Table 1: Characteristics of most common crystals and foreign bodies in synovial fluid or synovium*.

Crystal [Chemical formule] Breaking direction (Elongation) Breaking intensity Shape Size
Monosodium urate (MSU)-[C5H4N4O3]- Figure 2 negative intensive needle,rod, sperule Submicroscopic -40 μm 5-25 μm
Calcium pyrophosphate dihydrate (CPPD) [Ca2P2O7.2H2O]- Figure 3 positive strong rod, rhomboid < 40 μm
Calcium hydroxyapatite – HA [Ca5(PO4)3(OH)], [Ca10(PO4)6OH2] or [Ca5(PO4)3OH.2H2O] **- Figure 4 positive weak rod, clusters: shiny coins individual crystals:
submicroscopic [1]
50-500 nm [2]
clusters of crystals: 1-5 μm [2]
1.9-15.6 μm [3]
Octocalcium phosphate-[Ca8H2(PO4)6.5H2O] ND ND ND ND
Tricalcium phosphate-[Ca3(PO4)2] (whitelockite) ND ND ND ND
Dicalcium phosphate dehydrate [Ca2(PO4).2H2O] ND ND ND ND
Calcium hydrogen phosphate dihydrate-[CaH(PO4).2H2O] (orthophosphate, brushite) positive moderate rod 1-2 μm
Calcium oxalate [CaC2O4.H2O] positive variable tetrahedron, rod 1-2 μm
Cholesterol- [C27H46O]- Figure 5 negative and/or positive variable rhomboidal, notched, needle-shaped cloven -separate or clusters 5-40 μm [3]
Liquid lipid crystals- Figure 5 Maltese cross, positive variable spherules 0.5-30 μm [3]
Lithium heparin positive weak polymorphous 2-5 μm
Corticosteroid variable mostly strong polymorphous 1-40 μm
Talc Maltese cross, negative strong ovoid 1-40 μm
Glass fiber positive strong orbed-oval variable
Surgical sutures negative strong clusters variable
Methylmetacrylate intensive fragments variable
Metallosis (swarf) none none spotty deposits variable
Hemosiderin crystals Maltese cross, positive weak groups 5-40 μm
Osmium none none spotty deposits variable
Ochronosis (artifact) colored refractile collagen fragments positive variable fragments variable

*Modified from Gatter and Schumacher [3] and from Gardner and McClure [4].
ND- no data; **the terminology of hydroxyapatite is not uniform in the pertinent literature; the formula of HA is: [Ca5(PO4)3(OH)] or [Ca10(PO4)6OH2].