The bending human spine

Elegant and strong as the human back is, the job of bending and straightening is a tall order. The trunk and spinal muscles which actively control the movement are discussed further on. However, several other anatomical features help make bending possible, by working as a physical brake to control the free fall of the spine when it tips forward.

The first of these we have discussed already: the strong fibrous wall of the disc which holds the cotton reels together. This contributes about 29 per cent to the control of the segments going forward.

Figure 1.19 Unlike the discs the facet joints are wired for pain by the medial branch of the lumbar dorsal ramus.

(Illustration acknowledgement: N. Bogduk, 'Clinical Anatomy of the Lumbar Spine')

Figure 1.19 Unlike the discs the facet joints are wired for pain by the medial branch of the lumbar dorsal ramus.

(Illustration acknowledgement: N. Bogduk, 'Clinical Anatomy of the Lumbar Spine')

As the segments glide forward the stiff fibrous mesh of the wall retards the initial movement. When the spinal segments then tip forward and the back of the interspaces opens up, the same diagonal mesh pulls up, like stretching up a garden lattice.

Figure 1.20 As the spine bends, the facet surface of the upper vertebra slides up the stop-ramp of the one below, increasing the tension between the two segments and bringing movement to a halt.

However, even more important in controlling bend are the various structures of the back compartment. The bony facets contribute in two ways: a sloping stop-ramp, made by the joint surfaces and extremely tough capsular ligaments. When viewed from the side, the lower facet surfaces taper upwards towards the front of the spine. As the spine bends this means the upper vertebrae must travel uphill as they go forward. (They work like emergency stop-ramps beside steep downhill sections of highways, gradually bringing the vehicle to a halt as it nears the top of the ramp.)

Figure 1.21 The capsular tension of both the facet capsules (known as the 'capsular ligaments') and ligamentum flavum holds the facet surfaces pressed together and restrain the back of the spine opening as we bend.

In the case of the back, the escalating tension of the soft tissues gradually brings the upper vertebra to a halt, by which time the facet interfaces are firmly locked against each other and the ligamentum flavum and the facet capsule are tense at full stretch. It is a marvellously ingenious system with both bone and soft tissue complementing the workings of each other.

As we go further into a bend the upper vertebra then tips bodily forward by pivoting on the front edge, as the tail of the vertebra attempts to lift away. This second part of the movement is restrained mainly by the facet joint capsules. They contribute a powerful 39 per cent towards moderating bending. The ligamentum flavum contributes an initial 13 per cent. All up, the facets contribute 52 per cent of the ligamentous restraint on forward bending.

Figure 1.21 The capsular tension of both the facet capsules (known as the 'capsular ligaments') and ligamentum flavum holds the facet surfaces pressed together and restrain the back of the spine opening as we bend.

It is very significant that none of these facet ligaments exerts control over the forward gliding action of the segments, only forward tipping. Control of forward gliding is important because it is this action carried out to excess (when it is called 'forward shear') which constitutes the unstable element of a segment's movement. All segments must avoid shear, because it is potentially so devastating. And it is failure to control the forward tipping which allows too much shear.

The process works like this: as bending starts, the initial forward glide is only about 2 mm before the facets engage to stop it. As the bend continues, the facets disengage with the tipping forward action which makes the tail of the vertebra lift up and away, leaving a gap between the two facet surfaces. However, once the tail of the vertebra is away, the whole vertebra can glide forward more, until the bony block engages once again. In this way, more tip allows more glide. Incidentally, the tipping action is what both the facet capsules and the ligamentum flavum are designed to resist, while multifidus, the deepest intrinsic muscle of the spine, pays out to control it actively.

Figure 1.22 The segments have scant ligamentous shoring to control forward shear but plenty to restrain forward tip. Controlling shear of the segments is thus controlled by restraining tip.

and more forward slide

Figure 1.22 The segments have scant ligamentous shoring to control forward shear but plenty to restrain forward tip. Controlling shear of the segments is thus controlled by restraining tip.

and more forward slide

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