The more one begins to understand the magic behind movement in human body, the more one can appreciate the skills involved in conducting successful orthopedic surgeries like a knee replacement. In a perfectly aligned normal knee, you can imagine a straight weight bearing axis passing through the center from the femoral (thigh bone) head to the middle of the ankle.
In a deformity called Varus alignment, this load bearing axis shifts to the inside, causing stress on the inner part of the knee, causing a deformity called bow-legs.  When the load bearing axis shifts to the outside, it causes excess pressure on the outside part of knee. This causes the knee to bend inwards leading to the knock-knee deformity. This condition is also called Genu Valgum.
Such deformities can occur naturally as congenital defects but can also be trauma induced. Knee replacement, when not carried out properly, can also lead to joint mis-alignment. Keeping the alignment between the hip, knee and ankle neutral, that is maintaining a straight load bearing axis is critical for a successful knee replacement.
In a study that evaluated as many as 279 revision surgeries for TKR, mal-alignment of the limb or mal-positioning of the TKR was found to be an important cause of the failure. Of these 280 cases for revision surgery, 21 cases were caused by mal-alignment induced wear or osteolysis. [1, 3] A similar study conducted on this topic found that poor knee alignment posed an almost 7 times higher risk of failure!
A total knee replacement surgery involves several steps, one of which is cutting and shaping the femoral head followed by the implanting of the artificial femoral component. Watch this video to learn more – https://www.youtube.com/watch?v=m8LDBlZN-XM
This step is of critical importance to ensure that implant will be within the stipulated alignment range. Several research studies also suggest that computer assisted surgeries are known to have reduced instance of mis-alignment but the debate on this has continued in the medical community. [4, 5]
What does all of this mean for you as a patient?
First, you should be aware that one of the causes that can lead to failure of a knee replacement is mis-alignment. A mis-aligned joint can lead to a much shorter life of the artificial joint. Proper alignment of the implants is strongly associated with greater stability and a lower rate of loosening.
When the knee was not properly aligned as per the best practices, the failure rate was 1.5% in a sample size of 1044 patients, as opposed to the fail rate of 0.6% in case of normal alignment. 
The impact of mis-aligned joint was further exacerbated by the weight or BMI of the patient . The lowest failure rate of 0.65% occurred in patients with a BMI of 23 to 26 KG / m2.
This failure was 2.5 times higher in patients with BMI of 27 to 40 KG / m2 and almost 10.8 times higher in patients with BMI of above 41 KG / m2 .
This means that if you are over-weight, you are more likely at risk of failure when the joint is improperly aligned. A property replaced knee joint means that all forms of alignment, that is alignment of femoral head with the joint, the tibial bone and the overall alignment is appropriate as per the established practices. When any of these are off, it results in a sub-optimal outcome that often requires a corrective surgery.
Several studies have demonstrated that as the volume of surgeries conducted by a surgeon increases, the post-surgery complication rates tend to come down. Complications in case of surgeons performing fifty or more procedures in a year were at 1.5% compared to 4.2% complications that performed five or less such procedures. 
One hypothesis associated with this finding is that as a surgeon performs more and more procedures, he or she gets better acquainted with the minor differences knee anatomies presented in patient to patient. This experience could result in ensuring that there is a better alignment of the artificial knee, leading to a lower complication rate.
Therefore, as a patient considering a joint replacement surgery, it is important for you to ask questions on how many surgeries have been performed by your surgeon. This alone can be a strong determinant of how successful and long lasting your knee replacement turns out to be.
Another take-away from above studies is that BMI has an important role to play. A person with a higher BMI is more likely to need a revision surgery in the event of a mis-aligned joint. So if you fall into an over-weight or obese category, getting your knee replacement done by a high volume surgeon is more advisable than otherwise. Another option could be to try and reduce weight as much as possible through low-impact exercises, diet etc. prior to undergoing the knee replacement surgery.
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- Malposition and mal-orientation after total knee replacement, Tomoyuki Matsumoto and Ryosuke Kuroda
- Arthritis & Rheumatism, Volume 56, Issue 4, Article first published online: 28 MAR 2007
- Fehring TK, Odum S, Griffin WL, Mason JB, Nadaud M. Early failures in knee arthroplasty. Clin Orthop Relat Res. 2001; 392:315-8
- Kim JS, Choi Y, Kwon OR. Computer assisted surgical navigation does not improve the alignment and orientation of the components in the total knee arthroplasty. J Bone Joint Surg Am. 2009; 91(1):14-9
- Spencer JM, Chauhan SK, Sloan K, Taylor A, Beaver RJ. Computer navigation versus conventional total knee replacement: No difference in functional results at two years. J Bone Joint Surg Br. 2007; 89(4):477-80
- Ritter Miller A, Kenneth E, Meding John B, Pierson Jeffrey L, Berend Michael E, Malinzak Robert A, The effect of alignment and BMI on failure of total knee replacement, Journal of Bone and Joint Surgery, Volume 93 (17) – Sep 7, 2011
- Association between Hospital and Surgeon Procedure Volume and the Outcomes of Total Knee Replacement. Katz, Jeffrey N.; Barrett, Jane; Mahomed, Nizar N.; Baron, John A.; Wright, R. John; Journal of Bone and Joint Surgery , Volume 86 (9): 1909 – Sep 1, 2004