the Objective of This Research is to Compare the Clinical Effectiveness of the Conventional Lingual Arch Space Maintainers With Three-dimensional Printed Space Maintainers.

Inclusion Criteria: * Apparently, healthy cooperative children have recently prematurely loss of more than one mandibular primary molar in any of the quadrants, full erupted mandibular first permanent molars in both sides, and whose lower permanent incisors have erupted * The permanent successors of the lost primary molars were not expected to erupt within 6 months * Angle's class 1 occlusion with normal primary molar relation * Caries-free and periodontally healthy abutment teeth * Absence of parafunctional habits or abnormal occlusion conditions such as crossbite, open bite, or deep bite. * Children's age ranges from 7 to 9 years-old * Radiographically, presence of permanent successors, more than one millimeter of bone covering the successor tooth germ, and/or only about one-third of the root of the permanent tooth has been formed. * Parents and children were accepting the new treatment modality. Exclusion Criteria: * children exhibited poor oral hygiene, para-functional oral habits, severe crowding, space loss, and abnormal occlusion * Extraction of primary molars exceeding three months. * Radiographically, absence of permanent successor. * uncooperative children