calcium pyrophosphate, hydroxyapatite and corticosteroid crystals. Monosodium urate crystals are seen in gout or gouty arthritis and appear as needle-shaped negatively birefringent crystals varying in length from 2 to 20 μm. With negative birefringence, the crystals appear yellow in parallel light and blue with perpendicular light. Calcium pyrophosphate crystals are seen in pseudogout (also known as calcium pyrophosphate deposition disease or, CPPD). These crystals are rod-shaped or rhomboids varying in length from 2 to 20 μm and with positive birefringence (blue with parallel light, yellow with perpendicular light). Hydroxyapatite crystals are small and negatively birefringent. They are usually only detectable with an Alizarin Red S stain. Corticosteroid crystals may be seen following therapeutic corticosteroid injection into the joint space. They appear blunt, jagged, and show variable birefringence. Cracking joints When the two articulating surfaces of a synovial joint are separated from one other, the volume within the joint capsule is increased and a negative pressure results. The volume of synovial fluid within the joint is insufficient to fill the expanding volume of the joint and gases dissolved in the synovial fluid (mostly carbon dioxide) are liberated and quickly fill the empty space, leading to the rapid formation of a bubble. This process is known as cavitation. Cavitation in synovial joints results in a high frequency 'cracking' sound. Etymology and pronunciation The term synovia () came to English around 1640 (the anglicized form synovial is first recorded in the mid 18th century) from New Latin, where it was coined perhaps by Paracelsus from Greek συν- "with" and Latin ovum "egg" and -ia because it resembles egg white in consistency and external appearance. The term synovium is a much more recent pseudo-Latin coinage for what is less confusingly called the synovial membrane. It is not recorded in general dictionaries, and medical dictionaries only explain its meaning, not its etymology, but it is apparently derived from the term synovia, i.e. the obfuscated etymology of mixed Greek and Latin elements of the singular term synovia was misunderstood and the word was erroneously reinterpreted as the plural of the previously non-existent term synovium (perhaps in analogy to other plural terms for liquids such as "waters" for amniotic fluid). If one insists on using this pseudo-Latin term synovium for the synovial membrane, the non-Latinate plural synoviums is better and less confusing than synovia. References Further reading External links Hyaluronan: structure and properties Normal joint structure from