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Osteo-arthritis is a condition in which there is wear and tear of the smooth bearing surfaces of a joint. Initially this wear may not give rise to symptoms. Ultimately however, pain, stiffness, and even deformity may result. A range of treatments, from anti-inflammatory tablets through to replacement, are available to help in the various stages of this condition.
What is Arthritis?
The term 'Arthritis' literally means 'inflammation of a joint'.
There are many causes of this but, the end result of all of
these processes, is a joint where the smooth, low friction
bearing surfaces on the ends of the bone become worn out.
This special surface (made of hyaline cartilage) is the same
shiny white surface that can be seen on the end of a lamb
(or other animal) bone, and is responsible for the extremely
low friction bearing surfaces of our major joints (about ten
times less friction than the best man made bearing surface).
Wear or damage to this is generally known as 'osteo-arthritis'
and may be primary (where the surface just wears out
prematurely - i.e. degenerative), or secondary (where the
damage is caused by direct injury, inflammatory conditions
such as rheumatoid arthritis, infections and so on).
Arthritis secondary to injury
A major joint injury, where the surface is directly damaged
(as happens in at least 10% of anterior cruciate ligament
injuries), is essentially a seed for the onset of osteo-arthritis
proper. The damaged hyaline cartilage not only doesn't make
any attempt to heal itself, but rather, the area of damage
slowly deteriorates with time, gradually enlarging.
In that initial injury, a piece of the lining tissue (hyaline
cartilage) may be exploded out of the weight bearing surface
creating a pot hole. That pot hole then, like all pot holes,
gets gradually bigger because of the inevitable breakdown
of the edges. Eventually, the weight bearing part of the joint
becomes a large area of bare bone with no bearing surface
at all and, this in turn, wears out its counterpart, the surface
which it articulates with on the other side of the joint. This
goes on to become advanced osteo-arthritis and, as might
be expected, it is hastened by impact loading type activities
such as running and jumping.
Primary or Degenerative arthritis
Degenerative osteo-arthritis, rather than anything else, has
to do with inheriting a joint with a short 'use by' date. No
one knows why some people's joints wear out early and why
some do not, albeit that the pathologic changes that occur
are somewhat understood. The thing about degenerative
arthritis is that it is not caused by sport, exercise or physical
jobs. Indeed, there is no evidence that marathon runners'
knees wear out any quicker than couch potatoes'. In fact,
Knee Arthritis
A normal right knee Arthritis following knee injury
The gap between the bone ends is not a gap, but rather, it is the
thickness of the articular surface (hyaline cartilage) which cannot
be seen on an x-ray
This knee shows normal gaps
No gap at all indicates bone on bone arthritis. That is, there is no remaining lining (bearing surface)
and they do cause increased pressure to be applied to part
of the joint, leading to premature wear. A straight legged
person places 60% of their weight on the inside (medial
compartment) of their leg, and 40% on the lateral. That is
what the joint is designed to take. A bow legged (varus) person
places more than 60% of his weight on the inside half of the
joint, whereas a knock kneed (valgus) person places more
on the outside half of the joint. In each instance, one side of
the joint becomes overloaded and the other side becomes
unloaded or even unused. This means that the stresses on
the overloaded side of the joint are higher than normal, and
hence, premature wear may occur in that half of the joint.
This, in association with loss of meniscal function, can be
devastating for the joint in the longer term.
A common scenario is that there is degenerative breakdown
of the meniscal cartilage, presenting as a painful knee, often
of sudden onset. Degenerative tears of the menisci can be
very painful, they do not heal by themselves and, for the
most part, cannot be successfully repaired. Treatment, by
removal of the torn parts of that cartilage (partial or total
menisectomy), usually relieves the pain but, it also leads to
increased pressure on the articular or lining cartilage because
the stress is now focused on a smaller area, hence, wear of
the lining surface or osteo-arthritis may occur.
Without any deformity this increased pressure on the ends
of the bone may not be all that significant but, if there is also
Other causes of wear stem from overload of the hyaline
cartilage lining, rather than from direct injury. Such abnormal
forces are commonly seen after menisectomy, where a
meniscal cartilage is removed for a tear. Particularly important
with this, is the effect of any mal-alignment of the leg, which
can also change the forces within the joint.
Meniscal cartilage loss
The menisci (meniscal cartilages - sometimes known as 'the cartilages') function as fillers to spread the load between the surfaces of the femur and tibia. The ends of these bones are not the same shape, and thus, the menisci are needed to make up for that incongruity. They primarily function somewhat like shock absorbers but they also have a secondary role to enhance lubrication and nutrition of the articular or lining cartilage.
Loss of a meniscus (particularly the lateral one) leads to a poor spread of weight across the joint surface. This means that loads are taken over smaller areas of the joint, and hence, pressures are higher, causing increased rates of wear of the lining surface (the hyaline cartilage). It also follows, that the more meniscus that is lost, the faster that wear occurs.
Mal-alignment of the leg
Another cause of increased wear is a mal-aligned (crooked)
leg, such as a knock knee deformity (valgus) or a bow leg
(varus) deformity. These are not uncommon in the community
After loss of a meniscus, it can be seen that the lateral (outside) part of the knee has higher contact pressures because of the convex on convex structure of that part of the joint. Hence, wear in the lateral (outside) compartment of the knee develops more rapidly than it does in the medial (inside) compartment of the knee, where the joint is convex on concave. This wear (osteo-arthrits) ultimately leads to the end of impact loading type sports, including all running. Ultimately, it may also lead to knee replacement.
In the normal knee, the meniscus is a mobile structure that makes the joint spaces congruous and spreads the load over a wide area of the joint lining (articular surface). By increasing the area of distribution of the contact force, the local pressures are reduced and wear is prevented.
Loss of meniscus causes high pressures to be experienced at the point of contact of the femur and tibia. This pressure, especially at the time when impact loading is occurring, can exceed the breakdown strength of the lining of the joint (hyaline cartilage). This causes breakdown (wear) of that lining which, in essence, is osteo-arthritis, and which, once begun, will become progressive with time.
Meniscus filling in the gaps between the bone ends
Meniscus removed leading to point loading within the joint
Valgus (knock kneed) knees, which have gone on to lateral compartment osteo-arthritis. As the lateral side wears out, the bones on that side get closer togerther and hence the valgus deformity gets worse. This increases the load on that side of the knee even further and increases the rate of wear.
The 2 lines crossing the centre of the knee are the line from the centre of the hip and the line to the centre of the ankle. These should be in line. The angle between them is the angle of deformity.
an underlying deformity, then this extra increase in pressure
may become very significant. The combination of a varus leg
(bowed) and a resected or non-functioning (torn) medial
meniscus will always lead to arthritis on the inside part of the
knee (medial compartment arthritis). The time scale for this
depends on the amount of remaining meniscal function, the
degree of bowing of the leg, and the activity of the person.
Loss of the outside meniscus, associated with a valgus (knock
kneed) leg, leads to progressive osteo-arthritis of the outside
of the knee (lateral compartment arthritis) and, in general, this
progresses more rapidly than its medial counterpart because
of the relatively more important role of the lateral meniscus
compared to the medial. It is to be noted in passing that,
the commonest cause of retirement from elite sport in the
world today, is a lateral meniscal tear that requires resection,
leading to premature osteo-arthritis.
In the above cases, the arthritis that develops in a mal-
aligned leg following meniscectomy (or meniscal failure), is
likely to progress to the extent that, it will be visible on plain
X-rays within 10 years from the time of the original surgery.
Depending on the above factors however, this can be as
quick as a few months.
Progression of disease occurs as the wear increases, because
the deformity increases. As the wear causes the lining on the
ends of the bone to become thinner, so the bones on that
side of the knee become closer together. This then causes an
increase in the deformity which, in turn, causes increased
force to be exerted on that side of the knee, and hence, an
increased rate of wear. Essentially therefore, the problem
spirals. The worse it gets, the more rapid the progression.
When this progression becomes apparent, if the other side of
the knee is normal, and the patient is young, the treatment of
choice may be to re-align the leg: to put the weight through
the other side of the knee, thus sparing the worn side (rotating
tyres if you will). This is called osteotomy (cutting the bone).
What are the symptoms of arthritis?
Initially there may be no symptoms at all. Eventually however,
pain and swelling do become a feature. Sometimes this just
occurs gradually with no particular starting event. Other
times however, an injury may activate the arthritis, such
that a damaged or worn out joint which previously gave
little or no trouble, may suddenly start to be painful and
give considerable trouble. Frequently, the accident or injury
that initiates this is relatively minor and causes very little
new damage to the joint itself. For some reason however,
an arthritic joint becomes intolerant of injury and can be
rendered symptomatic through even a small event.
As the arthritis becomes more and more progressive, the
joint tightens up and the range of motion frequently starts
to become restricted. Initially there may be just a slight
inability to straighten the knee. With time however motion
is lost at both ends of the range, with both an inability to
fully straighten (extend) the knee, and an inability to fully
bend (flex) the knee, becoming apparent. In addition to
this, uneven wear may occur as described above, causing
the knee to become progressively valgus (knock kneed) or
progressively varus (bow legged).
Frequent symptoms of arthritis, are pain and stiffness after rest.
First thing in the morning therefore, the knee may not want to
work too well, but it warms up with use. It is also helped by
local heat, such as when in the shower. In contradistinction,
it is worse in cold wet weather. As the arthritis progresses,
the periods of flare up get worse, and closer together, until
it starts to hurt all the time, frequently disrupting sleep. In
addition, function starts to deteriorate such that walking
distance gradually reduces, ultimately to 100m or less.
Why does arthritis hurt?
Nobody really understands exactly why arthritis hurts. There
are some nerves in the ends of the bone and, potentially, these
may hurt. If this is the main cause of the pain however, then
Centre of knee
To centre of hip
To centre of ankle
This line from the centre of the hip
to the centre of the ankle should pass through the centre of the knee. This is the centre of weight