Long-Term Response to a Bioactive Biphasic Biomaterial in the Femoral Neck of Osteoporotic Rats

Abstract

Osteoporosis often leads to fragility fractures of the hip resulting in impaired quality of life and increased mortality. Augmenting the proximal femur could be an attractive option for prevention of fracture or fixation device failure. We describe a tissue engineering based strategy to enhance long-term bone formation in the femoral neck of osteoporotic rats by locally delivering bioactive molecules; bone morphogenic protein-2 (rhBMP-2) and zoledronic acid (ZA) using a calcium sulphate/hydroxyapatite (CaS/HA) biomaterial. A defect was created by reaming the femoral neck canal of osteoporotic (OVX) rats and they were treated as follows: G1. Empty, G2. CaS/HA, G3. CaS/HA + Systemic ZA, G4. CaS/HA + Local ZA and G5. CaS/HA + Local ZA + rhBMP-2. Bone formation was evaluated 6 months after treatment. Furthermore, radioactively labelled 14 C-ZA was used to study the bioavailability of ZA at the defect location, which was determined using scintillation counting. Micro-CT indicated significantly higher bone volume (BV) in groups G4 and G5 compared to the other treatment groups. This was confirmed qualitatively by histological assessment. Addition of rhBMP-2 gave no additional benefit in this model. Local delivery of ZA performed better than systemic administration of ZA. Mechanical testing showed no differences between the groups, likely reflecting that the addition of bioactive molecules had limited effect on cortical bone or the choice of mechanical testing setup was not optimal. Scintillation counting revealed higher amounts of 14 C-ZA present in the treated leg of G4 compared to its contralateral control and compared to G3 indicating that local ZA delivery can be used to achieve high local concentrations without causing a systemic effect. This long-term study shows that local delivery of ZA using a CaS/HA carrier can regenerate cancellous bone in the femoral neck canal and has clear implications for enhancing implant integration and fixation in fragile bone.

A combined fracture and mortality risk index useful for treatment stratification in hip fragility fractures

“…[26,27] Recently, Kok et al [15] reported that fixation could be improved with utilization of calcium sulfate/hydroxyapatite (CaS/HA) biomaterial injected through cannulated hip screws, increasing the pullout strength on synthetic bone models. Another recent in vivo study by Raina et al [28] found that the tissue engineering strategy of using CaS/HA ceramic biomaterial to locally deliver zoledronic acid (ZA) with or without recombinant human bone morphogenic protein-2 in femoral necks of osteoporotic rats had potential to regenerate cancellous bone in Nonetheless, there are several limitations to our study. Despite enrolling the same number of consecutive patients in each center and demographics, fracture and mortality rates being similar, as well as distribution of patients in different FAME categories, we consider selection bias and heterogeneity of patients as limitations related to the retrospective design.…”

“…With further developments in this field, modifications to the FAME index can be done to increase its ability to identify suitable patients for inclusion in registry-nested RCT trials. [27,28] In conclusion, the FAME index appears to be a useful stratification tool for allocating patients in a RCT studying augmentation of hip fragility fractures. Further well-designed, large-scale, prospective RCTs are warranted to draw more reliable conclusions on this subject.…”

An examination of two different approaches for the study of femoral neck fracture: Towards a more relevant rodent model

“…For these reasons, several groups have exploited the loading of osteoporotic bones in a constrained configuration. 8,11,14–16,22 These have ranged from loading the distal femur epiphysis in osteopenic rats, 23 to axial loading of the femoral neck of osteoporotic rats 9 and to measurement of the biomechanical properties of mouse femoral neck in a constrained axial load model. 8,11,14,15,18 There are also reports of modified biomechanical properties in the femoral neck following in vivo treatment with exercise or Vitamin D deficiency when loaded to failure.…”

“…9,11,14,15 They have also been used to evaluate whether biomaterial implants improve defect healing in the rat femoral neck, with increases in bone volume in the absence of any changes in biomechanical parameters reported. 16,17 Thus, it appears that constrained loading methods allow for precise and reproducible measurement of murine femoral neck mechanical properties, but to-date these have remained somewhat limited in their assessment of the effects of ovariectomy. 8,9,18…”

Bone mineral as a drug-seeking moiety and a waste dump

“…Radioactive active ZA was measured in the synthetic HA particles as well as in the contralateral femur when the animals were euthanized at six months. 29 It appears that bisphosphonates not only bind to the mineral, but remain bound in the bone for an extended period, potentially for years. The drug is eventually released in small amounts by osteoclast activity, only to be recirculated and rebound into vesicles that precipitate into new calcium phosphate mineral.…”