From the Research
Principle Behind Non-Contact Tonometry
The principle behind Non-Contact (NCT) tonometry is based on the deformation of the eye caused by a short air pulse. This deformation is estimated from the time when the eye becomes flat, which is derived from the monitored reflection of an incident infrared light 1. The eye deformation is influenced by the intraocular pressure (IOP), corneal thickness, and material properties of the cornea.
Key Factors Influencing NCT Readings
- Corneal thickness: NCT readings can be affected by anomalous central corneal thickness, with thinner corneas leading to underestimation and thicker corneas leading to overestimation of IOP 2, 3.
- IOP: The maximum apex displacement of the cornea shows a linear variation with IOP 4.
- Corneal material properties: The mechanical response of the cornea is influenced by its geometry, material behavior, and loading conditions 4.
- Air-puff loading: The cornea experiences bending when subjected to an air-puff loading, causing the anterior surface to work in compression and the posterior surface to work in tension 4.
Limitations of NCT
- NCT is less reliable in patients with elevated IOP, as comparative studies against Goldmann applanation tonometers have shown poorer correlations in the higher pressure ranges 5.
- Abnormal cornea or poor fixation can interfere with accurate pressure measurements 5.
- Corneal edema can reduce the effect of corneal thickness on NCT readings 3.