［摘要］全球大约有八千万人(约占总人口的1.5%)在配戴角膜接触镜，镜片的透氧性及配戴角膜接触镜后角膜的获氧状态是保证配戴安全性和有效性的关键。角膜需要多种营养物质以保证其正常的代谢功能，氧气是其中是重要的部分，只有在充分氧供的状态下，角膜才能保持稳定的78%的水合状态，保证角膜透明性。为了进一步了解氧气与角膜接触镜配戴的关系，本文着重对角膜接触镜所致的角膜缺氧的主要表现及角膜接触镜的透氧性能及其表达指标作一综述。

配戴角膜接触镜后，角膜的氧供明显减少，特别在闭眼状态下氧供减少尤其明显，角膜接触镜引起的角膜缺氧变化主要表现为：①角膜上皮：角膜上皮水肿，即因细胞间隙中水液充盈而发生的微囊样水肿，水肿明显时会影响视力并使角膜的敏感性下降；②角膜基质：角膜基质水肿达5%以上即可从裂隙灯检查中发现，主要表现为条纹或皱褶，长期慢性水肿会诱发角膜新生血管的发生，逐步发生角膜疤痕、屈光改变等一系列问题；③角膜内皮：慢性角膜缺氧最终会导致角膜内皮细胞形态的改变，而且无法代偿。保持角膜功能的最低需氧量称为“临界氧”，理想的角膜接触镜应该是通过该镜片后到达角膜面的氧供超过临界氧。

角膜接触镜的透氧性能表达指标主要有：透氧性(oxygen permeability,Dk)、氧传递性(oxygen transmissibility,Dk/t)和等效氧(equivalent oxygen permeability,EOP)，根据近期的研究结果，只有镜片的Dk/t高于(87.0+3.3)×10^-9(cmXmlo2/secXmlXmmHg)或EOP达到17.9%，才能保证角膜获得临界氧，目前的日戴镜片的Dk/t为(24.1+2.7)×10^-9(cmXmlo2/secXmlXmmHg)或EOP达到9.9%，所以不能用于长戴，而只能安全用于日戴。80年代初期，研究者致力于RGP镜片材料的开发，添加了硅和氟成分，从而极大提高了RGP镜片材料的透氧性，使得角膜接触镜长戴成为可能，并用于白内障术后等特殊镜片配戴选择。根据文献报道，日戴人群中角膜溃疡的发生率为0.04%，长戴人群中发生率大约为0.21%，长戴者发生率为日戴者的10～15倍，其主要原因是因为在软镜长戴过程中出现缺氧而导致角膜溃疡的发生。

新一代高透氧的角膜接触镜开始出现，其透氧性可以高达(100～175)×10^-9(cmXmlo2/secXmlXmmHg),含水量为26%～34%，据初步临床测试，过夜长戴后，角膜的水肿只达2.2%(非戴镜眼为1.8%)。当然理想的角膜接触镜除了高透氧性外，还需具备舒适、抗沉淀物、方便等特性。

Oxygen and Contact Lens Wear

Jianhua Wang,BA Med;

Desmond Fonn,Dip Optom,Moptom;Renee Du Toit,Dip Optom,MPhil

AbstractIn this article we review the role of oxygen and contact lens wear related to the metabolic processes of the cornea.The critical oxygen requirement of the cornea for maintenance of its normal hydration level to ensure transparency is discussed,as well as the subsequent reversible and irreversible changes to the corneal layers if this requirement is not met.We review the concept of oxygen transmission and its application to rigid gas permeable and soft contact lens material.Corneal responses to induced hypoxia related to extended wear of contact lenses as well as other complications,such as infection,are discussed.In conclusion,we comment on the oxygen performance of a new generation of silicone hydrogel contact lens materials and the future of extended wear.

1Introduction

There are approximately 80 million contact lens wearers world-wide which represents about 1.5% of the world's population.Between 5% and 12% of people in most developed nations use contact lenses and approximately 80%～90% of these people wear soft hydrophylic lenses.Soft lenses are generally more comfortable and therefore adaptation is easier,although rigid gas permeable (RGP) lenses supply much more oxygen to the cornea through transmission.

Contact lenses provide safe and effective vision correction if appropriate materials,lens designs and systems are prescribed.In addition patients have to wear and care for lenses appropriately to minimise the risks of complications.Some lens wearers however,find it inconvenient to insert,remove and care for their lenses on a daily basis and would like the convenience of “permanent" vision correction.This is probably why approximately 10% of contact lens weaters use their lenses on an extended wear basis,regularly sleeping with the lenses one to 6 nights per week.The majority of practitioners and patients are however concerned about the safety of extended wear and many practitioners are still reluctant to prescribe this mode of lens wear^［1］.

History has shown that extended wear is far from problem-free.One of the problems with soft contact lens extended wear has been corneal hypoxia induced by low oxygen transmission of contact lenses.The combination of reduced oxygen levels under the closed lid,with the restriction on available oxygen created by the presence of a low transmissible contact lens produces extended periods of hypoxia.Physiological changes of the cornea arise from hypoxia and may have both reversible and irreversible effects.To minimise these effects it is important to increase oxygen transmissibillity of lens materials^［2］.Until recently this was only possible with RGP lenses.

2Corneal oxgyen supply and metabolic process

The cornea requires a variety of nutrients for metabolic function and oxygen is probably the most important.Under aerobic conditions corneal metabolism generates sufficient energy for deturgence i.e.the cornea remains at a constant 78% hydration level which ensures transparency.Another reason for transparency is the fact that the cornea is avascular,unlike most tissues.It cannot therefore rely upon blood supply to transport oxygen and the other necessary nutrients to the site of metabolic activity.The aqueous is the principal source of the majority of metabolites such as glucose,amino acids,vitamins and minerals.

While the eye is open,the atmosphere is the major source of corneal oxygen by diffusion through the tear layer.The concentration of oxygen as a percentage of the atmosphere is 21% but this expression of oxygen may be misleading because it ignores pressure.Another term that is frequently used to represent oxygen supply is partial pressure (PO₂) The partial pressure of oxygen at sea-level is 155mmHg but at the top of Mt Everest,for example,it decreases to 53mmHg.Yet the concentration for both of those levels is 21%.

During eye closure,atmospheric oxygen is essentially eliminated and the cornea has to derive oxygen from other ocular sources.These are the limbal and deeper cilliary blood vessels but the bulk of the oxygen for epithelial needs is derived from the palpebral conjunctiva.The aqueous supplies oxygen to the endothelium which is more critical when the eye is closed.The PO₂ levels within the blood vessels of the palpebral conjunctiva and aqueous at the endothelial surface is approximately 55mmHg^［3～5］.This translates to an oxygen concentration of about 7% which means that the cornea has to function under anaerobic conditions during eye closure.Contact lens wear essentially simulates this condition,particularly if the lens has low oxygen transmission.

Anaerobic metabolism results in an energy reduction of about 90% compared to aerobic metabolism.This translates to an inefficiency of expulsion of the by-products of metabolism such as lactate,which is an important component in the etiology of corneal edema^［6］.Lactic acid accumulation induces increased osmotic pressure of the corneal stroma resulting in imbition of fluid.Lactic acid,in combination with carbon dioxide build-up,causes acidosis^［7］Hypoxia and therefore anaerobic metabolism results in a lower yield of adenosine triphosphate (energy) which decreases the cornea's fluid pumping function in the epithelium and endothelium.This results in additional corneal swelling^［8］.