The problem is that the alignment from the hip, knee, and ankle axis is secondary to restoring their pre-arthritic alignment, so you have to put a tibia in varus and a femur in valgus. This could be somewhat concerning.
Does alignment matter for total knee survivorship and is there a penalty for placing your knees outside the “safe zone”?
Mayo Clinic had a marquis paper in 2010 in JBJS-Am (Parratte, et al) which looked at the +/- 3-degree safe zone and the long-term outcomes for 398 cemented total knees. They reported no difference in survivorship at 15 years for total knees outside the “safe zone.”
But then you look a little closer, 91% of patients were within +/- 4 degrees and there were fewer than 20 patients who were in varus overall alignment. I think we should caution making broad generalizations on the risk factors of the safe zone when we have less than 20 patients.
In a study of over 3,000 patients (Berend, et al, CORR 2004), it paints a slightly different picture. Looking at the effects of putting a tibial component in >3 degrees varus, it results in a 17 times increased risk of failure over time and when you combine varus with a BMI greater than 33, which is many of those patients in the U.S., the failure rate goes up exponentially (168x).
There are multiple papers; historically, we have seen an association between varus alignment and total knee failure. You have a meta-analysis (Liu, et al., Knee Surg Sport Traumatol Arthrosc 2016) with more contemporary knee designs of over 12,000 knees showing that varus overall alignment shows a higher rate of failure. Alignment matters for survivorship.
Looking at retrieval data (Collier, JBJS-Am 2007 and Werner, J Biomechanics 2005), there is increased polyethylene wear when knees are in varus even when hips, knees and ankles are in neutral. Alignment matters.
Looking at finite element analysis data (Innocenti, J Arthroplasty, 2016), tibias in varus have 45% increased stress on medial bone and having a varus tibial component is always detrimental. So, alignment matters.
Cadaveric studies (Green, J Arthroplasty 2002); you place your tibial component in varus, you load your knee with a tester and you have a posterior-medial hotspot and that classic posterior-medial collapse. Alignment matters.
So, why is Dr. Howell still smiling?
Because in his one long-term outcome study of 233 kinematic knees at 6.3 years, his knees perform extremely well—97.5% implant survivorship at 6.3 years even though 80% of those tibial components were in varus, some of which were up to 7 degrees.
But, at 6.3 years in the other varus studies presented, those knees, also in 3 degrees of varus, were performing exceptionally well.
