In the present study, the geometry of the trochlear groove tracking in native and prosthetic knees was evaluated. Our results were consistent with the findings of previous researches, showing that native groove tracking 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 tracking was relatively consistent and smooth among different types, showed proximal-lateral to distal-medial orientation throughout the length of the trochlea, and had a prolonged proximal part compared to the native knee.
Current femoral trochlear geometry has evolved from symmetrical to asymmetrical, and the main design difference was considered existing on the anterior flange. Compared with traditional symmetrical component, in TKA with as asymmetrical component with a laterally orientated trochlear groove (more parallel to the orientation of the quadriceps force) and asymmetrical trochlear flanges, patellar “capture”, more stable and physiological patellar tracking could be expected during early stage of flexion (0°-30°; the supracondylar pouch/anterior flange) [6, 10, 12]. However, symmetrical and asymmetrical TKAs both have altered physiological patellofemoral kinematics. When compared with a symmetrical prosthesis, the asymmetrical component did not provide more patellar stability and improve the non-physiological tracking of the patella [6, 12]. This indicated that the groove of the prosthetic trochlea may still be different from that of the normal trochlea [10, 11].
Previously, Varadarajan et al. compared the groove morphology of 21 knee models and NexGen cruciate retaining femoral components via virtual TKA. Proximally, between 43.5–58.7% of the trochlear length, the prosthetic groove was more lateral than in the native knees (difference, 0.6–3.5 mm; average, 2.0 mm; p < 0.034) [11]. Similarly, in the present study, when compared with the proximal portion of the native tracking, the prosthetic tracking 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. It was 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, 22]. Thus, the prosthetic patellar initial position and engagement area might differ from that of the native patella, which might affect early stage patellar tracking and contribute to changes in the patellofemoral kinematics after TKA [12, 23].
It was believed that patellar tracking and patellofemoral kinematics could be affected by changes of the groove location after TKA [24, 25]. During knee flexion, patellar medial shift might lead to patellar periphery soft tissue imbalance and patellar lateral tilt, which may lead to pain impingement on the lateral edge of the trochlea (in the situation of a non-resurfaced patellar) and a laterally directed force on the patella [26, 27]. A biomechanical study by Barink et al. showed that an unsurfaced TKA patella was significantly displaced in high flexion angles, with about 3 mm more medially at 80°-90° of flexion compared with intact knees [6]. In the present study, for the distal trochlear groove, the prosthetic tracking was more medial than that in the native knees, with maximal discrepancy of 2.4 mm in the 69° cross section. Aside from prosthetic design, 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, 24].
Furthermore, in the present study, distally, Triathlon, NRG, and NexGen (the angle spans extended to 60°, 66°, and 78°, respectively) have shorter trochlear groove compared to that of the native one, MP and Stature (the angle spans both extended to 110°) showed similar trochlear groove length compared to that of the native one. Femoral components with a shorter trochlea appear to have increased incidence of patellar clunk syndrome, which has been associated with posterior-stabilized TKA [28, 29, 30]. In the study of Maloney et al., the prevalence of patellar clunk was 3.9% in 179 consecutive Insall-Burstein Ⅱ posterior-stabilized TKA, while with a longer trochlear groove extended distally, no patellar clunks developed in the patients with Advanced posterior-stabilized TKA [28]. In a recently published series, an incidence of 2.76% was observed with a modern posterior-stabilized implant, whereas an incidence of 6% was seen with the use of a different posterior stabilized design [30]. Lengthening the trochlea groove distally make it more difficult for a nodule to develop and become entrapped [28]. Besides, patella baja or alta, abnormal patellar tracking, anterior placement of the tibial tray, and an increased degree of postoperative knee flexion have also been associated with the development of patellar clunk syndrome [30, 31].
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 separately. The present study did not provide evidence to support the use of one prosthetic design over another, just showed the differences of the trochlear groove trackings between various prosthetic systems and between the native and prosthetic knees. As 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 [32], better patellofemoral function may be expected by a femoral component designed with physiological values in trochlear groove tracking; however, further studies are needed. Another limitation was that physiological features (e.g. the width and height of the lateral and medial femoral condylar facet, and trochlear bisector angle) are also important in prosthetic design and patellofemoral kinematics were not evaluated [11, 16]. Further studies are needed to explore these parameters. Third, CT-scanned knee models were used in the present study, neglecting the geometry of the articular cartilage. Although the difference was small (less than 1 mm) in the location between the osseous and cartilaginous grooves [33], the effect of articular cartilage on the morphology of the trochlear groove should be evaluated in further studies.