An investigation of the mechanism of adjacent segment disease in a porcine spine model

BACKGROUND: Fusion changes the biomechanics of the spine leading to the potential development of adjacent segment disease. Despite many studies on adjacent segment disease, it is largely unknown how spinal fixation affects the mechanical properties of the adjacent disc. The purpose of this study was to assess whether axial compression causes mechanical disruption to the annulus when the caudal spinal level is immobilized or injured. METHODS: Fifty-two porcine spines were assigned to one of four conditions: 1) control; 2) injured (18.5-gauge needle inserted into the nucleus of cervical 4/5); 3) immobilized (18-gauge steel wire wrapped around the transverse and spinous processes of cervical 4/5); and 4) injured+immobilized. Each specimen was then subjected to 0.5 Hz cyclic compression (300-1200N) for two hours. Post-compression, three annular samples were dissected from the cervical 3/4 disc (adjacent to immobilized and/or injured level) and mechanically tested. The same loading protocol and annular testing was also conducted in eight human cadaveric lumbar spines. FINDINGS: Immobilization and injury resulted in a reduction in adjacent disc lamellar strength including toe region stress (p < 0.001), initial failure stress (p = 0.03), and ultimate stress (p = 0.004), with immobilization having the greatest impact. Similar findings were observed in the human cadaver samples with reduced toe region strength in the injured+ immobilized samples compared to the control (p = 0.049). INTERPRETATION: The current study provides empirical evidence of decreased lamellar strength in the disc adjacent to an immobilized and/or injured level following prolonged cyclic axial loading, lending mechanistic insight into the development of adjacent segment disease.