Hi Xue,
In my mind calcinosis means abnormal deposition of calcium salts into a soft tissue structure.
Calcification of that soft tissue would mean the same thing so I think they are fairly interchangeable.
I use the word sesamoid where there is an expected ossification inside a tendon. For example the sesamoids in the two heads of flexor hallucis brevis in the great toe of the patella in the quadriceps tendon. These are mostly present in the majority of people.
I use the word accessory bone when it is a bone that has failed to fuse properly and has left an embryological gap such as the os acromiale.
But I also use it to describe an os tibial externum in there tib post tendon adjacent to the navicular.
These examples are not present in the majority of people.
Another way I define them is sesamoids are standard design features where a tendon must be toughened up into a bony formation to assist it turn a corner or withstand high forces.
Sesamoids have a reasonable length of tendon between them and the final insertion of the tendon.
Accessory ossicles are very close to the adjacent bone and often require a synchondrosis to articulate with them,
I hope this helps Xue,
Steve.