Carpal instability: Types, Place of VISI & DISI

 Key word: Carpal instability, Data base: Pubmed

Wrist anatomy- extrinsic & and intrinsic ligaments

The extrinsic (radiocarpal) and intrinsic (intercarpal) ligaments maintain carpal stability. The major extrinsic ligaments are the radioscaphocapitate, radiolunotriquetral, short radiolunate, and dorsal radiocarpal ligaments. The scapholunate and lunotriquetral ligaments are the most important intrinsic ligaments and the primary wrist stabilizers. The most common causes of carpal instability are unstable fracture of the scaphoid, scapholunate dissociation, and lunotriquetral dissociation (7).
Let us discuss the causes of carpal instability. Classification of carpal instability is presented below is based on anatomic and kinematic characteristics of the wrist. A classification of the subtle patterns of carpal instability is presented below.  

Navarro's concept of the carpus (1921):

Carpals of wrist are arranged in 3 vertical longitudinal columns: lateral (scaphoid); central (lunate and distal carpal row); medial (triquetrum).

Carpal dislocation occurs between:
1.    lateral and central columns- called lateral instability
2.    within the central column- called central instability
3.    central column and the triquetrum- called medial instability and
4.    between the entire carpus and the distal radioulnar articular surface- is called proximal instability

Classification of lateral instability: Lateral carpal instabilities are further subdivided according to the different components of the central column that articulate with the scaphoid. Therefore, 3 main lateral patterns may be identified:

1.    scaphoid-trapezium-trapezoid subluxation
2.    scaphoid-capitate diastasis
3.    scaphoid-lunate dissociation (rotatory subluxation of the scaphoid)

Classification of medial instability: Medical carpal instability may take place between the triquetrum and the lunate, or the triquetrum and the hamate. Dissociation between lunate and triquetrum results in static forms of instability, while disruption of triquetrohamate support leads to dynamic forms of instability.

VISI (volar-flexed intercalated segment instability): It is example of medial instability. In lunate-triquetrum instability, is believed responsible for VISI in which the lunate collapses into a volar-flexed position and there is longitudinal "crumpling" of the radiocarpal link.

Proximal instability:
Proximal carpal instability may lead to disruption at the level of the radiocarpal joint or at the level of the midcarpal joint. Proximal carpal or radiocarpal instability may occur in an ulnar (ulnar translocation), dorsal (dorsal subluxation), or volar direction (volar subluxation). It is usually associated with loss of the anatomic alignment of the distal radius.

Exclusive discussions on VISI & DISI:

Navarro’s carpus concept can be called column concept. But "The row concept best explains the behaviour of the carpal bones. 8 carpals comprising of 2 row of 4 carpals each. The proximal row acts as an intercalated segment- there is no direct control of this row. It is controlled by the bones surrounding it via the short interosseous ligaments. It has a natural tendency if isolated to 'pop out' and tilt dorsally."
Intercalated segment: The intercalated segment is the proximal carpal row identified by the lunate. The term 'intercalated segment' refers to it being the part in between the proximal segment of the wrist consisting of the radius and the ulna and the distal segment, represented by the distal carpal row and the metacarpals.

DISI: DISI, or dorsiflexion instability, the lunate is angulated dorsally. It is the most frequent mid carpal instability. It occurs due to complete tear of scapho-lunate ligament.

DISI pattern:
1.    when the scapholunate joint is dissociated, the scaphoid is palmar flexed and the lunate is dorsiflexed
2.    Scapho-lunate angle usually 30- 60degrees (average 46 degrees) and with DISI it is greater than 70degrees

What you look in lateral wrist X-ray if you suspect DISI: If you think the lunate is tilted, measure the scapholunate angle ( 30-60°is normal, 60-80°is questionably abnormal, >80° is abnormal) and the capitolunate angle (<30° is normal).

What you look in AP wrist X-ray if you suspect DISI: Guilula’ arc.

VISI: It is a palmar flexion instability where the lunate is tilted palmarly too much. While most DISI is abnormal, in many cases VISI is a normal variant, especially if the wrist is very lax. It occurs due to complete tear of lunotriquetral ligament. There is posterior subluxation of both lunate & scaphoid due to lack of union with triquetrum. Scaphoid & Lunate remain interdependent hence the scapho-lunate angle is normal.

VISI pattern:
1.    lunate palmar flexed
2.    if the lunate and triquetrum can be seen, the normal lunotriquetral angle of approximately -16 degrees becomes neutral or positive

Note: in conventional radiography: According to Toms et al (8) Conventional radiographic abnormalities are usually limited to volar intercalated segment instability (VISI) patterns of carpal alignment and are not specific.

Mid carpal instability:
Midcarpal instability (MCI) is the result of complex abnormal carpal motion at the midcarpal joint of the wrist.
Palmar, dorsal, ulnar midcarpal instability, and capitolunate or chronic capitolunate instability are all descriptions of types of MCI with often overlapping features.
It is a form of non-dissociative carpal instability (CIND) (see below for dissociative & non- dissociative carpal instability) and can be caused by various combinations of extrinsic ligament injuries that then result in one of several subtypes of MCI.
Palmar midcarpal instability (PMCI) is the most commonly reported type of MCI. It has been described as resulting from deficiencies in the ulna limb of the palmar arcuate ligament (triquetrohamate-capitate) or the dorsal radiotriquetral ligaments, or both.

Another broad classification of carpal instability:Dissociative & non- dissociative carpal instability.

The causes of carpal instability are dissociation of the intercarpal ligaments on either side of the lunate, are so-called scapholunate dissociation or a luno-triquetral dissociation. Carpal instability non-dissociative is generally due to a laxity or attenuation of the intrinsic ligaments of the carpus and are associated with deformity of the distal radius (6).
Ulnar translation of the carpus on the distal radioulnar articular surfaces occurs with shear stretching of the origins of the radiocarpal ligaments. The radial styloid attenuation of the ligaments may result in abnormal motions of the carpal bones going from ulnar to radial deviation at which time a catch-up click may occur (6).

Radiography choices in carpal instability (8):

1.    Stress view radiographs (To demonstrate carpal instability)
2.    Videofluoroscopy (To demonstrate abnormal carpal kinematics)
3.    Dynamic US can be also used to demonstrate midcarpal dyskinesia including the characteristic triquetral "catch-up" clunk.
4.    Tears of the extrinsic ligaments can be demonstrated with MR arthrography, and probably with CT arthrography.

Treatment approach:
Plan of management for carpal dislocations are based upon the following basic criteria: whether the dislocation is perilunate or lunate dislocation. In most cases they are managed identically (5).
Step 1: In all cases anatomic restoration of the 3 key elements (scaphoid, lunate, and capitate) is essential.
Step 2: Following initial closed reduction, rotary subluxation of the scaphoid and intercalary segment instability must be specifically looked for and corrected in the patient with perilunate or lunate dislocation without fracture of the scaphoid.
Failure to obtain or maintain anatomic position by closed methods is an indication for open reduction and internal fixation.

Operative methods & choices:
1.    Unstable carpal articulations can be treated with limited carpal arthrodesis.
2.    Ligamentous defects can be treated with capsulorrhaphy or ligament reconstruction.

Complications:As with all ligamentous injuries, early diagnosis and treatment are essential. Missing the concomitant injuries include median nerve damage, osteochondral fractures of the carpal bones, and fracture of the radial styloid. Pain & Post-op stiffness are a usual complications.

References: 
1.    Taleisnik J; Clin Orthop Relat Res. 1980 Jun;(149):73-82. (Post-traumatic carpal instability).
2.    http://www.wikiradiography.com
3.    LK Ruby; The Journal of Hand Surgery. Volume 13, Issue 1, January 1988, Pages 1–10 (Relative motion of selected carpal bones: A kinematic analysis of the normal wrist)
4.    Taleisnik J; Bull Hosp Jt Dis Orthop Inst. 1984 Fall;44(2):511-31.
5.    Green DP; Clin Orthop Relat Res. 1980 Jun;(149):55-72. (Classification and management of carpal dislocations).
6.    Linscheid RL et al;Orthopade. 1993 Feb;22(1):72-8. (Carpal instability).
7.    Timins ME et al; Radiographics. 1995 May;15(3):575-87. (MR imaging of the major carpal stabilizing ligaments: normal anatomy and clinical examples).
8.    Toms AP; Skeletal Radiol. 2011 May;40(5):533-41. Epub 2010 May 14. (Midcarpal instability: a radiological perspective).