The most important finding of this study was that the anterior prominence of the femoral condyle varied significantly depending on the implant design in either the anterior reference or posterior reference method. Journey II provided the least medial and central prominence among the 4 designs investigated. The anterior flange of Journey II is thinner than that of other models, suggesting the design influences the results significantly. In the posterior reference method, Attune and Persona were also associated with lower anterior prominence, probably because the generous pitch sizes of these models allowed for the selection of the size most closely attached to the anterior cortex.
Anterior prominence of the femoral component can lead to PF overstuffing and can affect ROM . Nishitani et al.  have reported that a decrease in the height of the anterior condyle from the original level is associated with increased postoperative flexion. Relatively good clinical outcomes in terms of postoperative ROM have been reported with Journey II. Brilhault et al. reported a mean ROM of 124 ± 9.8° . Vascellari et al. described improved mean ROM from 99.3° preoperatively to 119° postoperatively , and Taniguchi et al. reported a corresponding improvement from 117.7° (SD15.3°) to 129.0° (SD 9.6°) . Although these reports did not compare Journey II with other models under the same conditions, the relatively good ROM results reported with Journey II may be partially attributable to lower anterior femoral prominence than with other models. Selecting an implant with a thin flange may be a good strategy for gaining better postoperative ROM. In the anterior reference method, the degree of anterior prominence of the implant is likely affected by the implant design.
In contrast, using the posterior reference method, other factors are suspected to be involved. When an implant has been selected based on the best fit to the shape of the medial condyle, upsizing would be needed if an anterior notch is formed. In such cases, the use of models with generous pitch sizes is likely to result in a smaller mean anterior prominence. In fact, relatively small numbers of patients treated with Persona or Attune required implant upsizing, likely because of the generous pitch size and design of these models. Persona has a pitch size of 2 mm and Journey II and Attune have a pitch size of 3 mm. In contrast, Legion was associated with higher anterior prominence, likely due to its design and pitch size. More specifically, its pitch size of 4 mm and greater flange thickness compared with the other models appeared greatly impact on the outcome.
The decision to select the anterior reference or posterior reference method in an actual operation depends on the mechanical property of the implant and the surgical technique to be used. It is generally understood that when using the anterior reference method, notch formation can be avoided, but it is associated with difficulty in adjusting the posterior condylar offset and flexion gap. In contrast, the posterior reference method is associated with easier adjustment of the posterior condylar offset and flexion gap, but with higher risks of notch formation and PF overstuffing . The present study did not take into account compatibility to the size of tibial implant component or the flexion gap. Therefore, in actual surgery, surgeons should be cautious in selecting surgical techniques and procedures. Nevertheless, the fact that Journey II provided less anterior prominence in either the anterior reference or posterior reference method suggests that the implant design significantly affects it whichever method is to be used.
In this study, we defined implant placement on the coronal and sagittal planes perpendicular to the 3D functional axis. The mean valgus angle was 6.3°, which is comparable to that reported previously in a study using 3D-CT (5.4 ± 0.7°)  and in another study (6.3° when limited to knees with severe genu varum) . The transepicondylar axis (TEA) can be either the clinical epicondylar axis or SEA. In this study, the SEA was used based on previous CT-based studies [18, 19]. Meric et al.  defined the SEA as externally rotated by a mean of 3.3 ± 1.5° from the posterior condylar axis in a CT analysis of 13,546 knees. Victor  has concluded that the SEA is externally rotated by a mean of 3° from the posterior condylar axis based on a review of rotation data. These values are consistent with corresponding values obtained from the patients analyzed in this study. Mean implant angle on the sagittal plane against the distal femoral axis was 1.3 ± 1.9° extension, which was considered to be prone to notch formation. However, this was unlikely to affect the results of the present study because differences between implants were investigated using surgical simulation under the same condition of placing the implant perpendicular to the 3D functional axis.
There are several limitations to this study. First, we evaluated only 4 implant models. In this study, we sought to learn how implant design and sizing affect the anterior prominence rather than to search for the best implant available, and we selected models that are representative of the most commonly used models from each manufacturer in the registry. It is recommended that surgeons pay more attention to the design of anterior flange of the implant they use. Second, the femoral external rotation angle was only set against the SEA. We also evaluated all cases by setting the external rotation angle as being externally rotated by 3° from the posterior condylar line and obtained similar results. Therefore, only the data aligned to the SEA was presented to avoid confusion.