by Matthew Law

Bones grow in length from the epiphyseal plate, and grow in circumference by bone being laid down by the membrane periosteum. The similar membrane at the end of bones is called the endosteum.
There are two types of cell in involved in bone growth: osteoblasts - which lay down bone; and osteoclasts - which resorb bone.
In early years of life, bone growth is more prominent. Total bone mass increases until ages of 25 - 35.
The process of bone growth relates to the maintenance of calcium levels in the blood.
In some pathological conditions, there is bone loss or bone deposition in excess of the norm.

Any stress applied to the periosteum will result in bone being laid down. Gravity is one such stress. This is known as Woolf's Law (after Julius Woolf). You can particularly see this in the trabechulae on the head of the femur.

The only diseases you'll see in the skeleton are those that affect it. Most disease affects soft tissue. Very few bone diseases are fatal, so it is not usually possible to determine the cause of an individual's death from their skeleton. An estimated 30% of bone diseases are joint diseases, 10% infections, 10% trauma, 35% dental diseases, and 10% include things like metabolic disease. Osteoarthritis (OA) predominates.

Joints that move conform to a similar pattern - they are synovial joints. There are two or more bones, the ends of which are called articular ends. These are capped by hyaline (which means featureless) articular cartilage, which lacks a blood supply, instead containing cells called chondrocytes which obtain nourishment by diffusion. Attached around the end of this cartilage is the synovium or synovial membrane, which is vascular. Its purpose is to secrete synovial fluid. Within this joint, movement is virtually frictionless. Sometimes there are other structures within the joint, e.g. the cruciate ligaments. Surrounding the joint is the joint capsule, which is fibrous and very strong. There are also ligaments. In some joints, there are fluid-filled sacks called bursae, which act as shock absorbers. Muscles insert into bone with a tendon. This point is called the enthesis. This is often involved in joint disease. The enthesis inserts into a bone via Sharpey's fibres. Over this is a tendon sheath.

Osteoarthritis

OA is a disease of the articular cartilage. Initially, cartilage starts to shred slightly (fibrillate), becoming rougher and thinner
Events that follow are an attempt at repair: OA is not a degenerative disease.

The joint produces new bony growths around the margin of the joints. These are called marginal osteophytes.
There may be new bone formed on the surface. Bone contour will change. There may be pitting on the joint surface, connected to subchondrial cysts.
At some point the cartilage will have disappeared so much that bone is rubbing on bone.
Eburnation is a pathonemonic (i.e. having only one cause) effect of this. Eburnation is a polish on the surface of the bone.
There may be an inflammatory component: the synovial membrane may be enlarged. There may be marginal osteophytes or joint space narrowing. Radiologists call the thickening of bone sclerosis. Eburnated bone is thicker.
Sometimes osteophytes are so large they restrict movement.

OA in present populations:

· Highly age related prevalence and incidence (by age 70, roughly 75% of people have it)
· More common in females than males
· Knee more commonly affected than the hip
· Distal joints of the hands affected more in females
· Females more likely to have multiple joint involvement

HOW IS IT DIAGNOSED?

You need an operational definition: the ‘patient' has to fulfil all the criteria of this OD.

Finding eburnation is very easy, it is very smooth and very reflective.

PRECIPITANTS OF OA

Age - Sex - Genetic - Activity - Weight - Race - Trauma
▼
Interactions
▼
Joint failure

· Joints that don't move don't get OA
· Obesity is an important factor for OA of the knee and hand (but not the hip!)... possible metabolic factors
· Likelihood of getting OA in the past from a fracture to a joint was about 100%

DISTRIBUTION

· Predilection for some joints over others
· You can get OA on the odontoid peg, but it doesn't present sysmptoms clinically
· OA in the hand tends to be centred on the trapezium. There's also a lot of OA between the trapezoid and scafoid, which is not very painful. OA of the thumb base is painful.
· It is possible that there's an evolutionary factor - joints that were used later in development tend to be affected.

RANK ORDER AT DIFFERENT TIMES

Within the knee, the patella-femoral joint is most commonly affected, while the medial compartment is most painful.

OCCUPATION & OA

· Remember activity is needed for OA
· Information can be derived from modern epidemiological studies of occupational groups.

· BUT YOU CANNOT TELL THE PRECIPITANT FROM THE OUTCOME!

INTERVERTEBRAL DISC DISEASE

Between the vertebrae are intervertebral discs. These act as shock absorbers. The discs attach around the margins of the vertebrae by Sharpey's fibres. The insertion of Sharpey's fibres become inflamed with age. The disc space shrinks as osteophytes develop. Very common in the cervical and lumbar regions
May impinge on the nerve in the inter-vertebral foramen, leading to neurological signs in the hand and arm.
IDD is very common. In the neck it is almost certainly painful.

ROTATOR CUFF DISEASE

This is a very common cause of shoulder pain and decreased mobility of the shoulder joint.
Increases markedly with age
Due to extrinsic or intrinsic causes: traumatic tear in tendons, overuse injury, degeneration with ageing.
Causes pitting in the humerus
Sometimes insertion of transverse ligament will inflame
Sometimes biceps tendon ruptures - a bar of bone may develop in bicipital groove.
If rotator cuff ruptures, pull of the deltoid will cause the head of the humerus to impinge onto surface of the acromion, forming new bone and causing eburnation.
It's painful!

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