Modern total knee replacement has undergone remarkable advances over the past several decades, but few concepts have generated as much discussion—and as much research—as kinematic alignment (KA). In Episode #35 of The Joint Replacement Podcast, I had the privilege of sitting down with Dr. Stephen Howell, one of the pioneers of kinematic alignment, to discuss the origins of this philosophy, the biomechanics that underpin it, and how it has challenged conventional thinking about knee replacement.
Dr. Howell’s career uniquely positioned him to question long-held assumptions. Before becoming known for his work in knee arthroplasty, he spent two decades focused on ACL reconstruction. Those years taught him an important lesson: millimeter-level errors in tunnel placement could dramatically alter knee function. As he transitioned into total knee replacement, he began asking a simple but profound question: Could similar principles explain why some knee replacements felt natural while others did not?
The Origins of Kinematic Alignment
For many years, total knee replacement was based on the principles of mechanical alignment, which attempts to position implants relative to the mechanical axis of the femur and tibia. While this approach has produced excellent long-term implant survivorship, many surgeons have recognized that knee replacements often fail to reproduce the “forgotten joint” feeling patients experience after successful hip replacement.
Dr. Howell explains that kinematic alignment starts from a different premise: rather than making every knee look the same, surgeons should strive to restore each patient’s pre-arthritic anatomy. By resurfacing the joint and recreating the native joint lines and axes of motion, the goal is to preserve natural ligament tension and normal knee kinematics rather than altering them.
This philosophy grew out of decades of biomechanical research identifying the three principal axes around which the knee moves, work performed by researchers including Anne Hollister, Coughlin, Iranpour, and others that fundamentally changed our understanding of native knee motion.
From Engineering to the Operating Room
One of the most fascinating portions of our conversation centered on the development of patient-specific instrumentation (PSI).
Working alongside engineers at UC Davis, Dr. Howell helped develop one of the earliest MRI-based patient-specific cutting guides. Long before robotics became commonplace, this technology allowed surgeons to plan implant placement using three-dimensional imaging and customized guides designed specifically for each patient’s anatomy.
Although regulatory hurdles limited early adoption, these efforts laid much of the groundwork for today’s personalized approaches to knee replacement.
Why Millimeters Matter
Throughout our discussion, Dr. Howell repeatedly returned to one central concept:
Small errors create large consequences.
Drawing parallels with ACL reconstruction, he describes how seemingly tiny changes in implant position can alter ligament tension, knee motion, patellar tracking, and ultimately patient satisfaction.
Rather than relying solely on alignment angles, Dr. Howell advocates carefully measuring bone resections with calipers to verify that the femoral component truly restores the patient’s original anatomy. He argues that these measurements provide critical information that angles alone cannot.
This “calipered kinematic alignment” technique has become one of the defining features of his surgical philosophy and has been described extensively in the orthopedic literature.
Implant Design Continues to Evolve
The discussion also explored an important point that is sometimes overlooked:
Alignment is only part of the equation.
As Dr. Howell’s understanding of knee biomechanics evolved, so did his views on implant design. He explains why restoring native knee motion requires implants that better replicate normal anatomy, particularly with respect to the trochlea, medial stability, and lateral compartment motion. These concepts have helped drive the development of newer “KA-optimized” implant designs intended to better complement kinematic alignment principles.
Robotics and Alignment
One of the most thought-provoking parts of the interview centers on robotics.
Dr. Howell shares his perspective that the value of any technology ultimately depends on whether it improves patient outcomes—not simply whether it increases surgical precision. He discusses his views on robotics, navigation, and manual instrumentation while emphasizing that successful knee replacement still depends on understanding native anatomy and executing the surgical plan accurately.
As with many evolving topics in orthopedics, thoughtful surgeons may reasonably interpret the available evidence differently. This episode highlights one influential perspective from an individual who has helped shape much of the modern discussion surrounding knee alignment.
Education for the Next Generation
Although Dr. Howell has retired from active surgical practice, he remains deeply committed to education.
He has created the KA-Optimized TKA Educational Collective, a free online resource designed to teach surgeons the principles of kinematic alignment through structured courses, challenging cases, literature references, and an AI-powered educational assistant.
For surgeons interested in learning more, visit: https://www.kaoptimizedtka.com
Listen to Episode #35
Whether you’re a patient interested in understanding the evolution of knee replacement or a surgeon following the ongoing alignment debate, this episode provides a fascinating look into one of the most influential ideas in contemporary arthroplasty.
Search The Joint Replacement Podcast on YouTube, Spotify, Apple Podcasts, or wherever you listen to podcasts.
References
- Howell SM, Roth JD, Hull ML. Kinematic Alignment in Total Knee Arthroplasty: Definition, History, Principle, Surgical Technique, and Results of an Alignment Option for TKA. Arthropaedia. 2014.
- Howell SM, Papadopoulos S, Kuznik KT, Hull ML. Accurate alignment and high function after kinematically aligned TKA performed with generic instruments. Knee Surgery, Sports Traumatology, Arthroscopy. 2013.
- Kim KK, Howell SM, Won YY. Kinematically Aligned Total Knee Arthroplasty with Patient-Specific Instrument. Yonsei Medical Journal. 2020.
- Howell SM. Calipered Kinematic Alignment for Total Knee Arthroplasty. ISAKOS Newsletter. 2020.
- Howell SM, Zabiba A, Nedopil AJ, Hull ML. The Forgotten Joint Score after total knee arthroplasty with a kinematic alignment-optimized femoral component matches total hip arthroplasty. Knee Surgery, Sports Traumatology, Arthroscopy. 2025.