Gingival Phenotype Classification: AAP/EFP 2018 Criteria for Thin vs Thick Biotypes

For decades, dentists used the term "gingival biotype," coined in the 1970s. However, in genetics and biology, a biotype represents a genetically fixed, immutable profile. This was scientifically inaccurate because dental surgeons regularly alter soft-tissue dimensions through grafting.

At the 2017 World Workshop in Chicago, the AAP and EFP officially adopted periodontal phenotype. Phenotype accurately describes the observable physical characteristics resulting from the interaction between genetic predisposition and environmental factors, acknowledging that clinicians can modify tissue thickness surgically.

The 2017 AAP/EFP World Workshop standardized the terminology of periodontal phenotype, recognizing it as a composite of gingival thickness, keratinized tissue width, and alveolar bone morphotype. Clinicians assess phenotype objectively using standard periodontal probe transparency through the sulcular margin.

The 2018 classification stratifies periodontal architecture into three primary categories: Thin Scalloped, Thick Flat, and Thick Scalloped.

The Thin Scalloped phenotype features delicate, translucent soft tissue (thickness < 1.0 mm), a narrow zone of keratinized tissue, pronounced architectural scalloping, triangular tooth crowns, and a paper-thin underlying cortical bone plate. The Thick Flat phenotype features dense, fibrous tissue (thickness ≥ 1.0 mm), a broad keratinized band, flatter gingival contours, square tooth crowns, and a thick cortical plate.

Patients exhibiting a thin scalloped phenotype possess delicate, highly translucent gingiva, narrow zones of keratinized tissue, and thin, scalloped cortical bone plates. In contrast, thick flat phenotypes feature dense, fibrotic tissue, broad keratinized collars, and thick, square alveolar bone architecture.

To determine phenotype chairside, clinicians rely on the probe transparency method: inserting a periodontal probe into the sulcus. If the probe outline shines through the tissue, it is thin (< 1 mm); if invisible, it is thick (≥ 1 mm). High-frequency ultrasound and CBCT can also measure thickness to the tenth of a millimeter.

Identifying a thin phenotype allows periodontists to implement Phenotype Modification Therapy. By performing a connective tissue graft or placing a collagen matrix, the clinician thickens the delicate margin to > 1.5 mm, permanently converting a high-risk thin phenotype into a durable, recession-resistant thick phenotype.

Recognizing phenotypic characteristics is paramount before initiating implant placement, orthodontic movement, or cosmetic crown preparations. Thin phenotypes carry a significantly heightened risk of rapid recession and bone fenestration when subjected to surgical or mechanical trauma.

The 2017 World Workshop on Periodontal and Peri-Implant Diseases formally retired the historical term "biotype" in favor of "periodontal phenotype." This update reflects that while genetic factors establish baseline anatomy, the phenotype is dynamic and modifiable by environmental factors and clinical interventions.

The standardized classification assesses three core components: (1) Gingival Thickness (GT), clinically categorized as thin (≤ 1.0 mm) or thick (> 1.0 mm) using the probe transparency test or ultrasonic pulse-echo measurement; (2) Keratinized Tissue Width (KTW); and (3) Bone Morphotype, evaluated via CBCT.

Patients with a thin scalloped phenotype combine thin translucent gingiva, narrow KTW, and delicate underlying cortical bone with frequent dehiscences, representing the highest risk category for progressive recession under physical stress.

One of the most powerful capabilities of periodontal plastic surgery is phenotype conversion—the surgical transformation of a thin, vulnerable phenotype into a thick, protective tissue band. Placing a subepithelial connective tissue graft (SCTG) under the thin margin achieves this objective.

Histologic studies reveal that the donor connective tissue harvested from the palate retains its genetically programmed phenotypic memory. When grafted into the recipient bed, donor fibroblasts dictate epithelial differentiation, instructing the overlying mucosal cells to express cytokeratins characteristic of thick, parakeratinized gingiva.

This biological conversion permanently increases mucosal thickness beyond 1.5 to 2.0 mm, creating a dense collagenous barrier that resists mechanical toothbrush trauma and shields the underlying alveolar bone from inflammatory resorption.