quadratus labii superioris muscle

2 min read 19-03-2025

The quadratus labii superioris muscle, often overlooked, plays a crucial role in facial expression and lip movement. Understanding its anatomy, function, and clinical implications is important for healthcare professionals and anyone interested in facial anatomy and aesthetics. This article delves into the details of this vital muscle.

Anatomy of the Quadratus Labii Superioris Muscle

The quadratus labii superioris (QLS) is a complex muscle, often divided into three distinct parts based on its origin and insertion points:

1. Zygomaticus Minor: This portion originates from the zygomatic bone (cheekbone) and inserts into the upper lip, just lateral to the nasal ala (nostril). Its function is primarily elevating the upper lip.

2. Infratrochlear Part: This smaller part arises from the frontal process of the maxilla and inserts into the upper lateral part of the lip. Its contraction elevates the upper lip and contributes to nasal flaring.

3. Angular Head: Originating from the maxilla above the incisive fossa, this head inserts into the upper lip near the midline. It is responsible for elevating the upper lip and deepening the nasolabial fold (the crease between the nose and mouth).

The QLS is innervated by branches of the facial nerve (CN VII), specifically the zygomatic and buccal branches. These branches provide the motor innervation necessary for its various functions.

Function of the Quadratus Labii Superioris Muscle

The primary function of the quadratus labii superioris muscle is to elevate the upper lip. However, its contribution extends beyond this simple action:

Elevating the upper lip: This is the most prominent function, crucial for various facial expressions, including smiling and expressing surprise.
Nasal flaring: The infratrochlear part contributes to the widening of the nostrils, often seen during heavy breathing or intense emotion.
Deepening the nasolabial fold: The contraction of the angular head causes a deepening of the crease between the nose and upper lip. This contributes significantly to facial expressions of displeasure or disgust.
Synergistic action: The QLS works in conjunction with other muscles around the mouth, such as the levator labii superioris alaeque nasi and the zygomaticus major, to create a wide range of facial expressions.

Clinical Significance of Quadratus Labii Superioris Muscle Dysfunction

Problems with the quadratus labii superioris, while not always life-threatening, can affect facial expression and lead to cosmetic concerns. Here's what to consider:

Facial Nerve Palsy (Bell's Palsy): Damage to the facial nerve can result in paralysis or weakness of the QLS, leading to an inability to elevate the upper lip properly. This can significantly alter facial appearance and expression.
Surgical Considerations: Surgeons need to have a precise knowledge of the QLS during facial reconstructive surgery or procedures involving the upper lip. Damage to this muscle can lead to functional and cosmetic problems.
Botox Injections: Botox injections targeting the QLS can be used to reduce the appearance of nasolabial folds, however, precise injection technique is crucial to avoid unwanted side effects.
Cosmetic concerns: Weakness or asymmetry in the QLS can contribute to an uneven or less expressive upper lip.

Conclusion

The seemingly small quadratus labii superioris muscle is a complex and important component of facial expression and lip movement. Its anatomical variations and synergistic function with other muscles necessitate a thorough understanding for healthcare professionals involved in facial surgery, rehabilitation, and aesthetics. Understanding its function and potential for dysfunction helps in the diagnosis and treatment of related conditions and allows for informed decisions in cosmetic procedures.