1 Introduction

Typical anatomy of the posterior thigh muscles includes the semitendinosus muscle, semimembranosus muscle, and biceps femoris muscles. These muscles have proximal attachments onto the ischial tuberosity, including the long head of the biceps femoris (BFLH), whereas the short head has a proximal attachment on the linea aspera of the femur. Distal insertions are the medial surface of the tibia for the semitendinosus and semimembranosus muscles, and the fibular head for the biceps femoris muscle [1].

The tensor fascia suralis muscle (TFS) is a clinically relevant muscle variation that describes an accessory muscle of the posterior thigh and popliteal fossa [2,3,4,5,6]. It has a prevalence of 1.3% of 236 cadaveric legs studied, with only one donor possessing bilateral TFS [7]. The long head of the biceps femoris muscle (Type Ia) is the most common origin of TFS, but it may arise from the semitendinosus (Type Ib) or short head of the biceps femoris (Type Ic) [7]. Type II and III are two-headed and atypical variations [7]. Clinically, TFS may cause snapping knee, a palpable popliteal mass, and neuromuscular compression of regional nerves. We report the findings of bilateral TFS and a unilateral accessory muscle belly of the biceps femoris (AMBF) muscle in a 72-year-old female.

2 Case report

This case was identified during dissection of formalin-embalmed donors utilized in a medical school curriculum. The AMBF and bilateral TFS were further dissected to identify their origins and insertions. The TFS tendon insertion into the crural fascia (CF) was preserved for measurement and documentation. Digital calipers (Mitutoyo Corporation, Kanagawa, Japan) and a flexible plastic ruler were used to collect measurements on the bilateral TFS: the length from (1) the BFLH origin to the TFS origin, (2) from the superior tip of the calcaneus to the TFS insertion into CF, (3 and 4) length of the TFS muscle belly and tendon, and (5) diameter at the midpoint of the TFS muscle belly. All measurements were collected in triplicate by the same researcher (MM) and recorded in Microsoft Excel (Microsoft Corporation, Redmond, WA); mean measurements are reported in Table 1.

Table 1 Measurements of the bilateral TFS and unilateral accessory biceps femoris muscle
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The left TFS (Type Ia [7]) originated from the BFLH and inserted onto the CF overlying the lateral head of gastrocnemius muscle (Fig. 1). The right TFS (Type Ia [7]) originated from the BFLH and inserted onto the CF overlying the medial head of gastrocnemius muscle (Fig. 1). An AMBF, present in the left lower limb only, originated from the inferior most aspect of the greater trochanter. It inserted on the distal end of the biceps femoris long head.

Fig. 1
figure 1

Schematic drawing (a) and photographic image (b) of the donor’s left lower limb showing presence of TFS and accessory biceps femoris muscle. Schematic drawing (c) and photographic image (d) of the donor’s right lower limb showing presence of TFS

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3 Discussion

A recent review characterizing TFS variations noted 32 cadaveric reports, 26 were unilateral and three cases were bilateral [7]. An additional six reports were radiological in their identification; four of these reports were unilateral cases and one was bilateral [7]. The unilateral cases were predominantly right-sided (n = 16), with 10 cases present as left-sided variations; four reports did not include sidedness. TFS was overwhelmingly present in male cadavers (n = 28), with only three reports in female donors (seven of the cases did not report sex of the donor) [7]. This report documents an additional occurrence of bilateral TFS and a unilateral AMBF in a female donor.

3.1 Clinical considerations

The common peroneal nerve (CPN) branches from the sciatic nerve proximal to the popliteal fossa in the posterior compartment of the thigh [1]. This case report presents a unique possibility for entrapment of the CPN in the femoral region of the leg due to a unilateral accessory muscle belly of the left biceps femoris bifurcating the sciatic sheath, resulting in the CPN wrapping around this accessory muscle belly. Contraction of this muscle belly could have compressed the CPN, resulting in distal neuropathy of the common peroneal innervations [8], including decreased sensation of the lateral aspect of the leg distal to the knee, anterolateral distal third of the leg, and dorsum of the foot, as well as motor weaknesses such as foot drop [1]. Additionally, the donor was found to have bilateral TFS muscles with varied insertion points relative to the calcaneus. Possible complications from this presentation include bilateral snapping knee, a painful popping sound that usually occurs during knee flexion, and right-sided popliteal fossa bulge due to the proximity of the insertion of the right TFS to the popliteal fossa [9, 10]. A key limitation of this study is the unique anatomical presentation without record of any self-reported medical complications. As such, all complications resulting from this patient’s unique anatomy are conjecture based upon previous associated research but are unable to be confirmed.

4 Conclusion

To the best of our knowledge, this unique presentation has not been previously explained by cadaveric or imaging studies. This data characterizes the novel finding of bilateral TFS muscles with a unilateral accessory muscle belly of the biceps femoris in a female donor.