Abstract
A design modification to the DJO Linear hip stem was performed to facilitate use of the stem with the minimally invasive direct anterior approach. While the main design consideration was to reduce the overall stem length, it was also important to increase congruency of the implant and proximal cortical bone to ensure initial stability.
An initial design attempt produced a geometry that was difficult to insert into the femur; therefore, reconstructed digital models of the femur (ADaMs by Materialise) were obtained and used to delineate the best fit implant cross section. The ADaMs models were constructed from 74 CT scans taken from northern Europeans undergoing investigations for cardio-vascular conditions. Using equivalency points, models representing the bone mean, ±1σ, and ±2σ were constructed. The ADaMs models are pictured in Figure 1.
After importing the ADaMs models in the Solidworks CAD environment, the existing Linear stem was ideally positioned in the femur model and equally spaced planes parallel to the resection plane were defined as shown in Figure 2. At each plane, the shape of the cortical bone was determined and then used to define an implant cross section that was congruent to the bone, at least as large as the Linear hip stem, and symmetric about its midline. After using the base ADaMs models to drive the design's geometry, the final design fit was validated for very small patients using a hypothetical size −4σ extrapolation of the ADaMs models.
The digital reconstructions improved the design process by providing accurate, tangible models of the actual femur geometry. From these models, the design team was able to visualize how implant geometry should be constructed to optimize congruency, symmetry, and favorable insertion characteristics. Additionally, the ADaMs models served to validate the design for a challenging condition and as a starting point for computer simulations that were able to predict the insertion difficulty encountered in the initial, pre ADaMs model design. The final redesign was launched in the US in 2014 as the TaperFill hip stem.
