Coronal Plane Alignment of the Knee (CPAK) classification

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).

Methods: A radiological 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). Intraoperative balance was compared within each phenotype in a cohort of 138 computer-assisted TKAs randomized 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 most common categories were Type II (39.2% healthy vs 32.2% OA), Type I (26.4% healthy vs 19.4% OA) and Type V (15.4% healthy vs 14.6% OA). CPAK Types VII, VIII, and IX were rare in both populations. 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 vs 15% MA; p < 0.001), Type II (78% KA vs 46% MA; p = 0.018). and Type IV (89% KA vs 0% MA; p < 0.001).

Conclusion: CPAK is a pragmatic, comprehensive classification for coronal knee alignment, based on constitutional alignment and JLO, that can be used in healthy and arthritic knees. 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. Cite this article: Bone Joint J 2021;103-B(2):329-337.

Keywords: Arithmetic HKA; CPAK; Constitutional alignment; Coronal Plane Alignment Knee classification; Joint line obliquity; Kinematic alignment; Knee alignment.

Fig. 1

a) Lateral distal femoral angle (LDFA), medial proximal tibial angle (MPTA), and mechanical hip-knee-ankle angle (mHKA) in a knee with preserved joint space and mild constitutional varus alignment. b) The same knee following degenerative loss of medial joint space, showing a change in mHKA (with shift to further varus) but no change to LDFA and MPTA.

Fig. 2

Relationship between the lateral distal femoral angle (LDFA) and medial proximal tibial angle (MPTA) in varus, neutral, and valgus lower limb alignment with the arithmetic hip-knee-ankle angle (aHKA).

Fig. 3

Use of medial proximal tibial angle (MPTA) and lateral distal femoral angle (LDFA) to indicate joint line obliquity (JLO).

Fig. 4

Coronal Plane Alignment of the Knee classification (CPAK) with nine theoretical types of knee. Arithmetic HKA, arithmetic hip-knee-ankle angle.

Fig. 5

Plot of arithmetic hip-knee-ankle angle (aHKA) against joint line obliquity for a healthy population showing distribution by percentage in the nine Coronal Plane Alignment of the Knee (CPAK) types. LDFA, lateral distal femoral angle; MPTA, medial proximal tibial angle.

Fig. 6

Plot of arithmetic hip-knee-ankle angle (aHKA) against joint line obliquity for an arthritic population showing distribution by percentage in the nine Coronal Plane Alignment of the Knee (CPAK) types. LDFA, lateral distal femoral angle; MPTA, medial proximal tibial angle.

Fig. 7

Probability of achieving knee balance based on Coronal Plane Alignment Knee type for kinematic alignment and mechanical alignment at 10° of knee flexion.

Fig. 8

Box plot comparison of mean intercompartmental pressure differences at 10°, 45°, and 90° of knee flexion for kinematic alignment (KA) and mechanical alignment (MA). CPAK, Coronal Plane Alignment of the Knee; ICPD, intercompartmental pressure difference.