Voor kniechirurgen · For knee surgeons

Predicted HKA calculator

Waarom in het Engels? Deze pagina richt zich tot collega-kniechirurgen, in België en daarbuiten. Engels is de gemeenschappelijke taal van ons vakgebied, en ook de onderliggende publicaties verschenen in het Engels. De inleiding hieronder staat daarom in beide talen; de rekentool zelf is in het Engels.

Why English? This page is aimed at fellow knee surgeons, in Belgium and beyond. English is the common language of our field, and the underlying papers were published in English as well. The introduction below is therefore given in both languages; the calculator itself is in English.

Nederlands

Deze rekentool komt voort uit het wetenschappelijk onderzoek dat dr. William Colyn, orthopedisch chirurg bij Orthopedie Turnhout, de voorbije jaren deed naar de stand van de knie bij artrose. Zijn publicaties hierover verschenen tussen 2012 en 2023, samen met onder meer prof. Johan Bellemans. Je vindt ze onderaan deze pagina.

Het onderzoek toonde aan dat artrose meer doet dan kraakbeen wegnemen. Ook het bot aan de binnenkant van de knie slijt, en de banden aan de buitenkant rekken uit. De hoeken die je op een röntgenfoto van een knie met artrose meet, zijn daardoor niet meer die van vóór de slijtage. Op basis daarvan ontwikkelde dr. Colyn een formule die de oorspronkelijke stand van een O-been-knie (varusknie) voorspelt: de stand van vóór de artrose. De formule verscheen in 2022 in het vakblad Archives of Orthopaedic and Trauma Surgery.

Met deze calculator stellen we die formule ter beschikking van collega-orthopedisch chirurgen, bijvoorbeeld bij de planning van een totale knieprothese met kinematische uitlijning. Hij ondersteunt de planning, maar vervangt het klinisch oordeel van de chirurg niet.

English

This calculator stems from the research that Dr William Colyn, orthopaedic surgeon at Orthopedie Turnhout, carried out in recent years on the alignment of the knee in osteoarthritis. His papers on this topic were published between 2012 and 2023, with Prof. Johan Bellemans among others. You will find them at the bottom of this page.

The research showed that osteoarthritis does more than remove cartilage. The bone on the medial side of the knee wears as well, and the lateral soft tissues stretch. The angles you measure on a radiograph of an arthritic knee are therefore no longer those from before the disease. On that basis, Dr Colyn developed a formula that predicts the original alignment of a varus knee: its alignment before osteoarthritis. The formula was published in 2022 in Archives of Orthopaedic and Trauma Surgery.

With this calculator we make that formula available to fellow orthopaedic surgeons, for instance when planning a total knee arthroplasty with kinematic alignment. It supports the planning but does not replace the surgeon's clinical judgement.

Ben je patiënt? Deze tool is niet voor jou bedoeld. Meer uitleg vind je bij de totale knieprothese en bij knieartrose. Je orthopedisch chirurg bespreekt de planning van je ingreep met jou tijdens de raadpleging.

The calculator

Enter five measurements from a standing full-length radiograph and the Kellgren–Lawrence grade. The calculator determines the CPAK phenotype and estimates the HKA of the knee before the osteoarthritic process began: the HKAPRED according to the formula of Colyn et al. (2022).

The formula has been validated for CPAK types I and II, which together account for about 80% of varus knees. Everything is calculated in your browser; nothing is stored or sent.

Measurements of the arthritic knee

Filled in with an example
° varus
Enter varus as a positive number. Validated range: 0 to 15° varus.
°
°
°
Lateral opening = positive
°
Relative to the floor. Lateral opening +, medial opening −
Kellgren–Lawrence grade (tibiofemoral)

Measure on a standing full-length radiograph with the patella facing forward, a symmetrical shape of the lesser trochanters and a similar overlap of the proximal tibiofibular joint. A decimal point or comma both work.

Predicted constitutional HKA
3.9° varus
CPAKI

aHKA
MPTA − mLDFA
–
JLO
MPTA + mLDFA
–
ΔHKA
due to OA
–
Correction
CPAK × KL
–
    How this was calculated

    What you are looking at

    The limb and the CPAK phenotype

    mechanical axes joint lines predicted tibial axis (HKAPRED)
    Right lower limb, AP view, with the hip–ankle line vertical. Schematic: deviations from 90° and 180° are magnified 1.6 times to make them visible, and bone widths are slightly exaggerated. The numbers are the values entered.
    this knee (aHKA and JLO) covered by the formula
    CPAK according to MacDessi et al. (2021): neutral aHKA between −2° and +2°, neutral JLO between 177° and 183°, boundaries inclusive. As in the validation of the formula, the phenotype is determined on the arthritic knee. Only the highlighted types I and II are covered by the formula.

    Why today's radiograph is not enough

    Osteoarthritis changes the bone as well

    Kinematic alignment aims to restore the patient's constitutional, pre-arthritic alignment. In practice, we start from the mLDFA and MPTA measured on the full-length radiograph of the arthritic knee, or from the arithmetic HKA (aHKA = MPTA − mLDFA). That approach assumes that osteoarthritis only removes cartilage and leaves the bony angles untouched.

    This is not entirely true. On sequential full-length radiographs of the same patients, Colyn (2023) found that the mLDFA increases through erosion of the distal medial femoral condyle and that the MPTA decreases through erosion of the medial tibial plateau. On top of that, the JLCA increases as the lateral soft tissues become lax.

    Relying purely on the values of the arthritic knee therefore builds part of the disease into the target. As a result, varus is overestimated and the CPAK phenotype can shift as well. The HKAPRED reverses these bony changes and adds a correction for the grade of osteoarthritis.

    Per degree of change (Colyn 2023, linear mixed models on sequential radiographs): +1° mLDFA is associated with +0.61° HKA varus, +1° JLCA with +0.86° and −1° MPTA with +0.93°. The tibia and the joint line therefore contribute most.

    Mean HKA by KL grade
    KL 0 = varus knees of 250 young healthy adults (aged 20 to 27). KL 1 to 4 = natural evolution cohort. Source: Colyn et al. 2022, table 1. n per bar below the axis.
    1. 1

      Erosion of the medial femoral condyle

      The first shift towards more varus occurs on the femoral side, mainly between KL 1 and KL 2.

      mLDFA ↑
    2. 2

      Erosion of the medial tibial plateau

      At a later stage, from KL 3 onwards, the MPTA decreases markedly. This is the largest contribution to the increase in HKA.

      MPTA ↓
    3. 3

      Lateral laxity

      The lateral joint line opening increases progressively. Above about 5 mm of lateral opening (around 6° HKA), Colyn speaks of a high-grade varus knee.

      JLCA ↑

    Read this before using the result

    When the formula applies

    Within the validation

    • CPAK type I or II on the arthritic knee. The authors advise against using the formula for other phenotypes.
    • HKA between 0° and 15° varus and an MPTA below 89.9°.
    • KL grade 1 to 4. There is no correction factor for KL 0; the validation cohort contained KL 2 to 4.
    • Patients aged 50 and over.
    • No ipsilateral hip arthroplasty, previous femoral or tibial fracture, osteotomy or ligament repair.
    • A correctly rotated, standing full-length radiograph. Rotation and an extension deficit distort the angles.

    Keep in mind

    • The “accuracy of ≤ 0.5°” refers to groups. Per CPAK type, the distribution of predicted HKAs in an arthritic cohort (n = 99) matched that of young healthy adults. The true pre-arthritic HKA of an individual patient is unknown, so no individual error margin has been established.
    • The JLCA depends on weight-bearing and soft tissues, and all measurements are two-dimensional and static.
    • The reference values come from a young Caucasian population in Leuven, Belgium.

    This calculator supports preoperative planning and does not replace the surgeon's clinical judgement. It is intended for healthcare professionals, not for patients.

    What this is based on

    References

    1. Colyn W, Cleymans A, Bruckers L, Truijen J, Smeets K, Bellemans J. The pre-diseased coronal alignment can be predicted from conventional radiographs taken of the varus arthritic knee. Arch Orthop Trauma Surg. 2022. doi:10.1007/s00402-022-04709-6
    2. Colyn W, Bruckers L, Scheys L, Truijen J, Smeets K, Bellemans J. Changes in coronal knee-alignment parameters during the osteoarthritis process in the varus knee. J ISAKOS. 2023;8:68–73. doi:10.1016/j.jisako.2022.12.002
    3. Colyn W, Cleymans A, Bruckers L, Houben R, Smeets K, Bellemans J. The lateral joint line opening: a radiographic indicative parameter for high grade varus knees. J Exp Orthop. 2022;9:51. doi:10.1186/s40634-022-00489-5
    4. Bellemans J, Colyn W, Vandenneucker H, Victor J. The Chitranjan Ranawat Award: is neutral mechanical alignment normal for all patients? The concept of constitutional varus. Clin Orthop Relat Res. 2012;470:45–53. doi:10.1007/s11999-011-1936-5
    5. Victor JMK, Bassens D, Bellemans J, Gürsu S, Dhollander AAM, Verdonk PCM. Constitutional varus does not affect joint line orientation in the coronal plane. Clin Orthop Relat Res. 2014;472:98–104. doi:10.1007/s11999-013-2898-6
    6. MacDessi SJ, Griffiths-Jones W, Harris IA, Bellemans J, Chen DB. Coronal Plane Alignment of the Knee (CPAK) classification. Bone Joint J. 2021;103-B(2):329–337. doi:10.1302/0301-620X.103B2.BJJ-2020-1050.R1