Where Are We Now?
Although the Anderson Orthopaedic Research Institute (AORI) classification is the most-commonly used classification for bone defects in the femur and tibia [3, 6], it does not address diaphyseal bone loss. In the current study, Belt and colleagues [2] developed and tested a new radiographic classification system for bone loss in patients undergoing revision TKA, which is more detailed than the current AORI classification and also addresses bone loss in the diaphysis.
The authors of this study [2] found excellent inter-observer reliability with the new classification, suggesting that it is practical and capable of reliably describing the preoperative state of metaphyseal and diaphyseal bone loss in TKAs undergoing revision. Not surprisingly, the system was not reliable in assessing bone loss in the epiphyseal region, because of the presence of the implant.
While there is little long-term data to guide us, it is generally maintained that revision TKA must achieve stable distal fixation, especially in the metaphysis supplemented by intramedullary diaphyseal support. It is well-recognized that poor epiphyseal and metaphyseal fixation cannot be compensated for by press-fit or cemented intramedullary stems [4, 5]. However, metaphyseal sleeves and cones can provide stable fixation to metaphyseal bone in the setting of severe bone loss and offers support for long-term survivorship of complex revision TKA [1, 4, 5, 7]. The optimistic view is that this new classification may provide a research tool for comparing radiographically visible defects and a clinical tool as an aid to the process of preoperative planning for revision TKA.
Where Do We Need To Go?
The realistic view, however, is that assessing bone defects preoperatively remains a challenge. Epiphyseal defects are difficult to assess because they are covered by the metallic implants. Additional bone loss sustained during implant removal was, of course, not present before surgery and so cannot be assessed preoperatively; these surgeon-created defects may have a big impact on the revision procedure. Likewise, bone quality and osteolysis behind the implants cannot easily be evaluated before surgery.
The obvious difficulty of pre-operative assessment of bone loss has led to the development of robust, modular revision systems that provide solutions to the wide variety of challenges that may be encountered. I would suggest that the classification system proposed by Belt and colleagues [2] might only have value when retrospectively applied to help clarify the comparison of differing degrees of bone loss in observational studies.
We need to determine the clinical applicability of this new classification system. This leads to several questions: Is it realistic to suggest that any classification based on preoperative imaging can define the challenges of complex revision surgery, or help guide the surgeon to plan an upcoming revision accurately? Are there radiographic patterns of failed total knee replacements due to mechanical loosening and/or osteolysis that can accurately predict the degree of bone loss from the epiphysis and metaphysis and allow formulation of an accurate pre-operative plan?
How Do We Get There?
Researchers should work to correlate preoperative imaging patterns with the actual intra-operative assessment of the requirements for the fabrication of the revision prosthesis. The effectiveness of the preoperative planning process based on the preoperative imaging needs to be assessed for clinical practicality. Ultimately, if a classification system is to be adopted as a clinical tool, it must lead to better long-term clinical outcomes.
The value of this or any other radiographic classification system can only be tested by developing a prospective study in which the preoperatively determined classification (based on imaging) is compared to the actual intra-operative findings with inter and intra-observer reliability. If the classification is expected to predict the components of the revision prosthesis, then this comparison would be needed as well. The clinical utility of the classification system could be further tested by comparing improved patient reported pain and functionality as well as the long-term survivorship of the revision to a control group that undergoes surgery without application of the classification scheme. Long-term clinical follow-up would need to be gathered and compared. Until such studies are completed, these classification schemes can only serve to provide a means of comparing patients of similar complexity as dictated by the bone loss found at surgery.
References
1. Abdelaziz H, Jaramillo R, Gehrke T, Ohlmeie M, Citak M. Clinical survivorship of Asepctic Revision total knee arthroplasty using hinged knees and tantalum cones at minimum 10-year follow-up. J Arthroplasty. 2019;34:3018-3022.
2. Belt M, Smulders K, van Houten A, Wymenga A, Heesterbeek P, van Hellemondt G. What is the reliability of a new classification for bone defects in revision TKA based on preoperative radiographs? Clin Orthop Relat Res. [Published online ahead of print]. DOI:
10.1097/CORR.0000000000001084.
3. Engh GA, Ammeen DJ, Classification and pre-operative radiographic evaluation: Knee. Orthop Clin North Am
. 1998; 29:1424-1430.
4. Lachiewcz PF, Bolognesi MP, Henderson RA, Soileau ES, Vail TP. Can tantalum cones provide fixation in complex revision knee arthroplasty? Clin Orthop Rel Res. 2012;470:199-204.
5. Lachiewicz PF, Watters TS. Porous metal metaphyseal cones for severe bone loss: when only metal will do. Bone Joint J. 2014;96-B(11 supplA):118-121.
6. Qiu YY, Yan CH, Chiu KY, Ng FY. Review article: Bone defects classifications in revision total knee arthroplasty. Bone Joint J
. 2011;19:238-243.
7. Watters TS, Martin JR, Levy DL, Yang CC, Kim RH, Dennis DA. Porous coated metaphyseal sleeves for severe femoral and tibial bone loss in revision TKA. J Arthroplasty. 2017;32:3468-3473.
