The tibia tray has been a notable complication in TKA since 1984. It has been shown that metallurgy and stress rise have both contributed to tray fracture. A body mass index (BMI) of 37 kg/m², which was distributed heavily in the posterior thigh and calf areas, increased the load on the tibial tray as well as caused the posterior tibia to be loaded [11]. Fatigue fractures were majorly caused by a lack of bone support during the first implantation, most likely resulting from osteolysis. It is important to focus on the precise positioning of each component during the initial process [12]. The tibial component broke at the intersection of the base plate and stem resulting in total dissociation between the two parts. Clinicians should be cautious regarding catastrophic failure because this unusual occurrence is difficult to detect in radiography and necessitates careful observation for evidence of component subsidence [13]. There is a new modular design with a titanium alloy tray covered with a porous tantalum layer (trabecular metal) and a separate UHMWPE insert [14]. Due to failed knee arthroplasty, revision arthroplasty has become a significant clinical issue. Polyethylene wear, osteolysis, and material fatigue are the major causes of implant failure [15]. Microscopic study of the tissue beneath the broken portion of the tibial base plate disclosed a darkly stained metallic deposit that was verified to be tantalum and titanium by micro-proton induced X-ray emission (PIXE). The tibial plate fracture fits the requirements for a typical fatigue fracture, according to fractographic analysis and stress testing [16]. The patellar and femoral components were examined intraoperatively and found to have good bone fixation, requiring no alteration. A coronally oriented fracture was observed in the posteromedial tibial baseplate with little ingrowth. The polyethylene liner was also damaged, and the locking mechanism was rendered inoperable. The Anderson Orthopaedic Research Institute classified the osseous injury as a type 2A bone defect since it was localised to the medial tibial condyle. The osseous defect was filled with a tibial reaming autologous transplant. Fractured tibial components in both cemented and uncemented models, osteolysis, and long-term polyethylene wear were recognised as the predominant reasons for implant failure [17]. After the failure of cementless models, screw-based fixation was replaced by anchored tibial models as it eliminated screw holes and provided a larger surface area for implant fixation. As a result of this, the potential sites of osteolysis are eliminated. According to the report of the Australian National Joint Replacement Registry (ANJRR), only 9 patients (1.7%) needed revision knee arthroplasty out of 492 recorded cases, and all received revision surgery between 12 and 36 months after the initial procedure [18]. Nylon 645 polymer was developed specifically for the fabrication of biologically inert implants utilising typical fusion coating manufacturing procedures with 3D printers. This material has been used to make bespoke implants to repair damaged cartilage in the knee [19]. The edge-only ungrounded model has the greatest total damage points and line wear rate. No variations were reported in damage and wear amongst the other TKA designs. However, this inference may be used with caution due to the limited sample size [20]. The use of computer-guided TKA and patient-specific equipment increases TKA prosthesis insertion accuracy and results in the optimal positioning of mechanical axis at the postoperative lower extremity. Even though image-based preoperative planning can assist in mending of neutral mechanical alignment of the lower limb, it does not visibly depict the load and stress distribution of the TKA knee and does not consistently foresee TKA prosthesis retention [7]. The aim of the work is to extract patient anatomical properties of the knee joint using medical software, model patient-specific knee implants, and develop 3D printed implants for TKA using fused deposition modelling (FDM) 3D printer.

Femur bone, tibia bone, and kneecap (patella) constitute a knee joint. A smooth substance called articular cartilage protects the ends of these bones allowing them to move easily within the joint. In between the femur and the tibia, C-shaped wedges called menisci are present which serve as shock absorbers for the joint [21]. The knee is commonly thought to be made up of the tibiofemoral and patellofemoral joints. The knee joint is depicted in Figure 1. The tibiofemoral joint is separated into medial and lateral divisions [22]. There is a slight difference in cross-sectional size among lateral and medial condyles, but the former condyle is constant. When flexion occurs, the femoral and tibial condyles can slide and roll together [23].

Diseased bone and cartilage are removed in a TKR surgery. Femur distal and tibial bones are cut and reshaped to suit the femoral part and tibia tray of the prosthesis [25]. The femoral, the tibial, and the patella are the three basic components. As indicated in Figure 2, the tibial and patellar components could be solid polyethylene or have a metal backing that supports a polyethylene articulating surface [26].

TKA is commonly performed when the cartilage is injured, the joint is distorted, and worn cartilaginous bearing surfaces need to be replaced with artificial bearings [28]. The goal of arthroplasty is not to treat illness but rather to mechanically address a biological problem. Arthroplasties may be the only treatment for individuals with significant joint injury [29]. Obese individuals using TKA seems to have larger functional improvements than those with a normal BMI [30, 31]. However, the functional results of TKA can be enhanced by focusing on managing the patient’s excess weight concerns [32]. TKA resulted in weight reduction, better BMI, and functional results in 31% of instances [33]. On 781 patients, revision TKAs have been performed. Loosening (39.9%), infection (27.4%), instability (7.5%), periprosthetic fracture (4.7%), and arthrofibrosis (4.5%) are among the most common reasons for failure. Aseptic loosening, infection, instability, periprosthetic fracture, and arthrofibrosis are the typical causes of repeated TKA failures [34].

3D printing process produces objects using printing-based technology. It belongs to the material extrusion category in additive manufacturing (AM). This approach involves depositing the material selectively through a nozzle [35]. AM involves the creation of products from computer-aided design (CAD) data layer-by-layer using additive techniques [36]. Biomedical devices and tissue engineering (TE) benefit greatly from 3D printing due to its ability to manufacture low-volume or one-of-a-kind parts depending on patient-specific needs [37, 38]. FDM is an additive technology in which basic material is melted and moulded to produce new shapes. The nozzle acts as a guide and extrudes material precisely laying one layer over the other [39]. Generally used filament material types in FDM are polylactic acid (PLA), polyvinyl alcohol (PVA), acrylonitrile butadiene styrene (ABS), thermoplastic elastomer (TPE), polycarbonate (PC), and PLA with graphene [40].

Around 80% of medical implants are metallic and further classified as bio-degradable and non-degradable metallic implants. Metals, polymers, ceramics, and composites are employed in orthopaedic implants [41]. Fifty metallic alloys such as titanium (Ti) alloys, nickel (Ni) alloys, aluminum (Al) alloys, copper (Cu) alloys, tool steels, stainless steels, cobalt-chromium (Co-Cr) alloys, and refractory metals are employed in AM [42]. AM allows the fabrication of customized prosthetic patient-specific implants based on their size, shape, and mechanical qualities [43]. A variety of materials can be used in 3D printing like ABS plastic, PLA, polyamide (nylon), glass filled polyamide, epoxy resins (epoxy resins), silver, titanium, steel, wax, photopolymers, PC, cells, hydrogels, etc. Femur and tibia bones are made of cobalt, chromium, and molybdenum alloys [44, 45]. A spacer set consists of a femoral spacer, a tibial spacer, and a canal rod. UHMWPE spacers have been tested for biocompatibility [46]. The femoral component made of Co-Cr is 3D printed using selective laser melting. Titanium-aluminium-vanadium alloy (Ti6Al4V) and conventional polyethylene are used to make the tibia and tibial implants, respectively [47]. Due to their equivalent density with the human bone, magnesium alloys offer the highest biomechanical compatibility and generate the least degree of discomfort in comparison to stainless steel or titanium alloys [48].

Several softwares are available for identifying diseases and injuries. 3D Slicer v5.2.1 is an open-source software (www.slicer.org) which employs volume rendering as a visualization approach for displaying picture volumes as 3D objects. The visualization of a patient suffering from knee joint with degenerated cartilage and meniscus is depicted in Figure 3. A healthy knee joint has normal space among the femur and tibia bone, however, in an osteoarthritis knee, the joint space narrows with the breaking down of cartilage and damage of the meniscus [49, 50].

The knee joint of a 42-year-old lady suffering from osteoarthritis of the left knee is obtained using a computed tomography (CT) scan of transmissive type as shown in Figure 4A. In CT, images are stored in digital imaging and communications in medicine (DICOM) format by the devices. Using 3D Slicer v5.2.1 software, a complete set of images of the knee joint are separated by changing threshold values. The threshold values are set from 152 to 2,923.60 as exhibited in Figure 4B. With the help of Segment Editor tools, the model is edited to remove additional geometries and noise factors such as muscles surrounded by bone, as shown in Figure 5A. Segment Editor is a module in 3D Slicer v5.2.1 software that is used for specifying segments (structures of interest) in 2D/3D/4D images. It offers editing of overlapping segments, display in both 2D and 3D views, fine-grained visualization options, editing in 3D views, create segmentation by interpolating or extrapolating segmentation on a few slices, editing on slices in any orientation [51]. The anatomical features of the knee joint are obtained. The segmented DICOM file is then switched to a standard triangulation language (STL) file as shown in Figure 5B. Autodesk Meshmixer v3.5 (Autodesk Inc., San Rafael, CA, USA) software is used to identify the errors like open loops in the STL file as demonstrated in Figure 5C.

Post-repairing and optimization of the mesh are carried out using appropriate tools. 3D printing is performed on the FDM based Flashforge Guider II (Zhejiang Flashforge, 3D Technology Co. Ltd., Jinhua, Zhejiang, China). FDM is capable of 3D printing using different materials [52]. The working principles of FDM and Flashforge Guider II printer are illustrated in Figure 6.

Flashforge Flashprint v5.6.1 (Zhejiang Flashforge 3D Technology Co. Ltd., Jinhua, Zhejiang, China) software is employed for setting print parameters and slicing of the STL file where the 3D model is imported. The knee joint’s orientation is changed inside the print area. The printing variables are disclosed in Table 1.

STL file of the knee joint cannot be used for designing knee joint prosthetics directly in any CAD software. For processing of the STL file into the standard for the exchange of product data (STEP) file, knee joints are made into individual bone segments like femur, tibia, fibula, and patella. The individual STL and STEP files of the knee joint are shown in Figure 7. Conversion of STL to STEP files is carried out using Hypermesh v17.0 (Altair Engineering Inc., Troy, MI, USA) and Autodesk Meshmixer v3.5 softwares. STEP file is later imported into Autodesk Fusion 360 v2.0 (Autodesk Inc., San Rafael, CA, USA) software. These imported files are used for altering the part and adding materials to it.

Bone resection in CAD involves removal of bone wherein the resurfaced bone model utilized for constructing the patient-specific knee component is used for conducting an examination of off-the-shelf knee components [53].

There are three standard sizes for resection of the knee joint based on femur geometry where resection will take place. Size groups of femoral anterior-posterior (A-P) length are shown in Table 2 [53]. The measurement of A-P length is shown in Figure 8A. Femurs’ medial anterior posterior (AP_M) length is 53.351 mm and lateral anterior posterior (AP_L) length is 56.796 mm. The average length of A-P is found to be 55.072 mm which falls under the small category. When the “knee angle” (tibio-femoral angle) is 6° valgus, the knee is in neutral mechanical alignment. The femur is traditionally cut using an intramedullary reference system guided by the medullary canal, which is also the anatomic axis. The guide is set at an angle of roughly 6° between anatomic and mechanical axes. The depth of the distal femur is 9 mm. As a result, the distal femoral cut’s desired depth is 9 mm [54, 55]. Based on its small size, the average thickness of the bone resection in the ConforMIS CR implant (Conformis, Inc., Bedford, MA, USA) resection of distal medial and distal lateral thickness is 6.5 mm. The posterior medial and posterior lateral thicknesses are 5.9 mm and 5.8 mm, respectively. The resection of the femur bone can be seen in Figure 8B. Both the mechanical and the anatomical axes are parallel, therefore the tibia is cut at an angle of 0°. The joint line is unchanged if 11 mm of tibial bone is removed. This allows for a larger polyethylene liner of 11 mm instead of 9 mm, to be placed. The resection of tibia bone is shown in Figure 8C.

Knee implant files are imported into Flashprint v5.6.1 software where the process parameters are set for 3D printing. Slicing is performed after the addition of support structures on the femoral implant and tibia tray. Verification is carried out on knee implants. 3D printing process parameters are provided in Table 3. Flashprint v5.6.1 software generates the G-codes and is exported to Flashforge 3D printer for printing.

STL file of knee joint is imported in Flashprint v5.6.1 software. Different orientations are tried to achieve minimum support to the knee joint, better accuracy, and a good surface finish. The layer height of 0.20 mm, 20% infill with hexagonal pattern, and 2 shells are employed for printing of knee joint. It took 8 h and 36 min to print with supports using PLA material since it is biodegradable. A 3D printed knee joint prototype after the removal of support structures are shown in Figure 11A and 11B. Knee implants-femoral, tibial, and liner are 3D printed using PLA material in Flashforge FDM printer using the layer height of 0.20 mm, 25% infill with hexagonal pattern, and 2 shells as shown in Figure 11C–E. It took 2 h and 58 min for printing the femur and liner whereas 55 min for the tibia tray. The printed assembly components are shown in Figure 11F.