Fluidics and rheology in phaco surgery: what matters and what is the hype?

Abstract

Rheology can be defined as the study of materials flow behaviour under applied deformation forces (stresses). Inside the eye during cataract surgery, transient or reversible deformation (elasticity) and permanent or irreversible deformation (flow) can both apply. Although the fluidics of modern phaco machines is an important part of this talk, there are a number of issues relating to flow and fluids in the eye during cataract surgery, which are unrelated to the machine, which can hinder or help the operating surgeon in removing a cataract. In addition, coping with the numerous complications, which can occur in the eye at the time of surgery, relies on an understanding of how fluidics and rheology can help to control the pressurised eye and bring the surgery to a successful conclusion.

Figure 1

Diagram showing the general set up and principles of a phacoemulsification system. The pressure in the anterior chamber reaches a a fixed value dependant on the bottle height once the pinch valve opens (foot pedal position 1). In foot pedal position 2, aspiration of fluid from the eye starts as the pump is activated and then in foot pedal position three, vibration of the phaco needle commences to emulsify the cataract.

Figure 2

Iris prolapse occurring during hydrodissection in an eye with a shallow AC and a too-peripheral corneal wound.

Figure 3

(a) Picture showing excess fluid flowing out of the side port during phacoemulsification. This fluid is obscuring the surgeons view of the anterior chamber. (b) Having removed the second side-port instrument the leakage stops and the view clears.

Figure 4

Old style co-axial vitrector used through main wound after vitreous loss. The uncontrolled anterior chamber allows much leakage and enables flow of more vitreous out of the eye.

Figure 5

Modern bi-manual anterior vitrectomy. The anterior chamber is stable and no flow occurs out of the eye except through the vitreous cutter.