Dislocation

In materials science, a dislocation is a crystallographic defect, or irregularity, within a crystal structure. The presence of dislocations strongly influences many of the properties of materials. The theory describing the elastic fields of the defects was originally developed by Vito Volterra in 1907, but the term 'dislocation' to refer to a defect on the atomic scale was coined by G. I. Taylor in 1934. Some types of dislocations can be visualized as being caused by the termination of a plane of atoms in the middle of a crystal. In such a case, the surrounding planes are not straight, but instead they bend around the edge of the terminating plane so that the crystal structure is perfectly ordered on either side. The analogy with a stack of paper is apt: if half a piece of paper is inserted in a stack of paper, the defect in the stack is only noticeable at the edge of the half sheet.

There are two primary types: edge dislocations and screw dislocations. Mixed dislocations are intermediate between these.

Joint dislocation

A joint dislocation, or luxation, occurs when there is an abnormal separation in the joint, where two or more bones meet. A partial dislocation is referred to as a subluxation. Dislocations are often caused by sudden trauma on the joint like an impact or fall. A joint dislocation can cause damage to the surrounding ligaments, tendons, muscles, and nerves. Dislocations can occur in any joint major (shoulder, knees, etc.) or minor (toes, fingers, etc.). The most common joint dislocation is a shoulder dislocation.

Treatment for joint dislocation is usually by closed reduction, that is, skilled manipulation to return the bones to their normal position. Reduction should be done only by trained people, because it can cause injury.

Causes

Joint dislocations are caused by trauma to the joint or when an individual falls on a specific joint. Great and sudden force applied, by either a blow or fall, to the joint can cause the bones in the joint to be displaced or dislocated from normal position. With each dislocation, the ligaments keeping the bones fixed in the correct position can be damaged or loosened, making it easier for the joint to be dislocated in the future.

Dislocation (syntax)

In syntax, dislocation is a sentence structure in which a constituent which could otherwise be either an argument or an adjunct of the clause occurs outside the clause boundaries either to its left or to its right as in English They went to the store, Mary and Peter.

The dislocated element is often separated by a pause (comma in writing) from the rest of the sentence. Its place within the clause is often occupied by a pronoun (e.g. they).

There are two types of dislocation: right dislocation, in which the constituent is postponed (as in the above example), or a left dislocation, in which it is advanced. Right dislocation often occurs with a clarifying afterthought: They went to the store is a coherent sentence, but Mary and Peter is added afterward to clarify exactly who they are. By contrast, left dislocation is like clefting: it can be used to emphasize or define a topic. For example, the sentence This little girl, the dog bit her has the same meaning as The dog bit this little girl but it emphasizes that the little girl (and not the dog) is the topic of interest; one might expect the next sentence to be She needs to see a doctor, rather than It needs to be leashed. This type of dislocation is a feature of topic-prominent languages.