An Investigation of the Mechanism of Adjacent Segment Disease in a Porcine Spine Model

This study investigated how spinal fixation and injury affect the mechanical properties of adjacent intervertebral discs, using porcine and human cadaveric models. Fifty-two porcine cervical spines were divided into control, injured, immobilized, and injured+immobilized groups, subjected to cyclic axial compression (300–1200 N, 0.5 Hz, 2 hours). Annular tissue samples from the adjacent disc (C3/4) were tested for intralamellar and interlamellar mechanical properties. Immobilization significantly reduced toe-region and failure stresses of the intralamellar matrix (p < 0.03), particularly in the posterior annulus, while peel strength between lamellae was unchanged. Similar reductions in toe-region stress were observed in human lumbar discs. These findings reveal how immobilization weakens annular integrity, providing a mechanistic basis for adjacent segment disease following spinal fusion.