Plantar Fasciitis on Ultrasound: Threshold Plantar Fascia Thickness >4 mm and Associated Findings
Threshold Thickness and Localization
The most useful diagnostic sign of plantar fasciitis is plantar fascia thickness >4 mm. Measurement is performed at the fascia attachment zone to the medial tuberosity calcaneus. The process is bilateral in approximately 1/3 of cases.
Normally, the central bundle of the plantar aponeurosis, running from the calcaneal tuberosity to the metatarsophalangeal joints, is thick proximally and thin distally. Normal thickness at the calcaneal tuberosity is <4 mm. The lateral bundle (from the calcaneal tuberosity to the base of the 5th metatarsal bone) normally measures <2.3 mm in thickness.
Additional Ultrasound Signs
In addition to thickening >4 mm, plantar fasciitis presents with:
- hypoechogenicity of the fascia and loss of normal fibrillar echographic texture;
- minor dystrophic calcification of the fascia;
- perifascial edema, atrophy, and edema of the overlying heel fat pad;
- perifascial and intrafascial hyperemia;
- decreased elasticity on elastography.
Calcaneal Spur and Enthesitis
Calcaneal spur is detected on radiography in 50% of patients with plantar fasciitis but also occurs in asymptomatic individuals. On ultrasound, the spur is located deeper than the fascia (not within it) and may simulate the anterior edge of the calcaneal bone. In the presence of erosions and hyperostosis, enthesitis should be considered; localization is at the calcaneal attachment zone or the base of the 5th metatarsal bone.
Differential Diagnosis
Plantar fibromatosis differs from fasciitis by a discrete spindle-shaped nodule of the plantar fascia, separated from the calcaneal attachment, and predominantly involves the distal 2/3 of the fascia. The nodule is hypoechoic (75%) or isoechoic (25%) relative to the fascia. Mean nodule dimensions: length 13 mm (4–17 mm), width 10 mm (4–22 mm), depth 4 mm (2–10 mm).
| Sign | Plantar Fasciitis | Plantar Fibromatosis |
|---|---|---|
| Localization | Medial tuberosity of calcaneus | Distal 2/3 of fascia |
| Morphology | Thickening at attachment zone | Discrete nodule |
| Thickness/Size | >4 mm | Nodule (mean 13×10×4 mm) |
MRI Findings
On MRI in plantar fasciitis, fascia thickness >4 mm is present in nearly 100% of cases; subcutaneous or perifascial edema in 90%; bone marrow edema at the attachment zone in 40%. Marked bone marrow edema at the attachment, erosions, and hyperostosis are signs of enthesitis (peripheral spondyloarthropathy).
Frequently asked questions
What threshold fascia thickness indicates plantar fasciitis?
Plantar fascia thickness >4 mm at the calcaneal attachment zone is the most useful sign. Normal thickness at the calcaneal tuberosity is <4 mm.
Where exactly should fascia thickness be measured?
At the fascia attachment zone to the medial tuberosity calcaneus.
What additional signs support the diagnosis?
Hypoechogenicity and loss of fibrillar structure, minor dystrophic calcification, perifascial edema, hyperemia on Doppler, and decreased elasticity on elastography.
How to differentiate fasciitis from plantar fibromatosis?
Fibromatosis presents as a discrete spindle-shaped nodule separated from the calcaneal attachment, involving the distal 2/3 of the fascia; hypoechoic (75%) or isoechoic (25%). Fasciitis shows thickening precisely at the calcaneal attachment.
What is the significance of a calcaneal spur?
A spur is detected in 50% of fasciitis patients but also in asymptomatic individuals. On ultrasound, it is located deeper than the fascia, not within it, and may mimic the anterior calcaneal edge.