Crowther’s Tenth Martini, Chapter 9
Winter 2015
Chapter 9:
Joints
Chapters 6 through 8 covered bones; Chapters 10 and 11 cover muscles, which move the bones. Chapter
9 is a transitional chapter in which we focus on the joints, our usual frame of reference for
understanding these movements.
9.0: Outline
9.1: What are joints?
 A joint, or articulation, is a place where two bones connect.
9.2: Two ways of classifying joints
 Joints may be classified structurally, based on the materials between the bones, as bony,
cartilaginous, fibrous, or synovial.
 Joints may be classified functionally, based on how much they can move, as synarthoses,
amphiarthroses, or diarthroses.
9.3: Names of individual joints
 Individual joints are usually named according to the bones that comprise them. For
example, the radiocarpal joint is between the radius and the carpal bones.
9.4: Movement at synovial joints
 There are six types of synovial joints: ball-and-socket, condylar, gliding (or plane), hinge,
pivot, and saddle.
 Flexion is a movement that decreases the joint angle between articulating bones;
extension is a movement that increases the joint angle.
9.5: The structure of synovial joints
 Synovial joints include articular cartilage at the ends of bones and a joint capsule
containing synovial fluid.
 Synovial joints may also include accessory structures such as bursae, fat pads, ligaments,
menisci, and tendons.
9.6: The flexibility/stability tradeoff
 More flexible joints are less stable and vice versa.
 The flexibility and stability of a joint are affected by the way the bones fit together, the
number and strength of ligaments, restriction of motion by accessory structures, and joint
capsule strength.
9.7: An encapsulation of common joint problems
 Common joint problems include sprains, dislocations, arthritis, and bulging/herniated
discs.
9.8: Recommended review questions
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Crowther’s Tenth Martini, Chapter 9
Winter 2015
9.1: What are joints?
A joint is defined as any place where two (or more) bones connect. We usually think of joints as
being highly mobile, like the shoulder joint or knee joint, but the word joint also applies to pairs
of bones where the bones don’t move relative to each other, such as the fused bones of the
cranium (frontal, occipital, parietal, temporal).
The word articulation means the same thing as the word joint. Often we say things like, “the
scapula articulates with the humerus”; all we mean by this is that the scapula forms a joint with
the humerus. The word articulation should remind you of the word arthritis, as they share a
common root and start with “art.” Arthritis refers to any inflammation of a joint, as discussed
below.
To understand why having joints is useful, consider the alternative, i.e., NOT having joints.
What if your skeleton was one giant bone? What problems would you have? Movement in
general would be difficult, of course. It is advantageous to have the skeleton with parts that are
somewhat independent of each other. Also, it might be hard to develop pre- and post-natally if
you had a single giant, complicated bone that had to grow and ossify over time while meeting the
body’s needs at every stage.
9.2: Two ways of classifying joints
10th Martini notes that joints can be classified according to structure (what materials are between
the two bones?) and according to function (how much can the joint move?). The structural
categories are as follows:
 Bony: Nothing is between the bones; the bones interlock directly, and are often
considered a single bone. Examples: the left and right mandible and left and right frontal
bones of the cranium. In both cases, the left and right sides begin as separate bones and
then fuse together.
 Cartilaginous: Cartilage is between the bones. Example: the pubic symphysis between
the left and right pubic bones of the pelvis; the intervertebral discs, which are pads of
fibrocartilage between vertebrae.
 Fibrous: Collagen-rich connective tissue that isn’t cartilage (which can also contain lots
of collagen) is between the bones. Example: the distal joint between the tibia and fibula,
which are connected by a ligament (dense regular connective tissue).
 Synovial: A more complex joint capsule is between the bones – see 10th Martini Figure 91 (The Structure of a Synovial Joint). Examples include all joints with a wide range of
motion: ankle, elbow, hip, knee, etc. More information on synovial joints is given in
section 9.5 below.
When classifying joints by function, the choices are:
 The joint cannot move (synarthrosis).
 The joint can move a little (amphiarthrosis).
 The joint can move a lot (diarthrosis).
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Crowther’s Tenth Martini, Chapter 9
Winter 2015
Note the root “arth” in all of these words, again indicating that the words are about joints. The
prefix “syn” means “coming together” (think of synthesis, synergy, or synchronicity); here the
bones have come together so fully that no movement is possible. “Amphi” often means
“intermediate between two extremes”; an amphibian splits its time between land and water, and
an amphiphilic molecule is between hydrophobic and hydrophilic. Thus an amphiarthrotic joint
is between synarthrotic joints and diarthrotic joints in its extent of movement.
The structural and functional categories above can be interlaid as follows. Joints that can move a
lot (diarthroses) are synovial. Joints that cannot move much or at all (synarthroses and
ampohiarthroses) belong to one of the other three structural categories (bony, cartilaginous, or
fibrous).
9.3: Names of individual joints
Some joints have everyday names like “ankle,” “elbow,” “hip,” “knee,” and “shoulder.” These
joints, and others, also have formal names derived from the specific bones that form them. Here
are some straightforward examples:
 atlantoaxial joint: between the atlas (C1 vertebra) and the axis (C2 vertebra)
 atlantooccipital joint: between the atlas (C1 vertebra) and the occipital bone
 carpometacarpal joint: between carpals and metacarpals
 claviculosternal joint: between the clavicle and the sternum
 femoropatellar joint: between the femur and the patella
 intercarpal joint: between carpals
 interphalangeal joint: between phalanges
 intertarsal joint: between tarsals
 intervertebral joint: between vertebrae
 metacarpophalangeal joint: between metacarpals and phalanges
 metatarsophalangeal joint: between metatarsals and phalanges
 radiocarpal joint: between the radius and carpals
 radioulnar joint: between the radius and the ulna
 sacroiliac: between the sacrum and the ilium
 tarsometarsal joint: between tarsals and metatarsals
 temporomandibular joint: between the temporal bone and the mandible
 tibofemoral joint: between the tibia and the femur
 tibiofibular: between the tibia and the fibula
A few other joint names include parts other than bone names per se, but make sense if you know
that costal means “ribs” and that the glenoid cavity and the acromion are parts of the scapula
(remember acromial from Chapter 1?).
 acromioclavicular joint: between the scapula and the clavicle
 costovertebral: between ribs and vertebrae
 glenohumeral: between the scapula and the humerus
 sternocostal: between the sternum and ribs
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Crowther’s Tenth Martini, Chapter 9
Winter 2015
I will not ask you quiz or test questions in the format of, “What is the name of the joint between
the scapula and the clavicle?” However, if I refer to the acromioclavicular joint, or any of the
other joint names listed above, I will expect you to know which bones I’m talking about.
9.4: Movement at synovial joints
You have probably heard of joints being described as ball-and-socket joints or hinge joints.
These are two of the six types of synovial joints shown in the second page of 10th Martini Figure
9-2 (Joint Movement). Note that, while all six types are capable of significant movement, they
differ in their directions and axes of movement because of the way in which the articulating
bones fit together. The full list of synovial joint types, with examples, is:
 Ball-and-socket joint: hip, shoulder
 Condylar joint: radiocarpal joint; most metacarpophalangeal and metatarsophalangeal
joints
 Gliding/plane joint: acromioclavicular, claviculosternal, intercarpal, and sacroiliac joints
 Hinge joint: ankle, elbow, knee, and interphalangeal joints
 Pivot joint: atlantoaxial and proximal radioulnar joints
 Saddle joint: 1st carpometacarpal joint
While we will not memorize the number of axes associated with each type of joint, we can
appreciate that, for example, hinge joints (1 axis of movement) are more restricted in their
movement than ball-and-socket joints (3 axes of movement).
10th Martini lists many terms referring to types of movements that can occur at synovial joints:
pronation/supination, inversion/eversion, etc. Here we will focus on three pairs of terms that are
especially common and important.
 Extension/Flexion. Flexion decreases the joint angle between articulating bones;
extension increases the joint angle. When you kick a soccer ball, the kicking motion
extends the knee joint. The opposite of that – “resetting” the knee joint after a kick – is
flexing the knee joint.
 Dorsiflexion/Plantar flexion. Ankle joint terminology is confusing. The least
ambiguous, most widely accepted terms are dorsiflexion for drawing your toes toward
your knee and plantar flexion for pushing your toes away from your knee. When your
foot pushes off the ground while running or walking, that is plantar flexion.
 Abduction/Adduction. Abduction means moving away from the midline of the body,
whereas adduction means moving toward the midline of the body. For example, the
chicken dance consists of alternately abducting your elbows (moving them up and away
from your sides) and adducting your elbows (moving them down and toward your sides).
This should be easy to remember if you know that the word “abduct” means “take away”
(think of abducted children, alien abductions, etc.).
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Winter 2015
9.5: The structure of synovial joints
The structure of synovial joints is presented in 10th Martini Figure 9-1 (The Structural of a
Synovial Joint). Note that Figure 9-1a is a “generic” synovial joint, while Figure 9-1b shows the
knee in particular. Let us first notice the features common to all synovial joints, as shown in
Figure 9-1a:
 The ends of the articulating bones are covered in articular cartilage, which is very
similar to the hyaline cartilage that you learned about in Chapter 6.
 Unlike joints classified as bony, cartilaginous, or fibrous, synovial joints are bound by a
joint capsule – a covering that extends from the periosteum of the adjacent bones. On the
inside of the capsule is a synovial membrane (which we mentioned in Chapter 4 as an
area in which epithelial and areolar connective tissues come together).
 Within the joint capsule is synovial fluid, which 10th Martini describes as a “clear,
viscous solution with the consistency of egg yolk or heavy molasses.” The synovial fluid
provides lubrication, shock absorption, and distribution of nutrients to the articular
cartilage’s chondrocytes (cartilage cells). This distribution of nutrients is important
because the cartilage does not have its own blood supply.
Figure 9-1b shows accessory structures that individual synovial joints may or may not have:
 Bursa: a fluid-filled lubricating body.
 Fat pad: self-explanatory.
 Ligament: regular connective tissue connecting bone to bone.
 Meniscus: C-shaped pad of fibrocartilage
 Tendon: regular connective tissue connecting muscle to bone.
These accessory structures are noteworthy in part because they are frequent sites of injuries. At
the knee, the anterior cruciate ligament (ACL) is often overstretched and injured by athletes; torn
menisci are another common sports injury.
Note that synovial joints are a category distinct from cartilaginous joints and fibrous joints even
through they include both cartilage and other fibrous tissue such as ligaments.
9.6: The flexibility/stability tradeoff
There are advantages to a joint being flexible, and there are advantages to a joint being stable,
but a given joint cannot be both maximally flexible and maximally stable; the more flexibility
(freedom to move) is permitted, the less stability there is. This tradeoff is evident in the
classifying joints on a spectrum ranging from synarthrotic (very low flexibility, very high
stability) to diarthrotic (higher flexibility, lower stability). Even among the diarthrotic, synovial
joints, there is much variation in flexibility and stability, which stems from variations in the
components of these joints. One key factor is the way in which the bones themselves fit
together. The elbow is a stable joint in large part because of the tightly interlocking structures of
the humerus and ulna; likewise, the hip joint is fairly stable because the head of the femur fits
nicely into the deep pocket of the acetabulum of the pelvis. In contrast to the hip joint, the
shoulder joint’s pocket (provided by the glenoid cavity of the scapula) is fairly shallow, making
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Crowther’s Tenth Martini, Chapter 9
Winter 2015
the shoulder more flexible but less stable than the hip. Below are some additional factors that can
also affect the flexibility and stability of a joint:
 The number and strength of the ligaments
 Any restriction of motion by other nearby bones, tendons, and/or fat pads
 The strength of the joint capsule
9.7: An encapsulation of common joint problems
We can summarize common joint problems in the form of the table shown below.
CTM Table 9.1: Common joint problems.
Term
Nature of problem
Common treatments
Sprain
A ligament is stretched
and/or torn
Traditionally, RICE (Rest, Ice,
Compression, Elevation); now
often MICE (Movement instead
of Rest)
Dislocation
A bone is out of place
Have a medical professional put
the bone back!
Osteoarthritis
Articular cartilage, and
sometimes the bone tissue it
is protecting, gets damaged
and worn away
Pain medication; joint
replacement surgery; moderate
exercise
Rheumatoid
arthritis
Joints are attacked by the
immune system (this is an
autoimmune disease)
Disease-Modifying AntiRheumatic Drugs (DMARDs),
which work via diverse
mechanisms; anti-inflammatory
drugs; non-inflaming exercise
Gouty arthritis
Crystals of uric acid build
up in synovial fluid
Anti-inflammatory drugs; dietary
changes to lower uric acid levels
Intervertebral discs impinge
on spinal nerves
Education on body mechanics;
physical therapy; pain medication;
surgery (discectomy)
Arthritis: joint
inflammation
Bulging or herniated disc
9.8: Recommended review questions
If your understanding of this chapter is good, you should be able to answer the following 10th
Martini questions at the end of Chapter 9: #1, #5, #6, #7, #8, #10, #15, #16, #28, #29, #30. (Note
that these are NOT the Checkpoint questions sprinkled throughout the chapter.)
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Crowther’s Tenth Martini, Chapter 9
Winter 2015
Explanation
This file is my distillation of a chapter in the textbook Fundamentals of Anatomy & Physiology,
Tenth Edition, by Frederic H. Martini et al. (a.k.a. “the 10th Martini”), and associated slides
prepared by Lee Ann Frederick. While this textbook is a valuable resource, I believe that it is
too dense to be read successfully by many undergraduate students. I offer “Crowther’s Tenth
Martini” so that students who have purchased the textbook may benefit more fully from it. No
copyright infringement is intended.
-- Greg Crowther
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