What is the Difference Between Bursa and Synovial Fluid

The main difference between the bursa and synovial fluid is that bursa is a type of small sacs filled with synovial fluid. Here, synovial fluid is a viscous, non-Newtonian fluid, which occurs in the cavities of synovial joints. Furthermore, bursa occurs next to the tendon in a typical joint, while synovial fluid occurs deeper, surrounding the articular cartilage.  

In short, bursa and synovial fluid are two types of anatomical features that occur in a typical joint. Importantly, both reduce the friction of the articulating joints during movement. 

Key Areas Covered 

1. What is Bursa
     – Definition, Structure, Importance
2. What is Synovial Fluid
     – Definition, Composition, Importance
3. What are the Similarities Between Bursa and Synovial Fluid
     – Outline of Common Features
4. What is the Difference Between Bursa and Synovial Fluid
     – Comparison of Key Differences

Key Terms 

Articulating Joints, Bursa, Bursitis, Friction, Synovial Fluid, Synovial Membrane

What is Bursa 

Bursa is a type of fluid-filled sac made up of an outer membrane and inner fluid. The human body contains more than 140 bursae by wedging between bones and tissues. Importantly, the main function of bursae is to reduce the friction in joints during their movement.

Looking at the structure, synovial membrane is a type of membrane that surrounds the sac. Normally, it is a few cells thick. And, it produces synovial fluid, which fills inside the sac of bursae. The fluid is viscous and slippery, with an egg white-like consistency.

Figure 1: Typical Joint

Furthermore, there are three types of bursae; synovial bursae, superficial bursae, and adventitious bursae. Of these, synovial bursae occur in between bones and muscles, tendons, and ligaments. Whereas, superficial bursae occur just below the skin; between skin and bone. For example, the patella bursa at the knee and the olecranon bursa at the elbow are superficial bursae. On the other hand, adventitious bursae develop as a result of repeated irritation.

Figure 2: Bursae of the Knee Joint

Nevertheless, all types of bursae are susceptible to bursitis because of irritation and inflammation. Usually, healthy bursae are thin; 1-3 mm thick. But, bursitis results in the thickening of bursae by producing excess fluid. Bursitis is common in joints such as the hip, knee, shoulder, and elbow. 

What is Synovial Fluid 

Synovial fluid is the vicious, non-Newtonian fluid, occurring inside the cavity of synovial joints. Generally, it is ultrafiltration of the plasma. Therefore, it contains plasma proteins. The synovial membrane, which is the inner membrane of the synovial cavity, is responsible for secreting the synovial fluid. Primarily, fibroblasts in the synovial membrane secrete hyaluronan, which is one of the two main types of lubricants in the synovial fluid. And, the second type of lubricant in the synovial fluid is lubricin, which is secreted by the chondrocytes of the articular cartilage.

Figure 3: Synovial Fluid

Moreover, the synovial fluid makes up approximately 50 μm-thick fluid surface around the articular cartilage while seeping into microcavities and irregularities in the cartilage surface to fill all empty space. Apart from that, synovial fluid performs four different functions in a typical articulating joint. Firstly, it reduces the friction of the articulating joint by lubricating it. Also, it absorbs shocks, becoming more viscous under applied pressure. In addition to these, it helps to supply nutrients and oxygen to chondrocytes while removing their wastes. Furthermore, the synovial fluid serves as a molecular sieve by pushing hyaluronan against the synovial membrane under pressure, preventing the migration of cells into the synovial cavity and the migration of fluid out of the cavity. Importantly, synovial fluid analysis aids in the diagnosis of different joint-related, pathological conditions such as rheumatoid arthritis, osteoarthritis, gout, etc. 

Similarities Between Bursa and Synovial Fluid  

• Bursa and synovial fluid are two types of anatomical features that occur in a typical joint.  
• Both have an egg white-like consistency.  
• The two lubricating components in them are hyaluronan and lubricin.  
• Their function is to reduce the friction of the articulating joints during movement. 

Difference Between Bursa and Synovial Fluid 

Definition 

Bursa refers to a fluid-filled sac, or sac-like cavity, that counter friction at a joint. But,while synovial fluid refers to a transparent, viscid lubricating fluid secreted by a membrane of an articulation, bursa, or tendon sheath.  

Correspondence 

Bursa contains synovial fluid secreted by the synovial membrane. In which, the synovial fluid has an egg-like consistency. 

Occurrence 

Bursa occurs next to the tendon in a typical joint, while synovial fluid mainly occurs inside the synovial cavity. 

Pathology 

Inflammation and irritation of bursa result in bursitis, while the pathological conditions associated with synovial fluid include rheumatoid arthritis, osteoarthritis, gout, etc. 

Conclusion 

In brief, bursa is a sac filled with synovial fluid. It occurs next to the tendon and is lined by the synovial membrane. But, synovial fluid is the viscous fluid that occurs inside the synovial cavity of a typical articulating joint. Still, the main function of both bursa and synovial fluid is to reduce the friction of the articulating joint when moving. Therefore, the main difference between the bursa and synovial fluid is their occurrence in a typical articulating joint. 

References:

1. “Synovial Joints|Boundless Anatomy and Physiology.” Lumen, Available Here.
2. Funiciello, Marco. “What Is a Bursa?” Arthritis, Available Here.

Image Courtesy:

1. “Joint” By Madhero88 – Own work (CC BY-SA 3.0) via Commons Wikimedia   
2. “908 Bursa” By OpenStax College – Anatomy & Physiology, Connexions Web site. (CC BY 3.0) via Commons Wikimedia   
3. “Figure 38 03 03” By CNX OpenStax (CC BY 4.0) via Commons Wikimedia