INTRODUCTION
Worldwide reports regarding the basic science and clinical applications of fibronectin’s (FN) contribution to the healing of trauma and burns wounds have shown that FN can enhance the epithelial healing rate by promoting the adhesion and migration of epithelial cells at the wound site [1-3]. Clinical application of eye drops made of FN in treating burned corneal has obtained satisfactory results. However, we are still years from the widespread use of FN because the extraction and production of FN eye drops are time-consuming, expensive and offer specific preservation challenges. We have used MEBO since 1990 in the treatment of burned cornea and we have achieved satisfactory results in terms of improving corneal nutrition, promoting wound healing, ammeliorating pain, relieving irritation and reducing the incidence of corneal ulceration. Therefore, we embarked on an experimental study to verify the effect of MEBO as compared with FN in the management of corneal burns in rabbits.

MATERIALS AND METHODS
MEBO, developed by Beijing Guangming Chinese Medicine Institute for Burns, Wounds and Ulcers, is an ointment containing sesame oil, beeswax and other active ingredients derived from plant such as Cortex Phellodendri and Radix Scutellariae. FN produced by the Shanghai Institute of Biological Products was diluted to a solution containing 400g/ml with sterile normal saline.

Five healthy New Zealand white rabbit (body weight 2-3 kg) without eye disease were anaesthetized intramuscularly with 20mg/kg sodium thiopental and eye solution of 0.4 % Novesine two drops in each eye. 8-mm filter papers, previously soaked completely in 0.5N NaOH, were laid on the middle of both corneal, respectively. One minute later, the paper was removed and the eyes were rinsed immediately with sterile normal saline. Then eye drops made of FN were administered to the right eyes, four times daily and MEBO was applied on the left eyes, three times daily. Both courses lasted 2 weeks.

At 6, 24, 36 and 48 h postinjury, the corneal fluorescent staining zones were photographed with a DF Haiou camera (China) at fixed focus. After developing the film, we drew the fluorescent staining zones, analyzed and measured the areas at different h using a computer image pattern analyzer, and then calculated the epithelial healing rate (mm2/hour) by linear regression. Observations were made twice daily on days 3~14 postinjury and fluorescent staining zones were regarded as a positive indicator. Epithelial damage rate = (fluorescent staining positive number) / (number of eyes examined)  100%.

Besides corneal fluorescent staining zone, conjunctival congestion and corneal transparency were also observed. (1) Conjunctival congestion: + = palpebral conjunctival congestion; ++ = palpebral conjunctival and partial bulbar conjunctival congestion; +++ = whole palpebral and bulbar conjunctival congestion. (2) Corneal transparency: + = slightly opacity with distinct structure of underlying iridial texture; ++ = moderate opacity with blurred structure of iridial texture; +++ = severe corneal opacity with indistinct structure of iridial texture.

RESULT
In the early stage (at 6 h postinjury) epithelial healing was slow and at 6 - 48 h the healing rate remained constant The use of FN on eyes shows an epithelial healing rate of 1.2790.317 mm2/h compared to 1.2850.128 mm2/h with the MEBO treatment. There is no significant difference between the two groups although in the MEBO group treatment efficacy is a little faster.

Repeated corneal damage occurred from the early healing stage until 2 weeks postinjury and there is a significant difference in damage rate in both groups: 58.3 % in the FN group and 33.3 % in the MEBO group (u=5.56, P<0.01).

The cornea showed disk opacity a few minutes following burn. Conjunctival congestion with white secretion was observed at 3 h postinjury. On day 2 postinjury, eye irritation became obvious with increased secretion and corneal edema was noted until 1 week postinjury when edema was still observed in epithelium and matrix. At 2 weeks postinjury, the above symptoms were further relieved. The advantages of MEBO treatment as compared to FN in the management of burns eyes were demonstrated both in terms of corneal epithelial exfoliation and regarding conjunctival congestion and opacity.

CONCLUSION
Some advantages of MEBO include inexpensive cost, convenient application, safety and product stability. The effect of MEBO in the treatment of alkali-burned corneas compared favorably to that achieved with FN. MEBO proved more effective than FN in reducing the epithelial damage rate and was far easier to use given a widespread application. We suggest that MEBO treatment is remarkably advantageous in reducing the repeated epithelial damage rate of alkali burns eyes compared to FN treatment.

DISCUSSION
It has been reported that FN has the effect of promoting epithelial healing rate on corneal defects [4]. This study verified no significant difference in promoting corneal epithelial healing between the two groups. However, MEBO treatment is remarkably advantageous in reducing the repeated epithelial damage rate of alkali burns eyes compared to FN treatment. The repair of relative integrity of the corneal epithelium is beneficial to the stability of corneal parenchyma and endothelium. The cornea in the MEBO group showed slight transparency with slight congestion. Subsequent to alkali burns, the integrity of basement membrane underlying corneal epithelium was damaged. The appropriate replacement of exogenous FN may reduce the possibility of epithelium damage, as FN is needed for repair of basement membrane. This study demonstrated that MEBO is superior to FN with regard to firm adherence of corneal epithelium and maintenance and basement membrane integrity. In addition, MEBO supplies rich nutrients necessary for repairing and regeneration of alkali burns corneas.

FN is a kind of macromolecular glucoprotein which operates at the wound surface. The surge of enzymes released subsequent to alkali burn causes the degradation of FN at the wound surface. The paucity of FN resulting from enzymatic degradation can be compensated with exogenous FN, which reduces the epithelial damage rate by promoting epithelium adhesion. Many combining sites in FN molecular structure served as a bridge between epithelial cells and basement membrane, thereby improving epithelium adhesion. Affinity between FN and intracellular actin may cause the change of epithelial intracellular actin from sphere to fibriform and then induce cell migration.

As avascular tissue, the cornea receives the nutrition and oxygen necessary for metabolism mainly from diffusion of the vascular net in the corneal limbus, from tears and from aqueous humor. Glucose is the main source of energy for oxygenic metabolism of corneal epithelium and for intraparenchymatous anaerobic metabolism [5]. MEBO contains abundant glucose, organic acid, a variety of vitamins, proteins and enzymes as well, all of which directly provide energy and nutrition for alkali-burned corneal tissue. In this manner, MEBO serves to promote metabolism of the corneal, to accelerate the prompt removal of necrotic tissue and to facilitate swift growth of new epithelium. MEBO also has anti-inflammatory properties, bacterial inhibition, repercussive and analgesic effects [6]. By relieving local congestion and corneal irritation, blepharospasm due to pains and nictitation were alleviated, which reduced the friction to corneal surface and also improved local resistance.

REFERENCES
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5. Liu JQ, et al (eds): Practical ophthalmology. Beijing, People’s Health Press, 1984, p43.
6. Xu RX: The medicine of burn and ulcer: A general introduction. Chin J Burns Wounds  Surface Ulcers 1989; 1: 11.