The puzzle of growth rotation

This article presents a re-examination of the concept of "intramatrix rotation" as defined by Björk and Skieller in 1983 and also explored by Lavergne and Gasson. This concept is based upon stable landmarks in the mandible (metallic implants). Björk and Skieller postulated that "intramatrix rotation" is an expression of the remodeling at the lower border of the mandible and assumed that the rotation occurred in the corpus of the mandible. Lavergne and Gasson, on the other hand, contended that the rotation affected the ramus and the gonial angle and, consequently, the length of the condylion-pogonion diagonal. An alternative interpretation of mandibular rotation is presented here. It is based upon an exploration of two divergent hypothetical patterns of growth: (1) a circular growth direction with a center on the chin and no enlargement of the mandible but maximal rotation versus (2) a linear growth direction with maximal enlargement of the mandible but minimal rotation. In reality, most children will fall in between these two postulated extreme patterns. This mechanism for selective enlargement of the mandible in response to condylar growth increments may be termed counterbalancing rotation. An operational definition is presented as follows: counterbalancing rotation pertains to circular condylar growth, accompanied by selective coordinated remodeling, which does not contribute to the incremental growth of the mandible.