Malposition and malsizing may often be imperceptible, for both, the radiologist and the surgeon and predispose to loosening. As a rule of thumb any arthroplastic component should fit tightly into the osteotomized bone. Familiarity with the type of arthroplasty is crucial for the detection of malsizing and malposition. Malposition of a knee prosthesis can occur in six different degrees of freedom, including flexion, extension, varus, valgus, internal and external rotation. It has been recently shown that flexed and/or internally rotated femoral components of a TKA are seen in symptomatic patients [2]. The various types and usually low degree of malposition require accurate measurement methods, which cannot be precisely addressed on radiographs. However, radiographs provide a first impression and facilitate a comparison with previous images. For knee arthroplasties, the Berger method is a common measurement for rotational alignment [10]. On axial CT images, the femoral component is measured in relation to the epicondylar axis with normal values of 3° (anatomical axis) or 0° (surgical axis) internal rotation. The tibial component is measured in relation to the midpoint of the tibial tuberosity with a normal value of 18° ± 2.6 internal rotation. These measurements on axial images have a wide variability and only address the rotational alignment [11]. Measurements on 3D reconstructed CT images using specialized software yields highly reliable values and is significantly better than axial CT images alone [11]. In particular, all six directions can be addressed with this technique. The full-leg alignment can be measured with additional scans of the hip and ankle joints [12]. SPECT/CT is superior in the assessment of potential malposition of arthroplasties as the degree of malposition can be measured with the CT data and a malposition may demonstrate an increased bone tracer uptake. Flexion of the femoral TKA component typically shows an increased uptake in the region of the posteromedial femur and at the proximal patellar. An internal rotation of the femoral TKA component leads to patellofemoral overloading with an increased bone tracer uptake at the lateral patellar facet and at the posterolateral femur with a lateral lift-off of the femoral condyle from the polyethylene inlay, called mid-flexion instability (Fig. 2) [2, 9]. External rotation of the femoral TKA component shows increased uptake in the posteromedial and posterolateral femur and at the medial patellar facet [9]. An increased posterior tibial slope may show an increased uptake at the posterolateral tibia (Fig. 3). A varus malaligned tibial component typically shows an increased uptake adjacent to the lateral tibial component, a valgus malaligned component on the medial side. Accompanying new bone formation, which is referred to stress-shielding, and in a later stage osteolysis and component subsidence may appear. Values for an optimal TKA position have been reported, however, the range of the position is accepted by each patient individually and does not necessarily account for the patients` symptoms [2].

In hip arthroplasties, radiographic follow ups have the advantage of including the opposite hip on the anteroposterior image for comparison of the leg length and the offset. A small head or an increased ante- or retroversion of the femoral component in hip arthroplasties are prone to dislocation. The anteversion of the femoral stem and the acetabular cup can be measured on axial radiographs or on axial cross sectional images, normal values range for each measurement between 10° and 30° [13, 14]. The tip of the femoral stem is ideally centered in the shaft. Varus or valgus malaligned femoral stems may lead to early loosening (Fig. 4). Retroversion of the acetabular component or an anterior overlap are risk factors for iliopsoas tendinopathy with consistent tendon tearing.