Ankylosed and submerged primary molar

In the absence of ankylosis, primary molars without permanent successors may function for many years before exfoliation, preserving alveolar bone height and width. If a periapical radiograph shows flat bone levels between the submerged primary molar and adjacent teeth, the tooth may be maintained, preserving alveolar bone until facial growth is complete, and an implant can be placed. In this case, the mesial and distal surfaces of the mandibular primary molar can be disked to achieve premolar width. The mesiodistal width at the cementoenamel junction measured on a bitewing or periapical radiograph provides a good guideline for the amount of reduction, as does comparison to the contralateral side. The average width of the mandibular second premolar is 7.5 mm. Seven millimeters has also been recommended as the width to attain. This size can be marked with a pencil or marking pen on the occlusal of the primary molar to provide a guide for reduction. After administration of local anesthesia and rubber dam placement, a carbide fissure or diamond bur may be used to remove interproximal enamel, avoiding pulpal exposure. The bur is maintained in a vertical position. A layer of composite may be added on the mesial and distal surfaces to prevent caries. The occlusal surface is built up with composite to achieve a level occlusal plane, preventing super eruption of the tooth in the upper arch. Alternatively, a stainless steel, preveneered or zirconia crown may be placed. 

Because the roots of the primary molar diverge beyond the width of the crown, there may be unwarranted concern that the space cannot be closed adequately with interproximal reduction. The socket wall of the adjacent permanent teeth moves closer and is eventually in contact with the roots of the primary molar, causing root resorption. Bone replaces the roots, creating an ideal site for future implant.

With vertical bone loss, extraction is indicated. An orthodontic consultation will assist in determining whether to maintain or close the space after extraction. If the decision is to maintain the space for a future restoration, the orthodontic goals will include establishing the appropriate amount of space for an implant and the preservation of the alveolar ridge.

Delaying extraction of an ankylosed primary molar, until after orthodontic treatment will often result in a vertical defect in the alveolar bone, especially if there has been significant vertical development of the surrounding alveolar ridge. Bone grafting most likely will be necessary, increasing treatment costs and difficulty placing an implant.

The timing for extraction in a growing child is critical. If early extraction is performed due to observed early submerging, the alveolar ridge usually moves occlusally with eruption of adjacent teeth as the periosteum is stretched over the extraction site. Vertical crestal bony defects are prevented. Early extraction of the ankylosed mandibular second primary molar is preferred over late.

If substantial bone resorption with significant buccolingual narrowing of the alveolar ridge and loss of
vertical height occurs after extraction, the first premolar may be orthodontically moved into the extraction site. This movement will provide a suitable ridge for a single-tooth implant for the first premolar after facial growth is complete. An osseointegrated implant is biologically conservative and the first choice for a congenitally missing mandibular premolar. An implant can be placed after vertical facial growth is complete, confirmed by serial cephalometric superimpositions. On an average, girls’ facial growth may occur until the age of 17 years and boys’ until the age of 21 years. 

With early extraction, the width of the alveolar ridge may be reduced by 30% over 7 years. Although the ridges may still be wide enough for an implant, the implant must be placed toward the lingual because the ridge resorbs more on the facial side. The consequence is that the occlusion on the buccal and lingual cusps of the implant’s crown must be altered to avoid fracture of the abutment or the implant crown.

Appropriate management of infraoccluded mandibular primary molars is mandatory to preserve the alveolar bone and for future orthodontic and possible prosthetic considerations. Interdisciplinary collaboration is mandatory. Orthodontists should be given the opportunity to prepare and maintain alveolar bone throughout the mixed dentition. Inappropriate management will negatively impact orthodontic treatment, creating significant challenges, if the advantages and disadvantages of the approaches are not understood. Appropriate and timely management of infraoccluded primary molars will reduce treatment costs and prevent negative consequences that could last a lifetime.