Kidney Stone Models for In Vitro Lithotripsy Research: A Comprehensive Review

Introduction

Urinary lithiasis is a common disease with a lifetime risk of 12% in men and 5% in women. ¹ The incidence of disease varies by age, with peak occurrence between the ages of 20 and 40 years, sex with highest risk among men (∼10%), and ethnicity—with the highest risk among Caucasians. ¹

Renal calculi in humans are composed of calcium oxalate monohydrate and dihydrate, calcium monohydrogen phosphate, uric acid, cystine, and magnesium ammonium phosphate hydrogen (MAPH). Each of the aforementioned calculi in humans has a unique chemical composition and crystalline structure, which explains their distinct mechanical properties. ² By exploiting these properties, technological advances in intracorporeal and extracorporeal lithotripsy have reduced the morbidity of stone surgery, while continuously improving efficacy. To develop and evaluate these lithotripters, the generated forces and resulting effects on stones and surrounding biologic tissue must be understood. Although appealing, the use of physiologic stones yields many challenges (i.e., dehydration-rehydration of stored stones, size/shape dissimilarities, fragmenting during extraction, etc.). As such, many stone models or “phantom” stones have been developed for in vitro experimentation.

In this review, we will discuss the existing models described in the literature, and highlight the fabrication and unique properties of each.

Mechanisms of Stone Comminution

Development in surgical equipment and technologies, starting in the 1980s, has provided numerous tools for the intra- and extracorporeal fragmentation of urinary calculi for treatment. ³ Each relies on slightly different properties of a stone's crystalline structure to affect fragmentation. Initially, crushing forceps simply relied on compressive forces of the stone as a whole to produce fragments. With technologic advances, however, some newer lithotripters only act at the surface of a stone, while others exert both internal and external forces on calculi.

Urinary calculi are heterogeneous structures composed of organic and inorganic components. The interface between the organic and inorganic compounds in stones contributes to their unique crystalline structures, ultimately contributing to the strength and weaknesses of the stone. Fracturing of urinary calculi has similarities to the fracture of any brittle structure. Stress causes cracks along presumed fault lines within the structure of the stone, which grow and coalesce with continued application of a stressor. ^4,5

Pneumatic lithotripsy uses ballistic forces to transfer kinetic energy from the probe to the surface of the stone. This jackhammer-like action fragments stones at the point of contact. Similarly, ultrasonic lithotripters use a vibrating probe to transmit reverberations to the stone surface, producing fine debris as the oscillating metal tip contacts the irregular stone surface. As rigid objects are struck, local fracturing occurs, while fragments can also be cleaved along natural fault lines.

In contrast, shockwave lithotripsy (SWL) and electrohydraulic lithotripsy use both direct shockwave exposure and cavitation bubble collapse to produce both external surface and internal mechanical stress. Several postulated mechanisms include spallation (tension created by the interference of longitudinal waves and their inverted reflections), squeezing (narrow band of pressure encircling a stone), fracture, shear stress, superfocusing, acoustic cavitation, and dynamic fatigue.

Pulsed-dye laser lithotripters are theorized to generate pulses with duration of pico- to nanoseconds that may induce cavitation bubbles, which on collapse result in laser-induced shockwave lithotripsy. Conversely, holmium:yttrium-aluminum-garnet (YAG) laser lithotripsy depends on energy density, suggesting that stone communition is dependent on photothermal energy absorption, which leads to thermal breakdown and stone dessication. ⁶ Consequently, holmium:YAG laser lithotripsy is dependent on heat dispersion and thermal ablation.

Therefore, based on the mechanisms being studied, one may wish to consider the characteristics of the available models to best represent the in situ effects as closely as possible. Ideally, several model types may be needed to fully characterize the fragmentation capabilities of certain devices. For example, smooth round stones may provide challenges that would not be identified if only irregular rough stones were used.

Metrics for Measuring Urinary Calculi

Numerous metrics can characterize mechanical properties of a solid; however, the clinical value lies in understanding those that aid in efficient stone fracture. Mechanical durability in early artificial renal calculi was described using compression—“crushing”—strength. The compressive strength of urinary lithiasis has been described to range from 3.2 to 6.17 megapascals (MPa). ⁷ The comminution of renal calculi—which are brittle—is more a function of tensile strength, however. Tensile strength is the maximum stress an object can withstand while being stretched and is measured in pascals (Newton per meter-squared). Using BegoStone Plus (Table 1), Simmons and associates ³ demonstrated similar tensile strengths for phantom models in comparison with MAPH, uric acid, cystine, calcium oxalate, and calcium phosphate calculi in vivo.

In addition, mechanical strength can be measured using microhardness (Vickers hardness), ⁸ bulk modulus (K), Young modulus (E), shear modulus (G), and Poisson ratio. These are standardized metrics that describe the mechanical properties of a solid material under physical stress, which apply to forces exerted on a stone during shockwave fragmentation.

Vickers hardness is a measure of a material's ability to resist plastic deformation. The test uses an indenter and is a standardized method for measuring hardness of a solid material. Vickers hardness is a metric that may help practitioners determine the efficacy of treatment modalities that rely on the application of external forces such as pneumatic and ultrasonic lithotripters. K, also measured in pascals, refers to a solid's resistance to uniform compression and correlates to external forces such as squeezing described for SWL. E is a measure of the tendency of an object to deform along an axis when opposing forces are applied along that axis, measured in pascals. G, measured in pascals, refers to the stiffness of a material by evaluating the deformation of a substance when a force is applied parallel to its surface. Both E and G are surrogates for defining a material's resistance to external forces. It is important to recognize that higher values of metrics such as K, E, and G lead to more difficult stone fragmentation with SWL and other lithotripters. The Poisson ratio is a measure comparing the ratio of expansion to compression when a material is compressed in one direction and is an important mechanical property used to describe stone communition with any lithotripters that apply external compressive forces.

The acoustic properties of renal calculi can be measured using ultrasonography. Heimbach and colleagues ⁹ describe measurement of transverse and longitudinal acoustic speed and impedance—the degree of wave energy reflection and transmission between two dissimilar surfaces—with sonography. The higher the wave impedance, which is the product of wave speed and stone density, the harder it is to comminute a urinary calculi by SWL, and as such, the highest measurements of these two measures are seen in calcium oxalate monohydrate stones.

Evolution of Stone Models

Early calculi phantoms were developed from chalk, ceramic material, dental cement, and plaster-of-paris. ⁹ These artificial models provided structures with hardness properties that could be used to mimic in vivo urinary lithiasis, but had limitations in their value in simulating fragmentation properties of natural stones attributed to the fact that they were composed of elements not found in naturally occurring renal calculi. In the early 2000s, artificial stones produced from “natural” elements were developed and provided an in vitro model that more closely replicated fragmentation—especially with regard to SWL. These phantom renal calculi are an important instrument for research on lithotripsy, allowing scientists to provide standardized and reproducible results.

Early Models of Artificial Calculi

Many materials were used to develop early models of urinary calculi for in vitro experimentation, and a few persist as current kidney stones phantoms for research. Although these early stone models have provided useful data for SWL and lithotripsy, they are limited by the absence of natural elements and components seen in human urinary lithiasis. In particular, they typically represent a homogeneous substance, in contrast to the complex milieu of crystal, protein, and matrix that constitutes a human stone.

Discussion

Technologic advances, including endoscopic therapy, have driven stone fragmentation. New lithotripters have all exploited various aspects of crystal deformation and fracture. Well-designed in vitro and in vivo studies help clinicians and researchers compare the efficacy of different treatment modalities in a controlled setting, and for this purpose, phantom calculi are needed. It is clear that no one model is adequate to represent every stone type. Each crystalline composition possesses unique mechanical profiles, which may change their susceptibility to fracture.

Nonetheless, there are some general principles that can be derived from this review regarding which stone models can be used to mimic in vivo calculi. Simply, soft stones such as uric acid and MAPH can be simulated by Ultracal 30 with a ratio of 1:1 or 100:38 of powder to water. Harder stones, such as brushite and calcium oxalate, can be simulated with BegoStone, with varying mechanical and acoustic properties based on the ratio of powder to water (Table 1). In fact, the published data from the Preminger group show that BegoStone and BegoStone Plus models can be made that replicate all naturally occurring stones, with stones with a powder ratio of 15:3 replicating calcium oxalate monohydrate stones. ³ Begostone and Ultracal 30 are perhaps the most commonly used models for research and demonstrate ease of use with regard to manufacturing. Moreover, they can be molded to produce a wide range of shapes and surfaces.

By understanding the forces created with a lithotripter and clinical settings during which it is used, one can then select appropriate models for evaluation. Standardization of the stone models used in these studies would help facilitate a critical analysis of the literature. As such, it is important for clinicians and researchers to be familiar with the terminology and the unique characteristics of the most commonly used stone models.

The evolution of phantom models created from natural components holds promise. Given some refinement, a catalog of models could be created, allowing one to test new technologies, both diagnostic and therapeutic, against phantom stones possessing the entire range of characteristics found in commonly encountered compositions. The current concern regarding stone models that use elements of naturally occurring stones is the complexity of their production, which undoubtedly requires more resources and time. Nonetheless, advances in producing phantom calculi from naturally occurring stone elements will greatly facilitate the development of standard testing models for future research on urolithiasis. In addition, these stones could provide simulation models for training future clinicians in the use of various endoscopic lithotripters.