Background: The initial stability of press-fit acetabular components is partially determined by the reaming technique. Nonhemispherical (NHS) acetabular shells, which have a larger radius at the rim than the dome, often require larger reaming preparations than the same-sized hemispherical (HS) shells. Furthermore, deeper central reaming may provide a more stable press fit. Using a reproducible, in vitro protocol, we compared initial shell stability under different reaming techniques with HS and NHS acetabular components.

Methods: Cavities for 54-mm NHS and 56-mm HS acetabular components were premachined in 20-pcf Sawbones blocks. Acetabular cavities included diameters of 54, 55, “54þ,” and “55þ”. “þ” indicates a cavity with a 2-mm smaller diameter that is 2-mm deeper. A 4750N statically applied force seated shells to a height that was comparable with shell height after an orthopaedic surgeon’s manual impaction. Force required to dislodge shells was assessed via a straight torque-out with a linear load.

Results: Increased preparation depth (þ) was associated with deeper shell seating in all groups. Deeper central reaming increased required lever-out force for all groups. Overall, HS and NHS implants prepared with 55 þ preparation had the highest lever-out forces, although this was not significantly higher than those with 54þ.

Conclusions: In 20-pcf Sawbones, representing dense bone, overreaming depth by 1-mm improved initial seating measurements. In both HS and NHS acetabular shells, seating depth and required lever-out force were higher in the “þ” category. It is unclear, however, whether a decreased diameter ream increased seating stability (55þ vs 54þ). Clinically, this deeper central reaming technique may help initial acetabular stability.