Effect of anatomical variability on stress‐shielding induced by short calcar‐guided stems: Automated finite element analysis of 90 femora

Abstract

Short stem hip implants are becoming increasingly popular since they preserve bone stock and presumably reduce stress‐shielding. However, concerns remain whether they are suitable for a wide range of patients with varying anatomy. The aim of this study was to investigate how femoral anatomy influences stress‐shielding induced by a short calcar‐guided stem across a set of 90 CT‐based femur models. A computational tool was developed that automatically selected the optimal size and position of the stem. Finite element models of the intact and implanted femurs were constructed and subjected to walking loads. Stress‐shielding was evaluated in relevant volumes of interest of the proximal femur. After a detailed anatomical analysis, linear regression was performed to find potential correlations between anatomy and stress‐shielding. Stress‐shielding was found to be highest in the proximal regions on the medial and posterior side. A highly significant negative relationship was observed between stress‐shielding and bone density; a strong positive relationship was observed with stem size and the valgus orientation of the stem with respect to the femur. The results reveal how anatomy influences stress‐shielding, and they highlight the importance of evaluating new implant designs across a large population taking into account the anatomical variability. The study demonstrates that such large population studies can be conducted in an efficient way using an automated workflow. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 9999:1–8, 2019.

Cemented short‐stem total hip arthroplasty: Characteristics of line‐to‐line versus undersized cementing techniques using a validated CT‐based finite element analysis

“…The hip contact force was applied at the implant head center and distributed over the top surface of the stem. The muscle forces were distributed over node sets including the 10 closest nodes to the muscle force application points 7 . The nodes of the most distal elements were restrained to prevent rigid body motions.…”

“…However, using uncemented short stems in elderly patients with reduced bone quality increases the risk of postoperative periprosthetic fractures 5 . Thus, a cemented version would potentially offer a solution for patients with poor bone quality or uncommon anatomy 6,7 . Yet, clinical data on cemented short stems are still limited and concerns have risen about the risk of periprosthetic fractures and long‐term survival 8 .…”

Hip implants can restore anatomical and medialized rotation centres in most cases

“…For calcar-guided stems, the software aligned the medial border of the stem along the calcar region of the medullary canal. 23 As this step was automated, it was perfectly reproducible.…”

Sensitivity of the stress field of the proximal femur predicted by CT‐based FE analysis to modeling uncertainties

“…Stress shielding causes relative osteopenia around metal implants due to the preferential force transmission through the stiffer metallic implants. [1][2][3][4] Although the majority of total hip replacements are successful, according to a recent study, around 15% of the patients who receive a hip implant may require revision surgery within 20 years, with adverse outcomes significantly higher than primary surgeries. 5,6 Developing automated systems that are capable of quantifying and comparing the pre-and post-implant mechanical environment of the host skeleton tissue may help clinicians to use the ensued knowledge for designing patient-specific implants that tailor the mechanical response to the unique features of patient's anatomy and bone properties.…”

“…Because anatomy, bone material, and structural properties differ between patients, the current approach to implant selection has exacerbated clinical problems such as stress shielding. Stress shielding causes relative osteopenia around metal implants due to the preferential force transmission through the stiffer metallic implants 1–4 . Although the majority of total hip replacements are successful, according to a recent study, around 15% of the patients who receive a hip implant may require revision surgery within 20 years, with adverse outcomes significantly higher than primary surgeries 5,6 .…”