Single-leg-stance (flamingo) radiographs to assess pelvic instability: how much motion is normal?

BACKGROUND

Chronic pelvic instability is a relatively uncommon cause of pelvic and low-back pain. Patients present with feelings of instability and mechanical symptoms. Static radiographs are often inadequate to detect abnormal relative motion between the hemipelves consistent with chronic pelvic instability; dynamic views of the pelvis are required. We assessed the amount of physiologic motion present at the pubic symphysis in normal adult men and nulliparous and multiparous women with alternating-single-leg-stance radiographs.

METHODS

Forty-five asymptomatic adult volunteers (fifteen in each group) were evaluated with a standing anteroposterior pelvic radiograph as well as with anteroposterior pelvic radiographs made with the subjects assuming both right and left single-leg stance. The subjects completed a questionnaire to determine their eligibility for participation in the study, and an examination was performed to exclude certain physical anomalies that might alter the radiographic findings.

RESULTS

The mean total translation (and standard deviation) at the pubic symphysis, as measured by three blinded observers, was 1.4 +/- 1.0, 1.6 +/- 0.8, and 3.1 +/- 1.5 mm for the men, nulliparous women, and multiparous women, respectively. With the numbers available, we found no significant difference between the translation in the men and that in the nulliparous women (p = 0.63). The multiparous women had significantly more translation than did either the nulliparous women (p = 0.002) or the men (p = 0.0005). There was a significant positive association between the number of pregnancies and the total translation (p < 0.0001).

CONCLUSIONS

The use of anteroposterior pelvic radiographs made with the subject alternating between right and left single-leg stance demonstrated, with high interobserver reliability, that multiparous women had a significantly different physiologic range of pubic translation as compared with men and nulliparous women. The ranges of physiologic motion at the pubic symphysis measured on the single-leg-stance radiographs in this study can be used to identify pathologic amounts of motion at this site.

CLINICAL RELEVANCE

This investigation suggests that up to 5 mm of physiologic motion can occur at the pubic symphysis in asymptomatic individuals, as demonstrated by alternating-single-leg-stance radiographs.