**5. Preoperative evaluation**

A through clinical history must include questions about previous conservative treatment and surgeries, period of time that stiffness started, comorbidities, medications, and psychological profile. The physical examination must comprise the preoperative passive and active ROM (flexion and extension), patellar gliding, the amount of flexion contracture, scars, type and flexibility of the angular deformity, gait pattern, and extensor mechanism status (elongated or contractured). Osteoporosis is frequent in SKNs. Complete motor, sensory, and vascular assessment should be performed. Ankle/brachial index and Doppler ultrasound can be useful to estimate the function of blood circulation in the legs.

The imaging exams of the knee should include radiographic evaluation in anteroposterior (AP) and lateral at 30° of flexion (**Figures 1** and **2**). Special views with maximal and minimal flexion in the sagittal plane should be documented. Long-axis anteroposterior (AP) view can be useful to determine the mechanical and anatomical axis of the lower limbs. The sunrise patellar view at 45° of flexion can demonstrate a severe arthritic involvement, where the patella usually is fused with the anterior femur [2, 5]. A stress view in the coronal plane can be helpful to determine if the angular deformity is rigid or correctable. Presence of hardware is not uncommon in STK patients. Computed tomography (CT) scan may be used to assess bone stock and rule out infections [3].

#### **Figure 1.** *Radiography in anteroposterior view with stiff knee.*

The surgeon must select which type of knee prosthesis will be required. A broad assortment of modular systems are disposable according to each patient. More constrained implants can be considered in cases with bone loss, ligamentous insufficiency, or after extensive soft tissues releases. A custom prosthesis must be fabricated for a particular situation as a very small or large knees and ankylosed knees in rheumatoid patients. In a previous infected STK, a staged procedure can be recommended to decrease the risk of serious complications [14, 15].
