PATTERNS OF STRAIN IN THE MACAQUE ULNA DURING FUNCTIONAL-ACTIVITY

In vivo bone strain experiments were performed on the ulnae of three f emale rhesus macaques to test how the bone deforms during locomotion. The null hypothesis was that, in an animal moving its limbs predominan tly in sagittal planes, the ulna experiences anteroposterior bending. Three rosette strain gauges were attached around the circumference of the bone slightly distal to midshaft, They permit a complete character ization of the ulna's loading environment. Strains were recorded durin g walking and galloping activities. Principal strains and strain direc tions relative to the long axis of the bone were calculated for each g auge site. In all three animals, the lateral cortex experienced higher tensile than compressive principal strains during the stance phase of walking, Compressive strains predominated at the medial cortex of two animals (the gauge on this cortex of the third animal did not functio n). The posterior cortex was subject to lower strains; the nature of t he strain was highly dependent on precise gauge position. The greater principal strains were aligned closely with the long axis of the bone in two animals, whereas they deviated up to 45 degrees from the long a xis in the third animal. A gait change from walk to gallop was recorde d for one animal. It was not accompanied by an incremental change in s train magnitudes, Strains are at the low end of the range of strain ma gnitudes recorded for walking gaits of nonprimate mammals. The measure d distribution of strains in the rhesus monkey ulna indicates that med iolateral bending, rather than anteroposterior bending, is the predomi nant loading regime, with the neutral axis of bending running from ant erior and slightly medial to posterior and slightly lateral. A variabl e degree of torsion was superimposed over this bending regime. Ulnar m ediolateral bending is apparently caused by a ground reaction force ve ctor that passes medial to the forearm. The macaque ulna is not reinfo rced in the plane of bending. The lack of buttressing in the loaded pl ane and the somewhat counterintuitive bending direction recommend caut ion with regard to conventional interpretations of long bone cross-sec tional geometry, (C) 1998 Wiley-Liss, Inc.