Biological Basis of Overdentures

Overdentures have aspects that make them superior to common mucous-supported dentures due to the retention of natural teeth as part of the denture support. The retained tooth roots not only have improved support and retention force for removable partial denture, but also the retained tooth roots have a unique role in maintaining alveolar bone height and retaining periodontal proprioceptors, and these features improve the denture's role in restoring masticatory function, etc. Meanwhile, the retention form of overdenture has a health care effect on the abutment teeth themselves, and the vertical force of abutment teeth improves periodontal condition of abutment teeth, and prolongs abutment teeth's lifespan. In addition, the reduction of extractions has an important effect on the psychological acceptance of the patient.

1. Retained roots improve denture support and retention

Retained roots are used as overdenture abutments, which have a much higher supportive effect than mucosa-supported dentures.

The height of the overdenture abutments varies, as does the amount of retention provided by the denture. If the overlying abutments are less above the gingiva, they play a lesser role in retaining the denture and mainly play a supporting role. The higher the overlying abutments, the greater the retention. If additional attachments are placed on the abutments, retention is further enhanced.

The number of overlying abutments and the degree of their dispersion also play a role in the retention and stability of the denture, and the same as the ordinary partial denture, the more abutments, the greater the retention force; the more dispersed the abutments, the better the stability of the denture.

2. Root retention helps maintain alveolar bone height

Root retention not only preserves the alveolar bone around the roots, but also slows down the resorption of the bone tissue between the roots.
Crum and Roney in 1978 used cranial X-rays to compare 8 patients wearing full dentures with 8 patients wearing full overdentures. After five years of follow-up, those wearing overdentures lost an average of 1.7 mm of bone compared with 5.2 mm in patients wearing full dentures.

Tallgren's seven-year study of patients wearing partial dentures found that the vertical loss of the alveolar bone surface around the mandibular natural teeth was only 0.8 mm, compared with 6.6 mm in patients wearing full mandibular dentures.

It is clear that the difference in the level of bone loss varies with the level of resorption of the natural teeth. The difference in the level of bone loss improved significantly with the retention of the natural teeth.Crum & Rooney (1978) reported a four-year study that found that retention of the mandibular cuspids for an overdenture helped preserve the existing alveolar ridge. In addition, patients with retained cusps had less reduction in alveolar ridge height in the intercuspal region than patients without any natural teeth.

3. Preservation of periodontal proprioceptors

Multiple studies have shown that patients wearing overdentures are better able to differentiate between different thicknesses of stainless steel wires, and that patients wearing overdentures have a higher perception of occlusal thickness than patients wearing full dentures.

Because patients treated with overdentures retain the periodontal ligament proprioceptors, the range and type of masticatory cycles in the masticatory gag reflex are more effectively controlled. The root dispersion receptivity under overdentures serves as a signal to prevent bone resorption and to regulate the physiologic loading system. Therefore, if the periodontal and dental conditions of the abutment teeth and economic conditions allow, supporting the overdenture with more roots will be more favorable for the patient to extend the life of the abutment teeth.

4. Healthcare effect of overdenture on abutment teeth

When making overdenture, it is necessary to significantly reduce the height of clinical crowns, so as to improve the crown-root ratio of abutment teeth.

5. Retaining the roots improves the function of the denture

Studies have shown that patients with overdentures chew slower and more efficiently than patients with full dentures when they exert the same amount of masticatory muscle force.

6. The psychological impact of root retention

Retaining some of the remaining teeth not only provides retention, but is also a great psychological relief to the patient. Even with only a single root, the patient does not become edentulous. Especially young adults or postmenopausal women feel unable to accept the removal of all remaining teeth for the use of a full denture due to emotional and social pressures. The degree of psychological improvement varies. For some patients, the emotional depression associated with tooth loss is severe. The need to keep a glass containing dentures by the bedside as they age is unacceptable to some. Replacing a fixed restoration with a removable denture restoration is already emotionally traumatic for the patient, so the psychological improvement of retaining a root cannot be ignored.