If you look at some other studies (Kim, et al, J Arhtroplasty 2017 – a study looking at 182 simultaneous bilateral knee replacements, randomized one to a medial pivot, and the other to a cruciate retaining knee) the authors found that Knee Society, WOMAC, and range of motion were all worse in the medial pivot knee. There was 93% satisfaction with the cruciate retaining device versus 75% with the medial pivot—but no difference in long-term survivorship.
The authors concluded that the medial pivot actually performed worse.
A recent study (Meneghini, et al, J Arthroplasty 2017), looking at 140 primary knee replacements. Meneghini and his colleagues used sensor-embedded trials, they used CR, CS, and PS versions of the same implant design and assessed the range of motion and kinematic pattern. They found no difference in one-year Knee Society and UCLA activity scores whether or not these patients exhibited the medial pivot kinematic pattern.
I think that medial pivot designs do have good results. There is some merit to it. I think the concept makes sense, but all knee replacements require balance to have success.
If you’re not balanced, you’re not going to pivot around that medial side, you can even lift off that medial side despite the congruency that is present. I think it comes down to what’s best in your hands, what technique works for you, what implant works for you.
Moderator Thornhill: Bill, let’s start with you. What should drive the kinematics of a replaced knee? Should it be the prosthesis, or should it be the soft tissues?
Dr. Walter: That’s a good question. I think as with all the other designs it’s the soft tissues. With the medial ball and socket knee, the design actually drives the kinematics.
Moderator Thornhill: Denis, what should drive the kinematics of a total knee?
Dr. Nam: I think it should be both implant design and soft tissue. A medial congruent knee is a little more forgiving, but I would say you still have to balance it. If you overly externally rotate your component and you’re loose medially or you over-release the medial soft tissue envelope, I still don’t think you’re going to pivot around the medial side of that knee.
Moderator Thornhill: Bill, do you think the medial pivot knee more closely resembles the kinematics of the normal knee?
Dr. Walter: I do.
Moderator Thornhill: But you know the normal knee has menisci, they move, it has articular cartilage, we’re using a hard bearing. If you look at the fluoroscopic data from Rick Komistek of normal knees, they’re all over the place.
Dr. Walter: I mean if you think of a hip, hips are ball and socket and the center of rotation is constrained. The muscles act, the center of rotation stays where it is, and the muscles can produce a movement. With a knee, if it’s unstable the muscles act and the knee just slides. If you can constrain the femur on the tibia so that it doesn’t slide anteriorly, the quadriceps can act the way it should, it doesn’t need the hamstring to co-contract to try to stabilize the knee. I think it’s a better kinematic situation if you have a stable knee than if you have an unstable knee that’s sliding backwards and forwards.
Dr. Nam: I think Dr. Thornhill brings up a good point though that there’s such variability in how every patient’s knee moves and especially in the arthritic situation, we don’t know the way they’ve been for potentially several decades. Some patients will have a medial pivot kinematic pattern, some people will have a dual pivot, some people’s lateral condyle will slide anteriorly slightly when they’re in full extension. I don’t know if we know what’s best, similar to hip targets and alignments, I don’t know if we know what’s best for each knee replacement and that’s why I think we kind of shoot for the middle and say, “What’s going to work best for us,” when we do the surgery itself. I agree, I think there is variability, I don’t think every knee needs to move that way.
