Dorsal Intercalated Segment Instability (DISI)

What Is Dorsal Intercalated Segment Instability (DISI)? Causes and Symptoms

Dorsal Intercalated Segment Instability (DISI) is a specific type of static carpal malalignment where the lunate tilts dorsally (backward) and the scaphoid flexes (forward), leading to a collapsed wrist posture.

Causes of DISI

DISI primarily occurs due to ligament damage, specifically the failure of the scapholunate ligament complex and its secondary stabilizers. More detailed causes include:

• Disruption of the scapholunate ligament.
• Scaphoid nonunion.
• A complete tear of the scapholunate interosseous ligament (SLIL) with additional tears of secondary ligament stabilizers. To cause this specific deformity, SLIL injuries necessitate the associated disruption of at least one extrinsic ligament that stabilises either the scaphoid or the lunate. The dorsal intercarpal (DIC) ligament plays an essential role in stabilizing the scaphoid and lunate and preventing DISI deformity in cases of scapholunate instability (SLI).
• Osseous morphology, particularly the lunate type, can influence the development of DISI. A Type II lunate, which has a medial facet articulation with the hamate, has been associated with a significantly lower incidence of DISI in cases of scapholunate dissociation. This added lunohamate articulation is theorized to act as a bony restraint against abnormal lunate extension. The incidence of a Type II lunate in the general population varies, reported between 27% and 73%.
• Radius fracture malunion can contribute to DISI. This is a form of carpal instability nondissociation (CIND), where the DISI deformity exists because the ligaments are stretched due to carpal malalignment from a dorsally angulated distal radius, even if initially intact. If a secondary injury occurs in such a wrist, the already stretched and weakened SLIL can tear completely, resulting in advanced scapholunate dissociation. Dorsally displaced distal radius malunion is specifically known to aggravate or produce a DISI deformity, leading to more pronounced incongruity of the radioscaphoid joint due to greater dorsal translation of the scaphoid and distal carpal rows.

Symptoms of DISI

Patients experiencing DISI commonly report a range of symptoms that reflect the instability and altered mechanics of the wrist:

• Pain during weight-bearing activities, such as push-ups or lifting.
• Weak grip strength.
• A visible "step-off" deformity on the dorsal (back) of the wrist.
• Clicking or clunking sensations when performing radial or ulnar deviation of the wrist.
• General wrist pain, which can be chronic and often has a post-traumatic origin. Patients frequently present with chronic, post-traumatic wrist pain (lasting more than 6 weeks) that may be spontaneous or induced by palpation over the scapholunate joint.

Anatomy of DISI: Ligaments and Biomechanics

Dorsal Intercalated Segment Instability (DISI) is a static carpal malalignment where the lunate tilts dorsally (backward) and the scaphoid flexes (forward), resulting in a collapsed wrist posture. This specific deformity involves a complex interplay of ligamentous damage and resulting biomechanical failure.

• Primary Cause: The primary cause of DISI is the disruption of the scapholunate ligament complex. More specifically, it occurs after a complete tear of the scapholunate interosseous ligament (SLIL) and often requires the associated disruption of secondary ligament stabilizers to produce the full deformity. This ligamentous failure allows the lunate to dissociate from the scaphoid, leading to its abnormal extension. DISI is commonly known to occur after either scaphoid nonunion or disruption of the scapholunate ligament.
• Secondary Stabilizers: To produce the DISI deformity, injuries to the SLIL typically necessitate the associated disruption of at least one extrinsic ligament that stabilizes either the scaphoid or the lunate. The dorsal intercarpal (DIC) ligament plays an essential role in stabilizing the scaphoid and lunate, and its disruption contributes to preventing DISI deformity in cases of scapholunate instability (SLI). Other investigators have noted that radiographic signs of scapholunate instability become visible on plain radiographs once these other extrinsic ligaments are sectioned. The scaphotrapezotrapezoid (STT) ligaments also contribute to preventing DISI.
• Key Biomechanical Failure: The characteristic "zigzag collapse" described in DISI is rooted in the uncoupling of the flexion moment of the scaphoid combined with the extension moment of the lunate. In wrists affected by scapholunate dissociation (SLD) with DISI deformity, the scaphoid is palmar-flexed and pronated, with its centroid translating dorsally and radially**.** Simultaneously, the lunate is extended and supinated. This abnormal rotation and translation can be observed dynamically during radioulnar deviation.

This leads to the radioscaphoid joint becoming incongruent, with the contact area shifting dorsoradially towards the dorsoradial rim of the distal radius. Despite this, congruity is relatively retained in the radiolunate, lunocapitate, and scaphotrapeziotrapezoid (STT) joints. The distal carpal row, including the capitate, trapezoid, and trapezium, also translates dorsally, mimicking the scaphoid's movement due to their intrinsic ligamentous connections. Over time, this abnormal translation and increased contact pressure on the dorsoradial rim of the radius can lead to degenerative arthritis, typically beginning in the radioscaphoid joint.

Why DISI Requires Specialized Rehab

Dorsal Intercalated Segment Instability is a static carpal malalignment that, if left unaddressed, frequently progresses to degenerative arthritis, commonly known as scapholunate advanced collapse (SLAC) wrist. This progression occurs due to abnormal loading and movement of the wrist, where the dorsoradial translation of the scaphoid can increase contact pressure on the dorsoradial rim of the radius, leading to initial degenerative changes in the radioscaphoid joint. Over time, this degenerative process can also affect the lunocapitate joint, while the radiolunate articulation tends to be preserved. Abnormal lunate and scaphoid positioning in DISI are contributing factors to the development of SLAC wrist.

DISI treatment aims to restore normal carpal alignment and function. These corrective goals include:

• Reducing dorsal lunate tilt: A key objective in treating DISI is to restore scapholunate rotational malalignment and reduce the dorsally translated distal carpal row back to its anatomical position. DISI is characterised by an abnormally extended position of the lunate in the sagittal plane. Reducing this dorsal tilt is critical for correct scapholunate alignment.
• Counteracting scaphoid flexion: In DISI, the scaphoid is palmar-flexed and pronated. Corrective manoeuvres aim to pull the scaphoid palmarly by the intrinsic ligaments, which facilitates correct scapholunate alignment. Restoring the anatomical position of the scaphoid is part of realigning the carpal axis.
• Improving dynamic stability: Although DISI is primarily a static instability visible on radiographs, there are also dynamic scapholunate injury patterns that may not be apparent on standard static imaging10. Advanced imaging techniques, such as four-dimensional CT (4DCT), are used to analyze the carpus during various wrist movements like radioulnar deviation, providing dynamic and quantitative analysis of lunate displacement and identifying instability during motion. Correcting the underlying ligamentous and osseous issues aims to restore stable and physiological carpal kinematics during dynamic activities.

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Physiotherapy Treatment for DISI

Phase 1: Acute Stabilization

The fundamental goal in managing DISI is to address the instability and malalignment. This initial phase would conceptually focus on reducing acute symptoms and preventing further progression of the deformity.

• Reducing Deformity: The primary aim is to restore scapholunate rotational malalignment and reduce the dorsally translated distal carpal row back to its anatomical position. DISI is characterized by an abnormally extended position of the lunate in the sagittal plane. This involves efforts to decrease the dorsal lunate tilt and counteract the scaphoid's palmar flexion, which are fundamental to preventing advanced collapse.

Phase 2: Dynamic Strengthening

• Scaphoid and Lunate Realignment: In wrists affected by scapholunate dissociation (SLD) with DISI deformity, the scaphoid is palmar-flexed and pronated, and the lunate is extended and supinated. Corrective manoeuvres aim to pull the scaphoid palmarly by the intrinsic ligaments, which facilitates correct scapholunate alignment. Restoring the anatomical position of the scaphoid is part of realigning the carpal axis. The restoration of a more physiological relationship between the scaphoid and lunate is key.
• Restoring Carpal Axis: For realignment of the carpal axis of an advanced SLD wrist, it is critical to restore scapholunate rotational malalignment and reduce the dorsally translated distal carpal row back to the anatomical position.

Phase 3: Advanced Sensorimotor Training

Improving Dynamic Stability: While DISI is primarily a static instability visible on radiographs, there are also dynamic scapholunate injury patterns that may not be apparent on standard static imaging. Advanced imaging techniques, such as four-dimensional CT (4DCT), are used to analyze the carpus during various wrist movements like radioulnar deviation, providing dynamic and quantitative analysis of lunate displacement and identifying instability during motion. Correcting the underlying ligamentous and osseous issues aims to restore stable and physiological carpal kinematics during dynamic activities. The goal is to regain functional stability throughout the wrist's range of motion.

Prognosis: Can DISI Be Reversed?

• Early-Stage Instability (Dynamic): While DISI itself is a static instability, it can arise from earlier dynamic scapholunate injury patterns. These dynamic instabilities may not be apparent on standard static radiographs but can sometimes be perceived with stress views or dynamic imaging techniques such as four-dimensional CT (4DCT). It might be possible to identify and treat patients with an early stage of scapholunate instability (SLI), particularly those with partial SLIL tears combined with extrinsic ligament injuries. However, explicitly detail specific physiotherapy techniques or timelines like "6-12 weeks of intensive rehab" for managing these early-stage or dynamic instabilities, or for reversing a developing DISI deformity do not have a specific guideline. The overarching principle for such interventions would be to correct the underlying issues before the static deformity fully sets in.
• Chronic DISI (Static): Once DISI has progressed to a static carpal malalignment, where the lunate is permanently dorsiflexed and the scaphoid palmar-flexed, surgical interventions are often considered for its correction and treatment. These procedures aim to restore the carpal axis to a normal anatomical position. Surgical options include:
  • SLIL reconstruction.
  • Arthroscopic debridement.
  • Partial arthrodesis.
  • Corrective osteotomy of the radius, particularly when associated with dorsal radius malunion.
  • Arthroscopic repair of the scapholunate ligament.
  • Arthroscopic dorsal capsuloplasty.
  • Arthroscopic-assisted combined dorsal and volar scapholunate ligament reconstruction with tendon graft.
• Critical Factor: Prevention of SLAC Arthritis: A critical reason for timely intervention in DISI is to prevent its progression to scapholunate advanced collapse (SLAC) arthritis. Wrist instabilities, including DISI, are conditions that may result in progressive degenerative arthritis due to abnormal loading or movement of the wrist. In DISI, the dorsoradial subluxation of the scaphoid proximal pole leads to incongruity of the radioscaphoid joint. This dorsoradial translation of the scaphoid may increase contact pressure on the dorsoradial rim of the radius, initiating degenerative changes, typically first in the radioscaphoid joint. This abnormal positioning of the scaphoid and lunate directly contributes to the development of SLAC wrist. Therefore, early and appropriate treatment of DISI aims to restore normal carpal kinematics and prevent this debilitating progression to arthritis.

FAQs About DISI

Q: Is DISI the same as scapholunate dissociation?

A: Dorsal Intercalated Segment Instability (DISI) is not the same as scapholunate dissociation (SLD), but rather a consequence or deformity that develops from SLD. SLD is the most frequent post-traumatic carpal instability dissociative (CID) type, involving a complete tear of the scapholunate interosseous ligament (SLIL) and often additional tears of secondary ligament stabilisers. This allows the lunate to dissociate from the scaphoid and tend to extend abnormally. Gradually, the DISI deformity develops, which is characterised by the flexion of the scaphoid and extension of the lunate. In DISI, the lunate is in an abnormally extended position in the sagittal plane. This malalignment results in the scaphoid being palmar-flexed and pronated, and the lunate being extended and supinated.

Q: Can DISI be misdiagnosed on imaging?

A: Yes, Dorsal Intercalated Segment Instability (DISI) can be mimicked or misdiagnosed on various imaging techniques, especially on sagittal Magnetic Resonance (MR) images and standard plain radiographs, even in individuals who are asymptomatic and have otherwise normal findings

Q: Does DISI always need surgery?

A: No, DISI does not always require surgery, especially in earlier stages or if it's considered a dynamic instability. However, surgical intervention is often considered for chronic or static deformities. A critical factor for timely intervention, whether non-surgical or surgical, is to prevent the progression of DISI to scapholunate advanced collapse (SLAC) arthritis.

Our DISI-Specific Rehab Program

At our clinic, we use:

• Hand Therapy Certification: Expertise in ligament and tendon rehab.
• Advanced Modalities: Ultrasound, laser therapy, and joint mobilization.
• Sport-Specific Protocols: For athletes, musicians, and manual workers.

Book Your Assessment Today:

• Phone: 905-669-1221
• Address: 398 Steeles Ave. W Unit 201, Thornhill, ON L4J 3X6
• Online Booking: **https://vaughan.juvonno.com/portal/publicbook.php**

Serving Vaughan, Thornhill, Richmond Hill, Markham, and North York. Direct billing available.

Key Takeaways:

• DISI is a structural collapse requiring early intervention.
• Rehab targets volar muscle strength and proprioception.
• Untreated DISI leads to arthritis—don’t ignore persistent wrist clicks!