Bone and Tendon Ingrowth Prostheses

1 MODULAR ENDOPROSTHESES AND TENDON-INGROWTH PROSTHESES Osteosarcoma is the most common primary bone tumor of the appendicular skeleton, frequently affecting large breed dogs. While amputation is considered the gold standard for removing the tumor, many owners would prefer a limb-salvage technique and some dogs are not suitable candidates for amputation given co-morbidities such as neurologic disease, obesity, or debilitating osteoarthritis. Therefore these techniques are increasingly employed. The objective is to enable complete resection of the malignancy and restore pain free limb function. Techniques include longitudinal bone transport, microvascular anastomosis of bone transplant, bone transposition (ulna for radius), cortical allografts, endoprostheses, extracorporeal intraoperative radiation therapy and autograft pasteurization. Metallic endoprostheses may be technically easier to apply than allografts and do not rely on bone harvesting and banking. Other recently available options for pain palliation and short-term provision of functional limb use include stereotactic radiation therapy and plating of osteosarcoma-affected bone. Preoperative skeletal imaging for all cases includes orthogonal plane radiography, computed tomography and magnetic resonance imaging of affected limbs. Diagnosis may be additionally refined using pre-operative cytology, histopathology and appropriate staging for local or distant metastases. In the UK, neoplasia is overwhelmingly the most common cause of destructive osseous lesions with variable periosteal proliferation, since fungal and other neoplasia-mimicing osteomyelites are rare. Therefore frequently full-mass resection is preferred to staged biopsy in the interests of timely intervention. Radiographic and CT-scan documented absence of grossly discernable pulmonary metastatic disease is a prerequisite for limb salvage and adjuvant chemotherapy protocols are considered fundamental to standard of care. Limb salvage techniques have been most commonly employed for malignancies of the distal radius. Clinical outcomes at this site are often positive, with minimal compromise of limb function by pancarpal arthrodesis. To date simple metallic spacers with a single plate anchored to the proximal radius and a single metacarpal bone have been employed. Limb spare application at other sites including distal femur, distal and proximal tibia, proximal humerus and proximal femur have been associated with more variable outcomes. There have been limited reports of the usage of endoprosthesis for limb-sparing surgery in veterinary medicine. A biomechanical study showed superior axial strength of endoprosthesis limb-sparing surgery when compared with cortical allograft replacement. A clinical case series using a 316L stainless steel commercially available endoprosthesis showed that this could be used satisfactorily for limb-sparing surgery and there is one report of a custom tantalum implant with a good outcome. Interestingly, infection has been correlated with increased median survival times, but results in management issues. We have employed titanium and stainless steel modular endoprostheses with customized honeycomb collars and intramedullary pegs coated with hydroxyapatite to facilitate osseous ingrowth in addition to stainless steel peg-cemented implants at multiple sites, including arthrodesis of the carpus and hock and including a hinged knee joint for the distal femur and proximal tibia. The relative advantages of HA-coated versus cemented intramedullary stems have not been elucidated to date, and the advantage may best be realized with less rapidly terminal pathologies such as chondrosarcoma or osteoblastoma, where implant-bone construct resilience may be required for several years.