Morphological classification of anatomical variants of the intra-articular portion of the long head of the biceps brachii tendon and analysis of the incidence and the relationship with shoulder disease for each subtype. 
Although several cases of anatomical variants of the proximal portion of the long head of the biceps brachii tendon have been reported, most are case reports on single patients; few studies have investigated the classification and incidence of this phenomenon.  We aimed to classify the variants of the long head of the biceps brachii tendon by their morphology and characterize them in regard to incidence, demography, and the relationship with biceps tendon or other shoulder diseases.  We retrospectively reviewed 760 arthroscopic images spanning 31 months from October 2008 and found 45 variations of the proximal portion of the long head of the biceps brachii tendon.  We classified the variants using Dierickx classification system and calculated the incidence of each subtype.  We investigated age, gender, bicipital morbidity rate, procedure rate, and the frequency of related shoulder disease among the variant groups using patient data and compared the groups to normal patients.  The overall incidence of variants was 6.5% (67.4%, mesotenon; 32.6%, adherent; 0% split (SPL); and 0% absent (ABS) type).  Morbidity and procedure rates of the long head of the biceps (LHB) tendon in the variant group (27.9% and 25.6%, respectively) were lower than in the normal group (40.5% and 38.9%, respectively), but the difference was not statistically significant ( p = 0.102 and 0.082, respectively).  Rotator cuff tear was significantly less common in the mesotenon (44.8%) group than in the normal group (62.4%; p = 0.004).  Shoulder joint instability was more prevalent in the mesotenon (27.6%) group than in the normal group (14.9%), but the difference was not significant ( p = 0.305).  The distribution of other shoulder diseases in the adherent group was similar to that in the normal group.  The incidence of variants of the long head of the biceps brachii tendon was 6.5%, and most variants were mesotenon or adherent type.  The SPL and ABS types were rare.  Morbidity and procedure rates of the LHB were not significantly different between the variant group and the normal group, and rotator cuff tear was less common in the mesotenon group.