Central Slip Injuries

What is the central slip?
The central slip is the name given to the tendon responsible for pulling the middle knuckle joint straight. The anatomy of the extensor mechanism in the finger is complex and intricate. The main extensor tendon enters the finger over the back of the first knuckle, where it is easy to see through the skin. The tendon then merges with a flat sheet of tissue called the extensor hood. This extensor hood wraps around the back of the joint and merges with deep ligaments within the finger.

Two other tendons attach into the extensor hood. The first is from muscles which sits between the bones in the hand, called the interosseous muscles. The second is a small muscle called the lumbrical muscle. This muscle arises from the flexor tendon (the tendon responsible for pulling the finger into a fist), and inserts into the extensor hood. The tendons of the interosseous muscle and the lumbrical merge to form one of two lateral bands which run up either side of the back of the finger and these lateral bands merge just before the end joint of the finger to pull this joint straight.

How is the central slip injured?
The central slip can injured with either a CLOSED mechanism (no skin wound) or with an OPEN mechanism (an associated skin wound). The central slip can also be injured as a result of a FRACTURE DISLOCATION of the knuckle joint.

CLOSED INJURIES – The central slip is usually injured when a flexion or bending force is applied to the straightened finger, such as when a football hits the finger, or a fall when the back of the straightened finger hits ground. It can also occur as part of a twisting mechanism in association with ligament damage.

OPEN INJURIES – These commonly occur as part of a deep cut over the second knuckle joint of the finger, and are commonly associated with penetration of the joint.

What are the symptoms of a central slip injury?
Usually, the proximal interphalangeal joint or second knuckle joint becomes swollen and tender. The finger will not pull straight easily.

What is the natural history of the injury if untreated?
This can be a problematic injury if left untreated. When the central slip ruptures, the pull of the extensor tendon will retract is slightly from its usual insertion. When this happens, the lateral bands tighten up, and start to take a short-cut to the end of the finger, pulling the PIP joint into flexion. With time, the joint loses its mobility and becomes stuck in a bent position.

What is the treatment of a central slip injury?
CLOSED INJURIES – These injuries are treated with a splint, which is worn for around 6-10 weeks after the injury. The splint holds the finger straight, and allows the tendon to heal back down to bone with scar tissue. During this period of splinting, the joints on either side of the injury should be kept as mobile as possible.

When the splint is removed, the finger is usually fairly stiff, and often takes a while to loosen up and move again.

OPEN INJURIES – Where the central slip has been cut, then surgical repair is usually recommended. Once the tendon has been repaired, the finger is splinted in the same way as a closed injury above.

What happens if the injury is picked up late?
People tend to think the finger is just staved after a central slip injury, and often do not seek advice for some weeks after the injury. Dealing with these injuries weeks or months down the line can be very tricky. Generally, if the injury is less than 8 weeks old, splinting is recommended, but usually needs around 3 months to heal. Injuries older than this may not respond to splinting, and often the position of the finger with its consequent function impairment is best accepted. Surgery may be offered there is a major functional impairment.

What are the complications of this injury?
– FLEXION CONTRACTURE is the name given when the finger becomes stiff in a bent position. Unfortunately, this is relatively common following treatment for a central slip injury. Usually it is mild and does not interfere with function, but occasionally it is severe, and surgery may be recommended in this situation.

– LOSS OF MOVEMENT, particularly loss of the ability to pull the finger fully into a fist may be seen. This usually responds to physiotherapy.

© Fife Hand Service 2021

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