In the present study, the geometry of the trochlear groove in the native and prosthetic knees was evaluated. Our results were consistent with the findings of previous research studies, showing that the native trochlear groove followed a path that could be approximated by two consecutive straight lines, a bilinear approximation, composed of a laterally oriented proximal portion, and a medially oriented distal portion [20, 21]. The prosthetic trochlear groove was relatively consistent and smooth among different types, showed a proximal-lateral to distal-medial orientation throughout the length of the trochlea, and had a prolonged proximal part compared to the native knees.
Unlike in TKA with a symmetrical component in which the trochlear groove does not turn medially or laterally, in TKA with an asymmetrical component with a laterally orientated trochlear groove (more parallel to the orientation of the quadriceps force) and asymmetrical trochlear flanges, patellar “capture” and a more stable and physiological patellar tracking could be expected during the early stage of flexion (0°-30°; the supracondylar pouch/anterior flange) [5, 7, 12, 22]. However, both symmetrical and asymmetrical TKAs have altered physiological patellofemoral kinematics. When compared with a symmetrical prosthesis, the asymmetrical component did not provide better patellar stability and improvement in the non-physiological tracking of the patella [5, 12]. This indicated that the groove of the prosthetic trochlea may still be different from that of the normal trochlea [7, 8].
In the present study, when compared with the proximal portion of the native trochlear groove, the prosthetic trochlear groove extended along an opposite orientation, with its starting point located more proximal and lateral, with maximal discrepancy of 3.2 mm in the 0° cross section. One limitation of the present study was that it was based on CT-scanned knee models that neglected the geometry of the articular cartilage. Although the difference in the location between the osseous and cartilaginous grooves was small (<1 mm) [23], the effect of the articular cartilage on the morphology of the trochlear groove should not be neglected. Previously, Varadarajan et al. compared the trochlear groove morphology of NexGen cruciate retaining femoral components and 21 knee models, including the bone and articular cartilage, using virtual TKA. Proximally, between 43.5% and 58.7% of the trochlear length, the prosthetic groove was more lateral than the native trochlear groove (difference, 0.6-3.5 mm; mean, 2.0 mm; p < 0.034) [8]. The study of Stoddard et al. on TKA fresh-frozen knees with a resurfaced patella showed that the asymmetrical design (Triathlon) did not provide more anatomical patellar kinematics and stability than the symmetrical design [12]. The researchers found that soft tissue had an overriding influence, and the patella was disengaged from the trochlea by the medial patellofemoral ligament in the native knee near extension [12, 24]. Thus, the prosthetic patellar initial position and engagement area might differ from those of the native patella, which might affect early stage patellar tracking and contribute to changes in the patellofemoral kinematics after TKA [12, 25].
Patellar tracking and patellofemoral kinematics could be affected by changes in the groove location after TKA [26, 27]. During knee flexion, a patellar medial shift might lead to patellar periphery soft tissue imbalance and patellar lateral tilt, which may cause pain impingement on the lateral edge of the trochlea (in the case of a non-resurfaced patellar) and a laterally directed force on the patella [28, 29]. A biomechanical study by Barink et al. showed that an unsurfaced TKA patella was significantly displaced at high flexion angles, by approximately 3 mm more medially at 80°-90° of flexion, compared with the intact knees [5]. In the present study, the distal trochlear groove of the prosthetic knees was more medial than that of the native knees, with a maximal discrepancy of 2.4 mm at the 69° cross section. Individual variations (standard deviation) are considered as one factor that there is scarce information on how the anatomy of the normal trochlea is reproduced by the femoral component [1, 7, 11]. As knees within this natural variation will normally not experience problems, a mismatch between the native and prosthetic groove orientation within 3° is probably clinically irrelevant [7]. By extension, no clinically relevant could be observed within a certain mismatch between the native and prosthetic knees in terms of trochlear groove mediolateral location, which should be evaluated in further studies. Besides, the mediolateral position of the femoral and patellar button and how the surgeon should judge the best mediolateral position may also affect the groove position and patellar tracking [12, 26].
In the present study, distally, Triathlon, NRG, and NexGen (the angle spans extended to 60°, 66°, and 78°, respectively) had shorter trochlear grooves than the native knees, MP and Stature (with both angle spans extended to 110°) showed similar trochlear groove length as the native knees. Femoral components with a shorter trochlea appear to have increased incidence of patellar clunk syndrome, which has been associated with posterior stabilized TKA [30-32]. In the study of Maloney et al., the prevalence of patellar clunk was 3.9% in 179 consecutive patients who underwent Insall-Burstein Ⅱ posterior stabilized TKA. With a longer trochlear groove extended distally, no patellar clunk developed in the patients with Advanced posterior stabilized TKA [30]. In a recently published series, an incidence of 2.76% was observed with a modern posterior stabilized implant, whereas an incidence of 6% was found with the use of a different posterior stabilized design [32]. Lengthening the trochlea groove distally makes it more difficult for a nodule to develop and become entrapped [30]. Additionally, patella baja or alta, abnormal patellar tracking, anterior placement of the tibial tray, and increased degree of postoperative knee flexion have also been associated with the development of patellar clunk syndrome [32, 33].
The knee joint is a well-balanced system, and good function relies on coordination and cooperation of the femur, tibia, patella, and soft tissue during dynamic motion. A main limitation of the study was the static analysis of the femoral trochlea was performed separately. The present study did not provide evidence to support the use of one prosthetic design over another but showed the differences of the trochlear groove between various prosthetic systems and between the native and prosthetic knees. Owing to the sensitivity of ligaments and tendons to applied tensile loads such that stretching of these structures at very low loads may induce major changes in the response of their sensory receptors [34], better patellofemoral function may be expected from a femoral component designed with physiological values of the trochlear groove; however, further studies are needed. Another limitation was that physiological features (e.g. the width and height of the lateral and medial femoral condylar facets, and the trochlear bisector angle), which are also important in designing the prosthesis and patellofemoral kinematics were not evaluated [8, 11]. Further studies are needed to explore these parameters. Third, only a relatively small sample of Chinese subjects and implants were recruited for this study; thus, the results might not be generalizable.