• Disc stiffening allows the facet capsules to tighten
• Disc narrowing causes the facet joint surfaces to override
• A sway back causes the lower facets to jam
• Weak tummy muscles can jam the facets
• A shorter leg invokes a greater restraint role of the facets
Disc stiffening allows the facet capsules to tighten
The earliest form of stiffness of a facet joint may be nothing more than a fleeting protective spasm of its overlying multifidus muscle. A fluke odd movement or an awkward posture for a while, and the muscle locks the joint temporarily to protect it. It is noticeable as a sore tight patch beside your spine which feels like a crimped link but which passes off within a day or two.
Stiffness of the neurocentral core at the front of the segment can translate across to the strong capsular ligaments at the back. Because of the strength and toughness of their fibres, these ligaments are the first to lose stretch as the mobility of the front compartment declines. Even before obvious loss of disc height, disc immobility can greatly reduce the freedom of the facets.
In their less yielding state the capsular structures are much more vulnerable to injury. Being repeatedly yanked by everyday movement amounts to micro-trauma so that more and more capsular fibres are torn. On a microscopic scale there is oozing of blood and lymph into the interstitial spaces (between the fibres) where it lies about and gradually solidifies. This is scar tissue or adhesions. As the capsule
becomes increasingly cobbled by adhesions it stiffens and the facet joint underneath loses play.
This augments the chain reaction fuelled by the facet joint as well. From now on, both the back and front compartments of the segment contribute to the poor freedom of the upper vertebra articulating on the lower one. As the intervertebral disc loses vitality the stiffness of the link can be felt from the outside. It is less yielding to pressure as you roll back and forth over it and often feels like a plug of cement in a rubber hose. (Sometimes you can feel a stiffer facet to the side of the spine by tipping from side to side.)
Disc narrowing causes the facet joint surfaces to override
A disc of normal height provides a natural 'clearance' for the working facet joints of the back compartment which dictates that they bear only approximately 16 per cent of load. As a disc loses fluid—and height—the upper facet surface can ride so far down the lower one that it digs into the neck of bone at the base, making it take up to 70 per cent of the load through the segment.
Facet joints are not designed to bear load on this scale and breakdown picks up apace. The lining membrane of the capsules secretes greater amounts of synovial fluid to keep up the embattled exit foramen is semi-occluded exit foramen is semi-occluded
narrowed disc upper facet surface moves down
Figure 3.7 When a disc loses height the facet surfaces override and their load-bearing increases greatly. This is the main cause of facet breakdown.
bony ring of lower vertebra narrowed disc upper facet surface moves down
Figure 3.7 When a disc loses height the facet surfaces override and their load-bearing increases greatly. This is the main cause of facet breakdown.
lubrication and sluice the joint interfaces clean. The synovial fluid also carries large phagocytic cells which surround and devour each tiny particle of cartilage, and in this way the joints are kept running as best they can. The greater the internal friction of the joints the more synovial fluid is pumped in, like tears flooding the eyes to get rid of grit, until eventually the excess fluid becomes a problem in itself.
The trapped fluid can cause pain, but the tension of the joint may also invoke reflex contraction (spasm) of the multifidus muscle which lies immediately over the top. As the muscle fibres shorten, the joint is held more firmly and compressed, increasing the pressure from the trapped fluid inside. Although this protective response has not been documented, I suspect it may account for the rapid alteration in the feel of a tense facet when it is touched by probing thumbs. The typical dome of capsular swelling can subside so quickly it feels as if a release valve has let the fluid escape. This may be multifidus letting go, allowing the joint to move freely, thus evacuating its fluid. I am always pleased by how quickly mobilising can bring this about and alleviate pain.
If the angle of the sacrum tips forward more than its average 50 degrees the spine is forced to hollow more as it arches back to the upright again. This causes inordinate wear of the lumbo-sacral facets. In some people the sacral angle can approach almost 90 degrees (with the sacral surface nearly vertical), and the two opposing surfaces of the L5-S1 facets remain permanently jammed to keep the spine hooked on to the sacrum. In effect the whole spine hangs on to the pelvis at these two bony hooks, like sash window catches, and this takes its toll. The temporary sway back of advanced pregnancy causes pain for a similar reason.
The facet joints are not designed for this sort of heavy-duty wear. The closedness of the joint surfaces is bad enough but their excessive grinding is much more destructive. With a normal lordosis the facets are in similar contact only when the spine is fully bent forward, although the posterior ligamentous lock, which comes into force when the back is fully rounded, shares some of the load. When an overly lordotic spine bends forward, the ligamentous lock cannot operate as effectively because the lumbar hollowing puts it on the slack (stressing again why it is so important to bend and lift with the back humped, not arched).
Excessive use of the facet stop-ramp puts the facets under all-day duress and abrades a continuous spume of cartilage off the joint surfaces. This gritty debris floats around in the joint space, acting as a micro-abrasive which scours down the residual cartilage surfaces even faster.
In extreme cases of lordosis, the upper facet surfaces override so far down the lower ones that the tips of the upper bony pillars come to rest at the base of the lower ones. The two fine prongs of bone projecting down from the vertebra above (the front of which provides the articulating surface) are no match for a disc when it comes to spreading load, and breakdown escalates.
Along with the bony changes, there can be marked soft tissue contracture of the joint capsules, in effect creating a bow-string which keeps the spine over-arched. As the fibres shorten and the roominess decreases, the joint surfaces become so close-packed they find it hard
to pull away from each other to let the spine hump forward. This greatly reduces the bending freedom of the spine.
The backward arching action of the spine can become even more limited as the upper bony tips dig into the base, even jacking the interbody joint open as the spine tries to arch. If there is simultaneous impact of the foot hitting the ground as the back arches, the bony ring below can break. We see this as stress fractures of the spine with fast bowlers in cricket.
With the spine resting long term on the facets rather than the disc, the vertebral body can demineralise by being stress-shielded from normal gravitational forces. This is thought to be one of the processes whereby the vertebral bodies become osteoporotic and undergo spontaneous crush fractures. The same process can also take place in the absence of extreme lordotic postures, simply through the disc losing height and shunting more load onto the facets.
When the sacral angle remains marked over a period, there is adaptive remoulding of the bone of the lower facet surface to create a bony impediment to the spine slipping forward. This is similar to the way an unstable joint sprouts more bone around its edges to keep
the facet in joint (see Chapter 6). Nature comes up with an ingenious way of making these joints more secure. A bar of bone forms across the lower facets to bolster their stop-ramp function, like bolting a steel bar across railway tracks. This minimises forward trespass of the spine on the sacrum.
Strangely enough, the occupations which cause most facet trouble put the back into the fully stooped posture rather than an over-arched one. Shearers and farriers often spend hours bent double, with their facets fully engaged at the top of the stop-ramp. Initially this causes a ligamentous strain by stretching the fibrous capsules of the facets. Later on it causes bony degeneration from the grinding of the facet surfaces perpetually locked up against one another to keep the spine hooked onto the sacrum.
Weakness of the tummy can bring about a similar lordotic effect—but at least it is more under your control. As the tummy muscles weaken they often passively lengthen at the same time. As they stretch, they allow the front of the pelvis to tip down, causing a pronounced
Figure 3.12 A weak abdominal wall allows the pelvis to dip down at the front which causes the low back to arch and the facets to over-engage.
hollow in the low back. This causes the lumbo-sacral facets to engage, spending most of their time working as stop-ramps to prevent the rest of the spine sliding forward down the sacrum.
This is part of the explanation for backache which goes with ordinary old fat-tummy obesity. As the tummy gets bigger and more weight is carried in front of the line of gravity, the lower back goes into an even deeper hollow as you over-arch backwards to balance the weight out front. As the sacrum tips down, the lower lumbar facets lock into apposition to keep the spine on the pelvis. Simple abdominal strengthening is very effective at decreasing lordosis and is an important part of treating this problem.
A shorter leg invokes a greater restraint role of the facets
Most of us have one leg shorter by a millimetre or two, which I believe is a common cause of back pain. A shorter leg places great strain on the low facets to hold the spine locked in place on the pelvis. The greater the discrepancy, the greater the diagonal sideways-and-forwards trespass of the spine down the sacrum.
The spine slips forward as well as sideways because the hip joint lies in front of the centre of gravity. Thus on the side of the shorter leg the pelvis dips down at the front as well as laterally on that side. The combination of the two aberrant tendencies causes great wear and tear on all lumbar facets but the lumbo-sacral in particular.
The mechanics of distortion are more complicated than first imagined because the vertebrae then rotate around a central axis of movement, as well as slipping diagonally across the sacrum. A new centre of movement comes into effect as the facet on the downhill side engages and the vertebra swings around this new pivot to twist further. All this makes for complicated movement of the low lumbar vertebrae when one leg is shorter.
Furthermore, the hip joint of the longer leg develops trouble too. It acquires a tightness at the front because that leg always stands with the knee bent. This drops down that side of the pelvis and equalises the sit of the sacrum. Permanent contracture of the front of the hip of the longer leg makes length of stride uneven. Because it cannot angle back
Figure 3.13 With a shorter right leg, the spine tends to shear forward incrementally and sideways to the right. As the right lumbo-sacral facet permanently engages, the spine swings around it as it twists to the right.
fully, the spine has to compensate while walking by twisting more to that side to even up the steps. With all of us taking thousands of steps per day, you can see the spine must repeatedly twist one way as we walk. Ultimately, there is a cocktail of irritants where it joins the sacrum.
It is not really possible to be pedantic about which facet will be the most painful at the lumbo-sacral level, although there is a rule of thumb that the facet on the side of the shorter leg will be painful in the early years while the side of the longer leg will be more painful later on.
With extreme discrepancies, the downhill facet ingeniously remoulds, similar to the way it does in an over-lordotic spine. In what are termed 'wrap-around bumpers', the lower facet surface develops bony outgrowths to stabilise the lateral trespass of the upper vertebra. Although these excess knobbles of bone can look frightening on X-rays, they are not indicators of pain.
This 'facet tropism' or asymmetry of the lumbar facets is an issue which causes great excitement in academic circles. However, I believe the high incidence of differing leg lengths explains the prevalence of tropism. It is the readiness of the spine to adapt to the anomalous sit of the sacrum (especially if there is excessive lordosis as well) which accounts for the dissimilarity between two paired joints.
Tropism has been unearthed by researchers who have been quick to point out its strong link to back pain. As a shop floor clinician, I see things from the other perspective: the facets become different to help reduce back pain when the legs are born unequal in length, rather than the facets themselves being different at birth and causing back pain. This might banish your despair on being told you have back pain because your lumbar facets are dissimilar. I see leg-length inequality as a highly significant factor in the genesis of back pain— although conventional wisdom decrees that differences of less than 2 cm are unimportant—a verdict I deplore.
Correcting leg length discrepancies by using a heel raise in the shoe of the shorter leg is an early and mandatory part of self-treatment. Only an approximate adjustment is necessary; it is better not to jack up the difference to the nearest millimetre. Since you have coped for so long with one leg shorter, the discrepancy should be minimised rather than fully corrected—or it simply adds another set of strains to the pre-existing ones.
As a result of the rotation involved, golf can often traumatise the facet joints because of their role in limiting spinal twist. As a right-handed golfer swings to the left, the row of facets down the right side of the lumbar spine butts up against each other like doors against a door jamb. The facets down the left side of the spine pull apart. The forcible closing on one side and wrenching open on the other can cause breakdown.
Serious golfers will cause less damage, and get a better swing, if they take the twist higher in their back into their thorax. From waist level up, the facet alignment is different and no longer stops the vertebrae swivelling. If golfers employ the 'flying elbows' technique of taking the twist in an arching spiral up the back, they will not only avoid damaging the facets but hit a better shot.
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