Scaphoid Bone – Anatomy, Biomechanics and Mechanism of Fracture
Fractures of the scaphoid make about 50% to 80% of carpal injuries and occur in young, active individuals. The position of the scaphoid on the radial side of the wrist, as the proximal extension of the thumb ray, makes it vulnerable to injury.
The peak incidence of scaphoid fractures occur in the second and third decades of life.
Biomechanics of Scaphoid Bone
Intrinsic ligaments that attach scaphoid the lunate and distally to the trapezium and trapezoid stabilize the scaphoid.
These ligaments restrict the motion permitting a degree of rotation proximally and a degree of gliding distally .
Any shear strain that occurs across the midcarpal joint is transferred through the scaphoid, and may cause fractures and dislocations.
Mechanism of Fractures
Most of the patients have this fracture following fall on outstretched hand (FOOSH). These fractures are common in young men following falls, athletic injuries or motor vehicle accidents.
This results in forced dorsiflexion of the wrist. On geting dorsiflexed >95 degrees, the proximal pole of the scaphoid is tightly held between the capitate, the dorsal lip of the radius, and the taut palmar capsule resulting in fracture at the waist. Several other mechanisms are responsible for other types of fractures of scaphoid. For example fractures of the tubercle can be caused by either compression or avulsion.
Proximal pole fractures can be caused by an avulsion of the scapholunate ligament.
Scaphoid fractures in children and elderly are uncommon. In children, the force causes distal radial physis to fail first and in elderly the elderly, the distal radial metaphysis usually fractures before the scaphoid.
Scaphoid Fracture – Presentation and Diagnosis
A patient with scaphoid injury usually gives a history of falling on outstretched hand and presents with pain and swelling in the wrist.
On examination, there would be tenderness in the snuffbox. An associated swelling may However, swelling is often extreme occasionally be noticed. There is decrease in range of motion and extremes of motion may elicit pain.
The diagnosis of scaphoid fracture is usually made by x-ray. Following xrays are done in case of suspected scaphoid fracture for radiological diagnosis.
- Posteroanterior view with the hand in a fist [This puts scaphoid in extension]
- Lateral view
- Radial oblique view
- Ulnar oblique view
If required comparative views of the opposite uninjured wrist can be done.
In spite of this, undisplaced scaphoid fractures may not be visible on the initial set of x-rays.
If there is clinical suspicion of scaphoid fracture but x-rays are normal, a scaphoid cast is applied xrays are repeated after 10 days .
Sometimes the presentation is delayed and depending upon the period since injury, late presentation of a fracture or an established nonunion may show resorption at the fracture site, subchondral sclerosis, and displacement may occur.
Fractures of the distal radius, perilunate dislocation may occur be associated.
There is almost always a ligament damage that accompanies this injury.
Scaphoid Series Xrays
Scaphoid series is a series of xrays which can be filmed when there is a clinical evidence of scapjoid fracture but the lesion is not visible on routine projections.
Scaphoid series is helpful for better visualization of the scaphoid and other carpal and improves exposure of the intercarpal joint spaces.
Scaphoid series consists of a sequence of the following four radiographs-
- Wrist hyperextended and in ulnar deviation with the fist clenched and the thumb extended.
- 15° supination view.
- 15° pronation view.
- Lateral view in a neutral position.
Treatment of Scaphoid Fractures
Treatment of scaphoid fractures have evolved with time and it depends on type of fracture [Quick reference to classification of scaphoid fractures]
Treatment Options For Fractures of the Tubercle (Type A1)
These fractures represent an avulsion injury. Immobilization in cast for a period of four weeks is sufficient.
Undisplaced Scaphoid Fractures (Type A2)
Non Operative Treatment
Undisplaced scaphoid fractures are usually stable. Immobilization in plaster cast is the preferred treatment. Xrays are taken at regular interval and surgery may be considered if the fracture displaces in follow up.
Most of the stable scaphoid fractures unite in 6 to 8 weeks with cast immobilization but can take 12 to 16 weeks or some even more.
Many surgeons have repeatedly shown that fixation of undisplaced fractures gives significantly better results and a significantly lower rate of nonunion. It also means faster returns to work and can be considered depending on demands of the patient.
Unstable and Displaced Fractures (Type B2)
By definition, a scaphoid fracture is called displaced when
- There is 1 mm of step-off
- > 60 degrees of scapholunate angulation
- >15 degrees of lunatocapitate angulation
Non Operative Treatment
Unstable fractures have a high incidence of delayed union and nonunion. Most of these fractures would require surgical treatment
Conservative treatment should be considered in
- Metabolic diseases
- Poor compliance
- Medical comorbidities.
- Low functional demand
Non operative treatment includes closed reduction and immobilization.
Most of scphoid injuries occur in a young persons and require proper stabilization should be performed.
Percutaneus screw fixation after reduction of the fragments is the preferred. Postoperative cast immobilization for 4 weeks is recommended in displaced fractures.
Proximal Pole Fractures (Type B3)
All proximal pole fractures of scaphoid should be treated operatively.
Transscaphoid-perilunate fracture dislocation of carpus (Type B4), Comminuted fractures (Type B5), Delayed union (Type C) should also be treated surgically.
Scaphoid non unions (type D) are discussed in separate article.
Site of the fracture in scaphoid has been related to union rate.. Reported rate of fractures in scaphoid is
- Waist fractures – 80%
- Proxial pole – 15%
- Tuberosity – 4%
- Distal articular fractures – 1%
The scaphoid bone has its blood supply from distal to proximal direction. Scaphoid branches of the radial artery enter the dorsal ridge and supply 70% to 80% of the bone, including the proximal pole. The other group of vessels enters the scaphoid tubercle and supplies only the distal 20% to 30% of the bone.
Therefore, in cases with fractures through the waist and proximal third, the vascular supply would be restored only with fracture healing.
Herbert and Fisher classification serves as guide to treatment of scaphoid fractures. It is given below.
Type A: Stable Acute Fractures
These are incomplet fractures which unite rapidly with minimal treatment
Type A1: Fracture of tuberosity
Type A2: Incomplete fracture through waist
Type B: Unstable Acute Fractures
These fractures are likely to displace in plaster.Delayed union is common and internal fixation is the treatment of choice
Type C: Delayed union
There is a widening of the fracture line, formation of cysts adjacent to the fracture ande proximal fragment becomes relatively dense.
Type D: Established nonunion
Type D1: Fibrous union
Type D2: Pseudarthrosis
Fibrous union can occur after conservative treatment. It is a stable non union with minimal deformity and variable cystic change. It may progress to pseuarthrosis in time which is hallmarked by unstable progressive deformity and leads to development of osteoarthritis.
What Are Occult Scaphoid Fractures?
Occult fractures of the scaphoid are the fractures which are suspected clinically but do not show up in the xrays. Up to 30% of all scaphoid fractures may not be detected on initial xrays.
If an occult fracture of the scaphoid or severe concomitant wrist injury is suspected but not visible on initial x-rays, the wrist should be immobilized for 10 days in a forearm cast or splint, and the patient should be re-evaluated clinically as well raiologically.
MRI and radionuclide and MRI scans are reliable methods of diagnosing occult fractures of the scaphoid but are expensive investigations.
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