Master Techniques in Orthopedic Surgery Knee CH21

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21 Managing Bone Loss with Metaphyseal Cones

has shown early promise with its cost-effective approach. The construct is based on an intramedullary guided milling system that provides very precise bone preparation, and thus bony apposition. Similar to tantalum cones, the 3D printed cones are unlinked to a specific prosthesis and come in a variety of shapes and sizes. The symmetric tibial shapes and sizes are designed for simple type I and IIA defects where sleeves were traditionally utilized. On the other hand, lobe-shaped cones are available for type IIB and III defects. Femoral cones are bilobed in nature with the goal of bottoming out, and preventing longitudinal femoral fractures with preparation and implantation of the metaphyseal cone. An alternative process for creating porous titanium cones involves heating a titanium powder that contains additional compounds to high temperatures; the additives evaporate leaving a highly porous titanium structure (Zimmer-Biomet). Both femoral and tibial cones are fabricated using this process. In this system, preparation of the host bone is accomplished using a high-speed burr and metaphyseal broaches. Potential advantages of the porous tantalum and titanium cones versus structural bone graft include the following: ●● Availability of a wide variety of prefabricated shapes and sizes of augments ●● Quick and easy use ●● Ability to contour to obtain a custom fit ●● Immediate load bearing ●● Rapid bone ingrowth with stable long-term fixation In distinction, although structural grafts and impaction grafting techniques have been reported to have acceptable results in complex revision hip and knee arthroplasty cases, reservations persist. 17-19 Potential problems with large bone grafts include the following: ●● Limited availability ●● Risk of bacterial and viral disease transmission ●● Increased intraoperative time ●● Prolonged weight-bearing restrictions until graft incorporation has occurred ●● Graft resorption in 5% to 20% of cases 17,19 As a result of these ongoing concerns with the use of structural bone grafts, coupled with the in- creasing availability of tantalum and 3D printed titanium cones and good early results in the revision TKA setting, we have increased our use of porous metaphyseal cones in AORI type II and III bone defects, where we would have previously used bone graft material. CONTRAINDICATIONS Absolute contraindications for the use of porous tantalum and titanium metaphyseal cones include standard contraindications for TKAs including deep periprosthetic infection. Relative contraindications for the use of all metal augments regardless of size and composition include absence of host bone support. In cases where massive bicondylar bone loss exists with absence of any rim of cortical bone, allograft prosthetic composites or segmental replacement prostheses should be considered. PREOPERATIVE PREPARATION Accurate preoperative assessment and classification of bone loss is important to ensure that appro- priate augments or bone graft materials are available. However, owing to the potential for iatrogenic bone loss during component removal, the final classification of the bone defects must be based on the intraoperative findings after prosthesis removal and debridement. Therefore, in revision TKA, the surgeon must be prepared for several contingencies. Although not definitive, preoperative studies can provide helpful information about the type of defects that may be encountered intraoperatively. ●● Knee radiographs are low cost, but may fail to allow accurate assessment of the number and size of lesions, especially in the setting of osteolysis. 20,21 ●● Computed tomography (CT) is a more sensitive tool. In a study comparing radiographs to mul- tidetector CT, radiographs only identified 17% of the osteolytic lesions identified on CT scans. 20 ●● Metal artifact reduction suppression (MARS) magnetic resonance imaging (MRI) also appears to provide superior data to radiographs for evaluating osteolysis. 21 However, this capability may not be routinely available in all centers.