Objectives: To evaluate the e ect of various medial patellofemoral ligament (MPFL) ﬁxation techniques on patellar pressure compared with the native knee. Methods: A ﬁnite element model of the patellofemoral joint consisting of approximately 30,700 nodes and 22,200 elements was created from computed tomography scans of 24 knees with chronic lateral patellar instability. Patellar contact pressures and maximum MPFL graft stress at ﬁve positions of ﬂexion (0, 30, 60, 90, and 120) were analyzed in three types of MPFL reconstruction (MPFLr): (1) static /anatomic, (2) dynamic, using the adductor magnus tendon (AMT) as the femoral ﬁxation, and (3) dynamic, using the quadriceps tendon as the attachment (medial quadriceps tendon-femoral ligament (MQTFL) reconstruction). Results: In the static /anatomic technique, the patellar contact pressures at 0and 30were greater than in the native knee. As in a native knee, the contact pressures at 60, 90, and 120were very low. The maximum MPFL graft stress at 0and 30was greater than in a native knee. However, the MPFL graft was loose at 60, 90, and 120, meaning it had no tension. In the dynamic MPFLr using the AMT as a pulley, the patellar contact pressures were like those of a native knee throughout the entire range of motion. However, the maximum stress of the MPFL graft at 0was less than that of a native ligament. Yet, the maximum MPFL graft stress was greater at 30than in a native ligament. After 30 of ﬂexion, the MPFL graft loosened, similarly to a native knee. In the dynamic MQTFL reconstruction, the maximum patellar contact pressure was slightly greater than in a normal knee. The maximum stress of the MPFL graft was much greater at 0and 30than that of a native MPFL. After 30of ﬂexion, the MQPFL graft loosened just as in the native knee. Conclusions: The patellar contact pressures after the dynamic MPFLr were like those of the native knee, whereas a static reconstruction resulted in greater pressures, potentially increasing the risk of patellofemoral osteoarthritis in the long term. Therefore, the dynamic MPFLr might be a safer option than a static reconstruction from a biomechanical perspective. J. Clin. Med. 2019 ,8, 2093; doi:10.3390 /jcm8122093 www.mdpi.com /journal /jcm J. Clin. Med. 2019 ,8, 2093 2 of 12 Keywords: MPFL reconstruction; ﬁnite element model; patellar contact pressure; patellar cartilage degeneration after MPFL reconstruction 1. Introduction Chronic lateral patellar instability (CLPI) is a common ﬁnding of knee surgeons in their daily clinical practice. Although its etiology is multifactorial, the most important structure in the development of this instability is medial patellofemoral ligament (MPFL) deﬁciency [ 1,2]. Over the last 10 years, MPFL reconstruction (MPFLr) has become a standard surgical technique for the treatment of CLPI, either as an isolated technique or combined with other surgical techniques [ 1–6]. Many surgical techniques using various types of grafts (autografts, allografts, or synthetic) and ﬁxation techniques have so far been described. Based on the ﬁxation technique used, there are two types of MPFLr, static and dynamic [ 1,7–12]. A static MPFLr, which involves an anatomic femoral bone attachment and a patellar bone attachment, is currently more common [ 1]. In the less used dynamic reconstruction, only one of the graft’s extremities is ﬁxed to bone, while the other one is ﬁxed to soft tissues. This type of reconstruction is therefore a less rigid reconstruction [ 7–12]. Static and dynamic reconstructions show MPFL isometry between 0and 90[9,13]. In a static /anatomic MPFLr, the graft is isometric in all the cases between 0and 30of knee ﬂexion [ 13]. In 83% of cases, the graft is isometric from 0to 60[13]. Beyond 60of knee ﬂexion, the MPFL becomes progressively lax and isometry is lost [ 13]. Regarding isometry, a ligament is considered isometric when there is less than 5 mm of length change throughout the entire range of motion [14]. Both static and dynamic reconstruction techniques have been shown to yield satisfactory clinical results in the short and intermediate terms [ 1,7,8,12]. However, what is not known is whether one type of MPFLr is more likely than the other to lead to the development of patellofemoral osteoarthritis (PF OA) in the long term. We hypothesized that a static anatomic reconstruction might generate greater patellar contact pressure than a dynamic reconstruction and would therefore increase the risk of PF OA in the long term. Nevertheless, from a functional standpoint, a dynamic reconstruction would enable behavior that is more like that of a native MPFL than a static reconstruction. In a dynamic MPFLr, the ﬁxation point gives a bit before the graft starts to stretch when the patella moves laterally. Therefore, the dynamic MPFLr allows for patellar contact pressures that are closer to those generated in a native knee. After the ﬁxation of the MPFLr, it is important to verify that the patella can still be manually lateralized in full extension some 10 mm, to avoid any over-constraint, but with a ﬁrm endpoint. To avoid excessive tension on the graft when we perform an anatomic /static MPFLr, we have to ﬁx the graft at 30of knee ﬂexion as it is at this angle that the distance between the femoral and patellar attachments points is greatest [13]. Cadaveric experiments have been conducted that show the achievement of physiological patellofemoral biomechanics, like that of the native knee with static MPFL reconstructions [ 15–17]. Cartilage degeneration after a static MPFLr has been related to graft overtension or an incorrect femoral attachment point [ 16,17]. Several biomechanical studies based on the ﬁnite element (FE) method have evaluated the MPFLr [ 18–24]. Previous studies were mainly based on a single FE model of the knee. Sanchis-Alfonso et al. [ 18] recently created a 3D parametric FE model of the patellofemoral joint (PFJ) that allowed for the evaluation of di erent types of MPFL reconstructions through the full range of motion of the knee. Additionally, the model is patient-speciﬁc. The objective of this paper was to evaluate the e ect of di erent MPFL ﬁxation techniques on patellar pressures compared with the native knee. High patellar pressures would be a risk factor for patellofemoral cartilage degeneration in the long term. This evaluation used a clinically-validated 3D parametric FE model of the PFJ [18]. J. Clin. Med. 2019 ,8, 2093 3 of 12 2. Methods Investigation performed at the Department of Mechanical Engineering, University of Zaragoza, Zaragoza, Spain. 2.1. Parametric FE Model of the PFJ A parametric FE model of the PFJ was previously developed [ 18] based on the geometrical average of 24 knees acquired with a 64-detector multidetector computer tomography (CT) system (Philips, Best, The Netherlands) at the highest spatial resolution, without slice interpolation (0.255 0.255  0.672 mm3) [13]. The knee geometry was simpliﬁed (Figure 1). The main parts of the PFJ were the femoral condyle and patella bones, considered as rigid parts, and the femoral and patellar cartilages, considered as deformable components. A cartilage thickness of 3 mm was assumed [ 18,25]. Tendon and ligaments were also included to stabilize the patella and support the distribution of the patellar contact pressures better. The quadriceps tendon (QT), which consists of the vastus medialis (VM), vastus lateralis (VL), vastus intermedius (VI), and rectus femoris (RF) tendons, and the patellar tendon (PT) were modelled as a group of four and two truss elements, respectively (Figure 1), while the MPFL and the lateral retinaculum (LR) were deﬁned as beam elements. The QT was oriented from the insertion site on the patella to the muscle origin or the most distal wrapping point on the femur, while the PT was oriented from the distal patella to the tibia [ 19,20]. The tendon and ligament properties were taken from previous studies and are summarized in Table 1 [ 21,26,27]. The cartilages were modelled with an elastic modulus of 10 MPa and a Poisson’s ratio of 0.45 [ 22,28,29]. A contact between both cartilage surfaces was deﬁned with a friction coe cient of 0.02 [ 30]. Finally, the FE model of the PFJ consisted of around 30,700 nodes and 22,200 elements. J. Clin. Med. 2019 , 8, x FOR PEER REVIEW 3 of 12 2. Methods Investigation performed at the Department of Me chanical Engineering, University of Zaragoza, Zaragoza, Spain. 2.1. Parametric FE Model of the PFJ A parametric FE model of the PFJ was previous ly developed [18] based on the geometrical average of 24 knees acquired with a 64-detector mu ltidetector computer tomography (CT) system (Philips, Best, The Netherlands) at the highest spat ial resolution, without slice interpolation (0.255 × 0.255 × 0.672 mm3) [13]. The knee geometry was simplified (Figure 1). The main parts of the PFJ were the femoral condyle and patella bones, consider ed as rigid parts, and the femoral and patellar cartilages, considered as deformable components. A cartilage thickness of 3 mm was assumed [18,25]. Tendon and ligaments were also included to stabil ize the patella and support the distribution of the patellar contact pressures better. The quadriceps te ndon (QT), which consists of the vastus medialis (VM), vastus lateralis (VL), vastus intermedius (VI), and rectus femori s (RF) tendons, and the patellar tendon (PT) were modelled as a group of four and two truss elements, respectively (Figure 1), while the MPFL and the lateral retinaculum (LR) were defined as beam elements. The QT was oriented from the insertion site on the patella to the muscle origin or the most distal wrapping point on the femur, while the PT was oriented from the distal pate lla to the tibia [19,20]. The tendon and ligament properties were taken from previous studie s and are summarized in Table 1 [21,26,27]. The cartilages were modelled with an elastic modulus of 10 MPa and a Poisson’s ratio of 0.45 [22,28,29]. A contact between both cartilage surfaces was defined with a fr iction coefficient of 0.02 [30]. Finally, the FE model of the PFJ consisted of around 30,700 nodes and 22,200 elements. Figure 1. (A) Parametric geometry of the PFJ (femur, patella, and femoral and patella cartilages), ( B) static/anatomic reconstruction, ( C) dynamic reconstruction using AMT as the femoral fixation, and (D) dynamic reconstruction using the quadriceps te ndon (QT) as one of the attachment points. Figure 1. (A) Parametric geometry of the PFJ (femur, patella, and femoral and patella cartilages), (B) static /anatomic reconstruction, ( C) dynamic reconstruction using AMT as the femoral ﬁxation, and (D) dynamic reconstruction using the quadriceps tendon (QT) as one of the attachment points. J. Clin. Med. 2019 ,8, 2093 4 of 12 Table 1. Material properties considered for the FEM (Finite Element Model) simulation (from