The safe-zones for combined cup and neck anteversions that fulfill the essential range of motion and their optimum combination in total hip replacements.

Reduction of the range of motion (ROM) until prosthetic impingement of a total hip replacement may lead to frequent impingement, subluxation and dislocation especially for patients with good hip movement. The ROM until prosthetic impingement can be calculated using the technical ROM (theta) and the cup and neck positions by a previously created mathematical formula. A larger (theta) with proper cup and neck positions results in a larger ROM. However there was only one paper written in English, which revealed the optimum theoretical combination of cup and neck anteversions. ROM of more than 110 degrees flexion, 30 degrees internal-rotation at 90 degrees flexion, 30 degrees extension and 40 degrees external-rotation were defined as the criteria for essential ROM for ADL. The safe-zones for combined cup anteversion (betaanat) and neck anteversion (b) were defined as the areas that fulfill all the criteria of ROM without prosthetic impingement. The safe-zones were created for 35 degrees , 45 degrees and 55 degrees cup abductions (alpha) and for 120 degrees and 135 degrees (theta). The safe-zones for combined (betaanat) and (b) were much larger for a 135 degrees (theta) than a 120 degrees (theta). Their safe-zones showed that (b) should be reduced if (betaanat) is increased and choosing a lower (alpha) requires that the sum of (betaanat) and (b) should be higher and vice versa. A (theta) of more than 135 degrees is recommended as it further increases the size of the safe-zone and provides a larger ROM, and the optimum values of combined cup and neck anteversions can be estimated by the formula: (alpha) + (betaanat) + 0.77(b) = 84.3.