A pars fracture occurs when the bony pars interarticularis breaks. This type of spinal fracture causes spondylolisthesis; a forward slip or slide of one vertebral body over the one beneath. This educational article explains and illustrates the anatomy of the low back involved, and describes the causes, diagnosis and healing of these related lumbar conditions.
Facet Joints and Their Purpose
Facet joints are the paired joints in the back of the spine. They comprise two of the three regions where the two vertebrae join together (the disc being the other connection found in the front of the spine). The facet joints have two purposes. One is to simulate “railroad tracks” and guide the spine in only certain directions. The other is to act like a “door stop” to prevent the vertebra above from slipping forward on the one below.
Pars Interarticularis, Structural Forces and Stress
The joints above are connected to the joints below in one vertebra through the pars interarticularis. The pars interarticularis means the “part between the articulations” in Latin and is the bony bridge that joins these two upper and lower facets. As you remember from geography, an isthmus is a narrow portion of land that joins two larger bodies of land. Therefore, the pars interarticularis is the bony connection between the two facet joints and is considered an “isthmus." This term comes into play later.
The lowest two vertebrae in the spine (L4-S1) face downward like a sled on a ski slope and these two vertebra bear the entire weight of the spine above. The buttress that prevents the lower vertebra from sliding down is the inferior facets or “door stops." These “door stops” are joined to the upper structures (the pedicles) through the bony pars interarticularis.
As you can imagine, there is tremendous stress on these structures, especially with impact. Overload forces can occur with contact sports, jumping and especially with extension (bending backwards). This overload can cause stress fractures of the small bony structures (cortical bone and trabecula) that make up the pars interarticularis.
Development of Pars Fractures
Like bending a metal coat hanger enough times to cause it to fail, if there is continued overload of the pars without enough time between episodes to allow healing, the pars will eventually break (fracture) and become incompetent. Genetics also plays a substantial role.
These fractures normally occur on both sides of the pars but occasionally occur only on one side (unilaterally). If the fractures occur only on one side, the opposite pars bears all the stress and can eventually fracture. When the fracture occurs on both sides, there is no restraint to prevent shear forces (sliding forces) on this disc (remember the ski slope for the two bottom vertebrae).
The disc is an excellent shock absorber and is the main cushion for spinal impact. The disc, however, has poor shear (slide) resistance. Without the facets in back to protect it, the disc is exposed to abnormal forces that can injure it. It is the intact facets that prevent these forces from occurring and with pars fractures; the disc can start to fail.
Under abnormal stress, the disc can start to stretch and then tear. The vertebra above then starts to slip forward on the one below. (In the case of the L5 vertebra, it can start to slide forward on the sacrum).
Before the slide occurs, the pars defect is called an isthmic spondylolysis (lysis meaning to “cut” or “break”). If there is an accompanying slide, the condition is then called an isthmic spondylolisthesis (oliothesis means to slip in Latin).
Diagnosis and Healing
The problem with treating this disorder is that adolescents don’t typically communicate that they are in pain and the disorder is hard to diagnose early (requiring suspicion of the diagnosis and an MRI or CT scan). Pediatricians may not be fully aware of this disorder. Athletic trainers are much more liable to notice the symptoms and look for referral sources. If picked up late, treatment can be more difficult.
In general, children have very good bone fracture healing potential. Pars interarticularis fractures, however, are the one set of fractures, in particular, that are resistant to healing.
There are three reasons for this. One is that the surface area of these fractures is very small. Bone fractures heal best with large surface areas, and the surface area of this fracture is smaller than some of the smallest bones in the hand.
Bone also has better healing potential when large cancellous surface areas are exposed. Cancellous bone is the “spongy bone” inside the hard cortical bone and has many bone generating cells. Unfortunately, the pars interarticularis has almost no cancellous bone.
The third problem is that this area of the spine has some of the greatest motion and shear forces acting on it. This fact is one of the main reasons the bone fractured in the first place. Getting an active child or teenager to rest this area is difficult. A brace can slow a child down but will not immobilize the area and healing bone cells do not respond well to motion.
The type of pars fracture makes a big difference to healing. There are basically three types of fracture:
An atrophic fracture is a fracture that “is not trying to heal." The ends of the broken bone thin down like the burnt end of a matchstick and there is a poor attempt by the body to try to unite these ends. Similarly, the displaced fracture is one where the vertebra has slipped forward, and the ends of the fractured fragment are not near each other. Both have poor healing potential.
The hypertrophic fracture is noted where the ends of the fracture are thick, enlarged and coopted (right next to each other). These fractures have the best chance of healing in a brace. Unilateral (one-sided) fractures also have very good healing potential.
In the typical bilateral pars fracture, healing without a brace and without reduced activity has a very poor repair rate. Even with the best healing fractures (unilateral and hypertrophic), healing with a brace and three to six months of reduced activity has a success rate hovering about 50%. The chance of re-fracture is not known at this time, but children with healed fractures can occasionally refracture if they go back to their previous level and type of activity.
Depending upon the type of fracture and the slip amount, these fractures can be surgically repaired with a reasonable success rate.