Mandibular reconstruction: a review

Mandibular defects may occur from maxillofacial injury, inflammatory disease, benign or malignant tumour resections and osteoradionecrosis. Mastication, speech and facial disfigurement are often severely compromised without reconstruction leading to poor quality of life. The goal of mandibular reconstruction is to restore facial form and function, repair of mandibular continuity and soft tissue reanimation. Implant insertion should be feasible to allow occlusal rehabilitation and if possible the inferior alveolar nerve function should be restored. Mandibular reconstruction principles and techniques have evolved dramatically over the years. Refinements in techniques continue to improve patient quality of life. This paper reviews short history, current techniques and few promising future endevours related to mandibular reconstruction.

Website: https://www.banglajol.info/index.php/UpDCJWar I using grafting to treat mandibular defects 8 .The introduction of internal fixation using plate and screws post World War II with the advantage of using antibiotics gave surgeons high success rate over the previous procedures 9 .The next thing adopted were free, nonvascularized, bone grafts 10 , harvested in distant locations and held in place, at the receptor site, by metallic reconstruction plates, usually titanium.Several bones, as tibia, iliac crest, or ribs, were often used as donor sites 9 .Use of threaded Kirschner (K) wire in 47 patients were reported by Castermans et al. 11 in 1977.Bowerman used titanium plate to reconstruct the mandible in 17 patients 12 .Leuke and Rappaport, Schwartz and Albert and associates addressed the use of Dacron urethane mesh for holding the cancellous chips 13 .Wersal et al. 14 addressed splitrib grafts for the mandibular reconstruction.Bradley 15 in 1978 and 1982 reported a two-stage procedure for reimplantation of 'autogenous freeze treated mandibular bone'.It was further evident by Dong et al. 16 through a large series of mandibular reconstruction using 'autogenous freeze treated mandibular bone' for tumours of the mandible and floor of the mouth.Taylor, as well as Sanders and Mayou described the deep circumflex iliac artery and vein (DCIA/V) based free transfer of iliac bone and the overlying skin as a reliable and easily utilizable reconstruction option 17 .For the first time, Swartz et al. 18 introduced the scapular osteocutaneous free flap (SOFF) in 1986 for use in head and neck reconstruction.Another milestone achieved in 1989, Hidalgo 19 introduced the transfer of fibula bone to reconstruct a segmental defect of the mandible.Recently, in 2010, partial soleus muscle combined with fibula osteoseptocutaneous flap for dead space obliteration aws reported by Kuo et al. 20 .

GOALS OF RECONSTRUCTION
Mandible, the U-shaped bone which forms bony foundation of the lower face and extremely important for facial aesthetics 21 .It also serves as the attachment for tongue and muscles of the floor of the mouth.Functions of mandible include mastication, deglutition, airway patency and speech which need complex units of tissue.In addition, to bear the dentition is unique to mandible.Thus, any attempt to reconstruct a mandible would ideally need to reconstruct the height and shape of the missing part anatomically, to provide a platform for dental rehabilitation post reconstruction, to manage the forces that act on the mandible in normal functioning, to withstand similar fracture threshold to the intact mandible, to allow early or immediate masticatory function, restoration of the supporting muscle and soft tissue envelope, to allow normal sensation to the lips and tongue, to be simple, flexible, and cost effective and to be able to sustain repeated loading 22 .The gold standard of replacing like-with-like frequently needs the use of composite tissues as well.Taking all variables into consideration the principle of reconstruction for intra-oral hard tissue defects should establish continuity, restore soft tissue loss, establish alveolar height, width and form, improve facial contours and restore functions -dental rehabilitation, mastication, deglutition, speech and oral competence 23 .

CURRENT RECONSTRUCTIVE OPTIONS
Surgical management of several conditions occurring in mandible require the resection of the pathology along with good margin.After resection, the defect might be limited to hard tissue only or may affect both hard and soft tissue which mandates reconstruction not only to replace the missing structural component, but also to restore the associated function and aesthetics 23 .This restoration of form and the function becomes more and more difficult as the tissues resected become larger and complex in nature.The following reconstructive ladder is a list of options starting from the simplest to the more complex methods currently available: 1. Healing by secondary intention and/or primary closure 2. Skin graft 3. Skin graft substitutes 4. Reconstructive plate 5. Autogenous bone grafts -vascularized and -non-vascularized 6. Bone graft substitutes 7. Regional flaps and distant flaps eg.Pectoralis major myocutaneous flap, Latissimus dorsi flap etc. 8. Vascularized free flaps eg.Fibula flap, Scapula flap, Radial bone flap, Metatarsal bone flap etc. 9. Recent advances eg.Transport disc distraction osteogenesis, Modular endoprosthesis, 3D printed custom made prosthesis, tissueengineering, stem cell technology etc.

RECENT ADVANCES AND POSSIBLE FUTURE DEVELOPMENTS
Recently, new techniques for mandibular reconstruction have been tested, with a common aim of eliminating the need for harvesting bone from a donor site ensuring less surgical morbidities 1 .These include transport disc distraction osteogenesis (TDDO), modular endoprosthesis, tissue engineering and stem cells.Application of 3D printing also hold bright future in the reconstruction of mandibular defect along with innovative materials coming soon to be capable of printing biocompatible prosthesis.

TDDO
The first clinical cases of mandibular lengthening by distraction osteogenesis were reported by McCarthy in 1992 24 .Since then, Website: https://www.banglajol.info/index.php/UpDCJthis technique continues to evolve.Recently, a modification known as TDDO (Figure 2) is used to reconstruct mandible.A segment of bone is adjacent to the defect and moved gradually across the defect by a distraction device.New bone fills in between the two bone segments.The piece of bone being moved or transported is called as the transport disc 25 .In 1995, Costantin, et al. 26 successfully applied transport distraction to restore the continuity of a mandibular defect formed as a result of cancer ablation.The main drawback of distraction remains the time required to regenerate the new bone.Difficulties with the direction control and device dislodgement also harm the reconstruction sometimes.Better understanding with time and multi directional vector control will allow for greater use of this procedure.

MODULAR ENDOPROSTHESIS
Active communication between different specialties in the medical field can always bring wonderful results.'Modular endoprosthesis'-A concept which has been applied in the orthopaedic community for almost 10 years was recently introduced by Tideman and Lee 27 for reconstruction of monkey's mandible.An endoprosthesis is a metallic device that replaces diseased bone in long bones and is fixed internally with bone cement within the medullary space of the remaining healthy bone.No screw fixation is required.The variable length of the bone gap can be bridged by using modules that allow for accurate three-dimensional reconstructions.The modules are connected by a locking system.In principle, the mandible is also a suitable candidate for such an endoprosthesis because of the existing medullary space.Dental rehabilitation could be achieved by screwing implants into existing holes of the endoprosthesis.Immediate accurate three-dimensional replacement of the lost part of the mandible would be achievable, and after setting of the bone cement immediate function would be possible because no screws are involved.Whether this system will also work in patients with compromised soft tissues remains to be seen, but the principle is worthy of further research.

TISSUE-ENGINEERING
To date, there is only one published case of successful reconstruction of the mandible in human, using the principle of tissue engineering 28,29 .As reported by Warnke et al., it concerned a patient who underwent a secondary reconstruction after tumour resection.The engineered graft was allowed to heal in the trapezius muscle and subsequently transplanted to the recipient side, using microvascular anastomosis.This clinical application was largely based on research carried out on minipigs by Terheyden et al. 30,31 .Bone morphogenetic protein-2 (BMP-2) and BMP-7 were extensively used in tissue engineering.In principle, engineering a graft at the site of the defect would be more preferable.Moreover, it would require prolonged period of mandible immobilization with adequate soft-tissue coverage for healing.Although there is definitely a future for engineered grafts their routine clinical application is still a long way off.Apart from the technical problems there oncologic potential is a big question which requires further clarification.Another big factor is the high costs involved in using currently available bone morphogenetic proteins.However, autogenous growth factors, like those present in platelets, are mainly mitogenetic and are not known to be oncogenetic.

STEM CELLS IN MANDIBULAR RECONSTRUCTION
The regeneration of human tissues using stem cells from the patient, seeded in specially designed resorbable scaffolds and placed into a bioreactor which could simulate the natural conditions 32,33 , is an extremely exciting field that will certainly change not only the way in which mandible reconstruction takes place but also how medicine as a whole is practiced.Adipose stem cells have primarily been used in addition to tricalcium phosphate granules and recombinant human BMP.Sandor et al. 34 documented a successful reconstruction of a 10cm full defect of the mandible using adipose stem cells, tricalcium phosphate granules and recombinant human BMP without ectopic bone maturation.After 10-month maturation, dental implants were installed and bone formation was confirmed.

3D ADDITIVE MANUFACTURING AND NEW PROSTHETIC DEVICES
The application of computer aided design and computer aided manufacturing (CAD/CAM) using the high end 3D printers with bio compatible printing materials are capable of mimicking the complex anatomy of mandible with the perfect size, shape and contour.The customization of the device should go even further by reproducing the original geometry and weight, and can change the paradigm from simple titanium plates to customized prosthesis.Finally, the new prosthetic device could be prepared to include tissue regeneration strategies (Figure 3) as used by Zhou et al 35 .

CONCLUSION
This review confirms that there is no ideal solution for mandibular reconstruction.Each of the methods discussed has its shortcomings and limitations.Mandibular reconstruction cannot be completed perfectly without maintaining ideal soft tissue conditions that are frequently compromised.Another important issue is the sensory problems often noted with cancer patients who received ablative surgery.Quality-of-life studies have clearly pointed out the importance of above mentioned fundamental functions reported by the suffering patients [37][38][39][40] .Unfortunately, a reconstructed mandible with implants and occlusal rehabilitation on implants is not enough to restore these functions completely.Newer methods of mandibular reconstruction have been explored with varying degrees of success, but it remains to be seen whether these methods can overcome all the limitations.

Figure 1 .
Figure 1.Schematic image of the pedicle bone graft performed by Bardenheuer.

Figure 3 :
Figure 3: (a) Prosthetic device placed in a mandible model and (b) the prosthetic device including tissue regeneration strategies.Source: adapted from Zhou et al.35In the recent years, partial and complete face transplants have been conducted since 2005, the first of which was performed in France36 .Complete mandibular reconstruction has been documented in literature by Devauchelle et al. and the company Xilloc Medical BV manufactured and used a customized 3D printed lower jaw for complete mandibular restoration (Figure4).