CPAK vs Mechanical Alignment in Knee Replacement

Two Different Questions

Surgeons often discuss "CPAK versus mechanical alignment" as if they were competing recipes for positioning a knee replacement. They are not the same kind of thing. Mechanical alignment is an alignment strategy: a rule for where to put the implant. CPAK (Coronal Plane Alignment of the Knee) is a classification framework: a way to describe the knee a patient started with. One tells you what to do, the other tells you what you are working with.

Understanding that distinction clears up most of the confusion. This article compares the four main coronal alignment strategies used in total knee arthroplasty (TKA), mechanical, kinematic, restricted kinematic, and functional, and shows where CPAK fits as the shared language that underpins all of them.

For the underlying angular measurements (LDFA, MPTA, HKA) that every strategy relies on, see our primer on mechanical axis alignment and the LDFA, MPTA, and HKA angles.

The Four Alignment Strategies

Mechanical alignment (MA) aims for a neutral hip-knee-ankle angle of 180 degrees, with the femoral and tibial components each set perpendicular to their mechanical axes. It has been the reference standard for decades because it distributes load predictably and has a long track record of implant survivorship. The trade-off is that a neutral target ignores the patient's native anatomy, so knees that were constitutionally varus or valgus before surgery are pulled toward a shape they never had, sometimes requiring soft-tissue releases to make the knee balance.

Kinematic alignment (KA), described by Howell and colleagues in 2008, aims to restore the patient's pre-arthritic (constitutional) joint lines by resurfacing the bone rather than reorienting it to a neutral target. The goal is a more "native-feeling" knee with less ligament release. The concern is that unrestricted KA can reproduce extreme constitutional deformities, which some surgeons worry may load the implant unfavorably over time.

Restricted kinematic alignment (rKA) is a compromise proposed to set safe boundaries around KA. It reproduces constitutional anatomy only within a defined range (commonly cited as roughly 5 degrees of HKA and component angles within a safe corridor) and corrects outlier knees back toward that range. It tries to keep the "native" benefits of KA while avoiding extreme reconstructions.

Functional alignment (FA) is typically an intraoperative, technology-assisted approach (robotics or navigation) that fine-tunes component position and soft-tissue balance for the individual knee through range of motion, rather than committing to a fixed coronal target in advance. It treats alignment and balance as one problem to be solved together.

Where CPAK Fits

CPAK is not a fifth strategy. It is a classification that describes a knee's constitutional coronal phenotype, and it can be applied regardless of which strategy a surgeon ultimately chooses. It was described by MacDessi and colleagues in 2021 in The Bone & Joint Journal.

CPAK sorts knees into nine phenotypes using two calculations derived from the LDFA and MPTA:

• The arithmetic HKA (aHKA), which estimates constitutional limb alignment as MPTA minus LDFA and classifies the knee as varus, neutral, or valgus.
• The joint line obliquity (JLO), calculated as MPTA plus LDFA, which describes whether the joint line apex points distal, neutral, or proximal.

Three alignment categories times three obliquity categories gives the nine CPAK types (I through IX). Because aHKA is derived from the constitutional joint-line angles rather than the arthritic weight-bearing HKA, CPAK aims to describe the knee the patient had before cartilage wear and deformity set in.

The value of this is practical. CPAK gives surgeons a common vocabulary to answer the question every alignment strategy depends on: what shape was this knee to begin with? A mechanical-alignment surgeon uses CPAK to see how far they are moving a patient from their native phenotype. A kinematic or functional surgeon uses it to define the constitutional target they are trying to restore. In other words, CPAK is upstream of the MA-versus-KA-versus-FA debate, not a competitor in it.

What the Evidence Says

The honest summary is that no single strategy has been shown to be clearly superior for patient-reported outcomes across the board.

Randomized controlled trials comparing functional and mechanical alignment consistently show that FA requires fewer soft-tissue releases. In one reported trial, mechanical-alignment cases needed releases far more often than functional-alignment cases (65% versus 16%), and functional alignment produced a joint line that sat more parallel to the floor. But at two years, patient-reported and clinical outcomes between the two groups were largely similar. Meta-analyses and systematic reviews comparing kinematic and mechanical alignment report broadly comparable functional outcomes as well, with some trials favoring KA on early recovery, surgery time, or satisfaction, and others finding no meaningful difference.

Two evidence-informed takeaways are worth stating plainly:

1. Alignment philosophy is a genuine, unsettled clinical debate. Reasonable surgeons disagree, and the literature does not crown a winner.
2. Knowing the patient's constitutional phenotype (which CPAK captures) is useful under every philosophy, because it quantifies how far a given plan departs from the native knee.

There is also a population dimension. Bellemans and colleagues showed that constitutional varus is common in healthy adults, with roughly 32% of men and 17% of women naturally in 3 degrees of varus or more at skeletal maturity. Phenotype distribution varies by population: a CPAK study of a Turkish cohort found Type II (neutral alignment, apex-distal joint line) most common at around 41%, with the majority of knees apex-distal. This is precisely why a fixed neutral target does not describe most people, and why phenotype-aware planning has gained traction.

How Preoperative Planning Ties This Together

Whichever alignment strategy a surgeon prefers, the workflow starts the same way: measure the constitutional angles accurately, then decide how far to move from them. This is where preoperative imaging and planning software matter, because the LDFA, MPTA, aHKA, and JLO values that feed CPAK are only as good as the measurement.

Salnus is a software-only, browser-based preoperative planning tool (currently Research Use Only, not a cleared medical device) that performs CT-based 3D bone segmentation and automated coronal-plane alignment measurement, including the CPAK inputs, to support planning. A design choice worth noting: it runs client-side, meaning DICOM images are parsed and analyzed on the surgeon's own device rather than uploaded to a cloud server. In practice that supports going from scan to a draft plan in minutes without images leaving the machine. It is implant-agnostic and vendor-neutral, so the same phenotype and alignment analysis can inform a mechanical, kinematic, restricted-kinematic, or functional plan regardless of the implant system in use. That neutrality is also why the analysis can be useful to implant manufacturers evaluating planning workflows, without being tied to any one product line.

To understand the imaging step in more depth, see how AI-based CT bone segmentation supports surgical planning. For a controlled-laboratory example of how patient-specific planning translates into physical guides, see our summary of the OJSM study on 3D-printed guides in knee reconstruction. And for a look at where alignment thinking is heading in ligament work, see our research note on the PCL as the "forgotten ligament".

FAQ

Is CPAK an alignment strategy like mechanical or kinematic alignment? No. CPAK is a classification framework that describes a knee's constitutional coronal phenotype using nine types. It tells you what shape the knee started with. Mechanical, kinematic, restricted kinematic, and functional alignment are strategies for where to position the implant. CPAK can be used alongside any of them.

What is the difference between aHKA and the standard HKA angle? The standard HKA is measured from a weight-bearing radiograph and reflects the current, often arthritic, alignment. The arithmetic HKA (aHKA) in CPAK is calculated as MPTA minus LDFA from the constitutional joint-line angles, so it estimates the pre-arthritic alignment before cartilage wear and deformity distorted the picture.

Does functional or kinematic alignment give better outcomes than mechanical alignment? The evidence is mixed and unsettled. Trials show functional alignment needs fewer soft-tissue releases and produces a more floor-parallel joint line, but patient-reported outcomes at two years are often similar to mechanical alignment. No strategy has been proven clearly superior across the board.

Why does constitutional varus matter for alignment planning? Because it is common. Roughly a third of healthy men and a sixth of healthy women are constitutionally varus. Forcing every knee to a neutral 180-degree target moves many patients away from their native anatomy, which is the main argument behind kinematic and functional approaches.

Can Salnus tell me which alignment strategy to use? No. Salnus is a Research Use Only planning tool that measures constitutional alignment and computes CPAK inputs to inform planning. The choice of alignment strategy, and every clinical decision, remains with the qualified surgeon.

Which CPAK types are most common? It varies by population, but apex-distal, near-neutral or varus phenotypes tend to dominate. A Turkish cohort study found Type II (neutral alignment, apex-distal joint line) most common at around 41%, with most knees classified as apex-distal.

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Disclaimer: This article is for educational and research purposes only. Salnus tools are designated for Research Use Only (RUO) and are not cleared medical devices. Mention of third-party products is for educational context only and does not imply endorsement or comparison of clinical equivalence. Clinical decisions should be made by qualified physicians, and regulatory status should be independently verified for your jurisdiction.

References:

• MacDessi SJ, et al. Coronal Plane Alignment of the Knee (CPAK) classification: a new system for describing knee phenotypes. The Bone & Joint Journal, 2021.
• MacDessi SJ, et al. The Coronal Plane Alignment of the Knee Classification: How It Is Applied. JBJS Open Access, 2025.
• Coronal Plane Alignment of the Knee (CPAK) Type Shifts Toward Constitutional Varus with Increasing Kellgren and Lawrence Grade. JBJS, 2025.
• The John N. Insall Award: Functional Versus Mechanical Alignment in Total Knee Arthroplasty: A Randomized Controlled Trial. The Journal of Arthroplasty, 2025.
• Functional Outcomes and Patient Satisfaction in Kinematic vs Mechanical Alignment Total Knee Arthroplasty: A Systematic Review. 2025.
• Rivière C, et al. Restricted Kinematic Alignment, the Fundamentals, and Clinical Applications. Frontiers in Surgery, 2021.
• Bellemans J, et al. The Chitranjan Ranawat Award: Is Neutral Mechanical Alignment Normal for All Patients? The Concept of Constitutional Varus. Clinical Orthopaedics and Related Research, 2012.
• Phenotyping of the Turkish population according to Coronal Plane Alignment of the Knee classification: A retrospective cross-sectional study. Joint Diseases and Related Surgery, 2024.
• Cirdi YU, Serteser B, Mavi A, Ergun S, Akgun U. 3D-Printed Patient-Specific Guides Reduce Femoral Tunnel Convergence in Anatomic Knee Multiligament Reconstruction: Controlled Laboratory Study. Orthopaedic Journal of Sports Medicine, 2026.