What Is Scapholunate Dissociation

Introduction

A scapholunate dissociation is when two bones in the wrist, the scaphoid and lunate, move out of alignment. This can happen when there is damage to the ligament that joins these two bones together; the scapholunate interosseous ligament. It is important to understand the basic anatomy of the wrist to see how this injury can affect movement and why certain treatment methods are used.¹ 

Anatomy of the wrist

The wrist is a complex structure containing eight different small bones. Each of these bones are connected to each other by ligaments (a type of connective tissue) as well as to the bones of the upper arm and hand. This is why the wrist is able to perform many fine rotational movements which allow us to have great control of our hands.² Ligaments have a key role in maintaining the stability of the wrist joint. Ligaments connect different bones so that when they shift during movement they will always return to their original position. The scapholunate ligament is especially important as it is involved in most of the common movements of the wrist. 

The scaphoid bone has a key role in wrist movement because of its position near the bottom of the wrist joint. When the scaphoid rotates it allows the wrist to extend (imagine the position of the wrist when using your hands on a table to help you stand up from a chair). Therefore if there is damage to the scapholunate ligament then the risk of the scaphoid and lunate bones going out of alignment is much higher as they are bones that are very commonly used in wrist movement.³

Scapholunate dissociation mechanism

The most common cause of scapholunate dissociation, as mentioned before, is damage to the scapholunate ligament. This can occur due to over-extension of the wrist (bending the wrist backwards into an unusual position). The ligament may also stretch out over time if the same bending action is repeated. With old age ligaments get looser so there is an increased risk of overextending the ligament as you get older as well.⁴

Signs and symptoms

The most common symptom is pain in the wrist, typically around the back and bottom side. It may get worse on movement and you may even feel the bones moving excessively. Soon after an acute injury there is usually swelling around the wrist which can reduce the ability to move the wrist comfortably, making it stiff. In more severe cases there may be weakness, numbness or spasms (uncontrolled muscle contraction).⁵

Diagnosis

In order to make a diagnosis a doctor must first take the history of the individual to find out how the injury came about. Then they will perform a physical examination followed by radiological imaging to get an accurate picture of the injury. The physical examination will usually involve the doctor feeling the wrist joint for inflammation while asking if there is a particular part of the wrist that hurts when they touch it. The doctor may also perform the ‘Watson scaphoid shift test’.

This involves holding the injured wrist and pressing on the scaphoid to move it slightly out of alignment. This test will cause pain in the injured wrist and when they release their finger the bone there may be a ‘click’ sound. This test is very rarely used by doctors as it causes pain in the individual. It is also not very accurate as there are other conditions such as ‘synovitis’ that can also cause pain in the scaphoid area. Doctors instead will use radiological imaging to get an image of the bones for a more accurate diagnosis.⁶ 

A common imaging technique is a wrist X-ray. This allows the doctor to see the bones of the wrist and measure the distance between the scaphoid bone and the lunate bone. If this space is larger than normal then it is suggestive of scapholunate dissociation. They may put the wrist in two different positions, one neutral and one with the wrist in a position that puts stress on the joint. This will be helpful in showing if there is an increase in the space between the bones. A less common method is MRI to help image the ligaments themselves, however it is harder to see the small ligaments in the wrist compared to larger ligaments.⁷

The most common imaging technique is arthroscopy. This involves a small camera being placed through a ‘keyhole’ opening (a small cut in the skin). The camera is then passed to the position of the ligament to view the extent of the damage. Following an X-ray and arthroscopy the doctor is able to ‘stage’ the damage based on how serious it is.⁸ 

Classification and grading

Grading is useful in defining the extent of damage in the joint. It involves looking at how far the tear has damaged the ligament and if there is further damage to other bones and ligaments of the wrist. Grading is very important in helping to inform the doctor of the best method to treat a specific injury based on the location and severity of damage. There are two classification systems commonly used to grade wrist ligament damage.

Mayfield classification looks at all the bones of the wrist and whether they are aligned with respect to the scaphoid bone. A scapholunate dissociation is usually given a stage 1 classification.⁹ The second method is known as Gilula’s arc classification. This looks at the position of wrist bones relative to each other in the form of three imaginary arcs that can be drawn between sets of bones in the wrist. In scapholunate dissociation there would be a defect of the first arc which crosses those two bones.¹⁰

Treatment options

For less serious ligament damage the most common treatment involves wearing a cast for some time followed by physiotherapy. The cast prevents any further damage or pain by reducing movement of the wrist joint to allow proper healing. Physiotherapy is then used to slowly improve strength and rotation of the wrist joint.

For more serious ligament damage or bone misalignment surgery may need to be considered. It is important for surgery to be done as soon as possible as the longer the wait, the more damage can occur due to small movements. If the injury is picked up early then the ligament can be repaired with ‘keyhole’ surgery to repair the ligament. Keyhole surgery allows for better recovery as only a small cut is made.

However, if the ligament or bones are damaged too much, then open surgery may need to be performed. Following any type of surgery there will also be care from a physiotherapy team to build up strength in the wrist slowly. Most people will be able to return to a similar state of strength and joint movement as before but those with more severe ligament damage may see that they are not able to move the wrist as freely following surgery or physiotherapy.¹¹

Summary

Scapholunate dissociation is the misalignment of the scaphoid and lunate bones of the wrist following injury to the ligament that connects these bones. It can be caused by repetitive overextension of the wrist joint or sudden, severe extension in an unusual angle (falling onto the wrist).

Common symptoms include pain, swelling and reduced movement of the wrist. It is important to seek medical attention as a doctor will need to conduct an examination and imaging to determine the extent of damage. If it cannot be resolved with a cast and physiotherapy then surgery may be needed.

Following surgery and physiotherapy there is good prognosis for restoring the movement of the joint if action was taken quickly, however people may experience joint stiffness or reduced grip strength. These types of injuries can be prevented with actions that reduce the risk of falls such as keeping the house free from trip hazards, wearing good fitting shoes and using a walking aid when necessary.¹