**Question:** Callus formation is seen between what duration of fracture healing?

**Core Concept:** Fracture healing is a complex process involving inflammation, reparative phase, and remodeling phase. The process is essential for the restoration of bone continuity and strength. Callus formation is a crucial aspect of this process.

**Why the Correct Answer is Right:** Callus formation is primarily observed during the reparative phase of fracture healing. This phase is characterized by the proliferation of osteoblasts, chondroblasts, and fibroblasts, which leads to the deposition of new bone matrix and cartilage. The formation of callus allows the fracture site to be temporarily stabilized while the bone is being repaired.

**Why Each Wrong Option is Incorrect:**

A. **False:** Callus formation occurs during the reparative phase, not the early inflammatory phase.
B. **False:** The early inflammatory phase is crucial for the clearance of debris and the initiation of the reparative phase, but callus formation is not the primary outcome during this phase.
C. **False:** Although remodeling occurs after the reparative phase, callus formation is not a feature of this phase.
D. **False:** The late remodeling phase focuses on the remodeling of the callus into bone, not the absence of callus formation.

**Clinical Pearl / High-Yield Fact:** Callus formation is a key indicator of the reparative phase in fracture healing, representing the active phase of bone repair. Understanding the timing of callus formation can help assess the progression of fracture healing and guide clinical decision-making, such as determining when to apply external fixation or internal fixation, and when to expect union.

**Correct Answer:** D. The late remodeling phase (Option D) typically lasts from 6 to 9 months, during which the callus is remodeled into mature bone. This phase is characterized by the resorption of the callus by osteoclasts and the deposition of new bone matrix by osteoblasts, leading to the formation of mature bone at the fracture site.