Loading and Form in the Mammalian Jaw Joint

Abstract

The synovial articulation between the dentary and squamosal bones, usually referred to as the temporomandibular joint (TMJ), is a defining feature of the Class Mammalia. The mandibular component of this joint is a secondary cartilage that develops in a membranous matrix and contributes to growth of the mandible. While resembling the growth cartilage of long bones, the mandibular condylar cartilage differs in that it also must function as the articular surface of the mammalian jaw joint, a function mediated by a fibrous thickening of the perichondrium. Another difference from typical growth cartilage is that cells are closely packed and not aligned in columns. This feature permits the direction of growth to be altered. Perhaps because of its origin from membranous tissue, the mandibular condyle is notably sensitive to mechanical loading, particularly during the period of growth. Loading of the TMJ arises from contraction of the jaw muscles, but the magnitude and orientation of loading vary according to the muscle combination used. In this contribution we summarize and synthesize studies on condylar loading assessed using strain gages affixed to the condylar neck of rabbits (Oryctolagus cuniculus) and pigs (Sus scrofa) under normal and altered conditions, correlating the findings with condylar morphology. During normal mastication young adult rabbits evince nearly vertical compressive strain on the condylar neck. Reduction of muscle force using botulinum toxin diminished and reoriented the strain, and these changes were accompanied by a striking loss of bone in the condylar head and neck. The paralyzed side condyle tended to be smaller than that of the healthy side, suggesting that condylar surface area did not increase normally, but the thickness of the cartilage layer was unaffected, implying no change in length of the cartilage process. Like rabbits, pigs show compressive strain on the condylar neck during mastication. During the growth process, the orientation of masticatory compressive strain alters from posterior‐superior in younger animals to anterior‐superior in older pigs. Studies correlating condylar strain with cartilage proliferation showed a tendency for proliferation to decrease as condylar compressive strain increased. A procedure (distraction osteogenesis) used clinically to increase mandibular length was found to lower the magnitude and reorient condylar strain. Unlike the adult rabbits, these juvenile pigs responded to lowered loads with increased growth, but the direction was altered in correspondence with the reorientation of masticatory strain. Collectively, these studies illustrate the dependence of mandibular morphology on the specifics of species, age, and especially masticatory mechanics.Support or Funding InformationSupported by DE08513, DE14336, DE018142 from NIDCR.