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Relationship between the alpha and beta angles in diagnosing CAM-type femoroacetabular impingement on frog-leg lateral radiographs

  • Hip
  • Published:
Knee Surgery, Sports Traumatology, Arthroscopy Aims and scope

Abstract

Purpose

Alpha and beta angles are commonly used radiographic measures to assess the sphericity of the proximal femur and distance between the pathologic head–neck junction and the acetabular rim, respectively. The aim of this study was to explore the relationship between these two measurements on frog-leg lateral hip radiographs.

Methods

Fifty frog-leg lateral hip radiographs were evaluated by two orthopaedic surgeons and two radiologists. Each reviewer measured the alpha and beta angles on two separate occasions to determine the relationship between positive alpha and beta angles and the inter- and intra-observer reliability of these measurements.

Results

There was no significant association between positive alpha and beta angles, [kappa range −0.043 (95 % CI −0.17 to 0.086) to 0.54 (95 % CI 0.33–0.75)]. Intra-observer reliability was high [alpha angle intra-class correlation coefficient (ICC) range 0.74 (95 % CI 0.58–0.84) to 0.99 (95 % CI 0.98–0.99) and beta angle ICC range 0.86 (95 % CI 0.76–0.92) to 0.97 (95 % CI 0.95–0.98)].

Conclusions

There is no statistical or functional relationship between readings of positive alpha and beta angles. The radiographic measurements resulted in high intra-observer and fair-to-moderate inter-observer reliability. Results of this study suggest that the presence of a CAM lesion on lateral radiographs as suggested by a positive alpha angle does not necessitate a decrease in clearance between the femoral head and acetabular rim as measured by the beta angle and thus may not be the best measure of functional impingement. Understanding the relationship between these two aspects of femoroacetabular impingement improves a surgeon’s ability to anticipate potential operative management.

Level of evidence

III.

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Conflict of interest

The authors certify that they, or a member of their immediate families, have no funding or commercial associations (eg, consultancies, stock ownership, equity interest, patent/licensing arrangements, etc) that might pose a conflict of interest in connection with the submitted article.

Ethical standards

Local Research Ethics approval was obtained from REB # 13-318-C.

Author information

Authors and Affiliations

  1. Division of Orthopaedics, Department of Surgery, McMaster University, Hamilton, ON, Canada

    Moin Khan, Dale Williams & Olufemi R. Ayeni

  2. Hospital for Special Surgery, New York, NY, USA

    Anil Ranawat

  3. Division of Orthopaedic Surgery, University of Toronto, Toronto, ON, Canada

    Rajiv Gandhi

  4. Department of Radiology, McMaster University, Hamilton, ON, Canada

    Hema Choudur & Naveen Parasu

  5. Department of Clinical Epidemiology and Biostatistics, McMaster University, Hamilton, ON, Canada

    Nicole Simunovic

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  1. Moin Khan
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  2. Anil Ranawat
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  3. Dale Williams
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  8. Olufemi R. Ayeni
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Correspondence to Olufemi R. Ayeni.

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Khan, M., Ranawat, A., Williams, D. et al. Relationship between the alpha and beta angles in diagnosing CAM-type femoroacetabular impingement on frog-leg lateral radiographs. Knee Surg Sports Traumatol Arthrosc 23, 2595–2600 (2015). https://doi.org/10.1007/s00167-014-3182-3

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  • DOI: https://doi.org/10.1007/s00167-014-3182-3

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