Distal biceps tendon (DBT) tear or rupture is an acute injury in middle-aged men usually resulting from heavy-lifting activities or when the arm is used to break one’s fall. Conservative and surgical treatment of full and partial tears often results in diminished elbow flexion and supination (rotation) strength. This deficiency may be attributed to treating the DBT as a singular unit. Recent anatomic studies have shown that the DBT is comprised of two distinct tendons, a short head (SH) and a long head (LH), with each having discrete attachments on the radius. The individual contribution of each of the two heads, and hence their importance to elbow function, has not been well defined. There were two major objectives of this study. Firstly, to experimentally measure the intact contribution made by each of the heads to elbow flexion and supination and secondly, to assess the effect of anatomic and non-anatomic repair following rupture of the intact tendons.
Eight fresh-frozen cadaveric arms were mounted on an in-vitro elbow simulator developed for this project, while controlled static loads were applied to the individual biceps tendons. Isometric supination torque and flexion force were individually recorded with the forearm in 45 degrees supination, neutral rotation and 45 degrees pronation. This was done with the tendons intact to assess their native contribution and then repeated after a complete rupture was simulated and repaired both anatomically and non-anatomically.
In the intact biceps, the SH contributed 14% more than the LH to flexion for all forearm positions. In pronation and neutral positions the SH contributed 11% more than the LH to supination torque. In the supinated forearm, the LH contributed 2% more than SH to supination torque. When comparing anatomic and non-anatomic repairs, there was no difference in the supination torque when the forearm was in pronation. With the arm in neutral and supinated rotation, the non-anatomic repair generated less supination torque (15% and 40% less than intact, respectively). Anatomic repair reliably reproduced intact level supination torque in all forearm positions.
Discussion and Conclusion
This study demonstrates that the SH of the biceps is an especially important flexor, contributing 14% more to flexion force for the same load. The SH is also an important forearm rotator of the arm when it is in pronation and neutral rotation, with the LH playing a slightly larger role when the arm is in supination. Anatomic repair reliably restores distal biceps tendons function throughout the forearm range of rotation. If the tendon is repaired non-anatomically, the biceps will not be able to generate its full supination torque in neutral and supinated positions.
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