Joints

What is a joint?

Joints are the connection between two bones. Joints serve as the axis of human movements. For example, while drinking a glass of water your elbow joint acts as the axis for elbow flexion in the sagittal plane. Every joint has significant mobility and stability functions, which this blog discusses in detail.

Let me answer the curious question,

 

How many joints are in the human body? 

There are about 350 joints in the human body formed by 206 bones.

 

What are the types of Joints?

The joints are classified in two ways

 

Structural classification (based on the type of connective tissue forming the joint):

  1. Fibrous – bones connected by dense connective tissue with very little or no movement (example: skull sutures)
  2. Cartilaginous – bones connected by cartilage allowing minimal movement. (example: pubic symphysis) 
  3. Synovial – covered by a joint capsule and contains synovial fluid. It allows a wide range of movement. (example: shoulder joint )

 

Functional classification (based on the degree of movement allowed): 

  1. Synarthrosis (Immovable) 
  2. Amphiarthrosis (Slightly moveable) 
  3. Diarthrosis (Freely moveable). 
Types Of Joints

What is a fibrous joint?

A fibrous joint is formed by fibrous connective tissue joining two bones. It lacks a joint cavity (the space between bones in a joint). These joints generally do not allow movement. Even if allowed, only a little movement is possible. 

Before discussing the three types of fibrous joints, let’s define fibrous connective tissue.
It is a dense connective tissue primarily composed of tough protein fibers called collagen and specialized cells known as fibroblasts.

 

Types of fibrous joint: 

  1. Sutures are only present in the skull. They allow slight movement in early childhood, but later fuse and become immovable (synarthrosis).
  2. Gomphoses are only present between the teeth and their sockets in the jaw bones (mandible and maxillae). This joint is usually immovable (synarthrosis) but very slight movement can occur during activities like chewing. 
  3. Syndesmoses in this joint, bones are connected by long bands of fibrous tissue. It allows for slight movement, hence they are amphiarthrodial joints. Example: 
  • The interosseous membrane between the radius and ulna and
  • Distal tibiofibular joint. 

What is a cartilaginous joint?

Cartilaginous joints have cartilage that connects the bones, allowing limited to no movement. They lack a joint cavity and are classified into two types: 

  • (Primary Cartilaginous Joints): Bones are connected by hyaline cartilage, that ossifies over time and is immovable (synarthrosis).

Example: Epiphyseal endplates in long bones, costochondral joints in ribs.

  • Symphyses (Secondary Cartilaginous Joints): Bones are connected by fibrocartilage, which provides stability and allows slight movement (amphiarthrosis).  

Example: Intervertebral disc, pubic symphysis 

This raises a question, how does the spine have a greater degree of freedom, if the Intervertebral disc is slightly mobile? Although a single intervertebral joint has a limited range of motion, the collective motion of all the intervertebral joints provides a greater degree of freedom in the spine.

What is a synovial joint?

A synovial joint is the most common and highly mobile joint (diarthrosis) in the human body. This type of joint differs from other joints in having a joint cavity (a gap between two articulating surfaces) filled with synovial fluid.

 

Important structures of synovial joint:

 

The Articular capsule helps stabilize the joint passively.

Articular cartilage helps in minimizing friction and works as a shock absorber between the two bones.

Synovial fluid provides shock absorption, lubrication, and nutritional distribution for the joints. It is produced by the synovial membrane.

 

6 Types of synovial joint: 

  1. Pivot (radioulnar joint, atlantoaxial joint)
  2. Hinge (ankle joint, elbow joint, interphalangeal joint)
  3. Saddle (first capo metacarpal joint, ?)
  4. Ellipsoidal or condylar  (knee joint, wrist joint)
  5. Ball and socket (shoulder joint, hip joint)
  6. Plane (sacroiliac joint, plane) 

We can divide the 6 types into three classifications based on the degree of freedom it has. 

Joint movements occur in 3 cardinal planes, which are sagittal, frontal, and transverse. If the joint allows movement in any one of these 3 planes of movement, it is an uniaxial joint. Similarly, if the joint allows movement in any two planes, it is a biaxial joint, and if the joint allows movement in all three planes, then it is a multiaxial joint.

  • Pivot and hinge-type joints are categorized as Uniaxial joints.
  • Saddle and ellipsoidal-type joints are categorized as  Biaxial joints.
  • Ball & socket and plane-type joints are categorized as Multiaxial joints. 

 

Note: Even though plane joints move all three planes, they do not produce angular movement, instead they create a translatory type of movement (only linear sliding). However, the ball and socket joints create angular movements like flexion, extension, abduction, adduction, and rotations.

 

What are joints made of? 

A joint comprises many structures that function together to provide stability and mobility. The various connective tissues in joints include bones, cartilage, synovial membrane, synovial fluid, ligaments, tendons, bursae, and meniscus. 

 

Bones: Two or more bones align together to form a joint. 

 

Cartilage: It is a special connective tissue that is smooth and rubbery, covering the articular surface of the bones to reduce friction during movements. It is strong, and flexible, provides structural support, and enables smooth movement. 

 

Synovial membrane: It is a thin lining of connective tissue on the inner surface of the synovial joints. It produces synovial fluid to help with joint lubrication. 

 

Synovial Fluid: It is a slippery fluid produced by the synovial membrane to reduce friction between the bones and nourish the cartilage.

 

Ligaments: They are tough, fibrous tissues connecting the bones to provide joint stability.

 

Tendons: They are strong connective tissues that connect muscles to bones, allowing movement and transmitting force.

 

Bursae: These are small fluid-filled sacs that cushion and reduce friction between tissues, such as tendons and bones.

 

Menisci: These crescent-shaped cartilage structures are found especially in the knee joint to provide extra cushioning and stability. 

How Do Joints Work?

Joints are formed by the bones. Muscles are attached to bones. When muscles contract, they pull the bones closer and produce movements at the joints. Ligaments prevent excessive movement, ensuring stability. Cartilage and synovial fluid reduce friction and prevent joint damage. 

What are the roles of Synovial Fluid and Cartilage in Joint Movement? 

The primary role of cartilage and synovial fluid is to protect the joint’s articular surface and ensure smooth movement.

 

Cartilage is a thick, flexible tissue found at the ends of bones where they connect and form joints. It provides a smooth, slippery surface and acts as a cushion and shock absorber. 

Synovial fluid is a thick, viscous fluid found inside the synovial cavity. It is produced by the synovial membrane, which lines the inner surface of the joint capsule. It helps lubricate the joint, absorb and distribute the forces, providing oxygen and nutrients to the joint

What are the Importance of Ligaments and Tendons in Joint Stability?

Ligaments and tendons play a crucial role in maintaining joint stability. They vary in structure and properties, but both are essential for joint integrity. 

Ligaments are tough, fibrous tissues connecting the bones and holding the joints in place, thereby providing static stability. They restrict excessive movements in joints, preventing them from moving beyond their normal range. They also serve as shock absorbers and distribute forces across the joints. Additionally, they have a role in proprioception, sending signals to the brain about the joint position. 

Tendons are tough, elastic bands of connective tissue that connect muscles to bones, providing dynamic joint stability. They assist the muscles and stabilize the joints during movements by transmitting muscle forces, storing elastic energy, and releasing it efficiently to enhance movement.

Why do bones have joints?

Bones have joints to allow movement, provide support, and maintain structural integrity. Joints act as pivot points where bones connect, allowing various types of human motions. 

Without joints, the bones would be rigid and unable to perform daily activities. Joints help distribute body weight and other forces efficiently, reducing stress due to the presence of specialized joint structures like cartilage and synovial fluid. 

What are the common Joint-Related Conditions and Treatments?

There are several joint-related conditions, affecting the bones and other structures surrounding the joint. Many of them present with pain, stiffness, and reduced mobility. 

Some of the common joint-related conditions include, 

  • Dislocation and subluxations 
  • Osteoarthritis 
  • Rheumatoid arthritis 
  • Gout 
  • Bursitis 
  • Tendinopathy 
  • Ligament injuries 
  • Cartilage injuries
  • Adhesive capsulitis 

Dislocation and subluxations 

These occur when a joint moves out of its normal position due to trauma, instability, or ligament weakness. Common symptoms include pain, visible deformity, and inability to move the joint. 

Treatment: 

  • Joint relocation (by a medical professional)
  • Immobilization with a sling or brace 
  • Physiotherapy (strengthening, stabilization exercises)
  • Surgery (if the joint is very unstable and recurrent dislocations occur) 

 

Osteoarthritis 

It is inflammation and degeneration of joint cartilage, generally due to age-related wear and tear of cartilage over time. But so many other factors like repetitive overuse, previous injury or surgery, obesity, inflammatory factors, and family history can contribute to the development of osteoarthritis. Common symptoms include pain, stiffness, swelling, and reduced range of motion.

Treatment:

  • Physiotherapy (strengthening, stretching, joint mobilization)
  • Pain management (NSAIDs, corticosteroid injections)
  • Assistive devices (braces, orthotics)
  • Weight management
  • Joint replacement surgery (for severe cases)

 

Rheumatoid arthritis: 

It is an autoimmune inflammatory condition commonly affecting small joints. Usual symptoms include pain, joint swelling, and stiffness (often symmetrical). 

Treatment:

  • Disease-modifying anti-rheumatic drugs (DMARDs)
  • Biologic therapies (targeted immune suppression)
  • Physiotherapy (exercise, splints, education)
  • Lifestyle changes (diet, stress management)

 

Gout:  

It is an inflammatory arthritis caused by high uric acid levels leading to crystal deposits in joints. Symptoms include sudden, severe pain (often in the big toe), redness, and swelling. 

Treatment: 

  • Medications (colchicine, NSAIDs, urate-lowering drugs)
  • Dietary changes (reduce purine-rich foods, alcohol)
  • Hydration and weight management 
  • Physiotherapy (gentle range of motion exercises)

 

Bursitis

It is Inflammation of the bursae (fluid-filled sacs around joints). It presents with pain, tenderness, and swelling.

Treatment:

  • Rest and activity modification
  • anti-inflammatory medications
  • Physiotherapy (strengthening exercises)
  • Corticosteroid injections (if severe)

 

Tendinopathy 

It is caused by overuse or repetitive strain on tendons, presenting pain, tenderness, stiffness, and mild swelling. 

Treatment:  

  • Rest and modification of activities
  • Eccentric, isometric strengthening exercises
  • NSAIDs if required

 

Ligament injuries 

Ligament injury can range from a mild over-stretch to a complete tear of the ligaments. The symptoms of ligament injury or sprain include pain, swelling, instability, difficulty moving the joint.

Treatment: 

  • PEACE-LOVE protocol-  
    • PEACE: Protect, Elevate, Avoid anti-inflammatory medication/ice, Compression, Educate, 
    • LOVE: Load, Optimism, Vascularization, Exercise
  • Physiotherapy (strengthening, proprioception training)
  • Bracing
  • Surgery (for severe tears, e.g., ACL reconstruction)

 

Cartilage injuries: 

Cartilage gets damaged by trauma, overuse, or degeneration. Commonly affected is the special cartilage in the knee joint- meniscus. Common symptoms are Joint pain, clicking, and locking sensation.

Treatment:

  • Physiotherapy (muscle strengthening to reduce joint stress)
  • Activity modification
  • Surgical options (arthroscopy, cartilage repair procedures, in case the damage os severe or conservative management fails)

 

Adhesive capsulitis 

It is the inflammation and thickening of the joint capsule commonly affecting the shoulder joint. Usually referred to as the frozen shoulder, it presents with progressive stiffness and pain, and limited shoulder movement.

Treatment: 

  • Physiotherapy (range-of-motion, stretching, and strengthening exercises)
  • Corticosteroid injections
  • Manual therapy (joint mobilization- to be avoided in the painful stage)
  • Surgery (rare, only if other conservative managements fail) 

What are the roles of Joints in Specific Sports and Physical Activities?

Joints play an important role in movement, stability, force absorption, and power generation in sports and physical activities. Each joint type has a specific function depending on the demands of the sport. 

Each joint has a unique function. 

  • Shoulder, hip, knee – Mobility & Power Generation

Example:

  • Shoulder joint- basketball, volleyball, tennis 
  • Hip joint- soccer, basketball, running 
  • Knee joint- skiing, golf, running
  • Knee, ankle, spine – Stability & Impact Absorption

Example:

  • Knee joint- basketball, rugby, skiing 
  • Ankle joint- football, soccer, ballet, running 
  • Spine – weight lifting, martial arts, racing 
  • Wrist, hand, cervical spine – Fine Motor Control & Precision 

Example:

  • Wrist joint- baseball, cricket, boxing
  • Hand- archery, racquet sports
  • Cervical Spine – formula 1 racing, martial arts