Renal papillary calcification and the development of calcium oxalate monohydrate papillary renal calculi

Background: The objective of this study is to determine in a case series (four patients) how calcified deposits in renal papillae are associated with the development of calcium oxalate monohydrate (COM) papillary calculi. Methods: From the recently collected papillary calculi, we evaluated retrospectively patients, subjected to retrograde ureteroscopy, with COM papillary lithiasis. Results: The COM papillary calculi were found to result from subepithelial injury. Many of these lesions underwent calcification by hydroxyapatite (HAP), with calculus morphology and the amount of HAP in the concave zone dependent on the location of the calcified injury. Most of these HAP deposits grew, eroding the epithelium covering the renal papillae, coming into contact with urine and starting the development of COM calculi. Subepithelial HAP plaques may alter the epithelium covering the papillae, resulting in the deposit of COM crystals directly onto the epithelium. Tissue calcification depends on a pre-existing injury, the continuation of this process is due to modulators and/or crystallization inhibitors deficiency. Conclusions: Since calculus morphology and the amount of detected HAP are dependent on the location and widespread of calcified injury, all types of papillary COM calculi can be found in the same patient. All patients had subepithelial calcifications, with fewer papillary calculi, demonstrating that some subepithelial calcifications did not further evolve and were reabsorbed. A high number of subepithelial calcifications increases the likelihood that some will be transformed into COM papillary calculi.