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Can the Appearance of Canine Uroliths Reveal Its Composition?

Urolith shape and surface texture are influenced by many factors (e.g. location, growth rate, nidus shape, urine characteristics); however, stone composition is a powerful determinant of urolith appearance.  We evaluated the shape and surface texture of a random sample of 948 canine uroliths categorized by their composition (536 struvite, 172 calcium oxalate monohydrate (COM), 72 calcium oxalate dihydrate (COD), 66 urate, 99 cystine, and 3 silica); compound and mixed stones were excluded. 

various uroliths shapes

Most stones (61%) were round regardless of their chemical makeup.  When crystals are allowed to grow outwardly in all dimensions, growth progresses at similar rates along all planes forming a sphere, although imperfect. When multiple large stones are present, stone growth is constrained at areas of mutual contact.  By blocking growth sites, chemicals precipitate in the open spaces between stones.  This provides one explanation as to why 90% of pyramidal stones are composed of struvite, which is a fast-growing stone type.  In contrast, 78% of all cylindrical stones had a nidus consistent with suture.  In this instance, stone growth starts and continues outwardly mimicking the shape of the suture nidus.

We categorized surface texture as smooth (including fine granular), plates, bosselated (like a cluster of grapes), or spikes.  The surface of many stone types was smooth.  Of 412 smooth stones; 70% were struvite, 10% were urate, 9% were cystine, 9% were calcium oxalate monohydrate, and 1% were calcium oxalate dihydrate.   Other surface textures can also aid prediction of stone type. For example, of 252 stones with projecting plates, 52% were struvite, 25% were calcium oxalate monohydrate, and 20% were calcium oxalate dihydrate; other stone types were less than 2%.  Of 108 bosselated stones; 45% were calcium oxalate monohydrate, 35% were cystine, 11% were struvite, 6% were urate, and 3% were calcium oxalate dihydrate.  100% of spiked stones were composed of silica.

Although urolith appearance aids prediction of urolith composition, quantitative analysis is necessary to develop accurate therapeutic protocols to minimize recurrence. This is especially true when the outward appearance masks a urolith’s interior contents. 

This student-run research project was undertaken by Amy Blise (UMNCVM 2024) and supported by the Minnesota Urolith Center

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