LAUSR

Introduction Anatomic variations of the leg and ankle muscle origins and tendinous insertions are relatively common. The frequency with which these anatomic variations occur within the population is variably defined, resulting in literature that describes them simply as normal anatomic variants to truly anomalous structures (1–4). With respect to the triceps surae complex, which includes the two heads of the gastrocnemius muscle and soleus muscle, the accessory soleus muscle is the most frequently described anatomic variant with incidence ranging from 0.7% to 5.5% (5). The accessory soleus muscle itself can have variable insertional anatomy, but generally has been described with a low-lying muscle belly terminating on the superomedial aspect of the calcaneus or the deep aspect of the Achilles tendon itself (2,6). In certain individuals, the accessory soleus may result in clinical symptoms requiring surgical intervention (7). A third head of the gastrocnemiusmuscle, or gastrocnemius tertius, also has been defined on imaging and anatomic dissection, typically merging with the more proximal muscle bellies, and not described as a distinct distal tendinous structure or insertion (8,9). Similarly, an accessory plantaris muscle has been described, typically merging with the plantaris tendon or medial aspect of the gastrocnemius tendon distally (10,11). At the level of the tendinous insertion of the triceps surae complex on the calcaneal tuberosity, variability can exist with respect to the individual insertion of the Achilles tendon fascicles or subtendons on the calcaneus with a typical axial plane rotation noted (12–14). However, while these subtendons can be manipulated and subsequently isolated through anatomic dissection, they are not typically visualized or identified as distinct entities in clinical practice. How these subtendon variations contribute to Achilles tendon ruptures or partial ruptures remains speculative (15), but given the increased utilization of advanced imaging techniques and increased incidence of