Geoffrey N. Clark, DVM, Diplomate, ACVS Veterinary Surgery of New England
Bone grafting techniques have been an important part of small animal orthopedic surgery for many years. Autogenous cancellous bone grafts have long been considered the most effective graft material for accelerating bone healing. These grafts are useful when cellular transfer and osteoinduction are needed, but mechanical strength of the graft is not essential. Common indications for the use of cancellous bone grafts include fracture repair, management of non-unions, arthrodesis, and osteomyelitis. Harvesting autogenous cancellous bone requires a separate surgical approach during the primary procedure and is limited by the amount of bone graft material present at each donor site. Full cortical allografts have been used in small animal surgery to provide mechanical support and as a template for new host bone formation. These implants are useful in highly comminuted diaphyseal fractures and in limb sparing techniques; however, the allografts must be harvested prior to surgery and maintained in a bone bank. Clearly, alternative bone grafting options that offer more flexibility would be advantageous to the veterinary surgeon. Cancellous Bone Grafting Autogenous cancellous bone grafts enhance bone healing through three mechanisms. Osteogenesis occurs when viable transplanted cells continue to make new bone. Growth factors present in the transplanted bone matrix induce the differentiation of mesenchymal cells into bone-forming cells through the process of osteoinduction. A third mechanism, referred to as osteoconduction, occurs as the implanted bone provides a structural framework for bone and vascular ingrowth. The most common sites for collection of cancellous bone grafts in small animal patients include the proximal humerus, wing of the ilium, proximal tibia, and proximal femur. A separate surgical approach is used to expose the bone graft site. In cases of a potentially infected primary surgery site or when neoplasia is suspected, a separate instrument pack should be used to avoid contamination of the bone graft site with bacterial or neoplastic cells. The cortex is penetrated using a Steinman pin and graft is removed using a bone curette. Graft material is collected in a stainless steel cup or a blood-soaked gauze sponge. An alternative is to use a sterile syringe (3 mL or 6 mL) as a temporary graft container. Graft is placed along the ridges on the plastic plunger and the plunger is progressively inserted into the syringe, so that graft material is protected. When the graft is placed at the fracture or arthrodesis site, the plunger is withdrawn gradually to expose the graft. The donor site is closed following collection of adequate amount of graft material. The collection of cancellous bone grafts is not associated with significant complications in most small animal patients. Seroma formation, wound complications and fractures at the donor site are potential postoperative problems. This is in contrast to the morbidity reported in human patients undergoing autogenous cancellous bone grafting procedures. Persistent pain at the donor site is reported commonly in human orthopedic patients. Postoperative pain associated with a cancellous bone graft site is difficult to differentiate from pain arising from the primary surgical site in many veterinary patients.
In order to eliminate the issue of donor site pain and to avoid
other potential complications, one alternative is the use of allogeneic
cancellous bone graft. Cancellous allografts do not stimulate osteogenesis
as live cells are not transplanted, but these grafts are osteoconductive
and osteoinductive. It is not practical for most teaching institutions
or private surgical practices to maintain a bone bank with cancellous
allografts, but this graft material is currently available to veterinarians
from a commercial source. Veterinary Transplant Services (Seattle,
WA) provides frozen cancellous allograft as chips or mixed with
demineralized bone powder for use in dogs. The chips are used similarly
to fresh cancellous graft. They do not provide significant weight-bearing
capacity, but may be used to pack large defects. The allograft may
be combined with a smaller amount of autogenous graft to enhance
bone healing. Cancellous chips from VTS are packaged in sterile
pouches of 3, 5, or 10 mL. They may be kept in a residential freezer
(at least -20 degrees C) for up to 6 months. Demineralized Bone Matrix Another material that is available to enhance bone healing is demineralized bone matrix. This substance is prepared from cortical bone by extracting the mineral component with an acid solution. Demineralization of cortical bone exposes acid-resistant bone morphogenetic proteins (BMP), which have been shown to be potent inducers of bone formation in several species, including dogs and horses. BMP exert their effect through the differentiation of mesenchymal cells into bone-forming cells. Until
recently, the clinical use of demineralized bone matrix was not
practical. It is now available from Veterinary Transplant Services
with similar packaging to their cancellous chips. Veterinary surgeons
can purchase demineralized powder in 3, 5, or 10 mL pouches. This
powder may be used at fracture sites, non-unions, or in arthrodeses.
The demineralized powder is extremely convenient to use and it molds
nicely to host bone, especially as it mixes with blood. It may also
be combined with autogenous cancellous bone grafts. VTS also provides
packages of cancellous chips and demineralized powder mixed with
in a 2:1 ratio of chips to powder. Full Cortical Allografts Another bone grafting option is the use of full cortical allografts. These grafts are also referred to as alloimplants because no viable bone is transplanted. Cortical bone grafts are used primarily for mechanical support and to provide a template for new host bone formation. The two most common indications for the use of full cortical allografts is with comminuted diaphyseal fractures and in limb sparing procedures following excision of bone tumors. Collection of donor bone for full cortical allografts is not a benign procedure. Donors have recently died, are euthanatized after collection, or have lost a limb due to severe trauma.Several options are available for maintaining a cortical bone bank. Bones may be harvested after completion of an amputation in a clinical patient. All soft tissues are removed and the periosteum is stripped. The metaphyseal portion is removed with a sagittal saw. Bone grafts are placed in glass jars that were autoclaved on the same day as the bone is harvested. Jars are sealed, labeled, and stored in a residential freezer at -10 to -20 C. Cortical bone grafts may also be sterilized with ethylene oxide. Careful donor screening and strict adherence to aseptic harvesting technique are not necessary when using ethylene oxide to sterilize bones. These grafts can be stored at room temperature in the surgical supply area or in a freezer. If the use of full cortical allografts is an infrequent occurrence in your practice, then maintenance of a cortical bone bank is probably not practical. Cortical allografts are available commercially from VTS. Various grafts are available, ranging from short cortical grafts up to entire long bones. Overnight delivery is possible if a graft is needed on an urgent basis. This service has proven to be much more convenient than maintaining a bone bank in our hospital. In limb sparing procedures, the graft bone is matched as closely as possible to the host bone it will be replacing (e.g. right radius from an 80 pound dog) in order to facilitate bone plate placement.
Clark GN, Sinibaldi KR. Bone grafting options in small animal surgical practice. Proceedings of the 23rd Annual Veterinary Orthopedic Society. March 1996. Fitch R, Kerwin SC, Newman-Gage H, Sinibaldi KR. Bone autografts and allografts in dogs. Compend Contin Educ Pract Vet 1977; 19:558-578.
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