Orthopedics

Scapholunate Dissociation

Background

1. Definition
   • A pathologic disruption of scapholunate ligament leading to spreading apart of scaphoid and lunate
     • AKA: rotary subluxation of the scaphoid, scapholunate interosseous ligament (SLIL) injury
2. Synopsis
   • Scapholunate dissociation is the most common cause of degenerative arthritis of the wrist
   • Often implies severe injury to the scapholunate interosseous ligament and other stabilizing ligaments
   • Without early diagnosis and treatment ⇒ proximal migration of the capitate between the scaphoid and lunate
     • Results in SLAC wrist (scapholunate advanced collapse)
       • Carpal collapse, chronic pain, and eventual arthritis
     • Always consider scapholunate dissociation
   • Treatment
     • Must be differentiated from simple wrist sprain
     • Immobilize in thumb spica splint or cast
     • Surgical Rx can avert progression
       • Consider surgical repair of torn ligaments or selected fusion

Pathophysiology

1. Mechanism
   • Abnormal orientation of the scaphoid relative to the lunate caused by direct trauma to the hand/wrist
     • Typical pattern of scapholunate dissociation consists of:
       • Relative flexion (volar rotation) of the scaphoid
       • Relative extension (dorsal rotation) of the lunate
     • Major injury of the SLIL (i.e., complete tear of dorsal component) and radiolunate ligament ⇒ scapholunate dissociation
       • Complete SLIL transection does not result in permanent dissociation
         • Presence of secondary scaphoid stabilizers
           • Palmar radioscaphoid-capitate
           • Scaphoid capitate
           • Anterolateral scaphotrapeziotrapezoid ligaments
         • Long-term wear on these secondary ligamentous stabilizers ⇒ permanent deformity and/or SLAC
   • Anatomy
     • The scapholunate interosseous ligament (SLIL) is a U-shaped ligament is divided into 3 anatomic components
       • Dorsal component:
         • Strongest, 3 mm thick, composed of short, transversely-oriented collagen fibers
         • Important in resisting volar-dorsal translation
       • Intermediate component:
         • Primarily composed of fibrocartilage
         • Homologous to the meniscus of the knee
       • Volar component:
         • 1 mm thick
2. Etiology/Risk Factors
   • Most commonly results from trauma
     • Hyperextended wrist in ulnar deviation after fall on an outstretched hand
   • Risk factors
     • Spastic paresis
     • Rheumatoid arthritis
     • Congenital ligament laxity
3. Epidemiology
   • Incidence/Prevalence
     • ~5% of all wrist sprains have an associated SL tear
       • SL ligament injuries often associated with distal radius fracture (~40%)
   • Mortality/Morbidity
     • Most common cause of carpal instability
     • Acute carpal tunnel syndrome has been reported in up to 46% of dislocations
     • Leading cause of SLAC (scapholunate advanced collapse) wrist
       • SLAC wrist is most common pattern of osteoarthritis in the wrist

Diagnostics

1. History/Symptoms
   • Pain at the radial (thumb) side of the wrist
     • Pain with hyperextension
   • Swelling
   • Stiffness
   • Numbness
   • Clicking sensation
   • Weakness of the wrist
2. Physical Exam/Signs
   • Clinical findings vary significantly
   • "Snapping or clicking" sensation with wrist deviation
   • Joint instability
     • Scaphoid shift test
   • Tenderness at site of injury
   • Compromise of the nerve in the carpal tunnel
     • Median nerve injury
       • Reduced sensation in the thumb, index, middle fingers, and radial half of the ring finger
       • Decreased strength of thumb flexion, opposition, and abduction
3. Labs/Tests
   • Blood labs are usually not indicated unless suspect other etiology
4. Imaging
   • Xray
     • Standard PA and lateral wrist radiographs
     • Bilateral clenched-fist comparison views
       • Dorsal intercalated segment instability (DISI)
       • Relative widening of scapholunate interval on affected side
     • Often takes 3-12 months after trauma before SL dissociation is noted radiologically
     • SL angle > 60° and SL gap > 3 mm on clenched-fist or ulnar-deviation radiographs
     • SLAC ⇒ arthrosis pattern showing progression of injury
   • CT
     • Often unnecessary
     • Terry Thomas Sign (View image)
   • MRI
     • Often unnecessary
     • May reveal thinning of the articular surfaces of the proximal scaphoid
5. Other Tests/Criteria
   • Geissler's classification
     • Grade I
       • Attenuation and/or hemorrhage of the interosseous ligament
       • No incongruence of carpal alignment in MC space
       • Treatment by immobilization
     • Grade II
       • Attenuation and/or hemorrhage of the interosseous ligament
       • Incongruence and/or step-off as observed from MC joint
       • A slight gap (< 2 mm) between the carpal bones may be present
       • Treatment by reduction and pinning
     • Grade III
       • Incongruence and/or step-off of the carpal alignment
       • Gap > 2 mm between the carpal bones
       • Treatment by arthroscopic reduction or open reduction and pinning or repair
     • Grade IV
       • Incongruence and/or step-off of the carpal alignment
       • Gross instability with manipulation is noted
       • 2.7 mm gap between the carpal bones ("drive-through phenomena")
       • Treatment by open re-insertion or ligament reconstruction
   • Watson and Ballet SLAC stages
     • Stage I:
       • Localized arthrosis of scaphoid fossa begins at the styloid tip (radial styloid beaking)
     • Stage II:
       • Progressive joint space narrowing and sclerosis affecting entire scaphoid fossa of distal radius
     • Stage II:
       • Sclerosis and narrowing joint space between lunate and capitate
       • Eventual migration of capitate proximally into space created by scapholunate dissociation
       • Synonymous with "Terry Thomas sign"
         • Lunate dorsiflexed (dorsal intercalated segment instability) and scaphoid flexed
6. Differential Diagnosis
   • Carpal Tunnel Syndrome
   • Scaphoid fracture
   • Distal radial fractures
   • Dorsal ganglion cyst
   • Septic arthritis
   • Avascular necrosis of the scaphoid
   • Gout
   • Pseudogout
   • Rheumatoid arthritis
   • Tear of the triangular fibrocartilage complex
   • Kienbock disease
   • Tenosynovitis
   • Lunate fracture vs dislocation
   • Dorsal intercalated segmental instability

Treatment

1. Initial/Prep/Goals
   • ABCs, ATLS as needed
   • Splint affected wrist
     • Thumb spica splint
     • Radial gutter
     • Short arm volar splint
   • Pain medication
   • Urgent referral to hand specialist
     • Immediate reduction if lunate or perilunate dislocation
       • Immediate orthopedic or hand surgery consultation
       • Reduction, stabilization
         • Arthroscopically guided reduction
2. Medical/Pharmaceutical
   • Analgesia
3. Surgical/Procedural
   • May require urgent surgical intervention (< 6 weeks)
     • Decrease risk of severe/debilitating wrist dysfunction
     • Closed reduction with percutaneous pinning or open reduction
     • Short-arm cast postoperatively
4. Complications
   • Stiffness
   • Chronic pain
   • If untreated ⇒ SLAC

Disposition

1. Admission Criteria
   • Usually not indicated unless pre-Op
2. Consult(s)
   • Orthopedist
   • Hand surgeon
3. Discharge/Follow-up Instructions
   • K-wires are removed in 8-10 weeks
   • No heavy lifting using the wrist for 4-6 months post-injury

References

1. Bentley TP, Hope N, Journey JD. Wrist Dislocation. StatPearls [Internet]. Available at: https://www.ncbi.nlm.nih.gov/books/NBK507712/. [Accessed March 2024]
2. Stevenson M, Levis JT. Image Diagnosis: Scapholunate Dissociation. Perm J. Mar 1, 2019; 23(2): 18-237
3. Andersson JK. Treatment of scapholunate ligament injury. EFORT Open Rev. Sep 2017; 2(9): 382-393
4. Lane R, Tafti D, Varacallo M. Scapholunate Advanced Collapse. StatPearls [Internet]. Available at: https://www.ncbi.nlm.nih.gov/books/NBK537124/. [Accessed March 2024]

Contributor(s)

1. Ho, Nghia, MD

Updated/Reviewed: March 2024