Carpal instability

Ph. Saffar
Institut Français de Chirurgie de la Main - France

There is no muscular attachment to the proximal carpal row. It is an intercalated segment stabilized only by ligaments. Proximal carpal row motion is the consequence of movements of the distal carpal row and forearm bones. The intracapsular wrist volar ligamentous plane is thick and strong (Fig 1): stretching of this structure is difficult to assess but probably plays a role in carpal instability. The dorsal ligamentous plane is thinner and less important for carpal stability.

fig 1 fig 2
Figure 1 Figure 2

Description of the “carpal instability” entity is more than 25 years old (Linscheid et Dobyns, 1972). It was defined as carpal malalignment on X-rays or by carpal ligamentous tears and controversy still exists regarding this definition. We have been interested in pathology of carpal instability since it was described. New imaging techniques for the carpus have helped the surgeons to pre-operatively define carpal ligament injuries and to plan precisely the best treatment.
What are the principal features ?

1) New anatomical descriptions of carpal ligaments including their course and attachments (Taleisnik, 1976) were performed. Studies of their biomechanical role and pathomechanics followed. These intracapsular or intra-articular ligaments are not visible during the surgical approach of the wrist joint, which explains why they were described only recently.
2) Assessment of partial or total injury of each ligament and the consequences of each on this complex joint is possible. These injuries improperly called “wrist sprain” are each now well-known diagnoses and each ligament injury results in a particular pathology.
3) Wrist osteoarthritis (O.A.) is post-traumatic in 95% of the cases (Watson, 1984) and in 75% secondary to a carpal instability.

It is relatively easy to understand why this type of pathology has been overlooked for such a long time :
On a wrist plain X-ray, radius and scaphoid bones usually catch the eye
A quick examination does not allow one to note minimal modifications in carpal bone contours or positions occurred after an injury. These minimal changes may result in significant pathology.
Pain disappears or decreases in some days after injury and a period of time follows lasting for months or years without symptoms. When pain reappears, it is usually the onset of O.A. and it may be difficult to relate it to the initial injury.
This explains why a number of orthopedists and traumatologists tend to ignore this injury and do not treat it initially which is the best way to prevent the development of O.A. Early recognition and treatment is mandatory. However, since the wrist of a young manual worker is probably more important than the arthritic hip of an older person with lower demands.
The same type of trauma applied to a hyperextended wrist results in a distal radial fracture in an elderly woman, because bone is less resistant than ligaments, and a carpal ligament injury in a young man because bone is more resistant.
Wrist clinical examination is well known and signs of injury should be sought out during a precise physical examination. Patient’s elbow should rest on a table, with the forearm vertical (Fig. 3). The examiner should be seated in front of the patient and should stabilize the patient’s forearm to assess wrist motion, discover painful areas and evaluate the dynamics of each carpal and forearm bone. Bone landmarks are recognized (Fig 4): radial and ulnar styloid, Lister’s tubercle, capitate head fossa and proximal to it, the lunate bone. The scapho-lunate ligament may be palpated between these two bones, with the wrist in maximum flexion.
Clinical presentation includes wrist pain, clicking over the carpus area and decrease of motion and strength.

fig 3 fig 4
Figure 3 Figure 4

The initial traumatic event is often remembered by the patient; much time may have evolved since the injury. Initial X-rays often show abnormal findings. The mechanism of injury is not always evident. It may be a “sprain”, treated or not by a cast, a carpal dislocation which was reduced and immobilized in a cast or simply repetitive wrist strain. The diagnosis of scaphoid fracture may have been evoked without any radiological proof.
The diagnosis and type of instability are determined by clinical examination. The diagnosis helps in deciding the choice of radiological investigations and indicates to the radiologist what views are to be selected. After having tried many types of views, we now think that only the following are necessary :
P.A. and lateral views (Fig 5a,b)

fig 5a fig 5b
Figure 5a Figure 5b

Dynamic series : flexion and extension, radial and ulnar deviation (Fig.6a,b,c,d)

fig 6a fig 6b fig 6c fig 7
Figure 6a Figure 6b Figure 6c Figure 7

The clenched fist X-ray provides a view with the carpal bones in compression (Fig 7).
CT-Scan after arthrography (Arthroscan) is mandatory, although not used by many radiologists. It is the most useful investigation : it helps one to choose the surgical approach and the type of treatment. It will be described in the next paper. We have not found plain CT-scan and MRI to be useful. Cinearthrography may be useful in some types of dynamic and midcarpal instabilities.
A good arthrographic study may be useful if arthroscan is not available.
Clinical examination is performed one more time after having received the results of these examinations to correlate clinical and radiological findings. Surgical indications result from this comparison.
Is it useful to perform a preoperative arthroscopy ? We think that there is no point in performing a diagnostic arthroscopy if a good arthroscan is available. If there is not a local radiologist familiar with this technique or when the diagnosis is not clear after plain X-rays and arthroscopy, a radiocarpal and midcarpal arthroscopy may demonstrate ligament tears or static or dynamic instability.
It should be understood that a large range of injuries may be found, and with each type corresponding to a particular biomechanical mechanism of injury.
The scapholunate ligament is the most important : this interosseous ligament of the proximal row is comprised of three parts :
1. The anterior part is relatively large and is formed by twisted fibers due to the different movements of the scaphoid and the lunate : its section results in a dynamic instability.
2. The middle or proximal part is comprised of fibrocartilage which is devoid of any biomechanical role and which cannot heal after a tear. Dye may leak through a tear at this site during arthrography but this does not point to an injury to be treated.
3. The posterior part is the most important biomechanically, injury to this portion results in significant disturbances in carpal mechanism.

Two other features have to be considered :
Ligament elasticity : hypermobile joints may have their ligaments easily stretched without rupture after benign traumas.
The status of a ligament after injury :

  • perforation
  • tear with a “competent” ligament
  • tear with an “incompetent” ligament
  • dissociation of two adjacent bones

  • In the first two cases, there is no instability. An incompetent ligament results in a dynamic instability. Dissociation is a static instability.

    Classification.
    I. Scapholunate instability
    II. Lunotriquetral instability
    III. Combined first row instability
    IV. Midcarpal instability
    V. Proximal instability
    VI. Axial instability

    I. Scapholunate instability

    It is the most frequent problem and the main cause of post-traumatic wrist osteoarthritis. We will first consider scapholunate instability before the onset of osteoarthritis.
    A) Without osteoarthritis (Table 1).
    Clinical symptoms
    Pain is localized by the patient to the dorsal and lateral wrist. Clinical examination elicits provoked pain at the scapholunate joint, which should be palpated with the wrist in slight flexion.
    There is a click or a snap felt by the patient.
    Provocative tests such as

    a) Watson test : a painful snap is obtained when pressing hard on the scaphoid tubercle volarly and moving the wrist from ulnar to radial deviation, the wrist being maintained in axial compression.
    b) The same snap may be felt when palpating dorsally over the scapholunate joint with the same movement
    c) A scapholunate “ballotment” is present when moving the scaphoid and lunate in opposite directions (postero-anterior and antero-posterior) meaning that the links between the two bones are stretched.

    Table 1 Scapho-lunate instability
    STAGE INVESTIGATIONS
    "Predynamic" clinical diagnosis
    normal plain X-rays
    Dynamic Instability clinical diagnosis + dynamic X-ray series
    "Static" or constant (N.Barton) plain X-rays P.A. and lateral cartilage assessment : arthroscan or arthroscopy
    Static with O.A.: radio-scaphoid + luno-capitate (SLAC) plain X-rays P.A. and lateral +/- cartilage assessment : arthroscan or arthroscopy

    Table 2 Luno-triquetral Instability
    STAGE INVESTIGATIONS
    "Predynamic" clinical diagnosis
    normal plain X-rays
    Dynamic instability clinical diagnosis + dynamic series
    Static instability plain X-rays P.A. and lateral

    Plain X-rays
    P.A. X-rays may demonstrate :

    - a scapholunate gap > 3 mm
    - a “ring-sign” : the scaphoid is rotated on its axis and appears foreshortened and with a circle superimposed on its distal part.
    - Carpal height is calculated with the Mac Murtry or Natrass method. It is an important index for following the evolution of carpal collapse (Fig 6).

    Lateral X-rays may demonstrate :
    - A scapholunate angle > 70°, the scaphoid being in flexion and the lunate in extension, thus defining the DISI (dorsal intercalated segment instability), referring to the lunate position. The lunate is easy to identify on lateral X-rays, but it is the scaphoid position which is important (Fig. 7).
    - A lunocapitate angle > 15°

    Dynamic X-rays in flexion, extension, radial and ulnar deviation may demonstrate the decrease of rotatory movement of the scaphoid in radio-ulnar deviation and of the lunate in flexion-extension. A scapholunate gap may also appear . This defines the dynamic instability
    “stress views’” : “fist clenched” view may make apparent a gap non visible on normal P.A. views.
    Arthrography
    A midcarpal injection is first performed, associated with a distal radiolunate joint injection. A radiocarpal injection is added if information collected by the first injection is insufficient.
    A dye leakage is not always an indication of a significant tear in the ligaments. It may be a perforation without pathological consequences. Conversely a torn and healed ligament may give a normal image without leakage. Dynamic views during arthrography may make a leakage appear or lead one to suspect a stretched and incompetent ligament. Bilateral arthrographies have shown bilateral perforations in normal wrists. After 40 years of age, they increase in frequency.
    CTscan after arthrography or Arthroscan
    It is our favorite investigation. Using 1 mm cuts, It gives precise information on the site and length of ligament tears, existence of a ligament stump suitable for suture or reinsertion and cartilage status.
    It has to be understood that only radiological signs correlated with clinical signs should be taken into account.
    The major factor of severity is the horizontal scaphoid (Allieu) : its proximal pole is no longer congruent with the radio-scaphoid facet and impinges with the dorsal rim of the radius. This results in radioscaphoid osteoarthritis (O.A.) (Fig. 8). The lunate bone is congruent with the radius lunate facet in every position and there is no radiolunate O.A., which allows reconstruction around this joint.

    fig 8 Figure 1

    fig 9 fig 10
    Figure 9 Recherche du signe de Watson Figure 10 Recherche du “ballotement” scapho-lunaire

    B) With osteoarthritis
    Many patients are referred late at the arthritic stage because the diagnosis was first missed. The capitate bone pushes apart the scaphoid and lunate when making a fist and force transmission from hand to forearm tends to cause the capitate to slip into the joint space between these two bones. Carpal height decreases and the lunate bone rotates into extension (DISI) and shifts ulnarly. Cartilage wear is located in the beginning at the posterior and ulnar side of the capitate.
    The course was described by K. Watson under the denomination of SLAC (ScaphoLunate Advanced Collapse).
    Evolution of osteoarthritis (O.A.) always has the same pattern

    1. Stylo-scaphoid arthrosis SLAC type 1
    2. Radio-scaphoid arthrosis SLAC type 2
    3. Radio-scaphoid arthrosis + luno-capitate arthrosis SLAC type 3

    This evolution is of variable duration depending on patient occupation and magnitude of initial ligament injuries. It is possible to find total cartilage wear of the scaphoid proximal pole six months after initial injury. In other cases, it may take 15 years to develop a painful carpal arthrosis.
    The wrist is painful and clinical examination localizes at the scapholunate joint the maximum of discomfort at rest and during movement. Range of motion and strength are measured, comparing it with the opposite side. There is swelling and deformity at the dorsal and radial side of the wrist. This decrease in wrist function has developed slowly and the patient is often adapted to this impairment which was not noticed.
    Plain x-rays demonstrate alterations of scapholunate, radiolunate and capitolunate angles, carpal height and capitolunate shift angle (Sokolow) : angle between IIIrd metacarpal axis and the line from the metacarpal base and the lunate bone center. This angle is very important in determining the choice of limited carpal arthrodesis to be performed. Dynamic x-rays allow one to appreciate reducibility of the lunate on the capitate head.
    If a recent injury has provoked or increased wrist pain, it may create problems for a compensation worker, even if it is evident that it is an old injury. A.P, lateral X-rays and dynamic x-rays help to assess the extent of arthrosis. It is only at stage 1 that arthroscan or arthroscopy may be necessary.

    II. Luno-triquetral instability

    This injury is also mostly present in young patients, often manual workers (mean age : 34 y). It may be a partial or complete rupture or stretching of the luno-triquetral ligament. It results in a functional impairment but rarely in osteoarthritis. Associated injuries of the ulnar side of the wrist are frequent and should be diagnosed and treated at the same time or pain and wrist impairment will persist.
    The pathomechanics of the injury seems to be a fall on the hypothenar eminence, the hand being behind the plane of the body. It may also be the sequel of a carpal dislocation. A long ulna is an associated cause of these injuries.
    Pain and “ballottement” are found at the lunotriquetral jointspace. There is a click in pronation and ulnar deviation and in prono-supination.
    Plain X-rays are normal in more than half of the cases. The lunate may be rotated in VISI (volar intercalated segment instability) with a decrease of the scapholunate angle (less than 30°). A gap of more than two mm between the two bones is abnormal. A real dissociation is rare. Rupture of the Gilula’s angles is the most significant sign on P.A. views. It may only be visible on dynamic series but is less diagnostic in this case. Other investigations are mandatory here to assess the diagnosis of luno-triquetral tear and to elicit the associated injuries.


    III. Combined first row instability : SL + LT

    This association may combine in varying degrees of injury. The clinical diagnosis of the associated injuries is difficult except if visible on plain X-rays and dynamic series. Additional investigations are necessary.
    This association may be evident, particularly after reduction of a carpal dislocation. It may also be elicited by arthrography, arthroscan or arthroscopy when only one of the injury was suspected. It is a difficult problem : which one of the injuries is the cause of pain and discomfort and should the two injuries be repaired ?
    The two other types of instability are much more rare and should be known to differentiate them from the most common ones.

    IV. Midcarpal instability

    There are two different entities which are classified differently by the French and American Schools.

    1. Adaptive carpus. It is a loss of alignment of the first and second carpal rows which are no longer collinear on the lateral X-rays. This may be the consequence of a distal radius malunion, a scaphoid nonunion or other causes of carpal collapse : Kienböck’s disease at an advanced stage, capitate or scaphoid bone necrosis, etc.
    These are not true carpal instabilities since the ligaments are intact. However, a long-standing collapse may stretch the ligaments and result in a true carpal instability.
    2. Tear or stretching of the ligaments stabilizing the midcarpal joint.
    This happens often in hypermobile patients, mainly young girls. After a benign injury, or repetitive stress (tennis), the wrist becomes painful almost constantly.
    The clinical examination elicits a “clunk” which is a deep noise produced when moving the wrist in deviation and pronation. This may be bilateral but painful and accompanied only on one side by pain. A VISI deformity may be found, but sometimes the lunate is dorsiflexed.

    Another type of midcarpal instability is present after a significant injury with wrist torsion.
    The diagnosis is essentially clinical. Arthrography is normal and cinearthrography may demonstrate the sudden relocation of the first row on the second.
    V. Proximal instability

    It may result from several causes :

  • Ulnar shift of the carpus secondary to the rare radiocarpal dislocations or fracture-dislocations
  • Iatrogenic ulnar shift of the carpus after large resections of the distal ulna
  • Palmar or dorsal shift of the carpus secondary to malunion of the anterior or posterior margin of the distal radius.
  • Detachment of the radiocarpal ligaments by a radial styloidectomy.
  • Rheumatoid arthritis which will not be mentioned in this article.

  • Taleisnik has described two stages :
    1. Ulnar shift of the entire carpus
    2. Ulnar shift of the carpus with a scapholunate gap which should not be misdiagnosed for a scapholunate instability

    VI. Axial instability

    It is a fracture-dislocation of the carpus in the longitudinal axis.
    Fracture-dislocation of the carpus by a crush injury


    TREATMENT
    We will begin with the treatment of the rare instabilities

    Axial instability
    They will be treated by reduction and internal fixation of the fractures and longitudinal dissociation

    Proximal instability
    Isolated radiocarpal dislocations should be operated on by an anterior approach and the plane of the volar ligament sutured or reattached to the radius anterior margin. Pinning alone is not sufficient as a carpal ulnar shift will appear after pin removal.
    In the chronic cases, radiolunate arthrodesis may be useful to stabilize the carpus.

    Luno-triquetral instability

    1. Soft tissue repair
    Reduction of the gap and/or triquetral distal displacement and fixation by two K-wires without suture of the ligament. It may heal if relocated.
    Suture or reattachment of the lunotriquetral ligament by transosseous sutures or anchors
    Ligamentoplasty using a strip of ECU

    2. Limited carpal arthrodesis
    Luno-triquetral arthrodesis : after reduction of the triquetrum, cartilage is removed from the corresponding faces of the two adjacent bones and a corticoancellous graft is interposed between the two bones. The graft is harvested from the distal radius or from the olecranon. Fixation is performed using K-wires, screws or memory staples. Nonunion is frequent and should be avoided by a good technique.
    A four-corner arthrodesis fixing the lunate, triquetrum, capitate and hamate may also be performed. If there is a long ulna, it should be shortened during the same procedure to avoid abutment syndrome. TFCC tears should also be repaired when in a vascularized zone.
    Post-operative results are decrease in pain, strength improvement in 80% of the cases.
    Motion is slightly decreased.

    Scapholunate instability without osteoarthritis

    The aim is to restore a normal orientation to the scaphoid and to recover a normal congruency of the scaphoid proximal pole with the scaphoid facet of the distal radius.
    Many treatment have been proposed in the last 20 years.

    A. Soft tissue repairs
    B. Limited carpal arthrodeses.

    A. Soft tissue repairs

  • Reduction of the malrotated scaphoid and lunate and pinning.
  • Ligament suture
  • Ligament reattachment
  • Ligamentoplasty
  • Capsulodesis

  • These techniques may be isolated or associated and will not be described step-by-step. Pinning is mandatory for all techniques.
  • Several types of ligamentoplasties trying to reproduce the ligaments using slip of tendons have been reported but the failure rate at follow-up are too high and these techniques are no more performed. A new technique of ligamentoplasty stabilizing the distal scaphoid was recently reported by Brunelli but follow-up is not significant.
  • Capsulodesis may be used alone in dynamic instability but is often associated with ligamentous repair.
  • Ligament repair or reattachment after scaphoid reduction and pin fixation is indicated if ligament remnants are good enough. Intraosseous sutures or anchors are used and may be augmented using the intercarpal dorsal ligament left inserted to the scaphoid.
  • Association of ligament repair and capsulodesis is the most used technique for soft tissue repairs.

  • These techniques are indicated for dynamic instabilities and when the scaphoid is easily reducible. The ligament repaired is not as competent as a normal ligament but carpal dynamics is usually restored. A scapholunate gap may reappear at follow-up, but the scaphoid is congruent with the distal radius and evolution to osteoarthritis is avoided.
    For our first 40 cases of this type of treatment, on average : residual pain is absent or climatic, the range of motion obtained is 110° in flexion-extension, and 48° in radial and ulnar deviation. Strength is increased to 78% of the opposite side. The scapholunate angle is within normal values but the scapholunate joint space demonstrates a 4 mm gap on average.

    B. Limited carpal arthrodeses.
    It should be reminded that the biomechanichs of the carpus are modified by these procedures and that the load transfer from hand to forearm is modified. In the literature, there is no mention of onset of osteoarthritis in the previously normal joints. The wrist is a joint which is more often in a distracted position and only inconstantly loaded.
    There are several types of limited carpal arthrodeses :
    scapholunate (SL)
    scapho-trapeze-trapezoidal (STT)
    scapho-capitate (SC)
    scapho-luno-capitate (SLC)

    Some technical points should be stressed :
    cartilage and subchondral bone should be excised or healing will not be obtained.
    reduction helps to regain a normal orientation of the bones. Bone graft restores a normal distance between the bones and preserves the normal contour and height of the carpus to maintain normal kinematics.
    fixation will be performed using K-Wires, staples and particularly memory staples. Jointspaces are filled with cancellous bone
    Scapholunate arthrodesis has been abandoned because nonunion is too frequent (more than 50% in all publications).
    S.T.T. arthrodesis is most often used.
    S.C. arthrodesis is our favorite because it is easy to perform and the healing rate is high.
    Results with a significant follow-up demonstrated that pain is absent or mild. Range of motion is only slightly decreased from 99°6 preop to 92°4 postop in flexion-extension and 42°5 preop to 36°5 postop in radial and ulnar deviations.
    Strength has increased by 15% but is only 60% of the opposite side. We have to take into account that this type of surgery is aimed at cases more advanced than those repaired by soft tissue procedures.
    Limited carpal arthrodeses are indicated if the scaphoid is not easily reducible and for manual workers. “Static” instabilities are treated that way and they may also apply to painful dynamic instabilities They are reliable with a long follow-up and results improve with time.
    The most frequent complication is nonunion which may be reoperated with success by another arthrodesis.
    The choice between the soft tissue repair and limited carpal arthrodesis is often decided during surgery.
    In some cases, an unexpected small area of cartilage wear is discovered at the scaphoid proximal pole during surgery. It is sometimes possible to perform one of the two types of surgery if this small area is not in the loading zone after scaphoid reduction.

    Scapholunate instability with osteoarthritis
    The treatment of the “SLAC” wrist depends on its stage.

    SLAC1
    A limited stylo-scaphoid arthritis may be treated by radial styloidectomy. This procedure eliminates the worn area but also the attachment of the radiocarpal ligaments : radiocapitate and part of the radiolunate ligament may be impaired. This may destabilize the radiocarpal joint. To avoid this pitfall, it is possible to

    1. resect only the dorsal aspect of the radial styloid, retaining the anterior wall of the radial margin
    2. reattach the ligaments to the distal radius with an anchor.

    SLAC2
    Proximal row carpectomy may be indicated if the cartilage of the head of the capitate is preserved. This procedure will be followed by a four- week immobilization.
    SLAC 3
    The aim is to to relieve pain and preserve a useful motion and a good strength to the wrist. Reconstruction of the carpus is performed around the normal radiolunate joint. Several types of procedures may be applied. They all have in common a scaphoidectomy.
    This should be associated with a carpal stabilization by a limited carpal arthrodesis
    Capitolunate arthrodesis
    By a dorsal, longitudinal approach between the 3rd and 4th compartment, a longitudinal arthrotomy is performed. Ligaments and cartilage status are checked particularly the radius lunate facet and the proximal aspect of the lunate cartilage. Position and reducibility of the capitate relative to the lunate are assessed, the wrist being in radial deviation and pushing hard on the capitate.
    Osteophytes are often present at the distal radius margin and posterior horn of the lunate at the bone-cartilage junction due to lunate malrotation. These osteophytes have to be resected with a part of the distal posterior margin of the radius to allow better extension of the wrist.
    After excision of the intra-articular fibrosis, the scaphoid is totally excised and this bone will be used as a graft. The remaining cartilage of the lunate distal facet and of the proximal capitate are resected with the subchondral bone. The lunate is reduced on the capitate head and a provisional pin is inserted longitudinally maintaining the two bones in the reduced position. This pinning should be performed with the wrist in maximum flexion. The jointspace between lunate and capitate is filled with cancellous bone harvested from the scaphoid and distal radius. Fixation is completed using two memory staples. After closure of the capsule, dorsal retinaculum and skin, a palmar splint is applied for four days, followed by a short-arm cast until healing, generally two months.
    Four-bone or four-corner arthrodesis
    Approach and scaphoidectomy are performed the same way as that for the lunocapitate arthrodesis. Cartilage and subchondral bone are removed between the four bones.
    Reduction may be difficult. Fixation is performed by K-wires and memory staples.
    Shortening arthrodesis or decapitation of the capitate are variations of the precedent procedures. The triquetrum is excised when there is an abutment of the ulna.

    RESULTS
    Limited carpal arthrodeses
    Pain was disabling preoperatively and climatic or absent postoperatively.
    Range of motion obtained has been
    CL Arthrodesis 60° Fl-Ext 25° UD and RD
    Four-corner 53° Fl-Ext 24° UD and RD
    No change for strength : 50% of the opposite side
    The choice between the two types of arthrodeses is based on the severity of the displacement of the lunate relative to the capitate. When the lunate is significantly shifted ulnarly, its reduction is difficult and four-corner arthrodesis should be the choice. Nonunions were around 20% and decreased to 5% when applying these principles and using memory staples.
    Orthopedists and traumatologists should be aware of carpal ligament injuries to avoid post-traumatic wrist arthritis. The patients are young at the time of injury and disabling osteoarthritis appears around the age of 40.
    Total wrist arthrodesis is an obsolete procedure when arthritis is present. Numerous techniques help to keep a certain amount of wrist motion, which will preserve the automatic tenodesis movement of the flexor and extensor tendons when the wrist is moving.

    fig 11 fig 12 fig 13 fig 14
    Figure 11 Figure 12 Figure 13 Figure 14

    fig 15 fig 16 fig 17
    Figure 15 Figure 16 Figure 17

    fig 18a fig 18b fig 18c
    Figure 18a,b,c Les différents stades du SLAC

    fig 19 fig 20
    Figure 19 Figure 20

    fig 21a fig 21b
    Figure 21a Figure 21b

    fig 22 fig 23
    Figure 22 Figure 23

    fig 24a fig 24b fig 25
    Figure 24a Figure 24b Figure 25

    fig 26 fig 27 fig 27b
    Figure 26 Figure 27 Figure 27b

    fig 28 fig 29
    Figure 28 Figure 29

    fig 30 fig 31 fig 32
    Figure 30 Figure 31 Figure 32

    fig 33 fig 34
    Figure 33 Figure 34

    fig 35a fig 35b fig 35c fig 35d
    Figure 35a Figure 35b Figure 35c Figure 35d

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