Implants were placed 4–6 months after the grafting procedure. The implant installations were performed by different oral surgeons (not in the hospital) and different implant systems were used. The implants installed into the augmented bone were allowed to heal for an additional 4–6 months before loading.

The records of the original 69 patients were examined with regard to (1) grafting site (2), “graft-survival” determined by the ability to place implants in the grafted site(s) and (3) “implant survival” determined by the presence of functional implant-supported prostheses at the most recent follow-up. Reasons for implant failure were recorded when available.

A self-administered questionnaire (Additional file 1) was sent by post to all 59 patients, together with an information leaflet about the survey, a return envelope with prepaid postage and an informed consent form. Reminder letters were sent after 2 and 4 weeks if no response was received.

The questionnaire contained 36 previously validated questions, which were categorized and related to (1) demographic and lifestyle, (2) perceived general and oral health, (3) donor site and hospitalization, (4) implant and prosthesis, and (5) OHRQoL (OHIP-14) (Table 1). Responses to questions in categories 1–2 were recorded as “yes/no” or graded on a 3- to 5-point Likert scale [44]. Category 3 included information on the duration of hospitalization and sick leave. Category 4 included information on “graft survival,” i.e., whether implants (and prostheses) were delivered in the augmented site(s), and “implant survival,” i.e., the presence or “loss/loosening” of any implants after surgery. OHRQoL was assessed using a Norwegian version of the OHIP-14 [32]. These 14 questions addressed seven domains of OHRQoL and their responses were graded on a 5-point Likert scale ranging from “at no time” (0) to “all of the time” (4) (Table 1).

Data were anonymized and analyzed using SPSS v 24 (SPSS Inc., Chicago, IL, USA). Descriptive analyses were applied. Statistical significance was set at 5% level.