Background. Classically and based on necropsy observations, the muscles of the pelvic floor (the levator ani and coccygeus muscles) have been described as having the shape of a basin. In a previous study by Hugosson et al. (1991), magnetic resonance imaging revealed the dome shape of the pelvic floor in live subjects when muscular tonus is present.
Methods. The normal anatomy and the dynamic movements of the female pelvis were examined with magnetic resonance imaging including conventional and fast sequences. Six nulliparous and six parous healthy women without signs or symptoms of pelvic relaxation were studied in the supine position at rest, during voluntary pelvic contractions and during bearing down.
Results. The levator ani muscle was dome-shaped at rest. During voluntary pelvic contractions the muscle straightened, becoming more horizontal and during bearing down it descended, becoming basin-shaped. The width of the genital hiatus was the same in both groups at rest and it widened during bearing down in all but two 3-para women. The bladder base was lifted upward and forward during voluntary pelvic contractions but during bearing down it descended.
The rectum at rest had a posterior angle which decreased during voluntary pelvic contractions and increased during bearing down.
Conclusions. MRI of the female pelvis is valuable in anatomic and dynamic analysis in healthy women and offers new information about the female pelvic floor. The female pelvic floor is shaped like a dome - not like a basin.