The aim of this clinical trial is to evaluate the accuracy of digital implant impressions with or without prefabricated landmarks between scan bodies compared to conventional impression technique in mandibular full-arch implant cases.

Implant-supported full arch dental prostheses are crucial and reliable alternatives for individuals with missing teeth in their jaws. The initial phase in the production of implant-supported prostheses involves the impression-taking process. Inaccurate impressions can result in a misfit between the prosthesis and implant abutment, potentially causing both mechanical and biological complications that can adversely affect the long-term viability of the implants. Therefore, the precision of impressions is of utmost importance, particularly for prostheses supported by multiple implants.

The conventional approach to obtaining impressions for multiple implants involves the use of an open-tray technique, widely adopted in dental practice. In this method, impression copings are connected to prevent rotational movements, making it a preferable choice compared to the non-splinting technique. Nevertheless, this procedure is complex, less efficient, and demands considerable clinical expertise. It is advised for use by experienced dentists, as those lacking experience have shown a notably higher rate of unsuccessful outcomes when employing the splinting technique.6 Additionally, various factors, such as patients' pharyngeal reflex and the potential deformation of impression materials, can influence the accuracy of implant impressions in the conventional workflow.

With the advancement of computer-aided design/computer-aided manufacturing (CAD/CAM) and digital technologies, the intraoral scanner (IOS) has gained popularity for capturing implant positions. In clinical practice, IOSs are valued for their comfort, efficiency, and practicality. However, it's important to note that IOSs cannot completely replace traditional methods. When it comes to implant-supported fixed dental prostheses, intraoral scanners can effectively be used for single- and three-unit implant scanning.

In a study from 2020, Revell et al. recommended specific scanners, such as Primescan, for complete-arch implant impressions due to their low deviation values at the implant platform level. Nevertheless, the accuracy of older models of intraoral scanners may not be reliable when scanning edentulous spaces with long inter-implant distances or fully edentulous jaws, particularly in the hands of inexperienced operators.

Previous research has introduced custom-designed scan bodies featuring an extensional structure to achieve dependable digital full-arch implant impressions. Typically, these platforms necessitate the use of custom-made scan bodies or a secondary scan to obtain mucosal information. In this context, this study is conducted to present newly designed prefabricated landmarks between the scan bodies to offer a solution for digital scanning of a mandibular full-arch implant. Therefore, the aim of this study is to evaluate and compare the accuracy of digital intraoral scanning with or without prefabricated landmarks to conventional impression techniques in mandibular full-arch implant cases.

The null hypothesis posits that there would be no significant difference in accuracy between digital intraoral scanning with or without prefabricated landmarks between scan bodies and conventional impression techniques.

Inclusion Criteria:

• patients with completely edentulous mandible, non-smokers, 50-70 years in age, have Good oral hygiene and motivation.

Exclusion Criteria:

• patients with major systemic diseases that may affect osseointegration as uncontrolled diabetes mellitus, the need for extensive bone grafting in planned implant site, pregnancy, patients under bisphosphonate treatment, and limited mouth-opening for executing the guided implant surgery.