## 3 Results
### Clinical Case: Primary TKA in Polionyelitis  
A 58-year-old female patient suffered of poliomyelitis in her childhood, with subsequent left lower limb muscular deficiency. She came to our attention complaining about a left knee pain. The patient was a small lady, 150 cm in height, 47 kg in weight, with a Body mass Index (BMI) of 21 kg/m\({}^{2}\). Due to poliomyelitis, she underwent three different surgeries for lower limb elongation and a hindfoot fusion. She was diagnosed with left knee osteoarthritis, and, after failure of initial conservative treatment, she was recommended for TKA. Figure 1 shows pre-operative \(\chi\)-rays. At the clinical evaluation, the patient was able to fully extend the knee, with a flexion limited to 100\({}^{\circ}\) due to pain. There was a partially reducible valgus deformity, with severe lower limb muscular deficiency, a positive valgus stress and recurvatum and valgus thrust during gait. Furthermore, there was a mild-to-moderate extension lag mainly due to quadricep muscle deficiency. The main difficulties to be considered approaching this case were: (1) ligamentous deficiency with possible need for high constrain; (2) issues related to poliomyelitis, such as bone deformities, ligament laxities, recurvatum and quadricep deficiency [46]; and (3) the patient's small size, as detected with a digital templating. The patient's size was particularly worrying in this case, because digital templating already demonstrated very small distal femur and proximal tibia, which were not compatible with some available implants. Figure 1: Pre-operative X-rays evaluation: (**A**) long-leg weight-bearing view; (**B**) lateral view; (**C**) antero-posterior view; (**D**) patellar view. For these reasons, a 3D-printed model, based on a CT scan, was produced by [PERSON](r) (Turin, Italy). In this case, as shown in Figure 2A, available data on the femur were Posterior Condylar Offset (PCO = 28.7 mm), medio-lateral femur diameter (59.8 mm), antero-posterior medial (44.2 mm) and lateral (64.6 mm) diameter and distal and proximal medullary canal diameter (distal 15.3 mm \(\times\) 29.1 mm, proximal 12.08 mm \(\times\) 17.4 mm). On the tibial side (Figure 2B), available data were antero-posterior axis (36.9 mm), medio-lateral axis (53.2 mm), tibial slope (9.6\({}^{\circ}\)) and distal and proximal medullary canal diameter (distal 9.7 mm \(\times\) 15.4 mm, proximal 18.7 mm \(\times\) 30.5 mm). A bone thickness analysis was also included (Figure 3), revealing poor bone thickness in the distal femur (on both condyles, more on medial than on lateral condyle) and in the medial proximal tibia. In Figure 3, red zones correspond to 0.1 mm thickness, while green zones correspond to a maximum thickness of 6 mm. Figure 2: Pre-operative planning based on 3D-printed model: (**A**) femoral measurements; (**B**) tibial measurements. In this case, the 3D-printed model turned out to be extremely useful to plan the surgery. Considering knee dimension and the need for a constraint, the only implant that could fit these characteristics was the extra-small size of Edomodell(r) (Waldemar Link GmbH, Hamburg, Germany), which guaranteed both the right level of constraint and a very small dimension. With all this information, an accurate pre-operative planning, sizing, and surgery simulation was performed (Figure 4). Figure 3: Bone thickness evaluation: red zones correspond to lower thickness (0.1 mm) while green zones correspond to higher thickness (6 mm). (**A**) Anterior femoral side; (**B**) posterior femoral side; (**C**) medial femoral side; (**D**) lateral femoral side; (**E**) anterior tibial side; (**F**) posterior tibial side; (**G**) medial tibial side; (**H**) lateral femoral side. Post-operative X-rays for this patient are shown in Figure 5.  
Six months after surgery, the patient was satisfied with complete ROM, stable knee, no pain and no sign of loosening at the X-rays. Figure 4: Three-dimensionally-printed model for pre-operative planning and implant size evaluation.