Accessory heads of the biceps brachii muscle: A systematic review and meta-analysis

Abstract

The anatomy of the biceps brachii muscle has been a subject of interest to many researchers. In particular, the presence of one or more accessory heads has been reported to be the most common variation of the biceps brachii muscle. In fact, contemporary knowledge is quite inconsistent and lacks a definitive summary. Taking this into account, the present study aims to investigate the overall prevalence of the accessory heads and related questions, such as their broad morphological features, population variance, sexual dimorphism, side distribution or the changes in prevalence rates over time. A literature search of major scientific databases was conducted and produced 78 eligible articles, including 10,603 upper limbs, for our study. Relevant data were extracted and consequently analysed with the use of random-effects meta-analysis. As a result, the accessory heads occur with an overall prevalence of 9.6% (95% CI 8-11%) and by far the most common is the presence of a single accessory head (8.4%; 95% CI 7-10%). Additional sub-analyses revealed that accessory heads appear more frequently unilaterally and in males. Differences between the occurrence on the right or left side were not significant. Moreover, a decreasing trend in prevalence rates over time was observed, pointing towards an evolutionary adaptation. The innervation and blood supply of the accessory heads are nearly identical to that of the normal biceps brachii muscle. Although the accessory heads are usually asymptomatic, their potential presence must be kept in mind while interpreting various conditions. In addition, their direct clinical importance is speculated and is thought that they might be conductive to several pathological processes around the shoulder girdle and brachial region. While many of the morphological and morphometric aspects of the accessory heads are well documented, their functional value is still a matter for future investigations.

Keywords: accessory head; biceps brachii muscle; supernumerary head; third head.

FIGURE 1

PRISMA 2020 flow diagram showing the process of the study selection

FIGURE 2

Illustrative scheme of the five most common origins of the accessory heads of the biceps brachii muscle with their overall pooled prevalence and proportional prevalence that was calculated only from the sample with one accessory head (reports pooled prevalence estimate for the distinct origins deduced from upper limbs containing the accessory head)

FIGURE 3

Illustrative scheme of the two most common origin combinations in case of presence of two accessory heads resulting in a four‐headed biceps brachii muscle, including their pooled prevalence and proportional prevalence calculated only from the sample with two accessory heads (reports pooled prevalence estimate for the distinct origins deduced from upper limbs containing two accessory head)

FIGURE 4

Graphs summarizing the gender‐proportional analysis. Overall analysis shows a significantly higher prevalence in males (a). Continent‐related analysis revealed a higher prevalence among males in Africa (b), Asia (c), Europe (d) and South America (f). In contrast, a higher prevalence in females was reported only in North America (e). Noteworthy is that only one study was selected for this analysis each from Africa (b), North America (e) and South America (f). Therefore, these three continents must be carefully interpreted. Statistically significant differences are marked with an asterisk (*)

FIGURE 5

Graphs summarizes the side distribution proportional analysis. Overall analysis revealed that the accessory heads occur slightly more on the right side (a). The same outcomes were observed in Asia (c), Europe (d) and South America (e). Only Africa (b) shows a higher occurrence on the left side, although there was only one study originating from this continent, therefore these results must be carefully considered. Moreover, the accessory heads are significantly more prevalent unilaterally (f). Statistically significant differences are marked with an asterisk (*)

FIGURE 6

Scatter plot showing the exponential decrease in prevalence rates over time. Each dot represents one study. The low R² may indicate true noise in the data