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 appl ied to the articular surfaces of the jaw joint. Theoretical models of jaw m echanics can be manipulated to support either the presence or the absence o f loading, depending on the assumed contraction patterns of the muscles and the assumed occlusion. This paper synthesizes a series of studies on jaw j oint function using pigs as substitutes for humans. Bone strain (deformatio n) was directly measured on the lateral surfaces of the condylar neck and t he squamosal (equivalent to the human temporal) bone. Chewing strains indic ate that loading does occur and is not light. The peak strains on the condy le are indeed primarily compressive, but the situation is dynamic. Small te nsile strains can occur during chewing, and protrusive splints may decrease the strain resulting from muscle stimulation. The squamosal bone is even m ore surprising, in that the major strain is tensile. The most likely explan ation for this finding is that the squamosal bone is bent under the load. T hus, 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 tra beculae. The articular eminence has thick cortices and trabeculae oriented approximately transversely. in conclusion, the TMJ is loaded, but the situa tion is complex. The largest forces seen by the condyle are compressive, an d they arise from muscle contraction. These same forces serve to bend the s quamosal bone. Copyright (C) 2001 S. Karger AG, Basel.