Local treatment of a bone graft by soaking in zoledronic acid inhibits bone resorption and bone formation. A bone chamber study in rats

Abstract

Background: Bone grafts are frequently used in orthopaedic surgery. Graft remodelling is advantageous but can occur too quickly, and premature bone resorption might lead to decreased mechanical integrity of the graft. Bisphosphonates delay osteoclastic bone resorption but may also impair formation of new bone. We hypothesize that these effects are dose dependent. In the present study we evaluate different ways of applying bisphosphonates locally to the graft in a bone chamber model, and compare that with systemic treatment.

Methods: Cancellous bone grafts were placed in titanium chambers and implanted in the tibia of 50 male rats, randomly divided into five groups. The first group served as negative control and the grafts were rinsed in saline before implantation. In the second and third groups, the grafts were soaked in a zoledronic acid solution (0.5 mg/ml) for 5 seconds and 10 minutes respectively before being rinsed in saline. In the fourth group, 8 μL of zoledronic acid solution (0.5 mg/ml) was pipetted onto the freeze-dried grafts without rinsing. The fifth group served as positive control and the rats were given zoledronic acid (0.1 mg/kg) systemically as a single injection two weeks after surgery. The grafts were harvested at 6 weeks and analysed with histomorphometry, evaluating the ingrowth distance of new bone into the graft as an equivalent to the anabolic osteoblast effect and the amount (bone volume/total volume; BV/TV) of remaining bone in the remodelled graft as equivalent to the catabolic osteoclast effect.

Results: In all chambers, almost the entire graft had been revascularized but only partly remodelled at harvest. The ingrowth distance of new bone into the graft was lower in grafts soaked in zoledronic acid for 10 minutes compared to control (p = 0.007). In all groups receiving zoledronic acid, the BV/TV was higher compared to control.

Conclusions: This study found a strong inhibitory effect on bone resorption by bisphosphonates but also a limited inhibition of the ingrowth of new bone. Local treatment at surgery resulted in stronger inhibition of both resorption and bone formation compared to systemic treatment.

Figure 1

Bone conduction chamber. The chamber consists of a threaded titanium cylinder, formed out of 2 half-cylinders held together by a hexagonal screw cap. The interior of the chamber is 7 mm long with a diameter of 2 mm. One end of the implant is screwed into the proximal tibia. At this end, there are 2 ingrowth openings each measuring 0.75 mm², where tissue can grow in from the subcortical bone.

Figure 2

Remodelled bone graft. Inside the chamber, almost the entire graft has been revascularized at harvest with a fibrous tissue penetrating deepest into the graft and with the zone of new-forming bone (arrows) and resorbing osteoclasts lagging behind. Behind this layer of new-forming bone and close to the ingrowth openings, the new-formed bone as well as remnants of the graft is resorbed and replaced by a fatty bone marrow in the saline treated samples (a). In the zoledronic acid treated grafts this marrow still consists of non-resorbed bone and both graft bone and newly formed bone are retained (b). Hematoxyllin/Eosin x 4.

Figure 3

Representative histological sections. Representative histological sections of the five groups at 6 weeks. 1/Saline control. 2/Short soaking time. 3/Long soaking time. 4/Topical zoledronic acid, no rinse. 5/Zoledronic acid systemic injection at two weeks. Specimens are chosen to be representative of both median bone ingrowth distance and median bone density. The black line indicates the frontier between non-remodelled and remodelled graft bone and the bone ingrowth distance was less in group 3 compared to the other groups. Histologically, the increased bone volume fraction in the remodelled area can clearly be seen in all bisphosphonate treated groups.

Figure 4

Bone ingrowth distance. Ingrowth distance (mm) of new bone into the allograft bone in the bone chamber after 6 week as assessed by histomorphometric analyses in each of the five experimental groups. Individual data points and the median for each group are indicated.

Figure 5

Bone Volume/Total Volume. Bone Volume/Total Volume (BV/TV, %) in the remodelled part of the allograft bone in the bone chamber after 6 week as assessed by histomorphometric analyses in each of the five experimental groups. Individual data points and the median for each group are indicated.