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Long leg radiographs underestimate the degree of constitutional varus limb alignment and joint line obliquity in comparison with computed tomography: a radiographic study

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Knee Surgery, Sports Traumatology, Arthroscopy Aims and scope

Abstract

Purpose

The purpose of this study was to understand if differences exist between computed tomography (CT) and long leg radiographs (LLR) when defining coronal plane alignment of the lower limb in total knee arthroplasty (TKA). It aimed to identify any such differences between the two imaging modalities by quantifying constitutional limb alignment (arithmetic hip–knee–ankle angle (aHKA), joint line obliquity (JLO) and Coronal Plane Alignment of the Knee (CPAK) type within the same population.

Methods

A retrospective radiographic study compared pre-operative LLR and CT measurements in patients undergoing robotic-assisted TKA. The aHKA, JLO and CPAK types were calculated after measuring the medial proximal tibial angle (MPTA) and lateral distal femoral angle (LDFA). The primary outcomes were the mean differences in aHKA (MPTA−LDFA), JLO (MPTA + LDFA) and proportions of CPAK types between LLR and CT groups. The secondary outcomes were the differences in CT-derived MPTA values based on four different tibial sagittal landmarks.

Results

After exclusions, 465 imaging sets were analysed in 394 patients. There was a statistically significant mean difference between LLR and CT, respectively, for both MPTA (87.5° vs. 86.2°; p < 0.01) and LDFA (88.7° vs. 87.3°; p < 0.01). There were also statistically significant differences for aHKA (− 0.2° vs. − 1.1°) and JLO (175.1° vs. 173.4°) for LLR and CT, respectively (both p < 0.01). CT increased the proportion of patients with CPAK Type I (constitutional varus aHKA, apex distal JLO) and CPAK Type II (neutral aHKA, apex distal JLO), and decreased numbers of CPAK Types III–VI. There were significant mean differences in the MPTA using varying sagittal landmarks.

Conclusion

Alignment determined by LLRs underestimates the magnitude of both constitutional varus alignment and joint line obliquity compared to CT, differences that notably increase the proportions of patients included in CPAK Types I and II. These distinctions are primarily due to underestimation of proximal tibial varus when measured on LLRs compared to CT, which more specifically defines articular weight-bearing points.

Level of evidence

III.

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Data availability

The data that support the findings of this study are not openly available due to reasons of patient privacy. They are available from the Corresponding Author upon reasonable request.

Abbreviations

LLR:

Long leg radiograph

KA:

Kinematic alignment

CT:

Computed tomography

CPAK:

Coronal Plane Alignment of the Knee

CT:

Computed tomography

3D:

Three-dimensional

TKA:

Total knee arthroplasty

OA:

Osteoarthritis

HKA:

Hip–knee–ankle angle

MA:

Mechanical axis

LDFA:

Lateral distal femoral angle

MPTA:

Medial proximal tibial angle

JLO:

Joint line obliquity

aHKA:

Arithmetic hip–knee–ankle angle

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Acknowledgements

Stryker Mako Product Specialists (St George Private Hospital): Alec Becvarovski, Tom Donaldson, Alec Nethery.

Funding

No further funding to declare other than already stated in COI.

Author information

Authors and Affiliations

  1. Sydney Knee Specialists, Suite 201, Level 2, 131 Princes Hwy, Kogarah, NSW, 2217, Australia

    Payam Tarassoli, Luke E. Corban, Jil A. Wood, Darren B. Chen & Samuel J. MacDessi

  2. St George Private Hospital, 1 South St, Kogarah, NSW, 2217, Australia

    Darren B. Chen & Samuel J. MacDessi

  3. Stryker Australia, 8 Herbert St, St Leonards, NSW, 2065, Australia

    Andrew Sergis

  4. School of Clinical Medicine, University of New South Wales, St George and Sutherland Campuses, Sydney, NSW, Australia

    Samuel J. MacDessi

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Contributions

PT and SJM involved in study design, data collection, analysis, and manuscript preparation. AS and LC involved in data collection and manuscript preparation. DC and JW involved in manuscript preparation.

Corresponding author

Correspondence to Samuel J. MacDessi.

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Conflict of interest

No conflicts to declare: Payam Tarassoli, Jil Wood, Luke Corban. Andrew Sergis: Paid employee of company (Stryker). Darren Chen and Samuel MacDessi: Speakers’ bureau/paid presentations for a company or supplier (Stryker, Smith and Nephew); Paid consultant for a company or supplier (Stryker, Amplitude SAS); Research support from a company or supplier as a principal investigator for an unrelated study (Ramsay Hospital Research Foundation); Other financial support or material from a company or supplier (Research fellowship funding from Smith and Nephew, Stryker and Zimmer Biomet; IP pertaining to an unrelated study acquired by Stryker, with patent application lodged on key components of that IP).

Ethical approval

Ethics approval was granted by Ramsay Health Care to perform the analysis (approval number: 2022/ETH/0131). All investigations and procedures undertaken were in accordance with the ethical standards of the institutional research committee and with the 1964 Declaration of Helsinki and its later amendments or comparable ethical standards.

Informed consent

Informed consent was obtained in accordance with the ethical standards of the institutional research committee and obtained from all participants in the study.

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Tarassoli, P., Corban, L.E., Wood, J.A. et al. Long leg radiographs underestimate the degree of constitutional varus limb alignment and joint line obliquity in comparison with computed tomography: a radiographic study. Knee Surg Sports Traumatol Arthrosc 31, 4755–4765 (2023). https://doi.org/10.1007/s00167-023-07505-w

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