The superficial muscles of the posterior compartment of the leg include gastrocnemius, soleus, and plantaris (GM, SM, and PM, respectively). Together, they form the calcaneal tendon (CT), the thickest tendon in the human body, which attaches to the posterior surface of the calcaneus.^[1] The GM typically consists of two heads: a lateral head (LH) and a medial head (MH), which originate from the lateral and medial femoral condyles, respectively. Acting in conjunction with the SM, the GM primarily facilitates plantar flexion and supination of the foot and also contributes to knee flexion.^[1]

A well-recognized morphological variation of the GM is the presence of a third head (TH), referred to as the gastrocnemius tertius (GT), first described by Kelch in 1813.^[2] GT is the most prevalent variation of the GM, with reported prevalence rates varying considerably among different populations.^[3] Triantafyllou et al.^[4] performed a cadaveric study and meta-analysis, reporting a pooled prevalence of 4.34%. This finding highlights the remarkable morphological diversity and clinical relevance of this muscular variant. The additional head may arise from the popliteal surface of the femur, the lateral condyle, the knee joint capsule, or, less frequently, from the long head of the biceps femoris.^[3] Typically, the GT fuses with the MH of the GM.^[3]

Frey^[5] proposed a classification system comprising twelve morphological types of GT based on their origin and insertion patterns. Clinically, GT is important because specific variants traverse the popliteal neurovascular structures (PNVS), potentially leading to entrapment syndromes.^[3,5]

The present cadaveric case series investigates the frequency and morphological variability of the GM, particularly concerning the GT’s origin and insertion, as well as its anatomical relationship with the PNVS. It also discusses the developmental and comparative anatomical context, population-based prevalence, and clinical implications of this muscular variation.

Forty formalin-embalmed lower limbs were dissected to identify the presence and morphological variations of the GT. The specimens were obtained from the donated cadaveric collection of the Department of Anatomy and Surgical Anatomy at the Aristotle University of Thessaloniki, Medical School. Dissection began at the leg with the removal of the skin and superficial fascia to expose the GM. A detailed examination of the popliteal fossa (PF) was then performed. The following morphological features of the GM were systematically evaluated: the presence of a TH, its origin, course, and insertion, as well as its anatomical relationship with the PNVS.

All cadavers were donated to the Department for educational and research purposes, with informed consent obtained before death, in accordance with Greek legislation. The study was conducted in compliance with the Declaration of Helsinki and the institutional ethical guidelines for anatomical research.

The GT was identified in 6 (15%) out of 40 dissected lower limbs.

Type I variant (4/40; 10%): In four specimens, the TH originated from the posterior distal femoral surface, close to the midline, medial to the LH, and fused with it. In these cases, the popliteal vessels (PVs) were located medial to the TH (Fig. 1).

Type II variant (1/40; 2.5%): In one specimen, the TH originated from the LH and fused with the MH. The PVs traversed between the TH and MH, indicating potential for neurovascular entrapment (Fig. 2).

Type III variant (1/40; 2.5%): In another case, the TH did not fuse with the LH but instead formed an independent tendon, which inserted directly into the CT. This independent configuration highlights a unique anatomical course with clinical implications for the posterior leg compartment (Fig. 3).

Overall, the majority of GTs fused with the gastrocnemius heads, while a minority demonstrated independent insertion into the CT.

This cadaveric dissection series identified the GT in 15% of Greek specimens, representing the highest prevalence reported to date. Three distinct morphological configurations were observed: fusion with the LH, fusion with the MH, and independent insertion into the CT. In most cases, the tertius head fused with the LH; however, one specimen demonstrated a lateral trajectory associated with potential compression of the PNVS. Clinically, recognition of GT variants is essential, as they may contribute to neurovascular entrapment syndromes within the popliteal fossa. A thorough understanding of GT morphology, supported by targeted imaging, facilitates accurate diagnosis, prevents misinterpretation during radiological or surgical procedures, and aids in comprehensive preoperative planning.

The research was conducted ethically following the Code of Ethics of the World Medical Association (Declaration of Helsinki). The corpses belonged to the Anatomy and Surgical Anatomy Department (Aristotle University of Thessaloniki) through the Body Donation Program.

The authors declared that no clinical trials were used in the present study.

The authors declared that no experiments on humans or human tissues were performed for the present study.

The authors declared that they obtained a written informed consent from the donors or donors’ representatives participating in the study.

The authors declared that no experiments on animals were performed for the present study.

The authors declared that no commercially available immortalized human and animal cell lines were used in the present study.

The authors have declared that no competing interests exist.

No funding was reported.

No use of AI was reported.

All data used are referenced or included in the article.

Conceptualization: KN, JK, and MP; data collection: KN, GK, GTr, and TT; data analysis: GTs, AS, TT, and DC; writing–original draft: GTr and MP; writing–review: KN, GK, GTs, TT, and DC; supervision: MP; Approval: All authors.