Protracted systemic changes in bone biology after segmented unloading in the rat

A rat osteoporotic spine model for the evaluation of bioresorbable bone cements

BACKGROUND CONTEXT

As the aging population increases, the rising prevalence of osteoporosis-related spine fractures will have a dramatic impact on health care. At present, mainstay treatment relies on systemic medications intended to prevent diminishing bone mineral density (BMD) and bone mass. However, an adjunctive treatment strategy is to target specific areas of the skeletal system that are prone to clinically significant osteoporotic fractures. We term this strategy the "local treatment of osteoporosis" or osteoplasty. Potential use of osteoplasty involves the percutaneous injection of bioresorbable and bioactive bone cements into bones at risk of sustaining osteoporotic fractures. Calcium sulfate (CaSO(4)) is among the candidate bioresorbable bone cements with the material attributes desirable for potential application with osteoplasty, yet previous studies on the osteoconductive properties of CaSO(4) have been limited to animal models exhibiting normal bone biology and architecture. However, osteoporotic bone physiology may potentially interfere with the material properties of common osteoconductive biomaterials, such as that of CaSO(4). To further test this hypothesis, a suitable animal model is needed to evaluate the in vivo behavior of potential biomaterials in osteoporotic bone.

PURPOSE

The purpose of this study is to evaluate the caudal (proximal tail) rat vertebral body as an appropriate system for the in vivo evaluation of bone cement performance in the osteoporotic spine.

STUDY DESIGN

(1) Micro-computed tomography radiomorphometry study and (2) biomechanical vertebral compression analysis.

METHODS

Female Sprague Dawley rats were ovarectomized (OVX) at age 8 weeks and subsequently maintained on a low-calcium diet for 3 months. Normal nonovarectomized female rats (NL) of similar age and size were maintained on regular rodent feed. Micro-CT analysis was performed on both the lumbar and caudal vertebrae (levels 5-7) of both groups. The following bone radiomorphometric parameters were determined: bone mineral density (BMD), average cortical thickness (ACT), average trabecular thickness (TbTh), and average trabecular spacing (TbSp). Strength and stiffness of both NL and OVX vertebral bodies were assessed under axial compression at 0.1 mm/s, whereas displacement (mm) and force (N) were measured at 10 Hz until completion to failure. After the implantation of an injectable form of CaSO(4) bone cement into caudal vertebrae, radiomorphometric analysis of cement volume, based on its unique CT absorption profile, was performed over the 8-week time period, as well as the subsequent bone response of both NL and OVX caudal vertebrae to CaSO4.

RESULTS

OVX caudal vertebrae showed an 18% decrease in BMD, a 28% decrease in diaphyseal ACT, a 55% decrease in TbTh, and a 2.4-fold increase in TbSp compared with NL (p<.05). Additionally, lumbar vertebrae exhibited a 21% decrease in BMD, a 24% decrease in anterior body ACT, a 48% decrease in TbTh, and a 4.7-fold increase in TbSp (p<.05). Failure testing of OVX caudal vertebral bodies revealed a 29% decrease in strength and a 60% decrease in stiffness compared with NL (p<.01). After implantation into OVX caudal vertebrae, CaSO(4) cement exhibited a 50% decrease in initial cement volume at 2 weeks and complete resorption by 4 weeks, whereas CaSO(4) injected into NL vertebrae exhibited a 79% decrease in initial cement volume at 4 weeks, trace amounts at 6 weeks, and complete resorption by 8 weeks. At 8 weeks, NL vertebrae implanted with CaSO(4) cement exhibited increased cortical bone thickness compared with NL sham vertebrae. This CaSO(4) cement-mediated bone augmentation was altered in osteoporotic vertebrae that exhibited porous irregular cortical bone not noted in cement-treated NL vertebrae or OVX sham vertebrae.

CONCLUSIONS

Future investigation of potential biomaterials intended for the local treatment of osteoporosis will require their study within an appropriate osteoporosis animal model. The OVX rat caudal spine exhibits pathologic bone changes consistent with the osteoporosis phenotype, including decreased BMD, diminished trabecular network density, cortical thinning, and decreased mechanical strength. These derangements in bone microarchitecture and physiology may contribute toward the accelerated cement resorption and altered bone response to CaSO4 observed in this study. Important advantages of the OVX rat caudal spine are the rapid and minimally invasive surgical exposure of the vertebral body and the ease of cement injection. We propose that the OVX rat caudal spine represents a valuable and cost-effective tool in the armamentarium of investigators evaluating biomaterials designed for implantation into the osteoporotic spine.

Effects of spinal cord injury on osteoblastogenesis, osteoclastogenesis and gene expression profiling in osteoblasts in young rats

INTRODUCTION

Spinal cord injury (SCI) causes a significant amount of bone loss in the sublesional area in animals and humans, and this type of bone loss is different from other forms of osteoporosis such as disuse osteoporosis and postmenopausal osteoporosis. However, no data is available on the cellular and molecular changes of osteoblastogenesis and osteoclastogenesis during SCI-induced bone loss.

METHODS

SCI and SHAM rats were used in this study to investigate osteoblastogenesis and osteoclastogenesis in bone-marrow culture. We also measured bone mass and bone histomorphometry, as well as the expression of alkaline phosphatase (ALP), core binding factor alpha1 (Cbfa-1), osterix, receptor activator of NF-kappaB ligand (RANKL) and osteoprotegerin (OPG) in osteoblast-like cells in bone-marrow culture obtained from SCI and SHAM rats.

RESULTS

Bone mineral density (BMD) measurement showed serious bone loss in the tibial ephiphyses and metaphyses of SCI rats compared with SHAM rats. In addition, bone histomorphometry analysis of the tibial metaphyses of SCI rats demonstrated that bone microarchitecture in SCI rats deteriorated further than in SHAM rats, and increased eroded surfaces and bone formation rates were observed in SCI rats. The number of osteoclasts that developed from bone marrow of SCI rats at equal density was significantly increased compared with SHAM rats, and the area of the resorption pits formed in the bone marrow culture from SCI rats was significantly greater than SHAM rats, whereas the number of CFU-F and CFU-OB was similar in both groups. RANKL mRNA and protein levels in osteoblast-like cells in culture obtained from SCI rats were significantly higher than those from the SHAM rats, whereas OPG levels decreased slightly. The ratios of RANKL to OPG expression in SCI rats were significantly higher than those in SHAM rats. However, osteogenic gene profiling of Cbfa-1, ALP and osterix in SCI rats remained similar with SHAM rats.

CONCLUSION

These changes favor increased osteoclast activity over osteoblast activity, and may explain, in part, the imbalance in bone formation and resorption following SCI.

The effect of reloading on bone volume, osteoblast number, and osteoprogenitor characteristics: studies in hind limb unloaded rats

Skeletal unloading during space flight results in bone loss. In astronauts the extent to which bone is lost varies greatly between different bones of the skeleton as well as between different individuals. Following return to earth, recovery of bone mass during reloading also varies between different bones and different individuals. Due to this variability and the limited number of subjects it is difficult to study the effects of unloading/reloading on bone in humans. A viable alternative is to use the rat model of hind limb unloading developed at NASA. We have previously demonstrated that, in 6-week-old male rats, 14 days of unloading result in a decrease in osteoprogenitor number in cell populations isolated from the proximal femur. The goal of the present study was to determine the number of osteoprogenitor cells present in cell populations derived from the proximal femur of young rats after 14 days of unloading followed by 14 days of reloading and to characterize their proliferative capacity. To do this, we determined the number of alkaline phosphatase-positive colony forming units (CFU-AP) and osteoblast CFU (CFU-O). To establish whether the effects of unloading and reloading were specific for cells of the osteoblast lineage, we also determined the number of fibroblastic CFU (CFU-F). Effects on proliferation were evaluated by measuring the size of CFU-O. Unloading resulted in a 66% reduction in CFU-AP. CFU-O numbers were decreased by 76% and mean colony size was 33% less than controls. The decrease in osteogenic and osteoprogenitor cells in vitro paralleled the decrease in bone volume (- 50%), osteoblast number (- 35%), and bone formation rate (- 46%) observed in the proximal tibial metaphysis of unloaded rats. Unloading had no effect on osteoclast number or surface. Subsequent reloading for 14 days restored CFU-AP. CFU-O numbers were only partially restored at 14 days (83% of controls) but nodule size was 1.2-fold greater than controls. Neither unloading nor reloading had an effect on the total number of progenitors (CFU-F). Reloading restored bone volume back to control values, but osteoblast number and bone formation rate were still lower than those in corresponding controls. Both osteoclast number and surface were lower in reloaded animals than in age-matched controls. Our results indicate that 14 days of unloading result in a decrease in osteoprogenitor number and that reloading for 14 subsequent days completely restores CFU-AP and bone volume to control levels, while the number of CFU-O in vitro and osteoblasts in vivo were partially recovered but still lower than corresponding controls. Strikingly, osteoclastic bone resorption after 14 days of reloading was greatly reduced compared to controls, suggesting a significant contribution of this to the recovery process.

Effect of simulated weightlessness on osteoprogenitor cell number and proliferation in young and adult rats

Experiments with rats flown in space or hind limb unloaded (HU) indicate that bone loss in both conditions is associated with a decrease in bone volume and osteoblast surface in cancellous and cortical bone. We hypothesize that the decrease in osteoblastic bone formation and osteoblast surface is related to a decrease in the number of osteoprogenitors and/or decreased proliferation of their progeny. We tested this hypothesis by evaluating the effect of 14 days of HU on the number of osteoprogenitors (osteoblast colony forming units; CFU-O), fibroblastic colony forming units (CFU-F), and alkaline phosphatase-positive CFU (CFU-AP) in cell populations derived from the proximal femur (unloaded) and the proximal humerus (normally loaded) in 6-week-old and 6-month-old rats. To confirm the effect of unloading on bone volume and structure, static histomorphometric parameters were measured in the proximal tibial metaphysis. Effects of HU on proliferation of osteoprogenitors were evaluated by measuring the size of CFU-O. HU did not affect the total number of progenitors (CFU-F) in young or adult rats in any of the cell populations. In femoral populations of young rats, HU decreased CFU-O by 71.0% and mean colony size was reduced by 20%. HU decreased CFU-AP by 31.3%. As expected, no changes in CFU-O or CFU-AP were seen in cell populations from the humerus. In femoral cell populations of adult rats, HU decreased CFU-O and CFU-AP by 16.6% and 36.6%, respectively. Again, no effects were seen in cell populations from the humerus. In 6-week-old rats, there was a greater decrease in bone volume, osteoblast number, and osteoblast surface in the proximal tibial metaphysis than that observed in adult rats. Both trabecular thickness and trabecular number were decreased in young rats but remained unaffected in adults. Neither osteoclast number nor surface was affected by unloading. Our results show that the HU-induced decrease in the number of osteoprogenitors observed in vitro parallels the effects of HU on bone volume and osteoblast number in young and old rats in vivo, suggesting that the two may be interdependent. HU also reduced CFU-O colony size in femoral populations indicating a diminished proliferative capacity of osteoblastic colonies.