The OP mentions the femoral head, so it refers to the hip joint rather than the knee. A bit more context or a full sentence would help – is this a total joint replacement or, as I suspect, a resurfacing arthroplasty?

Modern hip-resurfacing systems involve placing a non-cemented porous metal cup into the hip socket and cementing a metal cap onto the femoral head. This results in an articulating metal-on-metal surface (IMAGE 3).

http://www.orthop.washington.edu/patient-care...

In which case it refers to the femoral head component (or cap) and the acetabular component (or cup) of the hip-resurfacing system -- an all-metal bearing couple:

http://www.smith-nephew.com/uk/products/ortho...

http://www.biomet.com/wps/portal/internet/Bio...