Imaging Studies

In the initial stages, when the fracture is forming, the plain films may be negative. Bone scan, however, which can detect areas of bone turnover (i.e., bone deposition even before the actual fracture has developed), may be positive and thus is an important diagnostic tool at the early stages of the condition. Single photon emission computed tomography (SPECT) may be the most sensitive test for detecting impending pars defects, as bone under stress with increased osteoblas-tic activity due to remodeling will have increased activity and appear hot. Later on, when the defect is established and is large enough, it can be spotted on various projections on plain films (Figure 5-2).

CT scan is the most sensitive method for detection of spondylolysis, whether it is unilateral or bilateral. The defects can be easily spotted in an axial cut (Figure 5-3). Sagittal reformats demonstrate the defect clearly. The key to identification of the defect is that it is separate from the facet joints. In the acute stage MRI may show, especially on T2-weighted images, increased signal around the pars defect due to bone marrow edema. Subsequently, focally decreased signal may be seen on sagittal and axial images. T1-weighted images with fat suppression may demonstrate the bony cleft (Figure 5-4). MR is an insensitive technique for pars interarticularis defects and spondylolysis detection. The reasons are basically that the slice thicknesses of the MR sagittal sequences are relatively thick, and the fracture does not have great contrast. Frequently the cuts are not centered on the fracture and it can thus be easily missed. In routine studies the axial images are often performed with the intention of visualizing disc and spinal canal anatomy rather than pars defects.

A simple and well accepted grading system for spondylolisthesis has been devised by Myerding and is named after him. The upper endplate of the vertebra below the slipping vertebra is divided into quarters. Grade 1 slip is when the vertebra above slips forward up to 25% of the diameter of the endplate below. Grade 2 is slippage of between 25% and 50%, grade 3 between 50% and 75%, and grade 4 over 75% (Figure 5-5). At times, L5 slips forward all the way and "falls" in front of S1. This condition is termed spondyloptosis.

Lateral X-rays clearly demonstrate the degree of slippage. In longstanding cases, bone remodeling and sclerosis may be seen, especially at the sacral dome (Figures 5-6A and 5-6B). In advanced L5-S1 spondylolisthesis AP views may disclose an inverted "Napoleon's hat." The hat's brim is made of the transverse processes of the L5 vertebra (Figure 5-7).

CT studies, especially on sagittal reformation, and sagittal MRI views can easily determine the degree of slippage. It should be noted that the forward slippage that occurs in the spinous processes secondary to the vertebral slip is found one level above the spondylolytic vertebra. Because the spinous process of the spondylolytic vertebra is not carried forward with the slipping vertebral body as it is separated from it by the pars defects, the "gap" in the posterior elements occurs one

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