Title Anterior Prominence of the Femoral Condyle Varies among Prosthesis Designs and Surgical Techniques in Total Knee Arthroplasty

Background Patellofemoral overstung after total knee arthroplasty can cause limited range of motion and anterior knee pain. This study compared anterior prominence of femoral components among different prothesis designs in surgical simulation models utilizing the anterior reference (AR) and posterior reference (PR) techniques. Methods Surgical simulations were performed using preoperative computed tomography data of 30 patients on a three-dimensional planning system. Four implant models were used: Attune, Persona, Journey II, and Legion. Rotational alignment was set parallel to the transepicondylar axis and size selection was based on absence of notch formation in the femoral anterior cortex and best t to the shape of the medial posterior femoral condyle. For each combination of surgical technique (AR or PR method) and implant model, measurements were taken of the maximum medial, central, and lateral prominence of the implant from the anterior femoral cortex (mm).


Introduction
Total knee arthroplasty (TKA) is widely known to provide good postoperative outcomes, with welldocumented effectiveness in relieving pain and achieving good range of motion (ROM) [1,2,3].
Postoperative ROM is one of the factors associated with postoperative physical function [4] and patient satisfaction [5]. Various factors have been reported to in uence postoperative ROM, including ligament balance, preoperative ROM, posterior condylar offset, and low anterior femoral condylar height [6,7].
Recently, it has been reported that reducing the size of the anterior condylar height after TKA can improve exion [8]. Anterior prominence of the femoral component has been suggested to in uence postoperative ROM because it can cause overstu ng of the patellofemoral (PF) joint. This overstu ng is known to affect postoperative ROM [9] and can cause anterior knee pain [10].
Various factors have been associated with PF overstu ng, such as surgical technique and component design and size. Component resizing has been associated with increased PF pressure during knee exion in the swing phase of gait [11]. Differential model designs have been associated with differential PF pressure and patellar kinematics [12]. Even when adopting the same surgical technique, implant selection can result in different degrees of anterior femoral prominence because of both the design and pitch size of implants. However, this issue has not been focused on, and no information is available about the difference in anterior prominence of the femoral component among implant designs.
The objective of this study was to investigate differences in anterior prominence of the femoral components among different models when the prosthesis was implanted in the same patients with the same surgical techniques. To achieve this, we adopted a three-dimensional (3D) surgical simulation model to measure the anterior prominence of femoral condyle of several widely used prostheses placed with the anterior reference or posterior reference method.
We hypothesized that anterior prominence is minimized with implant models that have a thin anterior ange when using the anterior reference method or models that have a generous pitch size when using the posterior reference method.
The clinical relevance of this study is that the ndings obtained would remind surgeons of the need to pay more attention to pitch size and design of the anterior ange in choosing or developing a new implant. preoperative computed tomography (CT) data on a 3D planning system (Zed Knee; LEXI, Tokyo, Japan).

Materials And Methods
A femoral implant was placed perpendicular to the 3D mechanical axis for coronal and sagittal alignment. Rotational alignment was set parallel to the surgical transepicondylar axis (SEA). Using the anterior reference method, the implant was placed so that the posterior of the anterior ange was in contact with the anterior cortex, and size was determined based on the best t to the medial posterior condyle. Using the posterior reference method, the implant was placed so that it was in contact with the posterior border of the medial posterior condyle, and the smallest size that did not form a notch in the anterior cortex was selected (

Statistical analysis
The Steel-Dwass test was used for multiple comparisons between the four implant groups; Wilcoxon's signed-rank test was used for paired comparison of the anterior reference versus posterior reference method with the same implants. Statistical signi cance was based on p-values of less than 5%. For the measurement of intra-observer reliability, the maximum medial, central, and lateral prominences were measured twice at an interval of ≥ 14 days for each implant model in 4 randomly selected patients. The resulting intraclass correlation coe cient (ICC 1,1) was 0.94. In statistical power analysis, assuming a power of 0.8, alpha of 0.05, and standard deviation of 0.5, the sample size to detect a difference in prominence between models of 0.5 mm, 0.5 mm, and 1 mm, respectively, was calculated as 27.

Ethical approval and consent to participate
All procedures involving human participants were in accordance with the ethical standards of the 1964 Helsinki Declaration and its later amendments. This study was conducted with approval from the ethics committees of Tokyo Women's Medical University (approval no.4578, December 12, 2017). Informed consent was obtained via an opt-out procedure.

Results
The anatomical parameters used for implant placement in the surgical simulation for 30 patients are listed in Table 2. Mean valgus angle against the distal anatomical axis of the femur was 6.3 ± 1.9 and mean external rotation angle from the posterior condylar axis was 3.8 ± 1.6°. The mean sagittal plane angle against the distal bone axis was 1.3 ± 1.9° extension. With the anterior reference method, the medial and central prominences were signi cantly lower with Journey II than with the other models (p < 0.001, Fig. 3). The lateral prominence was signi cantly lower with Attune than with the other models, with a 0.2-mm difference compared with Journey II (p < 0.02, Fig.   3).
With the posterior reference method, the medial and central prominences were signi cantly lower with Journey II than with the other models (p < 0.0001, Fig. 4). Lateral prominence was signi cantly lower with Attune, Journey II, and Persona than with Legion (p < 0.0001, Fig. 4), with no signi cant difference among the Attune, Journey II, and Persona models (p > 0.21, Fig. 4).
With the posterior reference method, component upsizing was needed to avoid anterior notch formation in 4, 3, 5, and 8 cases placed with Attune, Persona, Journey II, and Legion, respectively.
For each implant model, the medial, central, and lateral prominences were signi cantly lower using the anterior reference method than the posterior reference method (p < 0.05, Fig. 5).

Discussion
The most important nding of this study was that the anterior prominence of the femoral condyle varied signi cantly 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 ange of Journey II is thinner than that of other models, suggesting the design in uences the results signi cantly. 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.  [14]. 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 ange 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 t 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 speci cally, its pitch size of 4 mm and greater ange 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 di culty in adjusting the posterior condylar offset and exion gap. In contrast, the posterior reference method is associated with easier adjustment of the posterior condylar offset and exion gap, but with higher risks of notch formation and PF overstu ng [15]. The present study did not take into account compatibility to the size of tibial implant component or the exion 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 signi cantly affects it whichever method is to be used.
In this study, we de ned 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°) [16] and in another study (6.3° when limited to knees with severe genu varum) [17]. 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. [20] de ned 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 [21] 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 ange 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.

Conclusions
The degree of anterior prominence of the femoral implant is affected by implant design when the anterior reference method is used. The posterior reference method is associated with greater anterior prominence compared with the anterior reference method, with pitch size identi ed as an additional in uencing factor. Journey II is associated with the least anterior prominence when using either method. The various implant designs were shown to be associated with different degrees of anterior prominence, so additional attention is needed to the effects of surgical techniques and pitch size. In terms of relevance to daily clinical practice, our ndings highlight the need for surgeons to be familiar with implant designs, including the thickness of the anterior ange and available size selections, to ensure favorable postoperative outcomes.
Abbreviations CT, computed tomography; TKA, total knee arthroplasty; PF, patellofemoral; ROM, range of motion Declarations Ethics approval and consent to participate All procedures involving human participants were in accordance with the ethical standards of the 1964 Helsinki Declaration and its later amendments. This study was conducted with approval from the ethics committees of Tokyo Women's Medical University (approval no.4578, December 12, 2017). Informed consent was obtained via an opt-out procedure.

Consent for publication
Not applicable.

Availability of data and materials
The datasets used and/or analysed during the current study are available from the corresponding author on reasonable request.  Figure 1 Representative implant placement simulation (Journey II, right knee osteoarthritis) Femoral implant is placed perpendicular to the three-dimensional functional axis for varus/valgus deformity and extension/ exion. Rotational alignment is set parallel to the transepicondylar axis (surgical epicondylar axis). Using the anterior reference method, the implant is placed so that the posterior of the anterior ange is in contact with the anterior cortex, and size selection is based on the best t of the medial posterior condyle of the implant. With the posterior reference method, the implant is placed in contact with the posterior border of the medial posterior condyle, and the smallest size that did not form a notch in the anterior cortex is selected.  Comparison of medial, central, and lateral prominences (mm) using the anterior reference method with different implants *Steel-Dwass test With the anterior reference method, the medial and central prominences were signi cantly lower with Journey II than with the other models (p < 0.001). The lateral prominence was signi cantly lower with Attune than with the other models (p < 0.02).

Figure 4
Comparison of medial, central, and lateral prominences (mm) using the posterior reference method with different implants *Steel-Dwass test With the posterior reference method, the medial and central prominences were signi cantly lower with Journey II than with the other models (p < 0.0001). The lateral prominence was signi cantly lower with Attune, Journey II and Persona than with Legion (p < 0.0001), with no signi cant difference among the Attune, Journey II, and Persona models (p > 0.21).

Figure 5
Comparison of anterior and posterior reference methods *Wilcoxon's signed-rank test For all implants, the anterior reference method was associated with signi cantly lower anterior prominence (p < 0.05).