Cavitation bubble cluster activity in the breakage of kidney stones by lithotripter shockwaves

Abstract

Background and purpose: There is strong evidence that cavitation bubble activity contributes to stone breakage and that shockwave-bubble interactions are involved in the tissue trauma associated with shockwave lithotripsy. Cavitation control may thus be a way to improve lithotripsy.

Materials and methods: High-speed photography was used to analyze cavitation bubble activity at the surface of artificial and natural kidney stones during exposure to lithotripter shockwaves in vitro.

Results: Numerous individual bubbles formed on the surfaces of stones, but these bubbles did not remain independent but rather combined to form clusters. Bubble clusters formed at the proximal and distal ends and at the sides of stones. Each cluster collapsed to a narrow point of impact. Collapse of the proximal cluster eroded the leading face of the stone, and the collapse of clusters at the sides of stones appeared to contribute to the growth of cracks. Collapse of the distal cluster caused minimal damage.

Conclusion: Cavitation-mediated damage to stones is attributable, not to the action of solitary bubbles, but to the growth and collapse of bubble clusters.

Figure 1. Individual cavitation bubbles contributing to the formation of bubble clusters at the surface of an artificial stone

This series of frames captured at 70 μs steps shows cavitation bubble activity generated by a single lithotripter pulse. The images focus on the distal one-third of the stone. The axis of shock wave propagation was nearly vertical upward (see inset). (a): This frame shows the stone at the time of shock wave arrival, but prior to the formation of cavitation bubbles. (b): 70μs after passage of the shock wave, numerous individual cavitation bubbles have formed on the surface of the stone. (c-d): The bubbles have enlarged and have begun to coalesce into a cluster. There is a prominent bubble (arrow) visible at the center of the distal surface of the stone. (e): A portion of the bubble cluster has begun to move from the end of the stone to the lateral stone surface. (f): The bubbles in this field have now divided into two separate clusters; one, possibly a single bubble at the center of the distal end of the stone, and one (bracket) forming a ring or band that appears to encircle the stone. (g): In this final frame both the distal bubble and the cluster along the sides of the stone have collapsed.

Figure 2. Bubble cluster formation and collapse at the proximal face of a stone

These images, captured at 100 μs steps, show the growth of the bubble cluster that consistently formed at the leading face of the stone. Two different stones are shown and images were recorded using different illumination. In the left column illumination was from the side and in the right column the lamp was positioned at the back. By side lighting the stone is light and the bubbles are transparent. By back lighting the stone is dark and the bubbles are opaque. (a): These frames show the stone at the time of shock wave arrival, and thus before cavitation bubbles have formed. The arrow shows the direction of pulse propagation. (b-d): This series shows formation and growth of the cluster over the leading face of the stone. With side lighting (left) one can see numerous small bubbles at the surface of the stone and caught up by the expanding cluster. By back lighting (right) the surface of the cluster appears rough. (e): The cluster has now expanded to extend ~3 mm off the surface of the stone. With side lighting (left) one can see that the bubble both overlaps the edge of the stone and extends up the sides. It is difficult in either frame to resolve individual bubbles within the cluster. (f): The cluster has begun to collapse. It has retracted from the sides of the stone and is constricted at its base (arrowheads). (g): The cluster has collapsed at the center of the proximal face of the stone. Note: The time from impact of the shock wave to initial collapse of the proximal cluster was approximately 600 μs. This compares with a cavitation cycle for individual bubbles in the surrounding water of approximately 300 μs.,

Figure 3. Details of bubble cluster collapse at the proximal face of a stone

This series of images shows 15μs steps beginning at 700 μs after spark discharge. This sequence therefore corresponds, approximately, to the interval between frames “f” and “g” of Figure 2. The collapse times of the bubble clusters in Figures 2 and 3 differ slightly. This is typical of the shot-to-shot variability in shock pulse amplitude that has been documented for electrohydraulic lithotripters., (a-e): The bubble cluster constricts at its base (arrowheads), while the dome or cap gets smaller at a slower rate. (f): The cluster now consists of a cap and narrow stalk (arrow), with the stalk centered over the proximal face of the stone. (g): The stalk is no longer visible and just a portion of the cluster cap remains. The faint shadow visible between the stone and cluster may be debris eroded from the stone due to impact from collapse of the cluster.

Figure 4. Cavitation bubble rebound following cluster collapse at the proximal face of the stone

This set of images demonstrates bubble rebound following initial collapse of the bubble cluster. The frames were captured in 20 μs steps, and the first frame corresponds to approximately the same point in development of the proximal bubble cluster as is shown in Figure 3c. (a-b): The cluster is collapsing a little off center. (c): Collapse is nearly complete and just a wisp of the cluster remains. (d-g): The rebounding cluster grows outward. It is difficult to identify debris that might be carried within the cluster. Note: This figure shows the last of a series of 10 shock waves delivered at a rate of 5 Hz. Bubble cluster formation, growth, collapse and rebound were observed regardless of the pulse repetition frequency.

Figure 5. Cavitation damage to an artificial stone

(a): This image shows the proximal surface of a U-30 artificial stone treated with 50 shock waves delivered at 20 kV, 0.5 Hz. Cavitation bubble cluster activity has created a 2 mm diameter crater centered at the proximal end of the stone (arrow) (background grid 1 mm). (b): The distal end of the stone shows very little damage. There is no focus of erosion on the distal surface as is seen at the proximal end (frame “a”). Note: The distal ends of U-30 stones are always somewhat rougher than the proximal ends, as the distal end is not in contact with the plastic mold used to cast the stone.

Figure 6. Cavitation bubble activity at the distal end of a stone

This set of images captures cavitation bubble activity at the distal one-third of an artificial stone. This is the same stone shown in Figure 1, but with illumination from a different angle. The timing of frames is the same as Figure 1 (i.e. 70 μs steps, beginning at 180 μs). (a): This frame, included for reference, shows the stone at the time of shock wave arrival, but before cavitation bubbles have developed. (b-d): These frames capture the formation of individual bubbles and their subsequent growth. Backlighting helps one see the boundaries of individual bubbles at the edge of the stone (arrowheads), but obscures detail elsewhere. (e-f): The “bubble cluster” shown here atop the distal end of the stone looks like it may be a solitary bubble. At its maximum expansion, this bubble extends a little more than 1 mm above the surface of the stone. This distal bubble cluster is thus much smaller than the cluster that develops at the proximal end of the stone (Fig 2, 3, 9–11). The structure of this bubble seems very similar to the shape attributed to asymmetric bubble collapse. (g): This final frame has captured what remains of the distal bubble cluster (arrow) as it collapses at the center of the stone.

Figure 7. Bubble cluster formation and collapse at the sides of an artificial stone

This series of images shows the upper half (distal half) of a stone held in position with a rubber band (seen in lower left corner of each frame). The shock wave entered from the bottom of the stone. Frames were captured at 130 μs steps. (a): Image of the stone at the time of shock wave impact, but before bubbles are visible. (b-c): These frames show the formation and enlargement of a bubble cluster that surrounds the stone. (d): The bubble cluster has begun to retract, exposing the mid-portion (asterisk) and the distal edge (arrow) of the stone. (e-f): The cluster now forms a narrow ring or band of bubbles (arrow) 1–2 mm below the distal end. (g): In this final frame there is very faint dust near the line of cluster collapse, suggestive of fine debris released from the surface of the stone.

Figure 8. Bubble cluster formation on a stone not held by a ligature

This series of images shows an artificial stone that was positioned at the focus of the lithotripter by standing it upright atop a sheet of low density polyethylene. Frames were captured at 150 μs steps. These images demonstrate that formation of a bubble cluster at the side of the stone was not an artifact of the way in which the stone was held in the lithotripter (a): Stone at the time of shock wave arrival. (b-f): These frames show the formation of the bubble cluster to cover the surface of the stone, retraction of the cluster from the mid-portion (asterisk) and distal end of the stone to form a narrow band of bubbles (arrow), and collapse of those bubbles. (g): As in Figure 6, frame “g”, this image shows dust (arrow) that may be fine debris coming from the surface of the stone.

Figure 9. Bubble cluster collapse along an existing spall fracture in an artificial stone

This series of images captured at 140μs steps shows bubble cluster activity associated with a distal fracture that was present before the arrival of the shock wave. The stone is held in place by a rubber band. (a): This image shows the stone at 190 μs after spark discharge, approximately 10 μs later than the initial frames shown in figures 1, 2, 6, 7, and 8. As such, bubbles are already visible in the surrounding water, and some may be present at the surface of the stone as well. There is a transverse crack visible above the rubber band (arrows). This crack was present prior to this shot. (b-c): Bubbles grow at the surface of the stone and coalesce to form a large bubble cluster that surrounds most of the stone. (d): In this image the crack is visible (arrow) beneath the cluster at the side of the stone. The crack appears to have widened. Note also, the large proximal cluster in this frame (arrowhead). (e): The bubble cluster at the side of the stone has collapsed along the line of the transverse crack. A vertical crack is now visible as well and a portion of the cluster overlies this crack (arrow). The proximal cluster is retracting toward the leading face of the stone. (f): The crack is now considerably wider than before. (g): This frame shows bubble rebound at the proximal face of the stone (arrow).

Figure 10. Superposition of bubble clusters with fracture lines is not accidental

These images are of an artificial stone in which a spall-type fracture developed low on the stone. In this case the bubble cluster still formed along the crack and appeared to collapse into it. Steps after frame “b” are 140 μs. (a): Stone prior to the arrival of the shock wave. (b): Bubbles have begun to form at the surface of the stone and in the surrounding water. (c-d): Growth of a bubble cluster that appears to completely surround the stone. (e): This frame shows a bubble cluster at the side of the stone and one at the proximal end of the stone (arrow). The side cluster has a narrow portion that runs transversely across the stone. One cannot see what lies beneath the cluster, but this is the position of a crack that is visible in frame “f”. Thus, the bubble cluster appears to be collapsing into a crack. (f-g): The cluster is gone and the crack is now visible just above the rubber band (arrow).

Figure 11. Bubble cluster activity over different regions of a stone

This set of images captured at 100 μs steps illustrates cavitation bubble cluster activity over different regions of a stone following a single lithotripter pulse. (a): This initial frame at 190 μs shows small bubbles at the surface of the stone, and in the surrounding water. This stone has already been damaged by previous exposure to shock waves. There is a faint fracture about 1 mm from the distal end (arrow). (b-d): Bubbles grow, and coalesce with one another to form a prominent cluster at the proximal end, a smaller cluster at the distal end and a cluster surrounding the mid-portion of the stone. (e): The bubbles above the rubber band are lined up (arrow) along the transverse fracture first visible in frame “a”. The proximal cluster is very large. (f): The proximal cluster is smaller— has begun to collapse. (g): The crack near the distal end of the stone has widened (arrow) while the proximal cluster appears much smaller — consistent with continuation of collapse or subsequent rebound.

Figure 12. Cavitation bubble cluster activity at the surface of a natural kidney stone

These frames show bubble activity at the surface of a calcium oxalate monohydrate kidney stone. They demonstrate that bubble clusters form in association with natural stones, not just with artificial stones as illustrated in figures 1–11. The images are from two successive shock waves (left column = shot number 3; right column = shot number 4) fired several minutes apart, but imaged at the same framing rate (130 μs steps). Reading down, one can see the progression of bubble activity during one shot. Reading across, one can compare bubble behavior from one shock wave to the next. Note the similarity of size and location of the bubble clusters that formed with each of the two shock waves. Note also that the bubble cluster collapses along the line of a crack that becomes visible following the second shock wave and that the crack continues to grow after cluster collapse has occurred. (a): Stone at the time of arrival of the shock wave. (b-c): A bubble cluster forms at the side of the stone, retracts from the distal end of the stone and forms a band that appears to extend around the stone. (d): The cluster in the 4^th shot (right column) overlies a prominent crack (arrow) that is first visible in the previous frame, frame “c”. (e): The line of bubble collapse in these two frames is very similar. For shot 4 (right column) the location of the bubble correlates well with the crack that was visible in earlier frames, and is easily seen in frames “f” and “g”. In shot 3 (left column) no crack is visible, but the cluster (arrow) is collapsing along a line that seems to correlate well with the location of the fracture that developed during shot 4. (f): Frame corresponds to the time of collapse of the proximal cluster (out of field of view). (g): Crack has widened and a segment of this crack (arrows) is now visible after the time of cluster collapse.