Static vs dynamic loads as an influence on bone remodelling
Abstract
Remodelling activity in the avian ulna was assessed under conditions of disuse alone, disuse with a superimposed continuous compressive load, and disuse interrupted by a short daily period of intermittent loading. The ulnar preparation consisted of the 110 mm section of the bone shaft between two submetaphyseal osteotomies. Each end of the preparation was transfixed by a stainless steel pin and the shaft either protected from normal functional loading with the pins joined by external fixators, loaded continuously in compression by joining the pins with springs, or loaded intermittently in compression for a single 100 s period per day by engaging the pins in an Instron machine. Similar loads (525 N) were used in both static and dynamic cases. The strains engendered were determined by strain gauges, and at their maximum around the bone's midshaft were − 0.002. The intermittent load was applied at a frequency of 1 Hz as a ramped square wave, with a rate of change of strain during the ramp of 0.01 s−1. Peak strain at the midshaft of the ulna during wing flapping in the intact bone was recorded from bone bonded strain gauges in vivo as −0.0033 with a maximum rate of change of strain of 0.056 s−1.
Examination of bone sections from the midpoint of the preparation after an 8 week period indicated that in both non-loaded and statically loaded bones there was an increase in both endosteal diameter and intra cortical porosity. These changes produced a decrease in cross sectional area which was similar in the two groups (− 13%). Intermittently loaded bones however showed a 24% increase in cross sectional area resulting from new bone deposited predominantly, but not exclusively, on the periosteal surface. It appears that in this preparation, a static load sufficient to produce strains in the functional dynamic strain range has no effect on bone remodelling, whereas a similar load applied intermittently for a short daily period may be associated with a substantial increase in bone mass.
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Present address: Royal Veterinary College, London, NW1 OTV, U.K.