Courtesy: Prof Greg Bain, FRACS, Flinders University, Adelaide, Australia

Scapholunate Instability: Concepts of Scaffolding and Stability

Based on educational concepts in wrist biomechanics and ligament stability

Introduction

Scapholunate instability is one of the most important causes of carpal instability and wrist dysfunction.

Modern understanding of wrist biomechanics has evolved significantly over the past 10–15 years with the use of:

4D CT scans for dynamic wrist kinematics
CT-based wrist models for biomechanical visualization and research

The concept of “scaffolding and stability” emphasizes that wrist stability depends on coordinated ligament complexes rather than isolated ligaments.

Concept of Ligament Complexes

Wrist stability is best understood as interaction between multiple ligament complexes.

Important stabilizing complexes include:

Dorsal scapholunate complex
Volar radiolunate complex
Scaphotrapeziotrapezoid (STT) joint complex

Different ligament groups stabilize different functional zones of the wrist.

Injury may involve:

A single ligament complex
Multiple stabilizing complexes simultaneously

Normal Wrist Kinematics

In a normal wrist:

The scaphoid and lunate move synchronously
Carpal motion is coordinated and adaptive
Load is shared efficiently across the carpus

This synchronized movement maintains:

Stability
Smooth motion
Functional load transmission

Pathomechanics of Scapholunate Instability

Basic Problem

Scapholunate instability occurs when coordinated motion between the scaphoid and lunate is lost.

This produces:

Dissociation of the proximal carpal row
Abnormal intercarpal motion
Mechanical instability

Typical Biomechanical Findings

Dorsal Structures

Dorsal scapholunate ligament complex becomes disrupted

Distal Carpal Row

STT complex often remains stable

Volar Structures

Radiolunate and distal radioulnar joint stabilizers are frequently preserved

Resulting Biomechanics

Two relatively stable carpal blocks act across an unstable interval, leading to:

Deforming forces
Abnormal motion
Progressive instability

Dorsal Ligament Complex

The dorsal ligament complex includes:

Scapholunate ligament
Dorsal intercarpal ligament (DIC)
Additional distal stabilizers such as the triquetroscaphoid ligament

This dorsal complex acts like a stabilizing belt across the wrist.

It functions together with the volar ligamentous structures to maintain carpal stability.

Role of the Triquetrum

The triquetrum acts as an important proprioceptive and sensory hub within the wrist.

It receives input from:

Extensor Carpi Ulnaris (ECU)
Flexor Carpi Ulnaris (FCU)
Pisiform-associated structures

Functions include:

Coordination of carpal motion
Proprioceptive feedback
Dynamic stabilization

Biomechanical Changes in Instability

Scaphoid Changes

The scaphoid tends to:

Flex
Sublux dorsally over the radius

Lunate Changes

The lunate typically:

Extends dorsally
Develops DISI deformity (Dorsal Intercalated Segment Instability)
Demonstrates reduced motion arc

Clinical Features

Patients may experience:

Clicking
Catching
Pain
Reduced wrist range of motion

Primary Pathology

The principal problem in scapholunate instability is:

Radioscaphoid instability

Important concept:

Scaphoid instability usually occurs before lunate instability

Advanced untreated instability may progress to:

Carpal collapse
Degenerative arthritis
Chronic wrist dysfunction

Capsular and Ligament Injury

Typical injury pattern includes:

Intrinsic ligament tear
Extrinsic capsular or ligamentous avulsion

Important healing structures include:

Capsule
Ligaments
Periosteum

Principles of Healing

Importance of Periosteum

Periosteum is considered one of the strongest healing tissues in these injuries.

Synovitis

Inflamed synovial tissue should be debrided because persistent synovitis interferes with healing.

Goal of Repair

The main objective is restoration of:

Ligament-to-bone attachment
Capsuloperiosteal continuity
Stable carpal mechanics

Surgical Techniques

1. Arthroscopic Capsular Plication

Example

Mathoulin technique

Indications

Most useful in:

Geissler grade 1–3 injuries

Advantages

Minimally invasive
Preserves soft tissue
Avoids complications of open surgery
Converts symptomatic instability into functional stability

2. Expanded Capsular Repairs

These repairs incorporate a larger portion of the dorsal capsule to improve:

Global carpal stability
Force distribution

Arthroscopic techniques are increasingly favored.

3. Window Technique

Small dorsal exposures allow:

Targeted repair
Suture anchor placement
Limited soft tissue disruption

Useful when full arthroscopy is not feasible.

4. Suture Anchor Repair

Technique

Anchors placed into the scaphoid and lunate
Capsuloligamentous tissues reattached anatomically

Important Principles

Prefer cortical fixation
Respect native ligament footprint
Utilize tension-band concepts

5. Docking Technique

Anchors are placed into both bones, and sutures are tightened to:

Reduce the scapholunate interval
Restore ligament tension

This improves stability by decreasing interbone separation.

Limitations of Tendon Graft Reconstruction

Tendon graft reconstructions have several disadvantages:

High complication rates
Persistent scapholunate diastasis
Inability to restore normal wrist kinematics
Tendon necrosis

Current evidence suggests tendon grafts do not reliably maintain long-term reduction.

Lessons from ACL Reconstruction

Ligaments possess:

Broad footprints
Multiple functional bundles

Joint motion also involves variable centers of rotation.

Therefore:

A single tendon graft cannot replicate native ligament biomechanics accurately

Problems with Carpal Bone Drilling

Carpal bones possess:

Thin cortical bone
Fragile cancellous architecture

Risks of transosseous drilling include:

Avascular necrosis
Bone fragmentation
Disruption of vascular supply

Important point:

Ligament attachments are superficial, usually less than 1 mm deep

Deep drilling is therefore considered biomechanically unfavorable.

Issues with Synthetic Materials

Synthetic sutures and polyethylene materials may produce:

Micromotion
Bone erosion
Synovitis
Osteolysis
Cartilage injury

Because of these issues, synthetic intra-articular constructs are less desirable.

Vascular Considerations

Blood supply to the carpal bones depends on:

Extraosseous circulation
Intraosseous circulation

Drilling may disrupt:

Arterial inflow
Venous outflow

This increases risk of avascular necrosis of:

Scaphoid
Lunate

Denervation

Denervation is generally not recommended during reconstructive procedures because it removes:

Proprioception
Neuromuscular feedback

It may be considered only in salvage situations such as:

Partial wrist fusion

Role of Wrist Models and 3D Printing

Three-dimensional wrist models can reproduce:

Anatomy
Ligament relationships
Carpal mechanics

Applications include:

Surgical training
Arthroscopy simulation
Fracture fixation practice

These models provide a safe and repeatable educational environment.

Clinical Decision-Making

Distal Radius Fracture with Scapholunate Injury

After distal radius fixation:

Stability should be assessed under fluoroscopy

Management depends on severity:

Mild instability may be observed
Significant instability in young or high-demand patients may require arthroscopy and repair

Treatment According to Severity

Grade 1–3 Injuries

Arthroscopic capsular repair

Advanced but Reducible Injuries

Window technique with anchor repair

Irreducible or Degenerative Cases

Salvage procedures include:

Partial wrist fusion
Proximal row carpectomy

Key Takeaways

Scapholunate instability is primarily a radioscaphoid instability problem.
The dorsal ligament complex is essential for wrist stability.
Tendon graft reconstructions have significant limitations.
Excessive transosseous drilling should be avoided.
Capsular-based and minimally invasive repairs are increasingly preferred.
Successful reconstruction requires respect for:
- Ligament footprint
- Bone biology
- Vascular preservation

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