Sagittal Band Rupture (Traumatic Extensor Tendon Dislocation)
Matthew S. Torkington
David J. Warwick
The extensor mechanism to the fingers is a complicated arrangement of structures that contribute to finger extension. At the level of the metacarpophalangeal (MCP) joint, the extensor tendon is held in a central position by the sagittal bands. The sagittal band is part of the extensor retinacular system at the MCP joint and forms a close cylindrical tube around the metacarpal head.1
Sagittal band rupture resulting in dislocation of the extensor digitorum communis (EDC) was first described by Legouest; the condition is often referred to as Boxer’s knuckle after Gladden published his case report in 1957. While Gladden originally described an injury to the capsule or extensor tendon, his term of boxer’s knuckle has become associated with EDC instability over the MCP joint due to a sagittal band injury.2
The sagittal bands are separate from, and are superficial to, the collateral ligaments dorsally; as they sweep volarward they approach the accessory collateral ligament and blend with the volar plate.
The sagittal band has a thinner superficial layer and a deeper thicker layer containing a tunnel through which the extensor tendon runs.3
Section experiments4 show that
When the ulnar sagittal bands are completely divided, there is no extensor instability throughout flexion and extension of the digit.
If 50% of the proximal radial sagittal band was sectioned, then extensor subluxation occurred.
This was worse with increasing wrist and MCP joint flexion.
Disruption of the distal 50% of the radial sagittal band did not result in extensor instability.
With complete division of the ulnar sagittal band in cadaver specimens, EDC tends not to subluxate, although there is a greater tendency if the adjacent juncture tendinum is divided.5
Radial dislocation of the EDC is rarely reported. This is partly because of the normal tendency for EDC to slip ulnarward due to the ulnar inclination of the asymmetric metacarpal heads and soft tissue insertions, as well as the forward descent of the metacarpal heads with flexion.6
Mechanism of injury
The mechanism of injury can be closed or open
Closed rupture can occur with forced flexion of the digit and a flexed, ulnar-deviated wrist.
Another proposed mechanism in punch injuries is when a punch is landed on the relative narrow dorsal-distal edge of the index or long finger metacarpal rather than the relatively broad area of the dorsum of the flexed proximal phalanges.7
Atraumatic acute and chronic sagittal band rupture has been reported in the elderly; it is particularly prevalent in rheumatoid arthritis (RA) due to attenuation or even rupture secondary to the pathologic process of RA.8
The pathology in the atraumatic rupture is different from traumatic ruptures because only the superficial layer originating from its insertion point on the radial aspect is involved, whereas in traumatic ruptures, both the deep and superficial layers, originating a few millimeters radial to the extensor tendon, are affected.9
History and physical examination
The diagnosis is usually made through history and examination.
Presentation is usually with focal pain and swelling over the MCP joint, with the middle finger being most commonly affected.10
The patient may have a dynamic instability, with obvious extensor tendon dislocation during movement with clicking or crepitus.
Although the patient can hold the finger in the extended position, with flexion the tendon dislocates to the ulnar side. The tendon now runs palmar and ulnar to the axis of rotation and so the patient may then not be able to extend the tendon actively. This differentiates a sagittal band rupture from an extensor tendon rupture or radial nerve palsy.11
The possible differential diagnoses are shown in Table 33.1.
Plain radiographs are useful to rule out associated fracture or to exclude an osteophyte that snags the collateral ligament in flexion.
Ultrasound and Magnetic resonance imaging (MRI) are reliable. MRI can assess associated joint capsule injury; if present, the injury will respond more poorly to nonoperative management.12
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