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 (θ) and the cup and neck positions by a previously created mathematical formula. A larger (θ) 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° flexion, 30° internal-rotation at 90° flexion, 30° extension and 40° external-rotation were defined as the criteria for essential ROM for ADL. The safe-zones for combined cup anteversion (βanat) 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°, 45° and 55° cup abductions (α) and for 120° and 135° (θ). The safe-zones for combined (βanat) and (b) were much larger for a 135° (θ) than a 120° (θ). Their safe-zones showed that (b) should be reduced if (βanat) is increased and choosing a lower (α) requires that the sum of (βanat) and (b) should be higher and vice versa. A (θ) of more than 135° 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: (α)+(βanat)+0.77(b)=84.3.