Using a rat tibial fracture model, we have demonstrated that parecoxib given perioperatively for 1 week reduced the mechanical strength and BMD after 3 weeks. Accordingly, the mechanical strength in animals given indomethacin was reduced after 3 weeks, and BMD was reduced for 2 weeks.
A limitation of the present study might be the use of three-point cantilever bending test due to concerns related to possible deformation of the soft callus from the fulcrum. However, when doing the testing, the distal tibia is fixed, and the proximal fragment is bent in an anterior direction, the fulcrum is not moving or sagging into the callus. Three-point cantilever bending test gives standardized fractures and reliable results. Another limitation with our method concerning the BMD measurements was that we had to include the intramedullary nail. Due to this, the registered BMD values were artificially high. However, by subtracting the constant nail value, the real differences in fracture callus BMD between the groups could be reliably calculated.
We previously demonstrated that parecoxib exerted an inhibitory effect on the mineralization of fracture callus [16
]. These findings were confirmed in the present study; parecoxib had an inhibitory effect on the mineralization in the fracture callus. We also found that indomethacin reduced BMD at the fracture site, but to a lesser degree.
Our findings correspond with Beck’s rat study (2003) where BMD in the healing area of a tibial osteotomy after 3 weeks was lower in animals given diclofenac, a conventional COX inhibitor, compared with animals given tramadol and placebo. In contrast to their study, where BMD was calculated from computer tomography performed after euthanization and nail removal, we were able to measure BMD on anesthetized animals during the study using a bone densitometer specifically designed for measurements on small animals. With this method, we were able to record the development of BMD and differences between the groups during the healing process.
After 3 weeks we found a reduction in the ultimate bending moment and the bending stiffness for both parecoxib- and indomethacin-treated animals compared to placebo. Also, we found reduction of BMD in parecoxib-treated animals after 3 weeks and in indomethacin-treated animals after 2 weeks. According to this, it seems likely that the bone mineralization is involved in the mechanical strength of the healing fractures. Rø et al. [54
] concluded in their study in 1978 that the impaired mechanical strength after indomethacin treatment was not caused by an inhibition of collagen synthesis, but by production of lower-quality collagen. The results from our present study also suggest that the conventional COX inhibitors and COX-2 inhibitors interfere with the transformation of immature connective tissue to mineralized callus and bone. Since the osteoblasts produce alkaline phosphatase that initiate mineralization [53
], our results may suggest inhibition of the osteoblasts as one of the mechanisms for delaying union.
However, it seems likely that the reason these drugs inhibit bone metabolism and fracture healing is more complex. It has been postulated that COX inhibitors’ detrimental effects on bone metabolism and fracture healing are due to inhibition of prostaglandin (PG) synthesis [54
]. Vane [66
] showed that the therapeutic effects of sodium salicylate and aspirin-like drugs, the precursors of COX inhibitors, were due to inhibition of the synthesis of PGs. Later, several studies elucidated the role of PGs in the bone healing process. A direct link between PGs and all the characteristic signs of inflammation was demonstrated [33
], as well as increased release of PGs locally after a fracture [13
]. The dead cells in the area cause an aseptic inflammatory response, and without this inflammation the bone resorption and formation necessary for healing cannot occur adequately [58
]. Furthermore, PGs have a direct effect on bone resorption through increased osteoclastic activity [15
] and also increase the replication and differentiation of the osteoblasts resulting in enhanced bone formation [42
]. Thus, PGs are involved in regulation of the balance between bone resorption and bone formation [18
The ability of COX inhibitors to suppress inflammation depends mostly on inhibition of the COX enzymes [66
], and decreased production of prostaglandins elicited by COX inhibitors could potentially impair the bone healing process [33
Concerning the bone metabolism and fracture healing, it is questioned whether COX-1 or COX-2 is the enzyme responsible for the impairing effects. It is demonstrated that COX-2 plays a critical role in bone resorption [49
], and induction of COX-2 in osteoblasts is reported to be essential to the acute stress response in a bone remodeling system [51
]. Additionally, cytokines associated with inflammation have been found to induce COX-2 when added to bone marrow cells in culture [64
]. Furthermore, it is demonstrated that COX-2 is critically involved in bone repair and required for both intramembranous and endochondral bone formation in a study using mice genetically deficient for COX-1 and COX-2 enzyme (COX-1−/−
]. In another study involving COX-1−/−
mice, the authors also conclude that COX-2 function is essential for bone healing [59
]. Inflammation is present in the early phase after fracture, but does not occur during the later healing process. It was demonstrated that COX-2 mRNA levels showed peak expression during the first 2 weeks of fracture healing and then returned to basal levels by 3 weeks [29
]. In a review article on heterotopic ossification (HO), which may represent a mechanism comparable to bone formation in the fracture healing process, it was concluded that indomethacin given for 7 to 11 days prevented HO and was the drug of choice at present [24
]. This also supports that the initial inflammation is the critical phase of bone formation and gives further reason to believe that COX-2 inhibitors administered perioperatively even for only a few days will affect bone metabolism and be detrimental in the early phase of fracture healing.
In the present study, relatively large differences between the intervention groups and the placebo animals were found, whereas in a previous study [16
] we found no differences in mechanical properties concerning parecoxib animals compared to placebo after 6 weeks, only a tendency with weaker fractures in animals treated with parecoxib. This indicates that the effects of COX-2 inhibition administered in relation to surgery delays fracture healing, but that the effects on the healing fractures decreases, or even disappears, with time.
This is in accordance with the findings of Gerstenfields et al. [28
]. Even so, no definitive conclusions regarding clinical short term administration of COX inhibitors could be made on the basis of these experimental studies. In general, however, all fractures showing delayed union, have an increased risk to eventually end up in a non union. In a clinical setting this should be a concern, as short term administration of COX inhibitors might result in increased incidence of non unions, especially in long bone fractures [9
During the last few years, several studies have demonstrated that COX-2 inhibitors impair fracture healing and suppress bone formation [6
]. Controversies still exist, however, regarding the potentially negative effects of the COX-2 inhibitors compared to the conventional COX inhibitors on bone healing and bone repair. The impairing effect on bone healing is less significant with COX-2-specific inhibitors. Gerstenfeldt et al. studied the effects of ketorolac and parecoxib [29
]. Both had negative effects on fracture healing, but parecoxib only to a lesser degree. As pointed out by Aspenberg in a comment on this paper [4
], this was probably caused by the fact that the animals were given parecoxib by oral gavage only once daily. Due to this, the blood concentration was probably too low much of the time to induce proper inhibition of the COX-2 enzyme. Additionally, male rats, in which COX-2 inhibitors have shown to have a short half-life and are quickly eliminated [32
], were used. In Brown et al. [8
], there were similar findings; indomethacin delayed fracture healing at 4 weeks, but celecoxib, a specific COX-2 inhibitor, did not. Their use of male rats may be a likely explanation for not finding delayed fracture healing in the celecoxib animals. Recently Gerstenfeld et al. [28
] interpreted their previous findings so that inhibition of COX-2 was associated with impaired fracture healing when the medication was prolonged for 21 or 35 days, but not when given for 7 days. Once again, their use of oral gavage once daily commencing 24 hours after surgery and their use of male rats could have resulted in less notable findings compared to female rats or humans. However, they showed that the impairing effects were reversible when the drugs were administered only for 7 days, and this corresponds with our findings.
Another recent study elucidated the effects of celecoxib on fracture healing [60
]. Like our present study, they demonstrated that short-term treatment with a COX-2 inhibitor following fracture, delays healing. They also demonstrated that higher doses or longer duration of the celecoxib treatment affected bone healing even more.
The calculation of relevant parecoxib doses for rats is associated with some uncertainty. Due to the quick metabolism of the COX-2 inhibitors celecoxib and rofecoxib in rats [32
], Meunier and Aspenberg [45
] administered parecoxib in as high doses as 6.4 mg/kg daily using subcutaneous mini pumps with continuous release to compensate for the fast metabolism. However, to our knowledge, the half-life of parecoxib in rats is unknown. Furthermore, no direct correlation is observed between the plasma concentration of COX inhibitors and the magnitude of the pharmacological effects in chronic inflammatory conditions, and different COX inhibitors may also have various potency [35
]. Thus, the inhibitory effects on inflammation after fracture could differ between various COX inhibitors. We chose to administer 1 mg/kg daily to our rats, which is equal to the recommended human dosage, as this dosage has previously demonstrated a negative effect on bone mineralization.
The present study was designed to investigate the effects on bone mineralization and fracture healing of short-term perioperative treatment of parecoxib and indomethacin [16
]. We used female rats to eliminate the problem with a short half-life for drugs involved, and to ensure the best absorption and sufficient blood concentration, the drug was administered intraperitoneally twice a day. The first injection was given immediately before surgery and prolonged for a week to mimic the use of COX inhibitors in humans. No differences between parecoxib and indomethacin were demonstrated concerning mechanical properties. Our findings, where parecoxib had a higher potential for reducing BMD, might support the presumption that inhibition of the COX-2 enzyme is responsible for impaired fracture healing.