Coronal knee alignment · phenotyping

CPAK, Hirschmann & JLCA calculator with photo measurement

Load or photograph a full-leg radiograph, place the anatomical points, and MPTA, LDFA, the CPAK phenotype, the Hirschmann functional knee phenotype, the JLCA and the difference between aHKA and measured HKA are computed. You can also enter the angles directly.

For calculation and educational use only. A phone photo of an X-ray introduces perspective distortion that directly affects the angles. Measure on the original radiograph (PACS) where possible. Not a medical device; clinical judgement remains decisive.

1 · Image

Choose a file or take a photo. A loupe appears while placing for precision; a point that isn't placed yet is always dropped where you tap. The femoral and tibial axes each have their own knee centre (femoral notch and tibial eminence respectively), so subluxation is represented correctly.

Take photo Choose file

No image loaded yet.

Load an image first.

2 · Measured angles

Filled in automatically from the points. You can override any value.

MPTA — medial proximal tibial angle (°)

LDFA — lateral distal femoral angle (°)

Knee zoom — drag the points to adjust MPTA & LDFA

Place all knee points to see the zoomed view.

3 · CPAK classification

aHKA = MPTA − LDFA · JLO = MPTA + LDFA

aHKA · alignment

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JLO · obliquity

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Enter both angles

Phenotype appears here

Phenotype matrix — alignment across, JLO down

Varus

Neutral

Valgus

Alignment (aHKA): varus < −2° · neutral −2° to +2° · valgus > +2°

Obliquity (JLO): apex distal < 177° · neutral 177° to 183° · apex proximal > 183°

Thresholds per MacDessi et al., Bone & Joint Journal 2021.

4 · Hirschmann functional knee phenotype

Medial angles · TMA = MPTA · FMA = 180 − LDFA · HKA measured from the axis points (or estimated from aHKA)

Limb · HKA

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Femur · FMA

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Tibia · TMA

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Combined phenotype—

Reference means: HKA 180° · FMA 93° · TMA 87°. Each phenotype = mean ± 1.5°, in 3° steps.

VAR = below the mean, VAL = above, NEU = within ± 1.5°.

Per Hirschmann et al., KSSTA 2019; reference values from the CT-based distribution (KSSTA 2024).

5 · JLCA & aHKA-vs-mHKA

JLCA from the four joint-line points · mHKA from the axis points · relation mHKA = 180 + aHKA − JLCA

JLCA

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mHKA · measured

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mHKA − aHKA

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Place all points to compute JLCA and measured mHKA.

JLCA sign: positive = lateral joint opening (lines converge medially), typical in varus OA. Normal ± 2°.

The difference mHKA − aHKA reflects the intra-articular component (laxity / cartilage loss) that aHKA ignores by definition; it equals −JLCA.

Relation mHKA = 180° + aHKA − JLCA (e.g. Int J Surg 2025).