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Biology Body Dive: Joint Capsules, Synovium and What Is That Cracking Anyway?


To understand pathology, when things go awry, we have to have a foundation in the biology and behavior of the tissues we are thinking about. Histology is the study of cells and tissues. This study is extremely important in medical and PT school to understand the physiology- the behavior of tissues and organs.


When you observe a joint structure, the interface between the ends of two bones, several tissues play important roles in the health of that joint.

Let’s begin with the most external structure, the joint capsule.


The Joint Capsule and Synovium:

Made up of dense irregular tissue (very similar to ligaments), the joint capsule creates an envelope around the end of two interfacing bones.

This envelope thickens in places, creating articular ligaments, and can have tendons attaching in to it (as is the case with the long head of the biceps in the shoulder).


Ligaments provide stability against tensile forces (forces that are distracting, pulling apart- lengthening). This is why you sprain your ligament in your ankle when you roll it- you over stretch one aspect of it.

The capsule is highly vascular and innervated by a healthy nerve supply. As such, when capsule fibers are stretched, nerves communicate to send information around proprioception- changes in the position of the joint- to the cortex.


But the nourishment the capsule supplies continues.

The capsule is lined by the synovium- a series of cells that produce and create a very small volume (only about a twentieth of a millimeter thick) of nutrient rich fluid material that bathes the articular cartilage called synovial fluid.

When we take a closer look at cartilage, we come to find it is aneural (without nerve supply) and avascular (without blood supply).


Synovium is able to feed and nourish articular cartilage surfaces. Sometimes, movement is required to assist with this process.

By the way, the cavitation or cracking of joints is thought to proceed at this level (which will be discussed further later on).


In addition to helping cartilage with nourishment, the synovial fluid assists healthy joint function by:


1. Separation and friction reduction between moving tissues (such as the acromium and rotator cuff within the shoulder)

2. A fluid/ mobile, compressible medium for joints to move more securely through


The joint capsule forms a pressurized seal around the joint, so that no fluid escapes. When we have surgery that involves opening the joint capsule, this pressurized ligament seal is lost, and sometimes does not repair completely.


Cartilage:

Cartilage is an incredible connective tissue. It is perfectly designed to line our joint surfaces, making them more congruous with each other and dispersing load from weight bearing and muscle contractions.


Cartilage is made up of chondrocyte cells. These are puffy cells containing collagen and proteins.


Our entire skeleton is cartilage at one point in development, and gradually matures to bone through two different processes. Cartilage remains in several different places (famously at our ears, the end of our noses, end of ribs, and the ends of our long bones). Today we will focus on the type of cartilage that exists at the ends of our bones- hyaline cartilage.

Hyaline is derived from the latin Hyalinus- meaning glass. Like glass, this cartilage reduces friction and has a slippery feel to it that assists motion.


The shape of chondrocytes at borders (such as at the ends of bones) can get flattened and stretched out.


Unlike almost any other tissue in the body cartilage is avascular- it does not receive blood suppl