The Law of Laplace and its relevance to contemporary medicine and rehabilitation.

OBJECTIVE To show that the Law of Laplace is not only a historical curiosity but also remains relevant to our daily teaching and clinical activities. DATA SOURCES Comprehensive MEDLINE (1960-2000) and CINAHL (1982-2000) computer literature searches performed by using key words such as Law of Laplace, Laplace, and Laplace relationship. Additional references were obtained from the bibliographies of the selected articles. Supplementary searches were also made by using various Internet search engines. STUDY SELECTION Primary references were used whenever possible. DATA EXTRACTION A single reviewer assessed all references and extracted information relevant to the Law of Laplace. DATA SYNTHESIS Although the Law of Laplace is attributed to Pierre Simon de Laplace, Laplace may not deserve the credit for the discovery. Nevertheless, the relationship (T [tension] alpha P [pressure] R [radius]) is easily derived and improves our understanding of the physiologic basis of many of our medical and rehabilitation practices. For example, the Law provides an insight into the mechanism of action of compression garments and lumbosacral orthoses, an understanding of the role of uterine muscle during delivery, and a reason why cesarean sections are made in the lower uterus. In addition, the Law explains many aspects of such diverse phenomena as penile erection, compartment syndromes, and peripheral edema. Perhaps most important, the Law explains the basis of many common medical practices that we use to promote bladder emptying, to control edema, and to plan surgery. CONCLUSION The Law of Laplace explains the mechanism of a wide range of physiologic phenomena. Unfortunately, even though it was developed about 200 years ago, the insights it provides us are often underused. More consideration of its implications can improve our clinical practice, our teaching, and our enjoyment of medicine.