- significant forces are generated across the patellofemoral articulation on flexion and extension of the knee;
- these forces progressively increase as flexion angle of knee increase;
- contact area between the femur and the patella also varies in size with the angle of knee flexion and is greatest between 20 and 60 deg of flexion;
- there are 4 basic functions of the patella:
- provide a congruent articular surface opposing the femur;
- providing fulcrum to augment quads efficiency in knee extension;
- serving as a strong linkage within the quadriceps tendon;
- protecting the knee from direct trauma;
- references:
- Patellofemoral function in total condylar knee arthroplasty.
- Pathomechanics of the femoropatellar joint following total knee arthroplasty.
- Patellofemoral forces after total knee arthroplasty: effect of extensor moment arm.
- design considerations:
- large posterior bulge of femoral condyles is advantageous for knee flexion beyond 90 deg, but conflicts w/ need for a regular surface for patello femoral joint;
- requirements for knee design include need to restore a normal patello femoral condylar pathway, minimizing increases in patellofemoral joint pressure, and at the same time, maximizing joint ROM;
- patellofemoral contact:
- increasing contact area will decrease peak contact pressures and thereby reduce stresses in the underlying polyethylene cement interface;
- increasing the contact area between prosthesic patella & trochlea will not increase its stability;
- regardless of the configuration ofo the prosthetic patella and trochlea peak contact pressures that occur when knee is flexed far exceed compressive yeild strength of UHMWPE;
- although a faceted patella implant will distribute the forces to underlying bone more evenly than will symmetrical patella, it will only do so if it is exactly aligned with the trochlear facet of femoral implant;
- if it is only slightly malaligned, peak stresses in the bone will be increased locally;
- ref: One-peg versus three-peg patella component fixation in total knee arthroplasty.