Residency · Residency · Orthopedic Surgery

Carpal Instability and Scaphoid Fractures

Overview

The carpus is an intricate linkage system where stability depends on both intrinsic (intercarpal) and extrinsic (radiocarpal, midcarpal) ligaments. Carpal instability results from disruption of these ligaments, leading to abnormal kinematics and progressive arthritis. Scaphoid fractures are the most common carpal fracture and can lead to avascular necrosis and carpal collapse if not properly managed.

Carpal Anatomy and Kinematics

Carpal Rows

The proximal row consists of the scaphoid, lunate, and triquetrum (with the pisiform as a sesamoid). It functions as an intercalated segment with no tendon insertions, acting as a mechanical link whose position is determined by forces transmitted from adjacent bones, making it inherently unstable. The distal row consists of the trapezium, trapezoid, capitate, and hamate, and moves as a relatively rigid unit with the hand.

Key Ligaments

The scapholunate ligament (SL) has a dorsal component that is the strongest and most important portion, preventing SL dissociation. The lunotriquetral ligament (LT) has a palmar component that is strongest. Extrinsic ligaments include the palmar radioscaphocapitate (RSC), long radiolunate, and short radiolunate ligaments, as well as the dorsal intercarpal (DIC) and dorsal radiocarpal (DRC) ligaments. The Space of Poirier is a weak area in the palmar capsule between the RSC and long radiolunate ligaments and represents the site of perilunate dislocation.

Normal Carpal Angles

The scapholunate (SL) angle is normally 30-60 degrees (mean 47 degrees). The capitolunate angle is 0 plus or minus 15 degrees. The radiolunate angle is 0 plus or minus 15 degrees.

<image>Lateral wrist radiograph showing normal carpal alignment with scapholunate and capitolunate angle measurements</image>

Scapholunate Dissociation

Mechanism

The mechanism is typically a fall on an outstretched hand with wrist extension, ulnar deviation, and intercarpal supination. This injury represents Mayfield stage I of the perilunate spectrum.

Classification (Garcia-Elias)

Predynamic SL dissociation involves a partially torn ligament with no radiographic findings but positive arthroscopic findings. Dynamic dissociation has a complete SL ligament tear but intact secondary stabilizers, with findings only on stress views. Static reducible dissociation shows an SL gap and DISI pattern visible on standard radiographs but reducible. Static irreducible dissociation has fixed malalignment with secondary stabilizer failure. The final stage is arthritis (SLAC wrist).

Diagnosis

On radiographs, the Terry-Thomas sign shows an SL gap greater than 3 mm on the PA view (compared to the contralateral side). The SL angle exceeds 70 degrees on the lateral view (DISI pattern). The cortical ring sign shows the scaphoid "en face" due to flexion. Stress views using a clenched fist AP can widen the SL gap. The Watson (scaphoid shift) test involves applying pressure to the scaphoid tubercle while moving the wrist from ulnar to radial deviation; a clunk with pain indicates SL instability. MRI has limited sensitivity for partial tears, though MR arthrography is superior. Arthroscopy is the gold standard, directly assessing the SL interval using the Geissler classification from Grade I (attenuation without instability) through Grade IV (arthroscope can be driven through the gap).

Treatment

Predynamic/Dynamic injuries may be treated with arthroscopic debridement plus thermal shrinkage (questionable durability) or dorsal capsulodesis. Static reducible injuries require SL ligament repair plus dorsal capsulodesis, or SL reconstruction with tendon graft (modified Brunelli, RASL, bone-ligament-bone), or the emerging 3-ligament tenodesis (3LT) technique. Static irreducible without arthritis injuries require scaphoid reduction and association procedures, scaphoid-lunate-capitate fusion, or four-corner fusion. SLAC wrist is treated based on staging.

SLAC (Scapholunate Advanced Collapse) Wrist

SLAC wrist follows a predictable pattern of arthritis after chronic SL dissociation. Stage I involves arthritis at the radial styloid-scaphoid articulation. Stage II progresses to radioscaphoid arthritis. Stage III extends to capitolunate arthritis with midcarpal involvement. Treatment for Stage I is radial styloidectomy plus SL stabilization. Stage II and III are treated with scaphoid excision plus four-corner fusion (or proximal row carpectomy for Stage III). End-stage disease requires total wrist arthrodesis.

Lunotriquetral Instability

Pathoanatomy

LT ligament tear leads to VISI (volar intercalated segment instability). The lunate flexes because the scaphoid pulls it palmar without the triquetral counterbalance, producing an SL angle less than 30 degrees and a capitolunate angle greater than 15 degrees.

Diagnosis

Clinical tests include LT ballottement (Reagan test) to shuck the lunate against the triquetrum, the LT shear test, and the midcarpal shift test. MR arthrography or arthroscopy provides confirmation.

Treatment

Acute injuries are managed with LT ligament repair plus temporary LT pinning. Chronic injuries without arthritis may require LT fusion (which has a high nonunion rate) or ligament reconstruction. Injuries with arthritis require ulnar column procedures or limited carpal fusion.

Perilunate Injuries (Mayfield Classification)

Greater Arc vs Lesser Arc

Lesser arc injuries are purely ligamentous, progressing through the SL, capitolunate, and LT intervals. Greater arc injuries involve fractures through the scaphoid (trans-scaphoid perilunate), capitate, triquetrum, or radial styloid.

Mayfield Stages of Progressive Perilunate Instability

Stage I is SL dissociation. Stage II is capitolunate dislocation. Stage III is LT disruption producing a perilunate dislocation where the lunate remains in the lunate fossa and the carpus dislocates dorsally. Stage IV is lunate dislocation where the lunate rotates palmarly into the carpal tunnel, producing the "spilled teacup" sign on the lateral radiograph.

Diagnosis

These injuries are often missed on initial evaluation (30-50% at first presentation). The lateral radiograph is the key view, showing disruption of Gilula arcs and loss of colinear radius-lunate-capitate alignment. The PA radiograph shows disruption of the three smooth carpal arcs (Gilula lines). CT characterizes fractures and MRI assesses ligaments.

Treatment

This is a surgical emergency that should ideally be addressed within 1-2 weeks. Closed reduction is followed by open repair through combined dorsal and volar approaches. The dorsal approach addresses SL and LT ligament repair or reconstruction and fracture fixation. The volar approach provides carpal tunnel release (the median nerve is at risk from lunate compression) and capsular repair. Temporary K-wire fixation across the SL and LT intervals is maintained for 8-12 weeks. Delayed diagnosis beyond 6 weeks makes reconstruction more difficult with worse outcomes and may require salvage procedures.

<image>Lateral wrist radiograph showing lunate dislocation (Mayfield stage IV) with "spilled teacup" appearance</image>

Scaphoid Fractures

Epidemiology

Scaphoid fractures are the most common carpal fracture, representing 60-70% of all carpal fractures. They typically affect young adults after a fall on an outstretched hand. Waist fractures are most common (65-80%), followed by proximal pole (15-20%) and distal pole/tubercle (5-10%).

Blood Supply

The dorsal scaphoid branch of the radial artery enters the dorsal ridge at the scaphoid waist and supplies 70-80% of the scaphoid in a retrograde fashion. The proximal pole has limited blood supply and therefore the highest risk of avascular necrosis.

Diagnosis

Clinical findings include anatomic snuffbox tenderness, scaphoid tubercle tenderness, and pain with axial loading of the thumb. Initial radiographs may be negative in up to 20% of cases. When clinical suspicion is high with negative radiographs, MRI is the gold standard (greater than 95% sensitivity) and should be obtained within 24-48 hours ideally. CT is an alternative if MRI is unavailable. Repeat radiographs at 10-14 days are a traditional but delayed approach. The scaphoid series includes PA, lateral, 45-degree pronation oblique, and PA in ulnar deviation views.

Classification

The Herbert classification divides scaphoid fractures into Type A (stable: A1 tubercle fracture, A2 incomplete waist fracture), Type B (unstable: B1 distal oblique, B2 complete waist fracture, B3 proximal pole, B4 trans-scaphoid perilunate fracture-dislocation, B5 comminuted), Type C (delayed union), and Type D (nonunion: D1 fibrous, D2 sclerotic).

Treatment

Nonoperative

Nonoperative treatment is indicated for nondisplaced, stable waist fractures (Herbert A2, stable B2). A thumb spica cast is applied; while the choice of long arm versus short arm for the initial period is debated, traditional management uses long arm for the initial 6 weeks followed by short arm, though recent evidence suggests short arm thumb spica may be equivalent. Duration is 8-12 weeks or until CT demonstrates union. Union rates are 85-95% for nondisplaced waist fractures.

Operative

Operative treatment is indicated for displaced fractures (greater than 1 mm step-off or gap, greater than 15 degrees of angulation/humpback), proximal pole fractures, associated perilunate injuries, delayed presentation or nonunion, and athletes requesting faster return to activity. Percutaneous screw fixation is used for nondisplaced or minimally displaced waist fractures via volar or dorsal approach, using a headless compression screw (Acutrak, Herbert), with union rates exceeding 95% and faster return to activity than casting. Open reduction and internal fixation is used for displaced fractures requiring reduction, with a volar approach for waist/distal fractures and dorsal approach for proximal pole fractures, using a headless compression screw with or without bone graft.

Scaphoid Nonunion

Risk factors include proximal pole location, displacement, delayed diagnosis, smoking, and inadequate immobilization. The natural history is progressive carpal collapse (SNAC wrist -- scaphoid nonunion advanced collapse), following a pattern similar to SLAC wrist. Treatment without AVN involves ORIF with volar wedge bone graft (iliac crest or distal radius) plus headless compression screw (Fisk-Fernandez graft). Treatment with AVN requires vascularized bone graft (1,2-ICSRA pedicled graft from the distal radius or medial femoral condyle free flap). Established SNAC wrist requires salvage procedures (scaphoid excision plus four-corner fusion or proximal row carpectomy).

Scaphoid AVN

AVN is most common with proximal pole fractures. MRI shows decreased signal on T1 (loss of marrow fat); increased signal on T2 is not reliable. Treatment involves vascularized bone grafting if the scaphoid is reconstructable.

<image>PA wrist radiograph showing scaphoid waist fracture and CT scan demonstrating fracture displacement</image>

Clinical Pearls

Any young patient with wrist pain after a fall should be treated as a scaphoid fracture until proven otherwise; early MRI is more cost-effective than repeat radiographs and follow-up visits. On the lateral radiograph, the radius-lunate-capitate line should always be traced, as disruption indicates carpal instability or perilunate injury. Perilunate dislocations are frequently missed, and the lateral radiograph is the key view; if the capitate is not seated in the lunate, something is wrong. Scaphoid nonunion leads to predictable SNAC wrist arthrosis, making prevention through timely diagnosis and appropriate treatment paramount. The "humpback deformity" of the scaphoid (flexion through the fracture site) must be corrected with volar wedge bone graft to restore normal scaphoid length and alignment. Smoking is the single most modifiable risk factor for scaphoid nonunion, and patients should be counseled aggressively.

References

• Mayfield JK, et al. Carpal dislocations: pathomechanics and progressive perilunar instability. J Hand Surg Am. 1980;5(3):226-241.
• Watson HK, Ballet FL. The SLAC wrist: scapholunate advanced collapse pattern of degenerative arthritis. J Hand Surg Am. 1984;9(3):358-365.
• Herbert TJ, Fisher WE. Management of the fractured scaphoid using a new bone screw. J Bone Joint Surg Br. 1984;66(1):114-123.
• Garcia-Elias M, et al. Treatment of scapholunate instability. J Hand Surg Am. 2006;31(1):125-134.
• Dias JJ, et al. Should acute scaphoid fractures be fixed? A randomized controlled trial. JBJS Am. 2005;87(10):2160-2168.
• Zaidemberg C, Siebert JW, Angrigiani C. A new vascularized bone graft for scaphoid nonunion. J Hand Surg Am. 1991;16(3):474-478.

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