INTRODUCTION
Cubital tunnel syndrome (CuTS) is the second most common compressive neuropathy affecting the upper extremities27). There has been a significant increase in the number of surgical treatments for CuTS since 199533). Despite its prevalence, CuTS remains a condition that many neurosurgeons are less familiar with, often overshadowed by other neuropathies. This lack of familiarity is mainly due to the intricate and challenging anatomy of the ulnar nerve and its various entrapment sites. Regretfully, this under-recognition can lead to delays in diagnosis and treatment, prolonging patient discomfort and leading to less favorable outcomes.
In recent decades, the surgical management of CuTS has seen a significant rise, reflecting its clinical importance. However, ongoing debates about the most effective surgical approach highlight the complexities involved. The real challenge lies not only in achieving an accurate diagnosis but also in choosing the right intervention, which requires a deep understanding of the ulnar nerve's anatomy and the specific sites where compression can occur.
This review seeks to offer a thorough exploration of the anatomical nuances that are critical to understanding CuTS, evaluate the current diagnostic tools available, and examine the effectiveness of various surgical techniques. We aim to refine the clinical approach to CuTS, ultimately improving patient outcomes. We dedicate this work to the memory of our mentor, orthopedic surgeon, professor Keunwoo Kim, whose dedication to advancing the understanding of peripheral nerve disorders continues to inspire us and shape our work.
PATHOGENESIS
Repetitive trauma is the most common cause of CuTS. CuTS is prevalent among individuals who perform activities involving frequent elbow flexions, such as baseball players, telephone operators, and workers using vibrating tools. Diabetes mellitus also increases the risk of its development. It is also crucial to exclude progressive cubitus valgus deformities as a contributing factor. Other potential causes include nerve subluxation, osteoarthritis, rheumatoid arthritis, synovial cysts, anomalous muscles, trochlear hypoplasia, aneurysms, masses, and idiopathic entrapment28). CuTS may also occur as a complication following pectoral or supraclavicular flap surgeries31).
Ulnar nerve compression at the elbow, commonly known as CuTS, results from mechanical stressors such as compression, traction, or fraction as the nerve courses through the retrocondylar region. While traditionally associated with static sites of entrapment, recent investigations have underscored the significance of dynamic compression, which occurs during elbow movement, particularly flexion.
The ulnar nerve is susceptible to impingement at various anatomical landmarks, spanning proximally to distally from the elbow. These include the medial intermuscular septum, the arcade of Struthers, the medial epicondyle, Osborne’s ligament, the fascia of the flexor carpi ulnaris (FCU), and the flexor-pronator aponeurosis (Fig. 1)31). Additionally, an anomalous anconeus epitrochlearis muscle, present in approximately 20% of the population, has been identified as a potential source of compression in select cases of CuTS9). Although this anatomical variant is largely asymptomatic in most individuals10), its presence in CuTS patients is associated with more rapid and consistent symptomatic improvement following surgical decompression compared to patients without this anomaly11).
ANATOMY OF THE NERVE CONTROLLING THE MUSCLES OF FOREARM AND HAND
1. Ulnar Nerve
The ulnar nerve provides motor innervation to the forearm and hand as well as sensory innervation to the hand. Above the elbow, the ulnar nerve (Fig. 1A) originates from the C7, C8, and T1 nerve roots of the brachial plexus. It descends posteriorly through the arcade of Struthers (Fig. 1B), a deep fascial band located approximately 8 cm proximal to the medial epicondyle between the medial intermuscular septum (Fig. 1C) and the triceps. The nerve then lies more superficially along the ulnar groove and does not provide any significant sensory or motor branches above the elbow.
Between the elbow and wrist, the ulnar nerve passes through the cubital tunnel (Fig. 1D), the fibro-osseous canal between the medial epicondyle (Fig. 1E) and the olecranon (Fig. 1F). This tunnel is formed by the floor of the olecranon, elbow capsule, and posterior band of the medial collateral ligament. It consists of an aponeurotic band bridging the dual origin of the FCU (Fig. 1G), also known as Osborne's ligament26). In the forearm, the nerve travels between the humeral and ulnar heads of the FCU through the deep flexor aponeurosis, branching to the FCU and the medial half of the flexor digitorum profundus (FDP).
The palmar and dorsal branches of the ulnar nerve arise 5 to 10 cm above the wrist. The palmar branch supplies the skin over the hypothenar eminence, whereas the dorsal branch passes posteriorly to innervate the skin on the medial side of the dorsal hand and fingers. Dorsal digital nerves supply the ring and little fingers.
At the wrist, Guyon’s canal is formed by the floor of the flexor retinaculum and the origin of the hypothenar muscles, the medial wall of the pisiform, the lateral wall of the hook of hamate. Finally, the roof is composed of the volar carpal ligament, palmaris brevis muscle, and fibers from the palmar fascia. Within the canal, the ulnar nerve splits into superficial and deep branches28).
The superficial terminal branch supplies the palmaris brevis muscle and innervates the palm on the ulnar side and the fourth and fifth fingers. The deep terminal branch supplies the abductor and flexor muscles of the little finger, as well as the opponens digiti minimi (hypothenar muscle). In the palm, it gives off muscular branches to the third and fourth lumbricals, and interosseous muscles, ultimately terminating by supplying the adductor pollicis muscle25).
There are four potential sites of compression for the ulnar nerve:
(1) The arcade of Struthers and the medial intermuscular septum above the elbow;
(2) The cubital tunnel at the ulnar groove;
(3) Under the flexor aponeurosis, between the heads of the FCU in the forearm;
(4) Guyon's canal at the wrist
2. Posterior Interosseous Nerve (PIN)
At the elbow, the deep branch of the radial nerve transitions into the PIN as it traverses the two heads of the supinator muscle. The PIN primarily supplies motor fibers to several extensor muscles, including the extensor digitorum communis, extensor digiti minimi, extensor carpi ulnaris, abductor pollicis longus, extensor pollicis longus, extensor pollicis brevis, and extensor indicis proprius23).
3. Anterior Interosseous Nerve (AIN)
The AIN, the longest branch of the median nerve, runs along the interosseous membrane between the flexor pollicis longus and FDP. It supplies the flexor pollicis longus and the lateral part of the FDP, which are involved in flexion and opposition. Finally, it terminates under the pronator quadratus.
4. Medial Antecubital Cutaneous Nerve (MACN)
The MACN, derived from the medial branches of the brachial plexus (C8, T1), traverses the anterior aspect of the medial epicondyle, providing innervation to the skin around the elbow and forearm. Paresthesia resulting from damage to its branches is a well-documented complication of cubital tunnel surgery. Typically, two major cutaneous branches (ranging from one to four) are at risk: one crosses approximately 3.1 cm distal to the medial epicondyle, and the other crosses approximately 1.8 cm proximal to it20).
5. Analysis of Muscles Involved in Hand Movement
The median, radial, and ulnar nerves provide essential innervation to the upper extremities. The PIN primarily controls the extensor muscles of the hand. The AIN and ulnar nerves control the radial (lateral) and ulnar (medial) sides of the hand, in addition to the flexor muscles. The median nerve governs thumb abduction and opposition.
For an accurate differential diagnosis of neuropathy in the upper extremities, it is crucial to understand the functions of both the intrinsic and extrinsic muscles, which control interphalangeal and metacarpophalangeal movements. The FDP and flexor digitorum superficialis muscles are the main flexors of the interphalangeal joints (proximal and distal interphalangeal joints). The extrinsic extensors (extensor digitorum communis, extensor indicis proprius, and extensor digiti minimi proprius) are the principal extensors of the metacarpophalangeal joints. The intrinsic muscles (interosseous and lumbrical muscles) are essential for the extension of the interphalangeal joints and flexion of the metacarpophalangeal joints.
6. Clinical Manifestation of CuTS
Patients with ulnar nerve lesions at the elbow often present intermittent paresthesia in the fourth and fifth fingers, which worsens during elbow flexion23). Sensory loss in the dorsolateral hand without changes in the forearm is a classic localizing sign23). Early symptoms may be primarily motor-related and present as ill-defined loss of dexterity or strength3).
7. Physical Examination of CuTS
The elbow flexion test is positive, and Tinel’s sign is positive over the cubital tunnel28). Wartenberg’s sign is an early presenting feature, characterized by an abducted little finger due to weakness of the third palmar interosseous muscle26) (Fig. 2A). Wasting of the first dorsal interosseous muscle is also observed (Fig. 2B). The claw-hand deformity results from weakness of the interosseous and lumbrical muscles (Fig. 2C).
The Froment's sign is a reliable indicator of ulnar neuropathy (Fig. 3A). Owing to the weakness of the adductor pollicis, patients compensate by holding paper against the index finger with flexion of the interphalangeal joint rather than abduction of the thumb (Fig. 3A). This condition is characterized by hyperextension of the metacarpophalangeal joints and flexion of the interphalangeal joints of the ring and little fingers.
DIFFERENTIAL DIAGNOSIS THROUGH PHYSICAL EXAMINATION
1. PIN Syndrome
The ring and little fingers are initially affected, presenting with a pseudoulnar claw characterized by flexion at the metacarpophalangeal joints, owing to intact intrinsic muscles. As PIN syndrome progresses, the fingers can extend at the interphalangeal joints, but cannot fully extend at the metacarpophalangeal joints. The thumb cannot be extended to the plane of the metacarpals because of the paralysis of the extensor pollicis longus and brevis31).
Wrist extension results in radial deviation because the extensor carpi radialis longus and brevis usually branch off proximally. A wrist drop develops from a lesion proximal to a branch of the PIN.
2. AIN Syndrome
Patients typically present with pain and muscle weakness in the volar forearm, without sensory loss. Due to flexor muscle weakness, patients often experience difficulty forming an 'O' shape with the thumb and index finger (Fig. 3B).
4. Thoracic Outlet Syndrome
The medial components of the brachial plexus (lower trunk or C8 and T1) are the most frequently involved. A common symptom is paresthesia radiating from the inner aspect of the arm to the ring and little fingers. A positive Adson's test, in which symptoms are provoked by neck rotation away from the lesion with hyperextension, can aid in the diagnosis. Additionally, a positive percussion sign, hyperabduction maneuver, or the presence of an arterial bruit with abduction or extension may help to diagnose thoracic outlet syndrome.
DIAGNOSTIC METHODS
1. Nerve Conduction Study
CuTS is confirmed through nerve conduction studies11). Diagnostic criteria typically include an absolute motor nerve conduction velocity of less than 50 m/s from above to below the elbow, or a nerve conduction velocity at least 10 m/s slower in the above-to-below elbow segment compared to the elbow-to-wrist segment. Additionally, a reduction of more than 20% in the compound muscle action potential negative peak amplitude from below to above the elbow is indicative of CuTS3).
TREATMENT MODALITIES
1. Medical Treatment
Conservative treatment is effective in mild cases. This involves avoiding prolonged elbow flexion or resting the elbow on a hard surface. Using an elbow splint at night can also help to alleviate symptoms. A trial period of four to six weeks is recommended for these measures23). Steroid injections combined with splinting do not provide additional benefits29).
2. Surgical Treatment
Anterior Transposition of the Ulnar Nerve
The skin incision was made on the posteromedial side extending 8 cm proximal and 4 cm distally from the elbow. Careful undermining of the plane between the subcutaneous fat and fascia is performed to preserve the antebrachial cutaneous nerves. The ulnar nerve was identified as posterior to the medial intermuscular septum in the distal third of the arm.
The dissection and release of the ulnar nerve is commenced from the end of the cubital tunnel and proceed proximally. The medial intermuscular septum is cleared of muscular fibers posteriorly, to the level of the humerus while preserving the inferior ulnar collateral vessels that penetrates the intermuscular septum. The ulnar nerve is released into the cubital tunnel, extending distally from the medial epicondyle to the forearm (Fig. 1). The tendinous part of the humeral attachment of the FCU, the common fibrous aponeurosis of the FCU humeral head, and the flexor digitorum superficialis are then released to free the ulnar nerve in the proximal forearm.
If the ulnar nerve remains tethered after the anterior translocation, further untethering should be performed by releasing the arcade of Struthers or the distal forearm fascia. The nerve is then translocated and placed in the subcutaneous or the transmuscular plane (Fig. 5).
The primary distinction between transmuscular and submuscular transposition lies in the surgical approach. Transmuscular transposition involves the use of a Z-shaped incision to achieve muscle lengthening, whereas submuscular transposition requires an incision approximately 1 to 2 cm into the flexor-pronator mass21). The intramuscular transposition technique, described by Adson2) and Kleinman16), entails creating a groove within the flexor-pronator mass to accommodate the transposed ulnar nerve, followed by a mandatory three-week period of postoperative immobilization, which contributes to its disadvantages21).
Prior to wound closure, passive flexion and extension of the elbow are performed to confirm the complete relief of ulnar nerve compression. The wound is then closed with meticulous suturing of the fascial and dermal layers, ensuring secure stabilization of the ulnar nerve.
SIMPLE DECOMPRESSION
A 4 to 5 cm incision is made on the posteromedial side of the elbow, around the olecranon groove. The ulnar nerve is identified just proximal to the elbow and decompressed by opening the cubital tunnel retinaculum and tendinous portion of the FCU. Any occult lesions, such as ganglions, are removed without circumferential nerve mobilization. Further resections such as those of the medial intermuscular septum, are not performed.
PROGNOSIS
Recent randomized controlled studies and meta-analyses report clinical improvement rates of approximately 60% to 87% following surgical treatment for CuTS6,9,19). Prognostic factors such as age15,18), preoperative severity and duration of symptoms24), and nerve dysfunction on EMG10) have been reported. However, high-quality randomized controlled studies are needed to determine the best treatment and identify prognostic factors.
DISCUSSION
Surgical decompression is the most effective treatment for CuTS32). Wade et al.32) indicated that surgical intervention achieved an 87% success rate, with postoperative complications occurring in 3% of cases, a 2% incidence of revision surgeries, and a 3% recurrence rate. They advocated simple decompression as the most effective technique32). In contrast to the findings of Wade et al., a recent meta-analysis by Abourisha et al.1) concluded that there is no significant difference in outcomes among endoscopy, simple decompression, and anterior transposition, aligning with earlier research.
The optimal treatment for CuTS has been a topic of debate since Buzzard8) first recommended anterior transposition in 19225,19). Each surgical approach presents distinct advantages and limitations. Simple decompression offers the advantage of minimizing nerve manipulation, thus preserving blood supply. However, it may overlook compression at the proximal portion of the medial intermuscular septum or the distal segment of the cubital tunnel. Furthermore, nerve traction or subluxation may persist during elbow flexion, presenting a potential drawback22). On the other hand, anterior transposition involves substantial nerve manipulation, which can compromise blood supply. Furthermore, in cases requiring postoperative immobilization, recovery and return to normal daily activities are often delayed. The intramuscular and submuscular transposition are associated with a risk of nerve compression due to fibrotic scarring, while the subcutaneous transposition carries an elevated risk of nerve injury30). When considering factors such as operative time and the immediate return to normal activities, simple decompression or endoscopic approaches are considered more appropriate as first-line treatment options. Nevertheless, it is important to acknowledge that endoscopic decompression has been linked to a notably higher complication rate compared to simple decompression, and some studies report a high revision surgery rate of up to 19% for simple decompression1,17,32). These findings conclude that no definitive gold standard for CuTS surgery has yet been established.
While some surgeons advocate for minimally invasive techniques, we favor the anterior subcutaneous transposition approach. This technique not only facilitates decompression at any point of nerve entrapment around the elbow but also carries an acceptable risk of complications and effectively reduces ulnar nerve pressure during elbow flexion6,13). In the absence of definitive pathology within the cubital tunnel, we routinely perform a thorough inspection of the ulnar nerve at all common compression sites, ensuring wide exposure, followed by release and anterior transposition into the subcutaneous area using a soft tissue sling. Anterior transmuscular transposition is reserved for patients with an exceptionally thin subcutaneous fat layer.
In our experience, these critical considerations are pivotal to ensure optimal outcomes and minimize complications during CuTS surgery: First, to prevent the development of chronic pain, vigilant care must be taken during the incision and dissection to avoid damaging the two major cutaneous branches of the MACN, which typically emerge approximately 3.1 cm distal and 1.8 cm proximal to the medial epicondyle. Second, during simple decompression, meticulous intraoperative evaluation is critical to identify any potential sites of compression, not only within the cubital tunnel but also at proximal and distal segments. Performing elbow flexion and extension throughout the procedure is essential to ensure complete nerve decompression, thereby reducing the risk of revision surgery by confirming thorough nerve release. Third, it is essential to preserve key vascular structures associated with the ulnar nerve, such as the inferior ulnar collateral artery, the posterior ulnar recurrent artery and epineural vessels to prevent segmental ischemia of the nerve and ensure optimal surgical outcomes during anterior transposition14). For the last part, the use of a microscope is highly recommended, particularly in revision surgeries during the dissection of severely compressed lesions or the precise mobilization of the ulnar nerve motor branch within the distal cubital tunnel.
In summary, the key consideration for CuTS is not merely the choice of surgical technique. Comprehensive evaluation of nerve compression ensures not only the differential diagnosis from other conditions and also appropriate surgical treatment of optimal outcomes.
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