Isolated ossification of the coracoacromial ligament is a rare finding, and in this case most likely secondary to previous trauma.

Calcification of this ligament is most often due to trauma or renal failure, and if sometimes referred to as a hook osteophyte.  

Rotator cuff pathology can occur with this pattern of ossification and if suspected the type of imaging will depend on the institution's preference. Shoulder ultrasound in 70-year-old patients is a quick and low-cost option.  Non-arthrographic shoulder MRI could be considered but the MRI can be claustrophobic and if degraded by significant movement artefact, can be non-diagnostic.