Anterior Interosseous Nerve- The Benedictine and O.K. Sign

A patient with a complete anterior interosseous nerve injury or a high medial nerve injury should be asked to make a fist. The first and second digits will have difficulty in flexing, while the other digits will flex. The third digit will be weak, while the fourth and fifth digits are normal. This position of the hand is similar to the position taken during a hand blessing. The Benedictine sign is different from an “ulnar claw hand”. Ulnar claw hand refers to damage to the ULNAR nerve and is seen when attempting to extend all the digits (leaving the 4^th and 5^th digits flexed). The O.K. sign is used to check for paralysis of the anterior interosseous nerve due to entrapment or compression injury. A patient with paralysis of the anterior interosseous nerve will be unable to make the O.K. sign. This is due to weakness of the flexor pollicis longus and flexor digitorum profundus muscles. A typical pinch attitude is associated with anterior interosseous nerve injury.

The anterior interosseous nerve arises from the median nerve about 4-6cm distal to the elbow, which is about 1/3 of the way down the forearm. It exits from the anterolateral aspect of the median nerve and it runs between the radius and the ulna on the interosseous membrane between and below the muscles of the flexor digitorum profundus and the flexor pollicis longus.

The anterior interosseous nerve supplies the flexor digitorum profundus muscle for the index and long fingers. It also supplies the flexor pollicis longus and the pronator quadratus muscles. The flexor digitorum profundus muscle for the index and long fingers is supplied by the anterior interosseous nerve. The medial part of the FDP is supplied by the ulnar nerve (FDP has dual innervation). The anterior interosseous nerve passes dorsal to the pronator quadratus with the anterior interosseous artery and provides innervation to the volar wrist capsule. The terminal branch of the anterior interosseous nerve innervates the carpal joint capsule.

In patients with Martin-Gruber Connection, the median nerve, or anterior interosseous nerve to the ulnar nerve in the forearm may present with intrinsic muscle weakness. It may be differentiated also from Parsonage-Turner Synrome (acute brachial plexus neuritis) and patient may have pain in the affected extremity. In anterior interosseous nerve entrapment, the median nerve conduction study result will be normal, however the needle EMG of the anterior interosseous innervated muscles will be abnormal.

PIP Dorsal Fracture Dislocation

Proximal interphalangeal (PIP) dorsal fracture dislocations can be challenging in management. In PIP dorsal fracture dislocations, there is an involvement of the articular surfaces of the joint. These injuries are the most disabling PIP joint injuries. A reduction of the middle phalanx on the condyles of the proximal phalanx is the primary goal of treatment. The patient frequently presents with a chronic dorsal subluxation due to delay in seeking treatment or from failed treatment.

85% of motion for grasping objects occurs at the PIP joint. The PIP joint has the largest arc of motion (120 degrees) of the three joints in each digit.

There are three different types of injury:

1. Hyperextension
2. Impact Shear
3. Pylon

In cases of Hyperextension injuries, the size of the fragment can range from a small piece of bone, up to 30% of the joint surface with minimal comminution (usually stable).  In order to provide accurate treatment, the physician will want to test the stability of the fracture and apply a dorsal blocking splint. It is important to observe the lateral view for any subluxation.

Impaction/Shear injuries are produced by an axial load applied to a slightly extended or flexed PIP joint. The middle phalanx is driven over the head of the proximal phalanx with comminution and impaction of the base of the middle phalanx. These injuries are characterized by the loss of the volar plate tether. A 50% loss of the palmar of the middle phalanx base will make the PIP joint unstable. The extensor tendon and superficialis tendon will aggravate the dorsal subluxation. Splints will be inadequate if the injury is not stable. Treatment will include skeletal traction and early range of motion. An open reduction and internal fixation may be necessary if the fragment is large enough. A Palmar Plate Arthroplasty may be considered to advance the palmar plate into the defect. This procedure will support the palmar plate by filling the defect behind it with fracture chips, bone graft, or by a superficialis slip. Chronic impaction/shear cases are a difficult problem and can be treated with an arthrodesis or volar plate arthroplasty.

A pylon fracture results from an axial force that fractures the volar and dorsal articular surfaces with impaction on the central part. A pylon fracture is not a true fracture dislocation and will not have a good outcome. Dynamic external traction should be done in order to help in ligamentotaxis. Early range of motion is important for remodeling of the joint.

Classification of a PIP Dorsal Fracture Dislocation will depend on the degree of involvement of the articular surface.

• Type I (stable) <30%
• Type II (tenuous)= 30-50%
• Type III (unstable) > 50%

Treatment will consist of a Dorsal Extension Blocking Splint if the PIP can be reduced in less than 30% of flexion. If less than 40%, the joint involved and stable. For Type I and Type II fractures, decrease the flexion gradually every week. Type III fractures will require ORIF, volar plate arthroplasty, or hemi-hamate graft (if >40% joint involved and unstable).

With a Hemi-Hamate Graft, the damaged palmar lip of the middle phalanx with a size matched portion of the hamate bone obtained from its distal dorsal articular surface between the 4^th and 5^th metacarpals. It restores both articular congruity and osseous stability. Extension block pinning is probably a helpful technique. Treatment must provide stable reduction to allow for early mobilization.

Stability of the reduction depends on the size of the avulsed fragment and the amount of ligament remaining attached to the middle phalanx. If less than 40% of the articular segment is avulsed, some of the collateral ligament will be intact. This will keep the reduction stable. If more than 40% of the articular segment has avulsed, only very little or none of the ligament will remain attached to the base of the middle phalanx, rendering the reduction unstable.

Recognizing Subluxation

AP and lateral x-rays are necessary for diagnosis. An AP view will help to recognize impaction fractures.  Subluxation can be missed on x-rays. The physician will want to look for the characteristic “V” sign of diverging joint surfaces, which indicates injury and subluxation.

If a reduction of the avulsion fracture is achieved with less than a 30 degree bend, nonoperative treatment is a good choice. However, if it takes more than 30 degrees of flexion to reduce the fragment, this may be an indication for surgery.