The upper lip is more than just skin; it's a complex structure of muscle and specialized tissue that plays a key role in facial expression, eating, and speech. At its core is the orbicularis oris muscle, a circular band of muscle that surrounds the mouth. Think of it as a drawstring that allows you to pucker your lips, close your mouth, and hold in food and drink.

Several distinct features give the upper lip its characteristic shape. The colored part of the lip is called the vermilion, and its defined outer edge is the vermilion border. In the center of the upper lip, two subtle vertical ridges run from the base of the nose down to the lip. These are the philtral columns, and the grooved area between them is the philtrum. At the bottom of the philtrum, the vermilion border dips down to form a feature known as Cupid's bow.

These structures are supported by the nasal base, where the lip connects to the nose. The floor of the nostril (the nasal sill) and the sides of the nostrils (the alar bases) create a stable foundation for the lip. All these components work together seamlessly in a typical lip.

In a unilateral cleft lip, a gap or fissure appears on one side of the upper lip, failing to fuse during early pregnancy. This isn't just a simple split in the skin; it affects all the underlying structures.

The most significant change is to the orbicularis oris muscle. Instead of forming a continuous loop around the mouth, the muscle is interrupted at the cleft. The muscle fibers on both sides of the gap don't connect. Instead, they run vertically, parallel to the edge of the cleft, inserting into the nasal base and the skin along the fissure. This disruption prevents the muscle from functioning as an effective sphincter.

This anatomical change has both functional and aesthetic consequences. Functionally, the infant may struggle to form a proper seal around a nipple, making feeding difficult. Speech development can also be affected, as the lips are crucial for forming sounds like 'p,' 'b,' and 'm.'

Aesthetically, the cleft creates noticeable asymmetry. The Cupid's bow is pulled upward and distorted. The philtral column on the cleft side is either missing or rotated along with the muscle. Furthermore, the base of the nose on the affected side is often flattened and displaced, causing the nostril to appear wider and lower than the other.

A bilateral cleft lip involves two gaps, separating the central portion of the lip from the two side segments. This results in a more complex anatomical arrangement.

The orbicularis oris muscle is now in three distinct, unconnected segments. The two lateral (side) segments are separated from a central segment called the prolabium. The prolabium is the tissue that would normally form the philtrum, but in a bilateral cleft, it contains no muscle tissue. It's an isolated island of skin and connective tissue.

Because the muscle segments on either side are not anchored in the middle, they pull outwards, widening the clefts. The prolabium often appears small and can protrude forward, attached to a short columella (the tissue separating the nostrils). This lack of central muscle connection and anchorage leads to significant nasal deformity. The tip of the nose is typically very wide and flat, lacking definition and projection.

The functional challenges are often greater than with a unilateral cleft due to the complete separation of the muscle. Creating a seal for feeding is very difficult. Aesthetically, the symmetry of the central face is profoundly affected, involving not just the lip but the entire base of the nose.