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Citation:
Weerakkody Y, Knipe H, Hacking C, Alpha angle (cam morphology). Reference article, Radiopaedia.org (Accessed on 18 Apr 2024) https://doi.org/10.53347/rID-68548
The alpha angle is a radiological measurement proposed for the detection of cam morphology, which is sometimes associated with femoroacetabular impingement (FAI).
The term alpha angle is also used in a separate and unrelated context in the evaluation of developmental dysplasia of the hip (DDH) in children.
Plain radiograph
Measured on obliquely oriented radiographs. The Dunn projection of the hip is used.
CT/MRI
Measured on oblique axial images as the angle between 4:
- a line from the center of the femoral head to the center of the femoral neck at its narrowest point
- a line from the center of the femoral head to a point where the distance from the bone to the center of the head is greater than the radius of the cartilage-covered femoral head
There is no one value that can define cam morphology 5. Initially, in 2002, an alpha angle of ≥50° was proposed as pathological 4, subsequently, a value of ≥57° was proposed in symptomatic patients 2, and more recently (c. 2021) a value of ≥60° has been proposed as definitional of cam morphology 5.
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1. Jacob L. Jaremko, Myles Mabee, Vimarsha G. Swami, Lucy Jamieson, Kelvin Chow, Richard B. Thompson. Potential for Change in US Diagnosis of Hip Dysplasia Solely Caused by Changes in Probe Orientation: Patterns of Alpha-angle Variation Revealed by Using Three-dimensional US. (2014) Radiology. 273 (3): 870-8. doi:10.1148/radiol.14140451 - Pubmed
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2. Barrientos C, Barahona M, Diaz J, Brañes J, Chaparro F, Hinzpeter J. Is there a pathological alpha angle for hip impingement? A diagnostic test study. (2016) Journal of hip preservation surgery. 3 (3): 223-8. doi:10.1093/jhps/hnw014 - Pubmed
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3. Mohamed R. Nouh, Mark E. Schweitzer, Leon Rybak, Jodi Cohen. Femoroacetabular Impingement: Can the Alpha Angle Be Estimated?. (2012) American Journal of Roentgenology. 190 (5): 1260-2. doi:10.2214/AJR.07.3258 - Pubmed
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4. Nötzli H. P., Wyss T. F., Stoecklin C. H., Schmid M. R., Treiber K., Hodler J.. The contour of the femoral head-neck junction as a predictor for the risk of anterior impingement. (2002) The Journal of Bone and Joint Surgery. British volume. doi:10.1302/0301-620X.84B4.0840556
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5. Dijkstra HP, Ardern CL, Serner A, Mosler AB, Weir A, Roberts NW, Mc Auliffe S, Oke JL, Khan KM, Clarke M, Glyn-Jones S. Primary cam morphology; bump, burden or bog-standard? A concept analysis. (2021) British journal of sports medicine. doi:10.1136/bjsports-2020-103308 - Pubmed
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