Abstract
Objective
The purpose of this study was to investigate how rim poly locking scallop cutting depth could affect the rigidity of acetabular cup.
Materials and Methods
(11) generic FEA models including (5) 50mm OD Ti6Al4VELI hemispherical acetabular shells with thicknesses of 3.0, 3.5, 4.0, 4.5 and 5.0mm, and (6) 4mm thick hemispherical shells with standard rim poly indexing scallops varied in cutting depths from inner diameter of the cup in 1.0, 1.5, 2.0, 2.5, 3.0 and 3.5mm. All cups were analyzed in ANSYS® Workbench™ FEA software with a loading condition of 2000N applied to the cup rim per V15 ISO/TC 150/SC 4 N. Verification was carried out by the physical test of a same generic Ti6Al4VELI 50mmOD and 5mm thick solid hemispherical shell under 2000N rim directed load. The cup deformation was compared with FEA results. The maximum deformation of FEA scalloped cups were compared with that of solid hemispherical cups with different shell thickness.
Results
The average value for physical test was 0.265(0.282/0.268) mm. The FEA result of the same 50mm OD and 5mm thick shell was 0.2464mm representing a 7% difference. (Figure 1) FEA results for solid hemisphere cups and scalloped shells are shown in Figure 2 and Figure 3 respectively.
Conclusion and Discussion
Cup deformation increased in the average of 40% for every 0.5mm shell thickness decrease from 5mm to 3mm for 50mm OD solid cup. The increase of rim scallop depth from 0 to 2.5mm was equivalent to a 0.5mm shell thickness reduction for 4mm thick 50mm OD cup. The higher deformation reading in physical test was likely caused by the accumulations of the fixture deformations. Intraoperative acetabular cup deformation is a clinical issue. It increases the risk of ceramic liner fracture when the liner is inserted in the out of round cup. The increase of shell thickness is a simple and effective way to reduce the cup deformation. However it also reduces the poly thickness and that puts the adverse effects to the poly wear property and poly rim fatigue strength. The common design of the indexing scallops in the rim of cup helps the rotational locking for the poly liner. But the drawbacks including the reduction of cup stiffness must be noticed. The shallower scallop design may be warranted to maximize the cup rigidity in a given cup thickness and maximize poly thickness in a given cup rigidity.
