Applied Anatomy and Biomechanics of the Carpal Ligaments

The proximal and distal interosseous ligaments, together with the volar and dorsal extrinsic ligaments, are directly involved in scapholunate stability.

During wrist flexion or extension, both rows of carpal bones flex or extend collectively but to differing degrees. The primary flexion and extension lines (where the joint is most mobile) cross at the scapholunate ligament. Distal carpal compression is the greatest at the capitate bone. As the pressure is transmitted to the proximal row, it tends to separate the scaphoid from the lunate.

Intrinsic Ligament: Scapholunate Interosseous Ligament

The SLIL joins the scaphoid and lunate together. Its fibers are asymmetrical. The ligament consists of three separate, macroscopically continuous parts, at least before age-related degeneration sets in ( Fig. 14.1a, b ):

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  Dorsal: acts as a dorsal ligament between the posterior horn of the lunate and the scaphoid

  
  
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  Volar: thinner, but still strong

  
  
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  Proximal (intermediate): little to no vascularity, thus cannot be repaired (similar to central part of triangular fibrocartilage complex [TFCC]); fibrocartilage between the articular surfaces of the scaphoid and lunate ( Fig. 14.2a, b )

The dorsal portion is the strongest because of its thick, fibrous nature. It is considered the main scapholunate joint stabilizer.

Extrinsic Ligaments

Some of the extrinsic wrist ligaments provide additional stabilization to the scapholunate joint:

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  The dorsal intercarpal (DIC) ligament seems to be the second most important stabilizer. The proximal portion of the DIC ligament is also called the dorsal scaphotriquetral ligament. Spanning the distal scaphoid and the triquetrum, it combines with the dorsal radiocarpal (DRC) ligament to form the “dorsal V” described by Senwald and Segmüller. It is unique in that it has multiple insertions onto the lunate and the scaphoid ( Fig. 14.3 ).1 This ligament complex, which restricts intracarpal supination and ulnar translation of the carpus, has a particularly high number of nerve endings. Together, this dorsal V and the dorsal portion of the SLIL form a network of dorsal ligament attachments.2 Although this network’s length varies, it is always under tension, whether the wrist is in flexion or in extension.3

  
  
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  In a recent cadaver study, the authors showed that this scapholunate confluence plays a major stabilizing role; we called this structure the dorsal capsulo scapholunate septum (DCSS).2 The DCSS consists of a thickening of the capsule itself that connects the dorsal capsule with the dorsal portion of the scapholunate ligament ( Fig. 14.4a, b ).

  
  
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  The volar radioscaphocapitate (RSC) ligament spans the radius and the capitate ( Fig. 14.5a–c ). It inserts deeply on the anterior side of the scaphoid’s waist (isthmus), restricts intracarpal pronation, and stops dorsal translation of the proximal pole. It makes up the radial branch of the distal palmar V.1

  
  
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  The scaphotrapezial ligament stabilizes the radiovolar side of the scaphotrapezial joint. It is extremely strong and rarely tears.

Stability of the scapholunate joint is ensured by a combination of structures that constitute a ligament complex. The peripheral ends of the flexor and the extensor carpi radialis and ulnaris tendons further stabilize this complex by surrounding the carpus.

Arthroscopic Testing of Predynamic Instability

Predynamic or occult instability is evidence of an incomplete tear that can be detected by arthroscopy. The procedure is performed on an outpatient basis under regional anesthesia. The patient lies supine. Upward traction (5–7 kg) is applied to the arm, and a support pad is placed against the upper arm. A 30° scope is used for radial or midcarpal arthroscopy. A blunt trocar is essential to avoid damaging the cartilage and is inserted into portals once they have been located with needles. The standard radiocarpal (3–4 and 6R) and ulnar (ulnar midcarpal [MCU]) or radial midcarpal (MCR) portals are typically sufficient to assess proximal row stability. A hook probe is a critical tool for the testing.

Geissler and Dautel and Merle4,5 separately proposed classification systems for arthroscopic predynamic scapholunate instability based on midcarpal testing with a probe. This method was modified and improved by a European Wrist Arthroscopy Society (EWAS) group led by Messina.6 The basic concept rests on determining if the scapholunate joint opens spontaneously or can be opened with a probe.