The periosteum is a dense, fibrous membrane that covers the outer surface of all bones, except at the joint cartilage. Its outer fibrous layer, rich in collagen and fibroblasts, provides structural support, whereas the inner osteogenic layer contains progenitor cells capable of differentiating into osteoblasts and chondroblasts. Far from being a passive protective sheath, the periosteum functions as a biologically active interface between bone and its environment, playing a central role in bone development, remodeling, and repair throughout life.

The periosteum is critically involved in mechanotransduction, translating mechanical stimuli into cellular responses that drive skeletal adaptation. During growth, it contributes significantly to appositional bone growth by supplying osteoprogenitor cells that add new bone on the outer surface. In adults, it remains functionally active in adaptive remodeling, responding to mechanical stress and injury.

Because of its dense vascular network and abundant mesenchymal stem cells (MSCs), the periosteum exhibits remarkable regenerative potential and has been successfully applied in both preclinical and clinical studies for bone grafting, defect repair, and tissue-engineered periosteum. When transplanted into periodontal defects, periosteum generated alveolar bone in human subjects. In Beagle models, treatment with periosteal MSCs enhanced regeneration of alveolar bone, cementum, and PDL in periodontal defects, providing strong evidence that the periosteum may act as a compensatory osteogenic source capable of regenerating the full periodontal complex under suitable conditions.

Although traditionally viewed as separate entities, the PDL and the periosteum function as mechanically linked tissues during OTM, particularly in anterior regions in which the buccal cortical bone is thin and highly reactive ( Fig 1 ). Applied forces are transmitted across the alveolar bone and perceived by the PDL, periosteum, and osteocytes, each responding within distinct tissue contexts. The degree of periosteal engagement is strongly influenced by force direction.

• 1.

  Mesial-distal forces: primarily involve the PDL and endosteal bone within the alveolar socket, as seen during space closure after extractions, with minimal periosteal contribution unless cortical bone is affected. Consequently, mesiodistal OTM is generally well tolerated, even across edentulous ridges with substantial loss of alveolar height and width. However, clear limitations arise when incisors lie near or beyond the periosteal border of the alveolar cleft region before bone grafting.

• 2.

  Buccal-lingual forces: directly strain the outer cortical surface and may activate periosteum, especially when OTM approaches or exceeds cortical boundaries. In instances of overexpansion, periosteal response becomes critical.

• 3.

  Vertical forces: (1) extrusion elongates PDL fibers and transmits tensile strain to the alveolar crest, which may extend to the periosteum. Controlled orthodontic extrusion demonstrates coronal bone apposition, strongly suggesting periosteal involvement despite limited histologic confirmation; and (2) intrusion concentrates compressive forces within the apical PDL, typically inducing bundle bone remodeling without periosteal involvement. Periosteal activation may occur when roots approximate or penetrate thin cortical plates, particularly in anterior regions.

The periosteum at the maxillary buccal cortical bone surface: A, Thin buccal cortical bone and periodontal space on a sagittal dental radiograph; B, Buccal cortical bone with periosteum covering the outer surface ( red ) and bundle bone facing the PDL ( white ), magnification 25x; C, External fibrous layer ( red ) and internal cellular layers ( white ), magnification 40x (adapted with permission under CC BY from Consolaro ).

Evidence suggests that, under certain conditions, the PDL and the periosteum function as a coordinated entity in bone adaptation. Biologically, these mechanoresponsive tissues can be conceptualized as the PDL-periosteum axis.

In medicine, a biological axis describes coordinated, often reciprocal interactions between distinct yet functionally connected systems. Well-known examples include the gut-brain axis, describing bidirectional neural, hormonal, and immunologic communication between the gut and brain, and the bone-muscle axis, referring to the biochemical and mechanical crosstalk between muscle and bone.

Similarly, the PDL-periosteum axis represents a functional unit in which mechanical and biochemical signaling between the PDL and periosteum regulates alveolar remodeling, trauma repair, and periodontal regeneration.

The periosteum functions as both a mechanosensor and effector, detecting mechanical stress, recruiting progenitors, and driving osteogenesis. Mechanotransduction occurs through pathways such as Piezo1-mediated calcium influx, which converts mechanical forces into signals for bone remodeling, and microdeformations of alveolar bone activating periosteal MSCs. ^,

Although anatomically distinct, the PDL and the periosteum exhibit strong functional parallels. Both contain multipotent MSCs forming a multilevel mechanosensory network together with osteocytes, activating shared pathways including stretch-activated ion channels, integrin signaling, and transcriptional regulators (Runx2, driving osteoblast differentiation; and YAP [Yes-associated protein] and TAZ [transcriptional coactivator with PDZ-binding motif], mechanosensitive proteins guiding stem cells toward bone-forming cells). ^, Mechanical loading induces progenitor activation and paracrine signaling (eg, prostaglandin E ₂ , interleukin-6, tumor necrosis factor-α, and vascular endothelial growth factor), coordinating inflammatory and repair. Notably, YAP and TAZ nuclear translocation in both PDL and periosteal MSCs under cyclic loading suggests unified transcriptional responses.

Both tissues also share gene expression profiles related to extracellular matrix remodeling and osteoblastic recruitment. Periostin, preferentially expressed in both PDL and alveolar periosteum, is critical for matrix stabilization and osteogenesis, and PDL adaptation to orthodontic forces and tooth eruption. Together, these shared molecular pathways provide a mechanistic basis for viewing the PDL and periosteum as an interactive axis coordinating alveolar bone response to mechanical and biological cues.

Although the periosteum is well recognized for its role in trauma-induced bone healing, its physiological adaptation to static forces such as OTM is less understood. Notably, the interaction between the PDL and the periosteum appears reciprocal: PDL-derived signals activate periosteal MSC, whereas periosteal responses support PDL-driven remodeling, particularly near cortical boundaries. During buccolingual OTM, the periosteum modulates PDL-driven remodeling by sensing strain at cortical surfaces. Consequently, sclerostin in nearby osteocytes decreases, relieving wingless-related integration site inhibition, whereas PDL fibroblasts secrete parathyroid hormone-related protein, creating a proosteogenic environment.

In a rat model, maxillary expansion triggered periosteum-mediated inflammation, osteoclast activation, and cortical drifting, relocating the alveolar boundary—a process driven entirely by periosteal adaptation. Similarly an otherwise inactive periosteum can induce fibrogenic and osteogenic activity through controlled mechanical stimulation, promoting local bone formation. These observations confirm that the PDL-derived signals directly influence cortical periosteal responses and that the periosteum may act as a primary responder to orthodontic forces.

Nevertheless, experimental and clinical evidence indicates that, although buccal OTM can stimulate cortical apposition, net bone formation rarely exceed resorption, resulting in buccal bone thinning and dehiscence, indicating a limited compensatory capacity. Excessive expansion beyond the alveolar border can result in complete dehiscence, decreased bone thickness, and fusing of the PDL and periosteum, with the PDL, gingiva, and mucosa fusing into a single soft tissue layer. Surprisingly, gingival recession is rare, regardless of force magnitude ( Fig 2 ).

Complete bone dehiscence with PDL-periosteum fusion after excessive expansion in a rat model: A, A histomicrograph of a mesial molar root cross-section from a control rat, showing normal alveolar bone and periosteum ( black ); B, A mesial molar root after extreme expansion beyond the alveolar border, causing complete bone dehiscence and periosteum-PDL fusion ( yellow ); C, Almost complete bone dehiscence without gingival recession; D, With recession, magnification 10x. B, bone; T , tooth root; and P, PDL. (Reproduced with permission from Danz et al and adapted with permission under CC BY from Danz and Degen ).

These findings provide a histologic and functional basis to support the PDL and periosteum as a biological axis, reinforcing the mechanistic insights outlined previously.

The PDL exhibits a well-characterized, phase-specific response to orthodontic force, including acute inflammation, cytokine release, and recruitment of osteoclasts and osteoblasts. In contrast, the periosteum remains relatively underexplored in OTM. Functionally, it contains abundant osteoprogenitors and plays a pivotal role in cortical bone turnover, whereas the PDL’s regenerative capacity is largely confined to the tooth socket. This distinction becomes clinically important when OTM approaches or exceeds the alveolar envelope, a term often used without a clear biological definition. Here, it is defined as the anatomic and functional limits of alveolar bone, extending from the PDL to the outer periosteum, which constrain tooth movement. The PDL-periosteum axis coordinates alveolar socket and cortical adaptation, and movement beyond these limits risks dehiscence, fenestration, or periodontal compromise.

When periosteal osteogenic capacity is overwhelmed, tissue interface breakdown can occur. Clinical outcomes thus depend on the balance between tooth movement and periosteal compensatory bone formation, influenced by force regimen, tissue biotype, and vascularity. Therefore, periosteal activity acts as both a protective and limiting factor in OTM.

Despite its relevance, direct mechanistic links between orthodontic forces and periosteal remodeling are poorly defined. Given the PDL-osteocyte crosstalk and their regulation of periosteal MSCs-derived alveolar osteogenesis, osteocytes likely mediate the PDL-periosteum signaling. Although most periosteal activity follows cortical response, the periosteum can generate bone independently. Controlled periosteal distraction without corticotomy induces robust osteogenesis, significantly increasing bone thickness in rabbit models. This highlights its potential as an autonomous osteogenic driver. Local administration of synthetic parathyroid hormone-related protein analogs stimulates periosteal bone formation only when combined with orthodontic force, emphasizing that PDL-derived signals are indispensable. Targeting the periosteum may help counter cortical bone loss and enhance periodontal stability.