Loading of the temporomandibular joint: anatomical and in vivo evidence from the bones.

'Loading of the TMJ' is usually understood to mean a compressive force applied to the articular surfaces of the jaw joint. Theoretical models of jaw mechanics can be manipulated to support either the presence or the absence of loading, depending on the assumed contraction patterns of the muscles and the assumed occlusion. This paper synthesizes a series of studies on jaw joint function using pigs as substitutes for humans. Bone strain (deformation) was directly measured on the lateral surfaces of the condylar neck and the squamosal (equivalent to the human temporal) bone. Chewing strains indicate that loading does occur and is not light. The peak strains on the condyle are indeed primarily compressive, but the situation is dynamic. Small tensile strains can occur during chewing, and protrusive splints may decrease the strain resulting from muscle stimulation. The squamosal bone is even more surprising, in that the major strain is tensile. The most likely explanation for this finding is that the squamosal bone is bent under the load. Thus, the two elements of the TMJ are deformed in different ways by the same movements and muscle activities. Internal bony architecture reflects these differences. The condyle is filled with fine, vertically oriented bony trabeculae. The articular eminence has thick cortices and trabeculae oriented approximately transversely. In conclusion, the TMJ is loaded, but the situation is complex. The largest forces seen by the condyle are compressive, and they arise from muscle contraction. These same forces serve to bend the squamosal bone.