**Core Concept:** Benett's fracture is a type of diaphyseal fracture in which the proximal fragment is displaced by a deforming force. The deforming force is responsible for the translational or rotational displacement of the fragment.

**Why the Correct Answer is Right:** In Benett's fracture, the deforming force acts on the proximal fragment, causing a translational or rotational displacement. In this specific fracture type, the force applied results in the fragment being pushed medially (involving the medial cortex) or laterally (involving the lateral cortex). This leads to the translational or rotational displacement of the fragment, which is characteristic of Benett's fracture.

**Why Each Wrong Option is Incorrect:**

A. Deforming force on the distal fragment: This option is incorrect because in Benett's fracture, the deforming force acts on the proximal fragment, not the distal fragment.

B. Applied force on the shaft: This option is incorrect as well, as the deforming force in Benett's fracture is applied to the cortex of the proximal fragment, causing displacement.

C. No deforming force: This option is incorrect because a deforming force is indeed present in Benett's fracture, causing the translational or rotational displacement of the proximal fragment.

D. Deforming force on the opposite cortex: This option is incorrect because the deforming force in Benett's fracture acts on the cortex of the proximal fragment, not the opposite cortex.

**Clinical Pearl:** Understanding the specific pattern of deforming force in Benett's fracture is crucial for diagnosing and treating these fractures correctly. This concept also helps in distinguishing it from other types of diaphyseal fractures, such as Colles' fracture, where the deforming force is applied to the distal fragment, leading to a different fracture pattern.