Saddle Joints

Introduction

The human body is supported by a hard frame of a skeleton. Bones and cartilages are their main components. The skeletal system is divided into exoskeleton and endoskeleton. The human endoskeleton is made of 206 bones and some supporting cartilage.

The skeleton has various functions such as

• Supporting the body

• Giving definite shape

• Protecting internal organs

• providing a surface for attachment of muscles

• Help in movement and locomotion.

Bones are hard, immovable, and made of calcium salts. Cartilages are comparatively movable due to the presence of chondroitin salts. The study of various joints and their structure and function is known as Arthrology. Two bones at a joint are connected by ligaments with elastic, tough, and fibrous connective tissue. They help in keeping the bones in the correct position and help in various voluntary movements such as walking, running, swimming, etc. Joints make the rigid skeleton flexible and help in the locomotion of the body.

Types of Joints in Body

Based on the structure and amount of mobility there are 3 main types of joints:

• Synarthrosis/Fixed Joints: These are fixed joints and are completely rigid. Bones at the joints are connected by collagen. E.g. Sutures of the skull, peg, and socket joint, etc.

• Amphiarthrosis/Slightly Movable Joints: These are partially movable joints. They move only in response to compression, tension, or twisting. E.g. Symphysis, intervertebral joints, etc.

• Diarthrosis/Freely Movable Joints: These are completely movable and help in movement and locomotion. They contain special structures such as synovial membrane, synovial fluid, hyaline cartilage, and ligaments which enables the complete mobility of these joints. E.g. Saddle joint, gliding joint, condyloid joint, etc.

Typical Synovial Joint

A typical synovial joint is present between two bones and it assists the full movement of that bone. These bones have special features which makes them flexible.

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The following are features of a synovial joint:

• Articulate Capsule: It is a sac-like structure that encloses the synovial cavity by joining ends to the adjacent bone.

• Synovial Membrane: It is present inside the capsule and forms a synovial capsule. It secretes synovial fluid which protects the internal joint.

• Synovial Fluid: It contains hyaluronic acid and is a yellowish, clear, viscous fluid that is secreted by a synovial membrane. This fluid lubricates the joint areas and helps in the movements of the bones. It also contains phagocytes which prevents infections and removes cell debris.

• Hyaline/Articular Cartilage: These are cartilages that cover the ends of the bones and prevent direct contact and hence prevent friction between the two bones.

What are Saddle Joints?

Saddle joints are a type of synovial joints and are completely movable. As the name suggests, one bone is saddle-shaped and the other bone rests over it which provides free movement. E.g. The carpometacarpal joint of the thumb, formed between the first metacarpal and trapezium bone.

Features of Saddle Joints

Following are the features of saddle joints:

• Both surfaces of the bone have a convex and concave area.

• Each surface is concave in one area and its perpendicular area is convex.

• This concave and convex arrangement of the planes allows biaxial movement.

• They allow angular movement and are similar to condyloid joints.

• They allow movement in Flexion-Extension i.e. in and out the action of the thumb and Abduction-Adduction is moving away from the midline and moving towards the midline respectively.

• These joints are found in the thumb, sternoclavicular joint, and inner ear.

• These are highly flexible

• This joint has an evolutionary significance as it is responsible for increasing the grasping power of the hand and helps in doing various activities like, writing, drawing, catching objects, driving, etc.

Conclusion

Joints make the rigid skeleton movable. They are responsible for the various day-to-day activities that we do. Based on movement these joints can be fixed, partially movable, or completely movable. A type of completely movable joint is the saddle joint which is seen in the thumb and helps in performing important activities. The flexibility of the bones is due to the synovial fluid present in the cavity. The study of joints is known as Arthrology.

FAQs

Q1. What are fibrous joints?

Fibrous joints also known as synarthroses are immovable joints and do not allow any movement of the bones. These joints contain fibrous connective tissues known as collagen. These are formed to protect a particular organ and with age, they become fixed. Examples include sutures of the skull, and the syndesmoses- a joint which connects two bones such as the tibia, fibula, and peg and socket joints seen in teeth.

Q2. Explain ball and socket joint

Ans. Ball and socket joint is a type of synovial joint and is completely movable. It contains a spherical head that resembles a ball that is properly fitted into a C-shaped socket. Such an arrangement allows multiaxial or rotatory and straight movement of the bones. E.g. Shoulder joints, Hip joints, etc.

Q3. Match the following.

COLUMN A	COLUMN B
A. Suture	1. Cartilaginous joints
B. Synchondrosis	2. Tooth joint
C. Hinge joint	3. Fibrous joints
D. Atherosclerosis	4. Synovial Joint
E. Gomphosis	5. Deficiency of Synovial fluid

Ans: A-3, B-1, C-4, D-5, E-2

Q4. Explain Arthritis.

Ans: Arthritis means joint inflammation.

Common causes of it are:

• Reduced secretion of synovial fluid by the synovial membrane.

• Disruption of the hyaline cartilage which increases friction between the bones.

• A deficiency of vitamins and nutrition reduces the production of synovial fluid.

• Genetic disorders or hereditary.

Symptoms: Severe pain, burning, redness, swelling in the joint areas, and trouble in walking and movement.

Treatment: Medicines include painkillers, NSAIDs (Non-Steroidal Anti-Inflammatory Drugs) such as Aspirin, Ibuprofen, Corticosteroids, and DMARDs (Disease-Modifying Antirheumatic Drugs), Methotrexate is also used.

Q5. What are the sutures of the skull?

Ans: There are 4 sutures found in the skull.

• Coronal Suture: It is found between the Frontal and Parietal bones.

• Sagittal Suture: Found between Parietal bones.

• Lambdoidal Suture: Found between Parietal and Occipital bones

• Lateral Suture: Found between Temporal and Parietal bones.

These sutures are types of fibrous joints and contain collagen. Skulls of babies have 6 small gaps called fontanelles which are flexible during parturition but ossify after 2 years of age.