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Bone graft substitute: allograft and xenograft

Rapid bone graft incorporation for structural rigidity is essential. Early range of motion, exercise, and weight-bearing are keys to rehabilitation. Structural and nonstructural bone grafts add length, height, and volume to alter alignment, function,
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  Bone Graft Substitute Allograft and Xenograft Naohiro Shibuya,  DPM, MS a, *, Daniel C. Jupiter,  PhD b INTRODUCTION Fast incorporation of bone grafts to achieve structural rigidity is essential in most footand ankle surgeries, because early range of motion, exercise, and weight-bearing arethe keys to a successful rehabilitation process. Structural and nonstructural bonegrafts have been utilized in reconstructive foot and ankle surgeries for many years. 1–4 Theycanaddlength,height,andvolumetotheskeletalstructureofthefootorankletoalter alignment, function, and appearance (  Fig. 1  ).There are several different types of bone grafts utilized in foot and ankle surgerieswhere structural rigidity is necessary. These include corticocancellous autografts, al-lografts, xenografts, and synthetic bone grafts. An autogenic bone graft is ideal inmany situations because it is harvested from the patient himself or herself. It is thusless likely to be rejected and more likely to be incorporated than allografts or Disclosures: None. a Section of Podiatry, Department of Surgery, Texas A&M University Health Science Center, Col-lege of Medicine, Central Texas Health Care System, Baylor Scott and White Health Care Sys-tem, Temple, TX, USA;  b Preventive Medicine and Community Health, University of TexasMedical Branch, Galveston, TX, USA* Corresponding author. 5112 Wildflower Lane, Temple, TX 76502. E-mail address: KEYWORDS   Bone    Autogenous graft    Foot    Ankle    Incorporation    Union KEY POINTS   Structural integrity and fast incorporation are essential to foot and ankle surgery becauseof the importance of early weight-bearing and rehabilitation.   Both allografts and xenografts eliminate donor site complications, but osteogenicity andosteoinductivity can be sacrificed.   Even though allogenic bone grafts theoretically possess lower healing potential thanautogenous grafts, clinical outcomes may not differ.   A bovine-based xenograft may not be ideal in foot and ankle surgeries.   Retrospectivereviewsofsurgicalcasesusingbonegraftscanbesubjecttoselectionbias;well-controlled prospective studies are necessary to understand the effectiveness andsafety of each graft type. Clin Podiatr Med Surg 32 (2015) 21–34 0891-8422/15/$ – see front matter Published by Elsevier Inc.  xenografts. It also has both osteogenic and inductive properties that can assist bonehealing. However, harvesting an autograft adds an extra procedure to the reconstruc-tive surgery, and donor site complication is common. 5–9 In addition, in patients whohave multiple comorbidities, harvesting the compromised bone does not lend any Fig. 1.  (  A ) To correct pes planovalgus deformity in this pediatric patient, lateral columnlengthening and medial column plantarflexory opening wedge osteotomies were utilized.A structural freeze-dried allogenic calcaneal graft was used to maintain the correction.( B ) To restore the hindfoot height in this patient with neglected calcaneal fracture, afresh-frozen femoral head allograft was utilized. ( C  ) Autograft harvested from the resectedtibial plafond was packed into the subchondral cysts before implantation. Shibuya & Jupiter 22  bone healing potential to the operative osteotomy and fusion sites. Further, an auto-graft needs to have intact cortices to ensure structural rigidity.Other forms of bone grafts, such as allografts, xenografts, and synthetic grafts, elimi-natetheneedforsecondaryproceduresandobviatedonorsitecomplications.However,rejection and slower incorporation can be disadvantages of the use of these grafts. Inwell-vascularized bone, such as the calcaneus, it has been documented that there isno difference in the complication rate at the osteotomy site between autograft and allo-grafts. 10,11 However, in less vascularized areas, incorporation can be difficult. 12,13 OVERVIEW  Allogenic Bone Graft   Allografts are procured from humans and undergo vigorous sterilization processesbefore they are ready for surgeons to use. 14 They can be prepared using a combina-tion of different processing procedures. Because of this diversity in processing proce-dures, thepropertiesof theallograftscan varywidely. Ingeneral, allogenic bonegraftscan be classified into fresh, fresh-frozen, freeze dried, and demineralized types,depending on the preparation process. Although a more vigorous sterilization processcan eliminate the chances of disease transmission and infection, it can also reduceosteogenic and osteoinductive properties (  Fig. 2  ). In general, fresher grafts aremoreexpensiveandlessreadilyavailable thanothergraftsthathavealonger shelflife. Although there are no general rules, fresh grafts are mostly used for osteochondralrepair, namely in the talus (  Fig. 3  ) in foot and ankle applications. Because fresh graftspossess more viable chondrocytes and greater subchondral structural support, theyare more suitable when both osseous and chondral structures need to be replacedat the same time. 15 Fresh-frozen allografts are often used in places where structuralrigidity is necessary (  Fig. 4  ). Although they may not be as rigid as fresh grafts, theyare more easily obtainable by surgeons. The most abundant allografts, freeze-driedgrafts, are generally used in well-vascularized areas where the host can provideenoughnativeosteoinductivefactors(  Fig.5  ).Althoughthesegraftscanbesubstitutedwith fresher grafts, cost effectiveness is greater if freeze-dried grafts provide similaroutcomes. Demineralized allogenic grafts are commonly used to fill a void or a deadspace(  Fig.6  ).Manybelievethatdemineralizedallograftscanalsobeusedtoenhancebonehealinginareaswherebonehealingisdifficult,believingthatthesegraftsprovideextra osteoinductive properties, but the evidence of this is scarce. 16 Fig. 2.  Fresh allograft possesses the best strength and osteogenicity; however, shelf life, costeffectiveness, and availability are compromised. Bone Graft Substitute  23   Xenograft   A xenograft comes from a nonhuman species. Therefore, antigenicity is significantlygreater than that of allografts. Naturally, it requires more sterile processing, whichcan result in reduced osteoinductive properties. However, owing to the abundanceof donors, these grafts may be less expensive and more readily available. Alsobecause of the extensive sterilization processes, the shelf life is generally long. Themost common xenogenous bone graft used in orthopedic surgery is bovine based. Clinical Studies Allograft   Although autogenous bone graft is considered the gold standard, the efficacy of allo-genic graft in foot and ankle surgeries has been shown to be adequate in many Fig. 3.  A fresh talar allograft is often used for osteochondral defect repair. Fig. 4.  This patient underwent a revisional subtalar fusion for correction of decreased talardeclination. To maintain the correction under a tremendous amount of compression fromweight bearing and contracted soft tissues, a more rigid fresh-frozen allograft was usedinstead of a freeze-dried allograft. Shibuya & Jupiter 24  settings. Therefore, this may obviate the need for harvesting autogenic bone grafts inmany situations in foot and ankle surgery.There are multiple case series utilizing allogenic bone graft in foot and ankle surgerywithnoclinicallysignificantcomplicationrates,especiallyinawell-vascularizedareaof  Fig. 5.  For this first metatarsal lengthening procedure in a healthy young female, a freeze-dried allograft was used for the most cost-effective result. Because of the “Z” lengtheningtechnique utilized, there was still a good apposition of native bones, and the construct didnot have to rely greatly on graft incorporation for its short-term stability. Fig. 6.  Demineralized allograft was injected via a syringe in this arthroscopic subtalar jointarthrodesis to fill the void and space, mainly for good osteoconduction. Bone Graft Substitute  25
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