Pars Stress Fractures
Stress # of the pars interarticularis (Spondylolysis)
This affects 4-6% of the population and predominantly occurs in young people involved in sports requiring hyperextension, especially if combined with rotation. The incidence in a sports medicine setting of adolescent low back pain is as high as 47%
The # is usually opposite to the side performing the activity. The pars region acts as a bony fulcrum during spinal extension and is vulnerable to repetitive loading.
It occurs in males: females ~3:1.
Spina bifida occulta is a risk factor for dysplastic spondylosis.
Radiographically visualised spondylolysis is associated with spondylolisthesis about 25% of the time.
Pars #’s are most common at L5 (85-95%) and L4 (5-15%).
Longer standing #’s may appear on 45° oblique x-ray (but best avoided because of radiation exposure→ perform only AP and lateral views). Otherwise bone scan and SPECT (↑ sensitivity than bone scan alone) will confirm an active lesion (although positive SPECT has been found in asymptomatic athletes), and limited CT at the abnormal level will show the #. If there is no ‘hotspot’ than active remodelling is not occurring. CT best demonstrates # size and extent. MRI is touted as reliable in demonstrating pars defects, but has been shown to be inferior in grading lesions.
The #’s can be divided into 3 stages:
(a) Early- focal bony absorption or a hairline defect
(b) Progressive- wide defect with small fragments, and well corticated fracture
(c) Terminal- sclerotic change.
Many are asymptomatic. There may be unilateral backache, buttock pain, and aggravation by lumbar extension. Examination shows pain reproduction with the one-legged hyper-extension test. There may be excessive lordotic posture and hamstring tightness. Palpation shows unilateral tenderness over the # site.
- Type I: Dysplastic- L5 & upper sacrum congenital abnormalities allow anterior displacement of L5 on the sacrum.
- Type II: Isthmic- a lesion in the pars interarticularis occurs sub-classified as (a) lytic, representing a fatigue fracture, (b) elongated but intact pars, and (c) acute fracture.
- Type III: Degenerative- 2º to longstanding inter-segmental instability with articular process remodelling.
- Type IV: Traumatic- acute fractures in vertebral arch other than the pars.
- Type V: Pathological- due to generalised or focal bone disease in the vertebral arch.
Poor prognostic indicators:
The stage of the defect is associated with successful bony union with one study showing union resulting in 73% of early stage defects, 39% of progressive defects and in 0% in the terminal stage.
It is desirable to achieve # healing, however excellent clinical outcomes can be achieved even in the absence of healing.
Incomplete fractures on sagittal CT involve the inferior or infero-medial pars cortex and propagate superiorly or superolaterally. Fracture healing occurs from a superior to inferior direction. An intact superior cortex is a good prognostic sign.
A unilateral defect is more likely to heal than a bilateral one.
Defects in L4 have an improved union rate than L5, as do lesions closer to the vertebral body.
More pronounced lumbar lordosis has a reduced healing rate.
Spondylolisthesis >5% has reduced healing rate.
Overall, the risk of progression of spondylolysis with or without low grade spondylolisthesis to a more significant slip is small.
Participation in sports is possible even if defects in the pars have not healed, but there is ↑ risk of developing endplate lesions and subsequent vertebral deformities, including wedging or slipping of the involved vertebra. The response to conservative treatment varies considerably.
In a patient with an early pars # the initial period of modified rest may take 8-12 weeks. In patients with more advanced defects, healing is a less realistic outcome and their rehab may be commenced earlier, once pain has gone (around 4-6 weeks). There is a need to restrict the aggravating activities, stretch hamstrings and gluteals, and strengthen abdominal and back extensor muscles, as soon as this can be done pain free. The patient should not be given a set period to heal, but should undergo rehab initially involving pain free progressive exercises, but avoiding extension & rotation. Poor sport technique should be adjusted to prevent recurrence. RTS can take 5-7 months for an acute pars defect, and 2-5 months for a more advanced defect.
Anti-lordotic bracing is debated. Patients have been shown to heal with or without braces in a number of studies- although it has been argued that early or progressive lesions have the greatest chance of healing and should be braced. Persisting pain after 2-4 weeks of modified rest may benefit from bracing. Once patients can move into extension and rotation pain free, and there is no local tenderness, they can resume the activities gradually over 4-6 weeks.
Bracing involves using a Boston overlapping brace at 0° of lordosis worn for 23 hours per day, typically for 6 months. Sport can be performed whilst wearing the brace, provided the activity is pain free. If at 3 months healing is demonstrated, and the patient is asymptomatic, weaning of the brace over a further 6 months can occur.
Follow-up CT scanning may be performed at 3-6 months. If there is evidence of radiologic healing than return to sport should be delayed further whilst remaining compliant with ongoing rehab. If defects worsen or there is no sign of union than return to sport can occur once pain has gone and core stability has improved.
Potential indications for surgery include a progressive slip, neurological complications, and segmental instability with associated pain.