Radiographic classification of L5 isthmic spondylolisthesis as adolescent or adult vertebral slip.

STUDY DESIGN: A radiographic and morphologic study was conducted to investigate low-grade spondylolisthesis in cases with preexisting isthmic spondylolysis of L5.

OBJECTIVE: To distinguish radiographically between vertebral slips before and after skeletal maturity as determined by deformities of the sacral endplate.

SUMMARY AND BACKGROUND DATA: Very few reports have shown that spondylolisthesis with preexisting isthmic defects of L5 develops frequently in adulthood. The prognostic factors of the vertebral slip have remained unclear. It is hard to determine the onset time of low-grade spondylolisthesis.

METHODS: This study examined plain radiographs of 367 adult patients with pars defects of L5 (213 without slippage and 154 with Grade 1 or 2 spondylolisthesis) and 310 control subjects, ages 20 to 59 years at the first visit. The following parameters were measured and analyzed for each age decade: the sacral table index (anteroposterior width of the sacral endplate expressed as a percentage of the anteroposterior diameter of the upper L5 endplate), the sacral table angle (formed by the sacral endplate with the posterior wall of S1), the relative thickness of the L5 transverse process, and the iliac crest height.

RESULTS: The prevalence of patients with slippage who met deformity criteria (sacral table index > 102% [the mean plus 2 standard deviations of the controls] and sacral table angle /=89 degrees [mean minus 2 standard deviations of the controls]) was 0% in the third decade, but increased remarkably in the fifth and sixth decades. Of the 213 patients without slippage, 8 patients in whom new slippage developed during long-term follow-up evaluation all had a normally-shaped sacral table. The prevalence of patients without slippage decreased gradually with age, and elderly patients had relatively broader transverse processes and a higher iliac crest line.

CONCLUSIONS: The authors considered that the slips with and those without deformities of the sacral table had developed in adolescence and adulthood, respectively. Using new radiographic parameters that indicate widening and tilting of the sacral table, low-grade isthmic spondylolis thesis can be categorized into "adolescent and adult vertebral slips."