Microhardness properties of human gallstones and synthetic stones

Abstract

Extracorporeal shock-wave lithotripsy is a highly successful medical procedure and has become the preferred method of treatment for over 80% of patients with kidney stones [1]. However, approximately 75-85% of patients with gallstones are currently excluded from extracorporeal shock-wave lithotripsy based on the US Food and Drug Administration and European protocols [2, 3]. Among the patients who undergo gallstone lithotripsy, reported values of satisfactory fragmentation, based on the reduction of the stone(s) to fragments 3-5 mm in diameter or less, vary from 22 to 78% [4]. This variation is due, in part, to the variation in stone populations in the reported studies, with single stones and stones under 20 mm in diameter having higher rates of satisfactory fragmentation. Many factors may contribute to the lower overall success rate of gallstone lithotripsy, including the mechanical properties of the stones and the environment in vivo. The ease of fragmentation of kidney stones has been reported to be related to their chemical composition [5]. Also, the microhardness of kidney stones has been shown to vary with composition [6-8]. Although the precise relationship between the characteristics of the stone and its behaviour during lithotripsy has not yet been established, the response of a stone during lithotripsy depends on its mechanical properties. According to the fracture theory of failure, the mechanical properties (in particular, the fracture toughness and yield stress of a stone, along with its flaw size distribution) determine whether the stone will fragment when subjected to a prescribed stress. During lithotripsy the stress is applied by an acoustic shock wave as it passes through the stone. Knowledge of the mechanical properties of gallstones is needed to understand better the mechanisms of fragmentation and to improve the effectiveness of gallstone lithotripsy. Synthetic stones are now available to use when comparing lithotripters. These synthetic stones will be useful in determining the in vivo efficiency of lithotripters only if they behave in a manner similar to biological stones. We have shown previously [9] that the amount and type of damage to synthetic concretions during in vitro lithotripter experiments depend on the properties of the concretions used in the tests. Therefore, the mechanical properties of synthetic stone materials are also of interest. In this letter, microhardness measurements for both cholesterol and pigment gallstones stored dry and in saline are presented and compared with 554 published values for kidney stones. In addition, the hardness values of currently available synthetic stone materials were measured and compared with values for human stones. In some gallstones spatial variations of the hardness within the stone were also noted. The 20 gallstones from 18 patients used in this study were obtained at surgery for uncomplicated cholelithiasis. These gallstones ranged in size from 3 to 20 mm in diameter. Five stones were stored dry in airtight containers for at least 4 weeks. Measurements of the hardness of these dry gallstones were obtained for comparison with published data for dry kidney stones. The remaining 15 stones were placed in isotonic saline directly after surgical removal from the patients. Measurements on wet stones are believed to be more indicative of the in vivo hardness values. The set of stones stored dry consisted of four cholesterol and one pigment, whereas the set of stones stored wet consisted on nine cholesterol and six pigment. Determination of the stone type was made by visual inspection, as described by Trotman et al. [10]. Also, synthetic stones obtained from the American Lithotripsy Society (ALS) and from Lithotech, Atlanta, Georgia, USA (HMT) were tested, both dry (as supplied) or after soaking in tap water for at least 30 min and compared with values for concretions made from a 1:1 mixture by weight of plaster of Paris and to those for biological stones. To obtain a smooth, flat surface for testing, all gallstones were polished using silicon carbide paper (240-1200-grit) and/or alumina powder (5, 1 and 0.05/~m). The polishing technique was varied