The combination techniques were accurate and more precise than all but one of the single methods. Fifty-eight percent (375) of the 648 axes derived with combination techniques were rotated <5° from the reference axis, and five of the six combination techniques had a mean error of <5°. When we just considered the six combination techniques, we found that the combined digitized epicondyles and Whiteside line techniques and the combined digitized epicondyles and patella tracking techniques had a mean error of <2° and were more accurate than any of the other combination techniques (p < 0.02). When we collectively investigated all eleven alignment techniques in the study, post hoc comparisons demonstrated that the performance of these two combination techniques was not different (p < 0.08) than that of the anatomic posterior condyles technique and two additional combination techniques: the combined Whiteside line and screw axis techniques and the combined patella tracking and screw axis techniques. However, we were unable to determine a difference between the performance of the posterior condyles technique and the performance of those four combination techniques because of the large standard deviation of the posterior condyles technique; this anatomic technique was the least precise technique in the entire study (p < 0.001). The combination techniques and the “screw axis” technique were the most precise, and their standard deviations were not different from each other (p = 0.654).
No single alignment technique provided the most accurate rotational alignment and the best precision (smallest standard deviation). The combination techniques improved the accuracy of rotational alignment and were among the most precise. We found that incorporating data from various sources reduced the rotational alignment error associated with a single technique. We believe that this is not simply a function of increasing the number of data points; increasing the number of measurements with a single technique would likely result in the same error, since some surgeons are more accurate with certain techniques than others16. It is difficult to use the human eye to calculate kinematic axes and synthesize them with other data sources, but computer-based navigation systems can accomplish these tests and this represents an advantage over traditional techniques.
Although combining techniques improved accuracy, precision was still a problem. Only 58% of the axes derived with the combination techniques were rotated <5° from the reference axis. There is little clinical information relating magnitudes of femoral rotational malalignment to failure and rates of revision of total knee arthroplasties1,4,6-8,32. Studies relating rotational alignment to complications, failures, and revisions in association with different component designs are needed to address this question.
The present study revealed a high standard deviation for femoral rotational alignment, which is consistent with our previous results16, although some details in the present study were different from those in our prior study. These differences could be due to the different implementation of two of the techniques. The Whiteside line technique was carried out before distal femoral resection in the current study and after distal femoral resection in our previous study. Both are clinically relevant, as some surgeons determine rotational alignment before resecting the distal part of the femur33. Posterior condylar alignment was determined with use of a stylus in the current study, but a commercial alignment jig was used in the previous investigation. In addition, different surgeons participated in each study, and techniques for determining anatomic rotational alignment appear to be influenced by an individual surgeon's skill and preferences16.
The patella tracking technique is introduced in this paper. Because the lower limbs were disarticulated at the hip, we had to simulate the tension of the knee extensor mechanism, and this potentially affected the results derived with this technique. However, our results with the patella tracking technique were within 1° of results calculated in a previous radiographically based study26. We have used the patella tracking technique for patients undergoing total knee arthroplasty with none of the difficulties that were caused by the use of the disarticulated specimens. We are investigating this promising technique further.
None of the specimens had severe deformity, and only two of them showed signs of early, mild osteoarthritis. Therefore, the results of the kinematic techniques in this study may be different from what would be found in osteoarthritic knees25. The anatomic techniques may also be affected by osteoarthritis. Difficulty in accurately establishing the rotational alignment of the femoral component has been demonstrated in osteoarthritic knees17,20,34 and cadaver knees13,16,35-37, suggesting that the variability of alignment may not be primarily related to the disease but to challenges with visualizing the appropriate landmarks and with geometry, as a small linear error in identifying anatomic landmarks can lead to a large change in rotational alignment38.
There is debate surrounding the so-called gold-standard alignment axis for establishing rotational alignment of the femoral component. We chose to use the epicondylar axis as studies have associated undesirable postoperative outcomes with not aligning the component parallel to this axis1,6,7. Other alignment axes have been suggested13,40. Regardless of the reference axis selected, it is reasonable to suggest that high variability in alignment may lead to unpredictable surgical outcomes, and it is desirable to develop new techniques to reduce this variability to improve surgical reconstructions.
In this study in which multiple surgeons defined more than 100 alignment axes with each technique, we found that four combination techniques (the digitized epicondyles and Whiteside line methods, the digitized epicondyles and patella tracking methods, the Whiteside line and screw axis methods, and the patella tracking and screw axis methods) were accurate and reduced the number of rotational alignment outliers. These combination techniques were superior to any individual kinematic or anatomic technique for establishing femoral rotational alignment. However, precision, as reflected by the standard deviation and the number of outliers, continues to be a problem. As computer-navigation technology moves forward, further research and development should focus on improving accuracy and precision across multiple surgeons.
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