The volume of RTKA is rising as the aging of the population and the consequently increasing demand for primary TKA. 268,000 RTKAs are expected in the United States by 2030, six times the number in 2005. The restoration of the proximal tibial defect is one of the main challenges in RTKA. The reconstructing strategy usually varies according to the extent of bone defects. The Anderson Orthopedic Research Institute (AORI) classification is the most practical and widely accepted system among them for preoperative planning.
For mild proximal tibial defects (AORI type I and IIA), good clinical outcomes have been achieved by applying bone cement, metal screw, bone grafting, or metal augment. On the other hand, for more severe bone defects, especially AORI type III, each traditional restoration technique has its own prominent shortcomings.
The bone grafting technique is well adopted for massive bone defects reconstruction, including impaction bone grafting and structural bone grafting. It allows surgeons to restore various bone defects in different shapes and sizes and maintain the joint line. Compared with the expansive metal augment, bone grafting is a more economical solution, especially when autologous grafts are feasible. However, prosthesis collapse as a typical result of non-fusion grafting would be catastrophic. The study reported by Hilgen et al. shows that the 10-year survival of the prosthesis was only 50%, in which bone grafting was used for tibial AORI type II and type III defects. In another study, Engh et al. enrolled 46 RTKA patients with proximal tibial AORI type III defects who underwent structural bone grafting and followed-up for an average of 97 months. Although the 10-year survival of the prosthesis reaches 90%, four prosthesis failures occurred which caused by infection, aseptic loosening, osteonecrosis, or polyethylene wear. Besides, the bone grafting utilization has been further limited by the availability of allograft, the complexity of grafting procedure, and the requirement of experienced surgeons.
Recently, cones and sleeves are becoming the mainstream of tibial metaphyseal bone defects restoration. Cones have better performance in filling defects, while sleeves focus more on stability. The multiplicity of their size and shape allows them to fit various bone defects. High friction introduced by the porous metal surface facilitates immediate metaphyseal stability, which makes postoperative early weight-bearing probable. Meanwhile, the bone-ingrowth-friendly porous structure will lead to excellent long-term fixation and clinical outcomes. A study of 66 proximal tibial defects (AORI type IIB or III) restoration with tantalum cones in RTKA conducted by Kamath et al. shows 95% survival of the proximal tibial prosthesis on an average of 70-month follow-up. In another similar retrospective study reported by Burastero et al. on an average of 43-month follow-up, the cone-related survival rate was 97.8% of 60 cases of RTKA with massive bone defects. It also shows significant improvement in patients' postoperative joint functions and knees ROM. Klim et al. conducted a 6.3-year follow-up of 93 RTKA with bone defects restored with metaphyseal sleeves. Not even a single case shows aseptic loosening. Even though 17 cases underwent reoperation due to recurrent infection or periprosthetic fracture, 96.1% (73/76) of the rest cohort achieved excellent radiographic osseointegration with significant improvement of functions and ROM of the knee. Zanirato et al. systemically reviewed 37 published studies of RTKA with bone defects restoration, 927 cone cases (3.6-year average follow-up), and 1801 sleeve cases (4.5-year average follow-up) were enrolled. Both cone and sleeve groups showed promising clinical and functional outcomes. The aseptic survivorship of the implants was 97.3% in cones groups and 97.8% in sleeves group.
Although metaphyseal cones and sleeves have achieved good survivorship and clinical outcomes, some drawbacks have emerged. Precise anatomic reconstruction and biomechanic restoration are challenging to accomplish in that uncustomized cones and sleeves cannot perfectly fit into various bone defects. During the cone or sleeve insertion, further sculpturing and reaming of the host bone with a broach or high-speed burr are always required to optimize the bone-implant contact and enhance subsequent bone ingrowth. The additional bone sculpturing would multiples the procedure complexity, prolongs the operating time and introduces iatrogenic bone loss. The intraoperative fracture caused by implants insertion is another unneglectable risk, which increases the difficulty of the procedure even more.
Given the advancement of 3D printing technology, 3D-printed implants are also available in the market for bone defect reconstruction in RTKA. A study of 62 RTKA cases, in which metaphyseal tibial cones were applied for AORI type II or III tibial defects, followed-up on average of 27 months by Denehy et al. shows that the revision-free survival of the cones was 90.2% (the survivorship was 100% if the infection was excluded) with the notable improvement of patients' postoperative functions. Even though the 3D-printed cones were involved in this study, they were manufactured into several fixed sizes (symmetrical and unsymmetrical). Intraoperative bone sculpturing is inevitable to attain an ideal anatomical fit between the uncustomized cone and various bone defects. 24%(15/60) cases among them required additional operations. Yin et al. reported the application of individually customized 3D-printed porous titanium augment in a single RTKA case to reconstruct the proximal tibial defect (AORI type IIB), which was caused by previous periprosthetic infection. The prosthesis stability and the clinical outcome were satisfying at the 6-month follow-up. However, the porous alloy was only used as a filler to restore the bone defect in this study. Metaphyseal fixation was not obtained. Furthermore, the bone defect was relatively shallow, although it was classified as AORI type IIB. The short follow-up period is another limitation.
All the cones utilized in our study for RTKA with AORI type III tibial defects were modeled based on the patients' preoperative CT scans and printed layer-by-layer with titanium alloy. From our experience, immediate rigid metaphyseal fixation was much more accessible to achieve with the customized cones compared with fixed-size ones. Both operating time and procedure complexity were decreased. On the 25-month follow-up, clinical outcomes were excellent, without complications such as aseptic loosening, periprosthetic infection, and fracture.
Our study reveals several advantages of the application of 3D-printed patient-customized cones in RTKA with massive proximal tibial defects. High stability stands out firstly. By combining the excellent filling performance of conventional cones with the prominent stability of sleeves, customized cones can achieve reliable initial metaphyseal fixation. The pore size and porosity of the cones can be appropriately configured in modeling to increase the friction and promote the osseointegration, which respectively maximizes the initial stability and long-term stability. Thanks to their incredible immediate stability, the demand for diaphyseal stems fixation could be minimized or even eliminated to avoid the stem-related postoperative leg pain. Further searches, including finite element analysis, are required to verify this benefit, which is our next step.
Excellent fitness, both functionally and anatomically, is another main advantage. The 3D-printed patient-customized cone modeled according to the preoperative images fits the bone defects much more precisely compared to the conventional fixed-size cones, especially when dealing with massive bone defects in complicated revisions. Since it is an anatomical copy of the patient's knee, a customized cone can be implanted with minimal postoperative soft tissue interference. It obtains both bone and soft tissue fit.
Besides, As the extra bone cutting and reaming minimized by the utilization of customized cones, not only the iatrogenic bone loss is significantly avoided, but also the procedure greatly simplified by converting a complicated revision TKA into a straightforward "primary" TKA. Metal 3D printing technology also immensely reduces the cost of the customized cone and shortens its production cycle since traditional manufacturing techniques, such as casting, forging, and cutting, are no longer involved, making it a more practical and affordable solution. Moreover, the 3D-printed porous cone can accurately mimic the elastic modulus gradient from cancellous bone to cortical bone in order to diminish stress shielding and osteolysis.
There are some limitations to our study. We enrolled in seven cases without a control group; nevertheless, it is by far the largest case series regarding the utilization of the patient-customized 3D-printed metaphyseal cone in RTKA with massive bone defects. The length of the follow-up period is still short. Thus the comparison with long-term clinical and radiological outcomes of traditional cones in RTKA is unavailable. Coincidentally, all seven cases are female patients, which may induce selection bias to some extent. In order to further verify the advantages of this technique, the expansion of the sample size and follow-up time is needed for subsequent studies.