FORAMEN MAGNUM VARIABILITY AND OCCIPITAL DYSPLASIA IN THE DOG

Size and shape of the foramen magnum were studied in skulls from 80 Pe kingese, 12 Beagles, 9 Doberman Pinschers and 7 German Shepherds. Afte r maceration of the skulls, the total height (H), normal height (h), m aximal width (B) and area of each foramen magnum were measured, and sk ull indices were determined. The basic configuration of the canine for amen, magnum is ovoid, with a short dorsoventral axis (h) and longer h orizontal axis (B). The mean ratio between normal height and width (h: B) was 77% in small dogs and 75% in larger dogs. The foramen magnum ha d a dorsal extension with variable length (N) i n all but two Pekinges e, in 7 Doberman Pinschers and 5 Beagles and in none of the German She pherds examined. Prolapse of the cerebellum or brain stem through the enlarged opening was prevented in all cases by a fibrous membrane cove ring the dorsal occipital index (H:B) and foramen magnum index (B:H) w ere determined for each case. The size of the foramen magnum presented a large variability and was not significantly correlated with age or gender. The dimensions of the foramen magnum were better correlated wi th skull size than with the degree of brachycephaly. Comparative anato mic data and the absence of any neurologic problems in the dogs of thi s study indicate that the dorsal extension of the canine foramen magnu m is a normal morphologic variation rather than a pathologic condition .