Validation of a finite element model of the Kirschner-Ehmer external skeletal fixation system

To determine the validity of finite element analysis (FEA) as a means of examining biomechanical properties of the Kirschner-Ehmer external skeletal fixation system.

10 paired tibiae harvested from skeletally mature dogs weighing between 30 and 38 kg immediately following euthanasia for reasons unrelated to musculoskeletal disease.

A gap fracture was created in each bone; fragments were stabilized with 3 frame configurations (type I, type II, and type III), using enhanced-profile threaded pins. Each bone-frame construct was tested, using a materials testing machine in 3 modes of testing: axial compression (AC), mediolateral (ML) bending, and craniocaudal (CC) bending, for a total of 9 tests/bone. The elastic limit of the constructs was not exceeded during testing. Mean stiffness values were determined from load-displacement curves. A finite element model of each construct was created, using three-dimensional elastic beam elements, and stiffness values were calculated, using FEA. Correlations between experimental and FEA data then were determined.

Significant differences in stiffness were seen among all 3 constructs in CC bending and AC, with stiffness increasing with construct complexity. No significant difference in ML bending stiffness was seen between type-II and type-III constructs; however, both were significantly stiffer than the type-I constructs. The experimental and FEA stiffness data were strongly correlated (AC, r = 0.994; ML bending, r = 0.998; CC bending, r = 0.985).

Strong correlations among experimental and FEA data indicate that FEA is a valid method of comparing stiffness of Kirschner-Ehmer external skeletal fixation constructs.