Metaphyseal bone loss is common with revision total knee arthroplasty (TKA). The causes of bone loss include: osteolysis, loosening, infection, iatrogenic or a combination. Small defects can be treated with screws and cement, bone graft, and non-porous metal wedges or blocks. Large defects can be treated with bulk structural allograft, impaction grafting, or highly porous metal cones or augments. The AORI classification of bone loss in revision TKA is very helpful with preoperative planning. Type 1 defects do not require augments or graft—use revision components with stems. Type 2 defects should be treated with non-porous metal augments—wedges or blocks. Type 3 defects require a bulk structural allograft or large highly porous metal cone. Trabecular metal (TM) metaphyseal cones are a unique solution for large bone defects. There are both femoral (full or partial) and tibial (full or stepped) TM cones available. These cones substitute for bone loss, improve metaphyseal fixation, help correct malalignment, restore joint line, and perhaps, permit use of a shorter stem. The technique for these cones involve sculpturing of the remaining bone with a high speed burr and rasp, followed by press-fit of the cone into the remaining metaphyseal bone. The interface is sealed with bone graft and putty. The fixation and osteoconductive properties of the outer surface allow ingrowth and hopefully long term biologic fixation. The revision knee component is then cemented into the porous cone inner surface, which provides superior fixation compared to deficient metaphyseal bone. The advantages of the TM cone compared to allograft include: technically easier; biologic fixation; no resorption; and (?) lower risk of infection. The disadvantages include: difficult extraction and relatively short-term follow-up. The author has reported the results of 33 TM cones (9 femoral, 24 tibial) implanted in 27 revision cases at 2–5.7 years follow-up. One knee (2 cones) was removed for infection. All but one cone showed osseointegration. TM cones are a promising method for the reconstruction of large bone defects in revision TKA.
Patient-specific instruments for total knee arthroplasty shift the bone landmark registration and implant positioning of computer navigation from intraoperative to the pre-operative setting. A preoperative MRI or CT scan is mandatory, with the specifications determined by the instrument manufacturer. Default implant sizing and alignment targets must be templated by the surgeon and mapped onto the virtual knee. The surgeon must also review and modify the preoperative computer plan to incorporate any clinical findings, such as flexion contracture or fixed deformity. The finalised preoperative plan is sent back to the implant vendor for fabrication of patient-specific cutting blocks in 4–6 weeks. The supposed advantages of these instruments include more accurate coronal alignment, fewer outliers, no instrumentation of intramedullary canal, decreased operative time, and decreased hospital costs to clean-sterilise instruments. There are many disadvantages of patient-specific instruments, including: cost, preoperative scheduling of imaging, the learning curve for the surgeon, and the uncalculated preoperative planning time. A set of conventional instruments should be available if the custom instruments do not fit properly. One study of 66 knees using PSI reported that frequent surgeon-directed changes were required, 2.4 per knee, implant sizes were changed in 77% of femurs and 53% of tibias, and tourniquet time was not improved. A Markov model study reported an increased cost of $4600 for 4.6 QALYs for patient-specific instruments and that the rate of revision must be reduced by 50% or more for these instruments to be cost-effective. There is little evidence to support the claims made by the manufacturers of these instruments. We advise against the widespread use of these instruments for total knee arthroplasty.
We report six cases of contralateral limb involvement during total hip arthroplasty including swelling of the gluteal muscle compartments, rhabdomyolysis, myoglobinuria, and sciatic nerve palsy. The risk factors for such complications include obesity, prolonged operative time, and positioning in the lateral decubitus position. The laboratory and clinical findings are consistent with a gluteal muscle crush-injury with consequent compartment syndrome. The patients should be treated conservatively as symptoms can be expected to resolve.
