Preserving the peri‑implant bone level is a key factor in determining the success of dental implants. One common risk factor for soft tissue inflammation and crestal bone loss (CBL) surrounding teeth and implants is chronic hyperglycemia [1,2]. For evaluating chronic glycemia, glycosylated haemoglobin A1c (HbA1c) is the most reliable method [3]. To reduce their risk of glucose intolerance, the American Diabetes Association advises most patients to keep their HbA1c levels below 7 %. When achieving HbA1c < 6.5 %, a more stringent objective is recommended, provided that there is no notable risk of hypoglycemia or other negative consequences. In addition, numerous studies have documented elevated complications associated with levels over 8 % [4,5].A type 2 diabetes mellitus (T2DM) that is well controlled is indicated by a HbA1c result of less than 6.5 %, whereas inadequately controlled T2DM is indicated by a number greater than 6.5 % [3,4].

One of the biggest global health issues is the rising prevalence of DM. The incidence and death rates of diabetes are quite similar to those in developed countries due to the significant changes in people's lifestyles in recent years, as well as rising urbanization, industrialization, and population aging in emerging nations. The number of T2DM incident cases worldwide increased by over 2 times between 1990 and 2019: from 8.4 million in 1990 to 21.7 million in 2019, while the number of deaths increased by more than twice, from 606,407 to 1.5 million [6], [7], [8].

For the most susceptible, it becomes increasingly important to evaluate the advantages and disadvantages of implant therapy. Implant treatment may be particularly beneficial to those with poor glycemic control who experience the greatest oral debilitation, tooth loss, and reduced masticatory function. Additionally, those with in poor glycemic control are more reliant on food and oral hygiene, which may prevent them from benefiting from dental implant therapy [9].

According to the literatures, success rates are higher in the mandibular jaw and are improved when the implants are anteriorly located in the inter-foraminal area of the mandible [10], [11], [12]. Regardless of glycemic state, implant treatment may provide significant improvements in quality of life for T2DM patients [9]. According to Herrero et al. [9], dental implant therapy provided significant improvements in the oral health related quality of life of mandibular 2 implant overdentures (OVDs) for T2DM individuals, and these results were comparable to those reported for healthy edentulous people and included people whose blood sugar is inadequately managed as well.

Although functional and psychological parameters can be met with 2 implants, 4 implants improve prosthesis retention and chewing ability, reduce post-operative sore spots and pain in the long term, and reduce the number of prosthetic complications as well as the deterioration of the attachment system's male component [13], [14], [15], [16], [17]. Theoretically, the larger the surface area for osseous integration and the greater size and number of implants inserted, the greater the chance of stable implants during restoration [18]. According to studies, as the implant number increased, decreased stress values were observed in peri‑implant bone and implants in the implant OVD prosthesis [19]. For adequate load distribution to the bone and fixtures themselves, the anterior-posterior spread should be more than 16 mm, and implants should be distributed along a curve or any arrangement other than a straight line[12,20,21]

Different cultures and nations have different preferences for implant-supported prostheses, either fixed or removable. Some literature indicates that individuals treated with fixed implant-supported prostheses are not as satisfied with their definitive prostheses as patients who receive removable implant-supported OVDs [22,23].Four implant-retained OVDs show comparable marginal bone loss and quality of life scores to 4 implant-supported fixed detachable prostheses using the ALL on Four concept. On the other hand, patients believe that OVDs are more affordable and simpler to maintain [24].

According to various studies, diabetes and implant failure, or peri‑implantitis, are independent [25], [26], [27].On the other hand, peri‑implantitis and CBL are more common in individuals with diabetes than in non-diabetic individuals, according to previous research [28], [29], [30]. Consequently, many individuals whose diabetes is not well-controlled may not be able to benefit from implant therapy.

The association between diabetes and the status of peri‑implant tissues has been extensively explored in the literature, with heterogeneous and controversial results, and further research is still needed, especially in the long term [26,[31], [32], [33], [34]]. Clinical studies that evaluate the long-term peri‑implant parameters of 4 inter-foraminal implant-retained mandibular OVDs in individuals with T2DM are lacking [5,35]. Therefore, the purpose of this retrospective study was to evaluate the survival rates and the long-term impact on clinical and radiological peri‑implant characteristics of 4 implant-retained mandibular OVDs. such as plaque index (PI), bleeding on probing (BOP), probing depth (PD), and CBL in individuals with T2DM who were in well- and inadequately controlled T2DM. The null hypothesis was that after long-term function of 4 implant-retained mandibular OVDs, in terms of clinical and radiological peri‑implant characteristics, there would be no discernible change.