Digital MARPE appliance design around your miniscrew positions

The miniscrews of a MARPE appliance are placed into the palate, and the decision of where they go depends on what is beneath the mucosa: bone thickness, the position of the suture, the roots of the adjacent teeth, the nasal floor and the greater palatine vessels. That is a clinical judgement made from the CBCT, and it belongs with the treating clinician. The design service does not choose the anchor sites.

What the service does is make the marking precise and the consequences visible. The CBCT is aligned to the intraoral scan so that a position marked on the model corresponds to a known point in the imaging. The insertion axis and depth you specify are recorded with the position, and the guide slot in the appliance is built to that axis so that the miniscrew is inserted the way you planned it.

Where a marked position raises a question from the imaging, for instance thin bone at a paramedian site or proximity to a root, it is reported to you with the reason. The position is changed only when you change it.

In a tooth-borne expander the force path runs from the screw through the arms into the anchor teeth. In a MARPE appliance it runs through the miniscrews into the palatal bone, and in a hybrid design through both. The arrangement of the body and arms follows from that: the expansion screw is positioned relative to the anchor sites so that the halves of the appliance carry the miniscrews with them as they separate, and the arms are dimensioned for the load they carry.

Expansion screws differ in body length, guide pin arrangement and range, and the miniscrews differ in head design and driver. Both are specified by you and the design is built around the actual parts, so that the guide slots fit the miniscrew heads and the screw body sits where the arms expect it. The fabrication files state the specifications so the laboratory fits the same components.

Access is designed as a component in its own right. Each guide slot has clearance for the driver at the insertion angle, and the expansion screw's key access is checked with the appliance seated and the lower arch in occlusion, because an appliance that cannot be activated in the mouth has not been designed.

A bone-borne appliance sits close to the palate by definition, and the difference between an appliance that is tolerated and one that ulcerates is a matter of a millimetre or two. The body is designed to the vault with the clearance you prescribe, relieved over the rugae, and shaped so that the only points of contact with the mucosa are the guide slots at the anchor sites, where the miniscrews pass through into bone.

Contact simulation is run at each miniscrew position and through the expansion range. At each anchor it confirms that the guide slot meets the palate at the marked position and angle, with the clearance the miniscrew head and driver need. Across the range it confirms that as the halves separate and the arms swing, no part of the body approaches the teeth, the palate or the opposing arch. Any contact is reported with the position and the activation at which it occurs.

The insertion instructions that accompany the files are written from the same geometry: the order in which the anchors are placed, the axis for each, the driver access and what to verify before the expansion screw is first activated.