Applanation & Contact Tonometry

The Imbert-Fick Law

Applanation tonometry rests on the Imbert-Fick law: for a perfectly spherical, thin-walled, dry sphere, the internal pressure equals the force required to applanate (flatten) a given area, divided by that area, P = F/A. The cornea does not satisfy any of those assumptions. It has thickness, curvature, and rigidity, and its surface is wet rather than dry, which introduces tear-film surface tension into the measurement. Goldmann's instrument is designed around this: it applanates a fixed 3.06 mm diameter of cornea because at that specific diameter, the capillary surface tension of the fluorescein tear meniscus cancels the corneal rigidity force. The resulting reading is, in theory, independent of corneal rigidity at that one diameter.

Goldmann Applanation Technique

The eye is anesthetized topically and stained with fluorescein before measurement. The slit-lamp-mounted tonometer prism is advanced under cobalt blue illumination, which renders the tear film as two green semicircular mires. The examiner turns the calibrated drum until the inner edges of the two mires just touch at the center of the field: too much force and the mires overlap, too little and they pull apart. IOP is read directly off the drum at that touching endpoint.

Corneal Thickness & Biomechanics

Goldmann's algorithm assumes a standard central corneal thickness (CCT) of roughly 520 µm. A cornea thicker than that, say above ~590 µm, resists applanation more than the algorithm expects and needs extra force to flatten, so the tonometer overestimates true IOP. A thinner cornea does the opposite: less resistance, less force, and the tonometer underestimates true IOP.

Curvature and rigidity add a second, independent source of error. Steep corneas tend to read artificially high; flat corneas read low. Regular corneal astigmatism biases the reading in a specific direction: with the mires held in their standard horizontal orientation, with-the-rule astigmatism underestimates true IOP and against-the-rule astigmatism overestimates it, by roughly 1 mmHg for every 4 diopters of cylinder. Averaging a vertical and a horizontal applanation reading cancels this error without needing to know the astigmatism's axis. Corneal hysteresis, the viscoelastic energy the cornea absorbs during deformation and measured with the Ocular Response Analyzer, modifies the relationship between applanation force and true IOP through a mechanism separate from thickness altogether. Goldmann tonometry is therefore never measuring IOP in isolation; it is measuring IOP entangled with corneal biomechanics, and the gap between the two widens in eyes with atypical corneas.

Tonometry After Refractive Surgery

LASIK myopic ablation removes central corneal stroma, thinning the cornea and lowering both its rigidity and its hysteresis. A thinner, more compliant post-LASIK cornea needs less force to flatten than an unoperated eye at the same true IOP, so Goldmann tonometry underestimates pressure systematically after the procedure. That is a real clinical trap. A "normal" Goldmann reading in a post-refractive-surgery patient can be masking a genuinely elevated pressure, and glaucoma risk assessment in that population has to account for it.

Perkins & Tono-Pen: Portable Applanation

Perkins tonometry is a handheld version of Goldmann applanation. It uses the same optical principle in a device untethered from the slit lamp, which is why it is the standard for measuring IOP under general anesthesia in children and in supine or bed-bound adults who cannot be positioned at a slit lamp.

The Tono-Pen works differently: a handheld plunger tip takes several rapid measurements, averages them, and displays a confidence indicator for the result. It also needs no slit lamp, which makes it useful at the bedside, in pediatric patients, and postoperatively, when proper slit-lamp positioning is not practical.

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