Illustration of pars interarticularis
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Illustration of isthmic spondylolisthesis in a 35 year old. Thin arrow points to bone spur compressing the nerve. Medium arrow points to the slip of L5 on the sacrum. Thick arrow points to the non-healed fracture pannus
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Lateral X-ray of pars interarticularis fractures of L3 and L5 in a 15 year old boy who has failed conservative treatment
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CT scan of pars fractures L3 and L5. White arrows point to fractures of pars interarticularis
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CT axial scan of L5 pars fracture. White arrows point to fractures of pars interarticularis
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Anatomy
The pars interarticularis is the bony portion of the vertebra that connects the pedicles to the inferior (lower) facets. There are two pars interarticularis bony junctions- one on each side. The two lower joints below the pars articulate with (hook onto) the upper joints of the vertebra below.
The purpose of the lower joints, which are connected to the vertebra by the pars interarticularis, is two fold. One is that they act like railroad tracks to guide the motion of the vertebrae above on the vertebra below. The second is that they act as “doorstops” to prevent the vertebra above from sliding forward on the lower vertebra.
In the front of the two vertebrae, the disc is an excellent shock absorber and “bushing”. This disc is the main cushion for spinal impact. The disc however has no directional stability. Without the facets in back, the disc can twist, flex and extend, laterally bend and tilt and shear (slip forward and backwards). It is the intact facets that induce the spine to bend only in certain directions.
When the connections between the facets and the pedicles are damaged (pars interarticularis fractures), the disc is unprotected and can undergo motions that can cause damage. The disc is especially sensitive to shear forces, the forces that cause one vertebra to slide forward or sideways on the other. The facets prevent shear forces on the disc by acting as “doorstops”. The lower two vertebra (L4 and L5) sit on a ski slope pointing downhill and these intact “doorstops” prevent this slide.
The upper vertebra’s lower facets hook onto the lower vertebra’s upper facets. This means that the intact facets restrict the shear forces of the vertebral segments. If the facets wear out (as in the case of a degenerative spondylolysthesis) or break off (as in the case of bilateral pars fractures or isthmic spondylolysthesis), this protection is lost.
At what ages do these fractures normally occur? These fractures normally occur between the ages of 8 and 15 years. Why are these fractures not more commonly diagnosed in this age group? Many times the child that develops new fractures will not initially complain of significant symptoms or will ignore the pain.
Early Diagnosis and Potential Healing
If the fracture is diagnosed early in childhood, the pediatrician is the one that normally discovers the fracture but many injured children still go undiagnosed. This section deals with what to do if these fractures are caught early when the child still has a chance to heal or the fractures can be repaired.
In general, children have very good bone fracture healing potential. The pars interarticularis fracture is the one fracture in particular that is resistant to healing. There are three reasons for this. One is that the surface area of fracture 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 more 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 (the main reason the bone fractured in the first place). Getting an active child or teenager to rest this area is almost an act of God. A brace can slow a child down but will not immobilize the area and healing bone cells do not respond well to motion.
In bilateral pars fractures, healing without a brace and without reduced activity has a very poor repair rate. Healing with a brace and three to six months of reduced activity has a success rate of about 50%. The chance of re-fracture is not know at this time but children with healed fractures can occasionally re-fracture if they go back to their previous level and type of activity.
Although it is uncommon for only one side of the pars interarticularis to fracture, unilateral fractures do occur. If a unilateral pars fracture does arise, the opposite pars will be placed under more stress and therefore will hypertrophy (lay down more bone-Wolfe’s Law) to resist the greater forces placed upon it. If this fracture does not heal, the hypertrophy of the opposite side may allow an individual who has a unilateral pars fracture to live with it comfortably.
If only one side of the pars breaks, this fractured pars has a much better chance of healing with rest and bracing. Think of the vertebra and both pars interarticularis as a ring. If only one break of the ring occurs, the ring still has some rigidity. If there are two breaks, the ring is broken in half and has no rigidity. A one sided break allowed three months of bracing and rest will normally allow a unilateral pars fracture to heal.
Healing potential can be assessed by MRI, X-rays, bone scan and CT scan. There are particular appearances that may indicate an effort by the body to heal the fracture and conversely, a noted lack of healing potential.
Non-Healed Fractures and Altered Biomechanics
What about the bilateral pars fracture that does not heal in the adolescent? This creates a conundrum. Some adults with bilateral pars fractures present do not develop lower back symptoms in their lifetime. The question then rises if surgeons should surgically repair the non-healed pars fractures in children? There is not a well-developed answer to this question. Athletic children do seem to have a higher incidence of lower back pain with pars fractures but that does not mean that every athletic child needs a surgical repair.
If both pars are fractured, the biomechanics of the spine changes. There is more stress placed upon the disc and the entire spine can slip forward (spondylolysthesis). This leads to disc tears and degenerative changes of the disc. Some patients develop lower back pain and even leg pain if a pedicular bone spur develops (see isthmic spondylolysthesis section in website). To prevent degenerative changes from occurring, there is more evidence now that the pars fractures need to be repaired.
Mechanically, this pars repair could likely save the disc from destructive forces that would lead to degenerative changes but there is not enough research yet to confirm this. There have not been enough pars fractures repaired and followed long enough to see if this theory is correct.
Most individuals with bilateral pars fractures will develop degenerative disc disease. Many will also develop a slip of the vertebra (an isthmic spondylolysthesis). A certain percentage of patients will develop pain and some will have disabling pain. What would be the total percentage of disabled patients? That information is not known yet.
Read about Pars Fracture Repair
For more information on pars interarticularis fractures, please contact Dr. Donald Corenman at 970-479-5895.
