Quantification of temperature-induced corneal shrinkage

Purpose. To quantify thermally induced stromal collagen shrinkage and compare the results with laser photothermokeratoplasty (LPTK). Methods. Fresh Eye Bank eyes were used. Three 2 × 4 mm corneal strips were cut from each cornea. The strips were mounted in a novel digital displacement sensor apparatus designed for the measurement of corneal shrinkage at constant corneal hydration (76%) and mechanical tension (15mmHg) and controlled shock temperature (36-40 to 100°C to 36°C, transition time: 9 s). We quantified the shrinkage rate as a function of temperature, exposure time and donor age. Results. Collagen shrinkage started above 59°C and denaturation, seen as an elongation of the strip, above 85°C. Less heat, assessed in Joules, was required to induce a permanent shrinkage in 50 or more year old cornea but no permanent shrinkage could be induced in a 35 year old. Permanent collagen shrinkage threshold temperature was ≈63°C for a 78 years old. The % shrinkage increased with the shock temperature and exposure time but was independant of corneal thickness dependent. More than 25% permanent shrinkage was recorded in ≥60 year old donors. A cumulative shrinkage was recorded after sequential heat shocks. Conclusions. Local overheating (≥80°C of the cornea induces tissue expansion instead of shrinkage creating an opposite refractive effect. This pilot experiment also confirms the recent findings of US and European LTK clinical trials: For a given heat dose (J), the amplitude of the refractive effect is so strongly age and patient dependent that an accurate normogram cannot be constructed. An intrapulse laser feedback linked to the patient corneal topography system is necessarv for accurate correction.