Examples of these components can be found made of aluminum, stainless steel and titanium. Aluminum, in general, is considered as a lightweight alternative to steel. It's not as strong but depending on the particular application it is often strong enough to meet the design criteria and pass the necessary testing procedures. Certain knees are fabricated of aluminum, taking advantage of its light weight. Some of these knees are very strong and durable owing much of their strength to the geometry of the knee as well as the material used.
Steel is certainly strong but it is relatively heavy.
Because steel is strong, it can be used to create small components that may rely more on the strength of the material than the geometry of the design. Small knee units used for endoskeletal prostheses were originally made of steel. The material is fairly heavy however very little material is needed to construct these knees. Titanium is a strong, lightweight alternative. The penalty for using titanium is higher cost.