Cell-Based Approaches in Periodontal Regeneration: A Systematic Review and Meta-Analysis of Periodontal Defect Models in Animal Experimental Work

Abstract

Various cell types have been assessed for experimental periodontal tissue regeneration in a variety of animal models. Nonetheless, the efficacy of cell-based approaches for periodontal regeneration is still controversial. Therefore, the purpose of this study was to systematically review cell-based approaches for periodontal regeneration in animal studies including a meta-analysis to obtain more clarity on their efficacy. The results of this systematic review and meta-analysis revealed that cell-based approaches have a favorable effect on periodontal tissue regeneration, as displayed by the positive effect of cell-based approaches on new bone, cementum, and periodontal ligament (PDL) formation in periodontal defects. Moreover, subgroup analysis showed a favorable effect on PDL formation by PDL-derived cells, but not by bone marrow mesenchymal stem cells (BMSCs). However, meta-analysis did not show any statistically significant differences in effect between PDL-derived cells and BMSCs. These results provide important information for the implementation of cell-based approaches in clinical practice as a routine treatment for periodontal regeneration in the future.

Introduction

Periodontitis, a multifactorial disease caused primarily by dental plaque microorganisms,¹ is characterized by the destruction of the periodontium, which consists of the alveolar bone lining the tooth socket, the root cementum, the periodontal ligament (PDL), and part of the gingiva facing the tooth. The process of inflammation and related degeneration of periodontal tissues ultimately leads to periodontal defects. Without adequate treatment, periodontitis can eventually lead to tooth loss. Surveys have shown that ∼48% of adults have chronic periodontitis.¹ In addition, advanced periodontitis is more prevalent among the older age groups.^1,2

The ultimate goal of periodontal treatment is to prevent periodontal breakdown and to regenerate the destroyed periodontal tissues, including gingival connective tissue, PDL, alveolar bone, and cementum.³ Conventional treatment for periodontitis consists mainly of oral hygiene instructions, scaling, and root planing. This approach is usually successful in preventing further disease progression, but the regeneration of already lost tissues remains a clinical challenge.⁴ New therapeutic approaches include the introduction of bone grafts, alloplastic materials, guided tissue regeneration, and various growth factor-based therapies.⁴ Still, these strategies fail to reliably regenerate the complete periodontium damaged by severe periodontitis.^4–6

It has been postulated that stem/progenitor cells with the ability to self-renew and differentiate are the key factors in regenerative medicine.^7,8 In view of this, recent research has been focused on the development of cell-based approaches for periodontal regeneration to overcome the limitations of existing treatments. Cells can either be injected directly into the defect as a suspension or delivered by biomaterial scaffolds or cell carriers.^9–11 Various cell types, including but not limited to PDL-derived cells, bone marrow mesenchymal stromal cells (BMSCs), alveolar periosteal cells, dental follicle cells, and dental pulp cells (DPCs),^12–14 have been assessed for experimental periodontal tissue regeneration in a variety of animal models. Despite numerous publications in animal models, the efficacy of cell-based approaches for periodontal regeneration remains controversial. Some studies demonstrated that cell-based approaches had a favorable effect on periodontal tissue regeneration compared to their controls (e.g., cell-carrier alone group when cell carrier was applied),^9,15–17 while others reported that there were no significant differences between experimental groups with or without cells.^12,18,19 These conflicting results have raised doubt about the validity and efficacy of cell-based approaches for the enhancement of periodontal regeneration.

To increase the value of animal experimental work as proof of concept preparative evidence for clinical trials, systematic reviews have been proposed as the standard method for analyzing preclinical studies.²⁰ By means of a systematic review (particularly if it includes a meta-analysis), information relevant for evaluating the efficacy of treatments that cannot be directly obtained from individual studies may be obtained.^21,22 In the case of cell-based approaches for periodontal regeneration, a systematic review and meta-analyses of preclinical animal studies may provide valuable information for the implementation of cell-based approaches in clinical practice. Consequently, the purpose of this study was to perform a systematical review and a meta-analysis regarding cell-based approaches for periodontal regeneration in animal studies to obtain more clarity on their efficacy.

Materials and Methods

Search strategy and selection of articles

Original articles concerning the effects of cell-based approaches in periodontal regeneration were searched using two databases, PubMed and Embase (via OvidSP). Three components were included in the search strategy: cell, periodontal regeneration, and animal (see Table 1 for complete search strategy). Search filters to detect all animal studies were utilized in both PubMed and Embase.^23,24 The search was performed on March 19, 2014. Furthermore, the reference lists of the selected relevant articles were screened by hand for potentially relevant new articles. No language restriction was used and articles in languages other than English were translated by native speakers of that particular language within Radboudumc.

Table 1.

Search Strategy

PubMed
Component 1: cell	stem cells [MESH] OR cell culture techniques [MESH] OR cells, cultured [MESH] OR bone marrow cells [MESH] OR connective tissue cells [MESH] OR ((stem [tiab] OR mother [tiab] OR progenitor [tiab] OR periodontal [tiab] OR PDL [tiab] OR alveolar [tiab] OR dental [tiab] OR adult [tiab] OR IPS [tiab] OR fibroblast-derived [tiab] OR adipose-derived [tiab] OR induced [tiab] OR pluripotent [tiab] OR embryonic [tiab] OR embryonal [tiab] OR fetal [tiab] OR multipotent [tiab] OR mesenchymal [tiab] OR stromal [tiab] OR bone marrow [tiab] OR precursor [tiab] OR cultured [tiab] OR connective tissue [tiab]) AND (cell [tiab] OR cells [tiab])) OR MSCs [tiab] OR MSC [tiab] OR BMSCs [tiab] OR BMSC [tiab] OR PDLC [tiab] OR PDLCs [tiab] OR PDLSC [tiab] OR PDLSCs [tiab] OR ASCs [tiab] OR DFCs [tiab] OR DPCs [tiab] OR APCs [tiab] OR osteoblasts [tiab] OR osteoblast [tiab] OR stem cell transplantation [MESH] OR ((transplantation [tiab] OR transplantations [tiab]) AND (cell [tiab] OR cells [tiab])) OR cell engineering [MESH] OR tissue engineering [MESH] OR engineered [tiab] OR engineering [tiab] OR engineer [tiab] OR tissue-engineered [tiab] OR tissue-engineering [tiab] OR bioengineering [tiab] OR bioengineered [tiab] OR bioengineer [tiab] OR bio-engineering [tiab] OR bio-engineered [tiab] OR regenerative medicine [MeSH] OR regenerative medicine [tiab] OR tissue culture techniques [MeSH] OR ((tissue [tiab] OR tissues [tiab]) AND (culture [tiab] OR cultures [tiab] OR cultured [tiab] OR culturing [tiab])) OR cell-based [tiab]
Component 2: periodontal regeneration	Guided tissue regeneration, periodontal [MESH] OR ((periodontium [MESH] OR periodontium [tiab] OR periodontiums [tiab] OR periodontal [tiab] OR PDL [tiab] OR alveolar bone [tiab] OR alveolar ridge [tiab] OR cementum [tiab] OR parodontium [tiab] OR parodontiums [tiab] OR paradentium [tiab] OR paradentiums [tiab] OR tooth socket [tiab] OR tooth sockets [tiab] OR ((tooth [tiab] OR dental [tiab])AND (supporting structures [tiab] OR supporting structure [tiab] OR ligament [tiab] OR ligaments [tiab])) OR alveolar bone loss [MESH] OR (alveolar [tiab] AND atrophy [tiab]) OR (alveolar [tiab] AND atrophies [tiab]) OR (alveolar [tiab] AND resorption [tiab]) OR (alveolar [tiab] AND resorptions [tiab]) OR periodontal attachment loss [MESH]) AND (regeneration [MESH] OR regeneration [tiab] OR regenerations [tiab] OR regenerative [tiab] OR regenerated [tiab] OR regenerating [tiab] OR regenerate [tiab] OR reconstructive surgical procedures [MESH] OR reconstructive [tiab] OR reconstruction [tiab] OR reconstructed [tiab] OR reconstructing [tiab] OR reconstitution [tiab] OR reconstituted [tiab] OR reconstituting [tiab] OR reconstruct [tiab] OR repair [tiab] OR repairing [tiab] OR repaired [tiab] OR reparative [tiab] OR wound healing [MESH] OR healing [tiab] OR healings [tiab] OR healed [tiab] OR heal [tiab] OR rehabilitation [tiab] OR rehabilitative [tiab] OR rehabilitated [tiab] OR rehabilitating [tiab] OR rehabilitate [tiab] OR ((newly [tiab] OR new [tiab]) AND (formed [tiab] OR formation [tiab]) OR restoration [tiab] OR restored [tiab] OR restore [tiab] OR restoring [tiab]))
Component 3: animal	Search filter for animal studies (reference 23)
Embase
Component 1: cell	exp stem cell/OR exp bone marrow cell/OR exp cell culture/OR exp connective tissue cell/OR exp embryo cell/OR exp bone cell/OR exp cell transplantation/OR exp cell engineering/OR exp tissue engineering/OR exp regenerative medicine/OR exp tissue culture technique/OR (((stem OR mother OR progenitor OR periodontal OR PDL OR alveolar OR dental OR adult OR IPS OR fibroblast-derived OR adipose-derived OR induced OR pluripotent OR embryonic OR embryonal OR fetal OR multipotent OR mesenchymal OR stromal OR bone marrow OR precursor OR cultured OR connective tissue) AND (cell OR cells)) OR MSCs OR MSC OR BMSCs OR BMSC OR PDLC OR PDLCs OR PDLSC OR PDLSCs OR ASCs OR DFCs OR DPCs OR APCs OR osteoblasts OR osteoblast OR ((transplantation OR transplantations) AND (cell OR cells)) OR engineered OR engineering OR engineer OR tissue-engineering OR tissue-engineered OR bioengineering OR bioengineered OR bioengineer OR bio-engineering OR bio-engineered OR regenerative medicine OR ((tissue OR tissues) AND (culture OR cultures OR cultured OR culturing)) OR cell-based).ti,ab.
Component 2: periodontal regeneration	(exp periodontium/OR exp periodontal ligament/OR exp alveolar bone/OR exp alveolar ridge/OR exp cementum/OR exp alveolar bone loss/OR (periodontium OR periodontiums OR periodontal OR PDL OR alveolar bone OR alveolar ridge OR cementum OR parodontium OR parodontiums OR paradentium OR paradentiums OR tooth socket OR tooth sockets OR ((tooth OR dental) AND (supporting structures OR supporting structure OR ligament OR ligaments)) OR (alveolar AND atrophy) OR (alveolar AND atrophies) OR (alveolar AND resorption) OR (alveolar AND resorptions)).ti,ab.) AND (exp regeneration/OR exp healing/OR exp rehabilitation/OR (regeneration OR regenerations OR regenerative OR regenerated OR regenerating OR regenerate OR reconstructive OR reconstruction OR reconstructed OR reconstructing OR reconstitution OR reconstituted OR reconstituting OR reconstruct OR repair OR repairing OR repaired OR reparative OR healing OR healings OR healed OR heal OR rehabilitation OR rehabilitative OR rehabilitated OR rehabilitating OR rehabilitate OR ((newly OR new) AND (formed OR formation)) OR restoration OR restored OR restore OR restoring).ti,ab.)
Component 3: animal	Search filter for animal studies (reference 24)

For the selection of studies, two investigators (X.Y. and F.Y.) independently screened the titles and abstracts of the publications based on the inclusion criteria. Full texts of articles that were considered eligible for inclusion were obtained for further independent evaluation. Articles were included if they studied the effects of cells on periodontal regeneration and their outcome measures included the quantification of new bone, cementum, or PDL formation. The inclusion criteria made based on the study purpose and clinical experience were as follows: (1) The study was performed in animals in vivo; (2) Cell-based approaches should be used; (3) The study was an original article (e.g., not a letter or review etc.); (4) The study assessed periodontal tissue regeneration; (5) A periodontal defect model (e.g., not subcutaneous implantation etc.) was used; (6) Not a duplicate article; only one publication was included in case an article was found more than once in one of the databases; (7) Data should be acquired using an appropriate control group (e.g., data from cell-carrier alone group when cell carrier was applied); (8) Histomorphometrical data should be presented for new bone, cementum, or PDL formation; (9) Cells should be locally applied (e.g., not systemically applied cells). Criteria (1) to (5) were used only in initial screening phase based on titles and abstracts and criteria (1) to (9) were used in full-text selection phase. The inclusion criteria were specified in advance and documented in a protocol (www.umcn.nl/SYRCLE). Early Review Organizing Software version 2.0 (EROS, www.eros-systematic-review.org) was used for the selection process. Any disagreements between the two reviewers regarding inclusion of a certain publication were resolved by discussion.

Study characteristics and data extraction

The following study characteristics were extracted: animal species, strain, sex, age, and/or body weight of animals, defect type and size, description of experimental groups, number of defects per group, cell types, cell passage number, cell number per mL, cell number per defect, cell carrier/scaffold types, duration of treatment, outcome measures, number of defects excluded from statistical analysis, and reason for excluding samples (Table 2). Bibliographic details such as author, year of publication, and language were also registered.

Table 2.

Study Characteristics of the Included Studies

Reference	Language	Species/strain	Sex	Age/weight	Defect type/size	Experimental groups	Number of defects/group	Cell types	Cell passage number	Cell number/mL	Cell number/defect	Scaffold types	Duration	Quantification of newly formed tissues	Drop-outs/group+reason
Akizuki 2005	English	Dog/beagle	F	3-year/9.8–11.2 kg	Dehiscence/5×5 mm²	Cell+carrier/carrier	5	Canine PDLC	P4–6	N	1×10⁵	Hyaluronic acid sheets	8 weeks	Length of NC, NB, connective tissue attachment	No
Chen 2008	English	New Zealand white rabbit	M	3 kg	Alveolar bony defect/15×7×5 mm³	advBMP-2/advbgal/MSC/PF127	6	Rabbit BMSC	N	5×10⁷	1×10⁸	Pluronic F127 gel	6 weeks	NB volume	No
Dan 2014	English	Athymic rat/CBH-rnu	N	12-week	Fenestration defect/3×1.5 mm²	Blank/scaffold/GMC/ABC/PDLC	3	Human GMC/ABC/PDLC	P5	N	1×10⁵	CaP-PCL scaffold	1 and 4 weeks	Percentage of NB, NC, PDL	No
Ding 2010	English	Mini pig	F	6–8 months/30–40 kg	Periodontitis defect/3×7×5 mm³	Blank/HA-TCP/HA-TCP+autologous PDLSCs/HA-TCP+allogeneic PDLSCs/HA-TCP+autologous PDLCs	6	Mini pig PDLSC/PDLC	N	N	2×10⁶	HA-TCP	12 weeks	Percentage of NB	No
Dogan 2002	English	Dog	N	N	Class II furcation defect/5×2 mm²	Cell/blank	2	Regenerated canine PDLC	P4	4×10⁵	2×10⁵	Autologous blood coagulum	6 weeks	Length of NC, NB, connective tissue attachment, epithelial downgrowth	No
Dogan 2003	English	Dog	N	N	Fenestration defect/5×5 mm²	Cell/blank	2	Regenerated canine PDLC	P4	4×10⁵	2×10⁵	Autologous blood coagulum	6 weeks	Length of NC, NB, connective tissue attachment	No
Du 2014	English	Rat/ Sprague Dawley	F	4–5 weeks/80–100 g	Periodontitis defect/N	BMSC+NaCl/NaCl/blank	10	Rat BMSC	N	1×10⁷	1×10⁶	0.9% NaCl solution	12 weeks	Percentage of NB	No
Duan 2011	English	Nude mice	M	8-week/25–30 g	Fenestration defect/2×1.5mm²	Scaffold/scaffold+EMD/scaffold+EMD+cell	6	Murine iPS cell	N	1×10⁷	N	Apatite-coated silk	12 and 24 days	Percentage of NB	No
Fawzy El-Sayed 2012	English	Mini pig	F+M	18±1 months/46.9±4.6 kg	Periodontitis defect/3×7×5 mm³	Cell+DBCB/Cell+collagen/DBCB/collagen/SRP/blank	8	Mini pig GMC	P3	N	2×10⁷	DBCB and collagen	12 weeks	Attachment level; junctional epithelium length; connective tissue adhesion	No
Fu 2013	English	Mini pig	F	9–12 months/40–45 kg	Periodontitis defect/5×7×7 mm³	HA-TCP/HA-TCP+PDLSCs/HA-TCP+SPDs	4	Mini pig PDLSC/SPD	P3	N	1×10⁶	HA-TCP	12 weeks	Percentage of NB	No
Han 2013	English	Rat/Sprague Dawley	F	220–250 g	Fenestration defect/2×3 mm²	PDLSCs+Gelfoam/Gelfoam/blank	3	Rat PDLSC	N	1×10⁶	1×10⁶	Gelfoam	1, 2, 3, 4 weeks	Percentage of NB area, NB length, NC length	No
Hynes 2013	English	Athymic rat/CBH-mu	N	N	Fenestration defect/2×3 mm²	Cell+clotting factors/clotting factors/blank	6/3/3	Human foreskin iPS cell-MSC-like cells	N	1.25×10⁸	1.25×10⁶	Fibrinogen+thrombin	2 weeks	Percentage of NB	No
Iwasaki 2014	English	Nude rat	M	7 weeks	Class II furcation defect/2×1.5 mm²	PDLSC-amnion/amnion	6	Human PDLSC	less than P7	N	N	Amniotic membrane	4 weeks	Percentage of NB	No
Iwata 2009	English	Dog/beagle	M	10 kg	3-wall infrabony defect/5×5×4 mm³	PDLC+PGA/PGA	4	Canine PDLC	P3	N	9×10⁴	PGA sheets	6 weeks	Percentage of NB, NC; junctional epithelium length; periodontal score	No
Jiang 2010	English	Dog/beagle	N	9.5–10.5 kg	Class III furcation defect/3×4 mm²	e-PTFE+b-TCP+cells/e-PTFE+b-TCP/e-PTFE	8	Canine periosteal cells	P3	5×10⁶	5×10⁶	b-TCP	6 weeks	Percentage of NC, PDL; NB, soft tissue, nothing	2/all groups (over prepared defects)
Kawaguchi 2004	English	Dog/beagle	F	12–20 months/10–14 kg	Class III furcation defects/4 mm height	(0.2, 5, 10, 20)×10⁶ cells	N	Canine BMSC	P3	(2, 5, 10, 20)×10⁶	(2, 5, 10, 20)×10⁶	Atelocollagen	1 month	Percentage of NB, NC	N
Khorsand 2013	English	Dog/mongrel	M	1–2 years/14–22 kg	3-wall periodontal defect/3×5×8 mm³	Cell+carrier/carrier	10	Canine DPSC	P3	N	2×10⁷	Bio-Oss	8 weeks	Length of NB, NC, PDL	No
Lang 1998	English	Mini pig	F	4–8 years	Grade II furcation and 2-walled interdental defect/N	ABC/PDLC/cell carrier/membrane/flap surgery/blank	3 and 8	Mini pig PDLC/ABC	P1	2×10⁷	N	Bone gelatin	10, 30, 90 days	Percentage of NB, connective tissue, epithelial attachment	No
Li 2009	English	Dog/beagle	F	12–18 kg	Fenestration defect/5×5 mm²	Non-cryopreserved cell/cryopreserved cell/scaffold	9/9/8	Canine BMSC	P7	5×10⁶	1×10⁵	Collagen	8 weeks	Percentage of NB, NC, PDL	1/cryopreserved cell (infection)
Li 2010	Chinese	Dog/beagle	N	10 kg	Class II furcation defect/N	Gene modified cell+carrier/cell+carrier/carrier	8/8/6	Canine BMSC	N	1×10⁷	3×10⁵	Collagen	12 weeks	Percentage of NB and NC	No
Liu 2008	English	Mini pig	N	12 months/30–40 kg	Periodontitis defect/3×7×5 mm³	Cell+carrier/carrier/blank	12	Mini pig PDLSC	P3	N	2×10⁷	HA-TCP	12 weeks	Length of NB	No
Lu 2004	Chinese	Dog/mongrel	M	1–2 years	Class II furcation defect/3.5×4 mm²	Cell+carrier/carrier/blank	12	Canine PDLC	P2–4	5×10⁶	N	nHAC	8 weeks	Length of NC, NB, connective tissue attachment	No
Mrozik 2013	English	Merino ewes	N	3–5 years/63.5–72 kg	Dehiscence defect/10 mm deep	Cell+carrier/carrier/blank	7/7/6	Ovine PDLSC	P3	1×10⁸	1×10⁷	Gelfoam	4 weeks	NB, NC, PDL	1 and 2/no treatment (N)
Nakajima 2008	English	Rat/Wistar	M	7–8 weeks/230–270 g	Fenestration defect/N	ALP-expressing GF/GF/blank	3	Human GF	P4–6	N	9.5±2.6×10⁴	No	5, 10, 14, 28 days	Percentage and thickness of NB	No
Nunez 2012	English	Dog/beagle	M	1 year/10 kg	3-wall intrabony defect/3×4 mm²	CDC+carrier/PDLC+carrier/carrier	8	Canine CDC/PDLC	P4–5	>1.5×10⁷	>0.75×10⁶	Collagen sponge	3 months	Length of NB, dimension of NC	No
Paknejad 2014	English	Dog/mongrel	M	1–2 years/14–22 kg	3-wall intrabony defect/4×4 mm²	Cell+carrier/carrier	9	Canine BMSC	P3	1×10⁸	2×10⁷	ABBM	8 weeks	Percentage and length of NB, NC, PDL	No
Park 2011	English	Dog/beagle	N	10 months/10 kg	Periodontitis defect//3×3 mm²	PDLSC/DPSC/PAFSC/blank/no defect	8	Canine PDLSC/DPSC/PAFSC	P2–3	N	6×10⁶	No	8 weeks	Percentage and height of NB	No
Simsek 2012	English	Dog/mongrel	N	15 kg	Class II furcation defect/5×2 mm²	Blank/PRP/ACB/ACB+PRP/BMSC+PRP	6	Canine BMSC	N	1×10⁷	1×10⁷	PRP	8 weeks	Percentage of NB and NC	No
Suaid 2012	English	Dog/beagle	N	1.46±0.18 years/10–20 kg	Class III furcation defect/5 mm height	Blank/GTR/carrier+GTR/carrier+GTR+cell	7	Canine PDLC	N	N	3×10⁵	Collagen sponge	3 months	Length of NC, PDL, ECT, tissue free; area of NB, soft tissue, non-filled tissue	No
Suaid 2011	English	Dog/beagle	N	1.46±0.18 years/10–20 kg	Class II furcation defect/5×2 mm²	Cell+carrier/carrier	7	Canine PDLC	P23	N	3×10⁵	Collagen sponge	3 months	Length of NC, PDL, ECT, tissue free; area of NB, soft tissue, non-filled tissue	No
Tcacencu 2012	English	Rat/Sprague Dawley	M	350 g	Fenestration defect/2×(3–4) mm²	Cell+carrier/carrier/blank	6/12/6	Human BMSC	P4–5	7.5×10⁶	1.5×10⁶	Peptide hydrogel+collagen sponge	1 and 4 weeks	NB volume density, osteoclast number	No
Tobita 2013	English	Dog/beagle	N	9–10 months/8–10 kg	Class III furcation defect/5 mm height	Cell+carrier/carrier/blank	8	Canine ASC	P2	1.5×10⁷	1.5×10⁷	PRP	1 and 2 months	Percentage of NB and NC	No
Tsumanuma 2011	English	Dog/beagle	M	10 kg	1-wall intrabony defect/5×5 mm²	PDLC+carrier/BMSC+carrier/APC+carrier/carrier	4	Canine PDLC/BMSC/APC	P3	N	(9–15)×10⁴	Polyglycolic acid	8 weeks	Percentage of NB; length of NC, junctional epithelium; periodontal score	No
Xu 2006	Chinese	Dog/mongrel	M	1 year/13±2 kg	Horizontal defect/5×2 mm²	Blank/AP-C-PLA/C-PLA+BMSC/AP-C-PLA+BMSC	10	Canine BMSC	P3–5	1×10⁶	2×10⁸	AP-C-PLA/C-PLA	4 and 8 weeks	Percentage of NB	No
Yang 2010	English	Rat/ Sprague Dawley	M	8 weeks	Fenestration defect/1×3 mm²	Cell+carrier/carrier/blank	6/4/4	Rat BMSC	Day 25	N	N	Microcarrier gelatin beads	3 weeks	Percentage of NB and NC; new ligament density	No
Yu 2013	English	Nude rat	N	7 weeks	3-wall defect/2×2×1.7 m³	PDLC+carrier/GF+carrier/carrier	8	Rat PDLC, GF	P4	4×10⁶	1×10⁶	Gelatin sponge	6 weeks	Percentage of NB, NC, PDL	No
Zhan 2008	English	Dog/beagle	N	18 months	Class II furcation defect/5×3 mm²	Carrier alone/5×10⁸ L⁻¹ cell/5×10⁹ L⁻¹ cell/5×10¹⁰ L⁻¹ cell	7/7/8/7	Canine BMSC	P1	5×(10⁵/10⁶/10⁷)	N	Collagen membrane	12 weeks	Percentage of NB, NC	1/5×10⁸ and 10¹⁰ L⁻¹ cell (improper treatment)
Zhao 2004	English	Nude rat	N	250–300 g	Fenestration defect/3×2 mm²	Carrier/carrier+follicle cells/carrier+cementoblasts	5 for 3 weeks; 2/2/4 for 6 weeks	Murine follicle cells and cementoblasts	P2/N	6.7×10⁷	1×10⁶	PLGA	3 and 6 weeks	Length and percentage of NB	1/carrier (polymer is displaced from defect)
Zhou 2012	English	Dog/beagle	M	10–14 kg	Window defect/4×4×3 mm³	Cells+material+OPG/cells+material/material/blank	6	Canine BMSC-OPG/BMSC	N	1×10⁶	N	PLGA	6 weeks	Height of NB, NC, connective tissue	No

advBMP-2, adenovirus-mediated BMP-2 gene-infected MSC+PF127 group; advbgal, adenovirus-mediated bgal gene-infected MSC+PF127 group; MSC, mesenchymal stem cell group; PF127, Pluronic F127 control group; PDLC, periodontal ligament cell; ABC, alveolar bone cell; GMC, gingival margin-derived cell; PDLSC, periodontal ligament stem cell; HA-TCP, hydroxyapatite-tricalcium phosphate; DBCB, deproteinized bovine cancellous bone; SRP, scaling and root planning; HAL, histological attachment level; JE, junctional epithelium length; CTA, connective tissue adhesion; SPDs, allogeneic stem cells isolated from miniature pig deciduous teeth; e-PTFE, e-polytetrafluoroethylene; DPSC, dental pulp stem cell; nHAC, nano-Hap-collagen; ALP, alkaline phosphatase; GF, gingival fibroblast; CDC, cementum-derived cell; ABBM, anorganic bovine bone mineral; DPSC, dental pulp stem cell; PAFSCs, periapical follicular stem cell; PRP, platelet-rich plasma; ACB, autogenous cortical bone; ECT, epithelium/connective tissue extension; ASC, adipose tissue-derived stem cell; APC, alveolar periosteal cells; AP-C-PLA, astragalus polysaccharides-chitosan/polylactic acid; PLGA, Poly(DL-lactic-co-glycolic acid; OPG, osteoprotegerin; BMSC, bone marrow mesenchymal stem cell; GTR, guided tissue regeneration.

The outcome measures used for the meta-analysis were the quantification of newly formed bone, cementum, and PDL. For all studies, outcome data for experimental and control groups were extracted if mean, standard deviation (SD) or standard error (SE), and number of defects per group (n) were reported, or could be recalculated. If SE was reported, this SE was converted to SD for meta-analysis. If data were only presented graphically, data were remeasured based on the distances of figures using universal on-screen digitizer software (Universal Desktop Ruler v3.6.3481; AVPSoft.com) when possible.

Assessment of risk of bias

Two investigators (X.Y. and F.Y.) independently assessed the risk of bias of the included studies using an adapted version of the risk of bias (RoB) tool for animal studies developed by Hooijmans et al.²⁵ Items 9 and 10 of the tool were not scored; two reporting questions were added: (a) Was it stated that the experiment was randomized at any level? (b) Was it stated that the experiment was blinded at any level? The risk of bias for question 4 was always considered to be low in cases where all experimental interventions were present in one animal. Given the nature of periodontal regeneration, the risk of bias for question 6 was always scored as low in cases where the outcome of intervention and control groups was assessed at the same time points.

Data synthesis and statistical analysis

Data were analyzed using Review Manager Version 5.2 (Copenhagen, The Nordic Cochrane Centre, The Cochrane Collaboration, 2012). Meta-analysis was performed for the outcome measures new bone, cementum, and PDL formation, by computing the standardized mean difference (SMD). If a study contained two or more cell types, or outcomes were measured at several time points, these groups were analyzed as if they were separate experiments. However, if the same control group served more than one experimental group, the number of defects in the control group was divided by the number of experimental groups served.²⁶ Forest plots were used to display individual and weighted overall effect sizes. Data were presented as SMD and 95% confidence intervals (CIs). To account for anticipated heterogeneity, a random effects model was used.²⁷ I² was used as the measure of heterogeneity.

To explore possible causes of heterogeneity and to assess the influence of several variables on cell-based strategy efficacy, subgroup analyses were performed. Only subgroups that contained more than three experiments were included in the subgroup analyses. The variables studied were animal species (dog, rat, or mini pig), sex (male or female), and cell type (PDL-derived cells or BMSC). Sub-subgroup analyses were performed if the SD between the SMDs of the subgroups was larger than 1, to determine whether the trend observed was also present within all sub-subgroups. Only sub-subgroups that contained more than three experiments were included in the sub-subgroup analyses.

Publication bias was assessed by visually evaluating the possible asymmetry in funnel plots. Finally, to assess the robustness of the findings, a sensitivity analysis was performed for pooling outcome data using different units of measurement (percentage, length, or volume of newly formed bone).

Results

Description of the included studies

The search strategy described in Table 1 retrieved 3742 articles (2011 articles in PubMed and 1731 articles in Embase). After initial screening based on titles and abstracts, 116 articles were included for full-text evaluation. After studying these full-text articles, 77 reports were excluded for reasons mentioned in Figure 1 and 39 studies were included (Fig. 1).^{9–12,15–19,28–57} Three of the included 39 studies were translated, as these were published in Chinese.^42,44,53

FIG. 1.

Flow chart of study selection. The number of studies in each phase is indicated between brackets.

The characteristics of included studies as shown in Table 2 varied substantially. Six different animal species were used: 21 studies were performed with dogs, 10 with rats, 5 with mini pigs, and 1 with rabbits, mice, or merino ewes, respectively. Fourteen studies used only male animals; 8 studies used only female animals; 1 study used both male and female animals; and 16 articles did not mention the sex of animals. A variety of defects were induced, including surgical dehiscence (2 studies), fenestration (11 studies), intrabony (7 studies), furcation (13 studies), and defects induced by chronic inflammation (6 studies). Twelve different cell types were evaluated in periodontal regeneration, including PDL-derived cells (19 studies), BMSCs (13 studies), gingival margin-derived cells (GMCs, 2 studies), alveolar bone cells (2 studies), stem cells isolated from deciduous teeth (1 study), induced pluripotent stem (iPS) cells (2 studies), periosteal cells (2 studies), DPCs (2 studies), cementum-derived cells (2 studies), periapical follicular stem cells (1 study), adipose-tissue derived stem cells (ASCs, 1 study), and follical cells (1 study). Also the passage number, cell concentrations, and number of applied cells per defect greatly varied between the studies, ranging from passage 1 to passage 7, from 4×10⁵ to 1.25×10⁸ cells/mL, and from 9×10⁴ to 2×10⁸ cells per defect, respectively. Moreover, multiple types of cell carriers or biomaterial scaffolds were used, including natural (e.g., collagen, platelet-rich plasma, etc.) and synthetic biomaterials. The included synthetic biomaterials were ceramics (e.g., hydroxyapatite-tricalcium phosphate), polymers (e.g., polyglycolic acid), and composites (e.g., apatite-coated silk).

Risk of bias and quality of reporting

Figure 2 shows the overall results of the risk of bias assessment of the 39 studies included in this systematic review. Regarding selection bias, 64.1% of the included studies reported randomization of the experimental units across treatment groups. However, only 23.1% of the studies mentioned the method of randomization used, so that the adequacy of the method could be judged. 76.9% of the articles described that intervention and control groups were similar at the start of the experiment. With regard to the selection bias item “allocation concealment,” only 7.7% described whether or not the allocation to the different groups during the randomization process was concealed. In addition, 64.1% and 97.4% of the included studies were scored as unclear risk of bias with regard to performance bias items “random housing” and “blinding,” respectively. For the detection bias item “random outcome assessment,” 100% of the included studies were scored as low risk of bias, because the outcome of intervention and control groups were assessed at the same time points. 28.2% of the studies reported that the outcome assessment was blinded based on the item 7 of the RoB tool. For attrition bias, 51.3% of the studies were scored as low risk of bias, as they adequately addressed incomplete outcome data.

FIG. 2.

Risk of bias, averaged per item. Color images available online at www.liebertpub.com/teb

Meta-analysis of outcome measures

New bone formation

Thirty-eight studies of the included 39 studies evaluated the effect of cell-based strategies on new bone formation in periodontal regeneration, 37 of which could be included in the meta-analysis, as in one study the number of defects per group was not reported and could not be retrieved by contacting the authors.³⁸ The analysis contained 60 experiments or experimental groups, including data from 647 defects. In 30 experiments, the SMD and 95% CIs indicated that implantation of cells significantly increased new bone formation in the defect area (Fig. 3). None of the studies found a statistically significant negative effect of cell implantation on new bone formation. In the remaining 30 experiments, no statistically significant results were found. Overall analysis showed that cell-based strategies enhance new bone formation in periodontal regeneration, as displayed by the overall SMD and its 95% CIs (1.58 [1.19, 1.97]). However, overall study heterogeneity was moderate to high (I²=66%).

FIG. 3.

Forest plot of the included studies for outcome new bone formation. The forest plot displays relative weight of the individual experiments, the standardized mean difference (SMD), and 95% confidence intervals (CIs). The diamond indicates the global estimate and its 95% CI.

Subgroup analysis showed a beneficial effect of cells for all subgroups. No statistically significant differences between the subgroups were found for any of the examined variables (species, sex, cell type; Table 3). Two subgroups showed differences between SMDs larger than 1 SD (mini pig>rat, female>male). Only the results of the sub-subgroup analysis for sex was consistent with the trend found in the subgroup analysis. However, because of the small number of experiments in the subgroups and even more so in the sub-subgroups, these results must be interpreted with caution.

Table 3.

Subgroup Analysis of the Included Studies for Outcome New Bone Formation

Subgroup	Experiments	Defects	Effect estimate	I²
Overall	60	655	1.58 [1.19, 1.97]	66%
Dog	31	355	1.60 [1.06, 2.13]	70%
Rat	16	148	1.09 [0.49, 1.70]	41%
Mini pig	10	108	2.14 [1.16, 3.12]	60%
Female	15	151	2.48 [1.53, 3.44]	66%
Male	20	229	1.44 [0.72, 2.15]	75%
PDL-derived cells	23	242	2.00 [1.28, 2.71]	67%
BMSC	18	206	2.01 [1.27, 2.76]	69%

The effect estimate was displayed by the standardized mean difference (SMD) and 95% confidence intervals (CIs).

PDL, periodontal ligament.

New cementum formation

Twenty-three of the included studies evaluated the effect of cell-based strategies on new cementum formation, 22 of which could be included in the meta-analysis, as in one study the number of defects per group was not clear.³⁸ The analysis contained 33 experiments, including data from 344 defects. In 16 experiments, the SMD and 95% CIs indicated that implantation of cells significantly increased new cementum formation in the defect area (Fig. 4). In the remaining 17 experiments, no statistically significant results were found. None of the included studies showed a statistically significant negative effect of cell implantation on new cementum formation. Overall analysis showed that cell-based strategies enhance new cementum formation in periodontal regeneration, as displayed by the overall SMD and its 95% CIs (1.74 [1.13, 2.35]). However, overall study heterogeneity was moderate to high (I²=69%).

FIG. 4.

Forest plot of the included studies for outcome new cementum formation. The forest plot displays relative weight of the individual experiments, SMD, and 95% CIs. The diamond indicates the global estimate and its 95% CI.

Subgroup analysis showed a beneficial effect of cells for all subgroups, except for rat and female. No statistically significant differences between the subgroups were found for any of the examined variables (species, sex, cell type; Table 4). Two subgroups showed differences between SMDs larger than 1 SD (dog>rat, male>female). However, no sub-subgroup analysis was performed because of the small number of experiments in the sub-subgroups.

Table 4.

Subgroup Analysis of the Included Studies for Outcome New Cementum Formation

Subgroup	Experiments	Defects	Effect estimate	I²
Overall	33	344	1.74 [1.13, 2.35]	69%
Dog	27	302	1.94 [1.24, 2.64]	73%
Rat	5	28	0.48 [−0.63, 1.58]	0%
Female	4	41	0.79 [−0.32, 1.90]	0%
Male	12	137	2.60 [1.21, 4.00]	83%
PDL-derived cells	12	119	1.79 [0.72, 2.86]	72%
BMSC	13	136	2.46 [1.21, 3.71]	73%

The effect estimate was displayed by the SMD and 95% CIs.

New PDL formation

Nineteen of the included studies evaluated the effect of cell-based strategies on new PDL formation in periodontal regeneration. The analysis contained 29 experiments or experimental groups, including data from 318 defects. The SMD and 95% CIs from nine experiments indicated that implantation of cells significantly increased new PDL formation in the defect area (Fig. 5). One experiment found a statistically significant negative effect of cell implantation on new PDL formation.⁵⁷ In the remaining 19 experiments, no statistically significant results were found. Overall analysis showed that cell-based strategies enhance new PDL formation in periodontal regeneration, as displayed by the global estimate SMD and its 95% CIs (1.25 [0.64, 1.86]). However, overall study heterogeneity was moderate to high (I²=71%).

FIG. 5.

Forest plot of the included studies for outcome new periodontal ligament formation. The forest plot displays relative weight of the individual experiments, SMD, and 95% CIs. The diamond indicates the global estimate and its 95% CI.

Subgroup analysis showed a beneficial effect of cells for all subgroups, except for mini pig, female and BMSC. No statistically significant differences between the subgroups were found for any of the examined variables (species, sex, cell type; Table 5). One subgroup showed a difference between SMDs larger than 1 SD (rat>mini pig). However, no sub-subgroup analysis was performed because of the small number of experiments in the sub-subgroups.

Table 5.

Subgroup Analysis of the Included Studies for Outcome New Periodontal Ligament Formation

Subgroup	Experiments	Defects	Effect estimate	I²
Overall	29	318	1.25 [0.64, 1.86]	71%
Dog	17	186	1.46 [0.53, 2.38]	75%
Rat	5	38	2.16 [1.09, 3.22]	0%
Mini pig	6	80	0.68 [−0.37, 1.74]	75%
Female	8	107	1.32 [−0.47, 3.12]	89%
Male	10	113	1.91 [0.47, 3.34]	81%
PDL-derived cells	13	141	1.44 [0.31, 2.57]	82%
BMSC	7	76	0.81 [−1.05, 2.67]	71%

The effect estimate was displayed by the SMD and 95% CIs.

Publication bias and sensitivity analysis

The presence of publication bias was assessed for all outcome measures by visual analysis of funnel plots. As shown in Figure 6, the largest studies are plotted near the average (the dashed line) for all outcome measures. Funnel plot for new bone (Fig. 6A) indicates that small, negative studies appeared to be underrepresented (within the red box), while funnel plots for new cementum (Fig. 6B) and new PDL (Fig. 6C) were quite symmetrical.

FIG. 6.

Funnel plot of the included studies for outcome new bone formation (A), new cementum formation (B), and new periodontal ligament (PDL) formation (C). Solid blue lines are to guide the eye. The largest studies are plotted near the average (the dashed line), and small, negative studies appeared to be underrepresented (within the red box). Color images available online at www.liebertpub.com/teb

To assess the robustness of our findings, sensitivity analysis was carried out by separately assessing the different units of measurement for expressing new bone formation (the percentage, length, or volume of newly formed bone). The effect sizes did not significantly differ between these groups (data not shown).

Discussion

The aim of this study was to systematically evaluate the current evidence for the efficacy of cell-based approaches for periodontal regeneration in animal studies. The results of this systematic review and meta-analysis revealed that cell-based approaches have a favorable effect on periodontal tissue formation, as displayed by the positive effect of cell-based approaches on new bone, cementum, and PDL formation. Moreover, subgroup analysis showed a favorable effect on PDL formation of PDL-derived cells, but not of BMSC. However, meta-analysis did not show a statistically significant difference in effect between PDL-derived cells and BMSC.

The currently used approach of systematic review allowed the inclusion of a large number of studies, which enabled us to perform the meta-analysis and explore the effect of several subgroup variables. However, there are some potential limitations related to this approach. First, preferably, all experiments should be performed in a similar manner when their results are being combined in a meta-analysis. However, the publications display experimental variability for the utilized animal species and sex, the periodontal defect type and size, the used cell types and passage number, the number of cells per defect, the biomaterials applied as cell carrier, and the healing time after cell transplantation. Not surprisingly, substantial statistical heterogeneity was found, which might be explained in part by experimental variability.^58,59 To account for the expected heterogeneity, a random effects model was used. Moreover, subgroup analyses (animal species, sex, and cell types) were performed in an attempt to explain this heterogeneity, but these subgroup analyses did not notably reduce the heterogeneity. Second, the results of the meta-analysis may be subject to publication bias, as visual analysis of funnel plots revealed that small negative studies appeared to be underrepresented. The true effect of cell-based approaches may hence be smaller than the effect found in our meta-analysis. Importantly, funnel plot asymmetry can either result from non-publication of negative results, or be caused by other factors, such as true study heterogeneity, or differences in study quality.⁶⁰ Third, the study quality assessment (assessment of risk of bias) revealed that poor reporting of animal studies in scientific publications is of serious concern. Key measures to avoid bias, such as randomization and blinding, were infrequently reported. This seriously hampers drawing reliable conclusions from animal studies. For example, only 23.1% of the studies provided sufficient details to judge the adequacy of the method of randomization and only 28.2% of the studies reported that the outcome assessment was blinded. If the methods of randomization used in the other studies were inadequate and if the other studies did not actually blind the outcome assessment, there is a substantial risk that the effects of cell-based approaches in periodontal regeneration have been overestimated. To generate reliable and unbiased data, it is suggested that the standards of animal experiment reporting should be more like the standards routinely applied in human randomized controlled trials.⁶¹ Despite these limitations, the combined analysis of the included studies still generated extra and valuable information that could not be derived from individual studies.

To assess the influence of several variables on cell-based strategy efficacy, subgroup analyses (animal species, sex, and cell types) were performed. No statistically significant differences between the subgroups were found for any of the examined variables. As shown in this systematic review, dogs, rats, and mini pigs are the mostly used animal species for preclinical trials and hence subgroup analyses of these species were performed. Some subgroups did show differences between SMDs larger than 1 SD, for example, mini pig>rat in new bone formation, dog>rat in new cementum formation, and rat>mini pig in new PDL formation. However, the sub-subgroup analysis for animal species was either inconsistent with the trend found in the subgroup analysis or could not be performed because of the small number of experiments in the sub-subgroups. For the subgroup analyses of sex, we also observed that some subgroups showed differences between SMDs larger than 1 SD (female>male in new bone formation and male>female in new cementum formation). And the results of the sub-subgroup analysis for sex in new bone formation were consistent with the trend found in the subgroup analysis. However, because of the small number of experiments in the subgroups and even more so in the sub-subgroups, these results must be interpreted with caution and cannot be used for drawing any final conclusions. Still, it might be interesting to study the effect of sex on periodontal regeneration in future experiments.

One of the most important issues for clinical application of cell-based approaches is the type of cell used. As shown in this systematic review, PDL-derived cells and BMSCs are the mostly used cell types for preclinical trials and hence subgroup analyses of these two cell types were performed. Previous studies compared the regenerative potential between PDL-derived cells and BMSCs, and the results suggested that PDL-derived cells are the more suitable cell population for periodontal regeneration, as significantly more well-oriented periodontal PDL fibers, newly formed bone, and cementum were observed upon transplantation of PDL-derived cells.^12,13 A recent systematic review on PDL-derived cells for periodontal regeneration reported that 12 out of the 17 included studies demonstrated a statistically significant improvement in periodontal regeneration.⁶² However, no meta-analysis was performed and no information of BMSC in periodontal regeneration was provided in this systematic review.⁶² In our systematic review, 12 out of 23 included studies reported a statistically significant positive effect of PDL-derived cells on new bone formation. For BMSCs, 10 out of 18 included studies demonstrated a statistically significant improvement in new bone formation. The subgroup meta-analysis revealed that PDL-derived cells and BMSCs had a similar effect on the enhancement of bone and cementum formation. However, only PDL-derived cells but not BMSCs showed a favorable effect on new PDL formation, although there is no significant difference between these two subgroups. The enhancement of PDL-derived cells in PDL regeneration is probably caused by the fact that PDL-derived cells contain several subpopulations of cells, including osteoblasts, fibroblasts, and cementoblasts, and the combined subpopulations of cells can simultaneously synthesize both hard and soft periodontal tissues.^30,63,64 Our results indicate that PDL-derived cells may provide a promising treatment to regenerate periodontal tissues including bone, cementum, and PDL. However, for the implementation of cell-based approaches in clinical practice, either PDL-derived cells or BMSCs can be applied based on the specific condition of each individual patient (e.g., accessibility of cells and health condition of donor tissues).

The meta-analysis provided evidence for the enhancement of periodontal regeneration by implantation of either PDL-derived cells or BMSCs. It has to be emphasized that treatment of periodontal defects requires a large number of cells (9×10⁴ to 2×10⁸ cells are needed for one defect), which sometimes would be difficult to obtain from a single human subject. Although the in vitro expansion of stem/progenitor cells is necessary, those cells typically reduce their ability to self-renew and proliferate during passaging.⁷ Therefore, other sources of stem/progenitor cell types need to be sought. Among the evaluated 12 different cell types, GMCs, iPS cells, and ASCs are potential alternative sources. GMCs, which can be readily obtained during oral surgery, have mesenchymal stem cell properties and can differentiate into osteogenic phenotype under certain inductive conditions.^32,65 In addition, iPS cells, obtained after transfection of certain stem cell-related genes into adult somatic cells, have the ability to differentiate into a variety of cell types.⁷ Moreover, ASCs can be recovered easily in large numbers by liposuction under local anesthesia and show large similarity with BMSCs regarding gene expression and osteogenic capacity.⁶⁶ Regarding these three cell types, however, only few studies were included in this systematic review (two, two, and one for GMCs, iPS cells, and ASCs, respectively). Both iPS cells^17,31 and ASCs⁵² have given promising results in periodontal regeneration compared to cell-carrier only groups, while GMCs^18,19 did not promote any significant periodontal regeneration. Nevertheless, to further ensure the efficacy of the aforementioned cell types in periodontal regeneration, additional studies are required in future experimental work.

Cell passage number is another important factor for clinical application of cell-based approaches. For instance, primary PDL-derived cells used at early passages have the advantage of maintaining the rich phenotypic and functional heterogeneity of fibroblasts characteristic of the original tissue.⁶⁷ Nevertheless, characteristic changes of primary cells have been observed during passage. Alkaline phosphatase activity of PDL-derived cells gradually decreased and the expression of tendo/ligamentogenesis-related genes was downregulated as the passage number increased.⁶⁸ However, in the current review, only less than half of the included studies (18 out of 39) provided information on exact cell passage number, which made it difficult to perform subgroup analysis based on cell passage numbers. Therefore, it is highly recommended that the passage number of implanted cells is clearly reported in future studies.

Animal studies included in this systematic review indicate that cell-based approaches are effective for periodontal regeneration, providing some support for the implementation of cell therapy in clinical practice as a routine treatment in the future. Nevertheless, the longest evaluation time within the included studies was 12 weeks. Therefore, long-term in vivo studies are required to ensure preclinical safety. Moreover, various animal models were utilized in the included studies. An optimal preclinical animal model plays a crucial role in the evaluation of cell-based strategies for periodontal regeneration. Convenience and biologic similarity of the periodontium to that of humans are important considerations for the selection of an appropriate animal model. For animal species, six different animal species were utilized in the included studies and the most frequently used species were dogs (21 studies) and rats (10 studies). In general, defect size characteristics and biologic similarities favor the selection of large animals in periodontal regeneration before moving to clinical trials. In a study that compared bone composition, density, and quality in bone samples derived from seven vertebrates (human, dog, pig, cow, sheep, chicken, and rat), rat bone was most different whereas canine bone best resembled human bone.⁶⁹ The more commonly used animal models in periodontology have been canine and periodontal disease in rats has not been found to be as closely related to the human varieties.⁷⁰ However, due to ethical issues, the use of large animals should be reserved for the last phase of validation of a new treatment. Rodents are inexpensive and easy to house, and are often used as a starting point for preliminary screenings, followed by verification in large animals.⁶⁹

To create periodontal defects, either chronic⁹ or acute¹¹ periodontal defect models have been utilized. In chronic defect models, formation of natural periodontitis lesions can be induced by placing wires, ligatures, or impression material around the teeth. Although the clinical, histopathological, and microbiological features of chronic models were similar to those in human periodontitis, the onset of the lesions is difficult to predict and the defect area is difficult to be standardized.⁷¹ In contrast, the acute defect models can be surgically created including surgical dehiscence, fenestration, intrabony, and furcation defects, to mimic the defect morphology in humans and meet different experimental purposes. In this systematic review, periodontal defects were mostly surgically generated (33 studies) and only few studies used chronic inflammation (6 studies). To combine the advantages of chronic and acute models, periodontal defects have been created in an acute-chronic way. For instance, Yan et al. created an acute-chronic Class III furcation defect in beagle dogs by placing a cotton ball saturated with anaerobic bacteria in surgical defect to cause inflammation.⁷² Still, acute-chronic animal models cannot mimic some fundamental features including genetic background and risk factors (e.g., aggressive bacterial flora, tobacco, systemic diseases of host, etc.).⁶ Consequently, these features can only be evaluated in future clinical trials.

Conclusion

The current systematic review and meta-analysis indicates that, based on animal experimental work involving periodontal defect models, cell-based therapies have a favorable effect on periodontal regeneration compared to cell-carrier only therapies. Moreover, subgroup analysis showed a favorable effect on PDL formation of PDL-derived cells, but not of BMSC. However, meta-analysis did not show a statistically significant difference in effect between PDL-derived cells and BMSCs. These results provide important information for the implementation of cell-based approaches in clinical practice as a routine treatment in the future.

Footnotes

Acknowledgments

The authors thank the Royal Netherlands Academy of Arts and Sciences (KNAW; project number PSA 08-PSA-M-02) and China Scholarship Council (No. 2010622061).

Disclosure Statement

The authors report no conflicts of interest related to this study.

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