Allograft Ridge Augmentation: 3D Analysis of CAD/CAM Custom Milled and Prefabricated Conventional Allogeneic Bone Blocks and Dental Implant Follow-up

To rehabilitate patients after the loss of their own teeth, dental implants are nowadays the therapy of choice if the indication is suitable. In order to successfully place implants in the patient's jaw, a sufficient supply of bone in vertical and horizontal dimensions is necessary. Human bone is generally always in the process of reconstruction and changes over time. If there is not enough substance left for rehabilitation with implants due to previous degradation of the alveolar bone, bone augmentation can be performed. For this purpose, mainly autogenous, xenogenic or allogenic bone material is used.

Bone augmentation using allografts shows satisfactory results in clinical use and has advantages such as elimination of morbidity associated with autogenous bone graft harvesting. In addition to the use of conventional and prefabricated graft blocks, it is now possible to produce individual allogeneic bone blocks using computer-aided design based on the patient's radiological data. After this bone has been placed in the patient, it can be restored with implants after a healing phase of about 6 months.

In order to make the treatment as predictable as possible, it is crucial to know how a bone augmentation changes over time and whether bone remodelling or resorption occurs. How allogeneic bone blocks behave with regard to their stability and treatment success has already been investigated in some studies, but to date there is no study that deals with the difference between prefabricated, conventional and individually manufactured CAD/CAM allogeneic bone blocks.

By retrospectively analysing existing clinical data, the aim is to compare how the two methods, prefabricated bone blocks and CAD/CAM-fabricated bone blocks, perform during the healing phase.

A virtual volume analysis method of 3D digital imaging data (cone beam computed tomography scans) will be performed, which allows not only linear punctual evaluation but also area and volumetric analysis. Furthermore, the in the grafts inserted dental implants will be examined in terms of survival rate, complication rate and marginal bone loss by evaluation of the available follow-up records of up to 12 years.

A clinical retrospective cohort study is planned in a non-invasive, observational study design.

The study will be conducted at a single university centre: the Department of Department of Stomatology (Periodontology), Faculty of Dentistry, University of Santiago de Compostela, Spain.

The study will be conducted in the following steps:

1. Identification of the patient population:

   The cohort for the retrospective study will be filtered from the patient data of the Department of Stomatology (Periodontology) of the University of Santiago de Compostela. Data from all patients who have received one of the following two treatments in the last 12 years will be selected and examined:
   * Group A: horizontal and/or vertical augmentation of Puros® allogeneic bone graft material (Zimmer Biomet, Winterthur, Switzerland) in the upper and/or lower jaw; type: conventional prefabricated bone blocks.
   * Group B: horizontal and/or vertical augmentation of allogeneic bone graft material Puros® (Zimmer Biomet, Winterthur, Switzerland) in the upper and/or lower jaw; Type: individually planned, CAD/CAM-produced bone blocks
2. Data extraction:

   The following clinical data of the patients are collected for retrospective analysis:
   * The patients anamnesis with age at the time of surgery, gender, smoking habits at the time of surgery and during the period of follow-up examinations up to 12 years after surgery
   * The patients treatment history: Statements about the treatment and follow-up examinations during the period up to 12 years after the intervention, statements about adverse events and complications during the intervention or in the period of follow-up examinations up to 12 years after surgery, intraoral clinical photographs

   The following radiological data of patients from both groups are used for evaluation: cone beam computed tomography data sets for volumetric evaluation and conventional x-rays for implant follow-up investigation. In detail the last CBCT scan before surgery (preoperative, t=0); the first CBCT scan directly after surgery (postoperative, t=1); the CBCT scan at 6 months after surgery (control, t=2); all periapical X-rays in the follow-up period of maximum 12 years after the surgery (Follow-up 1, Follow-up 2, Follow-up 3...) of the bone blocks and the therein inserted implants.

   To ensure that no conclusion be drawn about the identity of the patient, the data are pseudonymised by the investigating clinicians. The following type of pseudonymisation of the data takes place: the evaluation of the data is pseudonymised, i.e. the patient's name is replaced by another characteristic (number and letter, e.g. patient 1a, patient 2a, etc.). In this way, cases can be distinguished from patients and no conclusions can be drawn about patients.
3. Digital data analysis:

   Standard tesselation langugage (STL) data are evaluated linearly and also volumetrically. First, STL models are created from the patients' DICOM data. The pre-operative model serves as the baseline model for each patient. This data is imported into CAD analysis software (GOM Inspect, GOM GmbH, Braunschweig, Germany). A region of interest is determined for each patient by precisely defining the margins of each graft when the reference model (t=0) is merged on the postoperative (t=1) and control model (t=2). Subsequently, the discrepancy at the ROI surface of both baseline and follow-up models can be calculated and is evaluated both linearly and volumetrically.
4. Implant follow-up and analysis of mean marginal bone loss:

   The implant survival rate and complication rate (biological and technical) will be analysed with the patient history data available in the follow-up period of maximum 12 years after the intervention. Mean marginal bone loss will be measured in orthograde periapical x-rays mesial and distal of the implants installed in the allogenic grafts. The periapical x-rays after loading serve as baseline and the radiological bone loss in the maximum follow-up period available is measured.
5. Statistical analysis:

To compare the change in the overlapping CBCT data of the two groups A and B after surgery and at follow-up examinations, a descriptive analysis is performed using SPSS Statistics (Version 22, IBM Corp., Armonk, NY) for the sum of the deviations on the x-, y- and z-axes. Statistical analyses are performed at both patient and material level. For each test, p-values with a significance level of p\<0.05 will be used.

Eligibility Criteria:
Inclusion Criteria:

* Treated with the defined allogeneic bone substitute material
* Received the defined standardized intervention
* Number of missing teeth in the bone defect region ≥ 3
* 3 CBCT scans recorded and available: baseline before surgery (t=0), after graft surgery (t=1) and 6 months after surgery (t=2)

Exclusion Criteria:

* Treated with a different bone substitute material than the one defined for investigation
* Received an other intervention than the defined standardized procedure
* Number of missing teeth in the bone defect region \< 3
* One of the following CBCT scans not available: baseline before surgery (t=0), after graft surgery (t=1) and 6 months after surgery (t=2)