Fig. 2
Mathematical notations about the true positions of the femoral head center. Head center coordinates relative to the cup center immediately after the operation were defined as \(\left({x}_{0}, {y}_{0}\right)=(0, 0)\). Penetration of the head center by the bedding-in \(\overrightarrow{b}=\left({b}_{x}, {b}_{y}\right)\) starts immediately after the operation and was presumed not to end until the first measurement period (t1). Penetration by wear starts immediately after the operation and continues at a steady pace throughout the follow-up period: \(\overrightarrow{w}=\left({w}_{x}, {w}_{y}\right)\) per year