[Thread design of screw-in acetabulum prostheses].

INTRODUCTION AND AIM OF STUDY

The screw-in behaviour and correct positioning of threaded cups is largely determined by the design of the thread. Up to now the various thread designs have not been systematically classified.

METHODS

The thread designs of 10 first generation and 27 second generation threaded cups were analyzed using a tool setter and a no-touch light section technique. The following parameters were evaluated: thread shape, pitch, number of turns, rows of teeth, tooth length and shape.

RESULTS

Threads of the first generation were V-cut or saw shaped. Thread depth was 3.1 mm on average; the mean width was 0.9-3 mm. Single-thread patterns predominated. The number of turns ranged from 1-7; the pitch was from 2.5-5 mm (single thread), or up to 20 mm (triple and quadruple threads). Cups had 3-16 rows of teeth. Threads of the second generation are V-cut, saw or flat shaped. Thread depth is 3 mm on average; mean width ranges from 0.3-2.2 mm. 72% of the threads have single-thread patterns. The number of turns ranges from 2-5; the pitch is from 2.5-6.2 mm (single thread). Cups have 4-24 rows of teeth.

CONCLUSIONS

There are many different thread patterns with widely varying parameters. Contemporary threaded cups have a narrow V-cut and saw shaped threads or flat thread with depths up to 3 mm, no more than 5 turns, and pitch values of approx. 4.5 mm.