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Fit of Modern Femoral Knee Implant Design to Native Trochlear Groove

Yifei Dai, PhD
Exactech, Inc.
Laurent Angibaud, Dipl. Ing.
Exactech, Inc.

INTRODUCTION

Total knee arthroplasty (TKA) is a mature surgical procedure for the treatment of endstage knee arthritis. Despite its overall high clinical success, many patients still report pain and discomfort after TKA, with approximately 20% of the patients not satisfied with the clinical outcomes.1,2 Among the complications related to TKA, patellofemoral pain and instability have been found to be one of the most common reasons for revision.1,3,4

The causes of patellofemoral complications are multifactorial, including improper surgical technique (implant positioning and sizing, soft-tissue balancing, etc.) and limitations in implant design.5-9 Numerous biomechanical studies suggest that even when the surgical technique is optimized, patellofemoral tracking is not always restored to physiological values due to the difference between the implant trochlea and the native trochlea.5-8

The ability of the implant to restore native trochlear groove morphology may be affected by the design philosophy. Currently, there are several designs in modern implant systems based on the orientation of the trochlear groove. One design philosophy (Philosophy I) employed by many device companies, is a trochlear compartment with a lateral groove orientation. With the rationale to capture perceived gender differences in Q-angle, a recent design refined this philosophy with “gender-specific” solutions. These solutions offer different amounts of lateral angulation in groove orientation based on the average Q-angle of male and female populations, respectively. Distinctly different, a second philosophy (Philosophy II) creates “forgiveness” for patella tracking by designing a neutral trochlear groove orientation with a widened proximal trochlear compartment on the femoral implant. The basis of this philosophy, encompassed by Exactech’s Truliant® Knee System design, is to respect the natural variable motion path of the patella by allowing a moderate degree of proximal mediolateral (ML) freedom, which gradually changes to a constrained trochlea in high flexion (intercondylar region) (Figure 1).

To date, there is a paucity of data regarding direct comparison between the design philosophies in the context of restoring native trochlear groove orientation. This study computationally assessed the native trochlear groove orientation in a dataset of healthy femora and compared the results to current modern femoral implants representing the two design philosophies.

Figure 1. Illustration of allowed range of ML positions of the patella (patella center location) at different levels of the trochlear groove. The allowed ML ranges were highlighted in red.

MATERIALS AND METHODS

Bone Data
CT scan based virtual surface models of 94 healthy right femora were used in this study. The data set contained 49 Chinese (24M/25F) and 45 Caucasian (23M/22F) femora.

Measurement of Native Trochlear Groove Orientation
An automated virtual workflow was developed to extract the trochlear groove region from the femoral surface (3-matic research, Materialise NV, Leuven, Belgium). A virtual plane was constructed passing through the anatomical transepicondylar axis (TEA) and the apex of the intercondylar notch. The plane was rotated 130° proximally in 5° increments (Figure 2).5 At each plane position, the intersecting curve between the plane and the femoral surface was generated and exported for further analysis.

Figure 2. A representative femur illustrating intersecting curves created by rotating a plane around the transepicondylar axis.
Figure 2. A representative femur illustrating intersecting curves created by rotating a plane around the transepicondylar axis.

 

Figure 3. A representative femur demonstrating the calculation of the orientation of the trochlear groove. A negative trochlear groove angle indicates that the groove was oriented laterally from distal to proximal direction, as illustrated to the left.
Figure 3. A representative femur demonstrating the calculation of the
orientation of the trochlear groove. A negative trochlear groove angle indicates that the groove was oriented laterally from distal to proximal direction, as illustrated to the left.

Custom software was developed to locate the deepest point on the trochlear groove on each intersection curve (Matlab, Mathworks Inc, Natick, MA, USA) (Figure 3). ML discontinuity (> 3mm) in the deepest point across the entire curve set was detected, and the corresponding location was determined as the proximal boarder of the trochlear groove. For each femur, the set of deepest points within the trochlear groove region were projected onto the coronal plane. The best-fit line, representing the trochlear groove path, was calculated from the projected point set. The trochlear groove orientation was calculated as the angle between the trochlear groove path and the line perpendicular to transepicondylar axis (Figure 3). Ethnic and gender differences in the trochlear groove orientation were investigated. The groove orientation was correlated with bone size (AP). Statistical significance was defined as p < 0.05.

EVALUATION OF MODERN FEMORAL DESIGNS

The trochlear groove orientation in five modern femoral designs was evaluated against the data on the native femur, including NexGen® Complete Knee Solution (Zimmer Biomet, Warsaw, IN, USA), Attune® Knee System (Depuy Synthes, Warsaw, IN, USA), GENESISTM II Total Knee System (Smith and Nephew, Memphis, TN, USA), Triathlon® Knee System (Stryker, Kalamazoo, MI, USA), and Truliant® Knee System (Exactech, Gainesville, FL, USA). It is worth noting that the trochlear groove angle in the Attune Knee System proportionally changes based on component size (ranging from 10° to 14° lateral) under the design assumption that a patient’s Q-angle and therefore their trochlear angle correlates with size. In contrast, the Truliant Knee System follows the philosophy of a fixed neutral groove orientation with a proximally widened trochlear compartment in order to provide more “forgiveness” to accommodate the naturally varying patella tracking (Figure 1), while the other four knee systems each present a fixed lateralized trochlear groove angle for patella tracking. The allowed range of trochlear groove orientation was measured on the Truliant femoral component based on tracking the center of the smallest sized patella component during simulated placement (Figure 4).

Figure 4. Measurement of the allowed range of trochlear groove orientation on the Truliant design.
Figure 4. Measurement of the allowed range of trochlear groove orientation on the Truliant design.

RESULTS

The pooled trochlear groove orientation in the native femur was near perpendicular to the transepicondylar line with only a slight tendency (~1°) of lateral orientation and quite variable from bone to bone (Table 1). Neither gender- nor ethnic- difference, nor correlation with AP dimension was found (N.S.). No significant difference was found between male and female femora (N.S.).

Among the five knee systems evaluated, only the Truliant Knee System closely matched the range of native groove orientation (Figure 5). In contrast, the other four knee systems each exhibited excessive lateralization of trochlear groove orientation, which was about 3°-13° more lateral compared to the native knee, depending on design and component size. The groove orientation was not found to be correlated with bone size (N.S.).

Table 1. A summary of native trochlear groove orientation. *Negative values indicate that the trochlear groove was tilted laterally in distal to proximal direction (illustrated in Figure 3).
Table 1. A summary of native trochlear groove orientation. *Negative values indicate that the trochlear groove was tilted
laterally in distal to proximal direction (illustrated in Figure 3).
Figure 5. Trochlear groove orientation in the native femur, compared to five modern femoral implant designs.
Figure 5. Trochlear groove orientation in the native femur, compared to five modern femoral implant designs.

DISCUSSION

The design of the femoral component trochlear compartment is one of the critical factors that affects patellofemoral outcome after TKA.10 This study demonstrated that the difference in TKA design philosophies may dramatically impact the restoration of native femoral trochlear groove orientation. Large variations in native trochlear groove angle orientation were found in this study, similar to data that has been reported by several morphological analyses (4°-6° in standard deviation).11-14 Furthermore, a comparison of coronal alignment between the TEA and the line perpendicular to the femoral mechanical axis in the dataset demonstrated a very close match (deviation in alignment: 0.02° ± 0.04°). This confirmed that the results found in this study are relevant to the in-vivo placement of the femoral component referencing the mechanical axis. Studies in the literature revealed that the trochlear groove has varying orientation throughout the flexion range. Barink et al. reported that the trochlear groove is neutrally orientated in the intercondylar region, while it has a medial orientation in the proximal flange area.15 This reported non-linearity in the groove orientation is accommodated by the Truliant design, which allows for moderate patella freedom in the ML direction in extension, accompanied by a gradually increasing ML constraint with more flexion. The evaluation revealed that the four designs following the philosophy of a lateralized trochlear groove angle did not capture the average native groove orientation. This finding has been confirmed clinically by previous studies on several such femoral designs, which found that oftentimes the normal patellar tracking was not restored.7,8,16,17 This altered patellar tracking may pose an increased risk of patellofemoral complications postoperatively.18-21 In addition, this data does not support the basis of designing a proportional trochlear groove angle with regard to femoral size as no significant correlation was found. On the contrary, in Truliant design, the femoral components’ inclusion of a neutral orientation and widened proximal trochlear groove, allows the patella to track at an angle similar to the native knee and matches the morphological data examined in this study.

CONCLUSION

Compared to a lateralized trochlear groove angle, the design philosophy with a neutral groove orientation and widened proximal trochlear compartment may offer improved capability to restore the native trochlear groove orientation in TKA.

References
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