Developments in bone grafting in veterinary orthopaedics part one

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1 Vet Times The website for the veterinary profession Developments in bone grafting in veterinary orthopaedics part one Author : John Innes, Peter Myint Categories : Vets Date : August 10, 2009 John Innes and Peter Myint discuss developments in bone banking and how such products can help the orthopaedic surgeon Bone grafting is a common procedure in veterinary orthopaedics particularly in dogs and cats. Bone grafts may be cancellous, cortical or corticocancellous. Cancellous grafts are generally used for osteoinduction and osteoconduction to encourage bone union in procedures such as arthrodeses, fracture repairs, spinal fusions and osteotomies. Cortical bone, as segmental grafts, is used following major ostectomies, such as limb sparing procedures, or sometimes for delayed and non-unions, or as augmentation grafts. While cortical grafts are usually allografts from donor animals, and supplied by a bone bank, veterinary surgeons have traditionally used autogenous cancellous bone graft when such grafts are required. However, although autogenous bone graft is the gold standard, there are many instances when it is not readily available, or is not available in sufficient quantity for the purpose in mind. Because of this, there is a demand for allograft cancellous bone, and these demands appear to be increasing in small animal orthopaedics. Allograft products provide several advantages: there is no need for donorsite clipping and preparation, which is an often overlooked, but time- 1/8

2 consuming procedure on the anaesthetised patient; there is no need for donor site surgery, reducing operative time and patient morbidity; potential complications from the donor site are avoided; and the use of allograft may well be cost-effective for the client, due to the reduced anaesthetic time and, importantly, provide working-time efficiencies for the busy veterinary surgeon. Bone banking basics The first step to bone banking is to retrieve suitable tissue from a donor animal. In the modern era, this must involve an ethical donor programme, whereby owners of potential donors give full, informed consent. Owners can find some solace in their time of loss through the gift of tissue donation and, in this way, one or more animals may benefit. Of course, the UK population is well educated in the principles of human organ and tissue donation, although tissues, such as the cornea, and organs, such as heart, kidney and liver, have a higher profile than bone. Not all potential donors are suitable of course. Each donor must be carefully screened. Medical history should include up-to-date vaccinations against infectious diseases, and the donor must be free of systemic disease and have healthy long bones. Most donors are likely to be younger to middle-aged animals that have died through trauma, or have been euthanised because of behavioural problems, such as aggression. Once consent and screening have been achieved, tissues are retrieved from the donor in an aseptic way. This is no different from the conditions in live orthopaedic surgery. Bones are then frozen in sealed, sterile bags, and labelled prior to processing (Figure 1). Records are kept of all donors for tissue tracking. Tissue processing should take place in a dedicated clean room. Generally, the long bone is stripped of attached soft tissues and cut into smaller segments of specific dimensions. For morcelised bone graft, the segments are ground into granules of various sizes using a bone mill. They are then subject to sequential processing steps of repeated agitation, ultrasonic treatment and centrifugation to remove bone marrow, fats and cellular debris. Various chemicals, such as hydrogen peroxide, alcohol and detergents, are used in these procedures. For demineralised bone matrix (DBM), which is a cortical bone powder of around 100(m particle size (Figure 2), the additional step of removing calcium is included to promote osteoinductive potential. All chemicals and water used are of pharmaceutical grade and procedures are carried out aseptically. The washed and cleaned bone is freeze-dried, packaged, labelled and sterilised by gamma irradiation, so that the final product is sterile and can be kept at room temperature. For 2/8

3 DBM, electron beam irradiation is used to minimise the adverse effect of radiation. Tissue tracking is an important part of the allograft process. The use of allograft should be recorded in the clinical notes (Figure 3), and good practice indicates that the graft provider should issue a tissue tracking code, which should also be noted in the clinical records. Ideally, the date and use of the graft should be reported to the graft provider for their records, by using a tissue-use report form. In summary, processing and irradiation of bone allografts provides ready-to-use, off-theshelf products that can be stored at an ambient temperature in the operating theatre ready for use at short notice. Types of bone allograft Cancellous chips Cancellous allograft chips are a convenient form of allograft that has many applications. They consist of morcelised cancellous bone that has been processed to various sizes for example, fine, medium and coarse (Figure 4). Such preparation makes them a convenient off-the-shelf solution for several orthopaedic applications. The various sizes available allow the surgeon to choose an ideal size for the application in hand. For instance, a large void may require large chips, whereas a small joint arthrodesis may require small chips. Cancellous chips can be stored at room temperature in the operating theatre, ready for use at short notice. The product is supplied in sterile glass vials within sterile peel-packs, providing a convenient system for aseptic opening. As the chips are freezedried, rehydration with sterile isotonic saline is recommended 10 to 20 minutes prior to implantation. If the chips are to be used as a graft extender, autogenous cancellous graft is mixed with the chips in a sterile gallipot. The chips provide a scaffold for osteoconduction in procedures such as tibial tuberosity advancement (TTA), arthrodeses, spinal fusions, fracture repairs, osteotomies and void filling (for example, corrective osteotomies). Cancellous chips may also be used to extend an autogenous graft when the graft is insufficient to cover a desired area. The chips also provide some structural properties while bone healing proceeds. Eventually, the chips will be resorbed, an advantage over many synthetic bone substitutes. Demineralised bone matrix 3/8

4 DBM is prepared from cortical bone that has been treated with hydrochloric acid to remove the mineral content. As such, DBM retains all the species-specific growth factors, such as bone morphogenic proteins (BMPs) and tissue growth factor beta (TGF-?). However, it appears that DBM has greater osteoinductive properties than can be explained by BMP and TGF-? content alone. DBM is traditionally tested by implantation into rat muscle with subsequent observation and assessment of ectopic bone formation. In addition, various in vitro studies have confirmed the osteoinductive properties of DBM. DBM has also been demonstrated to produce results similar to autogenous cancellous grafting in a case-control study in clinical canine patients undergoing various procedures1. Cancellous blocks and dowels Cancellous blocks can be used in any area where a large bone block is required as a load-bearing or structural support. Procedures where a cancellous block might be used are, for example, in spinal fusions, as an intervertebral spacer; and corrective osteotomies, where there is a gap requiring support. A cancellous block may typically be 10 x 13 x 15mm and consist of hard, dense cancellous bone. Cancellous blocks may be nibbled with rongeurs to customise the shape for the required defect. Cancellous dowels are similar in composition to blocks, but are cylindrical in shape. Procedures where a cancellous dowel might be used are, for example, in spinal fusions as an intervertebral spacer; and in the treatment of incomplete ossification of the humeral condyle, where the dowel might be used to span the humeral condyle. Cortical sections and struts These are used to buttress or add to existing cortical bone that may have been weakened, such as in revision femoral stem for hip replacement, limb-sparing procedures, or when fracture healing is expected to be slow. They are made from midshaft cortical bone from a long bone, such as the femur or tibia, and are supplied cut longitudinally to specific lengths and widths. Summary There are a variety of bone products now available to the small animal veterinary surgeon that can decrease patient anaesthetic time and postoperative morbidity. In part two of this article we will discuss some clinical applications of these products. Reference 4/8

5 1. Hoffer M J, Griffon D J, Schaeffer D J et al (2008). Clinical applications of demineralized bone matrix: a retrospective and case-matched study of seventy-five dogs, Veterinary Surgery 37(7): Figure 1. Canine freeze-dried bone allograft. Products are supplied within sterile peel-packs with tissue-tracking labels for easy recording of graft use. 5/8

6 Figure 2. Demineralised bone matrix is a bone powder. 6/8

7 Figure 3. Grafts should be tracked using a graft source code that is recorded in the clinical records. 7/8

8 Figure 4. Cancellous chips (coarse). 8/8 Powered by TCPDF (