Tissue-Engineered Mandibular Bone Reconstruction for Continuity Defects: A Systematic Approach to the Literature

Abstract

Background: Despite significant surgical advances over the last decades, segmental mandibular bone repair remains a challenge. In light of this, tissue engineering might offer a next step in the evolution of mandibular reconstruction.

Purpose: The purpose of the present report was to (1) systematically review preclinical in vivo as well as clinical literature regarding bone tissue engineering for mandibular continuity defects, and (2) to analyze their effectiveness.

Materials and Methods: An electronic search in the databases of the National Library of Medicine and ISI Web of Knowledge was carried out. Only publications in English were considered, and the search was broadened to animals and humans. Furthermore, the reference lists of related review articles and publications selected for inclusion in this review were systematically screened. Results of histology data and amount of bone bridging were chosen as primary outcome variables. However, for human reports, clinical radiographic evidence was accepted for defined primary outcome variable. The biomechanical properties, scaffold degradation, and clinical wound healing were selected as co-outcome variables.

Results: The electronic search in the databases of the National Library of Medicine and ISI Web of Knowledge resulted in the identification of 6727 and 5017 titles, respectively. Thereafter, title assessment and hand search resulted in 128 abstracts, 101 full-text articles, and 29 scientific papers reporting on animal experiments as well as 11 papers presenting human data on the subject of tissue-engineered reconstruction of mandibular continuity defects that could be included in the present review.

Conclusions: It was concluded that (1) published preclinical in vivo as well as clinical data are limited, and (2) tissue-engineered approaches demonstrate some clinical potential as an alternative to autogenous bone grafting.

Introduction

Mandibular continuity defects result from a variety of causes, including maxillofacial trauma, osteomyelitis, osteonecrosis, and resection of benign or malignant tumors.^1,2 Unrepaired defects are associated with defacement, reduced masticatory capability, and loss of speech, which severely affect the patient's quality of life. Ideally, mandibular continuity defect reconstruction should not only restore the anatomical height and contour of the missing part, but should, in addition, allow re-establishment of oral function.¹ Until now, autogenous bone transplantation—especially free vascularized tissue transfer—is considered a “gold standard of care” for mandibular reconstruction in patients undergoing major ablative surgery.^2–4 In principle, autogenous bone grafts provide all critical factors for bone regeneration, such as a scaffold for osteo-conduction, growth factors for osteo-induction, and cells for osteogenesis.⁵ However, the major problem of this approach is the requirement of autogenous donor tissue, which results, for example, in donor-site morbidity.⁶ Moreover, despite the availability of various reconstructive methods by means of autogenous tissue, perfect mandibular reconstruction, including restoration of continuity, sensation, dentition, soft tissue, function, and aesthetics, is still not achievable.^1,2 As a consequence, mandibular bone reconstruction still remains a challenge.²

However, development of reliable tissue engineering techniques might offer a next step in the evolution of mandibular reconstruction.^2,7 By definition, tissue engineering was defined as an interdisciplinary field that applies the principles of engineering and the life sciences toward the development of biological substitutes which restore, maintain, or improve tissue function.⁸ Bone tissue engineering is a relatively new method that uses scaffolds, bioactive substances, and/or cells/tissues with osteogenic potential.¹ Ideally, the scaffolds should be (1) three dimensional and highly porous with an interconnected pore network for cell growth and flow transport of nutrients and metabolic waste as well as (2) biocompatible and bioresorbable with a controllable degradation and resorption rate to match cell or tissue growth. Furthermore, these scaffolds should possess (3) suitable surface chemistry for cell attachment, proliferation, and differentiation, and (4) mechanical properties to match those of the tissues at the site of implantation.⁹ At present, a multitude of scaffolds made of various material^10–19 in combination with bioactive substances or osteogenic bone marrow stromal cells (BMSCs)^10,20–26 to initiate or enhance bone formation^{15–17,19,27–33} are under study.

In 2006, Ikada defined a concept on methodology in tissue engineering as (1) placing the construct scaffold in a bioreactor to reconstruct an engineered tissue in vitro called in vitro (or ex vivo) tissue engineering and (2) implantation of the construct scaffold in the body until a new tissue is regenerated in vivo called in vivo (or in situ) tissue engineering.³⁴ However, the construct completely lacks a pre-existing vasculature. Cell survival and tissue formation will depend on local vasculature and the speed at which a fully functional vascular supply will be developed.³⁵ This makes that the reconstruction of large-volume defects, such as mandibular continuity defects, remains challenging. Therefore, vascularization concepts gain interest and the combination of tissue engineering approaches with flap prefabrication techniques. This may eventually allow application of bone-tissue substitutes grown in vivo with the advantage of minimal donor site morbidity as compared with conventional vascularized bone grafts.¹² This review included the concepts of tissue engineering using axial vascularization in engineered bone tissues.

Nonetheless, to the best of the authors' knowledge, animal experiments as well as clinical case reports or studies on the subject of bone tissue engineering for mandibular continuity defects are currently neither systematically reviewed nor synopsized.

Therefore, the purpose of the present report was (1) to review systematically preclinical in vivo and clinical literature regarding bone tissue engineering for mandibular continuity defects, and (2) to analyze their effectiveness.

Materials and Methods

Study design

The scientific, preclinical in vivo and clinical literature regarding tissue engineered approaches for mandibular bone regeneration in continuity defects (i.e., segmental mandibular defects or total mandibular condyle replacements) was systematically reviewed.

Outcome variables

In principle, for animal experiments as well as human reports, macroscopical or histological or histomorphometric data on the amount of total bone defect bridging were chosen as a primary outcome variable. However, for human reports, clinical and/or radiographic evidence of the restoration of mandibular continuity was accepted as a surrogate outcome variable for the presently defined primary outcome variable. Concomitantly, histological or histomorphometric data of bone ingrowth, results of biomechanical testing, histological or histomorphometric records of scaffold degradation as well as clinical wound healing were selected as co-outcome variables.

Inclusion/exclusion criteria

In general, only animal in vivo experiments and human reports presenting macroscopical or histological or histomorphometric data on the amount of total bone defect bridging, histological or histomorphometric data of bone ingrowth, results of biomechanical testing, histological or histomorphometric data of scaffold degradation or information related to clinical wound healing, as well as human reports presenting clinical and/or radiographic evidence of restoration of mandibular continuity were included.

The following detailed inclusion criteria were used:

• 1. Research paper presenting in vivo animal data;

• 2. Research paper presenting human data;

• 3. Defect characteristics should be clearly stated;

• 4. Implantation site should be clearly mentioned;

• 5. Reconstructive technique (i.e.: tissue engineering) should be clearly stated;

• 6. Healing period should be clearly stated;

• 7. The animal model used should be described conspicuously (species, age);

• 8. Amount of total bone defect bridging, and/or percentage of bone ingrowths, and/or results of biomechanical testing, and/or percentage scaffold degradation, and/or information related to clinical wound healing had to be presented;

• 9. For human reports, clinical and/or radiographic evidence of restoration of mandibular continuity had to be presented

Studies that did not meet all the inclusion criteria mentioned earlier, for example, ex-vivo studies or studies not addressing tissue-engineered approaches for mandibular bone regeneration in continuity defects, were excluded.

Search strategy

An electronic search in the database of the National Library of Medicine (www.ncbi.nlm.nih.gov) up to September 30, 2012 was carried out. Only publications in English were considered, and the search was broadened to animals and humans. The following search strategy was applied: ((“tissue engineering”[MeSH Terms] OR (“tissue”[All Fields] AND “engineering”[All Fields]) OR “tissue engineering”[All Fields]) OR (“tissue scaffolds”[MeSH Terms] OR (“tissue”[All Fields] AND ”scaffolds”[All Fields]) OR “tissue scaffolds”[All Fields] OR (“tissue”[All Fields] AND “scaffold”[All Fields]) OR “tissue scaffold”[All Fields]) OR (“reconstructive surgical procedures”[MeSH Terms] OR (“reconstructive”[All Fields] AND “surgical”[All Fields] AND “procedures”[All Fields]) OR “reconstructive surgical procedures”[All Fields] OR “reconstruction”[All Fields]) OR (“bone morphogenetic proteins”[MeSH Terms] OR (“bone”[All Fields] AND “morphogenetic”[All Fields] AND “proteins”[All Fields]) OR “bone morphogenetic proteins”[All Fields] OR (“bone”[All Fields] AND “morphogenetic”[All Fields] AND “protein”[All Fields]) OR “bone morphogenetic protein”[All Fields]) OR (“bone marrow cells”[MeSH Terms] OR (“bone”[All Fields] AND “marrow”[All Fields] AND “cells”[All Fields]) OR “bone marrow cells”[All Fields]) OR (“intercellular signaling peptides and proteins”[MeSH Terms] OR (“intercellular”[All Fields] AND “signaling”[All Fields] AND “peptides”[All Fields] AND “proteins”[All Fields]) OR “intercellular signaling peptides and proteins”[All Fields] OR (“growth”[All Fields] AND “factors”[All Fields]) OR “growth factors”[All Fields])) AND ((“mandible”[MeSH Terms] OR “mandible”[All Fields]) OR ((“mandible”[MeSH Terms] OR “mandible”[All Fields] OR “mandibular”[All Fields]) AND (“Continuity”[Journal] OR “continuity”[All Fields]) AND defect [All Fields])).

In addition, the ISI Web of Knowledge database was searched operating the same MeSH terms. Again, only publications in English reporting on animal experiments and human studies were considered.

Furthermore, the reference lists of related review articles and publications selected for inclusion in this review were systematically screened.

Study selection

Two independent reviewers (N.C. and L.J.) initially screened the publication titles and abstracts as identified by the electronic as well as manual search for possible inclusion. Full texts of all papers that were considered eligible for inclusion by one or both of the reviewers were obtained for further assessment against the stated inclusion criteria (Fig. 1). Both reviewers used an identical data extraction form to acquire the data independently. Any disagreement between the reviewers regarding inclusion of a certain publication or data extraction was resolved by discussion.

FIG. 1.

Selection process.

Results

Study selection

The electronic search in the databases of the National Library of Medicine and ISI Web of Knowledge resulted in the identification of 6727 and 5017 titles, respectively.

As already mentioned, these titles were initially screened by two independent reviewers (N.C. and L.J.) for possible inclusion. In order not to exclude scientific reports unintended, title screening as well as abstract assessment was accomplished to identify articles reporting, in general, on mandibular defect reconstruction (i.e., noncontinuity as well as continuity defects). Title assessment and hand search resulted in the final selection of 128 abstracts, 101 full-text articles, and 40 scientific papers reporting on tissue-engineered reconstruction of mandibular continuity defects that could be included in the present review (Fig. 1, Tables 1–4). Regarding data extraction and interpretation, any disagreement between the reviewers was resolved by discussion.

Table 1.

Excluded Articles: Reporting on Noncontinuity Defects

No	Year	Authors	Journal
1.	1998	Schliephake et al.36	J Oral Maxillofac Surg 56, 616, 1998.
2.	2002	Fisher et al.37	Engineering in Medicine and Biology, 2002. 24th Annual Conference and the Annual Fall Meeting of the Biomedical Engineering Society EMBS/BMES Conference, 2002. Proceedings of the Second Joint 2002 Oct 1, pp. 827–828.
3.	2002	Chu et al.38	Biomaterials 23, 1283, 2002.
4.	2003	Gröger et al.39	Scand J Plast Reconstr Surg Hand Surg 37, 129, 2003.
5.	2003	Nakahara et al.40	Tissue Eng 9, 153, 2003.
6.	2004	Abukawa et al.41	J Oral Maxillofac Surg 62, 601, 2004.
7.	2004	Yamada et al.42	Cell Transplant 13, 343, 2004.
8.	2004	Yamada et al.43	Clin Oral Implants Res 15, 589, 2004.
9.	2005	Ito et al.44	J Biomed Mater Res Part A 73, 63, 2005.
10.	2005	Li et al.45	Aust N Z J Surg 75, 1017, 2005.
11.	2005	Marei et al.46	Tissue Eng 11, 751, 2005.
12.	2005	Meyer et al.47	Int J Oral Maxillofac Implants 20, 882, 2005.
13.	2005	Ren et al.48	J Biomed Mater Res Part A 74, 562, 2005.
14.	2006	Ito et al.49	Clin Oral Implants Res 17, 579, 2006.
15.	2007	Mylonas et al.50	J Prosthodont 16, 421, 2007.
16.	2007	Ren et al.51	J Biomater Sci Polym Ed 18, 505, 2007.
17.	2007	Rai et al.52	J Oral Maxillofac Surg 65, 2195, 2007.
18.	2007	Wang et al.53	Biomaterials 28, 3338, 2007.
19.	2007	Zhang et al.54	Biomaterials 28, 4635, 2007
20.	2008	Kuznetsov et al.55	Biomaterials 29, 4211,2008
21.	2008	Tang et al.56	Cell Biol Int 32, 1150, 2008.
22.	2008	Wang et al.57	J Clin Rehab Tissue Eng Res 12, 9762,2008.
23.	2009	Abukawa et al.58	J Oral Maxillofac Surg 67, 335, 2009.
24.	2009	Appleford et al.59	J Biomed Mater Res Part A 89, 1019, 2009.
25.	2009	d'Aquino et al.60	Eur Cells Mater 18, 75, 2009.
26.	2009	Guo et al.61	Acta Biomater 5, 268, 2009.
27.	2009	Jiang et al.62	Biomaterials 30, 4522, 2009.
28.	2009	Schliephake et al.63	Int J Oral Maxillofac Surg 38, 166, 2009.
29.	2009	Schuckert et al.64	Tissue Eng Part A 15, 493, 2009.
30.	2009	Shi et al.65	J Biomater Appl 23, 331, 2009.
31.	2009	Wang et al.66	Biomaterials 30, 2489, 2009.
32.	2009	Yao et al.67	J Biomed Mater Res Part B Appl Biomater 91, 805, 2009.
33.	2009	Yoshimi et al.68	J Craniofac Surg 20, 1523, 2009.
34.	2009	Zhang et al.69	Biomed Mater 4, 045007, 2009.
35.	2009	Zhang et al.70	J Controlled Release 136, 172, 2009.
36.	2009	Zhao et al.71	Bone 45, 517, 2009.
37.	2009	Zheng et al.72	J Dent Res 88, 249, 2009.
38.	2010	Gallego et al.73	Tissue Eng Part A 16, 1179, 2010.
39.	2010	Huang et al.74	J Biomed Mater Res Part A 95, 993, 2010.
40.	2010	Li et al.75	J Biomed Mater Res Part A 95, 973, 2010.
41.	2010	Parrilla et al.76	Arch Otolaryngol Head Neck Surg 137, 463, 2011.
42.	2010	Ribeiro et al.77	J Clin Periodontol 37, 1128, 2010.
43.	2010	Zhao et al.78	Oral Diseases 16, 46, 2010.
44.	2011	Dormer et al.79	J Oral Maxillofac Surg 69, e50, 2011.
45.	2011	Kohgo et al.80	Int J Periodontics Restorative Dent 31, e9, 2011.
46.	2011	Ito et al.81	Int J Oral Maxillofac Implants 26, 947, 2011.
47.	2011	Parrilla et al.82	Head Neck 32, 310, 2010.
48.	2011	Yamada et al.83	Cell Transplant 20, 1003, 2011.
49.	2011	Zhu et al.84	Osteoarthritis Cartilage 19, 743, 2011.
50.	2012	Yeo et al.85	Clin Oral Implants Res 23, 1322, 2012.
51.	2012	Vahabi et al.86	Chang Gung Med J 35, 28, 2012.
52.	2012	Zhou et al.87	Mater Sci Eng C 32, 994, 2012.
53.	2012	Zou et al.88	PLoS One 7, e32355, 2012.

Table 2.

Excluded Articles: Reasons Others Than Reporting on Noncontinuity Defects

No	Year	Authors	Titles	Journal	Reason(s) for excluding Articles
1.	1997	Schliephake and Langner89	Reconstruction of the mandible by prefabricated autogenous bone grafts—an experimental study in minipigs.	Int J Oral Maxillofac Surg 26, 244, 1997.	Defect characteristics not clearly indicated
2.	2001	Terheyden et al.90	Mandibular reconstruction with a prefabricated vascularized bone graft using recombinant human osteogenic protein-1: an experimental study in miniature pigs. Part I: Prefabrication.	Int J Oral Maxillofac Surg 30, 373, 2001.	No mandibular reconstruction
3.	2003	Abu-Serriah et al.91	The role of ultrasound in monitoring reconstruction of mandibular continuity defects using osteogenic protein-1 (rhOP-1).	Int J Oral Maxillofac Surg 32, 619, 2003.	Supplement report of Abu-Serriah et al.19
4.	2003	Marei et al.92	Fabrication of polymer root form scaffolds to be utilized for alveolar bone regeneration.	Tissue Eng 9, 713, 2003.	No mandibular reconstruction
5.	2008	Young et al.93	Development and characterization of a rabbit alveolar bone nonhealing defect model.	J Biomed Mater Res Part A 86, 182, 2008.	No mandibular reconstruction
6.	2008	Ko et al.94	In vitro osteogenic differentiation of human mesenchymal stem cells and in vivo bone formation in composite nanofiber meshes.	Tissue Eng Part A 14, 2105, 2008.	No mandibular reconstruction
7.	2010	Glied and Kraut.95	Off-label use of rhBMP-2 for reconstruction of critical-sized mandibular defects.	N Y State Dent J 76, 32, 2010.	Reconstruction technique not clearly mentioned
8.	2011	Matsushima et al.96	The nature and role of periosteum in bone and cartilage regeneration.	Cells Tissues Organs 194, 320, 2011.	No mandibular reconstruction

Table 3.

Characteristics of the Included Articles Reporting on Continuity Defects in Animal Experiments

				Defect		Osteogenic cells or tissues
No	Year	Authors	Species	model	Carrier	BMPs→[total dosage]	Healing period	Considered outcome variables
1.	1991	Toriumi et al.97	Dog	Body	Inactive dog bone matrix	rhBMP-2 [0.25 mg]	3 and 6 months	Clinical wound healing
								Bone bridging
								Bone ingrowths
								Biomechanics
								Scaffold degradation
2.	1996	Boyne29	Monkey	Body	Collagen	rhBMP-2 [not mentioned]	5 months	Clinical wound healing
								Bone bridging
								Bone ingrowths
3.	1999	Toriumi et al.98	Dog	Body	poly(lactide-co-glycolide)	rhBMP-2 [not mentioned]	3 and 30 months	Clinical wound healing
								Bone bridging
								Bone ingrowths
4.	1999	Boyne et al.99	Monkey	Body	collagen sponge	rhBMP-2 [not mentioned]	5 and 16 months	Clinical wound healing
								Bone bridging
								Bone ingrowths
5.	2001	Schliephake et al.100	Sheep	Body	Pyrolized bovine bone	Autogenous osteoprogenitor cells	5 months	Clinical wound healing
								Bone ingrowths
6.	2001	Boyne101	Monkey	Body	Collagen	rhBMP-2 [not mentioned]	5 and 6 months	Clinical wound healing
								Bone bridging
7.	2001	Seto et al.102	Monkey	Body	Poly-D,L-lactic coglycolic acid	rhBMP-2 [3.0, 2.5, or 1.0 mg]	16 weeks	Clinical wound healing
								Bone bridging
8.	2002	Seto et al.31	Dog	Body	Poly-D,L-lactic coglycolic acid-coated gelatin sponge	rhBMP-2 [not mentioned]	0, 4, 12, 24 and 48 weeks	Clinical wound healing
								Bone bridging
9.	2002	Marukawa et al.103	Monkey	Body	Poly-D,L-lactic coglycolic acid-coated gelatin sponge	rhBMP-2 [9 mg]	15 and 30 weeks	Clinical wound healing
								Bone bridging
10.	2004	Kontaxis et al.16	Sheep	Body	Collagen	rhBMP-7 [not mentioned]	3 months	Bone bridging
								Biomechanics
11.	2004	Wang et al.33	Minipigs	Body	Carboxymethylcellulose stabilized collagenous matrix	rhBMP-7 [3 mg]	12 weeks	Clinical wound healing
								Bone bridging
								Bone ingrowths
								Biomechanics
								Scaffold degradation
12.	2005	Fennis et al.104	Goat	Angle	Poly-(D,L- lactide) tray	Autogenous particulate bone graft mixed with platelet-rich plasma	6 weeks	Bone bridging
								Scaffold degradation
13.	2005	Abu-Serriah et al.19	Sheep	Body	Collagen	rhBMP-7 [≈7 mg]	3 months	Bone bridging
								Bone ingrowths
								Biomechanics
14.	2006	Xi et al.105	Goat	Body	Natural coral granules	rhBMP-2 [not mentioned]-induced autogenous BMSCs	16 weeks	Clinical wound healing
								Bone bridging
								Bone ingrowths
								Scaffold degradation
15.	2006	Wu et al.106	Dog	Body	β-TCP	rhBMP-2 [2.1 mg]-induced autogenous BMSCs	12 weeks	Clinical wound healing
								Bone bridging
								Bone ingrowths
16.	2006	Seto et al.18	Monkey	Body	Collagen beads Collagen sponge	rhBMP-2 [2 mg]-induced BMCs	24 weeks	Clinical wound healing
								Bone bridging
								Bone ingrowths
17.	2007	He et al.25	Dog	Body	β-TCP	Osteogenic-induced autogenous BMSCs	3 months	Clinical wound healing
								Bone ingrowths
								Biomechanics
18.	2007	Yuan et al.4	Dog	Body	β-TCP	Osteogenic-induced autogenous BMSCs	32 weeks	Clinical wound healing
								Bone bridging
								Biomechanics
19.	2007	Ayoub et al.17	Sheep	Body	Collagen	rhBMP-7 [3.5 mg] (applied in muscle for prefabrication)	3 months	Clinical wound healing
								Bone bridging
								Bone ingrowths
20.	2007	Spector et al.32	Dog	Body	Collagen sponge containing HA/TCP granules (compression-resistant matrix)	rhBMP-2 [2 mg]	26 months	Clinical wound healing
								Bone bridging
								Bone ingrowths
21.	2009	Nolff et al.107	Sheep	Body	β-TCP	Autogenous bone marrow and cancellous bone	12 weeks	Clinical wound healing
								Bone bridging
								Bone ingrowths
								Scaffold degradation
22.	2010	El-Bialy et al.108	Rabbit	Condyle	Collagen sponge inserted into biodegradable urinary bladder extracellular matrix	Autogenous, osteogenic, and chrondrogenic-differentiated BMSCs	4 weeks	Bone ingrowths
23.	2010	Nolff et al.11	Sheep	Body	β-TCP	Autogenous bone marrow and morselized cancellous bone	12 weeks	Clinical wound healing
								Bone bridging
								Scaffold degradation
24.	2010	Jégoux et al.26	Dog	Body	Biphasic calcium phosphate ceramic (HA/β-TCP)—wrapped in a resorbable porcine collagen membrane	Autogenous bone marrow	24 weeks	Clinical wound healing
								Bone bridging
								Bone ingrowths
								Scaffold degradation
25.	2010	Yuan et al.10	Dog	Body	Natural coral	Osteogenic-induced autogenous BMSCs	12 and 32 weeks	Clinical wound healing
								Bone bridging
								Bone ingrowths
								Biomechanics
								Scaffold degradation
26.	2010	Zhou et al.109	Monkey	Body	Demineralized freeze-dried bone allograft or Coralline hydroxyapatite	rhBMP-2 [4.5 mg] (applied in muscle for prefabrication)	26 weeks	Clinical wound healing
								Bone bridging
								Bone ingrowths
								Scaffold degradation
27.	2012	Busuttil Naudi et al.30	Rabbit	Body	β-TCP	rhBMP-7 [400 ng]	3 months	Clinical wound healing
								Bone bridging
								Bone ingrowths
								Scaffold degradation
								Biomechanics
28.	2012	Hussein et al.110	Dog	Body	Collagen sponge	rhBMP-2 [1.6 mg]	12 weeks	Bone bridging
29.	2012	Herford et al.15	Monkey	Body	Collagen sponge or Collagen sponge combined with HA/TCP or ceramic/collagen composite with HA/TCP (compression-resistant matrix)	rhBMP-2 [0, 6, 12 or 16 mg]	6 months	Bone bridging
								Scaffold degradation

BMSC, bone marrow stromal cell; β-TCP, beta-tri calcium phosphate; rhBMP-2, recombinant human bone morphogenetic protein-2; rhBMP-7, recombinant human bone morphogenetic protein-7.

Table 4.

Characteristics of the Included Articles Reporting on Continuity Defects in Human Studies

No	Year	Authors	Number of patients	Carrier	Osteogenic tissues/BMPs [total dosage]
1.	2001	Moghadam et al.111	1	Allogenic demineralized bone matrix gel	Native human BMPs [200 mg]
2.	2002	Ferretti and Ripamonti112	6	Allogenic bone matrix	Xenogeneic BMPs [2–8 mg]
3.	2004	Warnke et al.113	1	Xenogeneic bone mineral blocks	Autologous bone marrow/rhBMP-7 [7 mg]
4.	2006	Warnke et al.114	Supplement report of Warnke et al.113
5.	2007	Herford et al.115	2	Collagen sponge	Autogenous bone/rhBMP-2 [8 mg] or rhBMP-2 [8 mg]
6.	2008	Clokie and Sandor116	10	Allogenic demineralized bone matrix in a reverse-phase medium	rhBMP-7 [not mentioned]
7.	2008	Carter et al.117	4	Collagen sponge or Collagen sponge/allogenic bone	Autogenous bone marrow/rhBMP-2 [12 mg] or rhBMP-2 [8.4 mg]
8.	2008	Herford and Boyne118	14	Collagen sponge	rhBMP-2 [4.2–6 mg]
9.	2010	Kokemueller et al.119	1	β-TCP	Autogenous bone/autogenous marrow
10.	2010	Herford and Cicciù120	1	Collagen sponge	Autogenous bone/rhBMP-2 [4.2 mg]
11.	2012	Cicciù et al.121	1	Collagen sponge/allogenic bone	rhBMP-2 [not mentioned]

General characteristics of the included studies

In total, 29 papers reported on animal experiments. Twenty-seven of these articles presented data on segmental mandibular body reconstruction,^{4,10,11,15–19,25,26,29–33,97–103,105–107,109,110} one article reported on mandibular angle reconstruction,¹⁰⁴ and another one presented data on mandibular condyle reconstruction.¹⁰⁸ Research was done in rabbits,^30,108 sheep,^{11,16,17,19,100,107} goats,^104,105 as well as in minipigs,³³ dogs^{4,10,25,26,31,32,97,98,106,110}, and monkeys.^{15,18,29,99,101–103,109} In several studies, teeth were extracted in advance, and oral mucosa was allowed to heal completely before resective surgery and reconstructive therapy.^4,26,105,106 Beside the diversity in animal models, study design as well as healing periods after reconstructive surgery (range: 4–48 weeks) were not uniform. The follow-up periods were related to differences in animal species; for example, dog (12–48 weeks), monkey (16–30 weeks), sheep (12–20 weeks), rabbit (12–16 weeks), goat (6–16 weeks), and minipigs (16 weeks).

Furthermore, 11 out of the 40 articles presented human data on mandibular reconstruction.^111–121

The general characteristics of the included animal and clinical studies are summarized in Tables 3 and 4.

Animal studies

Autogenous bone precursor cells or autogenous osteogenic tissues

As described, bone tissue engineering is an approach that combines scaffolds with osteogenic cells/tissues and/or bioactive substances. In preclinical animal models, in principle, two different strategies for bone reconstruction in continuity defects have been intensively investigated, that is, the implantation of autogenous bone precursor cells or autogenous osteogenic tissues—which contain osteoprogenitor cells and/or mesenchymal stem cells—and the application of bone morphogenetic proteins. Both were combined with a range of different carrier biomaterials.

In total, 12 scientific papers^{4,10,11,18,25,26,100,104–108} reporting on autogenous bone precursor cells or autogenous osteogenic tissues were finally appropriate for inclusion in the current systematic review. Due to their experimental diversity, these studies are briefly summarized (Details can be found in Addendum No. 1 in Supplementary Data; Supplementary Data are available online at www.liebertpub.com/teb).

In summary, autogenous bone precursor cells or autogenous osteogenic tissues were primarily combined with calcium phosphate ceramic scaffolds, such as beta-tri calcium phosphate (β-TCP)^{4,11,25,26,106} and biphasic calcium phosphate ceramic,²⁶ or pyrolized bovine bone¹⁰⁰ or calcium carbonate, such as natural corals.^10,105 Considering the primary outcome variable bone bridging^{4,11,18,26,104–107,117} as well as the co-outcome variables bone ingrowth¹⁰⁰ and biomechanical testing,^4,10,25 autogenous bone precursor cells or autogenous tissues seeded onto calcium phosphate ceramic scaffolds showed in preclinical animal studies the potential of an alternative to autograft bone for mandibular bone reconstruction in continuity defects. Moreover, autogenous bone precursor cells or autogenous osteogenic tissues seeded onto or mixed with collagen sponges^18,108 demonstrated in preclinical animal studies promising results in terms of the primary outcome variable bone bridging¹⁸ or the co-outcome variable bone ingrowths.¹⁰⁸ In contrast, autogenous bone precursor cells containing tissues filled in preshaped poly–D, L–lactide trays did not show such a potential as an alternative to autograft bone for mandibular bone reconstruction.¹⁰⁴

Bone morphogenetic proteins

Furthermore, in total, 15 scientific papers^{15,18,29,31,32,97–99,101–103,105,106,109,110} reporting on recombinant human bone morphogenetic protein-2 (rhBMP-2) as well as five publications^{16,17,19,30,33} presenting data on recombinant human bone morphogenetic protein-7 (rhBMP-7) were eventually included in the current systematic approach. Thereby, the brief summarized studies of Kontaxis et al.,¹⁶ Abu-Serriah et al.,¹⁹ Boyne,^29,101 Busuttil Naudi et al.,³⁰ and Toriumi et al.⁹⁷ may give a good general impression of the effectiveness of rhBMPs regarding the quantity as well as quality of induced bone for the reconstruction of continuity mandibular defects (Details can be found in Addendum No. 2 in Supplementary Data).

In summary, rhBMP-2 and rhBMP-7 were studied in combination with collagen/collagen composite scaffolds,^{15–19,29,32,33,99,101,108,110} poly-D,L-lactide coglycolic acid based carriers.^{31,98,102,103} β-TCP³⁰, as well as coralline hydroxyapatite¹⁰⁹ and allogenic bone matrix.⁹⁷ Regarding the primary outcome variable bone bridging^{15–19,29–33,97–99,101–103,109,110} as well as the co-outcome variables bone ingrowths^{17–19,29,30,32,33,97–99,109} and biomechanical testing,^{16,19,30,33,97} rhBMP-2 and rhBMP-7 demonstrated in preclinical animal studies their potential as an alternative to autograft bone for mandibular bone reconstruction in continuity defects. However, the published results were not uniform. It should be mentioned that in different reports, rhBMP-2 or rhBMP-7 combined with demineralized freeze-dried bone allograft,¹⁰⁹ polyglycolic co-lactic acid,¹⁰² and a bovine collagen type I carrier wrapped into a sterno-occipitalis muscle flap¹⁷ were not associated with predictable defect bridging.

In line with these results are the reported effects of rhBMP-2^{15,31,32,98,99,103,110} and rhBMP-7³³ for mandibular bone regeneration in continuity defects. On the other hand, it should be mentioned that rhBMP-2, in the reports of Zhou et al.¹⁰⁹ and Seto et al.,¹⁰² as well as rhBMP-7, in the paper of Ayoub et al.,¹⁷ have not always been associated with bony union. Likewise, in the paper of Ayoub et al.,¹⁷ rhBMP-7 was not in all animals associated with complete bone regeneration.

Human case report

In addition to animal experiments, bone tissue engineering for reconstruction of mandibular continuity defects has been investigated in humans. Similar to preclinical models, considerable interest for therapeutic use has been focused on the application of autogenous osteogenic tissues or bone morphogenetic proteins, both of which are combined with a range of different carrier biomaterials. In total, 11 scientific papers^111–121 reporting on 10 different investigation entities were finally included in the current systematic review.

Autogenous osteogenic tissues and bone morphogenetic proteins (Details can be found in Addendum No. 3 in Supplementary Data)

In summary, transplantation of tissue-engineered autogenous osteogenic tissues without additional application of osteoinductive BMPs¹¹⁹ or in combination with rhBMP-2^115,117,120 as well as rhBMP-7^110,111 was associated with restored mandibular continuity in five cases, but in one case,¹¹⁷ no bony union was observed. Furthermore, in 16 patients in some reports,^115,118,121 osteoinductive rhBMP-2 loaded onto different biomaterials without concomitant transplantation of autogenous osteogenic tissue was followed by restored mandibular continuity. Again, this did not occur in one subject.¹¹⁷ Moreover, in 10 patients rhBMP-7,¹¹⁶ in one patient native human BMPs¹¹¹ and in two patients xenogeneic BMPs¹¹² without concurrent transplantation of autogenous osteogenic tissue were associated with reconstructed mandibular continuity. However, this was not observed in four subjects treated with xenogeneic BMPs.¹¹²

Discussion

Currently, bone tissue engineering can be considered a highly promising approach and as an alternative bone source. Well-performed in vitro and in vivo experiments are essential to determine the suitability of the chosen concept and to understand the risks before proceeding into the clinical trial.^122,123 In vitro studies require a desired monitored environment that mimics the dynamics of the in vivo condition by a controlled homogeneity of nutrients media (also in terms of pH/osmolarity), additional osteogenic stimuli(s), and providing a physical stimulation as relevant key components for bone construction.¹²⁴ However, the in vitro condition is unable to provide physiological function and is never exactly the same condition as the in vivo one.¹²⁴ The results from in vitro studies do not give direct information or can be difficult to infer from the in vivo situation,¹²⁵ but are rather considered baseline properties.¹²⁶ For this reason, the use of animal models is often an essential step in the testing of tissue engineering before clinical use.

The aims of the present report were to review systematically preclinical in vivo as well as clinical literature regarding bone tissue engineering for mandibular continuity defects and to analyze the effectiveness of this approach for the treatment of mandibular continuity defects.

In total, 29 publications reporting on animal experiments and 11 papers presenting human cases could be included in the present systematic review. The evaluated articles of the first part of the current review report on tissue-engineered reconstructions of segmental mandibular body, angle, or condyle defects in different animal species. Thus, autogenous bone precursor cells or autogenous osteogenic tissues were primarily combined with calcium phosphate ceramic scaffolds. Regarding bone bridging, bone ingrowth, as well as biomechanical testing, these tissue-engineered approaches demonstrated a certain potential as an alternative to autograft bone for mandibular bone reconstruction in continuity defects. In principle, these results were not unexpected and were in line with the literature for bone tissue engineering in general. It is well known that BMSCs are capable of self-renewal and differentiation into various osteogenic lineage cells.¹²⁷ Furthermore, their osteogenic potential has been demonstrated both in vitro and in vivo. Consequently, BMSCs became a major seed cell source for bone tissue engineering. Moreover, many previous studies have succeeded in repairing bone defects by using BMSCs in animal models as well as in humans.^24,127–129

Besides, due to their compositional similarities to bone mineral, their excellent biocompatibility, osteoconductivity, as well as drug delivery potential, calcium phosphates, especially tricalcium phosphate and hydroxyapatite, are the most widely used bone substitutes in bone tissue engineering.¹²⁷ Moreover, BMSCs seeded onto calcium phosphate scaffolds induced ectopic bone formation in a mice model.¹²⁹

However, the currently presented favorable data for bone tissue-engineered constructs as compared with scaffolds alone have to be interpreted with caution. In principle, sample size and thereby statistical power of the reviewed preclinical in vivo experiments tended to be low. For example, the compared 12 weeks of bone bridging and bone ingrowths¹⁰⁶ originate from only two animals/segmental defects without statistical analysis. Another example is the 3 months data.²⁵ Their statistically significant better biomechanical results (p<0.05) for tissue-engineered bone as compared with the scaffold alone originate from not more than three animals/segmental defects. Thus, with an assumed α-error of 0.05, post hoc analysis for, for example, compression strength reveals a statistical power as low as 0.385. Furthermore, next to autogenous bone precursor cells or autogenous osteogenic tissues, bone morphogenetic proteins (i.e.: rhBMP-2 and rhBMP-7) were studied. Predominantly, rhBMP-2 and rhBMP-7 were combined with collagen/collagen composite scaffolds. However, a few papers examined combinations with poly-D,L-lactide coglycolic acid as well as calcium phosphate carriers. Regarding bone bridging, bone ingrowth, as well as biomechanical testing, these tissue-engineered approaches displayed some potential as an alternative to autograft bone for mandibular bone reconstruction in continuity defects. However, the published results were not uniform. For example, rhBMP-2 or rhBMP-7 combined with a bovine collagen type I carrier,¹⁷ polyglycolic co-lactic acid¹⁰² as well as demineralized freeze-dried bone allograft¹⁰⁹ were not associated with predictable defect bridging. Overall, the published outcomes for bone morphogenetic proteins were not surprising and were in line with the reports for bone tissue engineering in general. The osteoinductive potential of BMP-2 and BMP-7¹³⁰ as well as the general importance of carrier selection in conjunction with growth factor application^{15,115,131–133} are well known. In addition, for these reviewed preclinical in vivo experiments, sample size and thereby transferability tended to be low. For instance, the 3 months bone bridging data and the found wide range of mechanical properties of Abu-Serriah et al.¹⁹ were obtained from not more than six animals/segmental defects. Another good example is the publication of Boyne.²⁹ Their 5 months of bone bridging and bone histology data originate from only three animals/segmental defects. Unfortunately, a meta-analytical approach to increase the power of statistical analysis by pooling the results of all retrieved available trials was not feasible. Research of results that are combined in a meta-analysis should preferably be done in a similar manner. As shown in Table 3, this is clearly not the case for the presently included papers. The publications, which were eligible for inclusion in the present study, display experimental variability for the utilized animal model, the anatomical site of reconstruction, the used bone tissue engineering approach, the number of enrolled animals/defects, and the healing time after reconstruction.

Apart from BMPs, alternative growth factors may serve as potential therapeutic agents to enhance bone and cartilage formation; for example, recombinant human platelet-derived growth factor (rhPDGF), transforming growth factor-beta (TGF-β), fibroblast growth factor, recombinant human growth/differentiation factor-5 (rhGDF-5), and insulin-like growth factor.¹³⁴ PDGF is known to simulate angiogenesis through activation of the macrophages,¹³⁴ which secrete factors cells to form new capillary sprouts. TGF-β1 has been proved to promote cartilage regeneration.⁷⁹ rhGDF-5 has the potential to grow the same type of tissues as where it is naturally present. Its possibility of being used in a tissue engineering approach has been reported for the regeneration of dento-alveolar tissues.^135,136

However, a single dose of an exogenous protein will not adequately induce a biologic response in compromised tissue conditions. Gene therapy is another concept in which genetic information is transferred into target cells. Subsequently, the cells synthesize the endogenous protein encoded by the gene.¹³⁷ The process that involves the transfer of functional genetic information into the target cell is known as transduction. This is accomplished when the recombinant vector (virus) which contains the therapeutic DNA binds to the cell, usually via a receptor-mediated process, and then enters that cell. The DNA passes into the nucleus of the cell, where it may become integrated into the host genome or may remain extrachromosomal. The transduced cells can then produce and secrete the growth factor encoded by the DNA98-100.^56,70 In this review, it was found that the use of gene therapy was being applied in the reconstruction of mandibular continuity defects in animals and humans. On the other hand, gene therapy has been reported for the repair of the mandibular condyle and temporomandibular joints and was found to support mineralized tissue formation.^138–140

In the second part of the review, 11 papers, presenting human cases regarding tissue engineered reconstruction of mandibular continuity defects, were eventually included. The review included the reports on microvascular tissue transfer of prefabricated bones in the study. Although, these techniques belong to tissue regeneration, they are important for the reconstructive surgeon. Therefore, transplantation of tissue-engineered autogenous osteogenic tissues without additional application of osteoinductive BMPs or in combination with rhBMP-2/rhBMP-7 produced restored mandibular continuity in five out of six cases. Furthermore, in 29 out of 34 patients, the application of native human BMPs, xenogeneic BMPs, rhBMP-2, or rhBMP-7 without concomitant transplantation of autogenous osteogenic tissue was associated with complete bony defect bridging. Unfortunately, no direct comparison of the results with autogenous bone transplantation can be done due to lack of direct control. However, Herford and Cicciù¹²⁰ stated that bone growth cytokines can be considered a predictable alternative to traditional grafting techniques. In general, these results were not astonishing and in line with the literature.^{15,115,141–144} Thus, it might be assumed that these tissue-engineered approaches may have, in certain selected patients, some potential as an alternative to autograft bone for mandibular bone reconstruction in continuity defects. However, it should be underlined that until now only a few successfully treated cases have been published. Furthermore, to date, the clinical predictability has to be questioned. An additional issue is the limited license of the use of rhBMP-2 in oral and maxillofacial surgery. According to the Center for Devices and Radiological Health (CDRH) of the U.S. Food and Drug Administration (FDA), rhBMP-2 is not licensed for use in surgery of mandibular continuity defects and may only be applied for sinus augmentation and localized alveolar ridge augmentation.

Conclusions

The reviews showed a various study methodology, review period, and different control groups. Not all studies compared the finding with a reconstruction with autologous bone substitute. None of the human studies were performed as a randomized control trial study. Within the limits of this systematic approach to the literature regarding tissue-engineered bone reconstruction in continuity defects of the mandible, we conclude that (1) published preclinical in vivo as well as clinical data are limited, and (2) tissue-engineered approaches demonstrate some clinical potential as an alternative to autograft bone. The future research in this area needs to include process evaluation research in order to define the characteristics contributing to the success and failure of any intervention.

Acknowledgment

This article was supported by the National Medical Research Council (NMRC), Singapore.

Disclosure Statement

No competing financial interests exist.