The transition from primary teeth to permanent adult teeth is known as the mixed dentition stage. This process typically begins around age six when the roots of the baby teeth naturally dissolve, allowing the primary teeth to shed (exfoliation). Following exfoliation, the permanent successor tooth emerges through the gum tissue (eruption). A significant delay in either the shedding of a baby tooth or the eruption of a permanent tooth often indicates a physical or developmental obstruction requiring professional attention.
Why the Primary Tooth May Not Shed (Delayed Exfoliation)
For a primary tooth to shed successfully, its root structure must be progressively dismantled through root resorption. The permanent tooth developing beneath the primary tooth stimulates specialized cells called odontoclasts. These cells break down the baby tooth’s root and surrounding bone tissue. If this coordinated resorption process fails, the primary tooth remains anchored in the jaw, blocking the path of the emerging permanent tooth.
Dental ankylosis is a specific cause of failed exfoliation where the tooth root fuses directly to the surrounding alveolar bone. This fusion eliminates the periodontal ligament, which normally suspends the tooth within the socket and allows for natural movement. Ankylosed teeth cannot move and often appear “submerged” or lower than adjacent teeth because they fail to keep pace with jawbone growth. Ankylosis most commonly affects primary molars in the lower jaw, often following trauma or infection that damages the root surface.
Another reason for retained primary teeth is the congenital absence of the permanent successor tooth, known as hypodontia. If no permanent tooth germ develops underneath to trigger root resorption, the baby tooth can remain in place past its expected shedding date. Dental trauma, such as a blow to the face, can also damage the primary tooth’s root or the permanent bud beneath it. This damage may lead to scar tissue formation that prevents normal root breakdown and exfoliation.
Why the Permanent Tooth May Not Erupt (Physical Barriers)
If a primary tooth has exfoliated but the permanent tooth fails to emerge, the issue usually stems from a mechanical or spatial obstruction. A tooth blocked from emerging into its functional position is termed impacted, a common finding in delayed eruption cases. Impaction is frequent with maxillary canines, or “eye teeth,” which are the second most common teeth to become impacted after wisdom teeth.
Insufficient space within the dental arch, often called severe crowding, is a common physical barrier. If the jawbone is too small or the teeth are too large, the emerging permanent tooth lacks the necessary room to move into its correct position. The tooth may become trapped between the roots of adjacent teeth or diverted onto an abnormal eruption path, causing it to remain embedded in the bone. This lack of space often results in the tooth being angled incorrectly or positioned too high in the jaw.
Supernumerary teeth, or extra teeth beyond the normal count, also represent a mechanical barrier. The most frequent example is the mesiodens, a small, conical extra tooth that forms between the two upper central incisors. These structures physically obstruct the normal path of the permanent incisor, causing the permanent tooth to become displaced, rotated, or impacted. Supernumerary teeth are often discovered incidentally on routine dental radiographs and require removal to allow the permanent tooth to proceed.
The permanent tooth may also be growing in the wrong direction due to a developmental anomaly, even without a physical obstruction. If a tooth is positioned at a severe angle or oriented horizontally, the natural eruptive forces are insufficient to guide it through the surrounding bone and soft tissue. This abnormal position is a reason for impaction, requiring diagnosis with advanced imaging techniques like Cone Beam Computed Tomography to determine its exact location.
Navigating Diagnosis and Treatment
A delay in tooth replacement can be a normal variation, but specific markers suggest the need for professional evaluation. Dentists commonly use the “six-month rule,” advising an examination if a tooth on one side has erupted but the corresponding tooth on the opposite side is delayed by six months or more. An appointment is also appropriate if a primary tooth remains long after the expected exfoliation window, or if a gap has been present for a year without a permanent tooth appearing.
The initial diagnostic tool is the dental radiograph, or X-ray, which allows the dentist to visualize the unerupted permanent tooth and assess the primary roots. These images confirm the presence, position, and developmental stage of the permanent tooth, revealing blockages like supernumerary teeth or signs of ankylosis. For complex cases, a three-dimensional Cone Beam Computed Tomography scan may be used to determine the exact angle and relationship of the impacted tooth to adjacent structures.
Treatment for a retained primary tooth or an impacted permanent tooth is tailored to the specific cause. If a primary tooth is retained due to ankylosis or failed resorption, extraction is often necessary to clear the eruption path for the permanent tooth. If the permanent tooth is impacted due to a physical barrier, such as a supernumerary tooth, the obstruction is surgically removed.
For a deeply impacted permanent tooth, particularly a canine, treatment often involves a two-step process combining surgery and orthodontics. An oral surgeon performs a surgical exposure, uncovering the crown of the impacted tooth and bonding a small orthodontic bracket and chain to it. The orthodontist then uses this chain to apply light, continuous traction forces over several months, gradually guiding the tooth into its proper alignment.