Previous studies have presumed a templated acetabular cup or stem to be accurate if within ± 2 sizes of the implanted size.(26–28) In our current study of 491 THAs, we found 83.3% of acetabular templates to be accurate within ± 1 size and 97.4% to be accurate within ± 2 sizes, data that is congruent with the previous literature. When examining accuracy of femoral stem templating, 69.4% were accurate within ± 1 size and 87.2% were accurate within ± 2 sizes. Again, this data is consistent with the findings from previous studies. Table 3 illustrates a current overview of the percent accuracy achieved for acetabular and femoral templates throughout the literature.
The greater accuracy of the acetabular components among all vendors could be explained in a couple of ways. First, when templating off a 2-D X-ray, the variation in acetabular cup size is reduced by templating to the visible cortical rim demarcating the supero-lateral and infero-medial borders of the acetabulum. Therefore, estimating final cup size is more reproducible if the templating surgeon stays within these landmarks. Conversely, there are several variables that can account for errors when templating the femoral stem side. Given the cone geometry of the proximal femur and the cone shape of the femoral stem implant, there is not only medial-to-lateral fit of the stem in the bone, but also the proximal distal shift in that cone of the proximal femur. For example, what may look like a good fit on a 2-D template, may fit better in the bone if the stem was more proximal or distal in the bone.
Another variable for greater differences in the femoral stem sizing from the templated size is the femoral rotation on the X-ray. The native hip has about 15 degrees of anteversion of the femoral neck. Although the X-ray image is taken with the patient’s lower extremity in 15 degrees of internal rotation to try and create an en fosse view of the femoral neck, this is sometimes not achievable. The inherent stiff nature of an arthritic hip joint may prevent the necessary femoral rotation for the X-ray. If the en fosse view of the femoral neck is not achieved, then the size of the proximal femur can appear more narrow on the X-ray, to have less offset, and can appear more valgus than reality. This inadequate profile can then lead to under-sizing of the femoral stem template.
One variable not elucidated in this study was femoral head offset and length differences when different femoral head offset sizes were used. For example, when templating the femoral stem size for “best fit” within the metaphysis, there is an ability to template different femoral head offset sizes onto the femoral neck. These different femoral head offset sizes allow the femoral head to be positioned deeper on the femoral neck, thereby reducing overall length and offset; or for the femoral head to seat more toward the tip of the femoral neck, creating more length and offset of the femur. During templating, not only is there a demand for best fit of the femoral stem within the metaphyseal bone, but consideration is equally given to the ultimate length and offset that a given femoral head and stem combination would produce. Therefore, a stem might be slightly smaller in size to achieve a better fit in the metaphyseal bone, but may require a “plus” offset on the femoral head to regain length and offset lost by the use of the smaller stem size. Conversely, a larger stem may fit better sitting higher in the femoral bone, but may need a “minus” offset head to achieve the desired length and offset.
This variability of femoral head offset sizes was not factored in to this study. Therefore, if a different stem size was used than what was templated, the length and offset may have been made up by the resulting femoral head offset, allowing for the best intra-operative fit of the femoral stem. This explains why there may be less accurate templating of the femoral stem sizes when compared to the acetabular cup sizes, as intraoperatively a different sized stem could have been used to achieve best fit.
When looking at a comparison of accuracy of templating over time, the accuracy of the first half versus the second half of the templated total hips were similar and not found to be statistically significant (Table 2). The consistency over time, despite different total hip component vendors, may be attributed to the very short learning curve and ease of incorporating digital templating into a total hip practice.
A limitation of this study was the inability to identify the cause of the inaccurately templated acetabular cups and stems as defined by those greater than two sizes of the templated size. This inaccuracy accounted for 2.6% of the acetabular cups and 12.8% of the femoral stems. We did not compare other factors that could possibly contribute to this inaccuracy such as BMI, gender, extreme sizes, or poor scaling device placement. An additional limitation was the accuracy of templating among the three different total hip component vendors. Comparing vendors was not the focus of this study, but could be considered in future studies.