True bone cysts, defined as a closed cavity lined by epithelium and containing fluid or semi-solid material, are relatively infrequently identified in the horse. Subchondral bone cysts (also called subchondral cystic lesions or osseous cyst–like lesions) are not true bone cysts because they lack an epithelial lining and frequently communicate with an articular surface. These characteristics have resulted in support for the use of the term osseous cyst–like lesions (OCLLs). The phalanges are the second most common anatomic location in which OCLLs are recognized in the horse (26.2% prevalence among all reported OCLLs).

Osseous cyst–like lesions are often found on the weight-bearing surface of joints, and most communicate with the joint through tracts of varying sizes. The lesions have marked variation in size (ranging from shallow defects to lesions >10 mm deep) and shape (dome shaped, conical, or spherical). On macroscopic evaluation, OCLLs have a fibrous lining and are filled with fibrous tissue, with or without synovial gelatinous material. Some OCLLs also contain fibrocartilage, which may be partially mineralized, and some may occasionally contain necrotic bone. Fibrous tissue within OCLLs produces nitric oxide, prostaglandin E₂, and neutral metalloproteinases. Conditioned media of OCLL tissue harvested arthroscopically induce the recruitment and activation of osteoclasts in vitro. This increased osteoclastic activity is likely to be associated with expansion of cysts in vivo. Osseous sclerosis of varying intensity may surround OCLLs, depending on the stage of development.

Physical examination abnormalities in horses with phalangeal OCLLs may be subtle, and specific details are described for each anatomic area below. Lameness in horses with phalangeal OCLLs is variable; some horses develop severe lameness, whereas others have mild intermittent lameness. Some horses have no lameness. The speed of onset of associated lameness is also variable. Lameness may not be appreciated until training is commenced in young horses, whereas there is often a history of previous or ongoing intraarticular inflammation in middle-aged or old horses. Lameness has been attributed to pain originating from the joint (synovitis) or the subchondral bone (increase in intraosseous pressure), an increase in intracystic pressure, or a combination of these. Osseous cyst–like lesions that are close to an articular surface are more likely to be associated with lameness than those that are distant from an articular surface. Lameness can subside with rest but often recurs when exercise resumes. Lameness may be exacerbated by distal limb flexion; however, most horses with phalangeal OCLLs are not responsive to hoof testers.