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The role of wrist arthroscopy in scapholunate instability
Relevant anatomy and biomechanics
The scapholunate interosseous ligament (SLIL) is a C -shaped structure connecting the dorsal, proximal, and palmar surface between the scaphoid and the lunate, leaving the distal aspect of the joint bare. The SLIL has three subcomponents: a stronger dorsal component, a biomechanically unimportant membranous component, and a palmar component. The dorsal and palmar portions of the SLIL are true ligamentous structures. The proximal portion is a membranous structure composed mainly of fibrocartilaginous tissue. The dorsal portion of the SLIL has a highest load at ultimate failure, followed by the palmar portion, and then the proximal portion. A number of cadaver studies have shown that sectioning the palmar and membranous SLIL causes shear stress between the scaphoid and lunate, which can manifest clinically as synovitis and wrist pain. A complete division of all three components causes significant kinematic alteration but no carpal malalignment. This is explained by the presence of secondary stabilizers of the scapholunate (SL) joint, which must be injured either acutely or chronically to demonstrate radiographic instability. This only occurs when there is a simultaneous failure or progressive attenuation of the secondary scaphoid stabilizers. Elsaidi and Ruch sequentially divided the radioscaphocapitate (RSC), long and short radiolunate, the SLIL, and finally the dorsal capsule insertion on the scaphoid. There was no appreciable change in the radiographic appearance of the wrist. When the dorsal radiocarpal ligament (DRCL) was then divided, a dorsal intercalated segmental instability (DISI) deformity occurred. Short et al. determined that the SLIL is the primary stabilizer of the SL articulation and that the DRCL, the dorsal intercarpal (DIC) ligament, the scaphotrapezial (ST) ligaments, and the RSC ligaments are secondary stabilizers. Dividing the DRCL alone only caused increased radial deviation of the lunate with the wrist in maximum flexion whereas dividing the SLIL after any of the ligaments tested produced increased flexion and ulnar deviation of the scaphoid and lunate extension. The SL joint stability is therefore dependent on a complex of ligaments, each having a separate role but working in concert with one another. The RSC acts as a fulcrum for the scaphoid to flex and extend. The DRCL and DIC ligament are stout restraints that prevent dorsal translation of the carpal bones. When the SLIL, along with any of the external ligaments, are injured, the scaphoid is free to flex over the RSC and pronate.
An isolated tear of the SLIL changes carpal loading and kinematics even without demonstrable radiographic abnormalities. It can lead to attenuation of the secondary stabilizers and progressive dissociation and rotation of the scaphoid and the lunate. With axial loading, over time the capitate migrates proximally, further driving the scaphoid and lunate apart like a wedge. This results in midcarpal instability, loss of carpal height, and changes in the radiocarpal, intercarpal, and midcarpal joint contact areas and loads. These lead to a predictable scapholunate advanced collapse (SLAC) arthritis 
( ). This begins with radial styloid beaking and radial styloid-scaphoid joint narrowing (stage 1), then progresses proximally to alter the radial scaphoid facet proximal pole scaphoid articulation (stage 2), and finally to the midcarpal capitolunate joint (stage 3).
Proprioception is also important to the stability of the SL joint. The average yield strength of the dorsal SLIL is roughly 260 Newtons. This cannot explain how a 180-pound gymnast can perform a handspring from a running position without disrupting the SLIL. The ligaments are the first line of defense against any destabilizing agent but the bulk of the wrist-stabilizing duty must be taken up by the muscles. The SLIL provides static stability and also contains proprioceptors innervated by the terminal branches of the posterior and anterior interosseous nerves that relay information to the sensorimotor system, which results in an automatic muscular response that helps to absorb the loads.
Diagnosis
Acute injuries often arise from a fall onto the outstretched hand. The patient will typically present with poorly localized wrist pain. The wrist is swollen and painful with restricted motion in the acute phase. Chronic injuries are more localized to the SL joint but with minimal swelling and often normal motion. With longstanding tears, however, chronic synovitis over the snuffbox may be misdiagnosed as a ganglion cyst. There may or may not be tenderness over the SL interval and/or snuffbox. The scaphoid shift test described by Watson is useful in making the diagnosis of a partial or complete SLIL tear. The examiner sits facing the patient with his/her thumb over the palmar scaphoid tuberosity. The examiner passively moves the patient’s wrist from ulnar deviation to radial deviation while applying increasing dorsal pressure to the tuberosity. A painful clunk due to dorsal subluxation of the proximal scaphoid pole is considered a positive test, but many authors have noted that pain may be present without a definite clunk. Dynamic fluoroscopy may show the SL interval opening and closing during active wrist motion and gripping, which may reveal SL instability that is not seen with static films ( ). 
AP and lateral radiograph views may be normal, even with a complete tear. An increased SL gap of more than 3 mm may be present only with ulnar deviation and pronated grip views ( Fig. 6.1 A–C). An increased SL angle beyond 60 degrees and a positive ring sign due to a flexed scaphoid with superimposition of the tuberosity and proximal poles is often indicative of a more chronic injury, where the secondary SL joint stabilizers have attenuated ( Fig. 6.2 A–C). An MRI diagnosis is highly dependent on the size of the MRI coil, the resolution of the grid, and experience of the radiologist. The axial view is useful in evaluating the integrity of the SL ligament. Unenhanced 3.0 T axial views with a dedicated wrist coil and a high-resolution grid may show a tear of the dorsal SLIL ligament but the palmar ligament is more difficult to visualize.
Treatment
A predynamic instability is an older term coined by Watson et al. to apply to SL instability that is diagnosed on physical examination by a positive scaphoid shift test but with normal radiographic studies. This most likely corresponds to a Geissler grade I or II instability (see Arthroscopic Classification of Ligament Instability ). The natural history of this type of lesion is unknown. However, O’Meeghan et al. demonstrated that milder grades of instability do not necessarily culminate in a static carpal instability. At a 7-year follow-up, 11 patients with untreated Geissler grade I or II SLIL instability diagnosed at the time of arthroscopy still showed no radiologic signs of instability, but they continued to experience considerable pain, loss of motion, and functional limitation. Patients with poorly localized pain and tenderness with normal static and dynamic radiographs can be treated with 4 to 6 weeks of immobilization with a thumb spica splint or cast, and then reexamination. Patients with pain and tenderness that persist beyond 3 months despite conservative measures with a positive Watson shift test can undergo further evaluation with an MRI. Many authors consider chronic pain beyond 6 months to be an indication for diagnostic arthroscopy. The patient that presents with pain and tenderness localized to the SL joint with an obvious SL gap are candidates for immediate exploration because complete SLIL tears will not heal with immobilization. The patient that presents with a fixed DISI pattern is also a candidate for surgery. In this case, however, one should maintain a high index of suspicion for an acute-on-chronic SLIL injury with a preexisting chronic SLIL tear. The patient should be counseled that some type of ligament reconstruction or salvage procedure might be necessary in the face of an old and unrepairable SLIL tear.
Diagnostic arthroscopy
Indications
Arthroscopic assessment of the scapholunate ligament complex (SLLC) is indicated in any patient with radial-sided wrist pain and with a positive scaphoid shift test who has not responded to conservative measures. It is also indicated when there is an SL gap without a DISI deformity. It then guides the subsequent treatment by allowing the staging of the degree of injury and the severity of instability.
Contraindications
Complete and repairable SLIL tears, static carpal instabilities, and those with associated radiocarpal and midcarpal osteoarthritis (OA) are best managed with open techniques. Arthroscopy is useful in assessing the articular surfaces, which can then guide the subsequent treatment.
Surgical technique
Under regional block or general anesthesia, the patient’s hand is suspended from either an overhead pulley or a traction tower with 10 to 15 pounds of countertraction. In cases where the tourniquet time is expected to exceed 2 hours, much of the arthroscopic survey can be performed under portal site local anesthesia without a tourniquet, as described by Ong et al ( ). In this case the portals are infiltrated with 0.5% bupivacaine with 1:200,000 units of epinephrine, before prepping and draping. The arm is exsanguinated and an upper arm tourniquet inflated to 250 mm Hg. I often perform dry arthroscopy, as described by del Pinal. Continuous fluid irrigation is replaced by intermittent irrigation through the scope using a 10-cc syringe. A full radius resector is used to suction the joint and maintain a dry field.
The surgeon is initially seated facing the dorsal surface of the wrist. The 3,4 portal is located in the concavity overlying the lunate between the extensor pollicis longus (EPL) and the extensor digitorum communis (EDC) is located just distal to Lister’s tubercle, in line with the second web space. The radiocarpal joint is identified with a 22-gauge needle that is sloped 10 degrees palmar to account for the volar inclination of the radius. A shallow incision is made to avoid injuring small branches of the superficial radial nerve (SRN) or superficial veins. Tenotomy scissors are then used to spread the soft tissue and pierce the dorsal capsule. The radial scaphocapitate (RSC) and long radiolunate (LRL) ligaments are slightly radial. The vascular tuft of the radioscapholunate (RSL) ligament is directly in line with this portal. Superior to the RSL is the membranous portion of the SLIL. The interval for the 4,5 portal is identified with the 22-gauge needle between the EDC and EDM, in line with the ring metacarpal. Due to the normal radial inclination of the distal radius, this portal lies slightly proximal and about 1 cm ulnar to the 3,4 portal. The 6R portal is identified on the radial side of the extensor carpi ulnaris (ECU) tendon, just distal to the ulnar head. The scope should be angled 10 degrees proximally to avoid hitting the triquetrum. With the scope in the 6R portal looking ulnarly, the dorsal capsular insertion onto the dorsal aspect of the SLIL ( Fig. 6.3 ) can be seen by rotating the scope. In the presence of a tear of the dorsal SLIL, an avulsion of the capsular reflection denotes a more severe injury. The 6U portal is found on the ulnar side of the ECU tendon. Angling the needle distally while radially deviating the wrist helps avoid running into the triquetrum. The MCR portal is found 1 cm distal to the 3,4 portal. Flexing the wrist and firm thumb pressure help identify the soft spot between the distal pole of the scaphoid and the proximal capitate. The scaphotrapezial trapezoidal (STT) joint lays radially and can be seen by rotating the scope dorsally. The SL articulation can be seen proximally and ulnarly, which can be probed for instability or step-off. Further ulnarly, the lunotriquetral articulation is visualized. The midcarpal ulnar (MCU) is found 1 cm distal to the 4,5 portal, and 1.5 cm ulnar and slightly proximal to the midcarpal radial (MCR) portal (in line with the fourth metacarpal axis). This entry site is at the intersection of the lunate, triquetrum, hamate, and capitate with a type I lunate facet, and directly over the lunotriquetral joint with a type II lunate facet (which allows preferential views of this articulation). Directly volar, the two limbs of the arcuate ligament can be seen.
Arthroscopic classification of ligament instability
Arthroscopy has perhaps its biggest role in the assessment of scapholunate instability. Geissler et al., however, proposed an arthroscopic grading scale of interosseous ligament instability which has been widely adopted because it quantifies the resultant instability and not the actual size of the tear ( ). Midcarpal arthroscopy is essential in evaluating the instability. Normally there is very little step-off between the distal articular surfaces of the scaphoid and lunate. Direct pressure from the scope combined with traction may force the carpal joints out of alignment. The traction should be released and the SL joint should be viewed with the scope in the MCU to avoid artificial separation of the SL joint by the arthroscope. In grade I injuries, there is loss of the normal concave appearance of the interosseous ligament from the scaphoid and the lunate as the ligament bulges with a convex appearance as seen with the arthroscope in the radiocarpal space. In the midcarpal space, the SL interval is still tight and congruent. It is thought these are minor wrist sprains and usually will resolve with simple immobilization. In Geissler grade II injuries, the interosseous ligament continues to stretch and a convex appearance is seen between the scaphoid and the lunate with the arthroscope in the radiocarpal space. In the midcarpal space, the SL interval is no longer congruent. The scaphoid starts to palmarly flex and its dorsal lip is rotated distal to the level of the lunate. This is best appreciated with the arthroscope in the MCU portal looking across the wrist to detect the amount of rotation between the scaphoid and the lunate. In Geissler grade III injuries, the interosseous ligament lesion has progressed from a stretch to a tear and a gap is seen between the scaphoid and the lunate with the arthroscope in the radiocarpal and midcarpal spaces. The tear usually progresses from a palmar direction to a dorsal direction. This gap can be appreciated both from the radiocarpal and midcarpal spaces. In the midcarpal space, a 1-mm probe may be passed through the gap and twisted between the scaphoid and the lunate. Part of the dorsal portion of the SLIL is still attached. In Geissler grade IV injuries, there is a complete tear of the SLIL. The arthroscope can be freely translated between the radiocarpal space and midcarpal space ( Fig. 6.4 A–C).
Dynamic wrist arthroscopy can also be performed, which is akin to motion studies under live fluoroscopy. With the scope in the MCU portal, the wrist is taken out of traction and passively moved into flexion, extension, radial deviation, and ulnar deviation while observing the relative motion of the scaphoid and lunate. This can be repeated with active wrist motion when combined with wide-awake wrist arthroscopy under local anesthesia. The SL joint is then observed while performing a Watson shift test. Without traction one gets a better sense of the normal and pathological wrist kinematics. For example, I have found that the midcarpal step-off disappears when the traction is released, which can account for some error in distinguishing between a grade II and grade III lesion 
( ).
In general, arthroscopic treatment is most applicable for relatively acute Geissler grade I to III lesions whereas a grade IV lesion requires open treatment ( ). Consideration should be given to open treatment with chronic grade III lesions. The current available literature consists mostly of level IV, retrospective case series, with short follow-ups and small numbers; therefore only limited conclusions can be made. Most of these studies predate the Geissler grading system; hence a comparison of the different treatment methods is difficult. Arthroscopic treatment options include the following, either in isolation or in combination: ligament debridement, ligament thermal shrinkage, transarticular K-wire fixation, capsuloplasty, and the RASL procedure. Complete repairable tears are best managed with open techniques.
Arthroscopic debridement
Indications
Arthroscopic debridement alone is indicated for acute or chronic partial, but stable, tears of the volar or membranous portion of the ligament in a patient with mechanical symptoms ( Fig. 6.5 ). These patients usually have focal reproducible mechanical wrist pain over the dorsal SL joint worsened by activity, and normal radiographs. It is common to treat these patients conservatively for several months with splints and activity modification. Arthroscopy in these patients typically reveals a stable Geissler grade I or II injury pattern with slight midcarpal incongruity and joint widening. The patient’s symptoms are due to tears in the substance of the ligament that, though not destabilizing, create mechanical impingement during wrist motion causing focal dorsal wrist pain and occasionally leading to a synovitis and dorsal capsular thickening. Debridement of these SLIL flap tears and partial synovectomy can ameliorate the symptoms.
Contraindications to debridement alone
An absolute contraindication is a complete repairable tear or a more advanced Geissler III or IV lesion that was underappreciated in the preoperative evaluation. Static instability patterns with preexistent arthritis require additional treatment.
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