Exam 4: Extensor Expansion (Module 8 Pt. 2)

Finger Extension: Which joint seems to move the most in the initial stage of finger extension from a full fist?

PIP initiates, DIP joint follows

Stages of Digit Extension

1. PIP joint initiates digit extension

2. DIP joint follows into extension as the PIP joint continues to extend

3. Intrinsic muscles help to bring PIP & DIP joints into full extension

4. Finally, MCP joints fully extend w/ the digit

Components of the Extensor Expansion: finger extension at (3) joints…

1. MCP extension: EDC tendon & sagittal band

2. PIP extension: Central slip & lateral bands

3. DIP extension: Terminal tendon (bands become 1) & triangular ligament

Extensor/Dorsal Hood

Triangular shaped web of CT structure that the EDC tendon & intrinsics (interossei & lumbricals) insert into

• Facilitates & completes finger extension

• Located between dorsum of MCP joint & dorsum of the proximal phalanx

MCP Joint: EDC Tendon & Sagittal Band

Sagittal bands: Originate from volar plate; wrap around MCP joint

• Function: centralize EDC tendon

PIP Joint: Central Slip & Lateral Bands transfer force from the EDC tendon & intrinsic muscles to…?

Extend the PIP & DIP joints

• After leaving MCP joint, the EDC tendon splits into the central slip & 2 lateral bands

PIP Joint: Central Slip Function

PIP extension

PIP Joint: Lateral Bands Function

DIP extension

• Transition between volar position (in flexion) to dorsal position (extension)

DIP Joint: Terminal Tendon

Lateral bands converge to form the terminal tendon @ dorsum of DIP joint

• Function: DIP extension

Digit Extension Mechanics

1. Central slip @ PIP joint initiates digit extension

2. Lateral bands dorsally migrate & extend DIP joint

3. Intrinsic muscles continue to extend both DIP & PIP joints through their insertions on the lateral bands

4. Sagittal bands extend the MCP joints as the EDC tendon migrates proximally

Lateral bands rest dorsally in extension (lateral view), and drops…

volar in flexion

Zones of extensor injuries are used to describe the area of…?

extensor injury (treatment & protocols differ)

Zone I/1 Extensor Injury “Mallet Finger”

Distal to or @ DIP joint

• MOI: direct trauma to fingertip, causing abrupt, forceful DIP joint flexion

• Soft mallet: terminal tendon tear

• Bony mallet: terminal tendon tear associated w/ avulsion fracture

Zone II/2 Extensor Injury

Middle phalanx

Zone III/3 Extensor Injury “Boutonniere”

PIP joint area

• MOI: Blunt trauma to PIP joint, causing abrupt, forceful PIP flexion

• Central slip tear

• Lateral bands stay volar: pull PIP into flexion & DIP joint hyperextension

• Wear static splint

Zone IV/4 Extensor Injury

Proximal phalanx

Zone V/5 Extensor Injury: Sagittal Band Injury

MCP joint area

• If injured/attenuated, sagittal band no longer effective @ keeping EDC tendon centralized on dorsum of MCP joint in MCP flexion

Mallet Finger Conservative Management (Zone 1)

24/7 DIP joint extension orthosis for 6-8 wks, then gradual splint weaning & gentle AROM

• If patient removes orthosis & starts moving fingertip, 6 wk wear time restarts!

Zone III/3 Extensor Injury “Swan Neck Deformity”

PIP joint area

• In people w/ joint hypermobility

• Can occur after PIP dorsal dislocation

• Volar plate insufficient @ limiting PIP joint hyperextension

• Lateral bands sublux dorsally & become ineffective @ extending DIP joint

• DIPJ flexion, PIPJ extension

Zone III/3 Extensor Injury “Swan Neck” - Oval-8 ring splints manage this deformity by…

• providing 3-point pressure system: prevents PIP (middle) joint from hyperextending

• By positioning the oval band on top of the finger, they restrict backward bending + allows flexion & maintaining DIP joint extension.

Zone V/5 Extensor Injury: Sagittal Band Injury “Boxer’s Knuckle” - What do you observe?

Present w/ pain, swelling, ecchymosis over MCP joint following trauma

• Visible or palpable "snapping" or subluxation of the extensor tendon into the intermetacarpal space during flexion

• Tendon typically deviates ulnarly during active extension

What structures are responsible for MCP extension?

EDC tendon & sagittal band

What structures are responsible for PIP extension?

Central slip & lateral bands

What structures are responsible for DIP extension?

Terminal tendon & triangular ligament