Abstract
The coccyx and associated structures are poorly described in standard anatomy texts. This is surprising given that localised coccygeal pain (coccydynia) is not uncommon and its cause is unknown in up to one-third of patients. There has been no systematic study of the coccygeal plexus or the sacrococcygeal (SC) cornual region. Both structures are potential sources of pain. Lack of morphometric data on the normal adult coccyx hampers current understanding of radiological abnormalities in coccydynia. This thesis aimed to study: 1) the coccygeal plexus; 2) the bony anatomy of the coccyx in normal individuals and patients with coccydynia; and 3) the SC cornual region.
In the first study, the coccygeal plexus was investigated in 16 embalmed cadaver specimens (mean age 78 years, 7 females) with no known pelvic disease. Ten specimens underwent microdissection and the remaining six were examined by serial histology. The second study consisted of a retrospective case-control analysis of computed tomography (CT) scans of the SC region in 112 normal adults (mean age 63 years, 50 males) and comparable magnetic resonance scans from 107 adults with coccydynia (mean age 43 years; 84 females). The following parameters were evaluated: coccygeal segmentation; intervertebral joint fusion; bony spicule formation, intercoccygeal subluxation; and sacral and coccygeal angles and curvatures. In the third study, the bony anatomy of the SC cornual region was analysed in: 33 high-resolution CT scans (mean age 64 years, 17 males); seven microCT scans of cadaver specimens (mean age 77 years, 3 males); and an archival collection of 105 Asian Indian adult skeletons. Soft tissues and innervation were also examined histologically in five cadaver specimens.
The coccygeal plexus was found to be formed within ischiococcygeus from the ventral rami of S4, S5 and Co1. It gave rise to branches to ischiococcygeus and terminated as anococcygeal nerves which pierced ischiococcygeus and the sacrospinous ligament to supply skin overlying the coccygeal region. The normal adult coccyx has the following features: usually consists of four segments (76%); shows SC joint fusion (57%); intercoccygeal joint fusion is increasingly common caudally; a bony spicule on the terminal segment in 23%; and is significantly longer in men than women (P<0.05). Compared to normal, women with coccydynia had a significantly more ventrally curved coccyx, lower prevalence of SC joint fusion, and higher frequency of bony spicule formation. Men with coccydynia had a significantly lower prevalence of SC and intercoccygeal joint fusion. SC cornual fusion was identified in 45% of CT/μCT scans and 20% of skeletons. The mean intersacrococcygeal cornual gap in unfused cases was 7.1 ± 2.4 mm. The SC intercornual ligament consisted of collagen fibres with a thin layer of elastin anteriorly and associated nerve fibres.
The coccygeal plexus, the bony morphology and morphometry of the coccyx, and the anatomy of the SC cornual region have been described in this thesis. Differences between the bony anatomy of the coccyx in normal individuals and patients with coccydynia were identified. These findings may offer new insights into the clinical problem of coccydynia.