The extensor mechanism of the knee is essential to ambulation and is subject to a number of traumatic, congenital, and inflammatory processes. In the pediatric population, the spectrum of pathologic conditions affecting the extensor mechanism is specific to skeletally immature patients. In addition, certain congenital and developmental disorders may further predispose the knee extensor mechanism to injury. The pathologic processes can be subdivided into categories: conditions of the attachments and insertions of the quadriceps and patellar tendons, conditions of the patella, conditions of the quadriceps muscle group, and avulsions of the superior attachments of the quadriceps. Cases of conditions affecting the extensor mechanism of the pediatric knee were collected at two large trauma centers, and the clinical and radiologic features were reviewed. Initial evaluation of these conditions is performed with radiography, but magnetic resonance imaging has evolved into a useful adjunct for assessing the soft tissues for a more precise evaluation of the true extent of an injury, thereby affecting decisions about surgical intervention and prognosis.

Major chest trauma in a child is associated with significant morbidity and mortality. It is most frequently encountered within the context of multisystem injury following high-energy trauma such as a motor vehicle accident. The anatomic-physiologic make-up of children is such that the pattern of ensuing injuries differs from that in their adult counterparts. Pulmonary contusion, pneumothorax, haemothorax and rib fractures are most commonly encountered. Although clinically more serious and potentially life threatening, tracheobronchial tear, aortic rupture and cardiac injuries are seldom observed. The most appropriate imaging algorithm is one tailored to the individual child and is guided by the nature of the traumatic event as well as clinical parameters. Chest radiography remains the first and most important imaging tool in paediatric chest trauma and should be supplemented with US and CT as indicated. Multidetector CT allows for the accurate diagnosis of most traumatic injuries, but should be only used in selected cases as its routine use in all paediatric patients would result in an unacceptably high radiation exposure to a large number of patients without proven clinical benefit. When CT is used, appropriate modifications should be incorporated so as to minimize the radiation dose to the patient whilst preserving diagnostic integrity.

Editor's Note.-Everyone who has taken the course in radiologic pathology at the Armed Forces Institute of Pathology (AFIP) remembers bringing beautifully illustrated cases for accession to the Institute. In recent years, the staff of the Department of Radiologic Pathology has judged the ''best cases'' by organ system, and recognition is given to the winners on the last day of the class. With each issue of RadioGraphics, one or more of these cases are published, written by the winning resident. Radiologicpathologic correlation is emphasized, and the causes of the imaging signs of various diseases are illustrated.

No abstract