In both chapters six and seven we will take a detailed look at our skeleton and the joints and attachments. We will briefly introduce the skeleton here in chapter six but discuss it in greater detail and specificity in chapter seven. Thus our focus in chapter six will mainly be on our joints, how they allow us to move and how they are classified. You will notice that the various joints in our body allow different ranges of motion. In general, the more mobile a joint, the less stable it is, making it more prone to injury. The shoulder joint is a nice example. However, there are other factors that affect our mobility and stability such as ligaments, tendons, skin, cartilage and daily activity. We will look at these in greater detail over the next few chapters. But let’s start by taking a simple look at the basic functions of the skeleton.
Basic Skeleton Function
The skeleton is our basic framework of support for all body tissues. It is our internal scaffolding that provides support for organs, allows movement, and gives us protection. The skeleton system is the name that is given to the collection of about 206 bones in addition to the joints and ligaments.
The skeleton has many important functions that vary in complexity but generally includes the following: (insert simple skeleton schematic)
1. It protects the body’s vital organs such as the brain, heart, lungs and other organs.
2. It gives us our shape, posture and support.
3. It provides sites for muscular attachment that allows us to move.
4. It provides a reservoir for the storage of minerals such as calcium, phosphorus, fat, magnesium and many other minerals.
5. It is an important site for the production of blood cells, specifically red blood cells that allow us to transport oxygen.
Our skeleton is a complex living system that is constantly changing. We will discuss many of these components in Chapter 7. However, at this stage we are interested in the role of the skeleton in how it allows us to move. In order to do this we need to take a look at our joint structure. The 206 bones in our body form approximately 230 joints.
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The joints are simply the place where two bones meet. Joints, which are also referred to as articulations, come in many different forms and not all are movable. The degree of mobility in a joint has a lot to do with its role or addition to its shape. The joints fall into three categories: synovial, fibrous or cartilaginous. They vary in movement and design.
Types and Classification of Joints
Joints are found anywhere that two bones meet. They have a specific and natural range of motion ranging from highly movable to unmovable. While most of our joints are freely movable, many are not. Joints are classified in several ways. For example, we classify some according to their architecture or their range of motion. Commonly, we use a mix of anatomical architecture and range of motion. In terms of movement, joints can also be classified according to the number of cardinal planes in which they can move. Therefore, joints can be non-axial (allowing no movement in any plane), uni-axial (movement in one plane of motion), biaxial (movement in two planes of motion), or tri-axial (movement in three planes of motion). Those joints that are freely movable are also referred to as synovial joints because at the end of the bone is a smooth covering layer called the synovial membrane. This membrane secretes a lubricating substance called synovial fluid which allows the joints to move in a smooth and fluid fashion. As this membrane breaks down over time we often experience more discomfort in our joints with movement. This is a form of arthritis.
(Chapter 8 in Seeley has great illustrations for all of this chapter)
Joints are normally classified as belonging to one of three sub-classes. These classifications are based on several factors, including:
a. the presence or absence of a joint cavity
b. the shape and nature of the connection
c. the degree of movement.
The three sub classes are as follows:
Synovial or Diarthrosis joints (freely movable).
Fibrous or Synarthrosis joints (immovable).
Cartilaginous or Amphiarthrosis joints (slightly movable).
Synovial or Diarthrosis Joints
These are the freely movable joints such as the shoulder, knee, ankle, etc. With this type of joint the articulating bones are covered with articular cartilage which is surrounded by an articular capsule which is lined with a synovial membrane. The articular surfaces are smooth and allow easy fluid movement. The synovial joint has two main functions. One is to allow movement, while the second is to transmit forces from one segment of the body to another segment, or one part of a limb to the other.
The interactions between bones at an articulation are regulated by several types of structures. There include the joint capsule, synovial membrane, ligaments, bone shape, articular cartilage and pressure. However, it is the general structure of the synovial joint that permits smooth movement. Synovial joints have five characteristic features. They all contain the following which facilitates their range of motion:
a. articular cartilage
b. joint cavity
c. articular capsule
d. synovial membrane
e. synovial fluid
These contents and arrangements allow the bones to move and glide across each other. This synovial arrangement allows for the greater range of movement of any joint types and movements permitted include the following: gliding, hinge, pivot, circumduction. Of these movement types gliding is the most common as it occurs in every synovial joint since it allows them to simply glide over each other. In some joints, like the carpal and tarsal joints, gliding is the only movement possible. The articular end of bones in a synovial joint are covered with hyaline cartilage (articular cartilage) and a surrounding tubular capsule which we call the joint capsule. The joint capsule is composed of an outer layer of ligaments and on the inside contains a synovial membrane which secretes synovial fluid. Some synovial joints have additional features. For example the knee contains small shock absorbing pads called menisci. Menisci are actually small pieces of fibrocartilage situated between the bones to absorb shock. Joints with menisci also have small fluid filled sacs called bursae. Bursae are also lined with synovial fluid and also help with smooth joint movement.
Let’s look at the synovial joints in more detail.
There are six types of synovial joints. If you read different textbooks you will notice several different terms for the same type of joint. Where appropriate the other terms are also provided.
(Insert Figs 8.8-8.12 from Seeley)
(Also insert table 7.4 from Shier)
a. Pivot Joint. This joint comprises a ring of bone that rotates around another. An example of this is found in the neck (the atlanto-axial joint). This joint is also referred to as a troichoidal or screw joint. This type of joint can also occur when two long bones fit against each other so that the bones roll around each other as with the radius and the ulnar in the forearm. The only type of movement that pivot joints allow is rotation. This movement only occurs in one plane and is therefore uni-axial.
b. Ball and Socket Joint (enarthrodial, spheroidal). This joint is the most mobile and allows movements in all directions. Examples include the hip and shoulder. The high degree of mobility also causes the joint to be less stable. In this type of joint, the head of a long bone fits into a cuplike structure of the other bone. If you think about the trailer and hitch setup on a car, the joint is highly mobile and allows movements in the three cardinal planes and is therefore referred to as tri-axial.
c. Hinge Joint (ginglymus). This joint allows flexion and extension (but not rotation). For this reason it is referred to as a uni-axial joint. Examples are the elbow and the knee joint. This joint structure contains strong ligaments and is therefore a very stable joint.
d. Ellipsoid (condyloid, ovoid). This joint is essentially a less flexible version of the ball and socket joint. This joint has an oval surface that fits into a reciprocally shaped concave disc surface. This joint allows movements in tow planes and is therefore biaxial. It allows flexion and extension movements, and abduction and adduction and therefore circumduction as these movements can occur together. Examples include the radiocarpal joints.
e. Saddle (sellar, carpometacarpal). The visual of a riding saddle is a good image to depict this joint. The bone surfaces are both shaped like a riding saddle and therefore fit over each other allowing flexion and extension, and abduction and adduction. Even though this joint has the same movement capabilities as the ellipsoidal, it has a greater range of motion. The joint is therefore biaxial. An example is the carpometacarpal joint of the thumb.
f. Gliding (plane, arthrodial). In this type of joint the articulating surfaces are almost flat and so the surfaces glide over each other. This motion is fairly limited and the joint is viewed as a non-axial joint. Examples of this joint include the intercarpal and intertarsal joints.
There are also other synovial type structures that are associated with the diarthrodial joints. They are called bursae and tendon sheaths. Like other aspects of the “joint capsule” these are susceptible to injury and breakdown over time causing discomfort and pain. The bursae are small capsules lined with synovial membranes that also contain synovial fluid. Their role is more for cushioning between the bones as opposed to providing a fluid lubrication surface (although they do that also). A primary role of bursae is to separate tendons and bone which reduces the friction during movement. Tendon sheaths are also synovial structures that surround tendons. Tendon sheaths are double layered structures and they also add an element of protection to many tendons that cross joints, such as those in the hands and wrist.
Fibrous or Synarthrosis Joints
These joints do not contain an articular cavity and are generally viewed as immovable. These joints are made mainly from fibrous connective tissue and can move very little. They are mostly concerned with absorbing shock. In this type of joint two bones are joined together by a fibrous connective tissue. There are two basic types of fibrous joints.
a. Sutures. This type of joint is found only in the skull. They are very rigid joints designed mainly to absorb impact. The design of these joints is such that grooved or serrated bone ends are attached by tightly connected fibers. This also allows skull growth. As an adult these fibers begin to ossify and are eventually 100% replaced by bone and then are basically immovable versus somewhat movable in growing children.
b. Syndesmoses. Like sutures, dense fibrous tissues bind the bones together allowing limited movement (although more than sutures). Examples in the body include the coraco-acromial joint. In this joint arrangement the bones are usually further apart than they are in sutures and are joined by ligaments. Hence, allowing some movement.
The teeth are also an example of a particular type of fibrous joint. Sometimes they are called peg and socket joints. This joint is more accurately referred to as a gomphosis joint. Gomphosis literally means a bolt in Greek. A gomphosis joint is an articulation via the insertion of a conic process into a socket (hence the bolt). If you envisage the root of your tooth into an alvestus (small hollow) in your jaw, this is the form it takes. A gomphosis is not really a connection between two bones even though it is considered a fibrous joint.
Cartilaginous or Amphiarthrosis Joints
These joints do not contain an articular cavity but are viewed as slightly movable. They allow a twisting or bending motion. There are two basic types of cartilaginous joints differentiated on whether they joint together using hyaline cartilage (sychondroses) or fibrous cartilage (symphyses):
a. Sychondroses. This literally means held together by cartilage. The bones are held together by a thin layer of hyaline cartilage. Examples include the sternocostal joints. The growth plates of bones are sychondroses. Interestingly, many sychondroses are temporary as bone eventually replaces the cartilage forming a synostoses (when two bones fuse together to form one bone).
b. Symphyses. These joints are connected by fibrous cartilage which allows slight compression. In these joints a thin layer of hyaline cartilage separates a disk of fibro cartilage from the bones. Again, the joints allow limited movement. Examples include the vertebral joints.
(Insert Table 8.2/8.1, Seeley)
The smooth movement of synovial joints is made possible by several features. We generally identify two forms of lubrication and refer to them as boundary lubrication and fluid film lubrication. The fluid film lubrication is really what allows us our day to day fluid movement as it comprises a thin film of lubricant that separates the bones. It functions for the most part under low loads with higher speeds. As an interesting side note; synovial joints can self-lubricate by shifting the synovial fluid back and forth under bone surfaces as the bones move. Boundary lubrication on the other hand is more important for higher stress loads over longer periods of time.
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In the earlier section on synovial joints we introduced the terms articular cartilage and articular capsule. We also discussed the role of the synovial fluid in lubricating these joints. Like any mechanical device, be it a bicycle chain or car engine or human joint, lubrication is vital for proper functioning. In humans, a white connective tissue known as articular cartilage provides this lubrication. This white dense cartilage coats the ends of the bones in diarthrodial joints allowing movement with minimal friction, wear and pain. It also spreads the load at the joint over a wider area decreasing pressure and stress at any contact point. Estimates suggest that articular cartilage can reduce the contact stress on bones and joints by up to 50%. Articular cartilage is somewhat unique in its design as a living substance. Articular cartilage contains no blood vessels, nerves or lymph vessels. Water makes up most of the mass of articular cartilage with estimates ranging from 65-80% of the weight of the time. Articular cartilage is also referred to as a viscoelastic tissue, sometimes this is referred to as biologically time dependent. What this means is that when you apply a constant load over time to the cartilage, its mechanical behavior (and shape) will also change over time. An example of this is an increase in the thickness of cartilage that occurs from exercise as greater volumes of fluid move in and out of the joint!
Cartilage is a connective tissue that comes in several forms. There are three recognized types:
c. white fibrocartilage
Hyaline cartilage is smooth with shiny physical properties of a glue-like substance (even though it lubricates). The term ‘hyal’ means glassy.
Articular cartilage lines the articular (smooth) surfaces of the bones allowing for efficient smooth movement.
White fibrocartilage is a strong fibrous tissue saturated with the glue-like cartilage that gives it a very strong tendon-like property.
Articular fibrocartilage is found in amphiarthroses joints. This articular fibrocartilage is found as a fibrocartilaginous disc known as a menisci. This is the design in the intervertebral discs. The role of menisci are somewhat unclear but are believed to help reduce shock.
The final articular component is that of articular connective tissue. Articular connective tissue includes both tendons and ligaments. The tendons connect muscle to bone and the ligaments connect bone to bone. These connective tissues are passive tissues comprised mainly of collagen and elastic fibers. These tissues are minimally extensible with no contraction ability and instead return to resting length as the muscle relaxes and the antagonist contracts. These tissues are elastic which helps them return to their original length.
Ligaments join bone to bone by inserting directly into the bone of the periosteum. Ligaments comprise fibers that are arranged in various directions. The major constituent in ligaments is the protein collagen and is very strong. The ligaments plan a major role in the stability of a joint. The arrangement of the ligaments varies according to the joint and the degree of mobility within the joint. In joints with greater mobility and larger ranges of motion there are usually multiple ligaments. The knee is a nice example in that its mobility necessitates four ligaments. This arrangement allows for a high degree of mobility while also maintaining its stability.
Joint Problems and Injuries
The basic shape of joints and they way they functions makes them prime targets for injuries. Some injuries are mild like a sprain or slight hyperextension while others are much more severe like shoulder dislocations or torn anterior cruciate ligaments. Sprains basically are a stretch of the ligaments and are usually very painful although not serious. While many joint injuries heal on their own, many do not and require either surgery or medications. Common joint injuries include the sprains and strains but also tennis elbow or nursemaids elbow. Nursemaid’s elbow occurs when the radius (one of the bones in the forearm) slips out of place from where it normally attaches to the elbow joint. It is a common condition in children younger than 4 years of age. It is also called pulled elbow, slipped elbow, or toddler elbow. The medical term for nursemaid’s elbow is radial head subluxation. A sudden pulling or traction on the hand or forearm causes nursemaid’s elbow. This causes the radius to slip out of the ligament holding it into the elbow. It can occur when an infant rolls himself or herself over, from a fall or from pulling, or swinging a young child by the hand. Tennis elbow is also a very common injury and contrary to popular belief doesn’t just result from playing tennis. Tennis elbow is a repetitive stress injury of the elbow that occurs when the muscles and tendons in the elbow area are torn or damaged. Tennis elbow is usually caused by repetitive activities that strain the tendons in the elbow area, such as using a manual screwdriver, using a hammer, gripping something repeatedly or of course hitting backhand in tennis. These types of injuries are usually acute and be treated effectively within a few days. Other conditions like arthritis are more chronic and require more long term treatment.
Much joint soreness is caused by some sort of inflammation and the biggest cause of joint soreness in humans is arthritis which can affect any joint in the body. Arthritis is basically an inflammation of any joint in the body. Perhaps you have noticed that you are a little more stiff and slow when you get out of bed in the morning. Although we don’t always feel it, arthritis is pretty much present in every person. As it develops it causes pain in the joints with movement especially after periods of inactivity. It is estimated to affect about 10% of the world’s population and 14% of the US population and is suggested as the leading cause of disability in people over 50 years old. There are many types of arthritis but the most common is probably osteoarthritis (OA). OA affects articular cartilage and results from the breakdown of the joint capsule and loss of synovial fluid. This means bones can end up ‘rubbing together’ which cause pain an inflammation. However, it is not just the joint capsule per se that is involved but also the ligaments, tendons and muscles. It has long been maintained that repetitive stresses caused arthritis but that is not always the case as we know that people who exercise regularly do not develop as much arthritis. It appears to be more due to trauma, age and infection.
In chapter six we provided a brief introduction of the general skeleton but paying particular attention to the role and classifications of articulations (or joints). We have learned that although there are many, many types of joints there are three basic classifications, namely, synovial, fibrous, and cartilaginous. The joints are classified according to their structure and also how much movement they allow. The joints that we are interested in the most in kinesiology are really the synovial joints. They are what permit the greatest range of motion. The structure of these joints is highly dependent upon synovial fluid which is a highly fluid lubricating substance permitting smooth movement. Joints vary in their range of motion whereby some joints do not allow any movement and some allow movement in all three cardinal planes. As a general rule, the more planes of movement a joint can move through, the less stable the joint, and the more likely we are to injure it. The shoulder joint is a nice example of this relationship. Ligaments are also present in joints and they play a varying role in the stability of a joint. Ligaments join bone to bone and are highly tensile structures. Generally, we find a greater arrangement of ligaments in joints with greater ranges of motion.
You may have noticed that women often become more flexible when they are pregnant. Naturally, this is to prepare for the action of childbirth and labor. However, for this increased flexibility to occur there needs to be structural changes in the joint structure. This is mediated by changes in hormones such as estrogen and progesterone. But there is another not so common hormone, relaxin, which increases and acts to improve mobility in the symphysis pubis, allowing them to stretch more. Although this action is most pronounced in the symphysis pubis, the hormone can act on all connective tissue in the body. However, while this is beneficial, it can also be problematic as this increased flexibility can cause injury such as back pain, or in worse cases torn ligaments during a fall. For the most part the hormone levels are restored to normal levels shortly after delivery.
Can you now answer the following questions related to joints and joint structure?
Differentiate between a fibrous joint, a cartilaginous joint, and a synovial joint!
2. Can you identify which of the synovial joints have:
a. 3 degrees of freedom?
b. 2 degrees of freedom?
c. 1 degree of freedom?
3. In your own words describe the 3 major classifications of joints and give two examples for each classification (if possible). Also write the synonym for the terms below.
a. Synarthrodial joint?
b. Amphiarthrodial joint?
c. Diarthrodial joint?
Can you list a motion/action that is allowed for each of the six diarthrodial joints:
5. Identify 5 primary functions of the skeleton!
6. Starting with the neck and working downwards, classify each joint.
Fingers (not thumb)
Trunk (bottom of spine)
7. Identify the type of joint shown below and then identify, where possible, two locations where one may find that type of joint on your body:
Insert a picture of 6 diarthrodial joints!
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