Abstract
Intervertebral disc herniation and internal disc disruption are both thought to be primarily mechanically based pathologies. Although several studies have previously disrupted discs in vitro, none have examined the resulting disruptions microscopically.
The technique of nuclear pressurization was used to mechanically disrupt ovine lumbar motion segments. A hollow injection screw was inserted longitudinally through the inferior vertebra of each motion segment, so that the injection screw’s tip was located in the centre of the nucleus. Through this screw, a radio-opaque gel was gradually injected into each segment’s nucleus until failure occurred, marked by a large drop in nuclear pressure, or focal change to the disc’s periphery. Following mechanical testing, the internal failure characteristics of each motion segment were assessed using micro-CT and microscopy. During nuclear pressurization, motion segments were held in one of four postures:
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0° flexion,
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7° flexion,
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10° flexion, or
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7° flexion plus 2° axial rotation.
Group I (0° flexion; n=12): Discs failed at a mean nuclear pressure of 13.2±2.1MPa. In most cases failure occurred in a diffuse manner via sequential circumferential tears within the posterior annulus. Group II (7° flexion; n=17): Discs failed at a mean nuclear pressure of 11.2±2.5MPa. Compared to the Group I discs, 7° flexion led to the creation of radial tears extending through the central posterior disc wall. Two types of radial tear occurred: mid-axial and annular-endplate. Mid-axial radial tears were confined to the annulus. Annular-endplate radial tears incorporated both annular and endplate failure; endplate failure in these tears always occurred adjacent to the mid-annulus at the cartilaginous/vertebral endplate junction. Group III (10° flexion; n=17): Discs failed at a mean nuclear pressure of 9.8±2.6MPa. Compared to the Group II discs, 3° of additional flexion increased the proportion of annular-endplate radial tears. Group IV (7° flexion + 2° axial rotation; n=25): Discs failed at a mean nuclear pressure of 7.9±2.4MPa. Compared to the Group II discs, the addition of 2° axial rotation significantly decreased the nuclear pressure at which discs failed, and reduced the occurrence of mid-axial radial tears.
Postures that reduced the disc wall’s ability to withstand high nuclear pressures were associated with an increase in the proportion of disc failures that incorporated tears of the cartilaginous endplates, specifically at the cartilaginous/vertebral endplate junction adjacent to the mid-annulus. The robustness of this junction appears to be intimately linked to the robustness of the disc wall.
Correspondence should be addressed to: Associate Professor N. Susan Stott, Orthopaedic Department, Starship Children’s Hospital, Private Bag 92024, Auckland, New Zealand.
