The osmotic variable.
Patients were found to differ in their prefitted tear osmotic baselines. Although such differences appeared small, they did represent the "safety margin" (i.e., against osmotic edema) each patient had against a "hypotonic shift" when receiving a hard (PMMA) contact lens.
D003261:Contact Lenses / Q000009:adverse effects*; D004487:Edema / Q000209:etiology*; D006801:Humans; D009997:Osmotic Pressure; D013666:Tears / Q000502:physiology*