An ultrasound study of the long posterior sacroiliac ligament in healthy volunteers and in patients with noninflammatory sacroiliac joint pain

Aim: To describe the sonoanatomy of the long posterior sacroiliac ligament (LPSL) in healthy volunteers and to assess by ultrasound the LPSL in patients with noninflammatory sacroiliac joint pain (SIP).Material and methods: We assessed 64 LPSLs of 32 healthy controls and 40 LPSLs of 40 patients with unilateral noninflammatory SIP and a positive Fortin finger test. LPSLs in both groups were assessed for the presence of alterations in their structure, continuity and echogenicity and their thickness was measured in three predefined points. All patients were examined in prone position following a strict scanning protocol.Results: Detailed sonoanatomy description and measurement of the LPSL in healthy volunteers are provided (length: 31.32±4.79 mm, width: 8.14±1.28 mm, thickness: 2.05±0.55 mm; 1.64±0.41 mm and 1.51±0.42 mm at the iliac and sacral entheses and in its middle part, respectively). The LPSLs were found to be significantly thicker in the SIP group, with an optimum criterion value of >2.0 mm in its middle part to identify pathologically thickened ligaments. In addition, LPSLs inthe SIP group presented significantly more often hypoechogenicity/altered fibrillar structure (57.5% vs.16%) and/or periligamentous edema (72.8% vs 28%). The combination of either altered structure or periligamentous edema, with thickening of theligament’s body showed the best diagnostic accuracy (sensitivity and specificity 83.9% and 94.7% for the first combination and 100% and 84.6% for the second combination) to identify LPSL pathology in noninflammatory SIP.Conclusions: LPSL could be assessed by ultrasound and sonopathological lesions could be identified in patients with SIP.