Synovial Joints

Image courtesy of pages.uoregon.edu

Over the past week I've posted two posts (Post 1, Post 2) on the feet that mention synovial joints, so I wanted to take the time to dig a little deeper into what a synovial joint is.

First, there appears to be three different types or classes of joints in the human body--synovial joints, fibrous joints, and cartilaginous joints (p 25-26)*.  Synovial joints are of particular interest to me because that is the classification of joint of nearly all of the limbs and the one that is used for locomotion (p 26)*.

So these synovial joints have some pretty cool distinguishing features about them.  There is this joint capsule that brings the joint together and is made up of fibrous layer which has a synovial membrane lining it (p 25)*.  This synovial membrane does three things:  1) it secretes synovial fluid, which lubricates the joint; 2) it closes off the joint cavity, which contains a small space for the synovial fluid to go once it is secreted by the synovial membrane; and 3) it covers all of the internal surfaces of the joint except for where the two bones articulate with one another.  Those areas are covered with articular cartilage (p 25)*.

There are also these periosteum and perichondrium tissues that surround the bones (periosteum) and the cartilage (perichondrium) of the joint, serving to provide nutrients for these areas as well as serve as areas for new formation of bone and cartilage (p 25)*.  Additionally, synovial joints usually appear to have some type of ligamentous material reinforcing them, which are either completely separate tissues (called extrinsic accessory ligaments) or are where part of the joint capsule thickens (called intrinsic accessory ligaments) (p 26)*.

Of the many synovial joints in the human body, Clinically Oriented Anatomy groups them into six classifications based off of how they articulate and the type of movement allowed.  The first type is a plane joint such as the acromioclavicular joint.  The second type is called a hinge joint such as the elbow joint.  The third classification of synovial joints is called a saddle joint and the fourth is called a condyloid joint.  The carpometacarpal joint is an example of the former and the metacarpophalangeal joints are an example of the latter.  The fifth classification is more easily recognizable with it being a ball and socket joint such as the hip, while the sixth is a little more obscure being a pivot joint such as the median atlantoaxial joint (p 27-28)*.

Interestingly, Clinically Oriented Anatomy does not mention hyaline cartilage in reference to synovial joints at all, instead referring to articular cartilage but highlighting hyaline cartilage by name as a characteristic of primary cartilaginous joints, an example of which is given as the hip or coxyfemoral joint (p 26)*.  So, according to this source, joints can fall in multiple categories.

Finally, just a few words on the nerve and blood supply of synovial joints.  It appears that the joint capsule, specifically the synovial membrane, is both highly innervated and highly vascularized, containing articular nerves and articular veins (p 28)*.

Your body.  Your training.

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*Moore, Keith L; Dalley, Arthur F; Agur, Anne M. R. Clinically Oriented Anatomy Sixth Edition.  Wolters Kluwer; Lippincott Williams & Wilkins. 2010.