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Lunate Dissociation

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Lunate Dissociation
Navid Mahooti
Thomas Trojian
Basics
Description
  • Dissociation, or dislocation, is classified by the pattern of carpal collapse, either dorsal or volar in nature, after wrist trauma or injury.
  • The carpal bones in the wrist are stabilized by multiple extrinsic and intrinsic ligaments. The scaphoid, lunate, and triquetrum have no tendon insertions and are described as an intercalated segment (motion depends on mechanical signals from ligamentously intact neighboring articulations) (1).
  • The dorsal component of the scapholunate interosseous ligament (SLIL) is the primary stabilizer of the lunate. Important secondary stabilizers include the short radiolunate ligament (SRL), which maintains the position of the lunate adjacent to the radius, and the dorsal intercarpal (DIC) ligament. Several other extrinsic ligaments serve as secondary restraints (1,2).
  • Injury typically occurs when an extended wrist undergoes a forced axial load, such as in a fall on an outstretched hand (FOOSH) with the hand in ulnar deviation or when pushing a heavy object.
  • Isolated carpal dislocations are rare. Most information on them comes from case reports and surgical technique papers (2).
  • A number of injury patterns can occur with this mechanism, depending on several factors: Position of the extremity at impact, quality of the bone, ligamentous strength, and direction of the force (2).
  • Disruption of the SLIL eliminates the scaphoid's flexion (volar) force on the lunate and allows the triquetrum to push the lunate into an extended (dorsal) position. Disruption of the SLIL and at least one other secondary ligament (eg, the DIC) is required to show static changes in scaphoid and lunate position [known as dorsal intercalated segment instability (DISI)] (3).
  • The lunotriquetral interosseus ligament (LTIL) is another important lunate stabilizer. LTIL disruption causes an unopposed volarly directed force on the lunate by the scaphoid. Volar intercalated segment instability (VISI) results (4).
  • The terms lunate dissociation, scapholunate dissociation, and perilunar dissociation and dislocation have overlapping features. Similarly, injury patterns to the lunate and adjacent structures have overlapping characteristics and nomenclatures.
  • Perilunate injuries are severe disruptions of carpal anatomy defined by dislocation of the capitate head from the concavity of the distal lunate. The spectrum of injury ranges from a perilunate dislocation (PLD), a soft tissue circumferential disruption around the lunate, to the transscaphoid perilunate dislocation (TSPLD), which involves a scaphoid fracture rather than a scapholunate ligamentous injury (5).
  • Mayfield initially described 4 stages of perilunate instability in 1980 that have since been modified (2,5):
    • Stage 1: Disruption of scapholunate articulation
    • Stage 2: Lunocapitate dislocation
    • Stage 3: Lunotriquetral disruption
    • Stage 4: Volar lunate dislocation
  • A 4-stage classification system is used commonly to describe scapholunate dissociation (SLD) (3):
    • Stage 1: Predynamic instability consists of the earliest scapholunate injury, typically a partially torn or attenuated scapholunate membrane that causes abnormal motion, synovitis, and wrist pain. Plain and stress radiographs are normal; arthroscopy may show attenuation of the SLIL or hemorrhage within the scapholunate joint. Untreated, secondary stabilizers may become attenuated and progress to dynamic or static instability.
    • Stage 2: Dynamic instability is characterized by ligamentous tear of the palmar or dorsal aspect of the SLIL. Plain radiographs will be normal, but stress views show widening of the scapholunate interval. An arthrogram may show abnormalities within the ligament.
    • Stage 3: Static instability occurs with injury to the SLIL and a secondary stabilizer and is evident on plain radiography (gap of 3 mm or greater between the lunate and scaphoid or a scapholunate angle >70 degrees on lateral views). DISI is often present.
    • Stage 4: SLAC refers to the final stage of SLD, the result of continued loads to a biomechanically altered joint that lead to progressive articular cartilage deterioration and eventually arthritis.
  • Synonym(s): Wrist sprain; Perilunate dissociation; Scapholunate dissociation; Scapholunate instability
  • Dislocation is often used interchangeably with dissociation; a dislocated bone is always dissociated, but a dissociated bone is not always dislocated.
Epidemiology
  • PLDs are the most common carpal dislocation (2).
  • More common in men in the 2nd and 3rd decades of life; less common in the elderly (distal radius fails/fractures before ligamentous injury occurs) and children (radial physis is weaker, resulting in Salter-Harris fractures) (2)
  • SLD is more common in football than in any other sport.
  • Wrist injuries account for an estimated 3–9% of all athletic-related injuries (4).
  • A 10-yr study done at the Cleveland Clinic revealed that 14.8% of athletic participants under the age of 16 yrs sustained upper extremity injuries, and of these, 9% involved the wrist.
  • The incidence of lunate dissociation is uncommon, but failure to identify the injury results in long-term disability.
Risk Factors
  • Any injury involving excessive wrist extension and ulnar deviation with intracarpal supination, typically a FOOSH injury
  • Chronic crutch-walkers, gymnasts, American football players, collision sports
  • Increased ulnar negative variance
General Prevention
  • Controversy exists as to whether braces or wrist guards effectively prevent wrist injuries (4).
  • One cadaveric model showed decreased carpal fractures, ligament, and capsular tears with bracing.
  • In-line skating-type guards have not been shown to prevent wrist fractures.
  • European studies of snowboarders show a significant decrease in the incidence of wrist fractures and injuries with wrist guards.
  • Some are concerned that bracing/guards simply alter the area of force transmission.
Commonly Associated Conditions
  • Scaphoid fractures occur with a similar mechanism of injury (FOOSH) and therefore must be ruled out.
  • Failure to diagnose and treat lunate dissociation increases the risk of arthritic degeneration and development of SLAC wrist.
Diagnosis
History
  • Determine duration of pain: Most patients present acutely, but some report a history of painful clicking, reduced grip strength, and giving way with activities.
  • Obtain detailed mechanism of injury to determine if SLD is a possible injury (typically a FOOSH with ulnar deviated wrist).
  • Rule out any distracting injuries (elbow, forearm, etc.).

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Physical Exam
  • Signs and symptoms:
    • 100% of patients have dorsal wrist pain, 91% have decreased grip strength, and 71% have decreased range of motion (ROM).
    • Clicking with wrist motion (nonspecific)
  • Physical examination:
    • Inspect for swelling and deformity; compare with contralateral wrist.
    • Tenderness is typically on radial aspect of the wrist; palpate just distal to Lister's tubercle, which is often tender as well.
    • Scaphoid tenderness (anatomic snuffbox) is a scaphoid fracture until proven otherwise.
    • Document ROM of wrist and neurovascular status (pulses, sensation, particularly median nerve distribution because it can be diminished in severe dissociations owing to mechanical forces).
    • Finger extension test: Hold wrist in flexion, and test active finger extension against resistance; causes pain over lunate.
    • Watson's scaphoid test may be positive. The examiner places the thumb on the scaphoid tubercle, and the 4 fingers wrap around the distal radius. While the wrist is in ulnar deviation, pressure is directed dorsally with the thumb at the volar scaphoid. The wrist is then radially deviated. Pain is the hallmark of a positive test result, although a dramatic “clunk” may be felt or heard.
    • Kleinman's shear stress test for lunotriquetral instability: Wrist in neutral position, examiner's contralateral thumb is placed over dorsal lunate while ipsilateral thumb loads the pisotriquetral joint with a dorsally directed force. Pain is a positive test.
    • The Reagan shuck test and Linscheid compression test are other maneuvers to determine lunotriquetral instability.
Diagnostic Tests & Interpretation
Imaging
  • Radiographs: Posteroanterior (PA), lateral, and oblique views of the wrist, clenched-fist view:
    • Gap of 3 mm on PA films (“Terry Thomas” or “Dave Letterman” sign, in reference to their gapped teeth) is classically used. However, Cautilli obtained PA radiographs on 100 normal wrists and found the mean gap in males was 4.0 mm and in females it was 3.6 mm. Zhu performed a similar study and found the mean gap to be 3.14 mm. Both studies revealed that gaps up to 5 mm were not necessarily indicative of carpal instability. Consequently, contralateral wrist films should be obtained for comparison.
    • Lateral views: Dorsal extension of the lunate is referred to as DISI and is highly suggestive of injury to the SLIL and at least one secondary stabilizer.
    • Scapholunate angle is normally 30–60 degrees on lateral view; an angle >70 degrees is diagnostic (6); clenched-fist view accentuates injury.
  • Follow-up and special considerations:
    • 3-phase bone scintigraphy is sensitive but nonspecific but has a high negative predictive value (3).
    • MRI (1,6):
      • Complete SLIL tears are characterized by distinct area of discontinuity within the ligament, outlined by fluid-like T2 hyperintensity, or by complete absence of the ligament. Fluid signal at attachments also can be seen.
      • Detection of LTIL tears is more difficult, with a decreased sensitivity and specificity given the smaller size of this structure.
    • MRI arthrography appears to have greater sensitivity at detecting partial tears of the SLIL than does MRI or traditional arthrograms. Extravasation of contrast material indicates a full-thickness ligamentous defect.
    • MRI arthrography has a sensitivity of 83–92% and a specificity of 46–100%.
Diagnostic Procedures/Surgery
Arthroscopic evaluation is the “gold standard” for identifying and grading scapholunate injuries (3).
Differential Diagnosis
  • Scaphoid fracture
  • Colles' fracture
  • Scaphoid impaction syndrome
  • Dorsal wrist ganglion cyst
  • Other carpal bone injury
Treatment
  • Acute treatment
  • Analgesia; NSAIDs, narcotics
  • Reduction techniques:
    • Only for least severe injury (eg, dynamic instability; see above)
    • Recommended for experienced providers only
  • Immobilization:
    • Partial tears of SLIL can be treated conservatively with splinting. Immobilize with splint in correct anatomic position. Symptoms guide management.
    • Patients with suspected ligamentous injury should be referred promptly to an orthopedist, preferably a hand specialist, because misdiagnoses can have significant consequences.
Additional Treatment
  • Long-term treatment
  • Surgical referral:
    • Standard of care is operative repair.
    • Prolonged immobilization for weeks to months after surgery is necessary to maintain carpal bone alignment and to prevent SLAC wrist.
  • Rehabilitation: Physical therapy after immobilization to improve ROM and strength
Referral
  • Timely referral to hand surgeon if any question in the diagnosis
  • Immediate referral to hand surgeon if abnormal neurovascular status
Additional Therapies
Typically considered a season-ending injury
Surgery/Other Procedures
Standard of care typically involves open reduction, ligament repair, and internal fixation (2).
References
1. Lau, Steven et al. Scapholunate dissociation: an overview of the clinical entity and current treatment options. Eur J Orthop Surg Traummot. 2009;19:377–385.
2. Grabow RJ, Catalano L. Carpal dislocations. Hand Clin. 2006;22:485–500; abstract vi–vii.
3. Manuel J, Moran SL. The diagnosis and treatment of scapholunate instability. Orthop Clin North Am. 2007;38:261–277.
4. Slade JF, Milewski MD. Management of carpal instability in athletes. Hand Clin. 2009;25:395–408.
5. Sauder DJ, Athwal GS, Faber KJ, et al. Perilunate injuries. Orthop Clin North Am. 2007;38:279–288.
6. Bencardino JT, Rosenberg ZS. Sports-related injuries of the wrist: an approach to MRI interpretation. Clin Sports Med. 2006;25:409–432, vi.
Additional Reading
Cohen MS. Ligamentous injuries of the wrist in the athlete. Clin Sports Med. 1998;17:533–552.
Mastey RD, Weiss AP, Akelman E. Primary care of hand and wrist athletic injuries. Clin Sports Med. 1997;16:705–724.
Nguyen DT, McCue FC, Urch SE. Evaluation of the injured wrist on the field and in the office. Clin Sports Med. 1998;17:421–442.
Rettig AC. Epidemiology of hand and wrist injuries in sports. Clin Sports Med. 1998;17:401–406.
Ritchie JV, Munter DW. Emergency department evaluation and treatment of wrist injuries. Emerg Med Clin North Am. 1999;17:823–842, vi.
Young D, et al. Physical examination of the wrist. Orthop Clin North Am. 2007;38:149–165.
Codes
ICD9
833.03 Closed dislocation of midcarpal (joint)
Clinical Pearls
  • Surgery results in ∼35% loss of flexion and extension. This should still allow an athlete to return to activity in most cases.
  • The ligament in the wrist has torn in this injury and the bones have shifted, so they cannot heal correctly without surgery.


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