Background: Healing of segmental bone defects can be induced
experimentally with genetically modified osteoprogenitor cells, an ex vivo
strategy that requires two operative interventions and substantial cost.
Direct transfer of osteogenic genes offers an alternative, clinically
expeditious, cost-effective approach. We evaluated its potential in a
well-established, critical-size, rat femoral defect model.
Methods: A critical-size defect was created in the right femur of
forty-eight skeletally mature Sprague-Dawley rats. After twenty-four hours,
each defect received a single, intralesional, percutaneous injection of
adenovirus carrying bone morphogenetic protein-2 (Ad.BMP-2) or luciferase cDNA
(Ad.luc) or it remained untreated. Healing was monitored with weekly
radiographs. At eight weeks, the rats were killed and the femora were
evaluated with dual-energy x-ray absorptiometry, micro-computed tomography,
histological analysis, histomorphometry, and torsional mechanical testing.
Results: Radiographically, 75% of the Ad.BMP-2-treated femora showed
osseous union. Bone mineral content was similar between the Ad.BMP-2-treated
femora (0.045 ± 0.020 g) and the contralateral, intact femora (0.047
± 0.003 g). Histologically, 50% of the Ad.BMP-2-treated defects were
bridged by lamellar, trabecular bone; the other 50% contained islands of
cartilage. The control (Ad.luc-treated) defects were filled with fibrous
tissue. Histomorphometry demonstrated a large difference in osteogenesis
between the Ad.BMP-2 group (mean bone area, 3.25 ± 0.67 mm2)
and the controls (mean bone area, 0.65 ± 0.67 mm2). By eight
weeks, the Ad.BMP-2-treated femora had approximately one-fourth of the
strength (mean, 0.07 ± 0.04 Nm) and stiffness (mean, 0.5 ± 0.4
Nm/rad) of the contralateral femora (0.3 ± 0.08 Nm and 2.0 ± 0.5
Conclusions: A single, percutaneous, intralesional injection of
Ad.BMP-2 induces healing of critical-size femoral bone defects in rats within
eight weeks. At this time, the repair tissue is predominantly trabecular bone,
has normal bone mineral content, and has gained mechanical strength.
Clinical Relevance: Direct administration of adenovirus carrying
BMP-2 could provide a straightforward and cost-effective treatment for large
osseous defects with adequate surrounding soft-tissue support. This local in
vivo genetherapy approach avoids the cost and complexity of ex vivo methods
that require artificial scaffolds and autologous cell culture.