The anatomy of the bones of the wrist leaves one of the bones vulnerable to injury. Or saying it another way, if you break this bone, you’re in for a long healing time and the potential that the fracture may not heal. A fracture that doesn’t heal puts the patient in the unenviable position of having long term pain, developing arthritis, and possibly having surgery.
The wrist is made up of 8 carpal (wrist) bones. They are arranged in two rows of four. The four bones farthest from the elbow and closest to the hand, listed from the thumb to the fifth finger are the trapezium, the trapezoid, the capitate and the hamate. The four bones of the second row, from thumb to the 5th finger are the scaphoid, the lunate, the triquetrum, and the pisiform. Of these bones, the scaphoid is the one you don’t want to break. Why? Because only 30% of the cartilaginous surface of the scaphoid is supplied with blood, and blood, and the nutrients in it, are needed for healing of a fracture.
The scaphoid bone was previously called the navicular bone. Curiously enough, the human foot also had a navicular bone, and for years they were called “carpal navicular” and “tarsal navicular” to avoid confusion. Eventually, however, the carpal navicular name was changed to scaphoid so the two could not be confused.
Another oddity of the scaphoid bone is the direction of blood flow: instead of running from near to far, it runs the opposite direction, from distal to proximal, or far to near. That means if the near end of the scaphoid is fractured, it heals poorly because of limited circulation to that end of the bone. That poses a problem for healing.
Scaphoid fractures occur when a person falls on their outstretched hand. The injury causes swelling in the wrist and pain and tenderness at the base of the thumb. On the thumb side of the wrist there is an indentation, formed by tendons of the wrist and forearm. It’s called the “anatomic snuffbox.” The scaphoid bone sits just beneath that indentation. If the scaphoid is fractured, the snuffbox is very tender when pressed on firmly. A fractured scaphoid also causes swelling in the wrist. It causes pain and weakness when trying to grasp with the thumb and index finger.
A fracture is normally diagnosed by an X-ray, but in the case of the scaphoid bone, it may not be. Scaphoid fractures are notorious for not showing up clearly on an initial X-ray. So, when a fractured scaphoid is suspected and the X-ray is negative, the plan is to splint the wrist and re-X-ray it two weeks later. In the two week interval, the bone on the line of the break deteriorates leaving a gap between the two edges that will now show up on an X-ray as a fracture. This happens often with scaphoid fractures so re-xraying after two to three weeks is advisable, especially if the patient is still having pain and tenderness.
Treatment is immobilization with long-term casting or splinting with the thumb immobilized as well. That type of cast is called a thumb spica. In cases where the fracture is in a location susceptible to non-healing, surgery to align and stabilize the bone fragments is sometimes recommended. A screw, wire, or other internal fixation device is used. Bone grafts are another surgical option.
After 6 months of casting or internal fixation, if the fracture has not healed, non-union must be considered. This happens because of poor circulation of the bone and is called avascular necrosis. Avascular, of course, means no circulation (or poor circulation) has caused necrosis, or death, of the bone. This problem results in chronic pain and arthritis, and is treated surgically by bone grafting or excision of the dead bone.
I think you can see why I said you don’t want to break this bone. It’s because it’s poor blood supply makes it very susceptible to non-union (non-healing) and avascular necrosis which is followed by arthritis and chronic pain. Why only the scaphoid and not the other seven wrist bones? The circulation of the other 7 is not limited to the degree of the scaphoid. Because of these limitations, doctors must be careful and avoid falling on to outstretched hands.
References: Scaphoid Fracture of the Wrist. Ortho Info Amer Acad Orth Surg.
www.orthobullets.com/scaphoid-fracture
