ACETABULAR BEARING DESIGN HAS A GREATER INFLUENCE ON JUMP DISTANCE THAN HEAD DIAMETER

Abstract

It is accepted that larger diameter heads are more difficult to dislocate due to the increased distance the head has to travel to come out of the cup. Currently larger femoral heads are being used for their resistance to dislocation however, there remains little reporting on the effect of design of cup on jump distance. Monoblock metal on metal cups, which were designed for hip resurfacing are typically less than a hemisphere internally in order to increase the range of motion (ROM) needed when the femoral neck is retained. This does however also reduce the jump distance. We investigated several designs of cup with a variety of head sizes in order to compare ROM using a computer range of motion tool and a two dimensional jump distance with the cup at 45 degrees inclination.

Jump distances were calculated for: Internally hemispheric cups in 28, 32 and 36mm bearing diameters; 28, 40 and 44mm polyethylene liners which were hemispheric but with an additional 2mm cylinder and a 0.7mm chamfer at the equator (Trident, Stryker, Mahwah, USA); 38, 48 and 54mm monoblock metal on metal resurfacing cups with a 3.5mm offset (BHR, Smith and Nephew, Memphis, USA); 40, 48, 58 dual mobility cups with an anatomic rim (Restoration ADM, Stryker, Mahwah, USA)

Range of motion modeling was carried out using custom-written software according to a previously published method2 with 5 degrees of pelvic tilt and a standard femoral component. For the present study, range of motion was assessed on a standard stem with a 132° neck angle. Inclination of the cup was set to 45° and anteversion to 20°. For each implant tested, the total ROM was computed in flexion/extension, ab/adduction, and int/external rotation.

Components tested for range of motion were: Trident 32, 36, 40 and 44mm Internal Diameter; Hemispheric 28 and 32mm Internal Diameter cups; MITCH TRH MoM Monoblock Resurfacing Cup (Stryker EMEA, Montreux, Switzerland) 46mm cup bearing diameter with a 2.75mm offset bore; Dual Mobility 40, 46 and 58mm cups. The metal on metal monoblock cups had a very high range of motion but a 48mm head has only a similar jump distance to a hemispheric 36mm design. The designs with the cylinder and chamfer have a markedly higher jump distance than their hemispheric equivalents but slightly reduced ROM. Interestingly, the dual mobility design has almost double the jump distance of an equivalently sized metal on metal resurfacing type cup and a higher jump distance than an equivalent head size in a conventional unipolar design. The dual mobility design has similar ROM to a 40mm head in the hemisphere with cylinder and chamfer design. ROM is slightly higher in the hemispheric and sub-hemispheric designs but this model does not take into account bony or soft tissue impingement. The role of design of ace-tabular component has a great effect on the range of motion and jump distance of bearings.