Pls help – osteomyelitis / bone abscess at prosthesis? (CT, MRI)

this is an elderly patient with a left knee arthroplasty (several years
old) and a distal pretibial leg ulcer at the distal calf.
CT done to rule out bone involvement at the leg ulcer showed lytic bone
changes at the tibial prosthesis with a medial cortical defect.
MR shows basically the same, the cystic looking bone lesions at the tibial
arthroplasty component extend out of the bone through a small defect. There
is no surrounding marrow or soft tissue edema at this level. (There is
diffuse subq edema of the entire calf though.)

To me this doen’t look like osteomyelitis or bone abscess, more like some
chronic cystic change. But why the cortical defect? Would this be some
periprosthetic lytic reaction? or infectious after all? The leg ulcer is
about 20 cm more distally (not shown; there are no bony changes there.)
What is your opinion?

Cheers,
Andrei