Yifei Dai, PhD
Laurent Angibaud, Dipl. Ing.
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.
MATERIALS AND METHODS
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.
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).
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.).
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 accomodated 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 often times 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.
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.
1. Australian Orthopaedic Association National Joint Replacement Registry. Annual Report. Adelaide: AOA 2013.
2. Baker PN, van der Meulen JH, Lewsey J, et al. The role of pain and function in determining patient satisfaction after total knee replacement: data from the National Joint Registry for England and Wales. J Bone Joint Surg [Br] 2007;89-B:893–900.
3. Baldini A, Anderson JA, Cerulli-Mariani P, et al. Patellofemoral evaluation after total knee arthroplasty: validation of a new weight-bearing axial radiographic view. J Bone Joint Surg [Am] 2007;89-A:1810–7.
4. Stiehl JB, Komistek RD, Dennis DA, et al. Kinematics of the patellofemoral joint in total knee arthroplasty. J Arthroplasty 2001;16:706–14.
5. Varadarajan KM, Rubash HE, Li G. Are current total knee arthroplasty implants designed to restore normal trochlear groove anatomy? J Arthroplasty 2011;26:274-81.
6. Varadarajan KM, Freiberg AA, Gill TJ, et al. Relationship between three-dimensional geometry of the trochlear groove and in vivo patellar tracking during weight-bearing knee flexion. J Biomech Eng 2010;132:061008.
7. Anouchi YS, Whiteside LA, Kaiser AD, et al. The effects of axial rotational alignment of the femoral component on knee stability and patellar tracking in total knee arthroplasty demonstrated on autopsy specimens. Clin Orthop Relat Res. 1993;170-7.
8. Ostermeier S, Buhrmester O, Hurschler C, et al. Dynamic in vitro measurement of patellar movement after total knee arthroplasty: an in vitro study. BMC Musculoskelet Disord 2005;6:30.
9. Healy WL, Wasilewski SA, Takei R, et al. Patellofemoral complications following total knee arthroplasty. Correlation with implant design and patient risk factors. J Arthroplasty 1995;10:197-201.
10. Kulkarni SK, Freeman MA, Poal-Manresa JC, et al. The patellofemoral joint in total knee arthroplasty: is the design of the trochlea the critical factor? J Arthroplasty. 2000;15:424–9.
11. Eckhoff DG, Burke BJ, Dwyer TF, et al. The Ranawat Award. Sulcus morphology of the distal femur. Clin Orthop Relat Res. 1996;(331):23-8.
12. Iranpour F, Merican AM, Dandachli W, et al. The geometry of the trochlear groove. Clin Orthop Relat Res. 2010;468(3):782-8.
13. Feinstein WK, Noble PC, Kamaric E, et al. Anatomic alignment of the patellar groove. Clin Orthop Relat Res. 1996;(331):64-73.
14. Varadarajan KM, Gill TJ, Freiberg AA, et al. Gender differences in trochlear groove orientation and rotational kinematics of human knees. J Orthop Res. 2009;27(7):871-8.
15. Tanzer M, McLean CA, Laxer E, et al. Effect of femoral component designs on the contact and tracking characteristics of the unresurfaced patella in total knee arthroplasty. Can J Surg 2001;44(2):127-33.
16. Barink M, Meijerink H, Verdonschot N, et al. Asymmetrical total knee arthroplasty does not improve patella tracking: a study without patella resurfacing. Knee Surg Sports Traumatol Arthrosc 2007;15(2):184-91.
17. Parker D, Dunbar M, Rorabeck C. Extensor mechanism failure associated with total knee arthroplasty: prevention and management. J Am Acad Orthop Surg. 2003;11(4):238-47.
18. Leblanc J. Patellar complications in total knee arthroplasty. A literature review. Orthop Rev. 1989;18(3):296-304.
19. Brick G, Scott R. The patellofemoral component of total knee arthroplasty. Clin Orthop 1988; (231): 163-78.
20. Mont M, Yoon T, Krackow K, et al. Eliminating patellofemoral complications in total knee arthroplasty: clinical and radiographic results of 121 consecutive cases using the Duracon system. J Arthroplasty. 1999;14(4):446-55.
21. Sun HJ, Choi D, Lipman D, et al. Comparison of trochlear grooves in contemporary total knee designs to native trochlear groove. Presented at International Society for Technology in Arthroplasty 2016 Congress.