Extensive hand injuries occur more frequently in otherwise healthy young males. Various methods of injury include hyperflexion direct blunt trauma and penetrating trauma. If left untreated, damage to the extensor mechanism on zone III and dislocation of the medial slip results in Boutonniere mild disability. This deformity is characterized by flexion of the proximal interphalangeal joint (PIP) and extension of the distal interphalangeal joint (DIP) due to volar subluxation of the lateral bands. It is an extensor tendon injury beyond zone III. Also called a buttonhole mild disability.
Clinically relevant anatomy
The Extensor Digitorum Communis (EDC) muscle on each digit splits into three cords or contracts. These slips are: the median muscle/slip underlying the medial phalanx; and two segments/slips that rejoin as a terminal tendon/slip to insert the distal phalanx. Active interphalangeal extension of the EDC muscles requires the support of two intrinsic muscle groups, the interossei and the lumbricals. These muscles also have attachments to the extensor hood and the lateral bands/slips. EDC tendon and all the hard active and the passive connections on and off the metacarpophalangeal joint are collectively known as the extensor mechanism. The surface of the extensor mechanism is formed by the roots of the EDC muscles (including the extensor indicis and extensor digiti minimi) and the median tendon/slip of the extensor hood and the external fibers/slips connecting the terminal tendon/slip. The triangular seams help stabilize the seams on the outside of the sleeve. The triangular ligament provides stability to the posterior ligament which prevents downward movement of the elbow during proximal interphalangeal flexion joints.
Central slip injuries in zone III are commonly caused by the following:
- direct blunt trauma
- penetrating trauma
Closed midline injuries usually result from hyperflexion and direct blunt trauma as commonly seen in athletes.
Boutonniere Deformity (BD) may develop secondary to trauma to the extensor mechanism rather than zone III or zone IV (including direct laceration to the extensor mechanism) secondary to rheumatoid arthritis (RA) and temperature conditions. Reports of congenital Boutonniere deformity are also have been documented. The pathogenesis of BD varies according to the etiology.
If the central slide of the digital dilation ruptures, the deformity is minimal as long as the dilated transverse fibers remain intact. If they are also torn, the PIP joint can become deformed. In this case, all extensor force will be transmitted to the distal phalanx through the intact A lateral band that produces hyperextension of the DIP joint. The PIP joint flexes and protrudes from the notch in the extensor mantle. The two transverse bands will now run on the volar side of the PIP joint and will exaggerate flexion. 
Signs and Symptoms
Signs of classic corsage deformity or interruption of central sliding appear immediately after a finger injury. Alternatively, they may occur 10 to 14 days after the initial injury , as the proximal interphalangeal extensor mantle and deltoid ligament may take time to heal Completely destroyed.  Typical signs are:
- Loss of extension of the PIP joint and hyperextension of the DIP joint 
- Inability to straighten fingers at intermediate joints (PIP joints) and to bend fingertips (DIP joints)
- Weak grip, inability to grasp and manipulate small objects with fingertips
- Pain and swelling at the top of the middle joint of the finger (proximal interphalangeal joint) 
Specific tests that are helpful in identifying injuries to the extensor mechanism include:
- Elson’s Test – Fix the PIP joint at 90° and ask the patient to lengthen the DIP joint. A loose DIP joint is a negative or normal finding despite the patient’s efforts to extend (loose lateral bands/slides prevent the DIP from extending when the PIP flexes if the central glide is intact). positive Discovery is a rigid DIP joint due to the added pull without resistance through the lateral strap/slide. 
- Modified Elson’s Test – The injured finger and its normal counterpart are placed in full proximal interphalangeal flexion, with the dorsal sides of each middle phalanx firmly against each other. Central slide if the injured finger has significantly better distal interphalangeal extension than the uninjured finger Likely to be injured. 
- Boyes test – prolongs PIP and asks to bend DIP; positive fails to actively bend DIP
Boyce’s test may only become positive at a late stage.
Management and rehabilitation of central slip injuries
Acute closed central slip injury – conservative management
Conservative management of acute closed central slip injuries is appropriate when the injury is less than 4 weeks old.  An injured finger can be splinted with the DIP MCP and the wrist mobilized for 3 to 6 weeks by splinting the injured finger together with the PIP. then 6 weeks Splint at night.  DIP joint flexion exercises may be prescribed if no lateral band/slip is involved. Flexion exercises of the DIP joint and extension of the PIP joint promote dorsal retraction of the lateral girdle from the volar subluxed position. This allows to tighten The deltoid ligament thus prevents the lateral band from moving from the volar side. It also prevents the development of corsage deformities. 
The PIP joint needs to be closely monitored for extensor lag. This sometimes happens when the splint is removed after 6 weeks.  If there is no extensor lag, the following exercises can be performed :
- Patient can start to come off the splint
- Begin Active Range of Motion Exercises for the PIP Joint
- Light functional use from 6 to 8 weeks
- If patient is doing well with range of motion exercises and pain, start therapeutic strengthening exercises at 8 weeks
If there is a prolonged PIP lag, the patient will need to continue to use the extended splint for two weeks and, if needed, an additional two weeks for an extended night splint. [twenty one]
Postoperative Rehabilitation for Central Slip Repair
Geoghegan et al (2018) recently performed a systematic review on the treatment of central slipped extensor tendon injuries. Different rehabilitation protocols are reported in the literature and can be broadly grouped :
- Isolated PIP flexion and extension after PIP arthrodesis
- Controlled early active short arc motion
- PIP joint immobilization followed by dynamic spring-coil finger splints
Evans et al. (1994)  reported significantly better functional outcomes in early short-arc exercise cohorts compared to long-term stationary cohorts.
Early Short Arc Motion Regime
The basis of the early short-arc movement mechanism is that adequate tendon deflection is necessary to prevent adhesions. A 30° arc of flexion at the PIP joint will achieve the desired tendon offset (3 -5 mm). With this protocol, patients are required to see weekly Monitor progress and monitor extensor lag. Modifications to sports splints are also required .
Patients who have undergone surgical repair of a central glide tendon injury require a different splint :
- Full Finger Palmar Stretch Splint – worn at all times except during exercise
- Exercise splints:
- Volar plate from distal metacarpophalangeal (MCP) joint to fingertips – allows 30° PIP joint flexion and 20° DIP flexion
- Short volar splint with the PIP in a neutral position but allowing the DIP to flex on top of the splint
These exercises are done on different practice cleats, usually made of thermoplastic. As the flexion angle increases, the splint is also reshaped. [twenty one]
- Week 1: 30° PIP and 20° DIP Flexion
- Week 2: 40° PIP and DIP flexion (if no extensor lag)
- Week 3: 50° PIP and DIP flexion
- Week 4: 70° – 80° PIP and DIP Flexion
- Week 5: full composite flexion
- Week 6 -8: light functional use allowed
- Week 8: Strengthening with Theraputty
- Do this exercise 3 times a day, 10 repetitions per exercise
- Scar management is critical for postoperative zone III tendon repair to prevent adhesions.
Some red flags to be aware of when managing patients with central sliding extensor repair are :
- Signs of infection
- PIP extensor lag
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