Abstract
A novel cementless tapered wedge femoral hip implant has been designed at a reduced length and with a geometry optimized to better fit a wide array of bone types (Accolade II, Stryker, Mahwah, USA). In this study, finite element analysis (FEA) is used to compare the initial stability of the new proposed hip stem to predicate tapered wedge stem designs. A fit analysis was also conducted. The novel stem was compared to a predicate standard tapered stem and a shortened version of that same predicate stem.
Methods
The novel shortened tapered wedge stem geometry was designed based on a morphological study of 556 CT scans. We then selected 10 discrete femoral geometries of interest from the CT database, including champagne fluted and stove pipe femurs. The novel and the predicate stems were virtually implanted in the bones in ABAQUS CAE. A total of thirty FEA models were meshed with 4 nodes linear tetrahedral elements. Bone/implant interface properties was simulated with contact surface and a friction coefficient of 0.35. Initial stability of each stem/bone assembly was calculated using stair-climbing loading conditions. The overall initial stability of the HA coated surface was evaluated by comparing the mean rotational, vertical, gap-opening and total micromotion at the proximal bone/implant interface of the novel and predicate stem designs.
To characterize the fit of the stem designs we analyzed the ratio of a distal (60 mm below lesser trochanter) and a proximal (10 mm above lesser trochanter) cross section. A constant implantation height of 20 mm above the lesser trochanter was used. The fit of the stems was classified as Type 1 (proximal and distal engagement), Type 2 (proximal engagement only) and Type 3 (distal engagement only).
Results
The mean % micromotion of the HA coated surface greater than 50 mm was lowest at 40.2% (SD 11.5%) for the novel tapered wedge stem compared to the clinically successful predicate stem design (Accolade TMAZ, Stryker, Mahwah, USA) at 44.9% (SD 13.2%) and its shortened version at 48.5% (SD 9.0%) as shown in Figure 1. Improved initial stability of the new stem was also confirmed for rotational, vertical and gap-opening micromotion. However, there was no statistically significant difference.
The novel tapered stem design showed a well balanced proximal to distal ratio throughout the complete size range. The novel tapered stem design showed a reduced percentage of distal engagements (2.8%) compared to the predicate standard stem (17.2%). In the 40 to 60 year old male group the distal engagement for the standard stem increases (28.2%), whereas the distal engagements for the novel stem remains unchanged (1.3%).
Discussion
It appears that through optimization of the novel tapered wedge geometry, a reduced length of a tapered wedge stem can be accomplished without jeopardizing initial stability. This data also shows that simply shortening an existing tapered wedge design may reduce the initial stability, albeit not statistically significant in this model. Optimising the shape of the stem has also significantly reduced the incidence of distal only type fixation in a computer model.
