In physics, energy transfer per unit of time equals power, or the amount of work done. The use of fluid as a tool to carry out work is known as hydraulics. In hydraulics, the use of the kinetic energy of a moving fluid is known as hydrodynamics, and the use of the static pressure of a fluid contained in a closed system with a reservoir and piping is known as hydrostatics.
Hydrodynamics and Hydrostatics
Phacoemulsification should be regarded as a complex system comprising a hydraulic machine with piping, providing work or power to a chamber – the eye. The work inside this chamber becomes a hydrodynamic process with fluid flow and pressure depending on the phaco machine itself. The whole system – made up of the machine and the eye – constitutes a hydrostatic power transmission system, with a reservoir for the fluid and tubing and a pump to drive the fluid. The difference lies in the pump – a vacuum pump, which is needed to provide the fluidics in the anterior chamber – and in the fact that the fluid is not re-used. Basically, however, the system uses the power transmission of the static pressure that stands inside the tubing.
From any handbook of physics we can learn that, since the connection between the elements is through the piping, the principal advantage of hydrostatic power transmission (see Figure 1) is that these elements can be positioned in whatever fashion is suitable for the machine manufacturer. Hydrostatic transmission also has the advantage of not depending on the size of components.
However, the main disadvantage of hydrostatic drive is that it may lose efficiency due to internal leakage, fluid-flow losses or mechanical frictionlosses (see Figure 2), all of which result in power being wasted. In addition, if for any reason any external leakage or fluid loss is apparent, more power is wasted and the system becomes more inefficient. Assuming that we are dealing with hydrostatic transmission, our aim in terms of phacoemulsification itself is to spare the energy, or power, lost through leakage. What happens, then, if our phacoemulsification is made bimanually with a sleeveless round needle?
Incisions – Size, Shape, Misconceptions and Frequent Mistakes
It is important, before going on to leakage, to understand common mistakes and misunderstandings surrounding bimanual phacoemulsification.
Most irrigating choppers currently in use are 19G. Until now, irrigating choppers and phaco needles have been round. This has meant the introduction into the elastic tissue of the limbus of a circular instrument with a certain gauge through a linear incision. Since it is very common to read or hear of surgery performed using a 1mm incision, we will take this measurement as our example. We will consider a 19G instrument with a circular section of 1mm in outer diameter. It is this circle that needs to be introduced through a 1mm incision. Using the formula 2πr, for a diameter of 1mm we calculate a perimeter of 3.1416mm. As the limbus is vertically elastic, the incision will open equally up and down, making the size of the incision half of the perimeter just 1.57mm. This means that for a 19G instrument (1mm in diameter) the real size of the incision needed would be 1.57mm. The same considerations work for a 20G 0.9mm instrument, which would require an opening of 1.41mm. This is a lot greater than 1mm; therefore, it is probable that when we hear or read about 1mm incisions, the author is actually referring to 1mm instruments. These disparities are due to the fact that the diameter of the instruments is reported to be equal to the size of the incisions. This in itself is based on the assumption that the incision fits perfectly around the circular instruments. This is the second mistake – as well as the size of the incision being misjudged, the shape of the incision is unclear.