Introduction:

There is a great need for suitable large animal models that closely resemble osteoporosis in humans, and that they have adequate bone size for bone prosthesis and biomaterial research. Previous investigations have shown that osteoporotic sheep model requires glucocorticoid (GC) treatment for a long period of time after ovariectomy (OVX) to induce osteoporosis (1). However, no information in literature is available whether osteoporosis (OP) in sheep can be induced by application of GC alone. This study aimed to investigate effects of GC alone without OVX on three-dimensional (3-D) microarchitectural properties and mechanical properties of sheep cancellous bone after a 7 months steroid treatment; and thus to validate a large animal model for orthopaedic implant/biomaterial research.

Materials and Methods:

Eighteen female sheep were randomly allocated into 3 groups: group 1 (GC-1) received GC (0.60mg/kg/day methylprednisolone) 5 days weekly for 7 months; group 2 (GC-2) received the same treatment regime for 7 months, and further observed for 3 months without GC; and group 3 served as the control group, and left untreated for 7 months. The sheep were housed outdoors, and received restricted diet, i.e. grass pellets (0.55% calcium and 0.35% phosphorus) and hay.

After sacrifice, cancellous bone specimens from 5^th lumbar vertebra, distal femur and proximal tibia were micro-CT scanned (vivaCT 40, Scanco Medical AG., Switzerland) to quantify their 3-D microarchitecture(2), and then bone samples were tested compressively (MTS Systems Co., USA) to determine their mechanical properties. At sacrifice, serum samples were collected, and biomarkers for bone formation (Osteocalcin) and resorption (Crosslaps) were determined. The results were analyzed statistically. One-way ANOVA were performed, A p-value <0.05 was considered significant.

Results:

After 7 months of GC treatment. Cancellous bone volume fraction of the 5^th lumbar vertebra in the GC-1  group was reduced by -35%, trabecular thickness by -28%, and changed from typical plate structure to a combination of plate and rod structure with increased connectivity by 202% (Fig 1 & Table 1). Bone strength was reduced by 52% (Table 1). Bone formation marker, serum osteocalcin of GC-1, was reduced by 71% at 7 months, but recovered with an increase of 45% at 10 month in the GC-2 group (Fig 1). Similar trends were also seen in the femur and tibia. At 10 months, the GC-2 group had microarchitectural and mechanical properties similar to the level of the control sheep.

Discussion and Conclusion:

For the first time we have demonstrated that 7 month high-dose GC on bone density and microarchitecture are comparable with those observed in human after long-term GC treatment. Moreover, we have shown that the bone quality with regard to strength and microarchitecture recovers after 3 months further observation without GC. This suggests that a prolonged administration of GC is needed for long-term observation to keep osteopenic bone. Interestingly, osteocalcin was significantly reduced after 7 months but a rebound phenomenon was observed 3 months after cessation of GC.