The distal phalanx is divided into three anatomical parts: proximally, the metaphysis (base), followed by the diaphysis (“shaft”), and finally the ungual tuberosity (“tuft”). The base of the distal phalanx has a prominent dorsal crest at the insertion of the extensor tendon. The tendon is also adherent to the distal interphalangeal (DIP) joint capsule. On the palmar surface is the insertion of the flexor digitorum profundus tendon. This is also adherent to the volar plate. The flexor tendon inserts into the whole width of the base of the distal phalanx. The volar plate is very flexible, allowing hyperextension of the DIP joint and pulp-to-pulp pinch.
The vascularity of the extensor tendons is more precarious than that of the flexor tendons. This prolongs extensor tendon healing time.
Discontinuities of the extensor insertion are often referred to as “mallet injury” or “baseball finger”. They can be purely tendinous, or bony avulsion fractures. Diagnosis is based on
the clinical history of the trauma
deformity, pain and swelling located at the dorsal aspect of the DIP joint
inability actively and fully to extend the DIP joint
AP and true lateral x-rays of the DIP joint are necessary for the diagnosis of fracture avulsions. Low-energy radiographs, as used to visualize soft tissues, can be useful in identifying small flakes of bone.
The commonest cause of these injuries is forcible flexion of the actively extended DIP joint, as when stubbing a straight finger against resistance.
Axial compression injury
Occasionally, the injury is the result of an axial overload of the terminal segment of the finger, causing joint impaction and a dorsal marginal fracture, which is retracted by the pull of the extensor tendon.
Fracture subluxation of DIP joint
An obliquely orientated axial compression force sometimes results in a dorsal marginal fracture, involving approximately half the articular surface, and can disrupt the collateral ligaments. In such instances the pull of the flexor digitorum profundus results in palmar subluxation of the distal phalanx. This injury represents a strong indication for ORIF.
Spectrum of injury – partial disruption
These injuries may be complete disruptions or partial disruptions. The nature of the disruption may be either a tear of the tendon insertion without fracture, or a bony avulsion of the tendon from the dorsum of the base of the distal phalanx, the bony fragment being of variable size.
In incomplete tendon injuries the resulting extension lag is no greater than 30 degrees. The patient retains a partial ability actively to extend the DIP joint.
Spectrum of injury – complete disruption
In complete disruption of extensor mechanism, the patient is unable actively to extend the DIP joint. The flexor digitorum profundus exerts a flexion deforming force on the distal phalanx, partly counterbalanced by the intact oblique retinacular ligaments and the collateral ligaments.
Dorsal avulsion fracture
A similar clinical picture is presented by bony avulsion of the extensor mechanism at its insertion. The dorsal avulsion fracture is of variable size.
In some patients, elasticity of the ligaments and a lax PIP joint can result in swan-neck deformity, because after disruption of the extensor mechanism at the DIP joint, all extensor forces are concentrated on the PIP joint, via the middle extensor slip.