Summary Statement

A large proportion of knee arthroplasty patients are dissatisfied with their replacement. Significant differences exist between preoperative, postoperative and normal kinematics. A better understanding of the inter-relationships between kinematics, shape and prosthesis placement could lead to improved quality of life.

Knee kinematics are altered by total knee arthroplasty (TKA) both intentionally and unintentionally. Knowledge of how and why kinematics change may improve patient outcome and satisfaction through improved implant design, implant placement or through rehabilitation.

In the present study we imaged and compared the 6 degree-of-freedom (DOF) patellofemoral (PF) and tibiofemoral (TF) kinematics of 9 pre-TKA subjects to the kinematics of 15 post-TKA subjects (Zimmer NexGen LPS implants) using a novel sequential-biplanar radiographic protocol that allowed imaging the postoperative patellofemoral joint under weightbearing throughout the range of motion, which has not been done previously to our knowledge.

There were clear, statistically significant differences between the pre-TKA and post-TKA kinematics: for the TF joint, the tibia was more posterior and inferior (max 20 mm and 15 mm, respectively) in the post-TKA group compared to the pre-TKA group (p<0.001), and had neutral alignment in the post-TKA group compared to varus alignment (max 9°) in the pre-TKA group (p<0.001). For the PF joint, the patella was shifted more posteriorly and medially, and tilted more medially in the post-TKA group compared to the pre-TKA group (p<0.001). There were no significant differences in PF superior/inferior translation and flexion/extension (p>0.5). Both groups showed differences from normal kinematics, based on the literature.

The kinematic differences are likely due to a combination of surgical, implant and patient factors. To investigate this further, we imaged the 9 pre-TKA subjects a minimum one year after their surgery; analysis of these data is in progress. Computed tomography (CT) scans and quality of life surveys were also taken before and after surgery. By comparing the preoperative and postoperative kinematics and shape for the same subjects, and analysing the interrelationships amongst these, we aim to determine if a different implant shape or different component positioning could create more normal kinematics, resulting in a better clinical outcome.