Posterior tibial slope considered as an important addition to the CPAK classification system

Zachary Morrisey; Jordan Cruse; Matthew Barra; Thomas Carroll; Christopher Drinkwater

doi:10.1016/j.jor.2024.01.008

Posterior tibial slope considered as an important addition to the CPAK classification system

J Orthop. 2024 Jan 14:51:54-59. doi: 10.1016/j.jor.2024.01.008. eCollection 2024 May.

Authors

Zachary Morrisey¹, Jordan Cruse¹, Matthew Barra², Thomas Carroll², Christopher Drinkwater²

Affiliations

¹ University of Rochester School of Medicine, 601 Elmwood Ave, Rochester, NY, 14642, United States.
² University of Rochester Department of Orthopedics and Physical Performance at Highland Hospital, 1000 South Ave, Rochester, NY, 14620, United States.

PMID: 38304145
PMCID: PMC10828574 (available on 2025-05-01)
DOI: 10.1016/j.jor.2024.01.008

Abstract

Introduction: Coronal Plane Alignment of the Knee (CPAK), is an informative way to classify native knee alignment types, but does not consider posterior tibial slope, an important variable in knee kinematics. We hypothesized that tibial slope would have a significant effect on knee kinematics and warrant consideration in addition to the CPAK system.

Methods: We examined 335 adult patients with osteoarthritis receiving total knee arthroplasty. We measured the lateral distal femoral angle (LDFA), medial proximal tibial angle (MPTA), and posterior tibial slope angle (PTS). Knees were categorized into CPAK classes and subdivided into types 'A' (PTS 8°) or 'B' (PTS >8°). We recorded pre-and-post operative knee flexion, and extension/flexion gaps in all subjects.

Results: CPAK classes VII-IX were not seen. Classes I and II comprised a plurality of all knees. One-third of all knees were type B. CPAK classes III, IV, and VI had greater type B proportions, but this was not statistically significant. Type B knees had greater flexion both pre-op (p < .001) and post-op (p = .043); type A knees had greater flexion improvement pre-to-post operatively (p = .045). Type A knees had greater medial and lateral flexion-extension gap change pre-operatively (p = .021) and (p = .027), respectively. Type B knees had greater medial-lateral gap imbalance preoperatively in both flexion and extension.

Discussion/conclusion: Our results suggest that there are important pre and post-operative differences in medial and lateral femorotibial gap balance between type A and B knees that require consideration for intra-operative balancing. Differences in knee flexion further solidify that PTS is an important variable that affects kinematics before and after TKA. We propose the addition of PTS types A and B to the existing CPAK classes. This is an easy and logical way to create a comprehensive classification system in both coronal and sagittal planes that captures further differences in knee kinematics.

Keywords: CPAK; Gap balance; Kinematics; Knee arthroplasty; Robotics; Tibial slope.