Perilunate dislocations and perilunate fracture dislocations are potentially devastating closed wrist injuries that are often missed on initial imaging.
These injuries involve dislocation of the carpus relative to the lunate which remains in normal alignment with the distal radius. They should not be confused with lunate dislocations where the lunate is dislocated in a volar direction and no longer has normal radiolunate articulation.
Overall, carpal dislocations account for less than 10% of all wrist injuries.
Perilunate dislocations typically occur in young adults with high energy trauma resulting in loading of a hyperextended, ulnarly deviated hand. Around 60% of perilunate dislocations are associated with a trans-scaphoid fracture which is then termed a trans-scaphoid perilunate dislocation.
Typical history is of a fall onto a dorsiflexed wrist. There may or may not be obvious clinical deformity. Occasionally median nerve injury, arterial compromise or compartment syndrome may be evident due to the dislocation.
Perilunate dislocation involves traumatic rupture of the radioscaphocapitate, scapholunate interosseous and lunotriquetral interosseous ligaments. Mayfield et. al. have proposed a four stage process to describe perilunar wrist instability (see carpal dislocation) where perilunate dislocation represents stage II 2.
Imaging plays an essential role in identifying perilunate and other carpal dislocations. Unfortunately, dislocations can often be missed by radiologists. Carpal alignment needs to be carefully assessed on all radiographs. The majority of cases involve dorsal dislocation of the capitate and carpus relative to the lunate which remains in near-normal alignment with the radius. Volar perilunate dislocation is rare.
In a trans-scaphoid perilunate dislocation the proximal scaphoid maintains its lunate relationship, and the distal scaphoid and remainder of the carpal bones displace dorsally 3.
- AP radiograph
- dislocation is often overlooked
- disruption of the normally smooth line made by tracing the proximal articular surfaces of the hamate and capitate
- increased overlap of lunate and capitate
- lateral radiograph 4
- dislocation more easily appreciable
- capitate not sitting within the distal articular 'cup' of the lunate
- line drawn through radius and lunate fails to intersect capitate
- lunate remains in articulation with distal radius (as opposed to lunate dislocation where it is usually in a volar position)
- abnormal scapholunate angle (normal 30-60 degrees, reduced in dorsal perilunate dislocation)
- abnormal capitolunate angle (normal 0-15 degrees, increased in dorsal perilunate dislocation)
In addition to stating that a perilunate dislocation is present, a number of features should be sought and commented upon:
- associated fractures
- present in 60% of cases
- thus these are trans-scaphoid-perilunate dislocations
- radial styloid
Plain film is normally sufficient to diagnose the dislocation (assuming there is no interpretation error), however CT plays an important role in assessing for associated occult fractures; the most common and important being scaphoid fracture.
Treatment and prognosis
Untreated there is high risk of median nerve palsy, pressure necrosis, compartment syndrome and long-term wrist dysfunction. As with other dislocations, perilunate dislocation should be reduced as soon as possible. Prompt open reduction with ligamentous repair is necessary.
Despite treatment, long-term risk of degenerative arthritis is high (~60%). Dissociative and non-dissociative carpal instability can occur with DISI or VISI pattern. Also, there is a higher rate of non-union of scaphoid fractures when associated with perilunate dislocation than with isolated scaphoid fractures.
The most important differential diagnosis is that of a lunate dislocation which can mimic a perilunate dislocation especially on AP projection. The key to not confusing the two is the lateral projection:
- in a lunate dislocation the radiolunate articulation is disrupted and the lunate is dislocated in a palmar direction
- in a perilunate dislocation, the radiolunate articulation is maintained
- 1. Schmitt R, Lanz U, Buchberger W. Diagnostic Imaging of the Hand. Thieme Medical Pub. (2008) ISBN:1588904539. Read it at Google Books - Find it at Amazon
- 2. Mayfield JK, Johnson RP, Kilcoyne RK. Carpal dislocations: pathomechanics and progressive perilunar instability. J Hand Surg Am. 1980;5 (3): 226-41. - Pubmed citation
- 3. Berquist TH. Musculoskeletal Imaging Companion. Lippincott Williams & Wilkins. (2006) ISBN:0781763746. Read it at Google Books - Find it at Amazon
- 4. Brant WE, Helms CA. Fundamentals of diagnostic radiology. Lippincott Williams & Wilkins. (2007) ISBN:0781765188. Read it at Google Books - Find it at Amazon
- 5. Schmitt R, Lanz U, Buchberger W. Diagnostic Imaging of the Hand. Thieme Medical Pub. (2008) ISBN:1588904539. Read it at Google Books - Find it at Amazon