Biomechanical Analysis of A Mini Force-Closed Cemented Hip Stem for Juvenile Arthritis Patients

Standard implants are available in a wide variety of models and sizes to meet the needs of the normal population. Such implants, however, cannot be used for patients with abnormal hip joint anatomy. This study assessed the mechanical strength and the range of motion of a mini stem, designed for juvenile arthritis patients with substantially small femoral bones. The geometry of the mini stem, designed based on the cemented force-closed philosophy, was determined such that it could accommodate in the very narrow intramedullary canal of a sample patient. A 3D finite element model of the bone-cement-implant construct was developed to assess the strength of the mini stem against yield and fatigue failure, the strength of the cement mantle against yield, and the subsidence of the stem in cement due to the cement creep. Also, the range of motion of the hip implant was estimated by an impingement analysis in which the interference of the femur-pelvis or stem-acetabular cup was examined while rotating the femoral bone in the anatomical planes. Results indicated factors of safety of 7.33 and 4.76 for the mini stem against yield and fatigue failure, and 4 for the cement against yield. The subsidence of the mini stem in the cement over time was obtained as 1.6 micrometers for a 50-hour loading period. Finally, the designed implant could provide large ranges of motion in different planes with no impingement between the implant components and the bones in daily activities. It was concluded that the designed mini stem could satisfy the biomechanical requirements for successful total hip arthroplasty of juvenile arthritis patients.