Primary Knee Symposium: Biomechanical Considerations in the Knee

Kinematic Conflict – Mark Kester, PhD

Dr. Kester began his presentation with a review of the definition of normal kinematics. PSlide A He discussed the challenges with multi-radius designs, suggesting that they can lead to kinematic conflict. According to Mikhulak et. al., kinematic conflict can cause:¹

• Soft tissue and implant impingement
• Reduced patellofemoral function
• Reduced mobility
• PSlide B

Mikhulak et. al also report that a PS TKA design shows that kinematic conflict can cause rotational constraint (box-post impingement), and rotational torque (and relative motion) to modular interface and to the fixation interfaces.¹

Dr. Kester reviewed the results of mechanical testing performed on a variety of fixed, PS TKA designs. Wear and stress were measured. Results showed that in extension, the anterior aspect of the insert and the clearance designed between the post and the box dictate the resistance to rotation.² In addition, in flexion, the insert geometry dominates and should be designed to accommodate rollback and internal/external rotation.² PSlide C

He then did a literature review on geometry and other factors that influence rotation, post wear, and post fracture. In a review of the 2009 Australian Registry, the data showed that a more constrained device, in this case the Smith & Nephew Journey Knee, had a higher revision rate.³ PSlide D

Dr. Kester then reviewed the Triathlon TS implant which was designed to avoid kinematic conflict, potentially increase range of motion, and decrease rotational stresses. The features include:

• Single AP axis from 10° - 110°
• Cam / Post engage at 40° degrees
• ±20° rotation

In summary, an optimal knee replacement device works with the correct arcs, the natural axes, and a shape more closely replicating the natural knee. It should also allow the knee to be guided by the soft tissue envelope.