Humeral torsion revisited: A functional and ontogenetic model for populational variation

Abstract

Anthropological interest in humeral torsion has a long history, and several functional explanations for observed variation in the orientation of the humeral head have been proposed. Recent clinical studies have revived this topic by linking patterns of humeral torsion to habitual activities such as overhand throwing. However, the precise functional implications and ontogenetic history of humeral torsion remain unclear. This study examines the ontogeny of humeral torsion in a large sample of primarily immature remains from six different skeletal collections (n = 407). The results of this research confirm that humeral torsion displays consistent developmental variation within all populations of growing children; neonates display relatively posteriorly oriented humeral heads, and the level of torsion declines steadily into adulthood. As in adults, variation in the angle of humeral torsion in immature individuals varies by population, and these differences arise early in development. However, when examined in the context of the developing muscles of the shoulder complex, it becomes apparent that variation in the angle of humeral torsion is not necessarily related to specific habitual activities. Variability in this feature is more likely caused by a generalized functional imbalance between muscles of medial and lateral rotation that can be produced by a wide variety of upper limb activity patterns during growth.

Bilateral Asymmetry of Humeral Torsion and Length in African Apes and Humans

“…This may explain why professional athletes such as baseball or tennis players have relatively lower torsion on the playing arm compared to the nonplaying arm [Pieper, 1998;Crockett et al, 2002;Osbahr et al, 2002;Whiteley et al, 2008;Myers et al, 2009;Schwab and Blanch, 2009;Taylor et al, 2009;Whiteley et al, 2010], and why individuals with brachial plexus injuries in which the medial rotators are unopposed by the lateral rotators have posteriorly oriented humeral heads (i.e. very low torsion) [Codine et al, 1997;Waters et al, 1998;Van der Sluijs et al, 2002;Pöyhiä et al, 2005;Cowgill, 2007]. Conversely, no statistically significant differences were found between right and left arms in non-athlete control groups [Pieper, 1998;Crockett et al, 2002] Aborigines [Martin, 1933], and that males possess relatively lower torsion than females [Evans and Krahl, 1945;Edelson, 1999].…”

“…Although the exact timing of the cessation of humeral torsion development is unknown [Cowgill, 2007], it is thought to occur at the time of epiphyseal fusion, between the ages of 16 and 20 [Krahl, 1976;Edelson, 2000]. While the natural process is for humeral torsion to increase throughout ontogeny, increased activity of the medial rotators (subscapularis, pectoralis major, latissimus dorsi and teres major) relative to that of the lateral rotators (infraspinatus and teres minor) limits the development of torsion, thus resulting in relatively lower torsion in these individuals [Cowgill, 2007]. Krahl [1976], for example, reports a correlation between the relative strengths of these muscles and the degree of torsion in the corresponding humeri.…”

Estimation of the angle of humeral torsion from digital images of dry humeri of South Indian origin

“…The distal end of the humerus was oriented along a transverse axis using reference lines marked on graph paper and was drawn using Adobe Photoshop version CS2 software after the image acquisition as mentioned below. [15] …”

Determining torsion angle of humerus head using MRI method

“…Humeral torsion angle (HTA) is the angle between the axis on the proximal articular surface of humerus and the axis on the distal articular surface (or transepicondylar line) (1)(2)(3)(4). Torsion of the humerus can be defined as a process of twisting in the proximal end of humerus.…”

“…Torsion angle can be measured when the axis of the humerus head (line between midsection of the tuberculum majus and the midsection of the caput humeri) and the transepicondylar axis in the distal end of humerus are arranged in a superposing fashion. The first anatomic studies about torsion of the humerus, conducted by Cowgill, focused on the origin and underlying reasons for studies (3). It was assumed that humeral torsion derives from 180° rotation of the radius around the ulna when embryologic rotation of forearm started and current localization and orientation of the humerus occurred during development of the extremity.…”