The importance of correct positioning of orthopedic implants

With this article, I would like to highlight the importance of correct positioning of orthopedic implants during the surgery. The hip joint is taken as an example because it is what I am most familiar with.

Simplified bio-mechanic of the hip

Center of rotation and lever arms in the hip joint

The hip is a classical ball-and-socket joint with the head of the femur moving in the cup-shaped acetabulum of the pelvis.

The geometry of the hip permits rotational motion in all directions with the center of the femoral head as the center of rotation. These movements are due to the forces induced by the muscles of the hip that are acting on different points of the pelvis and of the femur.

There are 22 muscles are acting on the hip joint, not only to contribute to stability but also to provide the forces required for movement of the hip. All the forces, or moments of forces induced by these muscles depend on the position of the center of rotation of the hip and/or the lever arm.

If the centers of rotation and the femoral lever arms are not symmetrical, the action of the muscles will not be similar on both hips.

The important angles in the hip joint

Several angles are important in the hip joint to ensure the stability and the range of motion.

On the pelvis side, the acetabulum has an orientation that is slightly different for each people. Angular position includes anteversion and inclination (abduction angle) of acetabulum (or cup). Different studies have focused on defining the values of anteversion and inclination where the risk of dislocation is minimized. The surgeon will try to implant the cup by respecting these angles.

On the femur side, the neck has an angulation in regards of the knee. The so-called version of the femur is one of the reasons why some people have in-toe or out-toe walking.

The femoral anteversion is a natural rotation of the femur. The neck has an around 15° angle to the knee (posterior condylar axis).

Due to the muscles attached to the femur, the neck will tend to the standard position even if the angle to the knee is different. If the angle is larger than 15° (increased anteversion), this can lead to a in-toe position of the foot. If the angle is less than 15°, it can imply an out-toe walking.

Summary:

The acetabular cup has inclination and anteversion angles, the femur a version angle. The combination of these angles will influence the movements that can be made without dislocation.

The version of the femur also has influence on the orientation of the foot.

The total hip arthroplasty (THA)

During a THA, the surgeon replaces the natural cup cavity (acetabulum) by an artificial component, the cup. The natural neck is cut and removed, and an artificial component, the stem, is inserted in the femur. An artificial head is then fixed on the stem to restore the anatomy of the femur.

Preoperative planning       

The surgeon will use X-Rays or CT-Scan images to select the different components, their size and their shape. He will then decide their position and orientation of implantation to restore the desired anatomy.

When selecting and planning the components, the surgeon also has to take in account the bone stock and the bone quality to be sure that the implants will be well fixed and stable.

The implanted cup in the pelvis will define the center of rotation on the pelvis side. The implanted stem will define the center of rotation on the femur side.

Surgery

At the end of the surgery, the femur is moved to put the head of the femur in the cup (reduction of the hip), both centers of rotation are then at the same position and the position of the femur relatively to the pelvis is restored.

The importance of correctly position the implants

A correct selection of the stem and head will allow to restore the adequate geometry of the femur only if the components are implanted as planned.

The acetabular cup also has to be implanted as planned to restore the center of rotation and to respect safe angulations.

Summary:

For a natural walk, the hips on both sides must have a similar geometry.

The restored geometry depends on the position of the different artificial components.

The tools provided by the technological advances

The advances in medical imaging (MRI + CT Scan) and in computer science bring new tools for the orthopedic surgeons: