This study included 228 patients, aged ≥ 65 years, who visited the Department of Oral Function Management, Showa University Dental Hospital, between June 2019 and December 2023 for an initial thorough oral function examination. The exclusion criteria were incomplete data and lack of consent for examination. Finally, 227 patients participated, of whom 111 and 116 were classified into the pre-dental implant group (those desiring dental implant treatment) and pre-bridge/denture group (those desiring prosthetic treatment other than dental implants) (Fig. 1). Consent was obtained from the study participants using the opt-out method. After a detailed explanation of the treatment options by the dentist in charge, the patient consulted with the dentist and selected his or her own treatment plan. This study was approved by the Ethical Review (DH2018-032).

Participant selection flowchart

Data on age, sex, comorbidities, smoking status, number of functional teeth, occlusal support status, oral hypofunction [encompassing oral hygiene, oral dryness, occlusal force, tongue-lip motor function (oral diadochokinesis), tongue pressure, masticatory function, and swallowing function], and the applicability of the diagnosis of oral hypofunction were collected from the patients’ medical records. Comorbidities were assessed using the Charlson Comorbidity Index (CCI) [19]. The presence or absence of psychiatric disorders other than hypertension and dementia, which are not included in the CCI but are common among the older and likely affect oral function (e.g., smoking), was also investigated. The number of remaining natural teeth, excluding those with mobility degree III and caries degree IV, and the number of functional teeth, which included remaining natural teeth plus prosthetic teeth (including pontics of bridges, dentures, and dental implants), were recorded. The occlusal support status, including the final dental implant prosthesis and pontics of already placed bridges, was evaluated using the Eichner classification. This classification, defined as a modified version of the Eichner classification, was applied to assess the occlusal support status.

Oral hypofunction was evaluated and diagnosed based on established guidelines [9]. Oral function was assessed and diagnosed by several well-trained and qualified dentists. All evaluators received 2 h of lecture and 2 h of hands-on training according to the manual to ensure consistency and accuracy in their evaluations. Oral hygiene was visually assessed to determine the degree of tongue coating using the Tongue Coating Index. The tongue surface was divided into nine sections, and the degree of tongue coating was evaluated at three levels (0, 1, 2) for each area. The sum of the scores for all areas ≥ 50% indicated poor oral hygiene. Oral dryness was assessed as the degree of mucosal wetness at the center of the dorsum of the tongue, approximately 10 mm from the apex of the tongue. An oral cavity moisture tester (Mucus; LIFE Corp., Saitama, Japan) was used for the measurements, and a special sensor cover was applied to the sensor to ensure uniform pressure contact. Measurements were performed thrice, and the median value was used for evaluation: a median value < 27.0 indicated oral dryness. Occlusal forces were measured using a pressure-sensitive sheet Dental Prescale II; GC (Dental Prescale II; GC Corp., Tokyo, Japan) and an analyzer (Bite Force Analyzer; GC Corp., Tokyo, Japan) for the entire dentition during clenching for 3 s in the occlusal–occipital fit position. A bite force < 500 N indicated reduced occlusal force. Tongue-lip motor functions were examined by asking patients to repeatedly pronounce /pa/, /ta/, and /ka/, and the number of repetitions per second was measured using an automatic measuring device (Kenko-kun Handy; Takei Kikai Kogyo Corp., Niigata, Japan). Patients were classified as having decreased tongue-lip motor functions if either repetition was < 6 per second. Tongue pressure was measured using a tongue pressure measuring device (JMS Corp., Hiroshima, Japan). Dentures were worn during the measurement. A maximum tongue pressure < 30 kPa indicates decreased tongue pressure. Masticatory function was assessed by measuring glucose concentration after chewing a gummy jelly. After 2 g of gummy jelly (Glucolam; GC Corp., Tokyo, Japan) was chewed for 20 s, 10 mL of water was added, and the gummy and water were discharged through a filter mesh. A glucose concentration < 100 mg/dL indicates decreased masticatory function. Swallowing function was assessed using a swallowing screening questionnaire (The 10-item Eating Assessment Tool). A total score ≥ 3 points indicates a deterioration in swallowing function. Oral hypofunction was diagnosed based on factors such as poor oral hygiene, oral dryness, reduced occlusal force, decreased tongue-lip motor function, reduced tongue pressure, decreased masticatory function, or deterioration in swallowing function, with a diagnosis made if three or more of these seven factors were present.

The t-test was used to compare the means of each of the seven continuous variables that were normalized using the Shapiro–Wilk test: age, CCI score, number of functional teeth, and means of each of the seven items in the detailed examination of oral function between the pre-implant and pre-bridge/denture treatment groups. Categorical variables, sex, occlusal support status, hypertension, mental disease, and smoking status were compared between the pre-implant and pre-bridge/denture groups using the chi-square test. Logistic regression analysis was conducted to clarify which factors were related to the presence or absence of a diagnosis of poor oral function and the presence or absence of each of the seven items of the detailed oral function test as objective variables. All statistical analyses were performed using IBM SPSS Statistics version 25.0 (IBM, Armonk, NY, USA), with a significance level of 5%.