Biomechanical study of hinged knee arthroplasty in patients with valgus and varus deformity

Rotating hinge (RH) knee prosthesis is a common solution used for revision procedure. Nevertheless, it is also used in complex primary surgery as an alternative of primary total knee arthroplasty. One of the most used RH implants is the Endo-Model rotating Hinged Knee Prosthesis, which enable knee flexion-extension and intra-extra rotation. The design of this prosthesis consists of a femoral and a tibial component, which are rigidly constrained, and a polyethylene insert placed between these two elements, which prevent luxation without reducing the range of motion. This kind of prosthesis thus is a highly constrained device able to provide the stability needed for arthroplasty in complex situations such as severe varus or valgus deformity. Deviation from neutral alignment could alter mediolateral force distribution, which could cause overloads the bone-implant interface as well as to the bone itself. A biomechanical study is conducted in order to evaluate the performance of Hinged Knee Endo-Model prosthesis in patients with severe Valgus and Varus deformity, with different lengths of stem and type of fixation, cemented or cementless. The aim of this study is to help clinicians and specialists to make a proper decision during surgery about the length and type of stems when a varus-valgus deformity is present. Starting from physiological configuration, static simulations were performed to analyse the stress distribution on implant components for different Varus and Valgus alignments (5°, 10°, 20° and 30°) at 0°, 5° and 90° of flexion. Three stem lengths (50, 120 and 160 mm) of femoral component and two types of fixation (cemented or cementless) were examined both for physiological and osteoporotic bone. Each configuration underwent was loaded with a constant axial force of 1000N applied on the proximal tibia and the proximal part of the femur was constrained. For all the stem lengths, when the knee has a Varus or Valgus configuration, it will cause an increase of stress on polyethylene component compared to the aligned configuration.