A corneal abrasion is an injury to the epithelium that is superficial enough not to involve the basement membrane. It is due to mechanical trauma. The defect is seen as superficial on slit lamp and takes up fluorescein, shining green upon illumination with cobalt blue light.
Treatment is usually prophylactic antibiotics with pressure patching over 24 hours to decrease the mechanical movement of the blinking lid and moving globe, which disrupt the epithelial network trying to heal the defect. Antibiotics are usually broad spectrum eye drops or ointments, like erythromicin, fucithalmic, polymixin/sulfacetamide. If the foreign object causing the abrasion is wood, twig, and so on, the use of polymixin/trimethoprim is more judicious to cover fungi as well.

Note that corneal abrasions generally do not cause scarring upon healing because the Bowman’s membrane is not violated.

Corneal ulcer is a defect that involve the stroma, past Bowman’s membrane. It is usually infected (with few exceptions, like the sterile ulcers seen in Vernal Catarrh due to releases of toxic inflammatory mediators), and leaves a scar upon healing. It shows as a more or less deep corneal defect with infiltrates in the ulcer bed as well as around it, with sometimes pus and tissue melting. It surely takes up fluorescein. Cultures should be taken from the ulcer prior to initiating treatment, which include specially prepared fortified doses of antibiotics eye drops, like Tobramicin and Kefzole together, or Amikacin and Vancomycin, to be used as frequently as Q1/2 hr to 1 hr, awaiting for the cultures to be out. Microorganisms recovered are Staph and Strep, Pseudomonas (especially in contact lens wearers and ICU patients), fungi (amphotericin eye drops hance to be used), or acanthameoba. An ulcer is NEVER patched, unlike an abrasion. If an ulcer is axial, the final outcome after healing could be poor vision because of scarring, necessitating corneal transplant (Penetrating Keratoplasty ). The use of topical steroids in conjunction to antibiotics once the ulcer approaches healing is beneficial to decrease scarring from the inflammatory reaction elicited and hence reach better visual outcome.