What Does "Thickening of the Spring Ligament" Mean on an Ankle MRI?
Thickening of the spring ligament on ankle MRI typically indicates ligament injury or insufficiency, most commonly associated with posterior tibial tendon dysfunction and progressive flatfoot deformity.
Understanding the Finding
The spring ligament (plantar calcaneonavicular ligament) complex is a critical stabilizer of the medial longitudinal arch of the foot, working in concert with the posterior tibial tendon and deltoid ligament 1. When you see thickening on MRI, you're observing the ligament's response to chronic stress, partial tearing, or degenerative changes.
Specific MRI Criteria for Spring Ligament Injury
The most reliable diagnostic criteria focus on the superomedial portion of the spring ligament complex 2:
- Thickening >5 mm (particularly proximally) is seen in 92% of surgically proven injuries
- Thinning <2 mm (particularly distally) occurs in 85% of proven injuries
- Increased signal intensity on T2-weighted or proton-density sequences accompanying the thickening
- Full-thickness gap in the ligament (seen in 79% of surgical cases) is the most specific finding for complete tears 3
Clinical Context and Associated Findings
Spring ligament thickening rarely occurs in isolation 1. You should actively look for:
- Posterior tibial tendon abnormalities (tendinosis, partial or complete tears) - present in the vast majority of cases
- Flatfoot deformity on weight-bearing radiographs
- Deltoid ligament complex injuries
- Talonavicular joint subluxation or malalignment
Important Diagnostic Pitfalls
The Gliding Layer Mimicker
A normal anatomic structure can simulate spring ligament thickening: the gliding layer between the posterior tibial tendon and the superomedial calcaneonavicular ligament measures 1-3 mm and can appear as apparent ligament thickening 1. This is a normal finding and should not be misinterpreted as pathology.
Medio-Plantar Portion Unreliability
The medio-plantar portion of the spring ligament complex is difficult to evaluate reliably on MRI due to its complex orientation 2, 4. Findings in this portion (thickening >7 mm, signal heterogeneity) are less reproducible and harder to correlate with surgical pathology. Focus your assessment on the superomedial portion, which shows consistent, reproducible abnormalities that correlate with surgical findings.
Clinical Significance and Management Implications
Spring ligament insufficiency is increasingly recognized as requiring surgical repair or reconstruction in the management of adult-acquired flatfoot deformity 4, 3. The presence of thickening with increased signal intensity has:
- Specificity of 100% for spring ligament insufficiency 4
- Sensitivity of 54-77% (moderately sensitive but highly specific) 4
When multiple abnormalities are present (thickening, increased signal, waviness, and especially a full-thickness gap), you can confidently diagnose a spring ligament tear 3.
What to Report
When you identify spring ligament thickening, your report should specify:
- Location: Superomedial vs. medio-plantar portion
- Measurement: Actual thickness in millimeters (>5 mm is abnormal)
- Signal characteristics: Presence of increased T2/PD signal
- Associated findings: Distal thinning, gaps, posterior tibial tendon status, flatfoot alignment
- Clinical correlation: Note that this finding suggests ligament insufficiency and may warrant surgical consideration in the appropriate clinical context
The finding of spring ligament thickening should prompt careful evaluation of the entire medial ankle stabilizing complex and consideration of weight-bearing foot radiographs to assess for flatfoot deformity if not already performed.