Spondylolisthesis refers to the relative translation of adjacent Vertebra in the spine. Anteriorlisthesis means forward translation of the upper vertebra. Although spondylolisthesis can be caused by various pathologic states in the spine, Isthmic Spondylolisthesis is produced when bones (pars interarticularis) connecting the facet joints in the posterior spine are fractured causing anteriolisthesis of the vertebra. The pars serves as a checkrein for translation movement of the vertebra and when fractured, the vertebra are allowed to move past each other producing symptoms ranging from mild low back ache to severe neurologic deficits. Although L5-S1 is most commonly affected, isthmic spondylolisthesis has been found at every lumbar level.
Why the pars fractures is incompletely understood, but research has revealed some telling facts. Approximately 5-8% of the ambulatory (walking) population older than age 5 has pars fractures. Pars fractures interestingly have not been found in two populations of humans: fetuses and patients who have been paralyzed and non-ambulatory from birth. Moreover, pars fractures are found in increased frequency in athletes who hyperextend their spine such as gymnasts and football inside lineman. These facts taken together indicate that pars fractures are in fact stress fractures produced when the predisposed, weakened pars interarticulars is subjected to repetitive hyperextension forces. Why certain patients become variably symptomatic is unknown.
The magnitude of symptoms caused by spondylolisthesis does correlate with the degree of anteriolisthesis of the vertebra. A grading system has been defined to characterize the degree of slip: the full front-back depth of the vertebra is divided into fourths and a spondylolisthesis is graded 1-4 based on the quarter percentage of slip. For example, a grade 2 slip is defined as the upper vertebra moving 50% on the lower vertebra. A grade 5 slip means that the vertebra has completely dislocated off the lower vertebra. In general, grade 4 slips may present with more low back pain and neurological problems than a grade 1 slip, although this rule does have exceptions. One study has suggested that 2/3rds of grade 2 slips could be treated non-operatively, while larger slips uniformly needed treatment including surgery. In general though, approximately 50% of individuals with Isthmic spondylolisthesis will seek treatment for low back pain at sometime during their life.
Low back pain and nerve compression symptoms are found in patients with isthmic spondylolisthesis. For the same reason that the vertebra translate relative to each other, the pars fracture permits abnormal vertebral motion creating low back pain. Chronic tearing and degeneration of the disc may actually be the pain generator, but this has never been proven. This chronic abnormal motion (sometimes referred to as instability) also creates a situation where the pars fracture cannot heal. New bone forms around the fracture and may actually compress the nerves which pass in proximity. This compression, plus the nerve stretch caused by the forward motion of the vertebra, is responsible for the nerve irritation and leg pain.
Treatment is designed to stop either the progression of the slip or the abnormal motion which creates pain. As with most spinal problems, children are treated for different reasons than adults. In children, low back pain and associated nerve irritation causing leg pain and hamstring tightness can initially be treated by rest and cessation of athletic activity. In some cases, bracing may be attempted. Documented progression of the slip and pain refractory to conservative measures are indications for surgery.
Adults should initially be treated with rest, anti-inflammatories, and physical therapy as symptoms subside. Slippage does not usually progress in adults. Studies have shown that subtle neurological deficits can exist in 18% of patients without surgery, but that severe neurological deficits are rare. It is for this reason that serial neurological examinations should be performed during the course of conservative treatment. Patients with symptoms refractory to conservative treatment may be candidates for surgery.
The current state of the art is fusion surgery for isthmic spondylolisthesis. To view an animation of the surgery, click here
How this fusion will be done depends on multiple factors. As the vertebra translates, it also tilts producing the characteristic slip angle. The slip angle produces kyphosis in the lumbar spine which may need to be corrected if severe. It is generally held that if correction of the spondylolisthesis is attempted, that it is more important to correct the slip angle than translation. These factors might determine whether a fusion surgery can be effectively done from the back only versus a front and back operation. In my hands it is impossible to correct slip angle without a front operation. The rate of fusion failure (pseudarthrosis) for posterior-only fusion is 25% meaning that only 75 out of 100 patients will obtain a solid fusion with this technique. In my practice, in approximately 300 patients with front and back surgery for spondylolisthesis, none required revision surgery for failure of fusion. Unequivocally, Isthmic Spondylolisthesis should not be treated with decompression surgery only. Anterior-only surgery for spondylolisthesis is risky because the potential instability created by the pars fractures does not lend itself to correction by the placement of dowel grafts or cages primarily. Each patient should be individualized and a specific operative strategy implemented.
In most cases the instrumentation and fusion for Isthmic Spondylolisthesis is done from L4 to S1. After 25 years of clinical experience, I am convinced that stopping a fusion at L5-S1 in an adult significantly increases the possibility of L4-5 adjacent segment degeneration. This is because the instrumentation from the L5 pedicle screws are very close to the L4-5 facet joints, changing their mechanics. Moreover, the L5-S1 inner spondylolisthesis is in kyphosis and there is a junctional hyperlordosis so that compensation can occur. This hyperextension causes significant alterations of facet contact structure and change of the IAR (instaneous axis of rotation) and constant pressure. Moreover, the connection of the L4 to L5 ligaments is incomplete because of the bilateral pars fracture and the Gill fragment being a free-floating entity. There, the adjacent segment normal tethering mechanisms are not intact. The L4-5 disk space is still under a significant amount of shear and because it is already predestined to degeneration because of its MR view, most adults are fused L4-S1.
For more information, you can read this research study that Dr. Pashman participated in: In Vivo Analysis Of Canine Intervertebral And Facet Motion.