Waterjetting 17e - Liposuction and other medical uses

One of the advantages of using high-pressure waterjet streams as a cutting or cleaning tool is that, unlike more mechanical devices, there is much less need to apply additional force through the tool in order for it to work. This was one of the considerable advantages that waterjetting had when it was first introduced, and married with robotic arms in cutting different materials. Early robots had little arm strength – it was a major constraint in the design of new equipment since we had to recognize that, for example, in order for the arm to keep its precision and accuracy we could apply no more than ten or twenty pounds of thrust through the nozzle. This limit was overcome when the power was applied from a jet at 50,000 psi pressure, over a very small nozzle diameter and meant that the combination of robot controlled manipulators and jet nozzles could cut complex shapes in materials ranging from shoe leather through carpet, through automobile and aircraft components.

That advantage remains as tool size is shrinking and waterjets are gaining an increasing role in the medical part of the waterjet market. I had mentioned, in previous posts, how waterjet systems are increasing being used to debride wounds, remove cancers and help expose blood vessels in liver surgery. But with the small sizes of nozzles that are now available, it is also possible for jets to be transmitted through the small tools used in laparoscopic (keyhole) surgery. As a result surgeries can be smaller in scale, less traumatic to the patient and faster to heal because of the smaller footprint of the surgery. In liver surgery, as an example, the increasing use of waterjets in resection has reduced the amount of transfusion liquid required by more than 75%.

Tool sizes have continued to shrink, and thus it is now possible to use the jets in surgeries beyond those that we originally envisioned. For example there have been a number of studies over the years on the use of waterjets to cut bone. One of the problems in hip, knee and other joint replacement surgeries is that the faces of the cuts need to be quite precisely aligned in order for the prosthesis to fit into place. With surgeons using a common saw to cut through the bone, the cut can deviate from that clean alignment. Further the saw can generate heat that can damage the bone and tissue along the cut line. High pressure jets both cool and help align the cut to minimize misalignment of the cut line.

Other tissues are more easily cut, and waterjets are being used more extensively in tumor removal and in cutting the ligaments that hold organs, such as the gall bladder or prostate in place, making endoscopic surgeries easier.

Other body tissue can also be removed. As I have updated the range of applications for waterjets in the medical field over the years it is only now that I am finding references to the use of waterjets in liposuction. Waterjet Assisted Liposuction (WAL) has been described as:

Using a fan-shaped laminar jet, the body-jet® simultaneously irrigates and aspirates fatty tissue. The gentle separation of fat cells from connective tissue minimizes trauma to the patient. At the same time, significantly less infiltration fluid is needed with the WAL procedure as compared to traditional methods, helping to reduce exposure to tumescent fluid, minimize swelling, allow real-time precise contouring, and dramatically shorten procedure times.

Other sites provide similar comments:

Body-Jet Water Liposuction relies on the power of highly concentrated water to gently dislodge and remove fat cells from the body. Using water-jet technology, fat is removed from the body with significantly less force than older liposuction techniques. The power of the water-jet detaches fat cells from their surrounding tissues, allowing the suction cannula to move freely. This limits the possibility of trauma to surrounding tissues, including skin, muscles, nerves, blood vessels, and septal attachments. . . . . . Because the process is so gentle, Body-Jet Water Liposuction is typically performed as an out-patient procedure under local anesthesia. On average, the entire procedure takes between 30 to 45 minutes for each part of the body that is treated. The majority of body-jet water liposuction patients find that they can return to their regular activities immediately following the procedure. In fact, Body-Jet Water Liposuction has been called “lunch break lipo” because most patients are able to have the procedure performed and immediately resume regular activities.

There is a video about the procedure here .

As I mentioned earlier in regard to back surgery the small amount of damage outside of the region where the jet is cutting means that many of these procedures can be carried out as outpatient surgery with the patient being able to leave without hospitalization. Further because the jet works by discriminately removing the desired tissue without damage to surrounding hard tissue, and it can reach “around corners” to flush out cavities it has been reported, for back surgeries:

In treating spines, where the disc is removed before carrying out spinal fusion the waterjet was able to get out some 96% more of the soft tissue that was achieved by conventional means.

More refined applications, such as in breaking up blood clots in thrombectomy have also now been developed, The technique appears to be more widely tested in Europe than in the United States at the moment. It has been approved for use in the United States.

Each year in the U.S., approximately 600,000 patients are diagnosed with deep vein thrombosis. Complications range from severe pain and limitation of mobility, to limb loss and even death. Moreover, 200,000 people die from pulmonary embolism (PE) each year, which occurs when venous thrombus migrates to the lungs and blocks blood flow. . . . . . The FDA's clearance of AngioJet thrombectomy . . . . . gives doctors a powerful tool to restore flow to blocked veins. Patients may benefit from faster resolution of leg pain and reduced risk of complications.

There are, in short, increasing applications for high-pressure waterjets in the medical field, and the advantages which the tool brings to cutting in more mundane applications seem also to carry over into surgical applications. It has only, however, been after smaller and more precise tools have been developed that this set of applications has evolved, and it will be interesting to see how much longer the list grows in the years to come.

Waterjetting 12d - The heat of a waterjet cut

In the last three posts I have been discussing the quantity of heat that is created when machine tools are used in the cutting of rock, metals and other materials. The amount that the temperature of both the cutting tool and work piece material will increase, and the effect that this has on the cutting tool and the finished part can, as I have shown, be reduced if a quite small stream of high-pressure water is directed into the small zone where the cutting is taking place.

But what happens if the cutting process doesn’t use the large scale typical mechanical cutting tools, but instead uses the very small particles embedded within the jet stream itself as part of an abrasive waterjet cutting system? For many years the evidence, after the cut was over, indicated that there was very little heat build-up in the part, and the process appeared to be a “cold cut,” but there was no immediate evidence, because of the rapidity with which the cut was made. However, with advances in technology that limitation was removed, and research scientists at the University of Hannover have now been able to make temperature measurements during cutting. (A Thermographical Map of Tool and Workpiece During the Cutting Process by Plain Waterjet and Abrasive Waterjet up to 900 MPa, H. Louis, A. Schenk, F. Pude and M. Mohamed, 17th International Conference on Waterjet Cutting Technology).

The group used an infra-red camera connected into a computer to capture images as an abrasive waterjet cut into a target work sample. The instrument had been calibrated to show the color temperatures that the image revealed.

Figure 1. Temperatures read through an infrared camera as an abrasive jet cuts into a target plate. (H. Louis et al, ibid)

The arrangement by which the images were obtained was relatively simple:

Figure 2. Experimental arrangement allowing capture of the temperature build-up in the cutting head, the abrasive jet and the work piece during an AWJ cut (H. Louis et al, ibid).

During the course of the experiment the size of the cutting jet and the pressure were changed to find how these controlled the temperatures that were generated in the different parts of the operation. The work first examined the results when only a plain waterjet, without abrasive particles, was used in cutting.

Figure 3. Temperature build-up when plain waterjets (at 125,000 psi) are being used to cut a piece. (H. Louis et al, ibid)

Note that there is not a large amount of heat generated in the part, in this case a temperature rise to 126 Deg F was measured, though the temperature rise in the nozzle holder was similar in range. When the effects of jet flow and pressure were plotted, the role that an increase in pressure played in raising the part temperature around the cutting zone is clear. Note, in Figure 3, the region over which the temperature has been raised in the work piece.

Figure 4. Temperature rise in the nozzle holder as a function of jet pressure. (H. Louis et al, ibid)

Note that at pressures of up to 100,000 psi (700 MPa) the temperature rise is only up to 86 deg F, much less than that in conventional mechanical cutting.

When abrasive is added to the jet stream, then the temperatures generated, as Figure 1 indicated, are higher in the nozzle holder, because of the impact of the particles with the focusing tube as part of the particle acceleration. The piece was moved under the jet at 1.2 inches/minute, with an abrasive feed of 0.06 lb/minute, with jet pressures varied from 42,000 psi to 115,000 psi. (300 to 800 MPa). The target was a metal alloy.

Not surprisingly as the pressure in the jet increased, so did the temperature in the focusing tube.

Figure 5. Temperature increase in the focusing tube, as a function of jet pressure (H. Louis et al, ibid).

Temperatures were measured at the top, middle and bottom of the cut which the AWJ made through the target material, and these are shown in the following plot:

Figure 6. Temperature build-up in the work piece during the cutting operation (H. Louis et al, ibid).

The graph shows that the temperature build-up is greatest in the middle of the cut, although this difference is small, and begins to disappear as the jet pressure increases. At 100,000 psi the temperature can rise to 150 deg F.

In most cutting work that temperature rise would not be enough to cause any damage to the part being cut. Where very temperature sensitive materials have been cut with the jet at lower pressures and higher speeds at MS&T the zone of influence of the cutting operation was measured in microns.

It is in living tissue, which can be more sensitive to temperature, where this can be a problem. The University in Hannover is internationally recognized for the work that it has been carrying out in to the use of high pressure waterjets in medical applications. While this is a subject for another day (or several since the range of applications continues to grow from year to year) the caution comes from work on cutting bone and reported at the 18th International Jet Cutting Conference in Gdansk by Biskup et al “Temperature measurement during abrasive water jet cutting of cortical bone measured by thermocouples”). Bear in mind, however, that one of the problems that the technology is seeking to address in these bone cutting experiments is to achieve a better quality cut than can be achieved with a hand saw, which has often been the tool used by a surgeon when dissecting bone, and the required edge quality is sometimes more difficult to achieve with that tool.

Figure 7. Temperature build-up in bone under varying conditions and for two bone thicknesses, as a function of residence time. (Biskup et al, ibid)

It can be seen that a thicker bone sample does become vulnerable to too high a temperature if there is a significant exposure time before the part is pierced. However, with an appropriate selection of parameters the temperature can be kept down in a range where the tissue does not die, and the considerable advantages to jet use can therefore be used.

Keeping the parts being cut cool is important in very delicate and precise work, where thermal distortion of the metal, particularly in thin but deep cuts, can otherwise lead to unacceptable failures to maintain tolerance.