Finite element analysis of mechanical stress in a cementless tapered-wedge short stem in the varus position

Background: In recent years, the use of tapered-wedge short stems has increased because of their ability to preserve bones and tendons. Surgical techniques occasionally result in a varus position of the stem, which is particularly pronounced in short stems. Although the varus position is not clinically problematic, there are reports of an increased incidence of stress shielding and cortical hypertrophy. Thus, we evaluated and examined the acceptable range of varus angles using finite element analysis. Methods: We selected patients diagnosed with osteoarthritis of the hip joint who had undergone arthroplasty and were classified into three types [champagne-flute (type A), intermediate (type B), and stovepipe (type C)]. Finite element analysis was performed using Mechanical Finder. The model was created using a Taperloc microplasty stem with the varus angle increased by 1° from 0° to 5° from the bone axis and classified into seven zones based on Gruen’s zone classification under loading conditions in a one-leg standing position. The volume of interest was set, the mean equivalent stress for each zone was calculated, and the mean value of the equivalent stress in each zone was calculated. Results: A significant decrease in stress was observed in zone 2, and increased stress was observed in zones 3 and 4, suggesting the emergence of a distal periosteal reaction, similar to the results of previous studies. In zone 2, there was a significant decrease in stress in all groups at a varus angle ≥3°. In zone 3, stress increased from ≥3° in type B and ≥4° in type C. In zone 4, there was a significant increase in stress at varus angles of ≥2° in types A and B and at ≥3° in type C. Conclusion: In zone 2, the varus angle at which stress shielding above Engh classification grade 3 may appear is expected to be ≥3°. Distal cortical hypertrophy may appear in zones 3 and 4; the narrower the medullary cavity shape, the smaller the allowable angle of internal recession, and the wider the medullary cavity shape, the wider the allowable range. Long-term follow-up is required in patients with varus angles >3°.