Calcium pyrophosphate crystal deposition disease

Below are some key points from a couple of free full text reviews [1] [2].

Terminology has changed and can be confusing

The current official term is calcium pyrophosphate crystal deposition (CPPD). Pseudogout, a term which historically referred to the acute attacks of CPPD, has been replaced by “acute calcium pyrophosphate crystal arthritis.” The plain radiographic finding known as chondrocalcinosis is frequently replaced by the term “cartridge calcification.” CC is not universal in patients with attacks nor is it entirely specific for CPPD.

Crystal identification is important in diagnosis

As opposed to the needle like negatively birefringent crystals of gout, the crystals of CPPD are variable in shape, often rectangular, and if birefringent at all, only weakly positively birefringent.

There are some disease associations

From the second reference above:

Metabolic disease associations of CPPD include haemochromatosis (18), hyperparathyroidism (19, 20), hypomagnesemia (21) and hypophosphatasia (22). Other diseases such as diabetes mellitus and hypothyroidism do not associate with CPPD once adjusted for age (22, 23). Haemochromatosis is the only metabolic disease associated with CPPD that results in structural arthropathy, and this commonly affects the knees, wrists, hips, MCPJs, and ankles (18, 24).

There are some genetic determinants as well.