The internal structure of the arthrosic and normal femoral heads are grossly different although both are roughly circular. In particular bone is destroyed or defective superolaterally in the former. The compressive strength of the femoral head depends upon the medial trabecular system which runs through the head at approximately 20 degrees to the vertical i.e. the plane of the resultant of the major loads borne by the hip. Ideally a femoral resurfacing prosthesis should be placed in this axis. The bone of the arthrosic femoral head can be sustained by blood reaching it within the neck. Thus vessels on the intracapsular surface of the neck need not be preserved.