UCLA

Maxillary palatal expansion has several relevant uses, including correction of transverse discrepancies and maxillary constriction, improving masticatory function, and improving the airway volume of the nasopharynx. Mid-palatal expansion can be completed by splitting the median suture with either a bone-borne or a tooth-borne expander. If the bone filled in is adequate, a permanent change in the transverse dimension of the maxilla can benefit the patient.

It has been noted that retention time plays a major role in the amount of relapse that takes place. A period of 90 days, or roughly 3 months, has been recommended for retention to allow for bone to adequately fill in. However, bony filling time can vary from individual to individual; for example, males and females have different rates of bone growth and bone densities.

To study the effects of different retention times, 28 Sprague-Dawley rats were divided into 10 groups (control n=2 male, control n=2 female, 1 week expansion male n=3, 1 week expansion female n=3, 1 week retention male n=3, 1 week retention female n=3, 2 week retention female n=3, 2 week retention male n=3, 4 week retention female n=3, 4 week retention male n=3). Rats were given a helical spring expander, placed to expand the maxillary incisors. After 7 days of expansion, 6 rats were sacrificed (3 male, 3 female) and the remaining rats had their expanders converted into retention devices. 6 rats (3 male, 3 female) were sacrificed at 1 week retention, 2 week retention, and 4 week retention each to observe bony changes. The bone volume was quantified by microCT analysis and histological analyses were completed to quantify mineralized area.

The relapse ratio was not significant, as the retentive device held the expansion adequately. It is noted that while bone mineral density (BMD) did not have any significant differences among groups, bone volume over total volume (BV/TV) and mineralized area over total area (MA/TA) were significantly different between the first and second weeks of retention. By week 4 retention, BV/TV and MA/TA values were at their highest.