Our study has limitations related to a retrospective analysis carried out on a heterogeneous group of patients. However the effects of the retrospective nature of the study are mitigated by the prospective collection of data. It is worth adding that this is one of the few studies of a series of cases with the use of 3D Cones in a large number of patients undergoing functional assessment. The use of implants was determined by surgeons with no preferred indication of one type. Another limitation is the imperfection of the KSS scale as a measure of clinical outcome that combines subjective outcomes with implant survival data. However, it is worth adding that it is the most frequently used classification for functional assessment of patients after TKA. Finally, the length of the observation is relatively short. However, 3D printing Cone implants were introduced in Poland in 2017. Therefore, further monitoring of our population will provide additional information.
Revision procedures after TKA are often a big challenge for surgeons. Additional problems arise especially when large bone defects occur (2 and 3 according to the AORI classification). According to our experience and the available literature, the key to the success of the procedure is appropriate planning, which takes into account e.g. zonal classification system. It determines three levels of implant fixation: joint surface, metaphysis and diaphysis . Most revision systems achieve stability due to the distribution of loads on the joint surface and diaphysis zones, however it is in the metaphyseal zone that the greatest forces occur, the uneven distribution of which leads to loosening . Understanding the role of fixation zones allows to create a methodology of conduct. The components should be firmly seated with proper distribution of mechanical forces, which is a decisive success factor. Lack of uniform load leads to movements exceeding 150 µm, which stimulates the formation of soft tissue, preventing implant osteointegration .
Methods of treating bone defects have been described in detail in the literature, especially in the short and medium-term follow-up period. Smaller, closed defects can be treated with a bone substitute or bone cement. Larger with bolt-reinforced cement or modular metal parts. The joint surface defects require the use of structural grafts or metal implants . The evolution of materials has contributed to the development of porous surfaces that improve mechanical stability. To provide structural and biomechanical reconstruction of the metaphyseal area, titanium implants can be used to increase the surface area and support the remaining implants. These solutions include Sleeve’s and Cone’s. They are two different technologies with different philosophies. In our practice, we decided to use Cones for several reasons. One of them is the independence of zone implantation, which enables the distribution of forces beyond the articular area, protecting it from excessive stresses. The fixation in the metaphyseal zone is independent of the joint surface and diaphysis zones. Ensuring the independence of the positioning of the tibial tray and the extension allows for > 85% of the adjustment of the stem to the bone shaft without disturbing the mechanical axis . In other cases, the fit can be achieved using a cemented stem. Another reason is the covering of the implant modeled on the structure of the trabecular bone. An average porosity of 80% combined with a modulus of elasticity (3 GPa), low stiffness and a high coefficient of friction increase osteoconductive, ensuring physiological load transfer and reducing the risk of loosening . A great advantage of Cones is the low potential of bacterial adhesion and high survival . Bonanzing et al. conducted a meta-analysis of 432 implanted Cones with an average follow-up of 42 months. Features of loosening were shown by 1.15% .
The change in bone density (on average by 27%) after primary arthroplasty, correlated with its severe loss and it is a serious challenge in TKA revision . 3D-printed titanium Cones, with anatomical shapes and porous structure can provide structural support while offering the potential for permanent biological fixation. The use of structural grafts has been associated with long remodeling times, limited graft availability, nonunion or resorption. Admittedly, Sandiford et al. proved comparable effectiveness in the application of both solutions, however, most reports indicate a high revision rate exceeding even 20% (over a 10-year period)  The first generation of Cones showed promising clinical and radiological results in TKA revisions. Their limitation was imprecise bone preparation and limited availability of sizes and shapes. The second generation offers a much wider range and safer preparation. 
The production technique based on 3D printing allowed the creation of a wide range of sizes for high fit, based on the use of a large CT database to determine the location and shape for optimal bone coverage and support . The tools adapted to the size significantly reduced the risk of uncontrolled milling without obtaining a guarantee of geometric fit and the need to use cement. Faizan et al. compared traditional and 3D printed implants. In the case of using traditional implants, the incidence of intraoperative fractures during preparation was 4%. By analyzing movements during valgus and simulated steps, the research showed similar values for the use of central tibial Cones for both technologies. In the case of asymmetrical tibial and femoral cones, they were much smaller compared to traditional .
The benefits of using an implant based on 3D printing were described by Patel et al., Assessing the stability and radiological results from the third to the sixth month after surgery as 100% . Denehy et al. Performed a multicenter review of 62 TKA revisions using 3D Cones, with at least two years of follow-up. There were no cases of aseptic loosening and the survival rate was 90.2% . In both studies the reason for the review was infections. The authors noted no signs of loosening in radiological examinations or progressive lines translucent to X-rays(after excluding patients with infection).
In our study the mean score in KSS questionnaire improved from 12.5 to 79.3 points. Similar results were reported by Girerd . Tetreault also showed 98% survival over a 2-year period. The authors observed four cases of unsuccessful osseointegration . Divano et al. Reported 100% aseptic survival in the mean 5 years and 2% revision due to infection, treated without the need for Cone removal. At the same time authors indicated 96% survival of the first generation femoral Cone in a series of 159 cases, but with a 24% fracture rate. The complication rate for 3D Cones was 2.1% . A potential failure may be the use of Cones in sclerotic bone, which is a serious obstacle to osteointegration, especially on the tibia side where the risk of loosening is greatest [14, 18, 23]. You JS et. all presented no deep infection in 17 cases with a mean follow-up of 3.5 years . Our study also reported a very low rate of infections − 2 cases (1.2%) within the first 6 weeks after surgery.
So far, severe bone defects, classified as type 3 according to AORI, were most often treated with the use of a structural graft or resection prostheses. By using the multi-zone fixation technique, we believe that Cones can also be effectively used in such cases. Their use allows to raise the level of implant fixations, ensuring the correct reconstruction of the joint line, while avoiding further bone resection and sharing the load forces between the metaphyseal and join surface zones. Such an application is described by Kukrejaa et al. presenting a series of 6 cases (AORI type 3) using the tibial Cones. Radiographic evaluation showed 100% of osteointegration recorded on final radiographs in all patients after a mean follow-up of 4.1 years . Similarly, Meneghini et al. described the use of Cone implants in the reconstruction of the tibia and joint line as an effective solution for delaying the use of a respectable endoprosthesis . Our experience also shows this possibility. In 9 cases, we reconstructed the metaphyseal zone of the tibia using the asymmetrical Cone with total reconstruction of the condyle. In 4 cases, we used the asymmetric Cone to rebuild the tibial surface, resigning from the use of a tibial augment. This decision was dictated by osteoporotic bone tissue and the belief in a more favorable, superficial force distribution into the diaphisis zone. In 3 patients, the broken bone fragments with ligament attachments were based on the femoral Cone and in one case reinforced with titan wire loops. The radiological evaluation of the image over a 2-year period did not reveal any signs of loosening. Physical examination confirmed ligamentous stability with a range of motion of 0-120. In one case, we used the femoral and tibial cones to fill the defects after removing the loosened sleeves, and in one case after removing the hinged prosthesis, which effectively allowed to fill the defects classified as AORI 3 type. We also believe that the use of Cones allows to reduce the need to use resection prosthesis in doubtful cases. Their use allows to rebuild destroyed metaphyseal zone and to restore the joint line. However, in the case of significant bone loss, reaching diaphysis, we recommend the use of resection prostheses with fixation in the bone shaft.
Conclusion The use of Cone implants has gained importance in the revision of the knee joint due to its biological and mechanical properties. Structural stiffness and stability due to the high coefficient of friction provide a structural reinforcement in the weaker spongy bone of the metaphyseal zone, ensuring the correct distribution of mechanical forces. Their use could reducing the risk of infection. The available literature and our experience confirm the assumption that Cones is a real option in the effective treatment of cavities classified according to AORI as type 2B and 3. Summarizing the available data, we conclude that in the short term 3D printed Cones provide good strengthening of the metaphyseal zone. These results are encouraging, although they must be confirmed by longer observation.