A 3 mm symphyseal width that remains unchanged between supine and standing positions does NOT indicate radiographic instability in this patient with chronic postpartum pelvic instability symptoms.
Radiographic Assessment of Instability
The key diagnostic finding is not the absolute symphyseal width, but rather the dynamic change between positions. A pubic symphysis width of 3 mm falls well within normal limits (normal range is typically 4-6 mm, with upper limit around 10 mm). More importantly, the absence of change between supine and standing positions argues against mechanical instability 1.
Diagnostic Criteria for Pelvic Instability
The most sensitive radiographic test for chronic anterior pelvic instability is single-leg-stance (flamingo view) radiographs, not simple supine versus standing comparison 1, 2:
- Positive finding: ≥5 mm (0.5 cm) of vertical translation at the symphyseal bodies between right and left single-leg-stance views
- In one study of 38 patients with chronic pelvic pain, 25 demonstrated instability averaging 1.98 cm of vertical translation (range 0.5-5 cm) 1
- Standard supine radiographs and even standing AP views are inadequate for detecting instability 1
This patient's imaging is incomplete. The unchanged 3 mm width between supine and standing does not rule out instability because the appropriate stress views (single-leg-stance) were not performed.
Clinical Context and Next Steps
Important Caveats
Symptoms do not correlate with radiographic findings: Studies show no significant difference in pain scores between patients with stable versus unstable pelves (6.4 vs 7.3 on VAS, p=0.28) 1. This patient's chronic instability sensation may represent:
- True mechanical instability not captured by current imaging
- Pelvic floor dysfunction without bony instability
- Chronic pain syndrome
Postpartum symphyseal widening: Recent evidence shows pubic symphysis width is independently associated with postpartum low back pain (OR 1.47 per 1 mm increase) 3, though 3 mm is below the pathologic threshold
Recommended Diagnostic Algorithm
Obtain single-leg-stance radiographs immediately 1, 2:
- If vertical translation ≥5 mm → confirms mechanical instability
- If <5 mm → instability unlikely; consider alternative diagnoses
If instability is confirmed (≥5 mm translation):
Consider CT to assess posterior pelvic ring involvement 2, as posterior instability may require additional surgical stabilization beyond anterior fixation alone 4.
Management Based on Findings
If Radiographic Instability is Confirmed
Initial conservative management (typically 3-6 months) 2, 5:
- Pelvic orthosis/binder
- Activity modification
- Physical therapy focused on pelvic floor and core strengthening
- NSAIDs or analgesics
Surgical intervention if conservative treatment fails 2, 5:
- Pubic symphysis plating is indicated for diastasis >2.5 cm in acute trauma 4, though this threshold is debated for chronic instability 6
- For chronic instability: symphyseal arthrodesis with double-plate fixation and iliac bone graft 5
- Alternative: reconstruction with autologous tendon graft (preserves physiological mobility) 7
- Critical: Assess for posterior ring instability—if present, posterior fixation must be performed concurrently 4, 2
If No Radiographic Instability
Focus on alternative diagnoses:
- Pelvic floor dysfunction (most likely given postpartum history)
- Chronic symphysitis/osteitis pubis
- Referred pain from lumbar spine or sacroiliac joints
- Consider MRI pelvis to evaluate soft tissue pathology 2
Key Clinical Pitfall
Do not assume normal symphyseal width equals stability. The diagnosis of chronic anterior pelvic instability requires dynamic stress radiographs, not static measurements 1, 2. This patient needs proper stress imaging before any treatment decisions can be made. The 3 mm measurement alone is reassuring but insufficient to exclude instability given her persistent symptoms.