Tuesday, January 21, 2014

Waterjetting Technology 17d - Some medical applications - don't blink!

Our discussion on the ability of high-pressure waterjets to remove some materials without damaging others has, as I noted in the last post, found some applications in the medical field. The example that I provided in that post dealt with the use of waterjets to dislodge material in the cleaning of wounds, or to carefully excise thin layers of burnt flesh without doing damage to the underlying tissue. I also pointed to the ability of a waterjet to remove weaker diseased tissue, such as brain and skin tumors, without damaging the surrounding tissue, which is healthy and thus able to resist the pressure of the cleansing waterjet.

There are a number of other applications in the medical field which have developed from these advantages. The first is in surgery on the liver and kidneys in particular. In these organs there are generally a large number of blood vessels that carry blood through the organ so that the blood can be cleaned of impurities. The difficulty that this creates usually occurs in cases where the organ becomes diseased. Medical treatment recommends that the diseased volume of the organ be removed, and the historic method for doing this has been to take a scalpel and carefully cut around the diseased region, so that it can be lifted out. This is often referred to as liver resection.

The problem that this creates is that, in the process of cutting out the diseased part of the organ, the surgeon must also cut through the blood vessels in that part of the organ. Because they are buried in the organ, it is not clear where these vessels are, and they are conventionally difficult to isolate. Early work in finding an answer was carried out in Japan.

However blood vessels are relatively tough (and when full of blood require pressures of around 2,000 psi or more to penetrate) whereas the surrounding tissue is much softer. As a result – using the same conceptual approach that I described last time for the removal of skin cancer, a surgeon can remove the tissue from around the blood vessels and along the projected line of dissection, without cutting through the vessels. Tests during brain operations have shown that as long as the pressure is kept below 300 psi there is no damage to any of the vessels in the brain, and similar conclusions are likely to hold for other organs in the body. The jet was, however able to remove diseased tissue, while leaving healthy tissue.(In these surgeries the jet is on the order of 100 microns in diameter, and as a result uses very little water during the operation.)

Figure 1. Japanese surgical removal of liver tissue around blood vessels of the liver, exposing them so that they can be tied off, before being cut, thereby significantly reducing blood loss.

Clinical trials rapidly spread around the world, Papachristou had published on the technique in 1982 when, after 75 trials with dogs it was tried in four male human patients. At the time it was noted that there was significant reduction in blood loss. This is fairly critical in older patients (who are more likely to need these operations) since large losses of blood can induce shock, and can be fatal. Blood loss is the most common complication of the surgery and consequent cause of death (which now runs around 5%, but was higher prior to waterjet introduction).

It also impacts long-term survival and post-operative complications. (Historically the problem was addressed using what is known as the Pringle manoeuvre in which all the blood flow to the liver is halted by clamping for the period of the operation. However this can only be applied for limited amounts of time and is of limited effectiveness.) Using a waterjet approach has the advantage, over ultrasonic and cavitating techniques, that the path cut through the liver is narrower and the vessels are more clearly delineated. At the same time, as figure 1 shows, the sides of the cut are relatively clean and well defined. The jet is able to handle the tougher tissue in a cirrhotic liver either through a longer residence time, or by raising the pressure of the jet by about 150 psi.

The very narrow cut achieved by the waterjet has another useful feature in that the jet will, in fibrous tissue, push apart the fibers rather than fusing them, as would be the case with laser cutting. This has advantage in eye surgery where any such fuse points can cause scarring that interferes with future vision, while the separation of the tissues with the jet does not carry that problem.

Figure 2. Precision cutting across the face of the eye at a jet pressure of 20,000 psi and with a jet diameter of 10 to 100 micron.

The technique was first announced in 1994 and animal tests had been successfully completed by 2001. Because the jet cuts so fast (less than a second per cut) there is no tissue loss. (The operation uses a device that fits to the eye to hold the lens in the right position to make the cut).

The technique has not, however, been that successful in the field as has another application, that of removing herniated disks in the spine. The technique uses a procedure known as Hydrocision. It is interesting to read a quote from an article last November on the technique:
"It's basically a high-velocity water jet eroding system," Kevin Staid said about the medical device that his North Billerica, Mass.-based company makes. "And this is our first entry into the area."

With HydroCision, a jet of saline solution comes out of a nozzle that is 0.005 inches in diameter -- "slightly larger than a hair" -- and can cut away protruding disc tissue that can cause the back and leg pain without an actual blade.

"Just the energy of the jet would be doing the cutting," said Staid, an engineer. "In our case, the water is going about 600 miles an hour and has the ability to cut quite effectively."

The advantages of the 20-minute outpatient procedure are: No hospitalization, quicker recovery times, less pain, no surgical trauma to the back muscles and no general anesthesia.

"There is no muscle damage, no bone removal, no nerve root manipulation ... and the size of the wound is approximately 4 mm," Kowalkowski said.
The use of the tool is sufficiently popular that in 2009 the company (Hydrocision) announced that the tool had been used in more than 40,000 procedures.

I’ll give other medical examples in the next post.


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