This 48-year-old male is eight months post surgery for Achilles repair and still has constant pain. You are initially shown three sagittal images and one axial image of the Achilles and repair site without arrows. The first is a fat or T1-weighted (image 1); the second is fat-suppressed or T2-weighted (image 2); the third is sagittal 3D additive gradient-echo / ADAGE (image 3); and the fourth is axial T1 (image 4). What is your differential diagnosis? After thinking about these questions, analyze the same images with arrows (images 5-8).
The Achilles itself is at least 1.5 times normal anteroposterior thickness with normal thickness being approximately 7-8 millimeters. This increase in size is compatible with prior surgical intervention. However, the internal striations (pink arrows, images 5-7) and hyperintensity seen on the water weighted image are typical of an active hypertrophic tendinopathy, or what we used to call “tendinitis”. Therefore, one explanation for the hind-foot pain is tendon inflammation. Tendons can also enlarge from infiltration as in gout, xanthoma, etc.
A second explanation is that the patient has reacted to the anchors placed in the calcaneus as there is swelling along the superior border of the calcaneus, and possibly between the anchors (lower blue arrow, image 5), which may be in part related to susceptibility artifact and signal distortion. However, a suture reaction was favored clinically, and based on the combination of Achilles swelling, pericalcaneal or subcalcaneal (upper blue arrow, image 5) swelling, and possible intramedullary swelling that is not at all destructive, this is a plausible favored diagnosis by imaging.
The other diagnosis to be considered is infection. Clinically, this should be obvious with draining wounds, erythema, a warm extremity. An experienced clinician usually suspects it. The pattern of marrow edema (usually geographically extensive) and loss of cortical margin, or destruction of bone, is not as prominent as would be expected in somebody with a pyogenic infection, even if such infection were caused by a less virulent form of skin bacteria such as Streptococcus viridans.
Differential diagnosis in descending order of likelihood:
1 – Anchor and/or sutural reaction.
2 – Active tendinopathy with granulation tissue from prior surgery.
3 – Smoldering infection least or unlikely.
Postscript: The axial T1-weighted sequence (images 4 or 8) with an oval 2 millimeter nidus of intermediate signal intensity. This is the only image on which it was seen. What would you do with it? We have presumed that it represents granulation tissue and have requested a nine-week followup.
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