MacDessi et al. propose the CPAK classification, a nine-phenotype system for coronal knee alignment based on two radiographic variables: arithmetic HKA (constitutional limb alignment) and joint line obliquity. The system was validated in 1,000 knees and tested prospectively in 138 computer-assisted TKAs randomized to kinematic versus mechanical alignment. The central question: can CPAK phenotype predict which alignment strategy achieves better intraoperative soft tissue balance?
Before CPAK, coronal alignment was described simply as varus, neutral, or valgus based on the mechanical HKA — a measure that shifts with arthritic cartilage loss and ignores joint line obliquity entirely. There was no standardized preoperative framework to predict which patients would balance poorly with mechanical alignment or to select an alternative strategy.
When you are planning a TKA, measure MPTA and LDFA on the long-leg film, calculate aHKA (MPTA minus LDFA) and JLO (MPTA plus LDFA), and assign a CPAK type. If your patient is Type I or IV (constitutional varus), this data says MA will fail to balance the knee in the vast majority of cases. Kinematic alignment should be your default strategy from the start of the case, not a rescue after discovering intraoperative imbalance. Type II knees (nearly 40% of your patients) also balance significantly better with KA, driven by the apex distal JLO even when limb alignment is neutral.
CPAK Type V is the only phenotype for which MA is truly the anatomically appropriate choice. And fewer than 1 in 6 patients have it.
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MacDessi et al. propose the CPAK classification, a nine-phenotype system for coronal knee alignment based on two radiographic variables: arithmetic HKA (constitutional limb alignment) and joint line obliquity. The system was validated in 1,000 knees and tested prospectively in 138 computer-assisted TKAs randomized to kinematic versus mechanical alignment. The central question: can CPAK phenotype predict which alignment strategy achieves better intraoperative soft tissue balance?
Before CPAK, coronal alignment was described simply as varus, neutral, or valgus based on the mechanical HKA — a measure that shifts with arthritic cartilage loss and ignores joint line obliquity entirely. There was no standardized preoperative framework to predict which patients would balance poorly with mechanical alignment or to select an alternative strategy.
When you are planning a TKA, measure MPTA and LDFA on the long-leg film, calculate aHKA (MPTA minus LDFA) and JLO (MPTA plus LDFA), and assign a CPAK type. If your patient is Type I or IV (constitutional varus), this data says MA will fail to balance the knee in the vast majority of cases. Kinematic alignment should be your default strategy from the start of the case, not a rescue after discovering intraoperative imbalance. Type II knees (nearly 40% of your patients) also balance significantly better with KA, driven by the apex distal JLO even when limb alignment is neutral.
CPAK Type V is the only phenotype for which MA is truly the anatomically appropriate choice. And fewer than 1 in 6 patients have it.