A dimple is a small, natural indentation found in the flesh, most commonly on the cheeks or chin. These features are scientifically defined as depressions in the skin caused by underlying anatomical variations. The specific causes and locations of these indentations vary significantly, leading to a nuanced understanding of what constitutes a dimple.
The Underlying Biological Mechanism of Dimples
The formation of the most recognized type, the cheek dimple, is rooted in a variation of the Zygomaticus Major facial muscle. This muscle typically functions as a single bundle to pull the corner of the mouth upward when a person smiles. In individuals with dimples, the muscle often presents as a double or bifid structure, split into two distinct bundles as it extends toward the mouth.
This muscular variation causes strands of connective tissue, or fascia, to insert into the overlying skin of the cheek. When the Zygomaticus Major muscle contracts during a facial expression, such as smiling, the tension created by the tethering fascia pulls the dermis inward. This creates the characteristic indentation. Dimples present only during muscle movement are described as dynamic, contrasting with static dimples visible even when the face is at rest.
Classifying Dimples by Location
Dimples are categorized primarily by their location, which indicates a difference in their underlying anatomical cause. Cheek dimples (Fovea Buccalis) are the most frequent type, resulting from the Zygomaticus Major muscle anomaly. These indentations usually appear around the corner of the mouth and can be unilateral (on one side) or bilateral (on both sides of the face).
The chin can also feature a distinct indentation, often referred to as a cleft chin or Fovea Mentalis. This Y-shaped fissure results from an incomplete fusion of the two halves of the lower jawbone (mandible) during embryonic development. Unlike cheek dimples, the chin dimple is a bone structure variation rather than a muscular one, and it is visible regardless of facial expression.
Moving away from the face, two symmetrical indentations on the lower back are known informally as the Dimples of Venus or Apollo (Fossae Lumbales Laterales). These benign indentations are located just above the gluteal cleft. They are formed by a short ligament stretching between the skin and the posterior superior iliac spine, a bony prominence of the pelvis. They are present in both men and women and are not caused by muscle tethering or bony fusion defects.
Genetic Basis and Inheritance
The inheritance pattern of dimples is more complex than often assumed, moving beyond the traditional textbook model of a simple dominant trait. While dimples tend to run in families, the trait is now considered an irregular dominant trait with variable penetrance. This means that while only one copy of a specific gene may be necessary for expression, other genetic or environmental factors significantly influence whether the dimple appears.
Current research suggests that dimples are likely a polygenic trait, meaning their expression is influenced by the interaction of multiple genes. This polygenic model explains the unpredictability of dimple inheritance, which is why a child may not have dimples even if both parents do. The genetic influence contributes to the anatomical variation in the facial muscles and connective tissue, the direct cause of the physical indentation.
When Indentations Require Medical Review
While most dimples are harmless cosmetic features, some indentations, particularly those on the lower back of infants, warrant medical review. A sacral dimple is a depression located just above the crease of the buttocks. These are common and usually benign, but they can occasionally be a cutaneous marker for an underlying spinal issue.
A medical review is recommended when a sacral dimple is deep, larger than five millimeters, positioned well above the gluteal crease, or accompanied by other skin abnormalities. Associated features that raise concern include:
- A tuft of hair.
- A skin tag.
- Skin discoloration near the site.
Such atypical markers may be linked to occult spinal dysraphism, including conditions like spina bifida occulta or tethered cord syndrome, where the spinal cord is abnormally attached to surrounding tissues. Medical professionals use the location, especially in relation to the gluteal crease, to determine if further diagnostic imaging is necessary.