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BMI, alignment associated with knee replacement failure

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The survival of a total knee replacement hinges heavily on a neutral overall alignment and component position as well as a body mass index under 41, according to a presentation at the 2011 Annual Meeting of the American Academy of Orthopaedic Surgeons.

“Over the years, multiple groups have looked at alignment as a cause of failure in total knee replacement, and not all the results have been similar,” study author Robert A. Malinzak, MD, said during his presentation. “So we wanted to identify the optimal ranges for anatomic, tibial and femoral alignment in total knee replacement … we also looked at body mass index, to see that role.”

Malinzak and his team performed a retrospective review of preoperative BMI and postoperative overall coronal tibiofemoral, tibial and femoral component alignments in 6,070 knees. They analyzed cases of failure that did not result from infection, taking into account overall alignment, component positions and patient BMI.

The researchers found the primary component angles most likely to be implicated in failure were tibial positions of <90° and femoral positions of >8°. If both component angles were abnormal, Malinzak noted, the failure rate was 8.7%. Those patients with the best overall survival were those who displayed both a neutral tibial (90° through 102°) and femoral component (<7.9°), as their failure rate was 0.2%.

The study also found that increasing BMI resulted in an increased failure rate for well-aligned knees, varus knees and valgus knees.

“In a cohort of patients with BMIs over 40, no matter what the alignment the success rates declined or the failure rates were worse,” Malinzak said. “For a BMI of 22 or under and a varus alignment, there was actually a significant increase in failure as well.”

“Malalignment is a major cause of total knee replacement in our group of patients,” Malinzak said. “Optimal tibial cuts and femoral cuts will improve our success. Our overall goal for coronal alignments is 2.5° to 7.4°, and in the big folks — BMI over 40 — that increases their chance of failure and that is something to talk about with them preoperatively.”

Reference:

• Malinzak RA, Davis K, Pierson JL, et al. Alignment and BMI, factors associated with total knee replacement failures. Paper #130. Presented at the 2011 Annual Meeting of the American Academy of Orthopaedic Surgeons. Feb. 15-19. San Diego.

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