EFFECT OF MEDIAL DISPLACEMENT OF THE TIBIAL TUBERCLE ON PATELLAR POSITION AFTER ROTATIONAL MALPOSITION OF THE FEMORAL COMPONENT IN TOTAL KNEE ARTHROPLASTY

A large Q angle induced by technical error such as an internally relat ed femoral component causes patellar failure after total knee arthropl asty. The effect of medial displacement of the tibial tubercle to decr ease the Q angle for patellar tracking was studied by evaluating the p atellar position relative to the patellar groove on the femoral compon ent in cadaver specimens. A 5 degrees internally related femoral compo nent caused the patella to shift medially about 5 mm and also caused t he tibia to rotate internally about 3 degrees at full extension. With a 5 degrees externally rotated femoral component, normal patellar trac king occurred. The distance of medial displacement was determined so t hat the patellar tendon was parallel to the longitudinal axis of the t ibia al full extension. This allowed the quadriceps tendon, the patell a, and the patellar tendon to form a straight line. The average distan ce of medial transposition of the tibial tubercle was 9.32 mm. Mediali zation of the tibial tubercle caused the patella to shift about 2 mm m edially from the patellar groove. The transfer also caused an external rotation of the tibia (2 degrees-5 degrees). Medial transfer of the t ibial tubercle changes patellar kinematics and corrects the tendency t oward lateral patellar dislocation caused by internally rotating the f emoral component; however, it also creates minor patellar and tibial k inematic changes that may have a clinical effect.