The cornea is a delicate tissue. It is thin, clear, avascular and susceptible to damage, particularly from the heat that is generated by the phaco probe.
Think of the corneal tissue like the albumin of an egg: With heat, these clear proteins denature, become opaque and contract. In the cornea, application of heat will cause the tissue to opacify and contract, which will induce a large degree of astigmatism and prevent the incision from sealing well.
During cataract surgery, the phaco probe generates heat due to friction. Ultrasound means that the frequency of the vibration is above the range of