The insertion of the long head was at a point farthest away from the rotation of the radius, potentially providing a greater lever arm to increase supination power (Fig. 7). Conversely, the tendon of the short head was attached more distally, providing it with the potential for greater flexion power.
We demonstrated that, in most individuals, the biceps muscles are two independent muscle bellies of the two heads, with two separate tendon areas. The remaining individuals had several interdigitations between both muscle bellies and again two easily defined tendons. No biomechanical or histological investigations were performed, and this is a potential limitation of the study.
The distinct pattern found in the majority of patients was described recently in a case report4. The authors reported a duplicated biceps tendon and failed to identify any evidence of fusion between the muscle bellies in the distal 8 to 10 cm4. This was an uncommon finding. It has been our clinical experience that an acute rupture of the biceps tendon often occurs with avulsion of the tendon from the bone as one unit, with the two heads often held together with loose areolar tissue, and with the lacertus fibrosus usually remaining intact.
The biceps tendon is controlled by the lacertus fibrosus, which is a fixed-length structure. As the forearm muscles contract, the flexor muscle mass migrates proximally, increasing its cross-sectional area. This tenses the aponeurosis, pulling the biceps tendon medially. This increased force on the biceps tendon may contribute to the etiology of rupture of the distal biceps tendon (Fig. 8).
In light of our findings in repairs of acute rupture of the distal biceps tendon, we use an Endobutton (Smith and Nephew, Memphis, Tennessee) and place Bunnell sutures in each of the two tendon bundles, as we originally described in 20005 and as has been subsequently reported by others6-9. Both tendon components are secured to the proximal part of the radius in their correct orientation. If there is tendon retraction of 2 cm, we may release the lacertus fibrosus to allow the tendon to be advanced onto the radial tuberosity. This ensures that no neurovascular structures become entrapped by the tight lacertus in the pronated position. With the arm in pronation and extension, the lacertus is then repaired to the biceps tendon, to reconstitute it without impinging on the neurovascular bundle.
The current study has had further direct implications on our clinical practice. In a delayed rupture with substantially greater retraction, we use a hamstring tendon graft to reconstitute the length of the biceps tendon10. The tendon unit is often scarred together to a single mass. Since performing this study, we have modified our technique so that the proximal end of each tendon graft is woven into each of the two separate muscle bellies. As it passes distally, the normal rotation of each tendon is recreated so that the short head inserts more distally when it is locked into the radial tuberosity.
The concept of two individual muscle bellies driving separate parts of the distal biceps tendon unit with active motion of the forearm has not been fully explored, and its clinical benefits are yet to be determined. However, the anatomical data from the present study encourage awareness and further development of this concept. ▪
Disclosure: The authors did not receive any outside funding or grants in support of their research for or preparation of this work. Neither they nor a member of their immediate families received payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity. No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, division, center, clinical practice, or other charitable or nonprofit organization with which the authors, or a member of their immediate families, are affiliated or associated.
Investigation performed at Modbury Public Hospital, Modbury, South Australia, Australia
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