Shoulder pain is a common medical problem and may lead to an inability to carry out daily living-activities. Luime et al. [1] reported that shoulder pain was present in approximately 14.7 persons per 1,000 persons per year in a primary care unit, whereas lifetime prevalence of shoulder pain was approximately 70% of the general population. In that review, the most common reported cause of shoulder pain was rotator cuff disease [1].

Due to its anatomical location and its long, oblique course in the shoulder joint, the biceps long head tendon is a frequent source of disorders associated with other shoulder pathologies, especially rotator cuff tear [2,3]. Murthi et al. [4] reported that the incidence of biceps pathology in patients is as high as 63% with rotator cuff disease, and Chen et al. [5] reported that a normal biceps long head tendon is observed in only 24% of patients with rotator cuff tears. Alpantaki et al. [6] identified sympathetic and sensory neural elements in the long head biceps tendon by applying immunohistochemical stains and indicated that those elements act as a major source of shoulder pain. Some studies have suggested that the outcome of a rotator cuff repair can be compromised if a concomitant biceps lesion is left untreated [7,8].

Problems associated with the biceps long head tendon may be placed in four groups. The first includes tenosynovitis and tendinitis due to degeneration of the tendon; the second involves tendon rupture caused by the progressive development of tendinitis; the third includes instability and dislocation caused by failure of the stabilizing process of the biceps pulley; the fourth and final problem involves rupture of the insertional portion of the tendon, often occurring in relatively young patients and/or those involved in sports activities.

The so-called Popeye deformity can be caused by distal movement of the ruptured biceps long head tendon. There were several reports indicating that the Popeye deformity produces cosmetic complaints, cramping, spasmodic pain, reduction of elbow flexion powers, and supination of the forearm [9-11]. However, there are no known reports on the pathogenesis of symptoms and histological changes that occur following the development of a Popeye deformity.

Research into the degeneration of the muscles of the shoulder has primarily focused on atrophy and fatty infiltration of the supraspinatus and infraspinatus muscles associated with chronic, massive rotator cuff tears. Moreover, there are reports that previously developed fatty infiltrations were irreversible even though the massive rotator cuff tear was repaired [12,13]. Previous research into the biceps long head tendon has been primarily related to describing specific techniques for surgical treatment and presenting subjective outcomes of patients after treatment. Unfortunately, there is an absence of reports on differences between such surgeries and describing good and bad clinical outcomes in patients.

Thus, the authors formed a hypothesis that atrophy and fatty infiltration occur after rupture of the biceps long head tendon. To investigate that hypothesis, we established a Popeye deformity animal model and evaluated the histological changes in muscle cells after tenotomy of the biceps long head tendon in the rat model.

The study was approved by the Institutional Animal Care and Use Committee of The Catholic University of Korea (No. PSPH-20130923-01).

Interestingly, muscle atrophy and fatty infiltration following the tenotomizing of the biceps long head tendon, which were hypothesized for this study, were not observed. However, all rats that underwent the tenotomy grossly exhibited the Popeye deformity.

The biceps brachii muscle has two origins; the short head originates from the coracoid process and the long head originates from the supraglenoid tubercle and glenoid labrum [3]. Due to its anatomical location and pathway, damage to the biceps long head tendon can induce shoulder problems, especially when accompanied by rotator cuff tear, resulting in long-term shoulder pain. It has been reported that the long head tendon prevents superior and anterior displacement of the humeral head, but that remains controversial. The occurrence of the Popeye deformity after a biceps long head tendon tear is reported to occur in 3% to 63% of cases [14-16], and prevalence of the Popeye deformity after failed fixation of biceps long head tears is approximately 2% [17]. With the Popeye deformity, symptoms such as weakened elbow flexion and forearm supination may occur. Moreover, there are many Popeye deformity patients that have complained of spastic pain [9-11], but the pathogenesis of that pain has not been fully described.

Muscle contraction followed by tearing of the biceps long head can be diagnosed grossly (e.g., the Popeye deformity) by identifying a mass-like lesion caused by the contracted muscle on the anteromedial aspect of the humerus. Ultrasonographically, the biceps long head tendon appears homogeneously hyperechoic, similar to normal tendons, and longitudinal long collagen fibers are visible. However, a contracted tendon appears unevenly hypoechoic with its collagen fibers showing an irregular pattern. The distal part of a torn biceps long head may be located distal to the biceps groove. Moreover, magnetic resonance imaging (MRI) of a torn biceps long head tendon may show absence of a portion of that tendon from its typical location, known as the ‘empty groove sign’, and retraction and atrophic changes to the distal part of the torn tendon can be seen [18,19].

Generally, atrophic changes to and fatty infiltration of the muscle occur after chronic, massive rotator cuff tears. These features are detectable through computed tomography, MRI, and/or pathologic results. However, such fatty infiltration does not reduce after repair of a torn rotator cuff [12,13,20,21].

In this study, an animal model was established by inducing biceps muscle retraction after tenotomy of the biceps long head tendon of rat. In that model, postoperative fatty infiltration, which is seen after a rotator cuff tear, was not observed. Since the biceps is formed of long head and short head branches, it is speculated that, after tenotomy of the long head branch, the short head branch was able to compensate and provide whole muscle reactions, thereby preventing fatty infiltration. Further investigation of that speculation is needed.

The rat Popeye deformity animal model is deemed to be successfully established because all rats undergoing tenotomy in this study showed the Popeye deformity. However, some human cases with biceps tendon damage do not show the Popeye’s deformity due to auto-tenodesis. In the rat model, retraction of the cut tendon end beyond the bicipital groove can be confirmed under direct vision. That creates a notable difference between some human clinical cases and the rat model.

The results of this study are important as this is the first study reporting muscle atrophy pathologically by showing reduced sizes of individual muscle cells and a reduced cross-sectional area of the long head muscle portion compared to that of the short head portion. This size difference could be a basis for using tenodesis to treat a biceps long head tear, which is still controversial between tenodesis and tenotomy. Surgeons who prefer tenotomy over tenodesis suggest that the clinical outcome of tenotomy is satisfactory [15,22,23], and that not all of their patients show the Popeye deformity. They support their assertions by suggesting the presence of autotenodesis of the biceps long head at the biceps groove; however, autotenodesis does not occur in all cases [24].

The limitations of this study are that although muscle cell size was affected, histologic changes after tenodesis were not detected; thus, further study is needed. Second, the absence of fatty infiltration, which was one of the hypotheses of this study, might have been caused by damage to fatty tissue during tissue dehydration in the process of placing samples in paraffin blocks. However, if there were dehydrated fat tissues present there should be empty spaces observed between muscle cells, and no such spaces were seen in the H&E stained sections. Therefore, we concluded there was no fatty infiltration.

Muscular changes after tenotomy of the biceps long head tendon include decreases in the size of individual muscle cells and a decrease in relative muscle mass. There were no posttenotomy changes in muscle cell nuclei and no evidence of fatty infiltration. Moreover, there were no changes observed following myostatin antibody immunohistochemical staining.