Ex vivo cyclic mechanical behaviour of 2.4 mm locking plates compared with 2.4 mm limited contact plates in a cadaveric diaphyseal gap model.

OBJECTIVES: To compare the mechanical properties of locking compression plate (LCP) and limited contact dynamic compression plate (LC-DCP) constructs in an experimental model of comminuted fracture of the canine femur during eccentric cyclic loading. METHODS: A 20 mm mid-diaphyseal gap was created in eighteen canine femora. A 10-hole, 2.4 mm stainless steel plate (LCP or LC-DCP) was applied with three bicortical screws in each bone fragment. Eccentric cyclic loadings were applied at 10 Hertz for 610,000 cycles. Quasistatic loading / unloading cycles were applied at 0 and 10,000 cycles, and then every 50,000 cycles. Structural stiffness was calculated as the slope of the linear portion of the load-displacement curves during quasistatic loading / unloading cycles. RESULTS: No bone failure or screw loosening occurred. Two of the nine LCP constructs failed by plate breakage during fatigue testing, whereas no gross failure occurred with the LC-DCP constructs. The mean first stiffness of the LCP constructs over the course of testing was 24.0% lower than that of constructs stabilized by LC-DCP. Construct stiffness increased in some specimens during testing, presumably due to changes in bone-plate contact. The first stiffness of LC-DCP constructs decreased by 19.4% and that of locked constructs by 34.3% during the cycling period. A biphasic stiffness profile was observed: the second stiffness was significantly greater than the first stiffness in both groups, which allowed progressive stabilization at elevated load levels. CLINICAL SIGNIFICANCE: Because LCP are not compressed to the bone, they may have a longer working length across a fracture, and thus be less stiff. However, this may cause them to be more susceptible to fatigue failure if healing is delayed.

Type IV Monteggia fracture in a cat.

A one-year-old neutered male cat was referred for a grade IIIA open radius and ulna fracture. The mid-diaphyseal radial and ulnar fractures were associated with lateral radio-humeral and radio-ulnar dislocations. From these abnormalities, a diagnosis of type IV Monteggia fracture was made. The annular ligament was found to be intact on examination and the radial head was correctly positioned and stable after fracture reduction and plate fixation. Eighteen months postoperatively, the cat did not exhibit any signs of lameness or any signs of pain on palpation and mobilisation. Signs of mild osteoarthritis were present on radiographs. Type IV Monteggia fractures are very rare in animals. The combination of a radial head dislocation with an intact annular ligament is uncommon. This pattern allowed a single reconstruction with two plates without a radio-ulnar screw.