If A and K are compact convex sets in Rn, a Steiner-type formula is valid for the erosion of A by K if and only if A is open with respect to K. The if part is proved in the general case. The only if part is conjectured, and proved only in the case n = 2.

Departments of Dermatology and Pathology and tory Medicine, Perelman School of Medicine at the ity of Pennsylvania and the Department of Medicine, of Dermatology, Pritzker School of Medicine at the ity of Chicago. ources: None. f interest: None declared. dence to: Caroline A. Nelson, MD, 3600 Spruce Street, 2 y Building, Philadelphia, PA 19104. E-mail: Caroline. @uphs.upenn.edu. JAAD Case Repor...

INTRODUCTION Erosions of the sacroiliac joints (SIJ) on pelvic radiographs of patients with ankylosing spondylitis (AS) are an important feature of the modified New York classification criteria. However, radiographic SIJ erosions are often difficult to identify. Recent studies have shown that erosions can be detected also on magnetic resonance imaging (MRI) of the SIJ early in the disease cours...

OBJECTIVE To compare a low field dedicated extremity magnetic resonance imaging system (E-MRI) with x ray and clinical examination, in the detection of inflammation and erosive lesions in wrist and metacarpophalangeal (MCP) joints in newly diagnosed, untreated rheumatoid arthritis (RA). PATIENTS AND METHODS Twenty five patients (disease duration < or =1 year) and three healthy controls entere...