For patients managed with revision total knee arthroplasty, obtaining successful outcomes often entails reconstructing bone deficiencies. Suitable methods exist to reconstruct small and medium defects. But the reconstruction of massive femoral defects remains challenging for those classified as Anderson Orthopaedic Research Institute (AORI) Type III.
An optimum method for treating these large femoral defects during revision knee replacement has not been established. Existing options include impaction grafting, large structural allografts and use of a tumor prosthesis.
However, the recent availability of a new alternative, metaphyseal-filling implant fabricated from porous tantalum may present orthopedic surgeons with more options in treating severe femoral bone loss. Due to their highly porous nature, tantalum implants may create a better environment for biological fixation. This is especially true in the setting of limited availability of host bone.
To test this alternative, a Mayo Clinic orthopedic surgery team implanted porous tantalum metaphyseal cones in 24 patients undergoing revision total knee replacements in the setting of severe bone loss. In all patients, bone loss was categorized as AORI Type IIB or greater.
The study group included 13 women and 11 men, with a mean age of 64 years. Patients were followed for an average of 33 months. Results published by David G. Lewallen, M.D., and Arlen D. Hanssen, M.D., et al., appear in the March 2, 2011, edition of The Journal of Bone and Joint Surgery, American Volume.
Results show an improvement of the average Knee Society clinical score, from 55 points preoperatively to 81 points at the latest follow-up exam. Upon postoperative radiographic evaluation of most patients (n=20) after 35 months, all femoral cones appeared well fixed. There was no evidence of cone complications.
The implants belong to a new class of materials known as highly porous metals (HPMs). Among researchers, HPMs are a promising topic of investigation because of their potential to improve implant technology and patient outcomes. This potential is related to HPMs' cellular architecture and mechanical properties. They enable it to form strong, rapid biological fixation of implants that improves spanning of large defects. The presence of structural microspikes gives the material a high coefficient of friction against cancellous bone.
Notes Dr. Lewallen: "Because of these properties, there is tremendous research under way to develop HPMs' potential for new and robust orthopedic solutions." Adds Dr. Hanssen: "The work with HPMs is great news for patients because we do have a need for larger and more complex implants. Our research is very much driven by the underlying patient need."
From these early short-term results, Mayo Clinic orthopedists conclude that the porous tantalum metaphyseal femoral cones provide effective structural support for the femoral implants of a revision total knee arthroplasty in this study group.
The potential for tantalum cones to provide long-term biological fixation and durable reconstructions may exist, but this prospect will have to be evaluated through comparison with alternative reconstructive techniques in further studies.