Tissue banking involves harvesting specific tissues, such as bone, from donated remains and processing and storing (‘banking’) these tissues such that they may be distributed and used as tissue grafts in recipient patients. Veterinary tissue banking and transplantation is still in its infancy but veterinary nurses have a key role to play in many aspects of this developing field from raising awareness with pet owners, to the process of tissue donation itself, post-donation support to pet owners and also the delivery of tissue grafts to patients, and the recording of graft use. Tissues that are suitable for banking and transplantation include blood, bone, tendon, ligament, meniscus, cornea, skin and heart valves (Pegg, 2006). While blood transfusion is well established in companion animals, the process of collecting blood and storing it in a blood bank is a more recent phenomenon in the UK. However, bone banking and distribution is also gaining in popularity (Hoffer et al, 2008; Innes and Myint, 2010) and it is likely that other tissues will also be increasingly sourced and used.
Bone is the second most transplanted material after blood in human medicine, and is by far the most transplanted tissue compared with other tissue transplants such as cornea, skin and heart valves (Eastland, 2003). More than half a million human bone grafting procedures are performed annually in the United States alone (Greenwald et al, 2001). In veterinary tissue banking, musculoskeletal tissue transplants follow the same trend and are a growing area of interest in veterinary orthopaedics. Traditionally, veterinary surgeons have mostly used autograft (bone harvested from the patient) but in small patients this can be problematic due to a lack of tissue and the donor site may be a source of pain or complications. Allograft bone (from one animal to another within the same species) offers off-the-shelf availability, unlimited quantity and desired shape and size, and avoids the time (for nurses and surgeons) and additional pain (for the patient) associated with harvesting autograft, and often a limited quantity of bone availability. In human studies, donor site pain is the most frequent complaint following autologous bone harvesting (Sassard et al, 2000; Hierholzer et al, 2006) and it is likely that animals also suffer pain associated with the donor site.
Veterinary bone transplantation
Although exact figures are not available, the use of bone allografts in veterinary orthopaedics is increasingly reported. This trend is set to increase as more and more complex procedures are now carried out by veterinary surgeons. Veterinary Transplant Services (USA) and Veterinary Tissue Bank (UK) are the two tissue banking organizations providing tissue grafts, including musculoskeletal allografts, to the veterinary community at present.
The aim of tissue banking is to provide readily available tissue grafts of high quality and safety, fit for purpose when used clinically. The successful clinical application is a balance between bone biology and biomechanics. In bone banking, the process demands a sound knowledge of bone graft biology, biomechanics, immunology, microbiology as well as donor selection criteria, preservation methods, sterilization technology and tissue recovery approaches. In the modern approach to tissue banking, one also requires an understanding of quality systems, good manufacturing practice, clean room technology, aseptic techniques, clinical application as well as logistics in storage and record keeping. Safety of grafts is paramount to avoid any risk to the recipient, and it is the combination of all the above procedures that makes the final product safe (Larkin, 1997; Tomford and Mankin, 1999). This article discusses tissue banking with specific reference to Veterinary Tissue Bank (UK).
The role of veterinary nurses
The role of nurses in the tissue donation process is one of utmost importance because nurses tend to have a closer relationship with pet owners and owners may find discussing the process of tissue donation somewhat easier with a member of the nursing team than with a veterinary surgeon. Nurses carry a great responsibility related to the care of owners, including their emotional state. For example, nurses may be involved in discussions with owners regarding ‘end-of-pet-life’ decision making; tissue donation can have a positive impact on the owner's family as it may provide some solace in the knowledge that, at a time of loss, other pets may benefit from the gift of donation (Simpkin et al, 2009). Successful tissue donation and procurement may depend on veterinary nurses' awareness of tissue donation, an understanding of the donation process, and knowledge of various types of tissue transplantation. Nurses must be:
Raising awareness and the pet tissue donor card scheme
The public in many developed countries is well aware of the subject of organ donation in human medicine and many people carry donor cards. Tissue donation is less well recognized although many people consent to this at the same time as organ donation. However, as an example, the UK's National health Service has a continual campaign to raise awareness of the benefits of organ donation because demand for donated organs and tissues usually exceeds supply (Balderson, 1995; Wellesley et al, 1997; Elding and Scholes, 2005). These issues are likely to appear in veterinary tissue donation. However, the public is less aware of this possibility in veterinary medicine and there is a need to increase pet owners' knowledge regarding the process and benefits of tissue donation. In the modern era, the internet and responsible use of social networking sites can be very helpful ways of raising awareness (www.facebook.com/home.php#!/pages/Veterinary-Tissue-Bank/361294441858). However, veterinary nurses often have significant contact with clients and, importantly, clients often feel more able to discuss sensitive issues with nurses than with veterinary surgeons. It is this trusting relationship that provides veterinary nurses with advantages when it comes to discussing subjects such as the pet tissue donor card scheme (www.petdonor.org); pet owners are now able to register their pet online on a UK pet donor register with Veterinary Tissue Bank. They then receive a pet tissue donor card and, if they consent, a letter will also be sent to their veterinary practice to ask for their registration on the scheme to be added to the pet's medical record. In this way, the practice is aware that an owner has registered their pet and, when the animal dies or is euthanized, the subject of tissue donation can be raised without fear of any upset. Of course, fully-informed consent is required at the time of actual donation, and nurses may be involved in the delivery of information to pet owners. Information for owners and consent forms are available for download (http://www.vtbank.org/donor-programme).
Pet bereavement and end-of-pet-life decision making
The role of veterinary nurses in pet bereavement is well established (Butler and DeGraff, 1996; Flynn, 1996; Thomas, 1997) but veterinary tissue banking brings a new dimension to this area. People who have signed on the Veterinary Tissue Bank Pet Donor Register will anticipate that veterinary healthcare professionals will be aware of this, and support them in following through their wishes; therefore, all veterinary healthcare professionals should be aware of donation issues as an integral part of pet bereavement and end-of-life decision discussions with owners. If the option of donation is presented to the owner in a positive and supportive style, the possibility of the owner embracing such a decision is much more likely (Simpkin et al, 2009). In the authors' experience, many pet owners have expressed a great amount of appreciation to veterinary staff when given the opportunity to give the ‘gift of life’. This giving can be consoling to the owner of the pet and may help them through their grief (Figure 1).
Donor identification and selection
Veterinary healthcare professionals are critical to the identification of potential donors. If a potential donor is identified, the Veterinary Tissue Bank can be contacted to answer any questions and make further arrangements.
Consent
The first step following identification of a possible donor is to obtain fully-informed and final consent from the pet owner. Clearly, ethical donation is important to veterinary tissue banking and, as such, the wishes of the pet owner must be observed at all times. The donor animal must have died through trauma, or through medical or behavioural reasons, and informed owner consent must be provided to allow tissues to be used to help other pets. Informed consent involves verbal and written delivery of information to the owner. The attending veterinary professional must be satisfied that the owner understands and agrees to anything they sign; owners should never be pressurized into donation. If for any reasons the tissues are found to be unsuitable for clinical use, owners may consent for the tissues to be used in ethically sound clinical research. It is crucial that fully-informed consent is provided by the owner for any intended use of the tissues. As such, Veterinary Tissue Bank provides printed information for owners to understand the whole process. If any questions arise, veterinary surgeons and veterinary nurses may need to respond but if there are further queries, Veterinary Tissue Bank in the UK can be contacted for any clarification needed.
Donor screening
Obviously, all candidate donors must be screened prior to being accepted as tissue donors. In general, the animal must have a full vaccination history, absence of history of cancer, systemic infection, immune-mediated diseases, or an idiopathic disease. In addition, a physical examination must be carried out to exclude any specific issues with the tissue(s) to be harvested. A post-mortem sample of blood is collected and, in cats, this is tested for feline immunodeficiency virus (FIV) whereas, in dogs, it is archived as part of the quality assurance protocols. The attending veterinary practice performs the initial screening of a donor and this is reviewed and authorized by the veterinary medical director of Veterinary Tissue Bank before further processing.
Record keeping
Accurate and comprehensive record keeping is an integral part of tissue banking. There must be full traceability from the point of tissue donation to the final clinical use of the graft. The records include donor details, tissue harvesting, processing, sterilization, stock inventory and despatch. All donations are anonymized and each graft carries a unique number which is recorded in the recipient form when implanted. Grafts are supplied to practices with graft code stickers which can be placed in the medical record of recipients to allow full traceability should any suspect adverse events occur; such events seem to be rare (Figure 2). Veterinary Tissue Bank have had no adverse events reported in nearly 1000 grafts supplied to date. Nurses are likely to be involved in the stock control of grafts in their practice and the recording of graft use. Any adverse event or adverse reaction that might potentially be attributed to the graft should be brought to the attention of the tissue bank and documented.
The process of tissue banking
Tissue harvesting, with specific reference to bone
Following informed consent, the Veterinary Tissue Bank will arrange for collection of remains from the veterinary practice — this is a UK-wide service. The remains are stored frozen until collected by a dedicated ‘same day’ courier to transport to the tissue bank. Tissue retrieval takes place in a clean environment and is carried out using aseptic procedures; cross contamination or additional contamination should not occur during tissue harvesting. The donor animal is treated with care and respect throughout the process. Generally, long bones from four limbs are removed from which proximal and distal ends provide the bulk of cancellous bone and mid-shafts are used for cortical bone. The owner can elect for routine cremation or individual cremation with return of ashes — the cost of this is paid by Veterinary Tissue Bank and, again, veterinary nurses may be involved in such discussions and decisions with owners.
Sterilization and preservation methods
The method of sterilization and preservation depends on the type of tissue and the intended procedure. Sterilization by irradiation is the most widely used method in musculoskeletal tissues. Irradiation is dose-dependent and is known to impair the biomechanical properties of tissues. For soft tissues such as tendons, low dose or aseptic processing without any sterilization is applied to prevent any changes to biomechanics. The same approach is adopted in demineralized bone matrix (DBM) preparation in order to preserve osteoinductive proteins, which are required for stimulation of bone formation. In musculoskeletal tissues, irradiation within a dose range of 15–25 kGy is generally adopted (Minamisawa et al, 1995; Hamer et al, 1999; Cornu et al, 2000). For osteochondral grafts, where donor cell viability is essential, a combination of antibiotics including fungicides are used to store the tissue.
Various chemicals such as methiolate and gluteraldehye have been used in the past. The method of preservation depends on the intended purpose of the tissue. For osteoarticular or cartilage grafts the preservation of cell viability and the integrity of tissue matrix are important, hence, they are either preserved in culture media or are cryopreserved. Culture media preservation limits the ‘shelf life’ of the tissue to a few weeks, whereas cryopreservation of the tissue enables storage for up to 5 years if stored in liquid nitrogen vapour (−196°C). Soft tissues such as tendons and ligaments need to maintain their biomechanical properties and the method of choice is fresh frozen. Bone such as cancellous, cortical or DBM is either preserved freeze dried or deep frozen. The shelf life of freeze-dried tissue is 5 years and those of frozen grafts are 6 months at below −20°C and 5 years at below −40°C.
Clinical applications of tissue grafts
Bone allograft products have wide-ranging uses in orthopaedic, neurosurgical and dental applications. DBM is an osetoinductive material which means it can induce bone formation in surrounding tissue, but is not intended to contribute structural support. As such, DBM is used alone in orthopaedics for small joint arthrodeses (e.g. carpus, tarsus and calcaneoquartal joints) and for osteoinduction around reconstructed diaphyseal fractures in adult dogs and cats (Figure 3). DBM may also be mixed with cancellous chips for void filling and osteoinduction in large joint arthrodeses (shoulder, elbow, stifle) and in buttress fracture fixation. In neurosurgery, cancellous chips may be mixed with DBM or autograft to produce the large volumes required to facilitate spinal fusions. Spinal fusions are used for atlantoaxial subluxation, caudal cervical spondylomyelopathy (Wobbler syndrome), cervical and thoracolumbar type II disc disease, and lumbosacral compression. Dental use of DBM and cancellous chips includes extraction socket grafting, ridge and sinus augmentation, treatment of bony defects and periodontal regeneration.
Nurses will be involved in the operating room in assisting with graft selection and preparation, as well as recording of graft use and tissue graft stock control in the theatre suite. Freeze-dried bone grafts come in sterile double peel packs and these can be opened in the usual manner by an unscrubbed assistant and passed to the surgical personnel. The surgeon, or scrubbed surgical assistant, can then open the graft vial and carefully rehydrate the graft in a small bowl using a small quantity of sterile saline. After a few minutes, the graft can be handled and packed into the recipient site. The purpose of bone grafts is to stimulate or facilitate bone formation; this might be to assist and speed fracture healing, or to create bone tissue to fuse a joint or bridge vertebrae. The potential uses of such grafts are wide raging and a variety of patients can benefit.
Future directions for tissue banking and biologic therapies
Over the last 15–20 years, trends in human surgery have seen a huge growth in the use of tissue allografts and it is likely that veterinary surgery will see a similar growth once the sector becomes familiar with the advantages and applications of such grafts. While bone products are most commonly used, soft tissue grafts of tendon and meniscus are also now being used in veterinary patients. In addition, corneal grafts are well established in human ophthalmic surgery and while they are used to a limited extent in veterinary ophthalmology, supply of grafts may be a limiting factor, making this another potential growth area in veterinary tissue banking.
Another rapidly developing area is that of ‘stem cell therapies’. Mesenchymal tem cells are pluripotential cells that can replicate and differentiate in to a variety of tissues such as bone and cartilage. Typically, mesenchymal stem cells are extracted from bone marrow or adipose tissue. There are many variations, uncertainties and controversies regarding the use of such cells such as the best source of such cells and the efficacy and safety of various delivery methods. The cells may be delivered back immediately to the patient, they may be sorted and certain cells selected, or they may be expanded in cell culture systems and delivered back to the patient in large numbers. The delivery back to the patient is also a controversial area as are the appropriate disease targets for such therapies. Essentially these treatment modalities remain ‘experimental’ at the current time but there is no doubt that this will be a developing area in the years ahead and tissue banks are well placed with appropriate expertise to play a role in this area.
Conclusion
Veterinary tissue banking is a relatively new discipline. Given trends in human medicine, it is likely that the field of veterinary tissue transplantation will grow substantially over the coming years as the veterinary sector becomes more familiar with the advantages and benefits of tissue grafts. Veterinary nurses should try to stay informed regarding developments in this field because it is an area where they can play a significant role.