Pirie's Bone

Abstract

There is a general misconception regarding the significance of a small accessory ossicle located on the dorsum of the astragalus, just proximal to the astragaloscaphoid joint. A roentgenogram taken following a traumatic incident about the ankle joint will reveal this ossicle, at times with a somewhat irregular surface, adjacent to the main body of the astragalus. This repeatedly occurs in compensation cases with a misdiagnosis of a chip fracture. A. H. Pirie, in 1921, first focused attention on this ossicle, which now bears his name. At that time he had collected 14 cases, in 4 of which the condition was bilateral. Proof of the existence of the bone was obtained postmortem. Notwithstanding Pirie's report, there has been a paucity of comment on the condition in the medical literature, and it is usually omitted from standard textbooks, being mentioned only in Ferguson's text. The distal end of the astragalus often exhibits developmental irregularities. The astragaloscaphoid ossicle is a frequent occurrence and causes no difficulties. The irregularity in contour in the form of an exostosis on the dorsum of the neck, called the astragaloid spur, is less well known. At times this spur may be present as a distinct ossicle, with cortex on each surface, and no demonstrable defect in the astragalus, in spite of the irregularity in appearance periodically observed on the surface adjacent to the main body of the tarsus. It has been shown that pathologically the line of demarcation from the talus is cartilage. This pitfall in diagnosis can often be avoided by taking the opposite ankle for comparison, since there is a distinct bilateral tendency. The fact that no essential change is demonstrable in the roentgenogram, after the repeated application of a cast for varying lengths of time, should definitely establish the diagnosis of an accessory ossicle in contradistinction to a chip fracture. Summary Attention is again focused on an accessory ossicle, called Pirie's bone, located on the dorsum of the astragalus. When demonstrated roentgenographically following trauma, it may be mistaken for a chip fracture. Its bilateral tendency helps in the correct diagnosis.