This Model is of the Knee Joint, but without the Patella covering the front. This lets you see the connection between the Femur and Tibia better. Note: This is in the Planar style, meaning that the corners of the forms are emphasized to allow you to 1) see the planes easier 2) Better understand which way the planes face. and 3) Light the form, and see the plane breaks easier (for artists).
This version is a bent knee. And you will note, that you are only seeing the bottom of the Femur and the tops of the Tibia and Fibula. The full bones are available for purchase, separately.