Craniopagus parasiticus is an extremely rare type of parasitic twinning occurring in about 2 to 3 of 5,000,000 births. In craniopagus parasiticus, a parasitic twin head with an undeveloped body is attached to the head of a developed twin. Fewer than a dozen cases of this type of conjoined twin have been documented in literature. Development
The exact development of craniopagus parasiticus is not well known. However, it is known that the underdeveloped twin is a parasitic twin. Parasitic twins are known to occur in utero when monozygotic twins start to develop as an embryo, but the embryo fails to completely split. When this happens, one embryo will dominate development, while the others development is severely altered. The key difference between a parasitic twin and conjoined twins is that in parasitic twins, one twin, the parasite, stops development during gestation, whereas the other twin, the autosite, develops completely.In normal monozygotic twin development, one egg is fertilized by a single sperm. The egg will then completely split into two, normally at the two-cell stage. If the egg splits in the early blastocyst stage, two inner cell masses will be present, eventually leading to the twins sharing the same chorion and placenta, but with separate amnions. However, the egg can split into two, but still have one blastocyst. This will lead to one inner cell mass and one blastocyst. Then, as the twins develop, they will share the same placenta, chorion, and amnion.
It rarely involves the temporomandibular joint (TMJ) and most publications are case reports.Synovial chondromatosis occurs twice as commonly in males as females and usually in their forties. However, online communities for synovial chondromatosis patients have yielded a stark contrast, with equal representation from both genders and members diagnosed as young as late teenage/early 20s. Familial synovial chondromatosis with dwarfism introduces characteristics of dwarfism. Cause
The exact underlying cause of synovial chondromatosis is unknown. Some evidence suggests trauma may play a role in its development as it mainly presents in weight-bearing joints. Infection has also been considered as a contributing factor. The condition is not inherited.Synovial chondromatosis can reportedly occur as either a primary or secondary form. Primary synovial chondromatosis, which is more rare, occurs spontaneously and does not appear to relate to any pre-existing conditions. Secondary synovial chondromatosis is the more common form and often occurs when there is pre-existent osteoarthritis, rheumatoid arthritis, osteonecrosis, osteochondritis dissecans, neuropathic osteoarthropathy (which often occurs in diabetic individuals), tuberculosis, or osteochondral fractures (torn cartilage covering the end of a bone in a joint) in the affected individual. Diagnosis
Diagnosis is by medical imaging; X-ray, CT scan and MRI. For diagnosis of the TMJ, Cone beam computed tomography (CBCT) can also be useful. Treatment
Treatment is frequently by means of removal of the loose bodies and of a partial or full synovectomy (removal of the synovium)
Full synovectomy is a moderately major operation and involves completely exposing the joint and removing the affected tissue. Partial synovectomy is normally done arthroscopically.