TMJ Surgery: The disc/capsule complex

 The above photos are of the human disc/capsule and the mandibular condyle along with the lateral pterygoid muscle which is the small but powerful muscle that opens the jaw. There are two views of this cadaver specimen which shows a sagittal (from the lateral, outside to inside) view and a coronal view (front to back)…the views that are taken with MRI examination of the jaw joint. I have found that visualizing the actual anatomy helps greatly in talking to patients about the objectives of treatment and for their own understanding. I hope this helps you as well.

There are many myths about “TMJ”….sometimes they are perpetuated by the professions that “manage” patients. Also, realize that this website is dedicated to orthopedic problems of the human jaw joint that create issues with well localized “ear” pain as well as a myriad of chronic conditions that arise out of misdiagnosis over time. This blog will serve as a first in a series relative to expanding into understanding other pain issues such as chronic headache associated with “TMJ”, muscular pain, etc. We will also talk (in another blog to come) about an ugly history in TMJ surgery in the 1980′s that was based on using artificial materials placed that were designed to act as an artificial “disc”….you can’t fool mother nature and she taught us a lesson once again in this era that paralleled a similar problem in the development of modern orthopedic hip replacement.

In our photos above, we are viewing only one joint. The mandible or jaw is the only bone and orthopedic system in the body with a dual joint operating system. Jaw movement is complex. Movement of the lower jaw is a complex orthopedic mechanism. Dual movement of both joints simultaneously, complinentary create the simple notion that the lower jaw is like a loosely held with a kind of fluid movement or “floating bone” , under the contol of pairs of muscle groups on either side of the midline. We have looked at biomechanical principles and the concept of angular momentum elsewhere in other blogs and the web site. What you see is a remarkable orthopedic apparatus.

Patients, who have received consultation and/or treatment for “TMJ” often are first presented that the joint has a “disc” or “meniscus” which are terms for other cartilage components of other joint systems of the body (spine and knee respectively). One fundamental misunderstanding that is prevalent is that the “disc” of the TMJ is somehow an entity of its own. Often what is visualized is a two dimensional view and this is incomplete.

As you see in the photos, the condyle (ball) of the jaw is in close adaptation to a 3-dimensional fibrous tissue cap(disc/capsule) that fits like a cup over the condyle. The complexity of jaw movements result in an infinite number of forces that are generated simultaneously in both joints by the movement of the jaw. The net effect that with an orthopedic system that has such complex mobility, only a tissue with natural elasticity can perform the job, day after day. If you were to take a mass of the “disc/capsule” you would get the feeling it was similar to the feel of soft leather or any similar material that when wadded into a ball, it would resume its original shape when let go. An analogy of the relationship of the disc/capsule to the condyle is that of a cap fastend on top of a soda bottle….the center of the cap is the “disc”, the crimped part of the cap below the rim of the mouth of the bottle the perimeter ligament attachments that fastens and hold the cap on the bottle. 

This fundamental property is the most important characteristic of the nature of the “cartilage” in the TMjoint. In fact it is not cartilage, it is fibroelastic tissue and is under constant load, movement and activity, and has to respond to increased demand under specific conditions. With mouth opening, the entire shape of the disc/capsule changes as it adapts to the various curvatures and movements of the condyle as it glides forward and past the various curvatures of the fossa or joint socket at the base of the skull.

With orthopedic problems of the jaw joint, a derangement is an incoordination, during movement and a dysfunction between the boney structures of a joint and the key cartilage components of the joint. Jaw popping,clicking, locking, inability to load the jaw joint with chewing force and mouth opening incoordination are all signs of a TMJ orthopedic derangement. Does this mean that every jaw joint that pops or clicks is or will be a problem? Not necessarily as the literature is clear that jaw popping or clicking can be a common (not necessarily normal) finding in humans….much of it is due to the complex movement of the joint…and the transformation of kinetic energy (energy of movement on soft joint tissues to sound when there is an occasional hesitation to movement from some cause. Also, with the ear directly next to the joint, sounds can be common….if our ear was anatomically located next to the knee….it would sound like a creaky floor every time we walk. So….noise by itself is not always a predictor of a pathology that will worsen.

What is critical is identifying the degree of instability of the disc/capsule and what is the cause of the instability in the first place. Biomechanics and studies are clear that when a derangement is created in the laboratory during studies, the most vulnerable region of the joint capsule attachment to the condyle is in the extreme lateral to lateral posterior portion of the attachment. Here is the “achilles heel” of this orthopedic system. An injury to the jaw, hyper extension injury can cause rupture of this region of the disc/capsule attachment to the condyle. Pain and joint popping (orthopedic instability) get worse over time due to the fundamental reality that the gliding movement of the jaw joint with attempted mouth opening creates shear and torque forces which cause further detachment and/or plastic deformity of the disc/capsule  over time.  Morphologic tissue changes in the character of the fibrous tissue are like a callous that forms overtime in the palms of our hands or soles of our feet.  These changes occur most frequently between the surfaces of the condyle where it contacts the naturally concave cupped surface of the disc/capsule.

 If the derangement is caused by a developing boney impingement from the socket or fossa (not pictured here…see previous blog with surgery photos of a lateral fossa impingement)  projecting into the disc/capsule during mouth opening, often the joint can get stuck such that it does not allow the joint to glide to allow mouth opening….or it can get stuck open creating a subluxation or dislocation. These are significantly painful events that are fundamentally orthpedic dysfunctions internal to the joint….not muscle spasticity as often suggested.

It has been shown that it takes over 640 Newtons of force to break the bone just below the condyle of the human jaw in the region called the condylar neck ( ) This is the most frequent site for a jaw fracture. It has also been shown, that it takes 10% or less of the force required to cause the fracture, to cause a tear or detachment of the disc/capsule from this lateral perimeter attachment of the disc/capsule to the condyle. ( ) Such forces are quite prominent in cases of childhood falls or with any blow to the jaw, or hyperextension injury (sprain) due to excessive mouth opening during a blow to the jaw, flexion/extension injury to the jaw etc…..that are incidents or forces not strong enought to cause fracture. We all have an intuitive understanding how ligament cartilage injury occurs in other joint systems (cruciate ligament tears in the knee, Ligament tears in the foot/ankle, disc rupture in the spine, and rotator cuff tears in the shoulder). Unfortunately, the mechanisms of potential myriads of TMJ injury is often ignored in all circles…..and this is unfortunate….especially in the growing child.

One of the important functions of the disc/capsule is  that its proper adaptation over and function with the condyle is critical for condylar growth and consequently,  with growth and development of the lower jaw. Our long bones grow from each end at the growth or epiphyseal plates….the jaw achieves most of its growth at the condyles of the jaw, there is no epiphyseal growth plate, rather an overall 3 dimensional volumetric increase in the size of the condyle during the skeletal growth years of life.  All jaw movements and even chewing of food creates reactive or tension forces delivered to the condyle by a properly adapted disc/capsule….this is another uniqueness of the jaw joint in that microstructure of the organization of fibrous tissue is designed to resist tension forces…where other joints of the body are designed to accomodate compression forces through weight bearing resistance design. If you remove this structure from a growing animal, growth at this joint can be  severely impacted. That is why identifying children at risk is extremely important…particularly those who have had jaw trauma and may have had a significant injury capable of creating a significant detachment of the disc/capsule from the condyle. Development of asymmetries or crooked jaws is a common association and sequela.

Lastly, Athanasiou and Detamore are two bioengineers who have dedicated much of their careers to the study of TM joint function and its structural design,. With the expertice of individuals such as these scientists, much of the myths and mistaken concepts of the past will eventually disappear. They have suggested that the proper way to look at the TMjoint disc/capsule and its relationship to its surrounding boney relationships with the condyle and fossa is to think of the disc/capsule as a trampoline…..when we chew food and exert compression loading to the joint, the “disc” portion of the structure is the trampoline center which we jump on….as we jump on it, it changes its shape to adapt to our weight compacting into it…..the springs of the perimeter of the trampoline is where much force is distributed and their integrity is key to the trampoline’s function….if one by one a spring breaks….the trampoline is no longer functional……it becomes loose and flacid and can no longer spring back….the trampoline becomes useless. In addition, if we cary this analogy further….a loose, poorly attached or weaking disc/capsule attachment (s) become even more damaged over time during the phenomenon of mouth opening where the condyle glides forward……an unstable capsule then is subjected to even more tissue damage due to the generation of unstable shear forces within the main body of the disc (trampoline).

Such physical realities point to the explanation as to why patients who develop significant orthopedic problems in the jaw joint do so much earlier (20-30 years on the average) compared to similar conditions in other joint systems of the body…..the tissue that makes up the human jaw joint “cartilage” must have significant inherent elasticity and “rebound” much like that trampoline….whenever the first few springs begin to break…the inherent forces generated in the joint will take over and progressive joint failure may result….if not perceived early enough.  A typical story of a patient with surgical orthopedic disease or dysfunction of the jaw joint is that they first began to notice the problem during their teen or skeletal developing years.  Instability of the jaw, popping, locking became more frequent and louder…sometimes loud enough for others to hear.  It was amusing for a while, but later pain began to develop, the fit of the teeth perceptually changed and other symptoms set in…..many begin to become impaired in their 20s and 30s. 

Next blog…the blood supply and nerve supply to the human jaw joint and its association with referred pain and headache.

Dr. W. Kirk, Jr.
Charlotte