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Surgeons debate utility of neutral mechanical alignment in revision TKA
ORLANDO, Fla.— Two surgeons debated the use of the neutral mechanical axis to correct varus and valgus deformities in revision total knee arthroplasty, according to a crossfire debate at the Current Concepts in Joint Replacement Winter Meeting, here.
Leo A. Whiteside, MD, said the key to maintaining neutral mechanical alignment is to look at how the native knee works, which involves utilizing the anterior-posterior (AP) axis.
“As the hip flexes and the knee flexes, the tibia stays in the AP axis and it rotates around the epicondylar axis,” Whiteside said. “As you flex the knee, you can see up the tibia, down through the AP axis of the femur right to the femoral head. You can do this every time. It is not perfect, but it is excellent.”
Leo A. Whiteside
For a varus knee, Whiteside recommends finding a reliable landmark, such as the AP axis, and creating a hole with a reamer down the cement center of the tibia. He warned that surgeons must aspirate the medullary canal of the femur thoroughly with a long, aspirating reamer.
Once the intramedullary rod is inserted into the tibia through the ankle, the AP axis marks the femur in flexion, which is still in varus at the tibia but correct at the femur, he said. Osteophytes should be removed carefully, he added. Since the knee is still malaligned, he recommended releasing tight ligaments and stabilize it with thicker polyethylene – tight ligaments in front cause flexion problems, while tight ligaments in back cause extension problems, he said. Once the polyethylene is inserted, the knee should be similar to a native knee.
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Regarding valgus knees, the procedure is similar, Whiteside said. Surgeons should follow the medullary canal, the AP axis and mark it. Even if there is a defect on the lateral side, once the medullary canal rod is inserted, surgeons should cut based on the intact surface. After applying cutting guides, finish the cuts correcting the femur deformity based on AP axis. For the intramedullary rod at the tibia, surgeons should apply the same philosophy using the AP axis and the ligaments should be released gradually front to back for flexion on the epicondyles, which creates a partial correct.
Whiteside said surgeons should avoid instruments or techniques that eliminate ligament balance, use computer navigation or implement minimally invasive custom instruments.
Michael J. Dunbar
Michael J. Dunbar, MD, FRCS(C), PhD, said in his presentation that some patients should not have their knees corrected to the neutral mechanical axis. Like in a bell curve, some patients are outside the curve and are healthy even though they have constitutional varus.
“What I would suggest to you is if we take this straight and balanced approach, [then] we are covering the majority of the curve but we are missing the tails,” Dunbar said. “The tails are the individuals who were never in neutral mechanical alignment, who do not like being in neutral mechanical alignment and are dissatisfied when we put them in neutral mechanical alignment.”
He added that several studies have indicated as many as 18% of patients report dissatisfaction with their knee implants. In patients with constitutional varus, surgeons will need to use additional asymmetrical distal and posterior femoral cuts to balance the flexion and extension gap created by aligning with the neutral mechanical axis, Dunbar said.
Reference:
Whiteside LA and Dunbar MJ. Papers #70 and #71. Presented at: Current Concepts in Joint Replacement Winter Meeting; Dec. 12-15, 2012; Orlando, Fla.
Disclosure:
Whiteside receives royalties from Smith & Nephew. Dunbar has no relevant financial disclosures.