Finger and Thumb Injury Imaging

Early attention to finger and thumb trauma such as sprains, open wounds, lacerations, nerve injuries, fractures, and dislocations is essential to prevent long-term sequelae. Traumatic hand and finger injuries account for approximately 20% of all emergency department visits. Common injuries are caused by activities such as sports and work-related events. ^[1]  

Ulnar collateral ligament (UCL)  injuries of thumb were first described in gamekeepers (ie, gamekeeper’s thumb), who sustained the injury by repetitive strangulation of small game such as birds and rabbits with the thumb and index finger. Hence, this was originally referred to as gamekeeper's thumb. ^{[2, 3]}  Posteroanterior, lateral, and possibly oblique radiographs of the are indicated in cases of suspected gamekeeper's thumb to identify avulsion fractures at the base of the proximal phalanx.

A Stener lesion occurs when the thumb is forcefully abducted and the distal attachment of the UCL at the metacarpophalangeal joint is traumatically avulsed from its insertion into the base of the proximal phalanx of the thumb. Plain radiographs fail to provide input into Stener lesions. In Stener lesions, there is significant retraction of the proximal UCL, so that the retracted UCL is seen superficial to the aponeurosis and the UCL is no longer seen in continuity. Both MRI and US imaging allow direct visualization of these lesions. The torn UCL is displaced, is folded on itself, and moves proximal to the adductor aponeurosis, producing the appearance of a "yo-yo on a string" on coronal images. If the injury is over a week old, MR imaging is usually preferred. ^{[4, 5, 6, 7, 8]}

The role of a radiologist is vital in providing a rapid and accurate diagnosis in guiding the attending physician. When conventional radiographic images are negative for fractures, stress radiographs are used. However, the results of stress radiographs are often difficult to interpret, and there is always the underlying concern that stress maneuvers may exacerbate the injury.

(See the radiographic images below.)

Preferred examination

Radiography is the initial imaging modality of choice for finger and thumb injuries.  Radiographs can reveal linear lucencies of various orientations and locations, avulsions, separation and angulation of fractures, dislocations, and the presence of radiopaque foreign bodies. ^[1]

Both magnetic resonance imaging and ultrasonography (US) are capable of not only visualizing the ulnar collateral ligament (UCL) directly but also classifying the tears into surgical and nonsurgical injuries (see the images below). Ultrasonography is a quick and noninvasive examination that can confirm the integrity of the UCL. Other pathologies affecting the metacarophalangeal (MCP) joint, such as tenosynovitis and tendon tears, can also be diagnosed by ultrasonography. Ultrasonography is also a useful technique in the follow-up of UCL injuries. ^{[9, 10, 11, 4, 12, 13, 14, 15]}

CT scanning is not routinely indicated but may be desirable in assessing joint alignment in dislocations (see the images below). CT and MRI are noninvasive techniques that can be used in the diagnosis of arteriovenous malformations. Plain radiographs, CT, and ultrasonography may be utilized in locating foreign bodies in fingers.

Radionuclide scanning is useful in stress fractures that may initially be negative on plain radiographs, although MRI can perform a similar function. Angiography may be useful and essential if embolization is planned for posttraumatic arteriovenous malformations and aneurysms. Avulsion fractures are usually assessed by plain radiographs, but because avulsion fractures may be seen in up to 12% of UCL injuries, plain radiographs cannot provide accurate assessment of ligamentous injury.

Conventional angiography has largely been replaced by Doppler ultrasound, magnetic resonance angiography, and computed tomography angiography, but angiography is required when embolization of an aneurysm or arteriovenous malformation is planned. Angiography remains the gold standard in the investigation of vascular abnormalities. Angiographic findings may be suboptimal in the presence of metal used in internal fixation of bone fractures.

Imaging Guidelines

The American College of Radiology (ACR) guidelines for acute hand and wrist trauma include the following recommendations ^[16] :

• Radiographs are usually appropriate for the initial imaging of adults with acute blunt or penetrating trauma to the hand or wrist.
• Repeat radiographs in 10-14 days, MRI without IV contrast, or CT without IV contrast is usually appropriate as the next imaging study for adults with acute hand or wrist trauma when initial radiographs are negative or equivocal.
• MRI of the hand without IV contrast or US of the hand is usually appropriate as the next imaging study for adults with suspected hand tendon or ligament trauma and when radiographs show acute hand fracture.
• MRI of the hand without IV contrast or US of the hand is usually appropriate as the next imaging study when initial radiographs show metacarpophalangeal, proximal interphalangeal, or distal interphalangeal joint malalignment in the absence of fracture.
• US or CT of the area of interest without IV contrast is usually appropriate as the next imaging study for adults with suspected penetrating trauma with a foreign body in soft tissues of the hand or wrist and when initial radiographs are negative.

Conventional radiographs are the mainstay of thumb and finger injuries (see the images below). They are simple, inexpensive, and noninvasive and provide rapid assessment of fractures and dislocations and exclude the presence of radiopaque foreign bodies. Injuries are simple to interpret, and rapid clinical decisions can be made.

Fracture and dislocations are quite obvious on plain radiographs and can be subdivided into crush fractures, linear coronal, oblique, and vertical fractures. Fractures may be angulated and compound. Avulsion fractures are also simple to diagnose and are located at sites of ligamentous attachments. Dislocations are usually assessed by the degree of displacement of apposing bones at the joint involved.

Fractures of the heads of the middle and proximal phalanges may be missed on plain radiographs, unless oblique views are obtained. Stress fractures may not become apparent until several days after the injury. Plain radiographs do not provide sufficient information on soft tissues and ligamentous injuries for clinical decision making.

Evaluation and treatment of basal thumb joint problems require a radiographic examination, along with a careful history and physical examination. The "true" anteroposterior (AP) and stress views provide useful information that supplements the standard plain radiographs. Additional maneuvers, such as local injection and other imaging modalities, are also sometimes employed. ^[17]

Computed tomography is not as elaborate in providing information about soft tissue injuries, ligamentous tears, and bone bruises as MRI, but CT appears to be more useful in helping determine the severity of dislocations. CT may miss ligamentous injuries and bone bruises. ^[18]

Rotational dislocation at the fracture site is a complication of long finger bone fractures of the metacarpals and phalanges. Differences in torsion angles exceeding 3º are strongly suspicious of a malrotation after fracture. These measurements may help in the planning of derotational osteotomy and in assessing the results of therapy. ^[19]

According to Clavero et al, an optimal MRI technique to provide important information for diagnosis and evaluation of soft-tissue trauma of the fingers should include proper positioning, dedicated surface coils, and specific protocols for the suspected abnormalities. Familiarity with the fine anatomy of the normal finger is crucial for identifying pathologic entities. ^{[12, 13, 20, 21, 22, 23, 24, 25]}

Connell et al, in a study of MRI appearance of supporting ligaments of the thumb carpometacarpal joint in patients following joint injury, found that the anterior oblique ligament was the most commonly injured ligament, usually on the metacarpal side. The dorsal radial ligament was occasionally avulsed or partially torn from the trapezoid. According to the authors, following chronic injury, MR imaging can evaluate ligamentous laxity, ganglion cyst formation, or osteoarthritis. Accurate evaluation of ligament injury may identify patients who would benefit from surgery. ^[10]

Spaeth HJ et al evaluated the use of MRI in ulnar collateral ligament (UCL) injury in cadavers and found that MR imaging depicted UCL displacement in all 10 specimens with displaced tears. Nondisplaced tears were diagnosed in 4 control specimens. Although MR imaging was only 67% specific for all tears, it was 100% sensitive and 94% specific for depicting UCL displacement and, therefore, was considered useful for evaluating gamekeeper thumb (see the image below). ^[26]  Romano et al described a number UCL injuries not previously described and therefore illustrated an existence of a subclass of UCL abnormalities. ^[27]

(See the images below.)

MRI is a powerful method for evaluating acute and chronic lesions of the stabilizing articular elements (volar plate and collateral ligaments) of the fingers and thumbs, particularly in the frequently affected proximal interphalangeal and metacarpophalangeal joints. As in other body regions, MRI is also useful for depicting traumatic conditions of the extensor and flexor tendons, including injuries to the pulley system. In general, normal ligaments and tendons have low signal intensity on MR images, whereas disruption manifests as increased signal intensity. ^[10]

Haramati et al examined 6 cadaveric thumbs to assess the utility of MRI in detecting surgically induced Stener lesions. ^[5] MRI examinations (2-dimensional short tau inversion recovery [STIR] and 3-dimensional gradient recalled acquisition in steady state [GRASS]) were performed identically on all specimens both before displacement (non-Stener) and after displacement (Stener lesion) of the UCL. They concluded that 2-D imaging is probably inadequate for the evaluation of Stener lesions. The most likely reason is that the STIR slice thickness of 3 mm limits resolution of small UCLs. The poor sensitivity and specificity of GRASS, as well as poor interobserver agreement, suggest that MRI may not be sufficiently accurate for Stener lesion evaluation.

^{[28, 29]} Ultrasonography (US), like MRI, is capable of not only visualizing the ulnar collateral ligament (UCL) directly but also classifying the tears into surgical and nonsurgical injuries. US is a quick, noninvasive examination that can confirm the integrity of the UCL. Other pathologies affecting the metacarpophalangeal (MCP) joint, such as tenosynovitis and tendon tears, can also be diagnosed by US. Ultrasonography is also a useful technique in the follow-up of UCL injuries. Ultrasound only deals with soft-tissue injuries and provides little if any useful information on fractures and dislocations. Doppler ultrasound techniques are noninvasive methods to assess posttraumatic aneurysms and arteriovenous malformations. ^{[11, 4, 14, 15, 30, 31, 6, 7]}

Ultrasonography has been found to be useful in the diagnosis of foreign-body–associated tenosynovitis. ^[32] It is a noninvasive technique that gives rapid diagnosis of ligamentous injuries that can be used on the playing field and provides clinical information allowing appropriate treatment by sports physicians.

The pattern of injury to the UCL varies from a strain, which is the mildest injury, to a full-thickness tear. Associated findings may be observed on US as well as MRI and include bone avulsions, joint effusion, volar plate injury, and the Stener lesion. Classification of the injury (see the image below) is important, because the management of these injuries is highly dependent on the type of injury. Since UCL is a capsular ligament, nondisplaced rupture is generally treated conservatively.

Ultrasound is able to depict the position of the torn UCL correctly in approximately 90% of cases. It is important to know about ultrasound-related pitfalls (eg, dislocation of the palmar joint capsule to the ulnar joint space, scalloping of the adductor aponeurosis due to the displaced UCL, and scar tissue or technical mistakes) because conservative treatment of a displaced UCL tear leads to instability. Therefore, the use of MRI is recommended whenever a nondisplaced UCL tear is suspected by US and conservative therapy is suggested. Splitting the diagnostic pathway between US and MRI and preferring conservative therapy in nondisplaced UCL tears should help keep costs down. ^{[33, 34]}

The hand is usually placed on the table, and a 12-13 MHz linear probe is used for examination following a liberal use of US gel. The probe is placed over the second finger and then slid onto the first MCP joint to obtain either axial or longitudinal images. The examination is relatively quick, typically taking less than a minute. Doppler studies can be added to assess the vascularity of the synovium and other vascular structures. The normal UCL appears hyperechoic on the ulnar side of the first MCP joint. Overlying the UCL is another, thinner hyperechoic band that represents the adductor pollicis aponeurosis.

The extensor pollicis longus tendon has a superficial location crossing over the extensor carpi radialis tendon at the dorsum of the wrist. Because of its superficial location, the extensor pollicis longus tendon is a highly vulnerable structure to trauma. A rupture in the extensor pollicis longus tendon produces loss of thumb extension. Detection of discontinuity of the tendon, retraction of the ruptured tendon, and limitation of tendon motion can be helpful for diagnosing a tendon rupture. Pseudomass formation may be more specific for diagnosing extensor pollicis longus tendon ruptures compared with other finger tendons. ^[35]

A cadaveric study demonstrated 99% accuracy in identifying full-thickness tendon lacerations and 92.7% accuracy in locating the tendon ends to within 1 cm using ultrasonography. ^[28]

Stress fracture is a common injury in healthy athletic patients and leads to local pain, often with no radiologic findings. Technetium-99m methylene diphosphonate (^99mTc-MDP) scintigraphy is a sensitive and a noninvasive technique in the diagnosis of such fractures. A negative scan 72 hours after an injury virtually excludes a fracture. Tc-99m-MDP scintigraphy is also useful in the differential diagnosis of a painful hand (eg, hypertrophic osteoarthropathy).

Radionuclide studies have been found to be useful in the advanced stages of repetitive strain injuries (RSIs). RSIs are characterized by lesions to the soft tissue after strainful work. An RSI requires rapid diagnosis and treatment, because it may lead to temporary or permanent functional impairment. Diagnosis is based primarily on clinical complaints, physical examination, and radiographic findings.

Technetium-99 pyrophosphate scanning has been found to be useful in the management of high-voltage electrical injuries. Occult muscle damage may not become apparent for 5-10 days. In one study, early scanning (within 3 days of injury) of 19 limbs in 15 patients demonstrated a sensitivity of 75% and a specificity of 100%. The predictive value of a positive test was 100%. However, compared to a control group of 17 patients treated without technetium scanning, scanning was not associated with reduced hospital stay or with a decrease in the number of surgical procedures. ^[36]