Purpose:
Investigate the assumption that measurements of the intraocular pressure (IOP)can neglect the compliance of the retropulsive structures (RPS) holding the eye.
Methods:
In measuring the RPS using a tonometer, we test and confirm that the RPS are linear elastic materials. The force, F, is then related to the distance, x, by F=kx, where k is the force constant and its reciprocal is the compliance X.To determine this parameter, a rigid corneal shield is placed over the cornea and sclera. The tonometer probe is touched to the center of the shield and pushes on the shield and RPS, while F and x are recorded in the range of F and x comparable to a Goldmann tonometer. Using the calibration of our tonometer, we difine X for the RPS by this measurement.
Results:
In the standard interpretation of IOP, the RPS are assumed to be rigid, or that their compliance is zero. If this assumption is not true, a corneal measurement (such as Goldmann) gives X(cornea)+X and X should be subtracted before diagnosis. Our measurements, Figure 1, show that a typical normal subject shows linearity in F vs. x with {X, standard deviation, coeff. regression} = {0.035mm/gm, 0.08,0.997}, for small F in the range 0-3gm (pressures in the range 0-30mmHg). A low-order, non-linear fit is not statistically preferable. The range of values of X is 0.035 to 0.045mm/gm
Conclusions:
Our measurements show that X is not zero. In the small sample of normal patients measured thus far, average X is only about 5 times smaller than the values we observe for X(cornea), so the error in neglecting X appears to be significant in normal subjects that we have measured (and it is not constant). The linearity is significant because it means that the size of the correction applies at any IOP. Further studies are required to determine the extent to which X can be ignored in diagnosis for a wider class of patients. We suggest that patients with normal pressure glaucoma warrant particular study because X may be unusually large and reduce the apparent IOP.
Keywords: cornea: clinical science • intraocular pressure