Segmental bone loss is a challenging clinical condition for both the clinician and the patient. It can result from high-energy trauma, infection or tumor resection, and revision surgery.1 A segmental long bone loss of more than 2 cm or occupying more than 50% of the bone's circumference results in a defect that is unlikely to progress to uneventful healing without some type of intervention.2,3 These type of injuries account for approximately 0.4% of all fractures, with the tibia being the most commonly involved anatomic area (68%).2 Bone loss >3 cm is more frequently encountered in distal tibia.4
The standard of care when the bone defect remains less than 5 cm is the application of autologous bone graft (ABG).5 In segmental bone defects, failure to natural healing accounts for 100% of the cases.6 There are only few documented cases of spontaneous healing of large bone defects of the femur and the tibia.7,8 For bone defects more than 5 cm, the application of only autologous bone has been proven to be suboptimal because of inability of the graft to incorporate successfully with the local environment, resulting in progressive clearance of the graft.9 In this article, we present the contemporary thinking on the biologic facet of posttraumatic segmental bone loss reconstruction with emphasis on the grafting materials used for reconstruction of such defects.
DESIRABLE GRAFT PROPERTIES
Despite its optimal biologic properties, pure ABG is often inadequate regarding the volume required for regeneration of long bone defects.10 Nonvascularized cortical bone grafts can offer immediate strength but have inferior biologic properties. Vascularized bone grafts are associated with significant donor site morbidity and complications, require expert technical skills, and currently their use seems to be in decline for reconstruction of posttraumatic defects.9,11
In the management of posttraumatic long bone segmental defects, a grafting material possessing optimal density and biological properties is needed. In this context, the use of ABG should be considered in every case. However, there are some important parameters that should be taken into account. First, regardless of the harvesting method, it is associated with donor site morbidity and potential complications. Second, the issue of inadequate volume is especially pertinent in bigger defects. Third, the biologic quality of the ABG depends on the harvesting method, the technique used, and the age of the patient. Fourth, the cost of harvesting is not trivial, and it has been estimated to be comparable with commercially available bone graft substitutes.9
In segmental bone reconstruction, expansion of the ABG volume and optimization of its biologic properties is desirable. Tissue engineering using bone substitutes along with the diamond concept of bone regeneration that incorporates the mechanical stability and vascularization has greatly contributed to our philosophy of management of segmental bone defects. Major categories of bone graft materials used in addition to ABG include osteoconductive grafts [allogenic bone graft, synthetic bone grafts demineralized bone matrix (DBM)], osteoinductive molecules [bone morphogenic proteins (BMPs) and platelet-rich plasma (PRP)], and the materials possessing osteogenic properties (concentrated bone marrow).12
Allografts are tissues from genetically nonidentical subjects from the same species and can be broadly categorized in corticocancellous and cortical grafts, osteochondral and whole body segments, morselized and cancellous chips, and DBM.13
Cortical and Corticocancellous Grafts
The main field of application of cortical and corticocancellous grafts is the revision joint reconstruction surgery while osteochondral and whole body segments are principally used in orthopaedic oncology.14 Orthopaedic fracture surgery represents another field of their application in the setting of segmental long bone defects and nonunions. The contemporary orthopaedic literature is devoid of high-quality studies documenting the technique, outcomes, complications, and challenges of their use in acute setting. A modified Cappana technique has been described for reconstruction of long femoral shaft defects.15 The use of large osteochondral allografts has also been reported in traumatic defects around the knee, but the literature is sparse and of low quality.16,17 Cortical grafts are incorporated at the bone ends by creeping substitution and intramembranous bone formation, a process that involves the initial weakening of the area with subsequent strengthening on its completion.12
Morselized and Cancellous Chips
In reconstruction of long bone defects, morselized and cancellous chips are typically used as graft “expanders” to increase its volume. They facilitate osteoconduction13 and have variable degrees of osteoinductive properties. They function as scaffolds, onto which the host lays new bone and incorporated to the host bone by endochondral bone formation, which after its completion enhances the strength of the construct.9,12,18 Allograft bone retains some osteoinductive proteins with variable properties depending on the preparation, sterilization, and storage of the graft.12 The major advantage of the use of allografts over autologous bone is their unhampered availability and the lack of donor site harvesting complications. Advances in the processing of the allograft have substantially decreased the risk of infectious disease transmission, making them a safe option in the expense of losing most their osteoinductive potential.
DBM is an allograft derivative that contains various types of osteoinductive molecules [BMP, Transforming Growth Factor beta (TGFβ) 1-2-3, and Insulin-like growth factor (IGF)].12,19 It is a highly osteoconductive material, but it is devoid of structural properties because of the demineralization process. BMP-2 and BMP-7 have shown to exist in DBM, but the concentrations are variable depending on the manufacturer and processing.20,21 Original donor site and the processing of the acquired bone affect the quality of its properties.22–24 Osteoinductive properties of DBM have been demonstrated in animal studies, but there are no high-quality clinical trials reproducing these findings.25–27 For the management of long bone defects, DBM is used as volume expander along with ABG and/or other grafting material.
Synthetic Bone Grafts
Synthetic bone grafts have similar consistency to the mineral phase of human bone, and they are made using sintering. They can be broadly categorized in calcium phosphate and calcium sulfate derivatives and bioactive glasses.19
Calcium phosphate and tricalcium phosphate demonstrate low resistance in tensile forces with the first being more brittle and latter undergoing faster degradation.19 They have been used in filling of small defects in tumor and fracture surgery, but there is limited literature available regarding their use in large bone defects.19,28 In vivo, calcium sulfate is degraded and quickly absorbed before the formation of new bone, which makes it unsuitable for grafting material in significant bone defects.28 Hydroxiapatite does not allow the formation of new bone and remodeling because is hardly absorbed. Beta-tricalcium phosphate is relatively balanced between new bone formation and graft absorption lacking though osteoinductivity and osteogenicity. Composite grafts consisting of β-TC and other materials enhancing its biological and mechanical properties [such as bone morphogenic proteins (BMP), PRP, mesenchymal stem cells, bone marrow, polycaprolactone, and hydroxyapatite] have also been used.29 For the reconstruction of segmental traumatic bone defects, β-TC is mainly used as volume expander.
OSTEOGENESIS PROMOTING MATERIALS
Osteogenesis can be defined as the generation of new bone from bone-forming cells.18 Musculoskeletal stem cells can be found in bone marrow, cartilage, periosteum, fat, muscle, and vascular pericytes.18 The presence of musculoskeletal mesenchymal cells that have the potential to proliferate and differentiate into osteoblasts that are able to produce bone matrix and bone-lining osteoblast are a desirable feature of any grafting material. The osteogenic potential of the ABG is subject to interindividual variability and genetic factors along with the age of the donor play a role in that.30–32 Sagi et al10 in a prospective study comparing the molecular and histological profiles of bone grafts from iliac crest and femoral debris from reamer irrigator aspirator (RIA) reported a similar transcriptional profile in relation to genes involved at the early stages of bone formation. Furthermore, Uppal et al33 demonstrated that iliac crest and femoral reaming from RIA have similar cell viability and osteogenic potential. On the contrary, der Bel and Blokhuis34 showed that the osteogenic capacity of RIA bone graft is superior compared with that of iliac crest. Nevertheless, the techniques of harvesting, preparation, and implantation as well as the harvest-to-implant time are essential factors related to potential osteonecrosis, a complication that compromises the osteogenic properties of the graft.30,35
In segmental long bone reconstruction, augmentation of the osteogenic capacity of the graft is desirable. Bone marrow aspirate has been used as a source of mesenchymal stem cells in reconstruction of long bone defects in animal models. Giannakos et al36 concluded that the beneficial effects of bone marrow concentrate aspirate (BMAC) are documented when it is used as the primary treatment or as an adjunct. The osteogenic activity of concentrated mesenchymal stem cells (MSCs) is three-fold more than the one from bone marrow aspirate.37 Hernigou et al used BMAC from the iliac crest for the treatment of tibial nonunions and demonstrated that more than 1500 progenitor cells per ml were required in the case of optimal clinical outcome.25 Desai et al38 combined BMAC with either BMP or DBM for the management of various types of delayed unions and nonunions and concluded that the late combination is more efficacious. Despite the fact that the use of BMAC has been established in the management of nonunions,39 its use in the graft for reconstruction of long bone segmental defect remains anecdotal.
Osteoinductive molecules have long been recognized as factor that enhance and promote the osteogenecity. Bone morphogenic proteins from the transforming growth factor β superfamily have been extensively used as osteoinductive substances in the reconstruction of long bone defects form more than a decade.37 The paucity of robust high-level evidence supporting their use coupled with various issues regarding the details of their clinical use including but not limited to the ideal dosing, timing of administration, clear indications, and potential risk for carcinogenesis have led to their current withdrawal from the market.37
PRP has been demonstrated to pose significant osteoinductive properties through the release of cytokines and growth factors that can enhance the bone regeneration process from activated platelets.40–42 PRP has also been linked to release of proinflammatory cytokines and angiogenetic factors.43–45 A recent systematic review concluded that despite the favorable preclinical evidence regarding the PRP on bone healing, the low quality and the heterogenicity of the available clinical studies does not allow to draw safe and meaningful conclusions for their use in the actual clinical setting.46 The remaining unresolved issues pertaining to the use of PRP as an osteoinductive agent are the optimal volume/concentration, the best combination with other grafting materials and the clarification of the indications of its use.47
Along with the obvious osteogenic, osteoinductive, and osteoconductive properties that a grafting material for reconstruction of long bone defects should possess, an optimal mechanical and biological environment should be provided for a successful healing process to occur. The “diamond concept” represents a conceptual framework for understanding the interactions between the aforementioned principles of healing in the context of bone grafting. The initial concept has been extended to include the need for optimal vascularity to provide a more comprehensive understanding in situations where tissue regeneration is needed (nonunions, bone defects). The mechanical macroenvironment in case of long bone defects reconstruction should be considered in the context of the location of the defect (diaphysis, metaphysis, and combination), its dimensions, and the best method of bone stabilization. Increased rigidity, high vascularity, and quick remodeling are observed in epi/metaphyseal areas of long bones while increased elasticity, scares vascularity, and slow remodeling characterize the diaphysis.48 Apparently, bone grafts for these different areas of long bone should have different characteristics.
Other contemporary questions regarding the ideal bone graft that are currently under consideration from the orthopaedic trauma community are the following: “Which is the best osteoconductive material?”, “What are properties of bioactive and osteoconductive materials in the clinical setting?”, “What is the optimal bioresorbability, porosity and mechanical strength of bone substitutes”.48 Allografts do not have the limitations of donor site morbidity and are available in larger quantities, but they have the drawback of variable osteoinductive properties and they carry the possible risk of disease transmission. The improvement in stability and handling of synthetic bone grafts along with their unlimited supply is expected to increase their clinical use.13 Combination of a materials consisting of an osteoinductive protein with an osteoconductive carrier or osteogenic mesenchymal cells with osteoconductive allografts are evolving and are currently being under review.9,49
The in vivo properties of osteoactive and osteoinductive materials are still contestable. The use of PRPs in the setting of grafts has not been yet sufficiently explored, and the only osteoinductive molecules proven to induce in vivo bone formation are the BMP's in supraphysiologic dosage.34 However, there is paucity of high-quality studies to investigate the real osteogenic efficacy of concentrated bone marrow aspirate, but there is an increasing amount of evidence supporting the osteogenic potential and capacity of the femoral cancellous bone graft harvested with the RIA.16
The bioresorbability, the porosity, and the mechanical strength of the calcium phosphate products, which is the main base of synthetic bone grafts, are also an area of contemporary research and possible improvement.50
In our practice for reconstruction of long bone defects, we apply the principles of the diamond concept in the context of the induced membrane technique. The graft that we use is a combination of autograft (most of the times obtained with RIA but occasionally harvested from iliac crest), concentrated bone marrow aspirate as an additional source of osteogenic cells, and bovine derived hydroxyapatite Orthoss (Geistlich Pharma AG, Wolhusen, Switzerland) as a graft volume expander whenever the harvested autologous bone volume is not of sufficient quantity. Moreover, we believe that the “Orthoss” bone graft substitute provides the RIA graft with more structural support, optimizing its “matrix related properties” for optimum cellular attachment. Since the discontinuation of BMP's, we use PRP as an inductive stimulus for long bone defects longer than 5 cm in patients with compromised physiologic regeneration footprint such as elderly patients, smokers, and diabetics. (see Figures 1–5, Supplemental Digital Content 1–5, http://links.lww.com/JOT/A162, http://links.lww.com/JOT/A163, http://links.lww.com/JOT/A164, http://links.lww.com/JOT/A165, http://links.lww.com/JOT/A166). We have recently published our results of treatment of 43 long bone defects with the use of the induced membrane technique and bone graft optimization according to the above strategy.51
The posttraumatic segmental long bone reconstruction dictates the application of a bone graft with optimal biologic potential. For defects more than 5 cm, the application of ABG is considered a necessity but is not sufficient in terms of volume. Volume expansion with allograft or synthetic bone substitutes can be used. Enhancement of ABGs osteogenic potential through the use of concentrated bone marrow aspirate and induction of bone regeneration with the use of PRP should be considered. The above strategy provides a maximum biological and structural substrate of the graft to be implanted. Nowadays, our knowledge on the clinical efficacy of the bone graft in segmental bone reconstruction mainly stem from the outcomes of the induced membrane technique (Masquelet technique). The quest for the “ideal bone graft” ie, the one that possesses all the above properties in the maximum degree and consistently leads to the best clinical outcome of union without complications, is still ongoing. Newer in vitro and clinical data are becoming available and with that our contemporary knowledge expands. Nevertheless, the clinicians should be alerted to interpret the literature with caution because only low evidence studies are available. Well-organized prospective randomized trials are desirable to compare the efficacy of different methods of reconstruction of segmental bone defects (induced membrane technique vs. vascularized fibular allograft vs. distraction osteogenesis) and also to compare the optimal graft composition in cases of the induced membrane technique application.
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