The first thing to address in this story is that most "bone cysts" are not typically "cysts," by strict definition. The medical dictionary defines a cyst as, "a structure lined with epithelium (a special cell type) and generally filled with fluid." The key part is that there is a lining of cells that secrete the fluid. With respect to a "bone cyst," there usually is not a secretory cell lining, and, therefore, it does not qualify as a true cyst; there are many authors and researchers who will call these bone lesions a "cyst-like" lesion. Now, with the pure terminology and definitions out in the open, I will take the liberty and refer to these lesions as cysts–some medical dictionaries will define a "bone cyst" as "a solitary bone lesion."

A surgical instrument has been inserted into the cyst shown at left during treatment. Courtesy Dr. Michael Ball.

It would be prudent to review some anatomy and growth specifics regarding bone in an effort to better understand the development of these cyst lesions. Bone is considered to be a special form of connective tissue and is made up of microscopic crystals of calcium and phosphate within a matrix of collagen. Remember, collagen is a protein structure that is also a major structural substance of skin and tendons. Within the collagen fibers are the bone crystals, which are made up primarily of a substance called hydroxyapatite (crystals of calcium, phosphate, water, and hydroxide)