Secondary Stabilization of Maxillary M-4 Treatment with Unstable Implants for Immediate Function: Biomechanical Considerations and Report of 10 Cases After 1 Year in Function

Purpose: Primary stability of dental implants, particularly when they are placed into immediate function in the maxilla, has been thought to be required. An alternative to primary stability is secondary stabilization, which can be obtained by a four-implant distribution pattern using 30-degree angulations for all four implants in the so-called "M-4" treatment scheme in combination with cross-arch stabilization from a prosthesis. If successful, the use of these two measures brings into question whether or not primary stability is required for immediate function in the maxilla. Materials and Methods: Patients were treated with the M-4 implant scheme with immediate function, despite the instability of at least one of the four implants. Instability was defined as less than 15 Ncm of insertion torque and palpable mobility, and an average anteroposterior spread of 15 mm between each implant was sought. The patients were followed for 1 year. Results: Ten patients were treated with a total of 40 implants. Composite insertion torque of the four implants was less than 100 Ncm in half of the patients; the average anteroposterior spread was 15.6 mm. After 1 year, no implants had been lost, and bone levels around all implants were at or near operative levels. There were no failures of provisional or definitive prostheses. Conclusions: M-4 distribution of implants with an average of 15 mm of anteroposterior spread and cross-arch stabilization did not require that all four implants had high insertion torque; in fact, all mobile implants stabilized and osseointegrated under these conditions. Keywords: All-on-Four, anteroposterior spread, composite insertion torque, cross-arch stabilization, immediate function maxilla, insertion torque, M-4, primary stability, secondary stabilization