Chapter 6. Bone Tissue and the Skeletal System

6.5 Fractures: Bone Repair

Learning Objectives

By the end of this section, you will be able to:

Explain how bone repairs itself after a fracture

  • Differentiate among the different types of fractures
  • Describe the steps involved in bone repair

A fracture is a broken bone. It will heal whether or not a physician resets (places) it in its anatomical position. If the bone is not reset correctly, the healing process will rebuild new bone but keep the bone in its deformed position.

When a broken bone is manipulated and set into its natural position without surgery, the procedure is called a closed reduction. Open reduction requires surgery to expose the fracture and reset the bone. While some fractures can be minor, others are quite severe and result in grave complications. For example, a fractured diaphysis of the femur has the potential to release fat globules into the bloodstream. These can become lodged in the capillary beds of the lungs, leading to respiratory distress and if not treated quickly, death (this is called a pulmonary embolism).

Types of Fractures

Fractures are classified by their complexity, location, and other features (Figure 1). Table 4 outlines common types of fractures. Some fractures may be described using more than one term because it may have the features of more than one type (e.g., an open transverse fracture).

Figure 6.51 – Types of Fractures: Compare healthy bone with different types of fractures: (a) open fracture, (b) closed fracture, (c) oblique fracture, (d) comminuted fracture, (e) spiral fracture , (f) impacted fracture, (g) greenstick fracture, and (h) transverse fracture.

Types of Fractures (Table 4)
Type of fracture Description
Transverse Occurs straight across the long axis of the bone
Oblique Occurs at an angle that is not 90 degrees
Spiral Bone segments are pulled apart as a result of a twisting motion
Comminuted Several breaks result in many small pieces between two large segments
Impacted One fragment is driven into the other, usually as a result of compression
Greenstick A partial fracture in which only one side of the bone is broken, often occurs in the young
Type of Fracture Description
Open (or compound) A fracture in which at least one end of the broken bone tears through the skin; carries a high risk of infection
Closed (or simple) A fracture in which the skin remains intact

Bone Repair

Depending on the type, severity of the fracture and distance between bone fragments, bones may heal directly by building new bone onto the fracture site (direct bone healing or contact healing) or may heal in a process like endochondral bone formation (indirect bone healing). Direct bone healing is essentially bone remodeling in which osteoblasts and osteoclasts unite broken structures. With indirect bone healing the process is more complicated and similar to endochondral bone formation in which broken bones form cartilaginous patches before regrowing new bone. In this process, blood released from broken or torn vessels in the periosteum, osteons, and/or medullary cavity clots into a fracture hematoma (Figure 2a). Though broken vessels promote an increase in nutrient delivery to the site of vessel injury (see inflammation process in blood vessel chapter), the disruption of blood flow to the bone results in the death of bone cells around the fracture.

 

This illustration shows a left to right progression of bone repair. The break is shown in the leftmost image, where the femur has an oblique, closed fracture in the middle of its shaft. The next image magnifies the break, showing that blood has filled the area between the broken bones. Blood has also filled in around the lateral and medial sides of the break. The influx of blood causes the broken area to swell, creating a hematoma. In the next image, the hematoma has been replaced with an external callus between the two broken ends. Within the internal callus, the blood vessels have reconnected and some spongy bone has regenerated in the gap between the two bone halves. In the next image, spongy bone has completely regenerated, connecting the two broken ends, referred to as the bony callus. The external callus still remains on the lateral and medial sides of the break, as the compact bone has not yet regenerated. In the final image, the compact bone has fully regenerated, encapsulating the bony callus and completely reconnecting the two bone halves. The bone has a slight bulge at the location of the healed fracture, which is clearly shown in the final image, which shows a zoomed out image of the completely healed femur.

Figure 6.52 – Stages in Fracture Repair: The healing of a bone fracture follows a series of progressive steps: (a) Broken blood vessels leak blood that clots into a fracture hematoma. (b) Internal and external calluses form made of cartilage and bone. (c) Cartilage of the calluses is gradually eroded and replaced by trabecular bone, forming the hard callus. (d) Remodeling occurs to replace immature bone with mature bone.

Within about 48 hours after the fracture, stem cells from the endosteum of the bone differentiate into chondrocytes which then secrete a fibrocartilaginous matrix between the two ends of the broken bone; gradually over several days to weeks, this matrix unites the opposite ends of the fracture into an internal callus (plural = calli or calluses). Additionally, the periosteal chondrocytes form and working with osteoblasts, create an external callus of cartilage and bone, respectively, around the outside of the break (Figure 2b). Together, these temporary soft calluses stabilize the fracture.

Over the next several weeks, osteoclasts resorb the dead bone while osteogenic cells become active, divide, and differentiate into more osteoblasts. The cartilage in the calluses is replaced by trabecular bone via endochondral ossification (destruction of cartilage and replacement by bone) (Figure 2c). This new bony callus is also called the hard callus.

Over several more weeks or months, compact bone replaces spongy bone at the outer margins of the fracture and the bone is remodeled in response to strain (Figure 2d). Once healing and remodeling are complete a slight swelling may remain on the outer surface of the bone, but quite often, no external evidence of the fracture remains. This is why bone is said to be a regenerative tissue that can completely replace itself without scars.

External Website

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Visit this website to review different types of fractures and then take a short self-assessment quiz.

 

Section Review

Fractures are classified by their complexity, location, and other features. Common types of fractures are transverse, oblique, spiral, comminuted, impacted, greenstick; they may also be classified as open (or compound), and closed (or simple). During indirect bone healing, fracture repair begins with the formation of a hematoma, followed by cartilaginous internal and external calluses. Osteoclasts resorb dead bone, while osteoblasts create new bone that replaces the cartilage in the calluses. Calluses eventually unite, and bone remodeling occurs to complete the healing process.

 

Review Question

 

 

 

 

 

Critical Thinking Questions

1. What is the difference between closed reduction and open reduction? In what type of fracture would closed reduction most likely occur? In what type of fracture would open reduction most likely occur?

2. In terms of origin, composition and cells involved, what are the differences between an internal callus and an external callus?

Glossary

closed reduction
manual manipulation of a broken bone to set it into its natural position without surgery
external callus
collar of cartilage and bone that forms around the outside of a fracture
fracture
broken bone
fracture hematoma
blood clot that forms at the site of a broken bone due to broken blood vessels
internal callus
fibrocartilaginous matrix, in the endosteal region, between the two ends of a broken bone
open reduction
surgical exposure of a bone to reset a fracture

Solutions

Answers for Critical Thinking Questions

  1. In closed reduction, the broken ends of a fractured bone can be reset without surgery. Open reduction requires surgery to return the broken ends of the bone to their correct anatomical position. A partial fracture would likely require closed reduction. A compound fracture would require open reduction.
  2. The internal callus is produced by cells in the endosteum and is composed of a fibrocartilaginous matrix. The external callus is produced by cells in the periosteum and consists of hyaline cartilage and bone. Both types are formed by stem cells that differentiate into chondroblasts (chondrocytes), but in different locations.