A geometric model of impingement during dislocation-prone activities in dual-mobility and standard acetabular cups

Dual Mobility (DM) Total Hip Replacements (THRs), are becoming widely used but function in-vivo is not fully understood.The aim of this study was to compare the incidence of impingement of a modular dual mobility with that of a standard cup.A geometrical model of one subject's bony anatomy \[1\] was developed, a THR was implanted with the cup at a range of inclination and anteversion positions (Corail® stem, Pinnacle® cup (DePuy Synthes)). Two DM variants and one STD acetabular cup were modelled. Joint motions were taken from kinematic data of activities of daily living associated with dislocation \[2\] and walking. The occurrence of impingement was assessed for each component combination, orientation and activity. Implant-implant impingement can occur between the femoral neck and the metal or PE liner (DM or STD constructs respectively) or neck-PE mobile liner (DM only).The results comprise a colour coded matrix which sums the number of impingement events for each cup position and activity and for each implant variant.Neck-PE mobile liner impingement, occurred for both DM sizes, for all activities, and most cup placement positions indicating that the PE mobile liner is likely to move at the start of all activities including walking.For all constructs no placement positions avoided neck-metal (DM) or neck-PE liner (STD) impingementevents in all activities. The least number of events occurred at higher inclination and anteversion component positions. In addition to implant-implant impingement, some instances of bone-bone and implant-bone impingement were also observed.Consistent with DM philosophy, neck-PE mobile liner impingement and liner motion occurred for all activities including walking. Neck-liner impingement frequency was comparable between both DM sizes (metal liner) and a standard cup (PE liner).