者身上之前，它還需要若干年的研究和改善?！钡抢聿榈隆h森卻認(rèn)為激光精密會引起一場革命。他說：“我們最近幾年所作的有關(guān)激光的研究就是為了制止在牙根處開洞的需要。也許很快我們就能夠徹底地廢除大多數(shù)成人牙科了?！?/p>