Summary

The new CPAK classification provides a simple and comprehensive system for describing knee alignment in the arthritic and healthy knee. In addition, CPAK allows determination of which patients are most likely to benefit from kinematic alignment when optimization of soft tissue balance is prioritized.

Abstract

Aims
A comprehensive classification for coronal lower limb alignment with predictive capabilities for knee balance would be beneficial in total knee arthroplasty (TKA). This paper describes the Coronal Plane Alignment of the Knee (CPAK) classification and examines its utility in preoperative soft tissue balance prediction, comparing kinematic alignment (KA) to mechanical alignment (MA).

Patients and Methods
A radiographic analysis of 500 healthy and 500 osteoarthritic (OA) knees was used to assess the applicability of the CPAK classification. CPAK comprises nine phenotypes based on the arithmetic HKA (aHKA) that estimates constitutional limb alignment and joint line obliquity (JLO). Intra-operative balance was compared within each phenotype in a cohort of 138 computer-assisted TKAs randomised to KA or MA. Primary outcomes included descriptive analyses of healthy and OA groups per CPAK type, and comparison of balance at 10° of flexion within each type. Secondary outcomes assessed balance at 45° and 90° and bone recuts required to achieve final knee balance within each CPAK type.

Results

There was similar frequency distribution between healthy and arthritic groups across all CPAK types. The commonest categories were Type II (39.2% healthy vs 32.2% OA) and Type I (26.4% healthy vs 19.4% OA). Across all CPAK types, a greater proportion of KA TKAs achieved optimal balance compared to MA. This effect was largest, and statistically significant, in CPAK Types I (100% KA versus 15% MA, p<0.001), Type II (78% KA versus 46% MA, p<0.02) and Type IV (89% KA versus 0% MA, p<0.001).

Conclusions

CPAK is a pragmatic, comprehensive classification for coronal knee alignment, based on constitutional alignment and JLO, that can be used in healthy and arthritic subjects. CPAK identifies which knee phenotypes may benefit most from KA when optimization of soft tissue balance is prioritized. Further, it will allow for consistency of reporting in future studies.