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The management of bone loss (whether it is primary or
revision TKR) depends on the type of bone defects.

 

AORI type 1

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This type of defect is rarely observed in revision TKR.
The defects are <5 mm in breadth and depth. A primary prosthetic implant with polymethylmethacrylate cement is a best option to fill the bone loss. In tibial bone defects, AORI type 1 provides stability at par with impaction bone grafting and the structural allografting. Stem fixation is suggested to offload the implant-host bone interface, while remaining bone stock can sustain primary components. Cement can be used only in content bone defect.6,7   AORI type 2 An AORI type 2 bone defect is generally larger than 5 mm therefore simple cement fill is not sufficiently effective. Hence, a cement reinforced with screws (5 mm or 6.5 mm) are also inserted into the defective condyle.7  In case of young patients, where further revision operations are predictable impacted morsellised bone grafts allow restoration of living bone stock. For restoring cortical or uncontained defects, impaction bone grafting is not advocated as it can hamper the stability of tibial tray. Thus, to integrate with the surrounding bone, the grafts size should be 0.5–1 cm. To carry the load sufficient impaction force is applied to make morsellised bone grafts sturdy. Bone remodelling via revascularisation is facilitated by cancellous bone allografts. When compared to demineralised cortical grafts, mineralised cancellous grafts have superior osteoconductive but feebler capacity. Due to radiation freeze-dried bone is more delicate and difficult to work with, but preserves its original mechanical properties. There is no immediate resistance with morsellised allografts but they can be incorporated and are remodelled in reaction to surrounding loading pressures.6Metal augments in AORI type 2 bone loss offer superior implant-host bone contact and enhances rotational and axial stability. In post implant removal, distal femoral and posterior condylar bone loss is frequently seen and metal augments up to 10 mm in size can successfully restructure the defects. Modular augments allows accurate placement and precisely sized femoral component in contact with host bone in distal and posterior femoral defects.7 Tibial augments are of different shapes and sizes like wedge or block and varying from 5 mm to 25 mm. They are designed to replace half or the entire width of the tibial plateau.Block augments are more stable than wedge augments, which have a higher shear force.Bone loss faced during the revision of a unicompartmental knee arthroplasty is successfully reconstructed by medial or lateral modular metal augments.7 The joint line must be elevated in some TKA cases to increase the extension gap to match the flexion gap but this should be considered only after other methods like upsizing and posteriorly translating the femoral component have been performed.7 The modern metal augments makes this metal conducive to biologic fixation as it provides 70%–80% volumetric porosity for bone ingrowth, with low modulus of elasticity and high frictional features.6  AORI type IIb/IIIFor the treatment of moderate to severe type IIb/III bone loss, the most suitable surgical options are impaction grafting, structural allograft and highly porous metal metaphyseal cones or sleeves.6 Impaction grafting is beneficial for younger population. Cement interdigitation and stem fixation is improved by utilising morcellised allograft as it changes smooth and sclerotic femoral or tibial bone into a more flecked surface by filling it with cancellous allograft. As wire mesh is needed to contain the graft managing peripheral cortical defects with impaction grafting becomes complicated. Therefore, structural allografts can be used to address these defects without the need for additional wire mesh.7  Frequent failure of structural allografts lead to the development of highly porous tantalum cones that work as a prosthetic structural allograft substitute. These devices attain robust primary metaphyseal fixation and subsequent biologic fixation without the risk of graft resorption or fracture. Presently, highly porous tantalum cones and titanium sleeves are the two metaphyseal prosthetic choices.6   Highly porous metal with low stiffness and a high coefficient of friction are used in manufacturing of tantalum cones. They are symmetrical or stepped and are available in a various sizes ranging from 48 mm to 67 mm in diameter and 15 mm to 30 mm in length.7   An interconnected porous surface for bone ingrowth is produced by coating titanium sleeves with titanium beads. To fill a variety of metaphysealbone defects, these sleeves are stepped in shape and are available in various sizes and lengths. Sleeve position varies from the tibial component rotation within 15°. Adequate axial and rotational stability must be attained in metaphyseal bone at the time of implantation in case of the sleeve to be uncemented.7

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