Temporomandibular Joint (TMJ): A Weight Bearing Joint?

Headnote

Abstract

There is an unresolved controversy concerning the forces transmitted at the temporomandibular joint. A hypothesis that the joint carries no load or at least, are subjected to little or no transarticulation compression during biting, has been supported by various researchers, where as others have opposed this fact. This controversy basically stems from the fact that the most of the researches are on the jaws of vertebrates other than mammals and no reliable data is available for humans. However, newer biomechanical analysis more or less, confirm that it is a weight bearing joint especially on the condyle opposite to one being used for biting.

Keywords: Temporomandibular joint; mandibular condyle; weight bearing; load bearing.

Introduction

From the anatomical and physiological point of view it is clear that weight bearing, load bearing and stress bearing are synonymous. Temporomandibular joint (TMJ) is defi- ned as the articulation between the temporal bone and the mandible. It is a bilateral diarthroidal, ginglymoid joint.1 It is the articulation of the condylar process of the mandible and the intra articular disk with the mandibular fossa of sq- uamous portion of temporal bone; a diarthroidal sliding hi- nge (ginglymus) joint.1 TMJ is a ginglymo-diarthroidal joi- nt which differs in its form and function from the other joi- nts of the human body. It is the most versatile and compo- und joint of the body which in most of the time is in work- ing state.2

Characteristic features of Temporomandibular Joint

1. Temporomandibular joint is in fact a double joint cons- isting of 2 synovial joint cavities separated by an articu- lar disc, each performing different functions. Rotatory movement in lower joint cavity between disc and head of condyle, whereas translatory movement in upper joi- nt compartment between disc and glenoid fossa (Figure 1).3

2. Other unique feature is the presence of dense avascular fibrocartilagenous disc between condyle and articular fossa. Articular disk which is devoid of nerve and blood vessels in it's centre , is interposed between the head of the condyle and slope of the condylar path and posterior slope of articular eminentia.

3. The third feature is that the 2 temporomandibular joint does not function independently, one joint is dependent on the other. One condyle cannot move in any manner without reciprocal movement on the opposite side.4

4. The fourth characteristic feature of this joint is the influ- ence of teeth on its movements. The functional movem- ents of the joint are guided by the nature of the occlusal surface of the teeth. There exists an occluso-articular harmony; any situation which interferes with this harm- ony may cause temporomandibular joint dysfunction. The positioning of the teeth against each other in occlu- sal rehabilitation or in planning occlusal scheme in rest- orations is important to the functioning of the joint and musculature (Figure 2).3

Different Opinions

Temporomandibular joint: Not a weight bearing joint

The glenoid fossa is not a functional load bearing unit of the joint but a mere pathway for the mandibular moveme- nts. The absence of the cartilage covering the glenoid fossa supports the fact that the glenoid fossa is not designed for bearing load. Ligaments and the disc are the bracing eleme- nts and not the glenoid fossa, which is neither the functio- nal part of articulation nor stress bearing. This is substanti- ated by the thin roof of the glenoid fossa, which is someti- mes translucent, having no articular cartilage covering it and the presence of minute foramina which traverse it for the passage of blood vessels.3 All these anatomical features point out that glenoid fossa is a fragile structure not capable of taking any load (Figure 3). Naturally, the temporoma- ndibular ligament prevents the displacement of the condyle superiorly to avoid injury to the delicate roof of glenoid fo- ssa. Therefore, from anatomical point of view, the roof of the glenoid fossa is not suitable as a stress bearing site.

Henry Hollinshead stated that the pressure produced by the closure of the jaw is normally exerted almost entirely on molar teeth, and temporomandibular joint receives little pr- essure. The thinness of the bone of the mandibular fossa, the fact that this may even be lined by synovial membrane containing nerves and vessels and sparseness of fibrocarti- lagenous tissues in the joint all indicate that it is not desig- ned to bear pressure.5

According to a hypothesis that joint carries no load or, at least, are subject to little or no transarticulation compress- ion during biting, has been supported by Wilson in 1920 and Scott in 1955.6,7 The weight of the cranium is beard by craniovertebral junction.8 Wilson in 1920 reported that the fibrocartilage of the TMJ condyle was softer than hyaline cartilage and therefore, could not be load bearing. Taylor in 1980 showed that no evidence was found of stress in glen- oid fossa and concluded that it is not stress bearing.

Temporomandibular joint: A weight bearing joint

The articular disk which is a dense avascular fibrocartilage interposes the articular surface, and is structured to bear the stress load. The disc fits on the condyle like a cap and mov- es together with the condylar head during its rotational and translatory movements.3 This integrity of condyle and disc functioning as one unit is very significant, as a disharmony in this relationship could give rise to clicking in the joint. The fibrous avascular disk together with the temporoman- dibular ligament which limit the rearward movement of the condyle are so tough and protective of the soft tissues beh- ind the joint, that even a sudden hard blow to mandible di- rected posteriorly or upward will cause either the fracture of the neck of the condyle or the mandible, rather than driv- ing the condyle posteriorly into tympanic plate or upward through the articular fossa. This explains the protective act- ion of the disk.

There is a theoretical evidence based on Newtonian mech- anics that "Jaw Joint is a weight bearing joint with a verti- cal bite force of 500N on the left first molar, the right cond- yle must support a load of well over 300N."9 Non working condyle is more loaded than working side condyle which explains why patients with a fractured condyle choose to bite on side of fracture.

Dawson in his book stated that the temporomandibular jo- int is designed to bear stress and must be capable of resist- ing forces that measure into 100pounds. The condyle serv- es as a bilateral fulcrum for the mandible, and so the joints are always subjected to stress whenever the powerful elev- ator muscles contract (Figure 4).4 The specific areas of re- inforcement of the fossa confirm with the bearing areas for the upward, forward and inward forces of the musculature. Further he evidenced that TMJ is a stress bearing joint, all the articular surfaces of condyle, the fossa and the eminen- ce are covered with avascular layers of dense fibrous conn- ective tissue. The absence of blood vessels is a sure sign that those specific areas are designed to receive considera- ble pressure. The avascular areas are also devoid of innerv- ations and this includes the bearing areas of the disk. So, if the condyle and the disk are in proper alignment in the fos- sa they can receive great pressure with no sign of discom- fort since there are no sensory nerves in the bearing areas to report discomfort.

Smith told that temporomandibular joint is a load bearing joint over the normal functional range of bite force positi- ons and angles. Temporomandibular loads were found to vary from a maximum appositional force of 60% of the bite force (when bite force are applied to the incisors) to a distr- action force of about 5% of the bite force (when applied to distal surfaces of third molars).10

TMJ loads tend to reach a minimum as a result of vertically directed bite forces positioned at second molars. In respo- nse to sagitally directed bite forces, the condylar loads we- re found to have small magnitudes. The force directions were restricted to a range nearly perpendicular to the artic- ular eminence and were appositional in nature.10

In response to a bite force which incorporated mediolateral component greater than 20% from vertical, the condylar lo- ads were found both to have large magnitudes and exhibit directions which were distractive in nature. Condylar loads were highly asymmetrical. These results indicate that mas- ticatory function which involves mediolateral bite force will bring stress to the TMJ in the form of large distracting loads. Large asymmetrical joint loads associated with med- iolaterally directed bite force are evidenced that masticat- ory muscles are incapable of completely resisting mediola- terally directed forces as none of the muscles of masticat- ion exhibit predominant mediolateral vector.10

Smith and Savage in 1959 and Turnbull in 1970 supported that temporomandibular joint is a weight bearing joint. Ga- ylord in his article stated that the 2 temporomandibular joints are unevenly loaded during unilateral biting or closu- res for posterior teeth. He saw the muscle activity pattern and control of temporomandibular joint loads and conclud- ed that, the neuromuscular system does not maintain balan- ced TMJ forces at all tooth positions.11 Kang in 2006, studi- ed microscopic changes in TMJ disk in response to tension and compression, and studied the mechanism of disk failu- re when subjected to mechanical stress. According to him the disk is loaded in function and heavier stresses can cause irreversible changes in the disk.12 Hylander WL in 1975 ca- rried out experiment in macaca monkey in which strain ga- uges were placed within the bone of the condyle below the joint surface and recorded strains of different degrees. The strains were greater on the contralateral side than the ipsil- ateral side.13

Temporomandibular Joint: A weight sharing joint

The 2 condyles of the TMJ are unevenly loaded. The non working condyle is more loaded than the condyle on the working side. The forces are transmitting, but not at equal magnitudes so the condyles share the weight.9 Hylander WL also stated that strains were greater in contralateral si- de than the ipsilateral side.13 Both the condyles work in uni- son but different amount of forces are shared between each other. Gaylord concluded that working/balancing ratio do not produce equal forces on the 2 TMJ. Experimental mea- surements of condylar loading in animals suggest that bala- ncing side condyle is indeed more heavily loaded during unilateral biting and chewing on the molar teeth.14-15

Conclusion

The skull does not transmit its weight on temporomandibu- lar condyle. Consequently for most of the situations it is not a weight bearing joint. However, during biting mechanism it takes load which is 60% of the biting force. If the left jaw is generating 500N of biting force, the opposite condyle is subjected to 300N of load bearing. In other words, the joint biting does not share the load bearing.

References

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