The vertebral endplate

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The vertebral endplate forms a structural boundary between the intervertebral disc and the cancellous core of the vertebral body. Comprised of a thin layer of semi-porous subchondral bone, approximately 0.5 mm thick, with an overlying cartilage layer of similar thickness, the principal functions of the endplate are to prevent extrusion of the disc into the porous vertebral body, and to evenly distribute load to the vertebral body. With its dense cartilage layer, the endplate also serves as a semipermeable interface, which allows the transfer of water and solutes but prevents the loss of large proteoglycan molecules from the disc. Finally, the dense subchondral bone of the endplate provides secure anchorage for the collagen network of the intervertebral disc.

The thickness of the endplate varies, with thicker bone found under the annulus than adjacent to the nucleus. The superior endplate is generally thinner than the inferior endplate. A positive correlation between the thickness of the endplate and the proteoglycan content of the disc has been shown, especially for the central endplate under the nucleus. This may be the result of a remodeling process whereby the endplate responds to a greater hydrostatic pressure in discs with higher proteoglycan content [32]. Therefore it is possible that the changes associated with aging and disc degeneration could result in a weakening of the adjacent endplate.

The local material properties of the endplate demonstrate a significant spatial dependence. Grant et al. [9] have shown that the strength and stiffness of the endplate are highest posterolaterally and lowest in the center of the endplate (Fig. 2). Sacral and inferior lumbar endplates are

Fig. 2 Spatial distribution of endplate material properties, normalized to maximum values measured. Endplate strength is greatest towards the posterolateral and lowest at the center of the endplate. Regional variation in endplate properties is more pronounced with decreasing bone mineral density. (Adapted from [9])

Cervical Vertebrae Labeled

Fig. 2 Spatial distribution of endplate material properties, normalized to maximum values measured. Endplate strength is greatest towards the posterolateral and lowest at the center of the endplate. Regional variation in endplate properties is more pronounced with decreasing bone mineral density. (Adapted from [9])

stronger than superior lumbar endplates, which may indicate an increased fracture risk in the aging spine for the superior endplate. The importance of the endplate for load transfer and the overall structural integrity of the vertebra has been highlighted in laboratory experiments which have shown a significant reduction in the local structural properties of the vertebral body following partial endplate removal [27]. Similar experiments have provided support for the hypothesis that the strength of the central endplate region decreases with increasing disc degeneration due to remodeling, and that logically the overall strength of the endplate decreases with decreasing BMD [10]. With decreasing BMD, the regional variation in material properties becomes more pronounced, which likely plays a significant role in the initiation of the endplate fractures which are a characteristic of the aging spine.

Of particular relevance for the aging spine, the morphology of fatigue fractures of lumbar motion segments has been investigated in laboratory experiments [11, 12]. Under repetitive cyclic loading designed to simulate vigorous physical activity, the weakest part of the vertebral body was shown to be the endplate; failure often occurred after only several hundred cycles. Two main types of fatigue failure occurred, both involving the endplate and the adjacent subchondral cancellous bone of the vertebral body. Fracture morphology was weakly correlated with disc degeneration grade. The development of Schmorl's nodes -the local extrusion of disc material through the endplate -was most often seen with normal intervertebral discs. Central endplate fractures were associated with moderately degenerated discs. In some cases crush or burst fractures were observed. These occurred always on the first loading cycle and were seen in specimens with low BMD.

Deformity of the vertebral body with aging is closely related to BMD loss, i.e., osteoporosis. With aging, increased concavity of the vertebral endplate is seen together with a loss of BMD [36]. The typical loss of stature, often attributed to disc thinning, is more likely a consequence of a fairly normal disc migrating into this concavity. Endplate fracture is significant in the initiation of vertebral body collapse, but is difficult to diagnose from conventional morphometric assessment of spinal osteoporosis; up to 80% of all endplate fractures are missed by conventional diagnostic radiography [21]. However, Schmorl's nodes, which generally evolve from significant traumatic events, are easily recognized on magnetic resonance imaging (MRI) as either a characteristic extrusion of disc material, or as a localized edema in the vertebral body adjacent to the fracture [39].

In contrast to the thinning of the endplate and increased fracture risk often observed with aging, endplate sclerosis with aging has also been reported and can be so substantial as to bias normal measurements of vertebral body bone density [29]. Ossification of the overlying cartilaginous layer has been observed with aging [31]. Localized calcification directly influences the permeability of the endplate, and it has been shown that this may lead to a potential reduction in the volume of fluid exchanged to the disc during daily activity, resulting in a disruption of the nutritional supply to the disc and possible dehydration of the disc [3, 8]. Degenerative changes to the disc are extremely important factors determining the function of the elderly spine, as is outlined below.

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