Hip research 

Ivan Zivkovic, F. Amirouche PhD., M. Gonzalez, MD.

Finite element of human hip and cementless acetabular implant.

Total hip replacement (THR) or arthroplasty, is a surgery performed to replace a severely damaged hip with an artificial joint. The artificial joint components consist of a shell (acetubular cup) and a liner that replaces the socket of the hip joint, and a femoral head and stem that replaces the ball of the hip joint and upper part of the thigh or femur bone.

Artificial implants of the joint can either be cemented or non-cemented.  Cemented implants are usually reserved for older patients with poor quality bones that lack strength to help support their body weight. Non-cemented implants are for young, healthy individuals with strong and healthy bones.  Their bone can readily attach to the implants without additional help (cement, screws) and will allow a revision surgery without severe damage to the body.

Objectives

Manufacturers are particularly interested in the micromotions and stresses produced among THR components in non-cemented implants, in order to ensure the stability of the implant, such as:

·         Hip bone/acetabular cup.

·         Acetabular cup/liner.

·         Liner/femoral head.

  To study the stability of the prosthesis both, experimental and computational studies have been performed.

Experimental Studies

Bone-Cup micromotion experiment setup

Cup-bone micromotion:

To obtain the micromotion occurred between the acetabulum and the cup fresh-frozen cadaver pelvises are cleaned of all muscles. A cementless acetabular prosthesis was inserted into each pelvis. The pelvis specimen is then potted into a metal box for better fixation with respect to the loading jig. Loads in the cup are applied through a custom loader, designed for such purpose. After positioning the cup, the liner is inserted in its socket. The loader is then connected to the cup attaching it to the liner. After the pelvis specimen is properly positioned, a fixture supporting six LVDT sensors is attached to the liner. These sensors enable us to measure micromotion along six different axes. Loads are then applied and the micromotion recorded for posterior evaluation.

Cup-Liner micromotion experiment setup

Cup-Acetabular Liner micromotion:

  To obtain the micromotion occurred between the cup and the liner during different loading conditions, commercially available cement-less acetabular components are tested. The experiment is used as well as to validate an FEM (Finite Element Model) of a cement-less acetabular component. The liner specimen was positioned in a custom jig designed to hold the acetabular cup at a constant angle. A commercially available femoral head is attached to a customarily designed loader. The femoral head-loader is then placed in an Instron machine and perfectly fitted into the liner’s socket. The supporting jig is fixed to the based of the Instron machine. A fixture supporting six LVDT sensors is attached to the supporting jig, and the sensors are put in contact with the liner. These sensors enable us to measure micromotion along six different axes. Loads are then applied and the micromotion recorded for posterior evaluation.

Computational Studies

Finite element of human hip and cementless acetabular implant.

A Finite Element Model (FEM) of the THR components based on actual components (DePuy) has been developed and validated to replicate the behavior of the implant. This model allows determining the conditions under which instabilities may occur by measuring micromotions and stresses over the components.

Several loading profiles are studied in an implicit and explicit manner using ANSYS. Some of these loading profiles include the analysis of subluxation of the femoral head, walking and stair climbing. Modal analysis of the components in a modular acetabular prosthesis is also being performed in order to analyze the contribution of external vibration sources in the stability of the THR implant.

Simulation results of femoral head subluxation using explicit finite element analysis.

Stress results occurred in the femoral liner due to the relocation of subluxation. Explicit finite element analysis.