Adipose tissue is loose connective tissue that is composed of fat cells, fibroblasts (as well as other cell types), small blood vessels, proteins and water. It serves many important bodily roles from the preservation of body heat, the protection of vital organs to an efficient means of energy storage. However, some people have too much of this necessary body tissue and the surgical role of liposuction has subsequently evolved for its extraction.
While many people think of liposuction as simple ‘sucking’, the science behind it is a little more complex. The components of traditional liposuction include a blunt cannula, a handle, suction tubing, a collection chamber, and a vacuum pump. The pump generates low negative pressure within the canister (approximately -1 atm). During liposuction, the aspirate moves through the system due to the pressure difference between the atmospheric pressure in the operating room and the lower pressure in the suction canister. How efficient liposuction can be (how well it is removed from the body) is affected by different altitudes, requiring more or less vacuum pressure in order to perform liposuction most efficiently
Prior to liposuction, a tumescent fluid is infiltrated into the fatty tissues prior to suctioning to allow for hemostasis. The tumescent fluid also has the added benefit of decreasing the frictional resistance of the tissues due to distention, making the cannula pass easier. It also helps homogenize the aspirate. As the aspirate moves through the tubing, it encounters friction due to the viscosity of the aspirate. (the tissue’s resistance to flow) A vented cannula is used to reduce the relative viscosity of the aspirate so it moves faster and continuously.
One of the very important principles of liposuction is that it results in the vaporization of water. The vapor pressure of water is the pressure at which water vapor is saturated. At higher pressures, water condenses. For normal room temperature (21C), water vapor pressure is 18.7 mmHg which is less than the 20 mmHg that a liposuction machine generates. This means that during the process of liposuction, the water in the adipose tissue is vaporized which allows for flow. This is why when you look at a liposuction canister it can seem that the extracted fat is ‘boiling’ even though it is not hot. This is the vaporization of water that is being seen.
The physics principle of Poiseuille’s Law applies in liposuction since this well known concept describes laminar flow of a Newtonian fluid (such as water vapor) through a pipe. (i.e. cannula) Q=dV/dt=?p?R4/8?L where Q is the volume flow rate of a fluid with viscosity ? through a cannula of radius R and length L when driven by a pressure difference ?p. Streamlined or laminar flow is assumed in Poiseuille’s law. Turbulence will dramatically decrease the volume flow rate. It is therefore evident that you will be able to liposuction faster with a larger, shorter cannula. But this is not done as large cannulas have a much higher incidence of inconsistent fat removal and skin irregularities. Contemporary liposuction uses small cannulas even if it decreases the rate of flow.
After liposuction, there is an increase in capillary permeability of the liposuction treated tissues due to the trauma. This results in leakage of large molecules out of the intravascular spaces into the interstitial spaces. The result is an increase in the interstitial osmotic pressures and a decrease in plasma osmotic pressure after liposuction. General anesthesia has the effect of increasing capillary hydrostatic pressure through the action of venous pooling from the loss of sympathetic vascular tone. According to the Starling equation,
J?=Kf([Pc-Pi]-?[?c-?i]) where:
J? = net fluid movement between compartments
Pc = capillary hydrostatic pressure
Pi = interstitial hydrostatic pressure
?c = capillary oncotic pressure
?i = interstitial oncotic pressure
Kf = filtration coefficient (constant)
? = reflection coefficient
this results in postoperative edema as fluid is driven into the interstitium. This physics principle is not as desirous as the others for the liposuction patient as it creastes a lot of fluid retention and swelling that takes many weeks to be reabsorbed.
Liposuction is a machine-based method of fat removal that employs mechanical and physics principles to exert its effects. While many different kinds of liposuction methods exist today (Smartlipo, Ultrasonic, Vaser, Hydrasolve), they still have to use these same basic principles to remove what is aspirated, no matter how it is aspirated.
Dr. Barry Eppley
Indianapolis, Indiana