Which length should the neck segment of modular revision stems have?

Background: Fractures of modular revision stems at the taper junction are rare but severe clinical problems. The purpose of this study was the estimation of taper loading to identify configurations which are less prone to failure.

Methods: A parametrical analytical 3-D model was developed to determine the influence of neck segment length, offset and anteversion on the loading at the modular taper junction between neck segment and stem. Published in-vivo hip joint forces were used to simulate different activities.

Findings: No unique ideal neck segment length can be specified due to the differences in loading magnitude and direction between activities. The best neck segment length for walking is longer than for high loading activities as stair climbing and jogging. A medium length between 70 mm and 90 mm appears to be a good compromise. A shorter offset (37 mm vs. 47 mm) reduces the stress by about 25% for walking and jogging. Retroverted implantation by 5° increases the loading whereas an anteverted implantation by 5° reduces it. A high offset (47 mm) combined with a short neck segment length (50 mm) reaches about 80% of the taper yield strength for jogging (taper diameter 13 mm).

Interpretation: Simplified 2-D modelling falsely predicts no bending at the taper junction for a long neck segment, whereas the 3-D model shows substantial stress load along the whole stem length. Stem tapers of short as well as very long neck segments are higher risk for failure. Neck segment length should lie in the range between 70 mm and 90 mm.