18 - Joints and Joint Tissue Flashcards
What do I need to know?
- Classification of joints and features of each
- Structure and function of the intervertebral joint and the changes that occur with ageing and damage
- Structure, function and pathology of a synovial joint
What is a joint?
The junction of 2 or more bones that allows for movement (may be small or large)
How are different joints classified?
According to the types and arrangements of the component tissues
What are the types of joints?
There are 4
- Synarthroses (fibrous joints)
- Synchrondroses (primary cartilaginous joints)
- Secondary cartilaginous joints
- Diarthroses/Synovial Joints
Describe synarthroses joints
- also called fibrous joints as the bones are united by fibrous tissue that insert into the bones (Sharpey’s Fibres)
- allows the bones to move relative to each other without moving apart
- permits little movement
What are the 2 types of synarthroses joints?
Joints that may fuse = Synostosis
Joints that don’t fuse = Syndesmosis
What is a synostosis joint?
A synarthroses/fibrous joint that may fuse for example the sutures of the skull
What is a syndesmosis joint?
A synarthroses/fibrous joint that is unfused for example the interosseous membrane
What are the 2 types of a synostosis synarthroses joint?
Squamous and serrate (interdigitate)
> occurs in the sutures of the skull
> there is LITTLE movement at these joints
What is a fracture of the epiphyseal plate called?
Salter Harris Fracture
What is a primary synchondroses joint?
- primary cartilaginous joint (bone - cartilage - bone)
- permit very small movement/immovable
What are 2 examples of a primary synchondroses joint?
- epiphyseal plate
- costal cartilages
What type of joint is the epiphyseal plate converted into?
Primary cartilaginous joint > synostosis (fused bones)
What percentage of childhood long bone fractures does a Salter-Harris Fracture make up?
15% (fracture epiphyseal plate)
> both the joint and left over line is a point of weakness
What is the significance of having synchondroses at the costal cartilages?
- need considerable flexibility for ventilation movements - calcifies and becomes stiff with age)
- between anterior ribs and sternum (posteriorly they are synovial)
Epiphyseal … eventually leaves as bone is ….
Epiphyseal lines eventually leaves as bone is remodelled
What are secondary synchondroses joints?
- bone > cartilage > FCT > cartilage > bone
- is a wider joint; has little movement but is flexible
What are 2 exmaples of a secondary synchondroses joint?
- Intervertebral disc
2. Manubrio-sternal joint
Describe the intervertebral disc as a secondary synchondroses joint
- there are collagen fibres on the outside of the disc that sweep from 1 body to the next (AF); the majority of these fibres run at angles
- there are 2 plates of hyaline cartilage (remaining from the fetal vertebral disc; the rest has ossified) with fibrous tissue (AF & NP) in between
Describe the intervertebral disc as a secondary synchondroses joint
- centre of ossification/micro-cancellous bone
- hyaline cartilage plate
- AF & NP
- hyaline cartilage
- bone …
Intervertebral Disc =
2 x plates of hyaline cartilage
(fibrocartilage)
AF
NP
What are Sharpey’s Fibres?
- collagen fibres from the AF that insert into the collagen and bone as it develops
- occurs especially at the periphery and are firmly embedded so it is hard to pull the AF away from the bone
Where do you find fibrocartilage?
At the point where the collagen fibres from the AF insert into the cartilage plates (sharpey fibres)
Are you more likely to pull off the bone or break the AF/sharpey fibres?
Break the bone
How do you identify the NP?
- lighter staining
- less dense/conc of collagen fibres
- hydrated
- loose CT
What happens to the NP with age?
- becomes less hydrated and more dense with age instead of a soft jelly ball bearing so restricts/stiffens range movement
Are longitudinal ligaments part of the intervertebral disc?
No but they also help hold the discs together
Does hyaline cartilage have collagen fibres?
YES but can’t see them as they are coated in more proteoglycan
Fibrocartilage is a …
Transition from fibrous tissue to cartilage. Will find all intermediates here
> shows how a change in mechanical forces can change a tissue from 1 to another
What does the nucleus pulposus acts as a …
Flat ball bearing between the plates of cartilage and bone. Gives support. Is highly hydrated with lots of PG and hyaluronan which binds H2O
How does the thickness of the intervertebral disc change during the day?
- when weight is applied consistently the H20 is extruded and over a day water is slowly squeezed out of the NP and the height of the disc decreases
- when you sleep/lie down the disc rehydrates and NP swells
(tallest in morning)
How does the ratio between disc thickness and vertebral body thickness change down the spine?
Cervical: 2/5
Thoracic: 1/5
Lumbar: 1/3
Describe the position of the NP at the thoracic vertebra
- slightly POSTERIOR but due to the curvature our COG is actually more FORWARD than the NP (we are not constantly in this position)
Describe the position of the NP at the cervical vertebra
- slightly posterior due to COG
Describe the position of the NP at the lumbar vertebra and what is the consequence of this?
- like cervical the COG passes through the NP but this is much more posterior
- this means the AF is much thinner at the posterior side
- means the posterior AF is weaker and NP is more likely to exude through cracks posteriorly
- posterior to the vert body especially in lumbar regions are nerve roots
- a bulge of NP will impinge both sensory/pain fibre axons and motor (slipped disc)
Where is a slipped disc more likely to occur and why?`
Lumbar region posteriorly onto nerve roots. COG passes through NP posteriorly so posterior AF is thinner and prone to damage. Lumbar is more weight bearing.
Besides a slipped disc posteriorly how else may the NP extrude?
Vertically down into the lower vertebral body. This is called a hernia.
What is the consequence of a vertically herniated NP?
- This collapses down the herniated disc and decreases its thickness so the discs/bodies move closer together
- this changes the diameter of the exit point of the spinal nerves and so can impinge on the nerves as the exit the vertebral foramen laterally
How does the posterior longitudinal ligament affect a slipped disc?
- adds support to posterior to prevent NP impinging on nerves
- instead the extrusion usually takes place laterally as is the path of least resistance
How does lateral/medial a NP extrusion is affected the damage?
- the more lateral the protrusion the more likely only 1 spinal nerve will be affected as they aren’t as closely approximated as they are when they first leave the canal
- the more medial the more likely it is several nerves will be affected
How can you trace back which nerve is impinged during a slipped disc?
- due to which field/area of skin/motor is affected i.e. myotomes/dermatomes to find level of lesion
- does NOT detail nature of lesion just what level
What are diarthroses joints?
Synovial joints
bone - cartilage - joint space (NOT FCT) - cartilage - bone
What are the 6 features of diarthroses?
- Joint cavity
- Articular cartilage
- Synovial Membrane (laterally)
- Synovial fluid
- Fibrous Capsule
- MAY have menisci or articular disc
Fibrous capsule purpose?
Holds joint together around outside so bones held together (make thicken/specialise to form ligaments)
Synovial fluid is produced by ..
Synoviocytes in synovial membrane (3-4 cells thick)
Where does synovium not cross?
Cartilage - except in arthritis where it can penetrate and get jammed
Where do fat pads sit in synovial joints and does synovium cover it?
Beneath the synovium so yes it does
Purpose of meniscus
Fibrocartilage comes in laterally to stabilise joint
Does synovium run over menisci?
NO
Describe a meniscus
- fibrocartilage so has chondrocytes far apart with lots of collagen organised in layers
- often a curved wedge/disc
- supports joints
- thin sharp medial edge
How is the menisci affected by rapid movement/change in direction?
Can TEAR, fold back on itself and jam in the joint and be very painful.
Results in inflammatory changes and needing to get some of it taken out
Only grows back to a certain degree but the thicker portion of the wedge is usually enough to support the joint
How is an intra-articular disc formed and how does it differ from menisci?
When menisci from either side project in far and join up to form a pad of fibrous tissue which divides the synovial cavity into 2 separate pools of synovial fluid
Where may you find an intra-articular disc?
In the temporo-mandibular joint where the mandible joins the skull. Enables you to shift your lower jaw forward in front of your upper teeth as can slide jaw back and forth at this joint due to the disc
(the 2 bones slide over a pad of fibrous tissue rather than cartilage)
Describe the synovium
Is a thin soft loose fibrous tissue well supplied with blood vessels.
How is the cartilage of synovial joints nourished?
Fluid leaks out of the vasculature and synoviocytes (HA) of the synovium to form synovial fluid that nourishes the cartilage
> the chondrocytes of cartilage only receive nourishment from DIFFUSION of this fluid
Cartilage turns over and forms waste … where does this go?
Is removed via diffusion into the synovial fluid, synovium and vessels
Besides from the joint cavity where else may you find synovial fluid and synovium?
BURSA and TENDON SHEATHS
> for example at the elbow. This can get inflammed, over secrete and swell
What are tendon sheaths?
Are essentially bursa wrapped around tendons to reduce the friction when tendons move up and down on bone when pulling on muscles
> tendon has a synovial lining surrounded by a fibrous membrane
Where would you see synovium at a synovial joint …
Folds up on itself to form a pouch below the borders of the articular cartilage
How do the chondrocytes at the SURFACE of articular cartilage
> at the surface you also get TENSION as well as pressure. Cells are therefore also elongated.
the surface of articular cartilage is also therefore easily damaged
Describe the make up/content of articular cartilage
- high proteoglycan content of 10% dry weight especially at load bearing regions (less at periphery unless stimulated/used so also thinner here)
What percent of cartilage dry weight is proteoglycan?
10% (high)
What happens to the cartilage and bone in arthritis?
It erodes especially at periphery leading to bone on bone
> the joint then tries to SPREAD the load by increasing surface area by growing OSTEOPHYTES (bits of bone growing out laterally)
- bone thins
- get a lot of damage at the joint surface leading to VERTICAL cracks in cartilage
- chondrocytes attempt to repair but can’t as get surrounded by matrix leading to chondrocyte clusters
What are osteophytes and why do they occurs?
When cartilage erodes, especially at periphery, then the joint tries to spread to load by increasing the SA by bits of bone growing out laterally (osteophytes)
