Here is a large superolateral humeral head erosion and synovial thickening and joint effusion.
Is this a Hill-Sachs Lesion?
Hill-Sachs lesions are strongly associated with anteroinferior glenoid fracture, anteroinferior glenoid labrum tear (Bankart lesion), and joint capsular injury.
However, there are several common non-traumatic bones and joint diseases that rarely mimic Hill-Sachs lesions.
Ankylosing spondylitis may exhibit erosions and bone destruction at the shoulder and hip.
In long-standing AS, glenohumeral joint involvement is not uncommon and demonstrates a large erosion of the superolateral aspect of the humeral head, producing a 'hatchet' deformity.
This lesion has a wedge shape as if it was chopped with a hatchet.
At the coracoid process level, the humeral head should have a circular shape, however, this large erosion is seen, like the Hill-Sachs defect.
In addition, there is extensive synovial proliferation
I think these findings are also shown in rheumatoid arthritis, but, In the absence of osteoporosis and the presence of sclerosis, this sign helps to differentiate it from rheumatoid arthritis.
Entheses are sites where tendons, ligaments, joint capsules or fascia attach to bone.
Inflammation of the entheses (enthesitis) is a well‐known hallmark of spondyloarthritis.
Enthesitis has been demonstrated in various peripheral locations including the Achilles tendon, pes anserinus, greater trochanter, ischial tuberosities, and the knee.
In the shoulder joint, intense entheseal bone marrow edema, particularly in the acromion and in association with erosion at the greater tuberosity, was the most specific finding in AS shoulders.
These images are virtually drawn by me to understand the typical case.
This patient also had a hip MRI.
Here we can see enthesopathy involving gluteus medius and hamstring tendons.
There is enthesitis‐related bone marrow edema at the ischial tuberosity, cortical irregularity of the greater trochanter, and edema within the enthesis of the hamstring and gluteus medius tendons.
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