Pars defects often lead to spondylolisthesis.
A pars defect of the lumbar spine involves a part of a vertebra called the pars interarticularis. Roughly translated, that means "the part between two joints." A defect in the pars is a break in this portion of bone. This break leads to a separation of the upper, front portion of the vertebra from its lower, back portion. This condition is a precursor to another condition called spondylolisthesis, a word that refers to the slipping of a vertebra.
Each lumbar vertebra has a large, blocky front portion called the "body." The rear portion has several knobby parts that project from it. Two of these project downward, where they form joints with the segment below, and two project upward to form joints with the segment above. The bony bridge that connects the upper and lower projections is the pars interarticularis, and there is one on each side of the vertebra. The pars interarticularis is sometimes just called the pars.
Pars defects sometimes develop on one side only, but if breaks occur that involve both the right and the left pars, the vertebra becomes separated into two pieces. Proper healing of bone breaks requires that the fracture ends be immobilized in close contact. These conditions are rarely achievable with pars injuries, so the breach fails to mend. This persistent separation is what is called a pars defect. For many years, it was assumed that pars defects were congenital anomalies, or birth defects. Today, we understand that most pars defects are really stress fractures that usually occur in the spines of young people. The presumed cause is repeated hyperextension, or backward bending, of the lumbar spine. Pars defects often affect athletes whose spines are frequently stressed in this manner, such as gymnasts, divers and football linemen.
The majority of pars defects occur at the lowest lumbar segment. This segment sits atop the sacrum bone. The sacrum is normally angled so the lowest lumbar vertebra is resting on a surface that slopes toward the front of the body. After a pars defect occurs, the front part of the vertebra is no longer restrained by its connection to the segment below. This situation often leads to a forward migration of the relatively unrestrained vertebra. This forward "slippage" is called spondylolisthesis. Many people cope quite well with spondylolisthesis, with nothing more than a slight statistical increase in lower back pain. But, in severe cases, the forward migration can be significant and the segment may even become unstable. Surgical fusion is sometimes recommended in these cases.
Thanks to improvements in imaging technology with MRI, stress injuries to the pars interarticularis may now be identified before actual fractures occur. An article published in 2005 in the "Journal of Pediatric Orthopedics" described 14 cases of children in whom such injuries were identified. The children were treated by bracing and activity restrictions for three months. In this group of children, pars defects were successfully prevented and MRI abnormalities were normalized. The study demonstrates how early detection through MRI can be key in determining the proper course of treatment and prevention.