The impact of intraocular pressure on elastic wave velocity estimates in the crystalline lens

Abstract

Intraocular pressure (IOP) is believed to influence the mechanical properties of ocular tissues including cornea and sclera. The elastic properties of the crystalline lens have been mainly investigated with regard to presbyopia, the age-related loss of accommodation power of the eye. However, the relationship between the elastic properties of the lens and IOP remains to be established. The objective of this study is to measure the elastic wave velocity, which represents the mechanical properties of tissue, in the crystalline lens ex vivo in response to changes in IOP. The elastic wave velocities in the cornea and lens from seven enucleated bovine globe samples were estimated using ultrasound shear wave elasticity imaging. To generate and then image the elastic wave propagation, an ultrasound imaging system was used to transmit a 600 µs pushing pulse at 4.5 MHz center frequency and to acquire ultrasound tracking frames at 6 kHz frame rate. The pushing beams were separately applied to the cornea and lens. IOP in the eyeballs was varied from 5 to 50 mmHg. The results indicate that while the elastic wave velocity in the cornea increased from 0.96 ± 0.30 m/s to 6.27 ± 0.75 m/s as IOP was elevated from 5 to 50 mmHg, there were insignificant changes in the elastic wave velocity in the crystalline lens with the minimum and the maximum speeds of 1.44 ± 0.27 m/s and 2.03 ± 0.46 m/s, respectively. This study shows that ultrasound shear wave elasticity imaging can be used to assess the biomechanical properties of the crystalline lens noninvasively. Also, it was observed that the dependency of the crystalline lens stiffness on the IOP was significantly lower in comparison with that of cornea.

The relationship between outflow resistance and trabecular meshwork stiffness in mice

“…Further, TM stiffness might be a surrogate marker for conventional outflow pathway function. Novel technologies are being developed to measure ocular tissue stiffness in situ 34 – 37 , and could thus hold promise for future diagnostic benefit in glaucoma if they could be adapted to the TM.…”

Does group velocity always reflect elastic modulus in shear wave elastography?

“…OCE has been used to evaluate tissue biomechanics 14 and, especially, in ophthalmology. 5 , 6 , 8 , 15 – 19 Of particular note is dynamic OCE to map corneal elasticity using noncontact mechanical loads based on air-puffs 6 , 8 , 11 , 20 – 22 or acoustic microtapping ( ). 5 , 15 , 23 …”

Optical coherence elastography of cold cataract in porcine lens

“… 21 , 24 In our previous work, we have used dynamic OCE and acoustic-based methods for noninvasively assessing lenticular biomechanical properties. 25 – 29 We have demonstrated that the biomechanical properties of the crystalline lens can be measured in situ and quantified variations in the stiffness of mammalian lenses as a function of age and intraocular pressure. 25 , 27 – 29 Shear wave elastography is one of the most common elastography techniques because it does not need a priori information about the sample or excitation, and it can be performed completely noninvasively.…”

“… 30 Ultrasound-based shear wave elastography techniques have been used to evaluate the biomechanical properties of various animal lenses. 25 , 31 , 32 However, the low resolution, poor contrast, and large excitation forces of ultrasound-based techniques may not be suitable for in vivo applications. Thus, we propose the use of OCE to detect elastic waves in the lens to characterize lenticular biomechanical properties.…”