Abstract
Iliopsoas tendonitis after total hip arthroplasty (THA) can be a considerable cause of pain and patient dissatisfaction. The optimal cup position to avoid iliopsoas tendonitis has not been clearly established. Implant designs have also been developed with an anterior recess to avoid iliopsoas impingement. The purpose of this cadaveric study was to determine the effect of cup position and implant design on iliopsoas impingement.
Bilateral THA was performed on three fresh frozen cadavers using oversized (jumbo) offset head center revision acetabular cups with an anterior recess (60, 62 and 66 mm diameter) and tapered wedge primary stems through a posterior approach. A 2mm diameter flexible stainless steel cable was inserted into the psoas tendon sheath between the muscle and the surrounding membrane to identify the location of the psoas muscle radiographically. CT scans of each cadaver were imported in an imaging software. The acetabular shells, cables as well as pelvis were segmented to create separate solid models of each. The offset head center shell was virtually replaced with an equivalent diameter hemispherical shell by overlaying the outer shell surfaces of both designs and keeping the faces of shells parallel. The shortest distance between each shell and cable was measured. To determine the influence of cup inclination and anteversion on psoas impingement, we virtually varied the inclination (30°/40°/50°) and anteversion (10°/20°/30°) angles for both shell designs.
The CT analysis revealed that the original orientation (inclination/anteversion) of the shells implanted in 3 cadavers were as follows: Left1: 44.7°/23.3°, Right1: 41.7°/33.8°, Left2: 40/17, Right2: 31.7/23.5, Left3: 33/2908, Right3: 46.7/6.3. For the offset center shells, the shell to cable distance in all the above cases were positive indicating that there was clearance between the shells and psoas. For the hemispherical shells, in 3 out of 6 cases, the distance was negative indicating impingement of psoas. With the virtual implantation of both shell designs at orientations 40°/10°, 40°/20°, 40°/30° we found that greater anteversion helped decrease psoas impingement in both shell designs. When we analyzed the influence of inclination angle on psoas impingement by comparing wire distances for three orientations (30°/20°, 40°/20°, 50°/20°), we found that the effect was less pronounced. Further analysis comparing the offset head center shell to the conventional hemispherical shell revealed that the offset design was favored (greater clearance between the shell and the wire) in 17 out of 18 cases when the effect of anteversion was considered and in 15 out of 18 cases when the effect of inclinations was considered.
Our results indicate that psoas impingement is related to both cup position and implant geometry. For an oversized jumbo cup, psoas impingement is reduced by greater anteversion while cup inclination has little effect. An offset head center cup with an anterior recess was effective in reducing psoas impingement in comparison to a conventional hemispherical geometry. In conclusion, adequate anteversion is important to avoid psoas impingement with jumbo acetabular shells and an implant with an anterior recess may further mitigate the risk of psoas impingement.
