Distal Biceps Tendon Anatomy: A Cadaveric Study

Eames, M.H.A. MD; Bain, G.I. MD; Fogg, Q.A. MD; van Riet, R.P. MD, PhD

Journal of Bone & Joint Surgery - American Volume:
doi: 10.2106/JBJS.D.02992
Scientific Articles

Background: The anatomy of the distal biceps tendon and aponeurosis has not been studied in detail.

Methods: Seventeen cadaver elbows were dissected with loupe magnification to identify the details of the distal biceps tendon and the lacertus fibrosus.

Results: In ten of the seventeen specimens, the distal biceps tendon was in two distinct parts, each a continuation of the long and short heads of the muscle. The remaining seven specimens showed interdigitation of the muscle distally. The tendon continued from each muscle belly. The short head inserted distal to the radial tuberosity and was positioned to be a more powerful flexor of the elbow, while the tendon of the long head inserted on the tuberosity further from the axis of rotation of the forearm and was positioned to be a stronger supinator. The bicipital aponeurosis consisted of three layers and completely encircled the ulnar forearm flexor muscles. The aponeurosis may be important in stabilizing the tendons distally.

Conclusions: The double tendon insertion may allow an element of independent function of each portion of the biceps, and, during repair of an avulsion, the surgeon should ensure correct orientation of both tendon components.

Author Information

1 Modbury Public Hospital, Smart Road, Modbury, SA 5092, Australia

2 196 Melbourne Street, North Adelaide, South Australia 5006. E-mail address: greg@gregbain.com.au

3 Department of Anatomical Sciences, University of Adelaide SA 5005, Australia

4 Department of Orthopaedic Surgery and Trauma, University Hospital Antwerp, Wilrijkstraat 10, Edegem 2650, Belgium

Article Outline

Unlike the anatomy and pathophysiology of the proximal end of the biceps muscle, the distal biceps tendon anatomy is poorly understood. Most anatomical descriptions1-3 have suggested that the muscle originates as two proximal heads that merge at the level of the deltoid tuberosity to form a single muscle belly. This muscle belly produces a single oval distal tendon that twists from a predominantly frontal plane to a sagittal plane before inserting into the bicipital tuberosity on the proximal part of the radius. As the tendon passes anterior to the elbow joint, a thin fibrous structure, the lacertus fibrosus or bicipital aponeurosis, fans out in an ulnar direction before merging with the superficial fascia of the ulnar side of the forearm1-3. This aponeurosis is said to protect the neurovascular bundle in the antecubital fossa, but its functional importance has not been described. The goal of this study was to assess the anatomy of the distal biceps tendon and its aponeurosis.

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Materials and Methods

Seventeen embalmed upper limbs (eight left and nine right limbs) from the Ray Last Anatomy Laboratory, University of Adelaide, were examined. The cadavers were perfused and fixed with a mixture of ethanol, glycerol, formalin, and phenol. The biceps muscle was identified and dissected with use of loupe (×2.5) magnification. The muscle bellies of the long and short heads were dissected, and their relationships to each other and the proportions of each relating to the formation of the distal tendon were observed and recorded. The bicipital aponeurosis, the bicipitoradial bursa, and the insertion pattern of the distal tendon were dissected and recorded. The size of the tendon was measured with use of a ruler (accuracy, 0.5 mm). The bicipital aponeurosis was followed from proximal to distal, and its components were recorded. In the dissected specimens, the actions of the biceps muscle were assessed by pulling on the musculotendinous junctions and observing the movements of the forearm.

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The results are described in relation to three zones, from proximal to distal, as preaponeurosis, aponeurosis, and postaponeurosis.

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Zone 1: Preaponeurosis

All of the specimens had two muscle bellies, a short head originating from the coracoid process of the scapula and a long head originating from the superior lip of the glenoid. In ten specimens (Group 1), these two muscle bellies continued along their entire length as separate muscles (Fig. 1). The two muscle bellies were surrounded by loose epimysial tissue. The short head remained on the ulnar side of the arm throughout its course. The long head ran parallel to the short head on the radial side of the arm.

The remaining seven specimens (Group 2) showed varying amounts of interdigitation of muscle into a raphe in the distal third of the muscle bellies. The maximum interdigitation occurred 5 cm proximal to the distal biceps tendon. In these seven specimens, the two bellies of the muscle could be easily separated with blunt dissection, by peeling them apart.

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Zone 2: Aponeurosis (Lacertus Fibrosus)

In Group 1, each muscle continued as a separate tendon distally (Fig. 2). The tendon of the long head continued on the radial side of the tendon of the short head. The cross section of the long head was crescentic in shape, and the short head was oval. The cross-sectional areas of the separate tendons appeared equal (Fig. 3).

In Group 2, the distal tendons continued in line with the respective muscle bellies and could be easily separated with blunt dissection. The tendons continued as for the other ten specimens described above.

The aponeurosis commenced at the level of the musculotendinous junction. All specimens demonstrated that the aponeurosis consisted of three layers (Fig. 3). These layers may be important in stabilizing the tendons distally. The superficial layer originated from the anterior radial aspect of the long head of the biceps just proximal to the commencement of the distal biceps tendon. This superficial layer macroscopically was the thickest layer in all specimens, and it passed in a distal and ulnar direction anterior to the musculotendinous junction of the short head. In some specimens, a rudimentary middle layer, which acted as a mesentery, was present. It was the only layer to attach to the short head. This middle layer passed in an ulnar direction to merge anteriorly with the superficial layer. The deep layer originated from the deep radial side of the musculotendinous area of the long head of the biceps. This layer passed in an ulnar direction deep to the tendon of the short head to merge with the other two layers.

These three layers merged and continued distally, superficial to the ulnar flexor muscles of the forearm. There were several strong fascial adhesions to the ulnar flexor muscles, tethering the aponeurosis. The aponeurosis also continued radially to the forearm flexor muscles as well as the median nerve and brachial artery. The aponeurosis was attached to both the radial and ulnar aspects of the proximal part of the ulna, completely encircling the forearm flexor muscles (Fig. 4). It inserted into the antebrachial fascia and reinforced it. There were several perforating holes in the radial side of the aponeurosis for the recurrent radial vessels.

The two distal tendons in the majority of the specimens (Group 1) were able to move separately from one another in a sliding action. The tendons in Group 2 followed the same line as those in the other group, but they did not have the ability to glide independently.

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Zone 3: Postaponeurosis

The two tendons continued distal to the aponeurosis and inserted into the proximal part of the radius. In both groups, the tendon of the long head passed deep to the tendon of the short head to insert more proximally. The insertion of the tendon of the long head was oval in shape, occupying most of the radial tuberosity. The tendon of the short head curved anterior to the tendon of the long head, to insert in a fan-like fashion into the distal portion of the radial tuberosity, and extended distal to it (Fig. 5). The attachments of the two distal tendons were surrounded by the bicipitoradial bursa (Fig. 6). This bursa completely encircled the distal tendons in all specimens. The bursa could be easily distended by injection of ≤7 mL of saline solution or latex on its deep radial side. The bursal membrane continued around the ulnar side of the tendons, where it was adherent to the tendon and would not distend. The bursa was attached proximally to the biceps tendon on the radial aspect. From this point, it draped down over the tendons, adhering to both tendons on the ulnar aspect. The bursa was attached along the proximal deep edge of the tendon of the long head to create a teardrop shape. Thus, the bursa lay between the groove in the brachialis muscle and the distal biceps tendons with the elbow extended, and between the proximal part of the radius and the biceps tendons during pronation of the forearm.

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.

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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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