Last Updated on November 22, 2023
Scapholunate instability is a spectrum of wrist instabilities that have occult scapholunate interosseous ligaments sprains on one side and scapholunate advanced collapse on the other side.
Often the term is used interchangeably with scapholunate dissociation but in a strict sense, the scapholunate dissociation is one of the types of scapholunate instability.
To summarize, acute disruption of the SLIL can lead to painful instability, which can progress to chronic degenerative arthritis
The scapholunate interosseous ligament is a c-shaped structure connecting the dorsal, proximal and volar surfaces of the scaphoid and lunate bones. Its dorsal fibers are thicker than volar fibers and provide the greatest constraint to translation between the scaphoid and lunate.
The proximal portion of the ligament, being a thin fibrocartilaginous membrane, does not contribute significantly to the stability.
The palmar and particularly the dorsal aspects of the SLIL are stronger.
The scaphoid, the lunate, and the triquetrum are joined by a strong interosseous ligament that causes these bones-the proximal row of the carpus-to move as a unit.
The scaphoid articulates with an irregular fossa at the end of the radius.
It flexes with radial deviation as it is compressed between the radius and the trapezium. With interosseous ligaments of the proximal row intact, the entire proximal row follows the scaphoid into flexion.
Similarly, in ulnar deviation, the triquetrum is forced into extension, and the rest of the proximal row extends with it.
After the scapholunate ligament is injured, the bones do not move in one unit.
The scaphoid still flexes, with wrist flexion and radial deviation. But it flexes more than it normally does because of the tendency of the triquetrohamate joint to extend the proximal row is not transmitted.
Also, the scaphoid no longer extends with wrist extension or lunate deviation. The lunate follows the triquetrum into extension, no longer balanced with flexion forces through an attachment to the scaphoid.
This leads to relative malpositioning of the bones. When this occurs with motion or loading, the instability is dynamic. If the deformity is at rest too, it is called static.
In acute instability, the abnormally increased motion of the scaphoid can lead to painful clunk.
Over time, the scaphoid can develop a fixed flexion posture
- No longer seated in the radial fossa
- Radioscaphoid joint cartilage gets eroded with repetitive motion
- First at the radial styloid/scaphoid interface
- then progresses to involve the entire radioscaphoid joint.
The lunate gets into a position of fixed extension but its rounded proximal surface will still articulate smoothly with the lunate fossa of the distal radius. Therefore, arthritic changes in the radiolunate joint as a result of scapholunate interosseous ligament injury are rare.
With further advancement, in the final stage of degeneration, the capitate migrates proximally into the gap between the fixed, rotated scaphoid and the lunate and erodes the cartilage of the capitolunate joint.
Mechanism of Injury
Scapholunate ligament injury occurs because of fall over outstretched hand injury leading to wrist extension, ulnar deviation, and intercarpal supination, with structures beginning to on the radial side of the wrist.
The severity of the injury determines the extent of the disruption of wrist structures. This pattern of injury has been called progressive perilunate injury because the structures fail in a sequence progressing from radial to ulnar around the lunate which usually is
The injury may begin with a fracture of the radial styloid, fracture of the scaphoid, tearing of the SLIL, or some combination.
Greater energy can cause injury to the capitolunate joint, occasionally fracturing the capitate, and progress through the lunotriquetral interosseous ligament (LTIL) or the triquetrum.
If the injury crosses to ulnar side, the ulnar styoid may be fractured, or the triangular fibrocartilage complex (TFCC) may be disrupted.
In its most severe form, the injury results in palmar dislocation of the lunate.
Scapholnate Instability can be considered a Mayfield stage I perilunate injury.
[Read more on perilunate injuries]
Acute injury occurs in approximately 10-30% of intra-articular distal radius fractures or carpal fractures.
Degenerative tears in >50% of people over the age of 80 years old.
Diastasis of the scapholunate complex occurs with complete SLIL tears and capsule disruption.
- Associated injuries are DISI (dorsal intercalated segmental instability)
Classification of Scapholunate Injury
The scapholunate instability can be stable or unstable and either acute or chronic.
Stable injuries may include partial tears of the SLIL with associated pain and swelling, occasionally with ganglion formation, but without findings of instability on radiographic or physical examination as described earlier.
Unstable injuries can be either static or dynamic.
An injury <12 weeks old is called acute and older are considered chronic. However, repair of the ligament hs been reported as late as years after an injury.
As we noted previously, scapholunate instability is type I of Mayfield Trumble’s subclassification of the Mayfield type I SLIL injuries separates the injuries into the following categories
- Ia: symptoms present, but radiographic studies negative
- Ib: arthrogram or arthroscopy positive, but no evidence of static or dynamic instability
- Ic: dynamic instability present;
- Id: static instability present.
Clinically these injuries can be classified as follows [Each type represents a higher stage of severity.
- Scapholunate Dissociation
- Scapholunate Advanced Collapse
Occult Scapholunate Instability
It is the mildest form of scapholunate instability.
It usually is initiated by a fall on the outstretched hand. The condition results from a tear or attenuation of a portion of the scapholunate interosseous ligament.
The x-rays are normal in these patients and most of them also have normal stress x-rays. The instability may be noted on fluoroscopy.
Occult scapholunate instability may benefit from conservative treatment such as casting, splinting and symptomatic medications.
In selected cases, arthroscopic debridement followed by pinning or capsule repair has been said to provide a satisfactory outcome.
Dynamic Scapholunate Instability
Patients with dynamic instability have a complete scapholunate ligament tear. The condition results from an injury of greater magnitude than that cause occult scapholunate instability. The x-rays might be absolutely normal like occult instability. But instability is noted in both the plains on stress x-rays.
Repair of the ligament and the capsule is necessary for the treatment of dynamic scapholunate instability.
Scapholunate dissociation or rotatory subluxation of the scaphoid may occur alone or in association with wrist fractures.
The injury ranges from grade I sprain of scapholunate interosseous ligament to complete scaphoid dislocation
Injuries to following ligaments may be associated with scapholunate dissociation
- Radioscapholunate ligament
- Radioscaphocapitate ligament
- Scaphotrapezial ligament complex
- Dorsal radiocarpal ligament
- Dorsal intercarpal ligament
Disruption of the scapholunate interosseous ligament results in separation of the motion between the scaphoid and lunate in the acute phase and the development of persisting widening of the scapholunate joint as a late clinical consequence.
Provocative stress test and Watson test may be positive. Watson test if present is highly suggestive of scapholunate instability.
Because scapholunate injuries may be associated with fractures of the radius [occurs in 10% of the cases], the x-rays of patients with fractures of the distal radius should be evaluated for ligamentous injury too.
A scapholunate gap >3 mm suggests scapholunate dissociation. If the gap is more than 5 mm it is considered a confirmatory sign. [Terry Thomas Sign]
If the scapholunate angle is more than 60 degrees or capitolunate angle is >15 degrees, it suggests scapholunate instability. If the scapholunate angle is >80 degrees or capitolunate angle is greater than 20 degrees, it confirms scpholunate instability. [Dorsal intercalated segment instability]
If routine x-rays do not show any abnormality, clenched-fist views or radioulnar stress x-rays should be done.
MRI is helpful in discriminating the extent of ligament injury and should be performed in cases with normal x-rays and clinical suspicion.
Repair of the broken ligaments, internal fixation supplemented by a plaster cast.
Lunate extends when there is a loss of radial ligamentous stability.
Dorsal Intercalated Segment Instability
In scapholunate dissociation, scaphoid rotates into flexion with an increase in the scapholunate interval. The rotation of the lunate becomes independent of the scaphoid. With time, the lunate assumes an abnormally extended posture. This occurs due to forces exerted by intact triquetrolunate ligament [causes extension] and the capitate [causes dorsal translational].
This leads to static deformity which can be visualized on static x-rays.
With further passage of time, a dorsal intercalated segment deformity [DISI] develops which is characterized by
- Flexion of the scaphoid
- Extension of the lunate and triquetrum
- Dorsal and proximal translation of the capitate and distal carpal row
Scapholunate Advanced Collapse
SLAC wrist or Scapholunate advanced collapsed wrist is the final stage of scapholunate instability spectrum, where there is a specific pattern of osteoarthritis and subluxation which results from untreated chronic scapholunate dissociation. The condition, however, can also occur in cases of chronic scaphoid non-union and degenerative changes abnormal loading.
The postural changes become irreversible with time and the changed kinematics [the way forces would be handled] lead to abnormal articular loading and progressive degenerative changes known as Scapholunate advanced collapse (SLAC).
Types of SLAC Wrist
Depending on the extent of arthritis, the SLAC wrist can be divided into
SLAC wrist I – Arthritis along the scaphoid facet of the distal radius.
SLAC wrist II – In addition to the above, arthritis develops along the proximal radioscaphoid joint.
SLAC wrist III – Arthritis also develops in the radial midcarpal joint.
SLAC wrist IV – Arthritis involves radiolunate joint and entire carpus.
The x-rays would reveal the deformities and arthritis depending on the stage of SLAC.
Clinical Presentation of Wrist Instability
The injury could be traumatic [fall on outstretched hand] or a degenerative rupture. Patient’s age, nature of the injury, duration since the injury, the degree of underlying arthritis, level of activity should be noted.
Chronic SLIL injuries sometimes present quite late, and often the patient will not recall a specific injury.
The patient usually presents with pain on the dorsal and radial side of the wrist which increases with the loading of the wrist.
Clicking or catching in the wrist may be felt. The patient may also complain of wrist weakness or instability.
On examination, there may be swelling over the dorsal and tenderness in the anatomical snuffbox or over the dorsal scapholunate interval which lies 1 cm distal to Lister’s tubercle.
Pain increases with extreme wrist extension and radial deviation.
For this test, the examiner locates the palmar prominence of the distal pole of the scaphoid (most easily identified when the wrist is in radial deviation) and applies dorsally directed pressure to the distal pole while moving the wrist from ulnar to radial deviation and back.
When SLIL is intact, it keeps the scaphoid seated in the scaphoid fossa of the distal radius.
But if but an incompetent ligament will allow dorsal subluxation of the scaphoid proximal pole over the dorsal lip of the radius with progressive radial deviation. Returning the wrist to an ulnarly deviated position allows for reduction of the subluxation; the subluxation and reduction generally cause pain and a palpable clunk.
The test must be repeated on the uninjured side for comparison because ligamentous laxity can lead to a positive test without injury being present.
AP and lateral views of the wrist are the recommended views. Radial and ulnar deviation views, flexion and extension views and clenched fist views may be done as additional studies.
- widening of the scapholunate gap > 3mm with clenched fist view (Terry Thomas sign) (maximized in the grip and ulnar deviation PA views)
- Shortening of the flexed scaphoid, resulting in the cortical “ring sign” as the proximal and distal poles overlie one another
- Arthritic changes in late cases
- Increased scapholunate angle
- Angle formed by the longer axes of the scaphoid and the lunate.
- Ranges from 300 to 600 [Average – 470]
- > 600 after SLIL inury.
- capitolunate angle > 20°
Arthrography documents communicating defects, but these may not represent functionally significant tears. It may be used as a screening tool for arthroscopy. Always assess the contralateral wrist for comparison.
It cannot determine the size of the tear. A positive finding of a tear may indicate the need for wrist arthroscopy
MRI has been used successfully to evaluate TFCC injuries but has a reported sensitivity [37%]
A diagnostic technique in which a camera is used to record images of internal body structures produced through radiography or fluoroscopy.
A simple alternative is to have the patient move the wrist through a range of motion under fluoroscopy device.
It is considered the gold standard for diagnosis but is the invasive technique.
Bone scintigraphy has been reported as highly sensitive for complete tear but much less so for partial ligaments injuries.
Treatment of Scapholunate Instability
This includes rest, pain medication and immobilization. It can be considered in acute, undisplaced scapholunate ligament injuries or chronic, asymptomatic tears
Scapholunate ligament repair
It is indicated in acute scapholunate ligament injury when there is ligament without carpal malalignment.
If the SLIL is repairable, the scaphoid reducible from its flexed posture, and there are no arthritic changes, most authors now recommend primary repair of the ligament using either drill holes or suture anchors, augmented with some type of dorsal capsulodesis. The procedure provides a checkrein that decreases palmar flexion of the scaphoid and prevents the scaphoid from shifting dorsally.
Repair of the ligament can be considered in chronic but reducible scapholunate ligament injuries. It has been done in cases as old as 18 months.
Scapholunate reconstruction is indicated when there is acute scapholunate ligament injury without carpal malalignment but the pathoanatomy is not amenable to repair.
It is also done in reducible scapholunate ligament injuries > 18 months from the time of injury.
Scaphoid Reduction and Fixation
This could be either open reduction and fixation of the scaphoid or closed reduction and percutaneous fixation.
This is indicated in scaphoid fractures resulting in instability [trans-scaphoid scapholunate instability]
Stabilization with wrist fusion (STT or SLC)
This is indicated in rigid and irreducible DISI deformity with or without arthritic changes.
Fusions may be scaphotrapezialtrapezoidal (STT) fusion or scapholunocapitate (SLC) fusion.
Surgeries for SLAC Wrist
- Four corner Fusion
- Excision of the scaphoid
- Arthrodesis of the capitate, hamate, lunate, and
- proximal row carpectomy
- Wrist Arthroplasty
- Wrist Arthrodesis