As passive fixed space maintainers do not replace the lost tooth and therefore do not provide a chewing function, this study aims to evaluate the efficiency of an innovative solution for maintaining space after the premature extraction of first primary molars using functional space maintainers based on CAD/CAM technology.

The innovative design of a functional space maintainer named KOS&MET (Key Orthodontic System and Materials Enhanced Therapy) was created using 3Shape Dental Designer (3Shape A/S, Copenhagen, Denmark). The functional maintainer features an intraoral window for monitoring the eruption of the successor tooth. In addition to its role in maintaining space, the maintainer is designed to restore bilateral chewing function. In addition to the loop encircling the second primary molar, the space maintainer includes a crown for the missing first primary molar. To achieve optimal anatomical details, we applied a mirror model of the molar from the contrary side.

Space maintainers were fabricated using CAD/CAM technology in two ways: 3D printing and milling.

1. 3D Print (Metal) - Additive Method: In this group, the functional space maintainer was fabricated from a metal alloy (Co 69%, Cr 25%, W 9.5%, Mo 3.5%, Si 1%, Scheftner, Germany) using a 3D printer (MySint100 Dual Laser, Italy).
2. Milling (Composite) - Subtractive Method: A dental milling machine (Wieland Zenotec Select Hybrid, Germany) was used to fabricate functional space maintainers from a polymer composite material (breCAM.HIPC, Bredent, Germany).

Inclusion Criteria:

• Indication for extraction or premature loss of the first primary molar in the upper or lower jaw.
• A prolonged time period from the loss of the primary tooth to the eruption of its successor (with bone tissue present above the successor tooth on the radiograph).
• A complete dental arch on the contrary side of the jaw.
• Intact or adequately restored vital second primary molars in the region indicated for the placement of a space maintainer.

Exclusion Criteria:

• Systemic disorders and uncooperative children.