I'm not sure about this, but it's worth suggesting. I looked at "cavity" and "space" and "site" also, and just don't find anything convincing.

" ... conventional shoulder prosthetics require insertion of a stem into the humeral canal of the humerus, "

"Take care to leave enough calcar to support the medial aspect of the prosthesis. Insert the prosthesis into the canal and remove any obstructing bone ..."

"Check the size of the prosthesis directly by inserting it into the acetabulum before inserting the stem into the medullary canal."

Another site says "Flexible reamers are utilized to progressively ream the canal to the appropriate diameter."

"The stem can also be inserted into the femoral canal using the stem pusher. "

But ... possibly "cavity". Here is one example that uses both terms: "The narrow canal of the bone marrow cavity in the femur poses difficulty when selecting appropriate joint prostheses" And later this site says: "For inserting the femoral stem into the bone marrow cavity, the contact surface between the stem and the proximal femur can be maximized and the stability can also be maximized."