Design/Failure Modes in Total Hip Replacement - Jonathan Jones 12/9/2000

Biomechanics :

                        Forces acting on Hip, Centralization of head, Length of Abductor

                        Lever arm, Neck Length, Offset, Head and Neck Diameter, Coefficient

                        Of Friction and Frictional Torque, Wear, Lubrication, Stress Transfer     

Design :

Femur- Cemented, Uncemented (Porous/Non Porous/Press Fit/HA )

Acetabulum- Cemented, Uncemented, Reinforcement rings         

Bipolar Components

Hip Resurfacing

"Charnley":   

Low friction torque Arthroplasty, Surgical Alteration of Hip

Biomechanics, Lubrication, Materials, Design, Operating room environment, Cold-Curing acrylic cement

Failure :           

Deviation from the stable femoral component where there is adequate fixation at each interface and intact material integrity.

Loosening :      

Radiographic interpretation of change in the mechanical integrity of the load-carrying cemented femoral component, specifically, fractured cement and an interface gap such as a radiolucent zone at the stem-cement or at the cement bone interface.

Wear :             

Loss of materials from surfaces of prosthesis as a result of motion between those surfaces (Abrasive, Adhesive, Fatigue and Third body)

Factors Associated with Wear :

            Coefficient Of friction Of Materials/Surface Finish        

                        Hardness of Materials

                        Applied Load

                        Sliding Distance for Each Cycle (Head diameter/motion of Hip)

                        No. Of Cycles

Creep :             (Plastic Deformation) Implant shape change without loss of material or production of debris

  Failure Modes (Gruen 1979 ):

                        (I)        Pistoning Behaviour

                                                (a) Stem within Cement

                                                (b) Cement Within bone           

                        (II)              Medial Midstem Pivot

                        (III)            Calcar Pivot