Magnetic Resonance Imaging Evaluation of the Distal Biceps Tendon and Lacertus Fibrosus at the Elbow: A Study of Normal Anatomy and Variants

Abstract The distal biceps brachii tendon (BB) and its aponeurotic extension, the lacertus fibrosus (LF), represent important anatomical structures of the elbow with functional and clinical relevance. The objective of this study is to describe the appearance, morphometry, and anatomical variations of the LF and distal biceps tendon on magnetic resonance imaging (MRI). Retrospective review of the MRI elbow joints of 81 patients. The presence and morphology of the LF and BB were analyzed utilizing high-resolution MRI and referencing anatomical classifications and morphometric parameters. The study included 81 patients (32 females, 49 males; mean age 33.7 ± 13.4 years). The LF was present in 87.7% and absent in 12.3% of elbows. Mean LF measurements were: thickness 0.63 ± 0.16 mm, width 17.54 ± 4.66 mm, and length 30.55 ± 8.37 mm. In 77.5%, the brachial artery lay anterior to the median nerve; in 19.7%, they were equidistant from the LF, and in 2.8%, the median nerve lay anterior to the brachial artery. Type I BB tendon was most common (n = 74), with Type II (n = 5) and Type III (n = 2). Mean BB tendon length and thickness were 59.25 ± 11.96 mm and 3.51 ± 0.70 mm, respectively. BB tendon thickness was the strongest predictor of LF absence (area under the curve [AUC]: 0.698), while adding variables like tendon length, age, and sex did not significantly enhance discrimination (AUC: 0.714). The LF and anatomical variations of the distal BB tendon at the elbow are often overlooked. We emphasize the relevance of understanding normal variants that could aid in accurate diagnosis and facilitate the recognition of possible entrapment syndromes.