Geometric Analysis of the Proximal Medullary Cavity of the Femur in the German Shepherd Dog

Summary

Forty-one cadaver femora of German Shepherd dogs were prepared for implantation of a cementless total hip prosthesis by reaming of the proximal medullary cavity with standard orthopaedic instruments. Silicone paste imprints of the cavity were taken and measured with a 3 D laser reflection system. Data were processed statistically according to a geometric model with these parameters: distal diameter Ø _dist , medio-frontal radius r, laterofrontal angle α, medio-frontal angle β, helix angle γ and increment eccentricity δ _e . Correlations between body mass and Ø _disv body mass and r, body mass and β were moderately significant (p <0.05, p <0.06, p <0.05 resp.). Other correlations between body mass and parameters were not significant. Simple weighing of a German Shepherd is not a useful predictor of the internal dimensions of the proximal femur.

The distribution of the most important parameter Ø _dist was compared with the range of Ø _dist sizes of the iso-elastic veterinary hip prosthesis (IVHP). This comparison showed that the test sample of 41 bones could be fitted with four IVHP sizes. However, 25 femora out of 41 do not fit into the available r values of the IVHP assortment. Less than adequate proximal canal fill will be the result if an IVHP is implanted, unless substantial reaming is done proximomedially.

It was shown that measurements on radiographs of intact dog cadaver femora do not yield equivalent results to the imprints. Moreover, comparison of the internal dimensions of the medullary cavity of the human femur showed that a press-fit human prosthesis cannot be geometrically proportional to the IVHP.

Dimensions of reamed cavities of forty-one cadaver femora of German Shepherd dogs were measured with a system of silicone imprinting and 3D laser reflectometry. Data were processed statistically according to a geometric model with seven parameters. Correlations between parameters and body mass were moderately significant for three parameters and not significant for the others. Distributions of the geometric parameters were calculated and implications with respect to prosthesis design are discussed. A comparison with geometric parameters obtained from radiographs was made and the significance of the differences is discussed. Finally, a comparison with the geometric parameters of the medullary cavity of the human femur is made.

Keywords

Dog - German Shepherd - hip prosthesis - internal geometry - femur

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