ABSTRACT
Under the influence of externally applied forces, or loads, a structure deforms or changes in dimensions. When a load in a known direction is imposed on a structure, the deformation of the structure can be measured and recorded on a load/deformation curve. The load/deformation curve is useful for determining the mechanical properties of whole structures such as whole bone or bone implant composites. The initial straight portion of the curve (a) is the elastic region. This portion of the curve defines the elas - ticity of a structure. As load is applied in the elastic region, deformation occurs but is not permanent; the structure returns to its original shape after the load is removed. If, how ever, loading continues, the outermost fibers of the structure begin to yield, defin - ing the yield point (b) beyond which the structure will not return to its original shape after the load is removed, i.e. the structure will have some residual deformation. This region of the curve beyond the yield point (c) is known as the plastic region. If load ing is increased, the structure will eventually fail, defining the ultimate failure point (d). The stiffness of the structure can be determined from the slope of the load/deformation curve in the elastic region. The steeper the slope, the stiffer the struc ture. Three par - ameters for determining the strength of a structure can be determined from the load/deformation curve. These include the load the structure can withstand before fail - ing, the deformation the structure can withstand before failing, and the energy the struc ture can absorb before failing. The strength, either in load or deforma tion, known as the ulti mate strength, can be determined from the curve by the ultimate failure point. The strength in terms of energy storage is determined by the magnitude of the area under the entire curve. The larger the area, the higher the energy storage before failure.
4 i. Bilateral carpal hyperextension injuries. ii. The palmar support for the carpus is derived from a myriad of small ligaments that support the surrounding bones and fibrocartilage. A major misconception is that carpal hyper extension injuries are caused by flexor tendon disruption. The tendon of insertion of the flexor carpi radialis muscle contributes minimally to the stability of the carpus. The main structure sup port ing the palmar aspect of the antebrachiocarpal joint is a complex of ligaments which con nect the distal radius, the distal ulna and the accessory carpal bone to the palmar aspect of the radial carpal bone. The intercarpal joint is also supported by an array of small, unnamed ligaments. The carpometacarpal joint is supported by the strong palmar carpal fibrocartilage, many small ligaments and two strong accessory carpal-meta carpal ligaments. Hyperextension injuries result from disruption of some or all of these anatomic structures. iii. In addition to dorsopalmar and lateral view radiographs, stress radiographs should be obtained to establish the level(s) of the instability. Stress radio graphs are necessary to identify the location of the instability. Standing, cross-table radio graphs are obtain ed with the dog weight-bearing (lifting the ipsilateral limb will increase weight-bearing) using a horizontal beam. If instability is confined to the intracarpal and carpometa car - pal joint, a partial carpal arthrodesis may be indicated; however, if there is instability of the antebrachiocarpal joint, a pancarpal arthrodesis should be considered.
