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Risorius

The risorius muscle is a slender, bilateral muscle of facial expression situated superficially in the cheek region, overlying the buccinator muscle. It typically originates from the parotid fascia, the fascia over the masseter muscle, and sometimes the zygomatic arch or platysma, before inserting into the modiolus—a fibromuscular hub at the angle of the mouth. Innervated by the buccal branches of the facial nerve (cranial nerve VII), its primary action involves retracting the corner of the mouth laterally and slightly superiorly, facilitating expressions such as smiling, grinning, and laughing.^[1]^[2]^[3] Notable for its high anatomical variability, the risorius may be absent in approximately 4-5% of individuals or exhibit atypical fiber arrangements, such as dual bands originating from the sternocleidomastoid or masseter fascia, as observed in cadaveric studies. This muscle is unique to humans and African great apes among primates, underscoring its evolutionary role in complex facial mimicry. Functionally, it coordinates with adjacent muscles like the zygomaticus major and depressor anguli oris to produce nuanced oral movements, though its thin structure makes it challenging to visualize on standard imaging modalities such as CT or MRI.^[1]^[4]^[5] In clinical contexts, the risorius's variability holds implications for aesthetic and reconstructive procedures, including botulinum toxin injections for masseter hypertrophy, where inadvertent involvement can lead to asymmetric smiling or unnatural expressions. It may also contribute to diagnostic challenges in facial nerve palsies, such as Bell's palsy, where impaired lateral mouth retraction signals buccal branch dysfunction. Despite its modest size, the risorius plays a key role in the dynamic harmony of facial aesthetics and emotional communication.^[4]^[5]^[6]

Anatomy

Origin and Insertion

The risorius muscle is a thin, narrow, ribbon-like band of superficial facial musculature that runs horizontally across the cheek, typically forming a slender fascicle or broad fan-shaped structure that narrows toward its anterior end.^[7]^[1] Its morphology is highly variable, with inconsistent presence and bilateral asymmetry observed in anatomical dissections.^[7]^[1] The origins of the risorius are notably variable, arising from multiple fascial and muscular attachments in the lateral face. Common sites include the fascia overlying the parotid gland, the fascia covering the masseter muscle, and the lower fibers of the platysma muscle.^[7]^[1] Less frequently, it originates from the zygomatic arch or the fascia of the temporalis muscle, contributing to its inconsistent anatomical presentation.^[1]^[8] In contrast, the insertion of the risorius is consistent, attaching to the modiolus—a dense fibromuscular hub at the angle of the mouth—where it blends with fibers of the orbicularis oris and adjacent facial muscles.^[1]^[6] This insertion into the skin and underlying structures at the mouth's corner anchors the muscle's horizontal trajectory.^[7]^[9]

Innervation

The risorius muscle receives its primary motor innervation from the buccal branch of the facial nerve (cranial nerve VII), which originates within the parotid plexus formed by the intracranial facial nerve as it traverses the parotid gland.^[7] These motor fibers enable voluntary contraction of the muscle, facilitating its role in facial expressions such as smiling or grimacing by retracting the corner of the mouth.^[1] The buccal branch typically emerges from the anterior border of the parotid gland and courses superficially over the masseter muscle and buccal fat pad to reach the risorius, often flanking the muscle superiorly and inferiorly before inserting into its fascicles. There is some variability in innervation, with possible contributions from the marginal mandibular branch of the facial nerve in certain cases.^[7] Sensory innervation to the region overlying the risorius is provided by the buccal nerve (a branch of the mandibular division of the trigeminal nerve, CN V), but this does not directly affect the muscle's motor control.^[7]

Blood Supply

The risorius muscle primarily receives its arterial blood supply from the superior labial artery, a branch of the facial artery that arises from the external carotid artery and courses deep to the muscle before supplying it superficially near its insertion at the corner of the mouth.^[1]^[6] Secondary arterial contributions may come from the transverse facial artery, a branch of the superficial temporal artery, particularly at the muscle's origin from the fascia over the masseter.^[7]^[10] Venous drainage of the risorius parallels its arterial supply, with blood from the facial and transverse facial veins draining into the common facial vein and ultimately the internal jugular vein.^[7] The facial vein runs superficially along the anterior face, collecting tributaries from the risorius region before merging with the anterior division of the retromandibular vein.^[9] Lymphatic drainage follows similar pathways to the submandibular and buccal lymph nodes, though no unique lymphatic features are specific to this muscle.^[7]

Anatomical Relations

The risorius muscle occupies a superficial position within the facial musculature, lying deep to the skin and subcutaneous tissue of the cheek while being superficial to the masseter muscle posteriorly.^[7] This arrangement places it in the superficial musculoaponeurotic system (SMAS), facilitating its role in facial mobility without deeper interference.^[1] Positioned laterally to the buccinator muscle, the risorius is separated from it by the buccopharyngeal fascia, which underscores its more external location in the cheek wall. Medially to the parotid gland, it originates from the overlying fascia and maintains close spatial proximity to the parotid duct (Stensen's duct), which runs anteriorly and may course superficially across or near the risorius before piercing the buccinator.^[1] This relationship is clinically relevant for imaging and surgical approaches in the lateral face.^[7] At its anterior extent, the risorius blends with adjacent muscles at the modiolus near the angle of the mouth, interweaving superiorly with fibers of the zygomaticus major and inferiorly with the depressor anguli oris to form a fibromuscular confluence that supports coordinated lip retraction.^[1]

Anatomical Variations

The risorius muscle exhibits significant anatomical variability, including differences in presence, size, symmetry, and morphological form across individuals. Cadaveric studies indicate that the muscle is inconstant, with reported absence rates varying by population; for instance, it was identified in 90.4% of 52 hemifacial dissections from Persian cadavers, suggesting unilateral or bilateral absence in approximately 10% of cases.^[11] In contrast, a study of 80 Asian hemifaces found the risorius present in all specimens, highlighting potential ethnic differences in prevalence.^[12] The muscle is often absent in individuals of African descent and occurs infrequently among Chinese and Melanesian populations.^[13] Asymmetry between the left and right sides is common, with one side frequently more developed than the other, leading to interindividual differences in bilateral symmetry.^[7] Such asymmetries can influence facial expression but are typically not associated with pathology. Morphological variations include multiple slips or bands of muscle fibers; for example, a cadaveric case report described the risorius forming two distinct bands on one hemiface, arising from the masseteric fascia and inserting into the modiolus.^[4] Rare fusions with adjacent structures, such as the transversus nuchae muscle, have also been documented in isolated dissections. Classifications of risorius form based on relations to nearby muscles reveal further diversity, with three main types identified in Asian cadavers: platysma risorius (most prevalent at 45%), zygomaticus risorius, and triangularis risorius.^[12] These variations underscore the muscle's inconsistent anatomy, as confirmed by multiple cadaveric investigations.^[7]

Development

Embryological Origin

The risorius muscle derives from the mesoderm of the second pharyngeal (branchial) arch, which forms during the early stages of embryonic development.^[14] This arch emerges as a bulge on the lateral surface of the embryonic head around the fourth week of gestation, with its mesodermal core providing the progenitor cells for the muscles of facial expression, including the risorius.^[15] Development of these muscular elements continues through weeks 4 to 6, as mesenchymal premyoblasts thicken and begin to differentiate within the arch.^[14] As the embryo progresses, the precursor cells of the risorius migrate from the second pharyngeal arch into the developing facial regions, contributing to the superficial musculoaponeurotic system.^[14] This migration is influenced by cranial neural crest cells, which populate the arch mesoderm and regulate the patterning and differentiation of head muscles by providing signaling cues for myoblast specification and organization.^[16] Neural crest-derived ectomesenchyme interacts with the mesodermal progenitors, ensuring proper spatial arrangement and connective tissue integration during this phase.^[17] Innervation of the risorius establishes concurrently with arch development, as fibers from the facial nerve (cranial nerve VII), the nerve of the second pharyngeal arch, extend into the mesodermal core to supply the emerging muscle precursors.^[14] These motor axons grow alongside the migrating myoblasts, maintaining topographic connections that persist into adulthood.^[7] Initially, the facial expression muscles, including the risorius, form as a continuous sheet of five laminae per side within the second arch mesoderm, extending from temporal to mandibular regions around weeks 6 to 8.^[14] Segmentation of this muscular layer subsequently delineates individual muscles, with the risorius arising from the mandibular lamina as it differentiates into distinct bundles.^[14]

Postnatal Development

The risorius muscle, as part of the facial mimetic group, experiences postnatal growth in size and fiber maturation throughout childhood, aligning with the downward and forward expansion of the facial skeleton during subadult stages.^[18] Muscle fiber diameters in human skeletal tissues, including those of the face, increase progressively from approximately 10-12 microns in infancy to 40-60 microns by late adolescence, reflecting hypertrophy and maturation driven by neuromuscular activity and skeletal remodeling.^[19] This development enhances the muscle's capacity for coordinated contraction, building on its embryological foundations from the second pharyngeal arch mesoderm.^[7] In infancy, the risorius contributes to early facial expressions, particularly through its lateral pull on the mouth corners during emerging social smiles around 6-8 weeks of age, which facilitate parent-infant bonding and social development.^[20] These smiles, often involving the risorius alongside the zygomaticus major, signal positive engagement and support emotional reciprocity, with observational studies showing increased frequency and intensity by 3-4 months as neural control refines.^[21] Postnatally, the risorius exhibits high anatomical variability, with potential for asymmetry arising from genetic factors or environmental influences such as habitual unilateral facial movements or orthodontic interventions.^[7] For instance, repetitive asymmetric muscle use can lead to differential fiber development, resulting in observable imbalances in smile symmetry by adolescence.^[22] During aging, the risorius undergoes atrophy, thinning, and weakening similar to other mimetic muscles, with reduced bundle definition and increased surrounding connective tissue contributing to perioral sagging and loss of facial contour in the elderly. This progressive decline in muscle density and contractile force exacerbates gravitational effects on overlying skin and soft tissues.

Function

Role in Facial Expression

The risorius muscle serves as a key contributor to facial expressions by retracting the angle of the mouth laterally through its contraction in an outward and slightly upward direction, which produces a characteristic grin or snarl.^[7] This action stretches the lips horizontally, exposing the teeth in a manner distinct from other smiling muscles, and is often observed in expressions of amusement or disdain.^[1] In coordination with the zygomaticus major muscle, the risorius enhances broad smiling by further pulling the oral commissures outward and upward, amplifying the width of the smile while maintaining a tense or effortful quality to the expression.^[1] Bilateral contraction of the risorius thus supports a full, toothy grin, integrating with surrounding facial musculature to convey joy or laughter.^[7] The risorius also plays a role in non-verbal communication, where its activation can signal sarcasm or tension, particularly through subtle co-activations that create a mocking or strained appearance in the lower face.^[23] In the Facial Action Coding System (FACS), this corresponds to Action Unit 20 (lip stretcher), which involves the risorius often in combination with the platysma to produce such nuanced emotional displays.^[24] Unilateral contraction of the risorius results in lateral deviation of the mouth angle toward the active side, facilitating asymmetric expressions such as a smirk that may indicate irony or one-sided amusement.^[25] These actions are enabled by its innervation from the buccal branch of the facial nerve.^[7]

Biomechanics of Contraction

The risorius muscle generates a primarily horizontal pull vector on the modiolus, the dense fibromuscular confluence at the corner of the mouth, by contracting its superficial fibers that originate from the parotid fascia and insert laterally into this structure.^[7] This lateral retraction displaces the modiolus outward, contributing to mouth corner extension with minimal vertical component due to the muscle's near-horizontal orientation.^[1] Electromyographic studies indicate low force magnitudes during contraction, reflecting the muscle's limited cross-sectional area and patterns observed in similar perioral muscles like the zygomaticus major, where maximal voluntary contractions reach approximately 2 N.^[26] Surface electromyography (sEMG) reveals moderate activation of the risorius during voluntary smiling, characterized by cocontraction with the zygomaticus major to amplify lateral mouth corner displacement, whereas grimacing involves notable cocontraction with other lower facial muscles.^[27] The risorius acts synergistically with the zygomaticus major and minor during smiling, where its horizontal vector complements the oblique pull of the zygomaticus muscles, enhancing overall smile width without dominating vertical lift.^[27] However, its thin structure imposes biomechanical limitations, restricting its role to subtle modiolus retraction rather than forceful deformation.^[7]

Clinical Significance

Involvement in Neurological Disorders

The risorius muscle, innervated by the buccal branch of the facial nerve, can exhibit paralysis in conditions such as Bell's palsy or other facial nerve injuries, resulting in paresis or complete loss of function on the affected side.^[7] This leads to an inability to retract the angle of the mouth laterally during smiling, manifesting as facial asymmetry with a one-sided or drooping smile.^[7] In Bell's palsy, the most common cause of acute facial nerve palsy, the risorius is among the muscles affected due to inflammation or compression of the nerve, contributing to overall lower facial weakness and emotional expression deficits.^[28] Hemifacial spasm, a neurological disorder involving vascular compression of the facial nerve root, can cause involuntary spasms in the risorius muscle, leading to repetitive, paroxysmal contractions that distort the mouth corner outward and upward.^[29] These spasms typically progress from the orbicularis oculi to lower facial muscles, including the risorius, zygomaticus, and depressor anguli oris, resulting in unilateral facial twitching that worsens with stress or fatigue.^[30] Electromyography (EMG) of the risorius serves as a diagnostic tool to assess facial nerve integrity in neurological disorders, measuring muscle activity and nerve excitability to evaluate the extent of palsy or predict recovery.^[31] For instance, needle or surface EMG can detect denervation potentials or reduced motor unit recruitment in the risorius during voluntary smile attempts, aiding in the differentiation of peripheral nerve damage from central causes.^[32] Following facial nerve trauma or palsy, recovery of the risorius often involves axonal regeneration, but incomplete or aberrant reinnervation can lead to synkinesis, where unintended muscle contractions occur, such as involuntary mouth retraction during eye closure or smiling.^[33] This complication, observed in up to 30% of Bell's palsy cases with severe initial paralysis, alters normal facial dynamics and may persist if nerve fibers misroute during the 3-6 month regeneration period.^[33]

Applications in Surgery and Cosmetics

The risorius muscle is frequently targeted with botulinum toxin type A (BoNT-A) injections to address excessive gingival display in gummy smiles, where hyperactive contraction elevates the upper lip beyond 3-4 mm upon smiling.^[34] These injections, typically administered at 2-4 units per side into the risorius near the modiolus, weaken the muscle's pull and reduce smile height by 2-3 mm, with effects lasting 3-6 months.^[35] In cases of facial asymmetry, such as post-paralysis or congenital imbalances, selective BoNT-A weakening of the contralateral risorius helps symmetrize smile dynamics by mitigating overpull from the unaffected side.^[36] During facelift procedures, precise knowledge of the risorius's variable origins—superficial to the superficial musculoaponeurotic system (SMAS) in 58.7% of cases, both superficial and deep to SMAS in 34.8%, or solely from the masseter tendon in 6.5%—guides safe SMAS dissection to preserve the muscle and avoid inadvertent facial nerve injury, which could impair smile function.^[37] Similarly, in parotidectomy for tumor excision, identifying risorius fibers within deep facial planes alongside the parotid duct and facial nerve branches minimizes iatrogenic damage, ensuring postoperative preservation of perioral mobility.^[37] In reconstructive surgery following tumor resection in the parotid or buccal regions, the risorius's function is often restored using free flaps, such as the serratus anterior flap, where muscle slips are configured to mimic risorius and zygomaticus actions for dynamic smile reanimation.^[38] These grafts, vascularized via the thoracodorsal system, integrate with recipient facial vessels and nerves to achieve satisfying dynamic smile reanimation in mid- and lower-face movements within 12-18 months post-transfer.^[38] Post-2020 advancements in minimally invasive neuromodulation for the risorius include ultrasound-guided BoNT-A injections, enhancing precision in targeting the thin muscle to treat hyperkinetic smiles or asymmetry while reducing diffusion risks to adjacent structures like the zygomaticus.^[34]

Comparative Anatomy

Evolutionary Aspects

The risorius muscle emerged as a specialized facial structure within the Homininae lineage, approximately coinciding with the divergence of the orangutan lineage from other great apes around 14-16 million years ago, as inferred from cranial morphology and muscle attachment sites in fossil primate skulls.^[39] This development is linked to the enhancement of complex social expressions in great apes, where the risorius contributes to subtle lip and mouth movements essential for nuanced facial signaling.^[40] In chimpanzees and gorillas, the muscle often appears as thin horizontal fibers integrating with adjacent structures, supporting displays that convey emotional states in group dynamics.^[40] The adaptive significance of the risorius lies in its role in non-verbal communication, facilitating social bonding through affiliative expressions like relaxed open-mouth faces and threat displays via bared-teeth gestures, which help maintain cohesion and hierarchy in primate societies.^[41] In great apes, these functions underscore the muscle's evolutionary value in promoting survival through visual cues that reduce aggression and foster alliances, with greater complexity observed in species exhibiting high sociality.^[41] Such adaptations highlight how facial musculature evolution prioritized communicative efficiency over mere locomotion or feeding in hominoid ancestors.^[39] Comparative analyses indicate that the risorius likely evolved from extensions of the platysma muscle. In primates, the platysma forms a broad superficial sheet across the neck and face, from which narrow, horizontally oriented slips like the risorius differentiate, particularly in hominoids, to enable precise lateral pulling of the mouth corners.^[39] This derivation is supported by homologous attachments and innervation patterns, reflecting a gradual specialization for facial mimicry rather than a de novo origin.^[40]

Presence in Non-Human Species

The risorius muscle is present and distinct primarily in chimpanzees (Pan troglodytes) among non-human great apes, where it typically appears as thin horizontal fibers originating from the orbicularis oris and depressor anguli oris, extending caudally toward the skin over the platysma.^[42] Dissection studies confirm its separation from the platysma in chimpanzees, though it is variable and not always well-defined, and note absence or rudimentary forms in gorillas (Gorilla gorilla) and orangutans (Pongo spp.), often without derivation from the platysma myoides or zygomaticus major.^[43]^[44] These structures support expressive functions akin to those in humans, such as lateral mouth movements during social displays.^[45] In contrast, the risorius is absent or rudimentary as a distinct muscle in most other mammals, including Old World monkeys such as macaques (Macaca spp.), where facial musculature lacks this specific formation and relies on other fibers for mouth retraction.^[43] Comparative analyses across primates indicate that while variable fibers resembling the risorius may appear in some hylobatids (e.g., gibbons), they are not consistently homologous or well-defined, highlighting its limited distribution beyond select hominoids.^[45] In non-primate mammals, such as carnivores, the risorius is generally lacking, but homologous platysma-derived slips contribute to snarling behaviors in species like dogs (Canis familiaris), enabling rapid lip retraction during threat displays.^[46] Primate dissection studies reveal correlations between risorius size and social complexity, with larger, more robust forms in species exhibiting extensive group interactions, such as chimpanzees in multi-male/multi-female communities, compared to smaller or absent variants in less socially complex taxa like gibbons in monogamous pairs.^[45] These findings, drawn from examinations of multiple specimens across genera, underscore how facial muscle elaboration aligns with phylogenetic and ecological factors influencing communication. Recent studies, including a 2024 analysis of gorilla anatomy, confirm the risorius's absence in gorillas, further emphasizing its specificity to the chimpanzee-human clade.^[43]^[44]

References

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### Risorius Muscle Summary
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