The digital extensor mechanism at the level of the MCP joint consists of the extensor tendon, sagittal bands, and volar plate. The sagittal bands are part of a complex extensor retinacular system that includes the triangular ligament between the lateral bands, the transverse retinacular ligament, and the oblique retinacular ligament at the proximal interphalangeal (PIP) joint level (FIG 1A).

The sagittal bands are dynamic structures that envelop the extensor tendons, centering them over the MCP joint during flexion, preventing bowstringing during hyperextension, and controlling tendon excursion. The sagittal bands insert onto the volar plate overlying the MCP joint (FIG 1B).¹⁵

The sagittal bands are the primary stabilizers of the extensor digitorum tendons at the MCP joints, and their integrity is essential for normal extensor tendon function.¹²,¹⁵,¹⁷,²⁰

When the MCP joint is maintained in neutral extension, the sagittal bands are oriented perpendicular to the tendon.

The sagittal bands are anatomically and physiologically distinct from the deeper collateral ligaments.

The radial sagittal band is often thinner and longer than its ulnar counterpart.

The greatest tension on the sagittal bands occurs with wrist and MCP flexion and radioulnar deviation.

The lumbrical muscles function to flex the MCP joint and extend the interphalangeal (IP) joint through the lateral bands. They originate on the flexor digitorum profundus (FDP) tendon and traverse on the radial aspect of the digit inserting into the extensor expansion.

The intermetacarpal ligaments are stout ligaments that originate and insert on adjacent metacarpal necks. These ligaments pass dorsal to the lumbrical tendons and volar to the interosseous tendons.

The mechanism of sagittal band injury commonly involves a direct blow to a flexed MCP joint.

Injury may result indirectly from forced flexion or directly from shear forces across the sagittal band.

Other described mechanisms include forceful deviation of the digit against resistance, usually with the MCP joint extended.

In open injuries, the sagittal band is usually lacerated.

Extensor tendon subluxation typically occurs with at least 50% disruption of the proximal sagittal band.²⁰ The extensor tendon no longer remains centralized over the MCP joint through flexion but rather subluxates ulnarly.

It has been suggested that frequency of injury among the digits is related to the cross-sectional diameter of the sagittal band, the extent of distal attachment, and the length of the sagittal band.⁷,¹⁴,¹⁵ The long finger is most commonly injured.

It has been suggested that traumatic subluxation occurs when there is tearing of both the superficial and deep layers of the sagittal band enveloping the extensor tendon.⁸

When underlying inflammatory conditions are present, the sagittal bands become attenuated and atrophic, allowing for atraumatic subluxation of the extensor tendons into the troughs (usually ulnar) between metacarpal heads.

Symptoms from acute injuries typically resolve within 3 weeks with appropriate treatment. However, pain can persist for up to 9 months before fully dissipating.⁶

When sagittal band injuries associated with discomfort, swelling, and subluxation are neglected, patients will experience ongoing symptoms that may worsen over time. The extensor tendon may become fixed in the valley between the metacarpal heads, leading to loss of MCP extension and deviation of the digit. These patients will require surgical treatment for resolution of their symptoms.⁷,¹⁴,¹⁶

This chapter deals with traumatic subluxation. Treatment protocols for inflammatory subluxation differ and are beyond the scope of this chapter.

A critical aspect of treatment involves understanding the circumstances surrounding the injury. This information will help identify those at risk for infection in open injuries (eg, clean laceration, fight bite) or the possibility of underlying systemic disease contributing to closed injuries caused by low-energy trauma.

Shortly after injury, soft tissue swelling may obscure the alignment of the tendon over the MCP joint.

Initially after traumatic injury to the sagittal bands with subsequent extensor tendon instability, symptoms and signs include the following:

Chronic cases of tendon instability often exhibit pain during MCP joint flexion, such as during grip, along with localized tenderness and swelling over the injured sagittal band.¹⁶

MCP extension can be actively maintained when the joint is passively placed into extension; however, difficulty is usually encountered when attempts are made to extend the MCP joint from flexion or when flexing the MCP joint from full extension.

Methods for examining extensor tendon instability over the MCP joint include the following:

Perform tendon and ligament instability examination.

Ask the patient to flex the MCP joint and wrist. This position places the maximum amount of ulnar force on the extensor tendon at the MCP joint. This will help to determine the amount of instability.

Pain provocation test: With the distal and proximal IP joints extended and the MCP joint flexed, ask the patient to try to extend the MCP joint against resistance.

A standard radiographic series, including posteroanterior, lateral, and oblique views of the MCP joint.

A Brewerton view (anteroposterior [AP] view with dorsal surface of the fingers touching the cassette and the MCP joints flexed 45 degrees) or stress views may be needed to rule out collateral ligament avulsion injury.

Magnetic resonance imaging (MRI) has been used with success to identify patients with sagittal band injuries, especially when the physical examination is obscured by swelling and patient discomfort.¹³ MRI with the injured MCP joint flexed facilitates the diagnosis.

Dynamic ultrasound has been reported as a useful modality for diagnosis of extensor tendon subluxation when swelling obscures the physical examination.¹¹