Background
Osteoarthritis (OA) is a degenerative joint disease, which causes pain and disability and is characterized by progressive damage of articular cartilage, changes in the underlying (subchondral) bone, and occasional mild synovial inflammation.
Increasing evidence suggests that subchondral bone plays an important role in the etiology of OA [
1,
2], but studies thus far do not provide a consistent view on this subject. Subchondral bone changes have been studied in both humans with OA and in animal models of OA. In human studies, an increase in trabecular bone volume fraction and trabecular thickness was found [
3,
4], as well as an increase in cortical subchondral plate thickness [
3]. However, other studies found a lower bone volume fraction and trabecular thickness in patients with OA [
5,
6] or a decrease in stiffness [
7,
8]. Even within one patient, areas with high and low bone volume fraction have been reported, depending on the condition of the overlying cartilage [
9]. A problem of the human studies is that mostly established (severe) OA is studied, and longitudinal data showing the changes from onset until full clinical osteoarthritic signs do not exist. A problem is that there are no objective criteria that indicate early OA with mild pre-clinical signs and therefore the design of longitudinal studies is difficult.
Several animal models have been developed to study osteoarthritis and changes in the subchondral bone. Some animal studies reported a decrease in bone volume fraction and trabecular thickness [
10‐
13], whereas in other studies these parameters increased [
14,
15]. These differences may be explained by the type of model used and the time at which the measurements were performed. Some bone parameters may occur in two phases: an initial decrease followed by an increase [
16].
A frequently used animal model of OA is anterior cruciate ligament transection (ACLT) in dogs. ACLT results in permanent instability of the knee joint, which is followed by osteoarthritic features [
17]. The ACLT model has been used for in-vivo evaluation of several treatment strategies [
18‐
21]. However, the instability remains present, and may counteract the possible beneficial effects of treatment.
For this reason, the canine groove model has been developed. In this canine model, surgically applied damage to the articular cartilage of the weight-bearing areas of the femoral condyles, not damaging the subchondral bone, is the trigger for development of OA features [
22]. The model is distinctive in that the osteoarthritic trigger is not permanent and the degenerative changes are progressive and not just the expression of surgically applied chondral damage, while synovial inflammation diminishes over time [
22‐
24].
In the current study, we report changes in the subchondral bone of the canine groove model and compare these with changes in the ACLT model. Because the cartilage damage induced in the groove model appeared less drastic than in the ACLT model, the groove model could be very useful to investigate the subtle relationship between bone and cartilage during the development of OA. Therefore, we studied the groove model also at a very early time point. Specifically we used micro-CT analyses to quantify subchondral trabecular bone volume and architecture, the subchondral plate thickness and porosity, and osteophytosis and related this to the changes in cartilage integrity.
Discussion
The thickness of the subchondral plate decreased very consistently in two different canine models of osteoarthritis: the groove model and the ACLT model. In contrast, the changes in the trabecular bone at the tibial epiphysis in the groove model were relatively small and not consistent over time whereas these changes in the ACLT model were larger, with up to 20% loss in bone volume fraction with significant changes in the corresponding architectural parameters. Due to the low number of animals in the groove model, the bone parameters could not reach statistical significance in this model. Although the trabecular parameters were not consistent, the changes in the subchondral plate were very consistent in the groove model, with a clear and early reduction of the plate thickness and an increase in plate porosity.
Although the grooves in the groove model were made in the femur only, the changes in subchondral bone were found in both the femur and in the tibia. This is in concurrence with the cartilage changes found in the groove model which also showed changes in both femur and tibia [
22]. Since the subchondral bone changes in the tibia cannot be caused directly by the grooves, we believe that these changes are part of the osteoarthritic process. This suggests an interaction between the bone and the cartilage through diffusive molecules that originate from the degenerated cartilage or the synovial fluid.
The cartilage changes in both models were similar to the changes previously described for larger groups of animals [
22,
24] and thus the data concerns a representative set of these earlier studies. The groove model showed only very mild changes in cartilage integrity at 3 weeks, which progressed at 10 and 20 weeks. In the ACLT model the changes were comparable to those in the groove model, but slightly more progressive.
Osteophytosis, visible on the CT-images, occurred in all the experimental ACLT knees at 10 and 20 weeks. This contrasts the groove model in which osteophytes only were detected at 20 weeks and not at 3 and 10 weeks. This corroborates the less progressive development of OA in the groove model compared to the ACLT model. However, a cartilaginous pre-form of the osteophytes may develop earlier, but is not detectable on the micro-CT images. In both models the osteophytes start below the rim of the medial tibia plateau and extend to more distant regions. This location is in line with osteophyte location in a rabbit ACLT model [
16]. In human osteoarthritis, osteophytes are found close to the joint surface; it has been suggested that the load bearing area increases as to compensate for instability [
33]. However, in our study, the osteophytes were also found in the groove model, in which the joint does not become unstable arguing against their role in joint stabilization. An explanation for the different location in comparison to humans may be that, in dogs, the ligaments are attached to the bone at a different location than in humans, thereby causing high stresses on the bone in a different location. In addition to this, cytokines such as TGFβ, which is elevated after OA induction [
34,
35], stimulate osteophyte formation [
36]. Since the synovial capsule in dogs extends more to the proximal and distal part of the joint than in humans, the interface between synovial capsule and bone is more distant from the joint space. Assuming synovial tissue derived cytokines to play an important role in osteophyte formation [
37], this may explain their location in dogs compared to humans.
The changes in the trabecular bone were not very pronounced in the groove model. However, in the ACLT model, the bone volume fraction and trabecular thickness were clearly reduced. This corroborates the difference in rate of development of cartilage changes in both models. The changes in the ACLT model fit with previous studies in this model in dogs as well as cats [
10‐
13]. The fact that other studies find an increase in bone volume fraction and trabecular thickness [
14,
15] may be explained by the use of a different type of model, evaluated at a longer time period. Irrespective of the different changes in trabecular bone, similar changes in cartilage and subchondral plate were found in both models. Thus, it seems that the trabecular bone changes are not directly related to the changes in subchondral plate and cartilage. Since the subchondral plate changes consistently follow the cartilage changes, and the trabecular bone changes do not, the subchondral plate may play a more important role in the OA process than the trabecular bone changes.
The subchondral plate thickness decreased in both models at all time points in all experimental knees. This is in line with findings from previous studies concerning various animal models for OA, where subchondral plate thinning was documented in the early stage of the disease [
10,
11,
38,
39]. In some of these studies, this early phase of thinning was followed by a later phase of plate thickening [
11,
38]. This also explains the discrepancy with the sclerosis seen in most human studies [
3,
4,
9], since such studies often concern patients with late osteoarthritis, whereas our present study examined only relatively early time points.
In order to justify the use of the contralateral knee as control, we calculated bone parameters in the diaphyseal and metaphyseal tibia, distal from the joint, containing cortical and trabecular bone, respectively. The bone volume of the metaphyseal tibia was significantly decreased in the experimental ACLT tibias, indicating disuse of the experimental ACLT knee. Thus, the trabecular bone loss in the epiphysis in the ACLT model may be explained by disuse. However, the tibias of the groove model showed hardly any changes in the diaphyseal and metaphyseal bone parameters. Hence, we have no signs of disuse in this model. Both the ACLT and groove model show similar subchondral plate thinning and increased porosity. Since the diaphyseal cortical bone showed no differences between control and experimental knee, we assume that in both models these subchondral plate changes are not caused by disuse of the treated leg.
The consistent decrease in subchondral plate thickness occurred already at 3 weeks post-surgery in the groove model, whereas the cartilage changes were only very mild at this early time point (Fig
5, table
1). This suggests that the subchondral plate changes occur fast. Taken together with the fact that this cannot be explained by disuse, this indicates (at least in the groove model) an interaction between cartilage and subchondral plate that induces bone resorption as a consequence of initiation of cartilage damage induced by the grooves. The thinning and drastically increased porosity of the subchondral plate may facilitate vascular invasion of the cartilage and diffusion of molecules from the damaged cartilage through the subchondral plate and vice versa, thereby enhancing the biochemical communication between bone and cartilage [
40]. It is not clear if this bone cartilage communication interacts with an intrinsic repair activity of cartilage [
41] or plays a role in the progression of the disease process [
42].
Competing interests
The author(s) declare that they have no competing interests.
Authors' contributions
YS carried out the bone analysis and drafted the manuscript. FI carried out the cartilage analysis. FL and SM designed the study. All authors were involved in interpretation of the data and revision of the manuscript. All authors read and approved the final manuscript.