Abstract
Purpose:
An intracorporeal lithotripsy probe tip was designed with a paraboloid shaped tip and compared with holmium laser for stone pulverization.
Materials and Methods:
The paraboloid tip concept was developed and designed using computer aided design (CAD), fabricated, and patented. CAD analysis and in vitro comparison (with laser) of pulverization and propulsion dynamics were performed in an underwater hands-free bench arrangement using phantom stones. SPSS analysis for different energy cohorts was performed.
Results:
CAD analysis: At “point contact” with the tip, the paraboloid lithotripter generated 3590 bars at generator settings of 4 bars. During “follow-up impacts,” the tip pressure exponentially decreased (graduated tip pressure) and the lateral/centrifugal forces increased, converting the probe into a side-firing energy source. Bench analysis: At point contact, the paraboloid lithotripter at 2, 3, and 4 bars was comparable to that of a 6, 10, and 15 W laser, respectively (P<0.005). The paraboloid lithotripter showed a statistically significant advantage in breaking the phantoms, as against a laser that always bored through the phantom. Stone propulsion was comparable within all energy cohorts (P>0.05).
Conclusion:
The paraboloid lithotripter generates highly focused impact force with low propulsion, at point contact. As stone pulverization progresses, the tip forces exponentially decrease and the probe converts into a lateral firing energy source resulting in pulverization into larger fragments. Thus, the paraboloid lithotripter has all the advantages of laser at point contact and advantages of pneumatic lithotripter at follow-up hits, akin to being a bimodal energy source.
Introduction
P
We designed, fabricated, and patented a novel paraboloid tip probe, essentially a further modification of the same, but with better vector physics. Hence, it was proposed to analyze and compared it with the holmium laser.
Materials and Methods
Computer aided design (CAD) and analysis
The conventional pneumatic lithotripter probe has a round disc-like tip. A lithotripter probe of 2-mm diameter with a paraboloid shaped tip (Fig. 1) was designed and evaluated. Analysis for “point contact” (impact dynamics at the first contact with the stone) was derived with the assumption of the metal probe loaded against an unyielding surface. “Follow-up hit” dynamics (impact dynamics when the stone is subsequently hit at the same point) were then derived assuming increasing volumes (paraboloids) of contact between the stone and the probe. Incremental paraboloid heights of 0.1 mm (starting from 0.1 mm until 2 mm) were considered. Graphs were then plotted, analyzed, and compared with a similar analysis performed for the conventional lithotripter.
Paraboloid lithotripter along conventional probe.
In vitro analysis
Inventory used: A hands-free in vitro underwater bench arrangement, round stone phantoms of calcium sulphate dihydrate (2 cm diameter and 30 g), a medical grade pneumatic lithotripter, and a holmium laser machine. A 800 μ fiber was used to replicate the clinical scenario of lithotripsy in settings of peercutaneous nephrolithotomy.
Study of the pulverization dynamics
The underwater laboratory bench arrangement (Fig. 2) was prepared using a fish tank. A nephroscope with a light source and endocamera was clamped vertically to the scaffolding using a stabilizer for the lower end. A 5 cm cube of fresh goat meat placed on a load cell in the fish tank served as a stone platform to replicate resilience of live tissue during stone pulverization. Margins of the stone were kept free to reduce artefacts of lateral extraneous pressures. The fish tank was filled with physiologic saline until the distal nephroscope was immersed. The operator sat in front of the fish tank with laser protection goggles.
In vitro bench for pulverization.
The energy source was introduced through the nephroscope channel lightly sandwiching the stone between the probe/fiber and the platform (maintaining the load cell reading below 1 g). The paraboloid pneumatic lithotripter (at generator settings of 2, 3, and 4 bars) and holmium laser (6, 10, 15 W with 800 μ fiber) were used to deliver single energy pulses (frequency of 12/min) onto the phantom. The number of impacts were counted. Pulverization was observed through the endocamera system.
Three predetermined end points were used to evaluate pulverization dynamics: (1) The number of impacts needed to create the first dent in the phantom (at point contact); (2) the number of impacts needed to break the stone into two pieces or bore through the stone (follow-up impacts). Twenty stone phantoms were used for each energy cohort.
Study of the propulsion dynamics
The underwater bench arrangement (Fig. 3) with a transparent acrylic tube (ramped up at 45 degrees to the horizontal) was used. The inner diameter of the tube being more than the diameter of the phantom allowed free roll of the phantom in the tube. The phantom was placed at the lower end of the tube in contact with the energy source. Twenty impulses each of the same energy cohorts (as for the pulverization module) were delivered. The maximum inline distance travelled by the stone from the probe/fiber was recorded using an attached scale. Once the stone rolled back to the probe, the subsequent impact was delivered.
In vitro bench for propulsion.
The Mann-Whitney U test for used to compare 2, 3, and 4 bars of pneumatic energy with 6, 10, and 15 W of holmium energy, respectively.
Results
CAD analysis
At point contact (when the stone is hit for the first time), the paraboloid lithotripter generated 3590 bars at generator settings of 4 bars (Fig. 4A). During “follow-up impacts,” as the contact area increased, the tip pressure exponentially decreased and the lateral/centrifugal forces increased (Figs. 4B, 5B). Similar pressure dynamics were observed at assumptions of generator pressures of 2 and 3 bars. The conventional probe showed constant tip pressures throughout the pulverization (Fig. 5A).
Computer aided design analysis:
Bench analysis
No malfunction was seen with either device, and both the energy sources could reach the end points of study (Table 1). It was observed that pulverization at point contact for the paraboloid lithotripter at 2, 3, and 4 bars was comparable to that of 6, 10, and 15 W of laser, respectively (P<0.005), indicating equivalent stone dusting properties of the paraboloid and the laser.
N=number of phantom stones; PL=paraboloid lithotripter; NA=not applicable.
A statistically significant difference was observed during the follow-up hits wherein the paraboloid lithotripter always broke, whereas the laser always bored through the phantom (an attribution to the lateral forces generated by the paraboloid). In clinical settings, with large stones, this would be a significant advantage for the paraboloid when the desired end point is stone fragmentation.
No statistical difference was observed in the number of impulses needed to reach these end points. Incremental energy levels and their efficiency to reach the study end points demonstrated a linear relationship.
Stone propulsion was comparable within all energy cohorts (P>0.05). This is because of the crumple zone effect at the point of contact.
Role in Endourology
Conventional probes have been traditionally used for stone pulverization, and the impact pressure generated at the tip of the probe (namely 16 bars for a 3 mm probe and 359 bars for a 2 mm probe when the generator settings is 4 bars) 2 is unidirectional/end firing. This force is directly proportional to the pressure transferred to the tip along the shaft of the probe from the lithotripter hand piece. This is independent of the diameter of the probe but inversely proportional to the area of contact between the stone and the probe. 2 Inconsistent efficiency, especially in hard stone scenarios, results in involuntary application of excessive axial pressure on the probe with the possible potential of renal trauma. This has led to a quest for better energy sources such as the laser, ultrasonic, and combination modalities, etc.
The paraboloid lithotripter acts as a highly focused end-firing source at point contact (namely 3590 bars at 4 bars). This causes visible dusting, leaving behind a pit on the stone, comparable to that of the laser. Dusting property was an observation made by all the researchers. This occurs only at first hit. During the experiment, one can see the stone dust emerging from the contact site of the paraboloid, leaving behind a pit on the stone. The same is very difficult to quantify (because dust dissipates in the water surrounding the underwater bench). Also, this high tip energy at point contact causes the stone to crumble, leading to dissipation of energy and low propulsion (the crumple zone effect). The high energy lasts only until a 0.1 mm depth pit is created in the stone (Fig. 5B), thus rendering tissue protective properties.
At follow-up hits, more surface area of the probe tip is in contact with the stone. This decreases the impact pressure at the tip, and the paraboloid converts into a lateral/side-firing source. This augments stone fragmentation into pieces as against the laser, which only bores through the stone. The conversion point of end firing into lateral firing correlates with the “axial total” and “centrifugal total” force plots crossing each other (Fig. 5B).
We believe that there are two scenarios in the kidney while pulverizing stones.
(1) Stone in a dilated system: In a dilated system, the stone migrates or moves so that it is not possible to hit the stone at the same place each time. Hence, each time the probe would end up hitting different places, implementing the dynamics of the first hit each time. (2) Stone in a nondilated system: In the case of large impacted renal stone and probably stones large enough to occupy most of the collecting system or a part of it, while fragmenting the stone, one tends to hit the stone at the same point to achieve the goal. This happens because the stone does not migrate. The bench design replicates the same.
The safety profile of the paraboloid lithotripter is likely to be similar to that of other metal devices and hence, as a universal precaution, it should be used only under direct vision, keeping away from the mucosa.
Also because of the favorable physics of the paraboloid lithotripter, it not necessary to learn special techniques such as the dancing, chipping, fragmenting, and popcorn techniques as done to make laser lithotripsy more efficient.
In our opinion, modifications in the probe tip head would open a new and economically less challenging avenue for enhancement of the pulverization properties of intracorporeal pneumatic lithotripsy.
Conclusion
The paraboloid lithotripter generates highly focused impact force with low propulsion (comparable to the laser) at point contact. As stone pulverization progresses, the tip forces exponentially decrease, and the probe converts into a lateral-firing energy source resulting in fragmentation in the setting of large stones. Thus, the paraboloid lithotripter has comparable advantages of the laser at point contact and a lateral-firing pneumatic lithotripter at follow-up hits, akin to being a bimodal energy source. We propose the paraboloid lithotripter as an energy modality comparable to laser for lithotripsy. Also, further study of the tip designs and their physics may open newer avenues to augment the efficiency of the conventional pneumatic lithotripter.
Footnotes
Disclosure Statement
No competing financial interests exist.