Oral rehabilitation in areas where bone width is insufficient is a complex issue. Insufficient bone width is common for edentulous patients, especially when alveolar fracturing occurs during dental extraction. When the bone loss results from a maxillofacial trauma, vertical dental root fracture, or from extensive periodontal/endodontic diseases, the effects are even more severe. These factors might result in insufficient vertical and horizontal support to install dental implants and may impair, or even limit, the options for prosthetic rehabilitation. In such cases, bone volume improvement has to be considered an effective alternative treatment.

Techniques that have been used successfully for the reconstruction of alveolar ridge bone volume include the use of onlay grafts harvested from the iliac crest, maxillary tuberosity, mandibular symphysis, or external oblique line. However, these procedures demand a second surgical site, which results in additional postoperative morbidity. Also, the receptor site often needs a healing time of 6–12 months before implant placement, and the risk of non-osseous integration of autogenous bone blocks is high. Guided bone regeneration (GBR) and osteogenic distraction are also adopted to improve the bone volume and enable fixed prosthetic rehabilitation. These two techniques also present potential disadvantages, such as tissue dehiscence, displacement or collapse of the membrane, inappropriate distraction vector, unpredictable bone resorption, and a delay prior to installation of the implants.

In 1986, Nentwig reported a bone crest division technique that simultaneously allowed the expansion of the alveolar crest and implant insertion. The surgical procedure divides the cortical bone crests, moving them to create an opening in the centre, which is then mainly occupied by simultaneously inserted implants. The remaining areas can be filled with biomaterials, autologous grafts, or autologous biological therapies such as plasma rich in growth factors. The main benefit of the split crest technique (SCT) is the simple, quick, and predictable way in which the alveolar atrophic crest can be expanded. This permits the use of bone grafts without the need for a second surgical site, thereby minimizing the risk of oedema, nerve injury, and pain.

Several adaptations and modifications of the SCT have been reported. Among these is the adoption of rotary and oscillatory instruments and, more recently, surgical ultrasound (US). The latter allows precise, clean, and smooth cutting of the bone tissue, with excellent visibility. It is also believed that the use of US could minimize the risk of complete fracture of the crests, which ultimately results in bone necrosis and implant loss. Complete fracturing of the cortical crest is more likely along the edges where the remaining bone is highly mineralized. The SCT is recommended in cases where the height of the alveolar ridge is acceptable, but its width is insufficient. Conversely, SCT should be contraindicated in those with atrophic ridges that lack elasticity due to a reduced volume of medullary bone tissue.

So far, two systematic reviews have investigated this technique for surgical division of the alveolar crest. Bassetti et al. evaluated the clinical, radiographic, and histological results of this technique in association with GBR. This review focused mainly on the crest bone loss, measured on the mesial and distal surfaces of the implant before or after loading. The review by Elnayef et al. focused on estimating the amount of horizontal bone gain promoted by SCT, in conjunction with implant placement, taking into account the flap type (partial-thickness flap or full-thickness flap). In addition, findings related to the role of the grafting material and/or membrane, intraoperative and postoperative biological complications, and the implant survival rate were also described. Complementary to these, Garcez-Filho et al. briefly presented an overview of the main clinical studies evaluating the SCT technique published from 1992 to 2007. These authors strongly believe that the SCT concept has raised great interest in recent years, especially due to the reduction in morbidity (no bone harvesting from a second surgical site, no risk of membrane exposure, and low risk of graft loss).

The present systematic review further examines the impact of the SCT on the bone volume obtained after separation of the cortical crests, and includes studies with implant installation simultaneous and posterior to the crest split. This review expands on the previous systematic reviews by (1) providing the most extensive quantitative assessment of bone volume gain to date, and (2) discussing the influence of methodological aspects on the gain in bone thickness. The latter was done specifically by investigating factors such as the type of surgical instrument used to separate the alveolar ridge. It was hypothesized that the use of surgical US instruments would be beneficial in terms of bone thickness gain and maintaining the peri-implant bone tissue compared to conventional instruments, because of the lower risk of surgical trauma.

Materials and methods

This systematic review followed the protocol for systematic reviews and meta-analyses outlined in the PRISMA statement. Two independent researchers (FF and JW) conducted an electronic search in the following databases: Cochrane Central Register of Controlled Trials (CENTRAL), ClinicalTrials.gov, Embase, PubMed/MEDLINE, Scopus, and Web of Science. The following medical subject heading (MeSH) terms indexed by MEDLINE were used: “split crest” OR “split-crest” OR “ridge expansions” OR “edentulous ridge expansion” AND “dental implants”. The search included articles published until October 2015. All articles were considered for review, regardless of language or publication year. The identification of duplicate articles in the databases was performed using EndNote Web reference manager software (Thomson Reuters, Philadelphia, PA, USA). An overview of the selection procedure is shown in Fig. 1 .

Flowchart of the systematic review.

A well-structured question in the PICO format was formulated to direct the literature search (P: population or problem of interest, I: intervention under investigation, C: comparison of interest, O: outcomes considered most important to measure results). The PICO strategy for the construction of the research question and evidence search was structured as follows: What is the expected increase in bone volume after the separation of cortical bone with crests of low thickness, but with sufficient height for regular implant placement? Is this volume increase related to the surgical instruments used for osteotomy?

After an independent reading of all titles and abstracts by two authors (FF and JW), the selected articles were read in full and included or excluded according to the criteria previously determined by the two reviewers. Articles that did not fully describe the inclusion criteria were retained at this stage. In addition, the reference lists of all manuscripts were hand-searched to check for additional papers not found through the database searches. Disagreements between the authors were resolved by consensus in a joint session. The assessment of the full-text articles retrieved after the initial screening was performed independently by the same two reviewers. Predefined data collection worksheets were used for the assessment of each selected publication.

Inclusion criteria encompassed the following: accessible articles applying the ‘split crest’ technique (surgical division of the alveolar ridge) in the maxilla, mandible, or both; study including at least five patients treated with the SCT; implant installation simultaneous or subsequent to the expansion of the alveolar crest. Outcome variables including the success rate, survival rate, and gain and/or loss of bone obtained after surgery (expressed as a percentage or in millimetres) were collected. Literature review articles were excluded, as were case reports ( n < 5), studies on cadavers or animals, laboratory studies, and articles that did not apply or evaluate the conventional surgical division of the alveolar crest.

The assessment of methodological quality for risk of bias was applied in order to check the strength of the scientific evidence in clinical decision-making. The classification of the polarization potential risk for each study was based on the criteria adopted by Clementini et al., namely random sample selection, definition of inclusion and exclusion criteria, reporting and monitoring the implant loss, validated measurements, and statistical analysis. Studies that included all of these criteria were classified as having a low risk of bias, those that did not include one of the criteria were classified as having a moderate risk of bias, and the remaining studies were assigned a high risk of bias.

The main characteristics of the studies and populations included in the systematic review are reported in Table 1 . The following information was collected from each of the included studies: study design, follow-up period, city/country and sample selection period, number of patients and implants, average age, type of implant, region in which they were installed, and the initial thickness of the alveolar ridge. The methodological characteristics of the surgical techniques used are presented in Table 2 ; these cover the intervention performed, instruments used to perform the osteotomy, associated biomaterials, evaluation methods, success criteria, bone width and height variation (millimetres), success rate, and implant survival rate.

Table 1

Characteristics of the studies included and their respective sample populations.

Study	Design Follow-up	Location and country Sample selection period	Number of patients (M/F)/implants	Average age (range) (years)	Jaw Region	Initial thickness of ridge (mm)
Crespi et al. 2015	Prospective 2 years	Department of Dentistry, San Raffaele Hospital, Italy January 2010 to May 2011	36 (13/23)/93	57.1 (36–71)	Max/Mnd Ant/Post	3.0 ± 0.8 (2.5–3.8)
Scarano et al. 2015	Prospective 3 months	University of Chieti-Pescara, Italy	32 (9/23)/64	57 (53–68)	Mnd Post	3.1 ± 0.6 (2.3–4.1)
Santagata et al. 2015	Prospective 3 years	University Hospital (AOU), Second University Naples, Italy January to November 2009	13 (6/7)/33	49.4 (32–68)	Max Ant/Post	4.7 (3.5–7)
Tang et al. 2015	Retrospective 3 years	Department of Oral Implants, School of Stomatology, Fourth Military Medical University, China 2004–2009	157 (92/65)/226	36.2 (17–74)	Max/Mnd Ant/Post	≥2.0
Garcez-Filho et al. 2015	Retrospective 10 years	Private practice, Brazil 2000–2002	21 (9/12)/40	55.5 (33–78)	Max Post	3.0–5.0
Abu Tair 2014	Retrospective 3 years	Oral and Maxillofacial Surgery Clinic, Israel 2007–2009	13/42	–	Mnd Post	2.0–4.0
Shibuya et al. 2014	Retrospective 2 years	Department of Oral and Maxillofacial Surgery, Kobe University Hospital, Japan April 2004 to March 2013	6 (1/5)/14	58.7 (25–71)	Mnd Ant/Post	3.4 (1.6–6.4)
Crespi et al. 2014	Prospective 2 years	Department of Dentistry, San Raffaele Hospital, Italy 2007–2009	46 (13/33)/118	53.8 (31–73)	Max/Mnd Ant/Post	2.0–3.5
Bassetti et al. 2013	Prospective 2 years	School of Dental Medicine, University of Bern, Switzerland Length: 30 months	7 (2/5)/17	57.9	Max/Mnd Ant/Post	≥2.0
Anitua et al. 2013	Retrospective 17 months	Private practice, Spain September 2007 to November 2008	15 (–15)/37	53.6 (19–72)	Max/Mnd Ant/Post	4.3 (1.8–6.2)
Rahpeyma et al. 2013	Prospective 6 months	Dental Research Centre of Mashhad University of Medical Science, Iran	25 (13/12)/82	50.2 (16–78)	Max/Mnd Ant/Post	3.0–4.0
Anitua et al. 2012	Retrospective 19 months	Private practice, Spain March 2008 to June 2009	6 (1/5)/9	61 (52–72)	Max Ant/Post	4.0 apical (1.5) and 3.0 occlusal (0.6)
Annibali et al. 2012	Retrospective 1 year	University of Rome, Italy May 2006 to January 2009	5/19	–	Max/Mnd Post	4.6 ± 1.3 (2.0–7.0)
Scarano et al. 2011	Prospective 3 months	University of Ferrara and University of Chieti-Pescara, Italy 2007–2009	22 (8/14)/44	59 (54–65)	Mnd Post	1.5–3.0
González-Garcia et al. 2011	Retrospective 2 years	Centre of Implantology and Oral and Maxillofacial Surgery CICOM, Spain	8/33	53 (38–69)	Max	3.0–4.0
Demetriades et al. 2011	Retrospective 2 years	Tufts University School of Dental Medicine, USA	15 (10/5)/34	–	Max/Mnd	3.0–5.0
Holtzclaw et al. 2010	Retrospective 6 months	Private practice in Texas and Pennsylvania, USA 2008–2009	13 (7/6)/31	35.2 (22–43)	Mnd Post	3.6 ± 0.8
Sohn et al. 2010	Prospective 3–4 months	Korea	32 (5/27)/84	48	Mnd Post	2.0–4.0
Blus et al. 2010	Retrospective 3 years	Italy January 2003 to September 2004	43 (20/23)/180	54.2 (26–82)	Max/Mnd Ant/Post	3.3 ± 0.7 (1.5–5.0)
Jensen et al. 2009	Retrospective 1 year	Private practice, USA Duration: 2 years	40/81	–	Max/Mnd Ant/Post	–
Danza et al. 2009	Retrospective 1 year	Italy May 2004 to November 2007	21/21	53	Max/Mnd Ant/Post	–
Bravi et al. 2007	Retrospective 10 years	Private practice, Italy January 1992 to December 2001	734 (233/501)/1715	48.6 (17–86)	Max/Mnd Ant/Post	–
Blus and Szmukler-Moncler 2006	Prospective 3 years	Italy January 2001 to May 2004	57 (28/29)/228	50.2 (23–82)	Max/Mnd Ant/Post	3.2 (1.5–5.0)
Ferrigno and Laureti 2005	Prospective 2 years	Private practice, Italy May 2002 to October 2003	40 (18/22)/82	47.1 (25–64)	Max	3.0–5.0
Sethi and Kaus 2000	Prospective 5 years	Centre for Implant and Reconstructive Dentistry, England 1991–1996	150 (72/78)/449	–	Max Ant/Post	2.0–4.0
Scipioni et al. 1994	Prospective 5 years	Private practice, Italy	170/329	–	Max Ant	–
Simion et al. 1992	Prospective 6 months	University of Milan, Italy	5 (1/4)/10	53.2 (39–71)	Max/Mnd Ant/Post	1.0–4.0

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