Fig. 16.1
Common PIN compression sites (“Copyright [2011] Mayo Foundation for Medical Education and Research”)
The Arcade of Frohse, which is the proximal margin of the supinator muscle, forms an opening of variable size for the PIN to pass through. The arcade is anchored laterally at the most lateral aspect of the lateral epicondyle. It then arches distally and reattaches at the medial border of the lateral epicondyle adjacent to the articular surface of the capitellum (Fig. 16.2). The lateral portion of the arcade is always fibrous while the medial portion is typically muscular. However, Spinner found that the medial portion could be fibrous in 30 % of patients [16]. Subsequent studies have shown that this anatomic variation may contribute to PIN compression [17, 18]. The origin of the ECRB muscle is a fascial structure that arises from the common extensor origin, and collateral ligament. It is continuous with the proximal aspect of the supinator muscle.
After exiting the radial tunnel, the PIN passes between the two head of the supinator as it winds around the proximal third of the radius. At the level of the bicipital tuberosity, there is a bare area between the insertions of the deep and superficial heads of the supinator [19]. In this location the nerve lies directly against the radius and is vulnerable to traumatic or iatrogenic injury. After exiting beneath the distal edge of the supinator, the PIN gives off superficial and deep motor branches [19, 21]. The PIN continues distally along the floor of the fourth extensor compartment and provides afferent fibers to the dorsal wrist capsule, and intercarpal joints. The superficial sensory branch of the radial nerve travels down the forearm deep the brachioradialis muscle. It then emerges between the brachioradialis and the ECRL, travels subcutaneously and supplies sensation to the dorsoradial aspect of the hand.
The branches of the radial nerve arise in a predictable pattern which can help localize a site of entrapment or injury. The first branch is the posterior brachial cutaneous nerve which originates in the axilla. As the nerve travels along the posterior aspect of the arm it supplies the triceps and anconeus muscles, then just before crossing the intermuscular septum the inferior lateral brachial cutaneous and posterior antebrachial cutaneous nerves originate. Proximal to the elbow the radial nerve proper supplies the brachioradialis and ECRL; although in some individuals the ECRL is innervated by branches of the PIN [22]. In 50 % of individuals the radial nerve also provides partial innervation to the brachialis along with the musculocutaneous nerve [22]. The branch to the ECRB may also arise in this region. In most individuals the ERCB is supplied by the PIN (45 %), but can arise from the radial nerve proper (24 %) or sensory branch (16 %) [22, 23]. The PIN innervates the supinator as it passes through this muscle, and supplies the remainder of the finger, thumb and wrist extensors upon exiting. The order of innervation from proximal to distal is the extensor digitorum communis (EDC), extensor digiti minimi (EDM), extensor carpi ulnaris (ECU) which are all supplied by the superficial branch, followed by the abductor pollicus longus (APL), extensor pollicus longus (EPL), extensor pollicus brevis (EPB) and extensor indicis proprius (EIP) which are supplied by the deep branch [19–21].
Compressive Neuropathies of the Posterior Interosseous Nerve
Although symptoms are entirely different, RTS and PIN compression syndrome are both compressive neuropathies of the posterior interosseus nerve. PIN compression syndrome results in motor weakness of the PIN innervated finger, thumb and wrist extensors, and is not associated with pain. In contrast, RTS is characterized by pain localized to the proximal lateral aspect of the forearm, and is not associated with motor deficits.
The potential causes of compression are similar for both disorders, and can be secondary to trauma, tumours and constricting anatomy. The most common aetiology for both disorders is compression by one of five anatomic structures: (1) Fibrous bands between the brachialis and brachioradialis, (2) the Recurrent Leash of Henry, which are the branches of the recurrent radial artery that cross over the radial tunnel to supply the extensor musculature (3) the tendinous origin of the ECRB, (4) the Arcade of Frohse, and (5) the supinator muscle. These are often remembered by the mnemonic FREAS. Tumors are more often encountered in PIN compression syndrome than RTS, and reported cases include ganglions, lipomas, periarticular synovitis, and arteriovenous malformations [24–28]. Compression might also result as a sequelae of trauma such as a fracture of the proximal radius or radial head dislocation [29]. Also, iatrogenic injury secondary to overly aggressive retraction or dissection during radial head surgery may cause PIN injury. Finally, microtrauma from repetitive pronosupination has been suggested. To this point, the literature contains a number of case reports describing palsies in occupations that require repetitive motion such as an orchestra conductor, violinist, and swimmer [30].
Clinical Pearl: Aetiology or Radial Tunnel/PIN Compression (“FREAS”)
Fibrous bands
Recurrent leash of Henry
ECRB origin
Arcade of Frohse
Supinator
Others – tumour, trauma
PIN Compression Syndrome
Clinical Evaluation
This can manifest as either a complete or incomplete paresis of the PIN innervated muscles, and it may be progressive over a period of days to weeks. It is a motor neuropathy only without a sensory component, and it not typically associated with pain. Patients will present with some combination of an inability to extend the metacarpal phalangeal joints of some or all fingers (EDC), extend the thumb IP (EPL) and radially abduct the thumb (APL). Wrist extension may be weakened due to ECU paralysis, and obligatory radial deviation with extension will occur if the ECRB is also affected. This is determined by the innervation of the ECRB which may be derived from the PIN in 45 % of individuals [23]. Forearm supination can be weakened but in order to detect this strength testing should be performed with the elbow extended to eliminate the biceps contribution.
The differential diagnosis for PIN compression includes extensor tendon ruptures such as Vaughn Jackson syndrome, which is an attritional rupture of the extensor tendons of the ring and small fingers caused by the caput ulnae syndrome seen in rheumatoid arthritis. Extensor tendon ruptures can be distinguished from PIN compression by examining for the tenodesis effect, which is lost in the setting of a rupture. A radial nerve injury proximal to the PIN can also produce a similar clinical picture, however sensation in the distribution of the superficial sensory branch will also be affected in that scenario. Other diagnoses to be considered are Parsonage-Turner (Brachial Plexus Neuritis), cervical radiculopathy and systemic disorders associated with neuropathy. These can often be identified by a careful and detailed physical examination and history.
When evaluating these patients plain radiographs of the elbow are obtained to examine proximal radius anatomy, sequelae of old trauma or occult radial head fractures and dislocations. MRI is recommended when clinical examination is suspicious for soft tissue causes of compression such as ganglion, bursa, or tumour. Electrodiagnostic studies can be used to confirm the diagnosis. Nerve conduction studies will show decreased conduction velocity within the PIN, but normal conduction along the superficial sensory branch. EMG demonstrates denervation changes in the PIN innervated muscles. Electrodiagnostic studies will also assist in differentiating the aetiology of the PIN injury from a brachial plexitis, proximal radial nerve lesion or cervical radiculopathy.
Non Operative Treatment
When an idiopathic or traumatic aetiology of PIN compression is identified, a trial of splint immobilisation may be attempted if the symptoms are not progressive and there is no underlying mass. The elbow should be splinted in a position of flexion and supination to relieve pressure on the nerve [32]. Dynamic splints with outriggers and rubber bands to assist in finger extension are useful in maintaining motion and preventing contractures. If muscle strength improves after a 6 week trial conservative treatment is continued. If there is no improvement, further evaluation, diagnostic studies and exam are performed. At this point, surgical release or tendon transfer surgery should be discussed.
Surgical Management
Multiple surgical approaches have been described for releasing the radial tunnel. Regardless of the approach used the goal is to visualise the nerve throughout its course, remove any masses, and release the five anatomic structures that may cause compression: (1) Fibrous bands, (2) Recurrent Leash of Henry, (3) ECRB, (4) Arcade of Frohse, (5) Supinator muscle.
Anterior Approach of Henry (Figs. 16.3, 16.4, 16.5, and 16.6)
Figs. 16.3, 16.4, 16.5, and 16.6
The anterior approach of Henry. The elbow is crossed in a zig zag fashion, and the incision is placed lateral to the brachialis and medial to brachioradialis. The radial nerve proper is identified between the brachialis and BR proximally and between the BR/ECRL distally. The radial nerve is traced distally releasing the four common compressive structures
The incision starts over the lateral aspect of the brachialis proximally, the elbow is crossed with either a zig zag or curvilinear course, and distally the incision continues along the medial border of the brachioradialis. The nerve is found in the interval between the brachialis and brachioradialis proximally, and between the brachioradialis and pronator teres distally. This approach has the advantage being extensile, but it may result in scarring particularly where the elbow is crossed. The lateral antebrachial cutaneous nerve can cross the incision site proximally and can potentially be injured.
Brachioradialis Split, BR/ECRL Interval, ECRL/ECRB Interval (Fig. 16.7)
Fig. 16.7
Surgical approach to the radial tunnel through the interval between the brachioradialis and extensor carpi radialis longus muscles achieves exposure to the proximal four sites of possible compression (“Copyright [2011] Mayo Foundation for Medical Education and Research”)
These three approaches provide a similar exposure and may be used interchangeably. They provide visualisation from the radiocapitellar joint to the distal one half of the supinator and there is very limited extensibility. They should not be used if impingement is suspected at the distal exit from the supinator. Of these the brachioradialis split was the first described. The incision is placed directly over the muscle beginning at the elbow crease and extending distally approximately 6 cm. The muscle is bluntly split, and the nerve is identified in a fat stripe beneath. The two interval based approaches (BR/ECRL, ECRL/ECRB) were described to avoid intramuscular dissection and the potential for hematoma formation. When utilising the BR/ECRL interval the incision is placed over the interval beginning at the elbow crease and extending distally 6 cm. It is easiest to identify the correct plain between the muscles distally and then trace back to the common extensor origin where it is less well defined. A similar tactic can be used for the ECRL/ECRB interval, but this incision is typically begun 4 cm distal to the lateral epicondyle. This approach may be useful for combined tennis elbow procedures.
Posterior Approach (Thompson) (Fig. 16.8)
Fig. 16.8
The posterior approach to the posterior interosseus nerve through the interval between the extensor digitorum communis and the extensor carpi radialis brevis muscles provides access to the entire supinator and can extend from the elbow to the wrist (“Copyright [2011] Mayo Foundation for Medical Education and Research”)
This approach uses the interval between the ECRB and EDC muscles. The incision is placed directly over this interval and extended towards the lateral epicondyle. It provides very good exposure to the distal portion of the supinator but is limited proximally and does not access the radiocapitellar joint.
Selecting an Approach
The anterior approach of Henry provides the broadest exposure and extensile capability. For this reason, it is recommended if mass excision is required or the site of compression is unknown. If surgical treatment of tennis elbow is being performed at the same setting the ECRL/ECRB interval approach provides convenient access for both purposes. Alternatively, an anterior approach may be performed in conjunction with a separate incision over the epicondyle for the tennis elbow release. Finally, surgical approaches can be combined. An anterior incision can be used proximally to identify the radial nerve between the brachialis and brachioradialis and a separate posterior approach can be used distally. By working between these two incisions complete exposure of the nerve can be obtained.
Post Operative Care
A soft dressing is applied post operatively and active range of motion is allowed as tolerated. Patients are encouraged to perform mobilisation. Lifting is restricted to less than 5 lb initially, but patients are allowed to perform all activities of daily living beginning at 2 weeks. Heavy labour and strenuous exercise is restricted until 6 weeks. If the patient fails to regain strength following nerve decompression, tendon transfers can be performed in the future. There are a number of potential surgical options when performing tendon transfers; however a discussion of these is beyond the scope of this chapter.
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