The annulus is the outer portion of the disc that surrounds the nucleus. It is made up of 15 to 25 collagen sheets which are called the 'lamellae'. The lamellae are 'glued' together with a proteoglycans. These sheets encircle the disc and, in concert with the nucleus, give the disc tremendous axial load strength.
The posterior portion of the annulus if further strengthened by the 'posterior longitudinal ligament'. This structure is the final barrier between the disc and the delicate spinal cord, and nerve roots.
The biochemical make up is similar to that of the disc only in different proportions. The annulus is 65% water, with the collagen, both type I and II making up 55% of the dry weight, and proteoglycans (mostly the larger aggregate type - 60%) making up 20% of the dry weight. 10% of the annulus also contain 'elastic fiber' that are seen near where the annulus attaches into the vertebral end-plate. …show more content…
The very outer lamellae are almost all Type I. As we move inward toward the nucleus the more Type II is seen and less Type I. The very inner layers are very hard to distinguish from the nucleus. There is not a clear boundary between the nucleus and the annulus.
A simply amazing fact about the lamellae design is that the collagen fibers that make-up each lamellae all run parallel at a 65 degree angle to the sagittal plane. Even more amazing is the fact that the each lamellae are flipped so that the 65 degree angle alternates between every lamellae, one to the right then one to the left. This design greatly increases the shear strength of the annulus and makes it had for cracks to develop through the layers of the annulus. This is just amazing if you think about it!! Brilliant
The posterior portion of the annulus if further strengthened by the 'posterior longitudinal ligament'. This structure is the final barrier between the disc and the delicate spinal cord, and nerve roots.
The biochemical make up is similar to that of the disc only in different proportions. The annulus is 65% water, with the collagen, both type I and II making up 55% of the dry weight, and proteoglycans (mostly the larger aggregate type - 60%) making up 20% of the dry weight. 10% of the annulus also contain 'elastic fiber' that are seen near where the annulus attaches into the vertebral end-plate. …show more content…
The very outer lamellae are almost all Type I. As we move inward toward the nucleus the more Type II is seen and less Type I. The very inner layers are very hard to distinguish from the nucleus. There is not a clear boundary between the nucleus and the annulus.
A simply amazing fact about the lamellae design is that the collagen fibers that make-up each lamellae all run parallel at a 65 degree angle to the sagittal plane. Even more amazing is the fact that the each lamellae are flipped so that the 65 degree angle alternates between every lamellae, one to the right then one to the left. This design greatly increases the shear strength of the annulus and makes it had for cracks to develop through the layers of the annulus. This is just amazing if you think about it!! Brilliant