The fibularis brevis muscle (FBM), also referred to as peroneus brevis, is one of the two principal muscles located in the lateral compartment of the leg. It originates from the distal two-thirds of the lateral surface of the fibula and inserts onto the tuberosity of the fifth metatarsal (Bergman et al., 1988; Moore et al., 2017). It is innervated by the superficial fibular nerve and plays a crucial role in eversion of the foot and in maintaining balance during locomotion, especially on uneven surfaces (Otis et al., 2004; Verma et al., 2011).

Clinically, the FBM and its tendon (FBT) are essential stabilizers of the lateral ankle joint. They contribute significantly to dynamic ankle control and are frequently involved in overuse injuries, lateral ankle sprains, and chronic ankle instability (Alanen et al., 2001; Philbin et al., 2009). FBT pathologies, including tears, subluxation, and tenosynovitis, are common in athletes and runners (Dombek et al., 2003; Seybold et al., 2016). Moreover, the FBT is a preferred graft source in various reconstructive procedures involving the lateral ankle complex, especially in cases of irreparable fibulariss longus or FBT ruptures (Clement et al., 2019; Housley et al., 2017; Olewnik et al., 2020).

Recent reports have also highlighted the surgical utility of accessory FBT slips and related variants, such as fibularis digiti quinti (FDQ) and fibularis quartus, as autografts in tendon reconstruction (Carlis et al., 2017; Yammine, 2015). However, anatomical variability of the FBT and its derivatives remains underreported in standard clinical guidelines and often underappreciated in surgical planning.

Radiological evaluation of FBM pathology poses diagnostic challenges, as accessory tendinous structures and variant insertion patterns can mimic longitudinal tears or soft-tissue masses (Sookur et al., 2008; Vohra et al., 2011). The FBM is frequently mistaken for fibularis longus in both static MRI and dynamic United States, especially in the presence of bifid tendons or accessory fascicles (Olewnik et al., 2020; Ruzik et al., 2022). Moreover, FDQ and fibularis quartus may be misinterpreted as abnormal masses if not properly recognized in radiological imaging (Chaney et al., 2017; Demir et al., 2015).

The aim of this review is to provide a comprehensive and structured reevaluation of the FBM, integrating data from developmental anatomy, evolutionary morphology, anatomical variation, comparative interspecies analysis, and clinical radiological findings. By synthesizing cadaveric, fetal, and imaging-based studies, we propose a unified framework for understanding the morphological variants of FB and their implications in clinical and surgical contexts (Diogo et al., 2010a; Olewnik et al., 2023; Olewnik et al., 2020).

The FBM develops from the dorsal muscle mass of the lower limb bud, which is responsible for forming the extensors and evertors of the lower extremity (Bardeen, 1906; Bardeen and Lewis, 1901). By the fifth week of gestation, myogenic precursor cells from the somites migrate into the developing limb bud, where they begin to organize into distinct compartments. FB originates posterolaterally and forms in parallel with fibularis longus, although each follows an independent developmental trajectory (Moore et al., 2017; Moore et al., 2020).

During fetal development, FBM gradually separates from fibularis longus and fibularis tertius, assuming its mature anatomical position and forming the myotendinous junction that inserts into the base of the fifth metatarsal. This differentiation occurs concomitantly with the medial rotation of the limb and is visible in second-trimester dissections (Olewnik et al., 2023).

The ontogeny of FBM, like other limb muscles, is governed by a well-characterized hierarchy of myogenic regulatory factors (MRFs). Pax3 initiates migration of progenitor cells from the dermomyotome, while Myf5 and MyoD drive their commitment and differentiation into skeletal muscle (Diogo et al., 2010a). These transcription factors are activated in response to limb-level morphogen gradients including FGF, Wnt, and BMP.

Although FBM specific expression studies remain limited, analysis of dorsal vs. ventral muscle development suggests that the posterolateral domain of the limb bud expresses a distinct profile of MRFs, facilitating formation of fibular musculature (Albay and Candan, 2017). Alterations in this tightly controlled sequence may lead to aberrant muscular differentiation, malformations, or asymmetries.

Disruptions in the developmental program can result in morphological anomalies of FBM. Errors in somite segmentation, progenitor migration, or fascial compartmentalization may lead to supernumerary muscle slips, atypical trajectories, or duplications. Embryologically, these anomalies may result from incomplete apoptosis of primitive muscle condensations or persistence of transient myogenic bridges.

Variants of FBM such as additional tendinous slips, aberrant insertions, or fusion with adjacent muscles are frequently reported in both adult and fetal specimens (Bhargava et al., 1961; Musiał, 1963; Olewnik et al., 2023). These findings support a developmental origin for many observed variations and highlight the need for embryological awareness in interpreting FBM anatomy. Although the anatomical variants of the FBT such as bifurcated (Type II) insertions are well-documented developmentally, no current evidence links these patterns to specific genetic mutations or congenital syndromes in humans. They are presently regarded as sporadic developmental anomalies rather than heritable traits. Nonetheless, further investigation into potential genetic predispositions may be warranted. Future molecular studies could clarify whether certain transcriptional or epigenetic variations subtly influence the formation or persistence of these morphotypes (Olewnik et al., 2023; Olewnik et al., 2020).

The development of FBM exemplifies the concept of muscle modularity, whereby muscles can evolve and adapt as discrete anatomical units. This modularity allows for plasticity in form and function across taxa, as muscles can undergo hypertrophy, fusion, or even regression without disrupting the global musculoskeletal architecture (Abdala and Diogo, 2010; Diogo et al., 2010a).

In the case of FBM, such modularity explains the presence of accessory fibular muscles in some individuals and their complete absence in others. These variations do not necessarily impair function but reflect an evolutionarily conserved mechanism for localized adaptation to biomechanical demands (Poonia et al., 2020; Yammine, 2015).

From a phylogenetic perspective, FBM likely originated from lateral digital extensor muscles that were prominent in early tetrapods. In non-human primates, especially arboreal species, FBM is robust and contributes significantly to dynamic foot stabilization and prehensile support. In contrast, terrestrial quadrupeds such as carnivores or ungulates often exhibit a reduced or absent FB, having functionally redistributed eversion roles to other muscles (Diogo et al., 2010a).

This cross-species variability highlights FBM as a developmentally plastic and evolutionarily responsive muscle, shaped by postural requirements and gait mechanics. In modern humans, its persistence in a distinct fascial plane with separate innervation reflects both its ancestral roots and its importance in maintaining lateral ankle integrity.

The FBM is a key component of the lateral compartment of the leg. It originates from the distal two-thirds of the lateral surface of the fibula, as well as from the anterior intermuscular septum and deep fascia. The muscle fibers converge into a strong tendon that runs distally and posterior to the lateral malleolus, finally inserting onto the tuberosity at the base of the fifth metatarsal (Moore et al., 2017; Zielinska et al., 2024). The FBM is enveloped in a common fascial sheath with the fibularis longus, with which it shares the lateral fibular groove and peroneal tendon sheath (Olewnik et al., 2023; Olewnikvet al., 2020).

• The muscle is innervated by the superficial fibular nerve, a branch of the common fibular nerve, and is vascularized by perforating branches of the fibular artery, (van Dijk et al., 2016). The presence of a myotendinous junction typically occurs in the lower third of the leg (Mateen et al., 2022).

Anatomically, the FBT is stabilized at the lateral ankle by the superior and inferior peroneal retinacula. These fibrous bands prevent subluxation or dislocation of the tendon during foot eversion. The superior peroneal retinaculum extends from the posterior border of the lateral malleolus to the calcaneus, forming the primary retaining structure for FBT and fibularis longus tendons. The inferior retinaculum, more complex in configuration, contributes to the pulley-like function of the retromalleolar path (Sookur et al., 2008).

Laterally, the FBT is in close proximity to the sural nerve and its lateral dorsal cutaneous branch, as well as to the small saphenous vein. The proximity of these structures poses a risk during lateral surgical approaches and must be considered during peroneal tendon repairs or reconstructions (Philbin et al., 2009). The peroneal tubercle on the lateral calcaneus serves as a distinguishing anatomical landmark separating the tendons of FBM and fibularis longus distally (Macalister, 1875).

On MRI, the FBT is visualized as a well-defined, low-signal-intensity structure on T1- and T2-weighted images. It courses posterior to the lateral malleolus and can be identified separately from the FLT in axial and coronal planes. Its normal myotendinous junction appears as a smooth, fusiform transition zone with intermediate signal, located proximally to mid-calf (Vohra et al., 2011).

In ultrasonography, FBT appears as a homogeneous echogenic fibrillar structure within the shared fibularis tendon sheath. Dynamic scanning can confirm the integrity of its gliding movement and detect intrasheath subluxation or early tendinopathic changes. Proper visualization requires axial and longitudinal planes just posterior to the fibula and around the retromalleolar region (Carlis et al., 2017; Dombek et al., 2003).

Recognition of the FBT’s normal anatomy on imaging is crucial to avoid misdiagnosing bifid tendons, longitudinal splits, or accessory insertions as pathological findings (Seybold et al., 2016; Yammine, 2015).

In a comprehensive cadaveric study of 102 lower limbs, the FBT was found to be present in all specimens (Olewnik et al., 2020). Two main types of distal insertion were identified based on the number and morphology of terminal tendon slips: a single insertion (Type I) and a bifurcated pattern (Type II). Type II was further subdivided into three morphologically distinct subtypes based on the course and insertion of the accessory tendinous band.

The FDQ muscle was identified in 18 feet (17.7%) out of the 102 dissected lower limbs (Olewnik et al., 2020). In all cases, the presence of the FDQ tendon was strictly associated with the Type I configuration of the FBT. No instances of FDQ co-occurring with bifurcated (Type II) FBT insertions were recorded, suggesting a developmental link between these structures.

Three distinct types of FDQ were classified based on the location of the distal tendon insertion:

Type 1 – The FDQ inserted onto the base of the proximal phalanx of the fifth toe. This was the most common subtype and was present in eight cases. This configuration may contribute to subtle functional eversion of the fifth digit and should be considered in anatomical studies of lateral foot musculature.

Type 2 – The FDQ inserted into the extensor aponeurosis of the fifth toe. This subtype was found in five cases. The anatomical course in this variant aligns with the dorsolateral axis of the foot, and its presence may be misinterpreted on imaging as a duplication of extensor tendons.

Type 3 – The FDQ was fused with one of the tendons of the extensor hallucis longus (EHL) muscle. This uncommon configuration was observed in five feet. The fusion with EHL may reflect a developmental or phylogenetic relationship and represents an unusual deviation from the typical lateral foot anatomy.

All three variants are clinically relevant, particularly in surgical approaches to the lateral foot and during interpretation of radiological studies such as ultrasound or MRI. The FDQ may mimic pathological tendon structures, contribute to pseudomasses, or complicate soft-tissue dissections. Preoperative recognition of such variants can help prevent iatrogenic injury and improve surgical outcomes.

The morphological diversity of the FBT has been noted for over a century. While current classification systems are based on detailed morphometric and imaging studies, earlier anatomical descriptions laid the groundwork for understanding the spectrum of insertional variability. Among the most influential early contributions are those by Macalister (1875), Bhargava et al. (1961), and Musiał (1963).

Macalister (1875) provided one of the first systematic surveys of muscular anomalies, including accessory tendons and fusion patterns in the lateral compartment of the leg. Although limited by the descriptive language of the time, his work documented numerous cases of accessory fibular insertions and connections with other foot musculature.

Bhargava et al. (1961), based on a study of 100 Indian cadavers, described the presence of split tendons and occasional dual insertions of the FBT. These observations closely resemble what would later be formalized as Type II configurations.

Musiał (1963) performed one of the earliest targeted studies of fibular muscle insertions in the Polish population, highlighting frequent deviations from the standard single insertion model. He reported examples of tendinous slips directed toward the cuboid, lateral cuneiform, or fascial structures now classified as Type II subtypes or atypical Type IV insertions.

To contextualize these foundational descriptions, the table below compares their major findings with the current classification system proposed by (Olewnik et al. (2020)) – Table 1.

A fetal anatomical investigation encompassing 84 lower limbs from 43 human fetuses (21 males, 22 females) revealed the consistent presence of the FBM and FBT (Olewnik et al., 2020). The study delineated three distinct tendon configurations during prenatal development, offering insights into the early organization and potential remodeling of the lateral ankle compartment (Olewnik et al., 2023).

Type I represented the predominant pattern, observed in 77% of specimens. It featured a single, direct insertion onto the lateral tuberosity at the base of the fifth metatarsal. This morphology mirrors the canonical adult form and likely reflects the default developmental trajectory in the absence of additional modulatory signals.

Type II, present in 19% of fetal limbs, exhibited a bifurcating tendinous pattern. While the main tendon consistently inserted in the typical location, secondary slips diverged in four anatomically distinct ways.

• Subtype A: A dorsal slip extended to the top surface of the fifth metatarsal base, often blending with fibers of the fibularis tertius tendon suggesting a shared developmental lineage or transient fusion.

Type III, the rarest variant (4%), was characterized by a trifurcated distal arrangement. In this form, two accessory tendons accompanied the primary insertion: one attaching to the metatarsal shaft, and the other extending dorsally into fascial structures. This configuration, absent in adult morphotypes, may reflect an intermediate or regressing stage in fibular tendon morphogenesis.

These findings illustrate that the FBT undergoes dynamic refinement in utero, with several anatomical patterns likely subject to postnatal regression, fusion, or selective persistence. The existence of fetal-only variants such as Type III reinforces the notion of a developmental plasticity unique to this muscle group.

Clinically, such prenatal anatomical diversity bears significance in the context of congenital foot deformities, pediatric tendon dysplasia, or early-onset peroneal pathology. An appreciation of these morphotypes may aid in refining pediatric imaging protocols and guiding corrective interventions in infants. The observed fetal variants of the fibularis brevis tendon demonstrate a clear developmental continuum, with certain configurations showing direct correspondence to adult types, while others appear transient or prone to postnatal remodeling. A comparative summary of these relationships is presented in Table 2.

The architecture of the FBM demonstrates strong functional alignment with the specific locomotor demands imposed by an organism’s ecological niche. Across vertebrate taxa, variations in muscle size, shape, insertion pattern, and degree of independence from fibularis longus reflect adaptations that optimize mechanical efficiency, stability, or precision.

In arboreal species, such as Pan troglodytes and Phalanger spp., FBM tends to be hypertrophied and well-differentiated, often maintaining an independent myotendinous trajectory. This configuration supports powerful foot eversion and active modulation of grasp during branch manipulation. The hypertrophy correlates with dynamic stabilization demands in climbing and suspension behaviors (Diogo et al., 2010a; Heilmann, 1926).

In cursorial mammals, including Canis lupus familiaris and Equus caballus, the FBM shows either increased fusion with FL or complete anatomical reduction. In canines, FBM remains functional and distinct, assisting with lateral foot stabilization during rapid directional changes. In ungulates, where limbs are optimized for straight-line, high-speed movement, FBM is commonly fused or absent, replaced by robust tendinous extensions of FL. This streamlining promotes energy-efficient elastic recoil and reduces muscular redundancy (Abdala and Diogo, 2010; Diogo et al., 2010a; Diogo and Abdala, 2010b).

Marsupials and climbers, such as Macropus rufus and Trichosurus vulpecula, exhibit variable FBM morphology, tuned to their respective locomotor strategies. Terrestrial hoppers show a simplified FBM, likely serving a limited proprioceptive or supportive role, whereas arboreal marsupials preserve a more distinct FBN for lateral control and postural adjustment on irregular substrates (Abdala and Diogo, 2010; Diogo et al., 2010a; Diogo and Abdala, 2010b).

Functionally, FBM’s anatomical variability affects.

• Tendon trajectory: direct vs. bifurcated insertions influence torque vectors on the fifth metatarsal.

These patterns underscore FBM’s evolutionary role as a modular structure selectively retained, remodeled, or minimized depending on biomechanical demands. Its form serves as both a reflection of locomotor ecology and a potential marker of developmental constraints. In addition to morphological diversity across taxa, the insertional pattern of the FBT itself particularly bifurcated configurations may have meaningful biomechanical implications. The FBM is responsible for approximately 60%–65% of the eversion force generated at the ankle. A bifurcated insertion pattern, involving multiple attachment sites, can provide a broader distribution of tensile force across the lateral foot, thereby enhancing subtalar joint stability and resistance to inversion stress. Conversely, a single-point (Type I) insertion concentrates mechanical loading at the base of the fifth metatarsal, which may increase the risk of focal degeneration or avulsion injuries. These biomechanical considerations underscore the potential clinical significance of anatomical variants, particularly in populations engaged in high-impact or lateral-movement sports (Olewnik et al., 2020). Subtle distinctions in FBM structure are also evident when comparing limb postures across taxa. In plantigrade species such as humans, bears, and primates the FBM tends to be more robust and muscular, reflecting its crucial role in stabilizing the entire foot during full-sole contact with the ground. In contrast, digitigrade animals like dogs and cats often exhibit a more streamlined, tendon-dominated FBM configuration. Their foot posture, which emphasizes toe-based contact and high-speed locomotion, favors mechanical efficiency over lateral fine-control. These morphological nuances align with broader locomotor adaptations and underscore the evolutionary plasticity of FBM in response to functional demands.

Understanding the evolutionary origins and morphofunctional variability of the FBM provides important insights for both developmental biology and clinical anatomy. Variant morphologies observed in humans may reflect retained ancestral traits or deviations arising during embryonic patterning.

The FBT typically appears as a low-signal-intensity structure on T1- and T2-weighted MRI sequences, located anteromedial to the fibularis longus in the lateral retromalleolar groove. It maintains a uniform thickness and course, inserting on the base of the fifth metatarsal. In United States, FBT demonstrates a fibrillar echotexture and lies within a shared peroneal sheath, becoming hypoechoic in longitudinal tears or tenosynovitis (Buckens et al., 2017; Sookur et al., 2008). Variant forms, such as bifurcated tendons or anomalous insertions, may alter the tendon trajectory or thickness, often appearing as accessory bundles or subtle duplications within the sheath (Zielinska et al., 2024).

Several FBM variants may mimic pathologic conditions on imaging. A bifid FBT, especially when the accessory slip is hypoplastic or curved, may simulate a longitudinal split tear on both MRI and United States, leading to unnecessary interventions. The FDQ tendon, when present, can appear as a small independent structure extending towards the fifth toe, and may be misinterpreted as a retracted tear or neuroma. Similarly, accessory fibularis muscles such as the peroneus quartus or fibularis accessorius may be mistaken for masses, particularly in the retromalleolar or calcaneal regions (Sookur et al., 2008; Yammine, 2015). Awareness of these variations is essential in patients presenting with lateral ankle pain, especially among athletes and individuals with overuse injuries.

To accurately differentiate variants from pathology, optimized imaging protocols are necessary. Dynamic United States should be used to assess for intrasheath subluxation, retinacular impingement, or split tendon motion. The ankle should be scanned in both neutral and dorsiflexed positions. For MRI, axial and oblique axial slices are preferred at the level of the distal fibula and cuboid, with high-resolution proton-density (PD) and STIR sequences for tendon evaluation. Key parameters include tendon thickness asymmetry >2 mm, signal heterogeneity, and anomalous insertion paths (Bencardino and Rosenberg, 2001; Mateen et al., 2022). For optimal visualization of FBT variants, we recommend a dedicated MRI protocol using proton-density (PD) or T2-weighted sequences with fat suppression. Axial-oblique slices aligned with the peroneal tendons particularly behind the lateral malleolus are ideal for detecting bifurcated or accessory slips. Coronal-oblique planes oriented parallel to the fifth metatarsal further aid in assessing insertional morphology. Ultrasound, when performed dynamically during active foot eversion, serves as a valuable adjunct for confirming variant tendons in real time and for assessing their relationship to surrounding structures (Wang et al., 2005).

Radiologists should consider anatomical variability in differential diagnoses of lateral ankle lesions, particularly when evaluating for peroneal tendon tears, tenosynovitis, or suspected soft tissue masses. When an unexpected bifid structure is noted, symmetry and continuity of both slips should be confirmed before diagnosing a tear. Sports medicine specialists and orthopedic surgeons should correlate imaging findings with clinical symptoms and physical examination, especially when considering surgical intervention. Preoperative imaging should aim to define not only pathology but also variant tendon anatomy to guide incision placement and graft viability (Heckman et al., 2008; Olewnik et al., 2020).

The FBT is commonly implicated in lateral ankle pathologies, including tenosynovitis, longitudinal tendon tears, and chronic ankle instability. FBT injuries often co-occur with peroneal tendon sheath disorders and are particularly prevalent among athletes. According to Mercer et al. (Mercer et al., 2021), surgical intervention for peroneal tendon tears, including FBT, led to favorable functional outcomes but was associated with a complication rate nearing 38.7%. Low-lying muscle bellies and bifurcated tendons may predispose to chronic friction and tendon degeneration (Sookur et al., 2008). These variants have also been shown to increase the risk of tenosynovitis and peroneal tendon subluxation (Koutsogiannis et al., 2022).

Anatomical variation of the FBT can complicate both diagnosis and surgical planning. For instance, bifid FBT may be mistaken for a split tear, and fusion with fibularis longus tendon can obscure clear visualization during tendon transfers or tenodesis procedures. Moreover, harvesting FBT or fibularis longus tendon for autografts in lateral ankle ligament reconstruction may endanger peroneal vincula, which are critical for vascular supply to the tendon (van Dijk et al., 2016). Ruzik et al. (Ruzik et al., 2022) reported that an unusual FBT insertion pattern may require modified dissection planes to prevent neurovascular injury, particularly to the sural nerve and small saphenous vein.

Preoperative imaging with ultrasound or MRI can help identify FBT variants, especially bifid insertions or anomalous accessory fascicles. During surgery, dynamic assessment of tendon movement and visual confirmation of insertion sites is critical. When variants are suspected, careful blunt dissection and extension of the surgical window may reduce the risk of iatrogenic injury. Tendon tension testing under anesthesia can help confirm integrity and differentiate between FBT and adjacent slips (Bianchi et al., 2010; Bianchi et al., 2007; Heckman et al., 2008).

A tailored approach is necessary for surgical procedures involving FBT, particularly in reconstructive contexts. We propose a safety-oriented preoperative checklist: (1) assess FBT morphology via imaging; (2) localize adjacent neurovascular structures (especially the sural nerve); (3) confirm absence or presence of FDQ or fibularis quartus muscles. Imaging should guide incision planning, and knowledge of variant configurations should inform intraoperative navigation. Such an approach reduces the likelihood of incomplete graft harvesting or inadvertent soft tissue damage.

The terminology surrounding the lateral compartment of the leg particularly in reference to FBM, its tendon, the FDQ, and accessory muscles such as the fibularis quartus remains inconsistent across anatomical atlases, radiological reports, and surgical literature. While classical texts such as those by Macalister (Macalister, 1875) and Anson (Anson and McVay, 1971) provided descriptive terms, they often lacked uniformity in defining morphological variants versus distinct muscular entities. Modern anatomical works (e.g., Bergman et al. (Bergman et al., 1988)) have attempted to consolidate these terms, but considerable confusion remains, particularly in distinguishing FDQ from fibularis quartus or fibularis accessorius.

The need for standardized nomenclature arises from both evolutionary and clinical perspectives. From an evolutionary viewpoint, Diogo and Abdala (Diogo et al., 2010a; Diogo and Abdala, 2010b) have emphasized the modular origin of the FBM and its relation to digital extensor systems in primitive tetrapods, which supports the view that FDQ and fibularis quartus are derived subdivisions rather than separate muscles. Embryologically, these structures share a common origin from the dorsal muscle mass of the limb bud and differentiate under the influence of Pax3, Myf5, and MyoD (Albay and Candan, 2017; Bardeen, 1906; Bardeen and Lewis, 1901). Clinically, imaging artifacts due to anatomical variability such as bifid tendons or partial fusions are often misinterpreted in MRI and United States, underscoring the need for precise, consistent descriptors (Sookur et al., 2008; Zielinska et al., 2024).

We propose the following standardized terminology.

• Fibularis brevis muscle should refer exclusively to the muscle originating from the distal lateral fibula, inserting on the fifth metatarsal, regardless of its internal subdivisions.

This classification aims to improve clarity across disciplines, facilitate radiological reporting, and support more precise anatomical education. Adoption of uniform descriptors in anatomical atlases and surgical protocols will reduce misdiagnosis and improve patient outcomes in cases involving lateral ankle pathology.

The FBT exhibits a range of morphological patterns, classified into four main types based on distal insertion, as described by Olewnik et al. (Olewnik et al., 2020) and further elaborated in fetal and adult anatomical studies.

• Type I: Single distal attachment to the tuberosity of the fifth metatarsal.

Additionally, the FDQ tendon occurs in approximately 18% of limbs and is closely associated with Type I FBT, with its own subclassification based on insertion site (e.g., proximal phalanx, extensor aponeurosis, extensor hallucis longus tendon fusion).

The classification integrates ontogenetic insights from fetal studies, where early developmental patterns often regress or remodel into adult configurations (Olewnik et al., 2023). Type II variants in adults likely reflect incomplete postnatal remodeling or persistence of accessory bands originally present in fetal Type II or III patterns. From a phylogenetic standpoint, FBT variability mirrors adaptations across species: hypertrophy in arboreal primates, fusion or reduction in ungulates and cursorial mammals, and vestigial absence in certain rodent taxa (Diogo, 2005; Diogo, 2007; Diogo, 2010; Diogo et al., 2010a; Heilmann, 1926).

Anatomical variants of FBT and FDQ are clinically significant. Bifid tendons may be mistaken for partial tears or soft-tissue masses on MRI or United States (Bianchi et al., 2010; Bianchi et al., 2007; Sookur et al., 2008). FDQ can mimic anomalous muscle bellies or accessory tendons. Awareness of these patterns is essential in preoperative imaging and intraoperative navigation, particularly in reconstructive procedures or tendon transfers involving the peroneal group. Variant configurations may also increase the risk of tenosynovitis or nerve entrapment due to altered biomechanics or fascial crowding.

To consolidate morphological, developmental, evolutionary, imaging, and clinical data, we propose a unified table (Table 4) summarizing each FBT and FDQ type. This integrative tool provides.

• Anatomical description and frequency

Such a classification framework can enhance anatomical education, improve diagnostic accuracy, and guide surgeons in pre- and intraoperative decision-making–Table 4.