Spondylolysis and Spondylolisthesis
Spondylolysis is an interruption of the pars interarticularis, this can either be unilateral or bilateral. Spondylolisthesis: an anterior displacement of a vertebral body in relationship to the segment immediately below, this can occur with or without a defect in the pars interarticularis.
Approximately 90% of all spondylolytic spondylolistheses involve the 5th lumbar vertebra. 5% at L4, 3% at L1 L2 and L3, and 2% at C5 C6 and C7.
In the white population there is a 5 -7% prevalence of pars defects with a male to female ratio of 2 to 1. In the black populations of South Africa there is a 2.4% incidence of a pars defect. In Alaskan Eskimos there is a 28-40% incidence of pars defects (? carrying upright in a papoose puts increased strain on the pars interarticularis).
In patients with a pars defect there is a high incidence of spina bifida occulta, almost 13 fold. The spinal defect does not necessarily occur at the level of the pars defect. The hypoplastic and asymmetric facets that occur with spina bifida occulta are thought to increase the stress at the pars interarticularis thus predisposing to a defect.
Causes of Spondylolysis and Spondylolisthesis
There is not a lot of evidence to support a congenital aetiology based on the non union of two separate ossification centres in the pars interarticularis or a defect in cartilaginous development.
The higher incidence in particular groups could be due to other factors predisposing individuals to develop the defect ie weaker cortical bone on a genetic level.
This is at the moment the most commonly proposed aetiological mechanism causing a pars defect. In an in vitro study on 28 human lumbar spines found that the pars interarticularis defect of the lumbar spine was related to hyperextention mechanical fatigue.
It is interesting to note that except for 1 case in a gorilla (C4 Spondylolysis) there has not been another mammal report of Spondylolysis, and it is thought to be related to the upright posture and associated repetitive hyperextention causing mechanical stress at the pars interarticularis.
This theory is supported by the fact that in patient populations who are confined to bed, paraplegics and cerebral palsy’s, the relative incidence of a pars defect is lower. In high activity sportsmen the incidence of a pars defect is higher ie diving, weight lifting and pole vaulting where there is repetitive hyperextention.
Acute traumatic fractures only involving the pars are rare, and certain disease processes which weaken the bone such as Osteopetrosis can cause spondylolysis.
Classification Of Spondylolisthesis
The most widely used classification is described by Wiltse, Newman and Macnab.
Type 1 Dysplastic
Associated with a congenital abnormality of the upper sacrum or the neural arch of L5. There is usually no defect of the pars.
Type 2 Isthmic (three subtypes)
Subtype A :A fatigue fracture of the pars interarticularis. Subtype B :There is elongation of the pars interarticularis without a defect. Subtype C :Rare, due to severe hyperextention leading to acute pars fractures.
Type 3 Degenerative
Also known as degenerative spondylolisthesis with an intact neural arch ie non spondylolytic spondylolisthesis. This is 10 times more common at L4, and no greater than 25% anterior displacement occurs.
It is rare under 50 years of age, it is 6 times more common in females over 60 years of age, 3 times more common in blacks as in whites, and is 4 times more likely if associated with a sacralized L5. The mechanisms of displacement are thought to involve arthrosis of the zygapophyseal joint, disc degeneration, and remodeling of the articular process and pars.
Type 4 Traumatic
Can occur as an acute fracture of a portion of the neural arch other than the pars interarticularis ie a hangman’s type fracture of C2.(note this type is different from the pars fracture – Isthmic subtype C)
Type 5 Pathologic
Generalised or systemic disorders may affect the neural arch of the spine and cause spondylolysis and subsequent spondylolisthesis. Some of the more common forms are Paget’s disease, metastastatic disease, and Osteopetrosis.
Radiological Investigations in Spondylolysis and Spondylolisthesis
Radiological evaluation is the definitive method of confirming the presence of spondylolysis and spondylolisthesis.
A complete plain film series of the lumbar spine and sacrum are recommended, and should include the following views.
- Weight bearing AP view
- Lateral view
- AP angulated (tilt up) / Ferguson’s view
- Bilateral 45 degree anterior oblique views
The most definitive imaging modality for demonstration of pars defects is CT, whereas the effects of the condition are best demonstrated by MRI.
For instability evaluation, lateral lumbar flexion and extension erect stress views should be performed. A displacement of 4mm or a difference in angular motion of greater than 11 degrees is regarded as being unstable and has a poor prognosis with conservative treatment.
Meyerding classification system
Meyerding classification system is used to determine the degree of anterolisthesis of the 5th lumbar vertebra in relation to the sacrum. The sacral base is divided into 4 equal segments, and the alignment of the posterior surface of the 5th lumbar vertebral body is ascertained. In this method spondylolisthesis can be graded 1 – 4, depending on the amount of anterior shift of L5 in relation to the sacrum.
Single-Photon Emission Computed Tomography
Single-Photon Emission Computed Tomography plays a major role in the identification and evaluation of early lesions, by sensitively localising increased activity due to bony remodeling caused by increased mechanical stress.
Magnetic Resonance Imaging (MRI)
MR imaging using parasagittal T2-weighted and axial T1 and T2- weighted images through the area of the pedicle / pars junction may demonstrate bone marrow oedema present in active spondylolysis. For the demonstration of an interruption in the pars interarticularis sagittal T1 and proton density studies which show better differentiation between cortical and medullary bone are more useful.
For assessment of the effects of the spondylolysis and associated spondylolisthesis on the surrounding soft tissues (nerve roots and contents of the spinal canal). These can be demonstrated by Sagital T1, Sagital T2 and Axial T2 weighted images.
Yochum and Rowe, Essentials of Skeletal Radiology.
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