Background
Osteoarthritis (OA) is a chronic joint disease that affects more than 100 million people worldwide [
1,
2]. Usually, OA most strongly affects the knee joint, a three-compartment structure that includes the patellofemoral joint (PFJ) and the medial and lateral tibiofemoral joints. Previous studies on knee OA have focused on the tibiofemoral joints, and little research has been done on the PFJ [
2]. However, isolated patellofemoral osteoarthritis (PFOA) is not only common but has a high prevalence in men (18.5–19.0 %) and women (17.1–34.0 %) aged 55–60 years [
3,
4]. Furthermore, PFOA is the main cause of anterior knee pain and disability [
5].
The PFJ is a unique and complex structure [
6], which is stabilized by a complex multivariate relationship of osseous joint geometry and force vectors generated by the capsuloligamentous stabilizers and quadriceps femoris [
7]. Patellar instability (PI) is frequently caused by pathological changes involving PFJ stabilizers, which increase the likelihood of lateral patellar dislocation and early OA [
8]. Additionally, PI is related to trochlear dysplasia [
8]. Schottle et al. demonstrated that trochlear dysplasia of the femur is a predisposing factor for recurrent PI [
9]. Huri also demonstrated that PI is related to trochlear dysplasia [
10]. In the general population, the incidence of primary PI is 5.8 per 100,000, with a higher incidence in more active and younger individuals [
11]. Some risk factors, such as lateralization of tibial tuberosity, trochlear dysplasia, and the medial patellofemoral ligament, are believed to promote PI [
12]. Biomechanical investigations have suggested that the medial patellofemoral ligament plays a crucial role in restraining lateral patellar translation and provides 50–60 % of PI; the retinacular fibers and patellomeniscal ligament were also demonstrated to be significant medial stabilizers. It is speculated that these ligament structures can provide proprioceptive signals for surrounding muscle tissues in addition to their biomechanical properties [
13]. Although the PFJ plays an important role in knee OA, studies on the pathogenesis of PFOA are scarce [
14]. The spontaneous PFOA model is characterized by slow disease progression, long study duration, and frequent changes in outcome [
15]. A rat model of joint inflammation consists of intra-articular injection of monosodium iodoacetate that chemically induces chondrocyte death, but may not be a representative model of OA [
16,
17]. The surgically induced OA model is mainly used to study tibiofemoral osteoarthritis, with only few studies on PFOA [
18‐
20]. Additionally, the influence of PI on PFOA remains unclear.
It is well known that collagen X [
21,
22] and MMP-13 [
23], markers of cartilage degeneration, lead to chondrocyte imbalance and matrix degradation. Compared with normal cartilage, the cartilage of OA patients exhibits up-regulated TNF-ɑ levels [
24]. Recently, the NF-κB signaling pathway was found to play an important role in regulating inflammatory mediators associated with OA [
25]. This pathway serves as a bridge between cartilage degeneration and development, and few studies have investigated the role of PFOA in early cartilage degeneration.
Therefore, we investigated the relationship between cartilage degeneration during PFOA and the NF-κB signaling pathway by employing immunohistochemistry to assess the histological characteristics of cartilage and quantitative real-time polymerase chain reaction (qRT-PCR) to assess the different expressions of NF-κB, MMP-13, collagen X, and TNF-ɑ in cartilage at different time points in a growing rat model of PI.
Discussion
In this study, we successfully established a rat PI-induced PFOA model and found that PI aggravated subchondral bone loss and cartilage degeneration, which worsened over time. Furthermore, we observed significantly elevated NF-κB protein and mRNA levels in the PFOA model, which indicates that PI-induced cartilage degeneration may be associated with activation of the NF-κB signaling pathway.
The present PFOA rat model exhibited obvious cartilage damage, which might be related to PI-induced patellofemoral hypertension and an abnormal load of PFOA-related structural damage [
33]. One study demonstrated that mechanical stress is an important factor affecting cartilage and bone development [
34]. PI-induced mechanical stress can destroy the integrity and dynamic balance of the PFJ. With the development of PI, the load in the PFJ redistributes, and the microstructure of the trabecular bone changes. The most obvious change was observed in the distal femur; the BV/TV fraction, TB thickness, TB number, and BMD decreased with time after surgery. This may be related to the abnormal pressure load that increase osteoclast activity, which may lead to a significant increase in osteoclast bone absorption in subchondral bone [
35]. Furthermore, cartilage degradation was apparent from 4 weeks after surgery. In the PI group, we observed cell aggregation, loss of cartilage structure and the Safranin O staining reduced significantly at 8 and 12 weeks after surgery, this is consistent with human OA. The safranin O and fast green histochemical staining showed that the cartilage surface was eroded and irregular, the number of chondrocytes in the superficial and middle zone was significantly reduced at 12 weeks after surgery. However, in the control group, the chondrocytes were flattened and the cartilage surface was smooth at different time points. These results suggest that the changes in the microstructure and morphology of subchondral bone may be accompanied by gradual cartilage degradation and may further aggravate cartilage degradation during the process of PFOA [
29].
The main feature of OA affecting the joint is the degeneration of articular cartilage. And, the altered phenotype of articular chondrocytes is the initial step in the progression of OA disease [
36,
37]. In patients with osteoarthritis, MMP-13 is the main member of MMP family expressed in cartilage, but not in normal adults [
38‐
40]. MMP-13 levels in patients with osteoarthritis indicate the degree of cartilage degeneration [
38‐
40]. Activated MMPs catalyze the decomposition of cartilage matrix and induces chondrocyte apoptosis, leading to cartilage damage [
41]. It has been found that MMP-13 can degrade collagen II in human OA [
42]. Additionally, the reduced load decreases the proteoglycan content and causes cartilage thinning, which leads to cartilage degeneration [
43]. Collagen X is a cartilage-specific collagen. Under normal circumstances, it is limited to the hypertrophic area of the growth plate, where it participates in endochondral ossification. Although Collagen X is not a component of normal articular cartilage, it is present in OA cartilage, especially in the deep regions where hypertrophic chondrocyte clusters are observed. Mutations in the Collagen X gene (COL10A1) lead to various forms of metaphyseal dysplasia [
44]. These studies are consistent with our findings. In the present study,we found that the expression of MMP-13 and Collagen X in immunohistochemistry and RT-PCR level were significantly higher. We believe that PI can significantly reduce the pressure in the PFJ, resulting in cartilage degeneration. In conclusion, our findings support the view that articular chondrocytes of the PI model exhibit the same molecular phenotype as the early stage of OA.
The pathogenesis of OA is not completely clear; however, it is closely related to many inflammatory factors [
45]. It has been confirmed that the levels of pro-inflammatory cytokines, such as TNF-ɑ and IL-1b, increase sharply during the early stages of OA progression. Animal and in vitro studies have confirmed that the inflammatory cytokine TNF-ɑ can induce bone resorption and enhance cartilage degradation in OA [
46,
47]. In our study, we found higher immunohistochemical and mRNA expression of TNF-ɑ, which can stimulate the production of other cytokines, such as IL-6, prostaglandin, and MMP [
46] as well as decrease type II collagen and proteoglycan levels [
48]. This may cause PFJ cartilage degradation after PI.
It is generally believed that the NF-κB signaling pathway plays a significant role in regulating inflammatory mediators associated with OA pathogenesis [
25]. NF-κB most likely combines with the inhibitory subunit IκBα and presents in the cytoplasm in an inactive form. Once induced by IL-1β, NF-κB activation requires sequential phosphorylation, degradation of IkBa, and NF-κB p65 translocates from the cytoplasm to the nucleus to induce the expression of inflammation-related genes, including TNF-ɑ, MMPs, COX-2, PGE2, NO, iNOS, IL-6, and ADAMTS [
49,
50]. In addition to regulating TNF-ɑ and IL-1b expression, NF-κB is also activated by these cytokines [
51], thus forming a vicious circle. In our study, immunohistochemistry and mRNA expression analyses revealed that NF-κB levels increased over time in the PFOA model of PI. Thus, persistent high expression of NF-κB in the PFOA model may explain the persistent existence of developmental factors in articular cartilage. However, in this study,we did not investigate the phosphorylation of NF-κB, so we only speculated that early patellofemoral articular cartilage degeneration in a rat model of PI is associated with activation of the NF-κB signaling pathway, further studies are needed to clarify how NF-κB contributes to PFOA pathology.
There are some limitations in our study. First, although this PFOA model cannot be translated to human OA, it can be used to study the therapeutic effects of drugs on OA. Second, because the shape of the patellar in rats is relatively small and difficult to obtain, we have not studied the changes of patellar cartilage degeneration. Third, we need to further study at the cellular level the molecular mechanism of the NF-κB signaling pathway in patellofemoral cartilage degeneration.
Publisher’s Note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.