Category: Orthopedics
Posted: 1/10/2026 by Brian Corwell, MD
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Plain films are often the first imaging test ordered in patient’s with knee injuries.
Plain films do NOT directly show the ACL but may reveal indirect signs of an ACL tear.
Key Plain Film Findings:
1) Effusion
ACL injury generates a large joint effusion (swelling/hemarthrosis). The presence of a fat/fluid interface (lipohemarthrosis) suggests associated bone injury.
This may be seen as joint fluid on the plain film lateral view. This view is best obtained with the patient in the supine position. This allows a layering of fluid in the suprapatellar bursa. A fat/fluid level indicates the presence of a lipohemarthrosis, signifying the likelihood of an intraarticular fracture.
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( A ) The normal suprapatellar bursa is seen on the lateral view as a line extending obliquely anterior and superior from the superior/posterior surface of the patella to the posterior surface of the quadriceps tendon.
(B) A small to moderate-sized effusion.
(C) A larger effusion.
In (D) the quadriceps tendon is bulged outward by an even larger joint effusion.
In the correct clinical setting, the presence or absence of a new fusion can be highly suggestive of ACL tear. Effusion can usually accurately be assessed on both physical exam and the lateral plain film. The absence of a knee effusion makes acute tear of the ACL unlikely as clinical practice suggests a rapidly developing hemarthrosis. Effusion of the knee may be associated with other entities such as intraarticular fracture or underlying arthritis. Typically, patients with acute ACL tears are young and do not have chronic infusions. Most acute meniscal tears and chondral injuries do not develop acute large effusions due to lack of significant vascularity.
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