Ultrasonic Vitrectomy Performance Assessment Using Micro-Extensional Rheology

Abstract

Purpose: The purpose of this study was to assess the performance of ultrasonic (US) vitrectomy devices by quantifying and comparing its impact on extracted vitreous properties to conventional pneumatic blade (PB) cutters using micro-extensional rheology. US vitrectomy is a new technology that offers an alternative to PB cutters used in vitreo-retinal surgeries.

Methods: Thirty-six porcine vitreous samples were extracted using US and PB cutters. Each sample was kept at 4°C and tested within 24 hours postmortem and 4 hours post-vitrectomy. A recently developed micro-extensional rheology technique is used to infer the relative protein fragment size of extracted vitreous by quantifying the extensional relaxation time.

Results: US-extracted vitreous exhibited extensional relaxation times orders of magnitude lower than PB-extracted vitreous (0.37 ms and 27.25 ms, respectively). Relaxation time is directly correlated to the fragment size of the collagen fibers in the vitreous. The formation of beads-on-a-string droplets within the PB samples indicates the presence of larger collagen fragments. These droplets were not seen on US samples.

Conclusions: This new micro-extensional rheology technique can identify significant differences in physical properties of extracted vitreous. Long relaxation times and beads-on-a-string droplets within the PB vitreous samples indicate larger protein fragments compared to the US samples.

Translational relevance: Higher fragmentation of vitreous and lower extensional relaxation times may improve retina safety due to a reduction in vitreo-retinal traction resulting from the continuous shear action and aspiration applied by ultrasonic vitrectomy technology.

Figure 1.

Comparison of pneumatic and ultrasonic mechanisms of action. (Left) Conventional pneumatic mechanisms aspirate intact vitreous and use inner needle blade to shear the tissue inside the needle lumen. (Right) Ultrasonic mechanisms shear the vitreous at port entrance, aspirating only ultrasonically sheared tissue. Figure courtesy of Bausch & Lomb.

Figure 2.

Micro-extensional rheology test apparatus. (a) Photron High Speed camera, (b) Kruss K-100 tensiometer with 1 mm platinum probe, (c) hydrophobically treated surface, and (d) liquid sample.

Figure 3.

Radius evolution images starting at approximately 60% radius, taken 5 ms apart. (a) Balanced saline solution, containing no polymer or protein and showing no extensional relaxation behavior. (b) Vitesse ultrasonic cutter vitreous. (c) BiBlade pneumatic cutter vitreous at longer time steps. Beads-on-a-string phenomenon was observed at the end of the liquid bridge evolution (at 165 ms).

Figure 4.

Radius evolution plots of representative US and PB samples. The elasto-capillary regime is identified with dotted lines. The first data points in the graphs represent the inertia-capillary regime governed by a power-law decay. Extensional relaxation time, λ_E, is calculated by fitting Equation 1 to elasto-capillary regime.

Figure 5.

Box plots of sample relaxation times extracted using ultrasonic and pneumatic blade cutters. The average extensional relaxation time for US and PB samples were 0.37 ms and 27.25 ms, respectively.