Magnetic resonance imaging morphology and morphometry of the coccyx in coccydynia

Retrospective case-control study.

To determine if differences exists between the bony anatomy of the coccyx in patients with coccydynia and that in subjects with no known coccygeal pathology.

Numerous bony characteristics of the coccyx have been described in patients with coccydynia but their significance is uncertain. This study aimed to evaluate these in patients with coccydynia and compare them with normal coccyges.

Magnetic resonance scans of the coccyx from 107 adults (mean age, 43 ± 12 yr; 84 females) with clinically diagnosed coccydynia were retrospectively analyzed and compared with 112 computed tomographic scans from sex-matched adults with normal coccyges. The following were assessed: coccygeal segmentation; sacrococcygeal and intercoccygeal joint fusion, angles, and curvature; bony spicule formation; and subluxation.

Compared with normal, females with coccydynia had a more ventrally curved coccyx (curvature index, 85 ± 7 vs. 90 ± 5; P< 0.01), a lower prevalence of sacrococcygeal joint fusion (27% vs. 58%, P< 0.01), and a higher frequency of bony spicule formation (44% vs. 19%, P< 0.01). Males with coccydynia showed a trend toward a more ventrally curved coccyx (curvature index: 86 ± 6 vs. 89 ± 5, P< 0.08), and a lower prevalence of sacrococcygeal (27% vs. 60%, P< 0.02) and intercoccygeal (0% vs. 22%, P< 0.02) joint fusion. Combining statistically significant coccygeal parameters in a logistic regression model yielded sensitivity, specificity, and positive predictive values of 72%, 71%, and 73%, respectively in females and 52%, 92%, and 73%, respectively in males.

This is the most detailed comparison of coccygeal morphology and morphometry in adults with and without coccydynia investigated using cross-sectional imaging. Anatomical differences in joint fusion and coccygeal curvature may either predispose to the development of coccydynia or arise as a result of this condition.