Dental implant treatment has been frequently applied in dental practice as the most important prosthodontic procedure with long-term predictability to restore oral function, maintain occlusion, and improve the quality of life (QoL) of a patient [1]. Clinically, dental implants are mainly applied to correct mandibular distally extended edentulism [2]. However, implant placement in the molar region of a mandible occasionally has some anatomical difficulties, such as lingual concavity of a mandible, small distance to the mandibular canal, insufficient space between the alveolar ridge and opposing teeth, and lack of keratinized mucosa. Especially, lingual concavities in an edentulous mandible appear to be related to risk of perforation in the lingual cortical bone during dental implant insertion, which may lead to hemorrhages or infections in the parapharyngeal space [3, 4]. Lingual concavities have a prevalence of 68% in the molar region and occur at a significantly higher rate in the second molar (90%) than in the first molar (56%) [5]. To avoid these risks, implants may not always be suitable to repair second molar defects.

Dental implant treatment is also associated with higher initial costs in general [6, 7]. Therefore, a cost-effective treatment is desired. Removable partial dentures with a single posterior implant could be a possible treatment option in the case of inappropriate implant placement in the second molar region. However, the least amount of Oral Health Impact Profile improvement was observed in patients with removable partial dentures compared with patients with implant-supported fixed prostheses [8]. Thus, patients who desire a fixed prosthesis may not be satisfied with a removable overdenture.

The shortened dental arch (SDA) is known as an acceptable concept in natural dentition for its lower cost than restoration of missing teeth. Kayser et al. found no significant differences between subjects with SDA of three to five occlusal units (OUs) and those with complete dental arches with regard to masticatory ability, signs, and symptoms of temporomandibular disorders, migration of remaining teeth, periodontal support, and oral comfort [9,10,11]. Therefore, according to this concept, second molar defects may not need to be replaced. Fueki et al. identified that only 3% of subjects missing just the second molar(s) sought prosthetic treatment compared with 58% of subjects missing first and second molars [12]. Baba et al. investigated the relationship between patterns of missing OUs and oral health-related QoL in subjects with SDA and reported that a significant difference was observed between groups with and without first molar occlusal contact [13]. Although these studies have examined SDA in natural dentition, there are few studies on SDA including dental implants.