[Impingement Free Motion in Total Hip Arthroplasty - How Can We Implement It?]

Prosthetic impingement and insufficient soft tissue tension are still the most important factors responsible for early dislocation after total hip arthroplasty. Optimal positioning of both prosthetic components, the stem and the socket, optimising their design and restoring individual hip biomechanics, are of the upmost importance in reducing the risk of impingement. This study describes the concept of the combined safe zone (cSafe-Zone) that provides guidelines for the optimal positioning of both components.

A computerised CAD model of a total hip prosthesis was used to systematically investigate the effect of design parameters, such as head-to-neck ratio, CCD shaft angle, as well as positioning parameters, such as cup inclination and cup anteversion and stem antetorsion, on the range of motion. We looked for all positioning combinations that allow the predefined range of movement (= iROM, intended range of movement) and thus define the combined safe zone. The analysis was carried out with straight stems, anatomical and short stems. The size of the cSafe-Zone was chosen as the optimising criterion and the largest cSafe-Zone was considered to define the optimal component positions. These optimal relative orientations of cup and stem were engraved onto the surface of the navigation trial head and used to position the cup during surgery.

This new combined safe zone is not static but dynamic; it varies in size and position and is specific for each prosthesis system. High stem antetorsion should be combined with lower cup anteversion and vice versa. Thus, cup anteversion and stem antetorsion are complementary. CCD shaft angles above 135° reduce the size of the cSafe-Zone and are therefore not recommended. Larger head sizes allow lower cup inclinations, i.e. the recommended cup inclination for a 28 mm head is 40 to 45°, for 32 mm 38 to 42° and for 36 mm 35 to 40°. This also increases the so-called jumping distance. Anatomical stems require less cup anteversion than straight stems.

The concept of combined safe-zone delivers clear guidelines how to position both components of a total hip prosthesis in order to maximise range of movement and to reduce the risk of prosthetic impingement. It is the basis of the stem-first surgical technique. Computer-based navigation or mechanical instruments can be used to implement this new concept in surgical practice.