Photoelastic Evaluation of Reduced Alveolar Support and Elastic Space Closure

Objective: Orthodontic appliance treatment may require space closure in areas of reduced periodontal support. The selection of closing elastics may be significant for areas of compromised periodontal support. A reduction in active forces has been recommended. The purpose of this study is to compare space closure mechanics in conditions of reduced periodontal support. Methods: Two dentate photoelastic mandibular models with idealized tooth positions were fabricated. Model-1 simulated normal bone height (NBH). Model-2 reduced periodontal support (RPS) by 2.5mm in the mandibular anterior region. Orthodontic brackets were bonded from first premolar to first premolar. Intermodel bracket placement was confirmed using a resin-positioning stent, ensuring uniform idealized bracket placement. Using a round nickel-titanium archwire, individual elastic ligatures were used for the non-closing condition, then open power-chain elastics (OCE), followed by closed power-chain elastics (CCE). The resulting stresses were observed and photographed in a circular polariscope. Results: Models were photographed with and without individual ligatures to record appliance passivity in a non-closing condition. Comparison of CCE and OCE elastics demonstrated higher closing force between both the tooth root areas and surrounding simulated bone apically for closed elastic condition. Comparison of NBH and RPS models with identical closed chain elastics showed significantly more resulting forces in RPS. The OCE with RPS demonstrated lower, more even stress distribution than CCE. RPS created a higher level of predicted stress distribution for teeth with attached orthodontic appliances. The OCE condition showed lower order stress distribution for a space closure condition compared to CCE. Conclusions: The OCE demonstrated a lower order of closing force to attached teeth. In situations of reduced osseous support, the open elastic material may provide an even level of space closing force. Proper periodontal diagnosis may determine clinical appliance design and prescription.