Synovial joints: Classification

Classification of jointsClassification and general structure of articulations
~ 6 min

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

    Synovial joints
    Synovial joints
  • And the synarthroses contain no cavity, and the bones are held together with connective tissue.

Synarthroses
Synarthroses

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).

Articular cavity or joint gap (cavitas articularis)
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).

Articular surfaces of the bones (facies articularis)
Articular surfaces of the bones (facies articularis)

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.

Joint capsule (capsula articularis)
Joint capsule (capsula articularis)

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.

Ligament (ligamentum)
Ligament (ligamentum)

The internal layer is the synovial membrane (membrana synovialis), which produces the already mentioned synovial fluid. It lubricates the articular surfaces, eliminating their friction.

Synovial membrane (membrana synovialis)
Synovial membrane (membrana synovialis)

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)

    Articular disc (discus articularis)
    Articular disc (discus articularis)
  • The articular meniscus (meniscus articularis)

    Articular meniscus (meniscus articularis)
    Articular meniscus (meniscus articularis)
  • The glenoid labrum (labrum articulare)

    Glenoid labrum (labrum articulare)
    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:

  • Uniaxial

  • Biaxial

  • And multiaxial

    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.

    Cylindrical joint (articulatio cylindrica)
    Cylindrical joint (articulatio cylindrica)
  • 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.

    Hinge joint (ginglymus)
    Hinge joint (ginglymus)
  • The ellipsoid joint (articulatio ellipsoidea) has one ellipse-shaped articular surface, and the other one is shaped as a fossa that matches it.

    Ellipsoid joint (articulatio ellipsoidea)
    Ellipsoid joint (articulatio ellipsoidea)
  • 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

    Saddle joint (articulatio sellaris)
    Saddle joint (articulatio sellaris)
  • 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.

    Ball and socket joint (articulatio spheroidea)
    Ball and socket joint (articulatio spheroidea)

    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.

    Cotyloid joint (articulatio cotylica)
    Cotyloid joint (articulatio cotylica)
  • 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.

Plane joint (articulatio plana)
Plane joint (articulatio plana)
Dictionary

Classification of joints

synovial joints
articulationes synoviales
articular cavity
cavitas articularis
articular surface
facies articularis
synovial fluid
synovia
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
meniscus articularis
labrum
labrum articulare
synarthrosis
synarthrosis
amphiarthrosis
amphiarthrosis
synovial joint/diarthrosis
diarthrosis
cylindrical joint
articulatio cylindrica
hinge joint
ginglymus
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
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