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 1 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 9- 1 (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). 2 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 3 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.). 4 Crowther’s Tenth Martini, Chapter 9 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).