Abstract
Most discussions of alignment after TKA focus on defining “malalignment”; the prefix mal- is derived from Latin and refers to bad, abnormal or defective and thus by definition malalignment is bad, abnormal or defective alignment. No one then wants a “malaligned” knee. The intellectually curious, however, might switch the focus to the other end of the spectrum and ask what does an ideally aligned knee look like in 2015? Is there really one simple target value for alignment in all patients undergoing TKA? Is that target broad (zero +/−3 degrees mechanical axis) or is it a narrow target in which a penalty, in regard to durability or function, is incurred as soon as you deviate even 1 degree? Is that ideal target the same if we are evaluating the functional performance of the TKA versus the durability of the TKA or could there be 2 different targets, one that maximises function and one that maximises durability? Is that target adequately described by a single 2-dimensional value (varus/valgus alignment in the frontal plane) as measured on a static radiograph? Is that value the same if the patient has a fixed pelvic obliquity, a varus thrust in the contralateral knee or an abnormal foot progression angle?
It is revealing to ask “do we understand TKA alignment better in 2015 than in 1979…?” Maybe not. We allowed ourselves over the past 2 decades to be intellectually complacent in regard to questions of ideal alignment after TKA. The constraints on accuracy imposed by our standard total knee instruments and the constraints on assessment imposed by 2-dimensional radiographs made broad, simple targets like a mechanical axis +/− 3 degrees reasonable starting points yet we have not further worked to verify if we can do better. It is naïve to think that the complex motion at the knee occurring in 6-dimensions over time can be reduced to a single static target value like a neutral mechanical axis and have strong predictive value in regard to the success or failure of an individual TKA. We assessed 399 knees of 3 different modern cemented designs at 15 years and found that factors other than alignment were more important than alignment in determining the 15-year survival.
Until more precise alignment targets can be identified for individual patients or sub-groups of patients then a neutral mechanical axis remains a reasonable surgical goal. However, the traditional description of TKA alignment as a dichotomous variable (aligned versus malaligned) defined around the broad, generic target value of 0 +/− 3 degrees relative to the mechanical axis is of little practical value in predicting the durability or function of modern TKA.
