The influence of the oscillation angle and the neck anteversion of the prosthesis on the cup safe-zone that fulfills the criteria for range of motion in total hip replacements. The required oscillation angle for an acceptable cup safe-zone

A normal hip joint has more than 120° flexion. The reduced range of motion (ROM) of total hip arthroplast leads to frequent prosthetic impingement, subluxation and dislocation. Prosthetic impingement may be more serious for metal-on-metal and ceramic-on-ceramic total hip prosthesis (THP). A larger oscillation angle of THP (OsA) and proper cup and neck positions make a larger theoretical ROM of a patient's artificial hip joint. But what OsA is required and what range of cup positions is kinetically accepted are not clearly understood. A ROM of more than 120° flexion, 45° internal-rotation at 90° flexion, 30° extension and 40° external-rotation was defined as severe criteria for an acceptable ROM. Theoretical cup safe-zones were created that fulfill the severe criteria of ROM for (OsA=110°, 120°, 135°) by the mathematical formulas. The size of the cup safe-zone mainly depends on the size of the OsA. There is no cup safe-zone for 110° OsA, an extremely small safe-zone for 120° OsA and an acceptable safe-zone for 135° OsA. Each THP has its own OsA, because OsA is the function of head and neck diameter and cup design. More than 135° OsA enlarges the safe-zone of the prosthetic position, so it extends the acceptable range of error that surgeons cannot avoid completely. However, few THPs with more than 135° OsA are currently clinically available. Both surgeons and manufacturers must realize that OsA is as essential as cup and neck orientations for ROM.