Ranawat v. Dunbar: Neutral Mechanical Alignment: Stickin’ With the One that Brung Ya!
OTW Staff • Tue, April 10th, 2018
Moderator Trousdale: The next debate is I think one of the hottest topics in knee surgery and that’s where do we put the mechanical axis or align our limbs. Professor Ranawat from New York is going to be in the affirmative of the Neutral Mechanical Alignment: Stickin’ With the One that Brung Ya!
Dr. Ranawat: The principles of total knee replacement are: Proper alignment in all three planes; soft tissue balance; maintain the joint line; and obviously, proper cement technique.
Neutral position of the proximal tibial cut: I have been practicing this technique for a long time. In extension you have a rectangular space with the soft tissue balanced. Same thing in flexion: you have a rectangular space with the soft tissue balanced. I wrote about this in JBJS in 2012.
Bringing the tibia in front of the femur displaces the neurovascular bundle behind the tibia and avoids injury. And you will not leave cement behind in the posterior part of the knee.
With the tibia exposed I size the tibial component based on the dimension of the lateral tibial plateau. I remove all overhanging bone on the medial side as needed and then put the baseplate back down and check whether the cut is correct.
I don’t use a tourniquet, I do use hypotension and tranexamic acid. I do all my dissection with a cautery and hardly any blood loss. I identify all the significant vessels and cauterize them. I bring the tibia in front. Then identify the axis of the tibia. Align the tibial cutting block and cut at least 8-9mm off the uninvolved side. Size it properly and remove the overhanging bone.
I present to you the results of 68 consecutive varus knees: the postoperative mechanical alignment was 0-3 degrees. There were no outliers.
And here I want to make a point.
When you take a long film, if you don’t control your external rotation of the foot, you can have wrong information.
To summarize, total knees with restored mechanical axis have been proven to last 15-20 years in 90% or more of the patients. The technique is reproducible. Fixation of the tibia with metal-backed or all poly component has been durable and is a matter of choice in the hands of most surgeons.
Moderator Trousdale: Thank you Dr. Ranawat, that’s a strong argument to restore mechanical axis in our total knees as far as durability goes, for sure. Michael Dunbar from Canada is going to talk to us more about some functional results and argue, maybe, against putting the mechanical axis right down the middle on everybody.