Abstract
Introduction
Primary stability is achieved by the press fit technique, where an oversized component is inserted into an undersized reamed cavity. The major geometric design of an acetabular shell is hemispherical type. On the other one, there are the hemielliptical type acetabular shells for enhanced peripheral contact.
In the case of developmental dysplasia of the hip (DDH), the aseptic loosening may be induced by instability due to decreased in the contact area between the acetabular shell and host bone.
The aim of this study was to assess the effect of reaming size on the primary stability of two different outer geometry shells in DDH models.
Materials and methods
The authors evaluated hemispherical (Continuum Acetabular Shell, Zimmer Biomet G.K.) and hemielliptical (Trabecular Metal Modular Acetabular Shell, Zimmer Biomet G.K.) acetabular shells. Both shells had a 50 mm outer diameter and same tantalum 3D highly porous surface.
An acetabular bone model was prepared using a solid rigid polyurethane foam block with 20 pcf density (Sawbones, Pacific Research Laboratories Inc.) as a synthetic bone substrate. Press fit conditions were every 1 mm from 4 mm under reaming to 2 mm over reaming. To simulate the acetabular dysplasia the synthetic bone substrate was cut diagonally at 40°. Where, the acetabular inclination and cup-CE angle were assumed to 40° and 10°, respectively.
Acetabular components were installed with 5 kN by a uniaxial universal testing machine (Autograph AGS-X, Shimadzu Corporation).
Primary stability was evaluated by lever-out test. The lever-out test was performed in 4 mm undersized to 2 mm oversized reaming conditions. Lever out moment was calculated from the multiplication of the maximum load and the moment arm for primary stability of the shell. The sample size was 6 for each shell type.
Results
The hemisphererical acetabular shell had the maximum lever out moment in 3 mm under reaming condition (7.4 ± 0.4 N·m). The hemielliptical acetabular shell had the maximum lever out moment in 1 mm under reaming condition (8.7 ± 0.8 N·m). Furthermore, the lever out moment of the hemielliptical acetabular shell was significantly 1.2 times greater by the t-test than the hemispherical acetabular shell under the maximum primary fixation conditions.
Discussion
The risk parameter of the acetabular loosening is indicated the lack of lateral bony support. The hemielliptical shell was not adversely effected more than the hemispherical shell. Furthermore, the reaming condition of the most primary fixation on the hemielliptical shell was 1 mm under reaming, and was a more general operating procedure than the hemispherical shell (3 mm under reaming). From this study, it was suggested that the hemielliptical shell might be expected excellent clinical outcomes in severe acetabular dysplasia hips.
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