Introduction. In complex primary and revision total knee replacement (TKR) the operating surgeon may encounter proximal tibial bone defects. The correct management of such defects is fundamental to both the initial stability and long-term survival of the prosthesis. Cement or metal augments have been used to address some such type II unconstrained defects [1]. Aim. The aim of this finite element (FE) study was to analyse the comparative behaviour of cement and metal based augments and quantify the stresses within these different augments and underlying cancellous bone. Materials and methods. A three-dimensional FE model was constructed from a computer tomography (CT) scan of the proximal tibia using SIMPLEWARE v3.2 image processing software. The tibial component of a TKR was implanted with either a block or wedge-shaped augment made of either metal or cement. The model was axially loaded with a force of 3600N and testing was conducted with both evenly and eccentrically distributed loads. Results. Upon loading the FE model, the von-Mises stresses in the cancellous bone underneath the augments were found to be higher with cement based augments in comparison their metal based counterparts. This was evident with both block and wedge-shaped augments. The FE model demonstrated that compressive stresses within the metal based augments were greater than those within the cement based augments. This was evident with both block and wedge designs. Upon even loading the maximum recorded compressive stresses within the metal augments were 5 times less than the endurance limit of the material [3]. However, the maximum recorded compressive stresses within cement augments were only half the endurance limit of the material [4] and upon eccentric loading compressive stresses in excess of the endurance limit were recorded. Discussion. The FE model has demonstrated that cement based augments undergo a greater deformation when loaded and therefore transfer greater loads to the underlying cancellous bone. This is a result of the inherent flexibility of the cement based augment in comparison to the stiffer metal counterparts. The greater transference of load to cancellous bone with cement based augments may reduce the possibility of stress shielding. However, the compressive stresses within cement based augments are too close to the endurance limit of the material and with uneven loading even exceed it. This would imply that cement based augments are more prone to fatigue failure than their metal counterparts. Conclusion. This FE study supports the use of metal based augments over cement based augments in augmented and revision TKR surgery

The orthopaedic surgeon is often consulted to manage pathologic fractures due to metastatic disease, even though he or she may not be an orthopaedic oncologist. A good understanding of the principles of management of metastatic disease is therefore important. The skeleton remains a common site for metastasis, and certain cancers have a predilection for bone, namely, tumors of the breast, prostate, lung, thyroid, and kidney. Myeloma and lymphoma also often involve bone. The proximal femur and pelvis are most commonly affected, so we will focus on those anatomic sites. The patient may present with pain and impending fracture, or with actual fracture. Careful preoperative medical optimization is recommended. If the lesion is solitary, or the primary is unknown, the diagnosis must be made by a full workup and biopsy before definitive treatment is planned. For patients with known metastasis (the most common situation), the options for treatment of pathologic lesions of the proximal femur generally center on internal fixation versus prosthetic replacement. Patients with breast or prostate metastasis can live for several years after pathologic fracture, so constructs must be relatively durable. If fixation is chosen, it must be stable enough to allow full weight bearing, since the overwhelming majority of pathologic fractures will never heal. In general, long constructs are chosen to protect the entire length of the bone. Nails should protect the femoral neck as well, so cephalomedullary devices are typically chosen. Megaprostheses can be useful in situations where bony destruction precludes stable internal fixation. Postoperative radiation is recommended after wound healing. Acetabular involvement typically requires reinforcement rings or cement augmentation with the Harrington technique. Careful multi-disciplinary medical management is recommended to minimise complications