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Neuroradiology Case of the Week

Case 413

June 2009

Derek Lasher, Ashwani K. Sharma, MD, and P-L Westesson, MD, PhD, DDS

Clinical Presentation: A 13-year-old male presented with three month history of intermittent pain and difficulty bearing weight on the left.

Imaging Findings: Sagittal T2 fat-saturated images show a high-signal intensity in the left L5 pedicle. Adequate evaluation of the pars interarticularis of L5 was not possible.
     Alignment of the lumbosacral spine is preserved, and there is no significant central canal stenosis or neural foraminal narrowing. The vertebral bodies are of normal height. The intervertebral discs are of normal configuration. The conus medullaris ends at L1. There is no abnormal signal in the distal spinal cord. There is no significant abnormal finding in the paravertebral musculature.

Diagnosis: Stress fracture of pars

Discussion: Spondylolysis is a leading cause of back pain in children. It is a stress fracture of the pedicle, which occurs most commonly at L5 [1]. This becomes particularly evident during the adolescent growth spurt, during which there is increased physiological bone remodeling. The resulting stress fracture may be complete or incomplete at the time of presentation [4].
     The identification of incomplete pars fractures has important implications regarding patient management. The failure to restrict the precipitating activity may result in fracture propagation leading to a complete pars defect, with possible progression to a chronic non-union [5].
     Spondylolysis can be, but is not necessarily, associated with spondylolisthesis, the misalignment of a vertebra, or a pars defect [1]. In about 15% of pars interarticularis lesion cases, there was a progression to spondylolisthesis [2]. Extension of the lumbar spine causes the inferior articular process of the cranial vertebra to impact the pars interarticularis of the caudal vertebra. Prolonged repetition of these impacts can produce a stress or fatigue fracture of the pars interarticularis [4].

Spondylolysis is classified by a scale developed by Wiltse et al. (1976) as follows [1,5]:
Type I:	Dysplastic —	Congenital abnormalities of the arch of L5 or the upper sacrum allow the listhesis, or anterior displacement of L5 on the sacrum, to occur.
Type II:	Isthmic —	Occurrence of a lesion in the pars interarticular. This is further classified as either (a) lytic, representing a fatigue fracture of the pars, (b) elongated but intact pars, or (c) acute fracture.
Type III:	Degenerative —	Resulting from a long standing intersegmental instability with associated remodeling of the articular processes.
Type IV:	Traumatic —	Resulting from fractures in vertebral arch other than the pars.
Type V:	Pathological —	Due to a broad or focal bone disease affecting the vertebral arch.

     Pars defects are found in approximately 5% of the general population, with occurrence in the black population being 2-3 times higher than that of the white population [4].
     Those patients who do not respond to conservative treatment and who progress to complete pars fractures may be candidates for surgical fusion. This is therefore an avoidable situation if appropriate imaging and clinical management pathways are followed.

References:

1. Standaert CJ, Herring SA. Spondylolysis: a critical review. Br J Sports Med. 2000 Dec;34(6):415-22. [PubMed]
2. Ralston S, Weir M. Suspecting lumbar spondylolysis in adolescent low back pain. Clin Pediatr (Phila). 1998 May;37(5):287-93. [PubMed]
3. Coady CM, Micheli LJ. Stress fractures in the pediatric athlete. Clin Sports Med. 1997 Apr;16(2):225-38. [PubMed]
4. Hu SS, Tribus CB, Diab M, Ghanayem AJ. Spondylolisthesis and spondylolysis. J Bone Joint Surg Am. 2008 Mar;90(3):656-71. [PubMed]
5. Wiltse LL, Newman PH, Macnab I. Classification of spondylolisis and spondylolisthesis. Clin Orthop Relat Res. 1976 Jun;(117):23-9. [PubMed]
6. Hollenberg GM, Beattie PF, Meyers SP, Weinberg EP, Adams MJ. Stress reactions of the lumbar pars interarticularis: the development of a new MRI classification system. Spine. 2002 Jan 15;27(2):181-6. [PubMed]