Human bones are connected to each other with various formations that differ in their structure and functionality. All joints can be divided into two large groups:
The synovial joints contain a cavity between the bones
And the synarthroses contain no cavity, and the bones are held together with connective tissue.
We have learned about synarthroses in a separate abstract. In this abstract, we will learn about the synovial joints (lat. articulationes synoviales).
As already mentioned, their main difference from synarthroses is the presence of an articular cavity, or joint gap (cavitas articularis).
It separates the articular surfaces of the bones (facies articularis) from each other and is filled with synovial fluid (synovia).
The articular surfaces in most joints are covered with hyaline cartilage, which prevents the bones from touching each other, thereby protecting them from mechanical destruction.
The external surface of the joint is covered with a joint capsule (capsula articularis), which consists of two layers.
The external layer is a fibrous membrane (membrana fibrosa). It is usually strenghened by several ligaments (ligamenta, sing. ligamentum). There are extracapsular and intracapsular ligaments. That means that they are located, respectively, on the external or internal surfaces of the articular capsule.
The internal layer is the synovial membrane (membrana synovialis), which produces the already mentioned synovial fluid. It lubricates the articular surfaces, eliminating their friction.
Sometimes the synovial membrane can form various recesses (recessus articularis), bursae (bursa synovialis) and folds (plicae sinoviales).
Sometimes there may be various additional structures in the joint.
The articular disc (discus articularis)
The articular meniscus (meniscus articularis)
The glenoid labrum (labrum articulare)
And the already mentioned intracapsular ligaments.
The additional structures contribute to the achievement of joint congruence to one degree or another. Congruence means that the articular surfaces match each other.
Let’s learn about the types of motions possible in the joints.
Around the frontal axis: flexion and extension (flexio et extensio)
Around the sagittal axis: abduction and adduction (abductio et adductio)
Around the longitudinal axis: pronatio (internal rotation) and supination (external rotation) (pronatio et supinatio)
With a shift from one axis to another, circular motion (circumductio) may take place
There are several other types of motions, but in fact they are different combinations of the basic options already mentioned.
The extent of motion, or rather the mobility of the joint, is also the basis for one of the classifications.
The synarthrosis (synarthrosis) is a joint with either low mobility or no mobility at all
The amphiarthrosis (amphiarthrosis) is a joint with moderate, but still poor mobility
The diarthrosis (diarthrosis) is a joint with great mobility. Such joints perform a significant number of various motions.
Depending on the number of axes around which the motions are performed, the joints are divided into:
Depending on the shape of the articular surfaces, several types of joints are also distinguished:
The cylindrical joint (articulatio cylindrica) has one articular surface that is convex, cylinder-shaped, while the other is concave, with a shape that matches the cylinder.
The hinge joint (ginglymus) has one cylinder-shaped articular surface with a groove or crest, and the other one is shaped as a notch that matches that groove or crest. The cochlear joint is a separate type of hinge joint.
The ellipsoid joint (articulatio ellipsoidea) has one ellipse-shaped articular surface, and the other one is shaped as a fossa that matches it.
The bicondylar joint (articulatio bicondylaris) is essentially the same as the ellipsoid joint, but one of the articular surfaces is located on the condyle of the bone.
The saddle joint (articulatio sellaris) has articular surfaces shaped as two “saddles” located on top of each other
The ball and socket joint (articulatio spheroidea) has one ball-shaped articular surface, while the other one is shaped as a socket that matches it.
The cotyloid joint (articulatio cotylica) is a type of ball and socket joint. The difference is that the fossa of the cotyloid joint, unlike the classic ball and socket one, covers the head almost completely.
The plane joint (articulatio plana) has flat articular surfaces, where one bone seems to slide over the surface of the other. Even though plane joints belong to the multiaxial, the extent of their motions is poor, for the most part they do not take place at all.
Classification of joints
- synovial joints
- articulationes synoviales
- articular cavity
- cavitas articularis
- articular surface
- facies articularis
- synovial fluid
- joint capsule
- capsula articularis
- fibrous membrane
- membrana fibrosa
- ligament (ligaments)
- ligamentum (ligamenta)
- synovial membrane
- membrana synovialis
- articular recess
- recessus articularis
- synovial bursa
- bursa synovialis
- synovial folds
- plicae sinoviales
- articular disc
- discus articularis
- meniscus articularis
- labrum articulare
- synovial joint/diarthrosis
- cylindrical joint
- articulatio cylindrica
- hinge joint
- condylar joint/ellipsoid joint
- articulatio ellipsoidea
- bicondylar joint
- articulatio bicondylaris
- saddle joint
- articulatio sellaris
- ball and socket joint
- articulatio spheroidea
- cotyloid joint
- articulatio cotylica
- plane joint
- articulatio plana