Revisiting the Anconeus using Functional Ultrasound Imaging

Abstract

IntroductionThe anconeus is a small triangular muscle originating on the posterior aspect of the lateral epicondyle of the humerus with a broad insertion along the postero‐lateral proximal ulna. Cadaveric studies have assessed its potential involvement as an elbow joint stabilizer, whereas electromyographic (EMG) studies have focused on its contribution to elbow extension. However, changes in fascicle length (Lf) and pennation angle (Pa) during voluntary contractions have not been assessed to help elucidate its function. Clinically, the anconeus has been sacrificed for use in various muscle flap transplant and reconstructive surgeries, but the impact of this loss on elbow joint function has not been resolved. Thus, the purpose of this study was to assess the relative architectural changes during in vivo elbow extension and flexion contractions to better understand its functional role.MethodsIn six healthy participants to date (2 females and 4 males) aged 22‐30 years, ultrasound imaging was used to assess Lf and Pa changes in the anconeus. Participants were seated in a Humac Norm dynamometer with their shoulders abducted to 90 degrees and the elbow joint aligned to the axis of rotation. The anconeus was imaged at four different elbow joint angles (35, 90, 135, and 180°) at rest, and during ramp (~5s) isometric maximal voluntary contractions (MVC) of both elbow extension and flexion. Participants also performed maximal‐torque isokinetic (20 degrees/s) extension and flexion contractions through the same 145° range of motion (35 to 180°). For both isometric and isokinetic contractions, Lf and Pa were measured from the ultrasound images using ImageJ software at the same four elbow angles.ResultsAt the four elbow angles (35, 90, 135 and 180°) at rest Lf was 2.92 ± 0.32 cm, 2.77 ± 0.25 cm, 2.52 ± 0.16 cm, 2.29 ± 0.31 cm, whereas Pa was 10.66 ± 2.49°, 12.19 ± 1.98°, 13.88 ± 1.68°, and 16.26 ± 2.13°, respectively. During isometric extension MVCs, Lf decreased by ~10%, 23%, 22%, 10%, whereas Pa increased by ~50%, 59%, 78%, and 46%, at each respective elbow angle. Changes in these parameters made during the isokinetic (dynamic) contractions matched those taken during the isometric contractions at the same four joint angles. During isometric and dynamic isokinetic maximal elbow joint flexion, there were negligible changes in both architectural parameters at the four joint angles.ConclusionDuring elbow extension actions there was a significant decrease in Lf and a significant increase in Pa at all joint angles, however no changes occurred during elbow flexion. The largest change of both parameters was at 90 and 135°, suggesting this is the optimal position for anconeus function. Lack of changes during elbow flexion indicates a minimal functional role during this task which may relate to the anconeus having less importance for overall elbow joint stability. These results highlight the significant architectural changes of the anconeus muscle and illustrate its presumed functional importance during elbow extension. Future studies using in vivo EMG recordings would be useful to further understand its role in elbow joint actions that may have clinical implications.