Lag screw fixation

The distal phalanx is divided into three anatomical parts: most proximally, the metaphysis, 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

1. The clinical history of the trauma
2. Deformity, pain and swelling located in the dorsal aspect of the DIP joint
3. Inability actively and fully to extend the DIP joint
4. X-rays

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.

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.

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.

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.

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 onto the distal phalanx, partly counterbalanced by the intact oblique retinacular ligaments and the collateral ligaments.

A similar clinical picture is presented by bony avulsion of the extensor mechanism at its insertion. The dorsal avulsion fracture is of variable size.

Swan-neck deformity
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.

Nonoperative treatment can produce excellent results, with near-normal range of motion, due to bone remodelling, even in the presence of persisting fragment displacement.
Nonoperative treatment is based on immobilization of the DIP joint in extension, leaving the PIP joint free.

Duration of immobilization
The DIP joint should be immobilized in extension for 8 weeks. It must be impressed on the patient that splintage should be uninterrupted during this period.
It must be kept in mind that the vascularity of this area is precarious, even in healthy patients, and healing is slow. Any shorter period of immobilization risks re-rupture. Joint stiffness is unusual after these injuries.

Result
Frequently, patients may complain of a dorsal prominence, and of loss of the last 10-20 degrees of extension of the DIP joint. Dorsal tenderness rarely persists for more than a month or so after removal of the splint.

Using a dorsal splint has the advantage of leaving the patient with the ability to pinch while the digit is immobilized.
However, proponents of palmar splintage argue that the palmar aspect is better cushioned than the dorsal and thereby can tolerate the splint better.

The advantage of a custom thermoplastic splint is that it is better adapted to the shape of the finger, and easier to change.

Instruct the patient to keep the finger in extension by pinching it with the thumb when the splint is taken off for cleaning.
Flexing the finger may delay the healing process.

Hyperextension of the DIP joint
Do not attempt to immobilize the joint in hyperextension. This can compromise the precarious vascularization of the skin of the dorsal aspect of the joint and might provoke ischaemia and possible necrosis.

Immobilization of the PIP joint
Do not immobilize the PIP joint as this may result in flexion contraction of the joint.

Inexperienced handling of this area may harm the germinative matrix of the nail and cause permanent deformity. Consider that nonoperative treatment is almost always a viable alternative in these fractures, often with comparable results. Operative treatment should only be attempted by experienced hand surgeons, in selected cases.

Absolute indications for surgical intervention are:

• Open fractures
• Palmar subluxation of the DIP joint

It is wise always to use magnifying loupes in these surgical procedures.

For this procedure a dorsal approach to the DIP joint is normally used.

Flex the DIP joint. In order to gain better visualization of the fracture, use a syringe to clean out blood clots with a jet of water.
Often the degree of comminution is not apparent from the x-rays, and can only be determined under direct vision.
Use a dental pick to carefully free interposed tissues, and to remove blood clots and other debris.

Extend the DIP joint. Put manual pressure on the palmar side of the distal phalanx.
Now complete the reduction with help of a dental pick.

The maximal permitted diameter of the screw is one third of the diameter of the avulsed fragment.
Screw length needs to be adequate for the screw to penetrate the opposite cortex.
Most commonly, a 1.0 mm screw is used. 1.3 mm screws can also be used for large fragments.

Be very careful when drilling. When not drilling gently, comminution of the fragments may be caused.

Drill a gliding hole
Maintaining the reduction with a drill guide, drill a gliding hole using a 1.0 mm drill bit for a 1.0 mm screw.

Now use a corresponding drill bit to drill a thread hole into the opposite fragment, penetrating the far (trans) cortex.

Use a depth gauge to measure for accurate screw length.

When measuring for screw length in oblique drill holes, the measurement for the far cortex may be different from the measurement for the near cortex. This problem increases with the degree of obliquity.
Always measure both cortices and use the longer measurement. Keep in mind that a too long, protruding screw puts the soft tissues at risk.

Insert a self-tapping lag screw and tighten it. The screw should penetrate the opposite cortex.

Check under image intensification. Reduction must be anatomical.

Be careful not to overtighten the screw as this may result in comminution of the fragment.

In very large fragments, a second screw can be added to improve stability.

If the collateral ligament and the volar plate are ruptured, palmar subluxation usually persists.

In such a case, the DIP joint needs to be immobilized with a K-wire.

Aftertreatment depends on the size of the fragment, the quality of the bone, and the stability gained by the fixation.

The DIP joint is immobilized in extension in a palmar splint, leaving the PIP joint free.
PIP joint movement is encouraged immediately to avoid extensor tendon adhesion.
A Coban bandage to reduce swelling is recommended.

If the fixation is strong enough, the patient is encouraged to take off the splint 2-3 times daily, and to commence with gentle active exercises.

After 3 weeks, the splint is removed, and unrestricted active flexion and extension is permitted.
Passive motion is only permitted after 4 weeks.

For ambulant patients, put the arm in a sling and elevate to heart level.
Instruct the patient to lift the hand regularly overhead, in order to mobilize the shoulder and elbow joints.
Mobilization of the PIP and MCP joints are encouraged from the very beginning.

The screw may need to be removed in cases of soft-tissue irritation.