In idiopathic calcium oxalate stone-formers, unattached stones show evidence of having originated as attached stones on Randall's plaque

Abstract

Objective: To analyse the structure and composition of unattached stones in idiopathic calcium oxalate (CaOx) stone-formers (ICSF) and compare them to attached stones from the same cohort, to investigate whether there is more than one pathogenic mechanism for stone formation in ICSF.

Patients and methods: ICSF undergoing percutaneous nephrolithotomy or ureteroscopy for the treatment of nephrolithiasis gave consent to participate in this study. All accessible renal papillae were endoscopically imaged using a digital endoscope. All stones were removed and determined by the operating surgeon to be attached or unattached to the underlying papilla. Micro-computed tomography (micro-CT), which provides three-dimensional analysis of entire stones, was used to compare the structure and composition of attached and unattached stones.

Results: Of 115 stones collected from nine patients (12 renal units), only 25 stones were found not to be attached to renal papillae. Of these 25 stones, four were lost and 12 showed definite morphological evidence of having been attached to tissue, probably having been displaced from papillae during access. For the remaining nine stones, micro-CT analysis showed at least one internal region of calcium phosphate within each of these unattached CaOx stones, i.e. the internal structure of the unattached stones is consistent with their having originated attached to Randall's plaque, and then having become detached but retained in the kidney, with new layers of CaOx eventually covering the original attachment site. CONCLUSIONS; Micro-CT analysis supports the hypothesis that in ICSF, both attached and unattached stones occur as a result of a common pathogenic mechanism, i.e. in this type of stone former, CaOx stones, even those not showing morphology that betrays attachment, all originate attached to interstitial plaque on the renal papilla.

Figure 1

Example of a stone found loose in the collecting system of a patient, but which shows morphological evidence of having been recently attached to tissue. A: View of stone immediately after removal, showing region (*) that appeared to be covered with a mixture of tissue and mucus. Background grid shows 1 mm squares. B: Opposite surface of stone, presumably the surface exposed to the urine; clot of blood on the left is that seen in panel A clinging to right side of stone. C: Surface image of micro CT reconstruction of stone; closest to the viewer is the right edge of view A (left edge of view B). Arrow indicates surface corresponding to marked region in panel A. This region also showed material at the surface that was x-ray bright, indicating the presence of calcium phosphate (CaP), most likely in the form of apatite.

Figure 2

Examples of two stones that showed no evidence of having been recently attached to tissue. A and C: Micrographs of stones on mm background grid. Images of other sides of these stones looked very similar to those shown. B and D: Micro CT section through each stone, shown below its corresponding micrograph. Stone in A shows significant region of interior CaP (white region in panel B), surrounded by calcium oxalate monohydrate. Stone in C displayed smaller regions of interior CaP, one of which is indicated by arrow in D.

Figure 3

Demonstration of interior position of internal CaP regions. A: Surface rendering of same stone as in Figure 2A/C. B: Maximum intensity projection with same orientation as in panel A; white regions within the stone indicate CaP. C and D: Similar surface and maximum-intensity views as A and B, but image stack was rotated 90° about the horizontal axis; note that regions of CaP are completely interior, without any portions that extend to the stone surface.