Mechanical vs Arithmetic Definitions of Coronal Plane Alignment of the Knee (CPAK) Measures Have Different Distributions: An Assessment of 3947 Cases

The Coronal Plane Alignment of the Knee (CPAK) classification has been used to describe healthy and arthritic knee alignment as well as to predict phenotypes which could benefit from kinematic alignment using soft tissue balancing during TKA. At our institution, we have access to a large database of navigated TKA procedures including intra and postoperative mechanical hip-knee-ankle angle (mHKA) measurements, which are defined differently than the aHKA. It has been previously recognized that these alternative, but related, measures of coronal alignment may have different distributions. The primary aim of this study was therefore to determine if the CPAK classification frequencies described in the original publication by MacDessi et al. for the aHKA are similar to frequencies acquired using the mHKA. A secondary aim was to categorise postoperative TKA alignment at our institution utilising the mHKA-based CPAK classification. We analysed data from 3947 total knee arthroplasty procedures undertaken using surgical navigation at our institution between March 2007 and October 2022. The mHKA was measured directly during the registration process while JLO was calculated using the mHKA and LDFA (JLO = HKA + 2xLDFA). This was completed twice for each case using the pre and postoperative mHKA and LDFA. Each case was then categorized as one of the nine CPAK phenotypes. The pre-operative mean mHKA was 2.00 varus using surgical navigation (compared to 0.80 varus reported by Macdessi et al. using the aHKA). The pre-operative mean JLO was 1750 (versus 1740). Using the mHKA as opposed to the aHKA resulted in more knees being categorized as Class I (34.0% vs 19.4% ) or Class IV (17.5% vs 19.8%) and fewer in Class II (19.0% vs 32.2%) and Class V (6.3% vs 14.6%). All other differences in class frequencies were within 4%. For postoperative CPAK classification, a large majority of knees (72.7%) were categorized as Class V. Our study using mHKA determined during navigated TKA showed that the majority of preoperative arthritic knees were Class I, II, and IV in contrast to the original CPAK publication where most preoperative knees were Class I, II, and III. For TKAs at our institution, the goal was to mechanically align knees to neutral mHKA and JLO. This reflects in our postoperative results in that 73% of all postoperative TKAs were categorized as Class V.