An analysis of a custom expanding total femoral prosthesis with a rotating hinge knee

Osteosarcoma is the most common malignant bone tumor in the pediatric age group. The authors present an analysis of a custom total femoral prosthesis implanted in March 1992 in a 9 year old black male suffering from osteosarcoma. The custom design included a bipolar, expanding tube-in-tube mid-shaft component, and a rotating hinge knee used in the reconstruction of the patient´s right lower limb after surgical resection of the femur. The areas analyzed were (i) the trochanteric attachment, (ii) the expanding component, and (iii) the rotating hinge knee. The greater trochanter was cabled around the prosthesis in order to maintain abduction and to provide proximal stability. The muscle maintained the vascularity of the bone. A porous surface on the prosthesis allowed bone ingrowth and stability. The stresses acting on the stainless steel multifilament cables reconnecting the greater and lesser trochanter were determined for bipedal stance, single-legged stance, stance phase of gait and running of the patient. They were calculated to be 0.5 MPa, 17 MPa, 23 MPa and 51 MPa respectively. On comparing these stresses to the fatigue data for stainless steel cables, it is estimated that the cables will last for 10 ⁸ cycles which is approximated to be 10 years of usage. The expanding midshaft was designed to provide the required lengthening of the femur as the patient grows. A telescoping tube-in-tube with a cross pin was the design chosen because of minimal micromotion and wear