The effects of dexamethasone on bone fusion in an experimental model of posterolateral lumbar spinal arthrodesis.

OBJECT: The use of corticosteroid agents during the healing phase after spinal arthrodesis remains controversial. Although anecdotal opinion suggests that corticosteroids may inhibit bone fusion, such an effect has not been substantiated in clinical trials or laboratory investigations. This study was undertaken to delineate the effect of exogenous corticosteroid administration on bone graft incorporation in an experimental model of posterolateral lumbar fusion. METHODS: An established, well-validated model of lumbar intertransverse process spinal fusion in the rabbit was used. Twenty-four adult New Zealand white rabbits underwent L5-6 bilateral posterolateral spinal fusion in which autogenous iliac crest bone graft was used. After surgery, the animals were randomized into two treatment groups: a control group (12 rabbits) that received intramuscular injections of normal saline twice daily and a dexamethasone group (12 rabbits) that received intramuscular dexamethasone (0.05 mg/kg) twice daily. After 42 days, the animals were killed and the integrity of the spinal fusions was assessed by radiography, manual palpation, and biomechanical testing. In seven (58%) of the 12 control rabbits, solid posterolateral fusion was achieved. In no dexamethasone-treated rabbits was successful fusion achieved (p = 0.003). Tensile strength and stiffness of excised spinal segments were significantly lower in dexamethasone-treated animals than in control animals (tensile strength 91.4+/-30.6 N and 145.3+/-48.2, respectively, p = 0.004; stiffness 31.4+/-11.6 and 45.0+/-15.2 N/mm, respectively, p = 0.02). CONCLUSIONS: The corticosteroid agent dexamethasone inhibited bone graft incorporation in a rabbit model of single-level posterolateral lumbar spinal fusion, inducing a significantly higher rate of nonunion, compared with that in saline-treated control animals.

Stability of the craniovertebral junction after unilateral occipital condyle resection: a biomechanical study.

OBJECT: The authors sought to determine the biomechanics of the occipitoatlantal (occiput [Oc]-C1) and atlantoaxial (C1-2) motion segments after unilateral gradient condylectomy. METHODS: Six human cadaveric specimens (skull with attached upper cervical spine) underwent nondestructive biomechanical testing (physiological loads) during flexion-extension, lateral bending, and axial rotation. Axial translation from tension to compression was also studied across Oc-C2. Each specimen served as its own control and underwent baseline testing in the intact state. The specimens were then tested after progressive unilateral condylectomy (25% resection until completion), which was performed using frameless stereotactic guidance. At Oc-C1 for all motions that were tested, mobility increased significantly compared to baseline after a 50% condylectomy. Flexion-extension, lateral bending, and axial rotation increased 15.3%, 40.8%, and 28.1%, respectively. At C1-2, hypermobility during flexion-extension occurred after a 25% condylectomy, during axial rotation after 75% condylectomy, and during lateral bending after a 100% condylectomy. CONCLUSIONS: Resection of 50% or more of the occipital condyle produces statistically significant hypermobility at Oc-C1. After a 75% resection, the biomechanics of the Oc-C1 and C1-2 motion segments change considerably. Performing fusion of the craniovertebral junction should therefore be considered if half or more of one occipital condyle is resected.