Strontium ranelate causes osteophytes overgrowth in a model of early phase osteoarthritis

In this study, although subchondral bone was significantly enhanced by SR (625 mg/kg/day), it did not protect cartilage at this dose from degeneration in meniscectomized guinea pigs. Besides, SR stimulated osteophytes overgrowth with cleft formation and/or osteophyte collapse in meniscectomized guinea pigs. The dose used in this study, considering strontium bioavailability would be reduced by ~60–70% if taken with calcium or food, is similar to that used by Pelletier et al. [15]. They demonstrated therapeutic effects of 75 mg/kg/day in ACLT dogs at least 2 h before feeding. Yu et al. [16] reported that 1,800 mg/kg/day showed therapeutic effects in MNX rats. Bruel et al. [22] showed that mean serum Sr of 8990 μg/L results from 8 weeks of 900 mg/kg/day for rats, thus serum Sr in Yu’s study should be no less than this value. On the contrary, Meunier et al. [23] reported the mean serum Sr is 117.9 μg/L in patients who regularly receive SR at 2 g/day. Because a potential risk of venous thromboembolism (VTE) may occur in SR-treated patients [24], high SR dose may encounter obstacles when adapting the drug to human OA treatment. In this view, a lower dose as in this study and Pelletier et al.’s tends to be more appropriate.

Although several in vivo studies are described in the literature [15, 16], none have focused on SR in a guinea pig model which has been described by Bendele in detail [19]. This model is similar to human OA, and features cartilage degeneration, osteophytes formation, and fibroblast proliferation in the synovium. Surgically induced OA models, such as anterior cruciate ligament transectomized (ACLT) [25] and MNX animals, generate cartilaginous lesions rapidly but different surgical approaches cause various rates of cartilage degeneration. Generally, cartilage in the MNX model degenerates more rapidly than in the ACLT model due to more joint instability [19]. Thus, the MNX model might narrow the therapeutic window for drug intervention. And they might be obscured if there exist any potential therapeutic effects of SR.

Instead, SR stimulated osteophyte growth and subsequent collapse. The method of osteophyte measurement is adapted from the method for rats reported by Gerwin et al. which only measured cartilaginous tissue [26]. Osteophytes often grow upward and outward on tibial plateau edge and guinea pigs’ osteophytes contain more bony tissue, so data from this method should be more representative with respect to the extent of osteophytes growth direction. Our data show that osteophytes were larger in SR-treated animals, which indicates that osteophytes may be stimulated by SR. This finding is supported by work published by Panahifar et al. [6] who used SR as a tracer and mapped strontium with electron probe microanalysis (EPMA). They reported strontium incorporated toward the osteophytic margins in meniscectomized rats. Panahifar et al. further reported that strontium was heavily incorporated in mineralizing osteophytes at 4, 8, and 12 weeks post-surgery, whereas it was incorporated in subchondral bone only between weeks 8-12 [18]. However, an earlier study in which radioactive strontium isotope ⁸⁵Sr was used as a tracer for monitoring patella bone turnover in the medial or lateral femorotibial articulation in severe symptomatic OA patients [27], suggesting that osteophytes were not associated with high uptake of ⁸⁵Sr. These contradictory data may be explained by different phases of OA progression aimed in these studies. Panahifar et al. suggested that in early phases of OA progression, rather than the knee plateau, osteophytes have greater affinity for strontium. Furthermore, SR can strongly stimulate human cartilage matrix formation in vitro by a direct ionic effect [28]. From strontium accumulation, chondrocytes in osteophytes could be specifically stimulated and the chondral aspect of the osteophyte grows, forming larger osteophytes in early OA. Chondrocytes in the knee plateau were merely stimulated, thus cartilage degeneration may be unaffected.

Collapsed osteophytes observed here were related to condensed subchondral bone beneath the osteophyte, not only related to osteophyte overgrowth stimulated by SR. Figures 3f and 6c show that canals almost disappeared in bony osteophytes and the subchondral bone plate. Condensed subchondral bone could transmit increased loads to overlying cartilage, although its reason is unclear, leading to further cartilage degeneration [29]. In contrast, because the articular cartilage is avascular, the permeability of calcified cartilage and subchondral bone plates allows crossover communication, providing connecting channels between subchondral bone and cartilage [30]. Canal reduction in subchondral bone or osteophytes would affect nutrition and oxygen and induce chondrocyte apoptosis, which could contribute to osteophyte collapse.

It was noted that chondrocytes alignments diffused radially from the cartilage-subchondral bone interface to the cartilage surface at the transitional area between plateau and osteophyte. This suggests that this region is under abnormal mechanical stress, which may be due to stretching by osteophyte and plateau cartilage. This transverse force may be derived from the bearing load and transduced by excessively grown osteophytes. Then, this region would have exhibits catabolic changes. AGG and Col-II, two major components of the cartilage matrix [31, 32], were absent in chondrocytes located in the center of collapsed osteophytes, and this may suspend the repair process. Also, ADAMTS4 and MMP-13, enzymes responsible for degrading AGG and Col-II [33, 34], were expressed by chondrocytes, which will accelerate cartilage degeneration locally. These findings were supported by Su et al. [35] who reported that an in vitro model mimics mechanical stress and undergoes similar changes in chondrocytes. Abnormal mechanical stress can also induce chondrocyte apoptosis, which was shown by increased expression of caspase-3 which is associated with mechanical stress according to Kong et al. [36]. Thus, these changes contribute to the formation of a cleft in the region between the osteophyte and the knee plateau.

The limitation of the current study include the small number of animals and the single time point. To avoid diffused staining in immunohistochemistry procedure caused by 100% ethanol fixation, we had to randomly divide each group further into two subgroup. Consequently, the number of animals available for microCT and immunohistochemistry analysis were reduced. Another consequence is randomization could not completely eliminate sampling error between subgroups, which result in potential weaken the power of results. The study conducted on a surgical induced OA model, the progress of cartilage lesion is rapid. Single time point observation may be insufficient to discover potentially existed therapeutic effect of SR. And the last, the secondary outcomes, such as osteophyte overgrowth and osteophyte collapse, should be tested and verified by further research.