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Orthodontic relapse in dentitions with reduced periodontal support: an experimental study in dogs.

The purpose of the investigation was to assess whether supra-alveolar connective tissue formed following periodontal surgery carried out subsequent to orthodontic tooth movement is an effective barrier to relapse.

The experiments were carried out on five beagle dogs. At the start of the study (Day 0) the mandibular third premolars (3P and P3) were extracted and experimental periodontal breakdown was initiated around the lower fourth premolars (4P and P4) by placing cotton floss ligatures around the neck of the teeth.

After 180 days of plaque formation the cotton floss ligatures were removed. The diseased periodontal tissues around 4P were removed surgically and silver markers, serving as future reference points, were placed in the buccal bone plate close to the apex of the roots of 4P. At the same session amalgam fillings were placed in the buccal surface of the mesial and distal roots of 4P and P4. During the next 34 weeks (between Day 180 and Day 420), the dogs' teeth were brushed three times a week.

On Day 225 orthodontic appliances were inserted bilaterally to tip 4P and P4. Active orthodontic forces were used between Day 225 and Day 300. Between Day 300 and Day 360 the orthodontic appliances served as retainers for the moved teeth.

On Day 360 the periodontal tissues around P4 were removed surgically and silver reference points were placed into the bone as described earlier. On Day 360 the orthodontic appliances were removed. The dogs were sacrificed on Day 420.

The difference in position of 4P and P4 was measured on radiographs taken on days 300, 360 and 420 and it was found that (1) during the 'period of retention' the difference in position of 4P and P4 was close to zero and (2) during the ‘period of relapse’ the movement of 4P and P4 was small and the difference between the right and left jaws not significant.

The investigation demonstrated that no significant differences exist between supra-alveolar connective tissue formed prior to or after orthodontic tipping movements regarding the prevention of relapse.