Last updated on May 11, 2020
The lunate bone is aptly named because of its crescent shape on the lateral view (‘luna’ means moon in Latin). It is bounded by the scaphoid radially and the triquetrum ulnarly. The carpal bones are divided into a proximal and distal carpal row; the proximal carpal row, which includes the lunate, demonstrates considerable intercarpal mobility, contrasting with the comparatively immobile distal carpal row (1). Intrinsic and extrinsic ligaments further stabilize the wrist; of particular relevance to this entry are the scapholunate and lunotriquetral ligaments, which can often be injured in perilunate dislocations (2).
Mechanism of Injury and Classifications
Perilunate dislocations and perilunate fracture-dislocations are high-energy injuries resulting from loaded hyperextension and intracarpal axial rotation. All-terrain vehicle or motorcycle accidents are often common mechanisms in these patients. Mayfield and colleagues classified perilunate injuries into four stages as follows (3), see Figure 1:
- Stage I involves scapholunate ligament dissociation or scaphoid fracture.
- Stage II describes displacement of the carpus relative to the lunate. The lunocapitate articulation may be disrupted resulting in a dorsal perilunate dislocation, or in the case of concomitant scaphoid fracture, the wrist may undergo a transscaphoperilunate dislocation.
- Stage III involves disruption of the the lunotriquetral ligament or triquetral fractures.
- Stage IV denotes a true lunate dislocation, involving a complete avulsion of all perilunate ligaments. This devastating injury forces the lunate out of the lunate fossa of the radius, often dislocating palmarly.
Often, the vector of force producing the perilunate dislocation travels through the waist of the scaphoid rather than the scapholunate ligament (see Figure 1, top right panel). We will discuss the details of this case further in our example below.
It is worth reiterating that there is a well-defined difference between a perilunate dislocation and a complete lunate dislocation. Often these two terms are used interchangeably, which is incorrect. Although both injuries lie on the same spectrum, a true lunate dislocation (Mayfield IV) involves disruption of the radiolunate articulation and forces the lunate out of the lunate fossa of the radius usually in a palmar direction.
Patients usually will have swelling and perhaps even a visible wrist deformity. Ensure full trauma work-up is completed when appropriate as these injuries involve high energy mechanisms. Paresthesias in the median nerve distribution should be carefully noted, as acute carpal tunnel syndrome is an urgent operative indication.
Physical Examination and Radiographic Studies
Visual inspection of the wrist often reveals swelling of the affected area. It is important to assess for snuffbox and volar tubercle tenderness with the understanding that the exam may be complicated by diffuse tenderness. Sensory examination should focus on evaluation of any potential traumatic compression neuropathy of the median nerve; gross sensory and two-point discrimination should be documented thoroughly. Motor exam is often also limited; a wrist block is helpful after sensory examination to facilitate.
Radiographic studies are essential for diagnosis. Standard 3-view wrist films are helpful; of particular relevance is measurement of the scapholunate interval on the PA projection (normal is usually 5mm), assessment of Gilula’s lines and the position of the lunate relative to the capitate and radius on lateral projection. Radiographs of the contralateral wrist for comparison can be helpful to confirm scapholunate widening, also called a ‘Terry Thomas sign.’
Moreover, CT scans can be helpful for operative planning (see images from example case below), especially when involving complex carpal pathology such as perilunate dislocations. CT scans can also be helpful when scaphoid fractures are suspected clinically but are not detected on plain film radiographs, which is relevant to our discussions here.
An attempt should be made to restore alignment in the Emergency Department through closed reduction. The following is a brief list of materials needed to accomplish this:
- Lidocaine 2% plain
- 10cc syringe
- 18G blunt needle
- 27G injecting needle
- Kerlix Roll
- IV pole
- 10-15 pounds of weights (can string together IV bags if weights are unavailable)
- Bedside fluoroscopy (mini C-arm)
- Morphine and valium or conscious sedation (i.e. propofol, if cleared by Emergency Medicine provider) for reduction maneuver
- 4 inch plaster for dorsal and volar slab splint (can also fashion a sugartong post-reduction)
- Cast padding
- Bucket of warm water
- Two 3 inch ace wraps
- Establish adequate analgesia by performing a wrist block.
- Administer pain medications; morphine and valium are useful here
- Hang in traction for 30 minutes using the kerlix roll fashioned in a ‘Chinese fingertrap’ configuration (see Figure 3).
- Reassess pain level and forearm muscle fatigue status. Reduction of these injuries requires considerable force, which is why conscious sedation is helpful. Speak to your colleagues in the ED and work with them to establish a safe plan for the patient.
- Use bedside fluoroscopy to ensure appropriate placement of fingers over the lunate volarly
- In perilunate dislocations, the capitate is often dorsally displaced relative to the lunate. Reduction maneuver involves volar pressure over the lunate, full extension of the wrist followed by complete flexion
- You should feel a palpable clunk with appropriate reduction
- Review bedside radiographic films and obtain formal post-reduction radiographs after applying splint
- Observe for any residual post-procedural median neuropathy after blocks wear off
Operative indications: perilunate injuries are almost always followed by surgical stabilization of the region with possible scapholunate ligament repair, fixation of fracture fragments and carpal tunnel release if indicated. The dorsal Berger capsulotomy offers excellent visualization of the wrist for further operative interventions.
The patient is a 22 y.o. male admitted after fall off a dirtbike. Driving a quad unhelmeted in the woods when he fell off and hit his head and left forearm. Brother drove him to ED where he noted left wrist pain. On radiographs, found to have a left perilunate dislocation and scaphoid fracture.
Elbow and hand radiographs otherwise unremarkable. Pan scan otherwise unremarkable.
Left wrist visibly swollen. Sensation largely intact in radial and ulnar distribution. +snuffbox tenderness. Reports tingling in the median nerve distribution; 5mm two point discrimination. Vascularly intact with capillary refill < 3 seconds in all finger tips. Palpable radial and ulnar pulse. WWP throughout. Flexion/extension limited by pain at wrist and fingers. Median/radial and ulnar motor function grossly intact with “OK” sign, thumbs up and finger adduction/abduction.
There is no acute fracture, dislocation, or effusion of the elbow joint. Soft tissues are unremarkable.
The capitate is dislocated dorsally relative to the lunate. There is a fracture through the scaphoid waist with approximately one bone width of displacement (Figure 4).
Reduction maneuver performed as detailed above in the Emergency Department. Post reduction image below (Figure 5). For post-operative radiographs demonstrating improved carpal alignment and fixation, please see Figure 6.
The patient was then admitted for monitoring of carpal tunnel symptoms and underwent operative intervention the following day. Scaphoid fixation and an open carpal tunnel release was performed. Thumb spica splint with volar slab was applied and the patient was instructed to follow-up in clinic in 2 weeks for further evaluation. Post-operative fixation films at 2-week follow-up seen in Figure 6.
- Green, D. P. Green’s operative hand surgery, 7th ed. Philadelphia, Pa.: Elsevier/Churchill Livingstone; 2016.
- Stanbury, S. J., Elfar, J. C. Perilunate dislocation and perilunate fracture-dislocation. J Am Acad Orthop Surg 2011;19:554-562.
- Mayfield, J. K., Johnson, R. P., Kilcoyne, R. K. Carpal dislocations: pathomechanics and progressive perilunar instability. J Hand Surg Am 1980;5:226-241.