Stress protection due to external fixation.
Bone is sensitive to mechanical influences. The presence of an orthopaedic device will impose constraints on the mechanical environment that may influence subsequent remodelling and repair. An Oxford External Fixator was applied to six intact ovine tibiae. The strains engendered during normal walking were then recorded from strain gauges applied to the mid-shaft of the bone. The fixator configuration was then altered such that the intact fixator bar connecting the pins was replaced with a sectioned version that did not permit load transfer through the fixator, and the strain environment re-recorded. The peak strains recorded with the intact bar were significantly lower than those recorded with the sectioned bar. This showed that the use of a fixator with an intact bar resulted in significant stress protection of the underlying bone. A fixator was then applied to both the right and left tibiae of a further six animals and the resulting strain environment and corresponding remodelling response was observed over 16 weeks. In each case one fixator was configured with an intact bar (the stress protected limb), whilst the other utilised a sectioned bar (the unprotected limb). The results showed that over this period the bone mineral content fell by 9% in the stress protected limb compared to the unprotected limp. Quantitative assessment of the bones showed that this bone loss occurred as a direct consequence of resorption on the periosteal and endosteal surfaces. In addition, strain recordings at week 16 showed that the fixator was still stress protecting the tibia.
O'Doherty
D M
DM
School of Engineering, University of Wales College of Cardiff, U.K.
Butler
S P
SP
Goodship
A E
AE
D000818:Animals; D001696:Biomechanical Phenomena; D015519:Bone Density; D001858:Bone Nails; D016723:Bone Remodeling / Q000502:physiology; D001842:Bone and Bones / Q000503:physiopathology*; D016267:External Fixators; D008124:Locomotion / Q000502:physiology; D011859:Radiography; D012756:Sheep; D013314:Stress, Mechanical; D013977:Tibia / Q000000981:diagnostic imaging / Q000473:pathology / Q000503:physiopathology; D016474:Weight-Bearing