3T Magnetic Resonance Imaging of the Thumb-normal Appearances and Common Injuries
How to cite this article: Chakrabarty N, Daftary A, Lawande M. 3T Magnetic Resonance Imaging of the Thumb-normal Appearances and Common Injuries. Indian J Musculoskelet Radiol 2020;2(1):44-51.
Thumb is unique not only in its orientation and function compared to the other fingers but also in its bony and soft-tissue anatomy. It is important to understand methods to acquire appropriate image of the thumb along its axis, become familiar with the normal magnetic resonance imaging anatomy of the thumb and develop an understanding of the appearance of common injuries, all of which help in proper management. With proper scanning technique, it is possible to identify all the ligaments, tendons, and pulleys of the thumb. Checklist ensures that all the structures are systematically seen and injuries are consciously looked for in a case of thumb trauma.
In this pictorial review, we will discuss the basic tenets of optimal image acquisition, including positioning, sequences, and other technical parameters. We will demonstrate the appearance of the normal structures, common injury patterns to the carpometacarpal (CMC), metacarpophalangeal (MCP), and interphalangeal (IP) joints as well as the flexor and extensor tendons and demonstrate their appearance. Finally, we will discuss the relevance of these findings in relation to clinical management.
The checklist should include: Bones, ligaments, volar and dorsal plates, tendons, adductor aponeurosis, and pulleys so that all the structures are systematically evaluated. In this pictorial review, we will focus on the anatomy and injuries of the soft-tissue structures of the thumb.
MR SEQUENCES AND PLANNING
We have a 3T Magnetic Resonance Imaging (MRI) and use a dedicated hand coil for obtaining high resolution images of thumb. The thumb axis is taken as the trapeziometacarpal (TMC) joint in approximately 80 degrees pronation and flexion with respect to the other metacarpals.
[Figure 1a] shows the thumb axis along the TMC joint to the distal phalanx. We obtain coronal images parallel to the widest axial dimension of the proximal phalanx, followed by sagittal images perpendicular to the coronal image [Figures 1b-d]. Proton density fast spin echo (PD FSE) and T2-weighted images (with and without fat saturation), each in axial, coronal, and sagittal planes of the thumb are the sequences used with 2 mm slice thickness. We use 320 × 320 matrix with a field of view of 130 mm on sagittal and coronal planes and 80 mm in the axial plane on 3T MRI.
The following five ligaments are imperative for stabilizing the TMC joint: (a) The anterior oblique ligament (AOL) (both portions; superficial [AOLs] and deep [AOLd]), (b) posterior oblique ligament (POL), (c) ulnar collateral ligament (UCL), (d) dorsoradial ligament (DRL), and (e) intermetacarpal ligament (ICL). The DRL, POL, ICL, and AOL are best evaluated in the sagittal plane [Figures 2-5], whereas the UCL is best demonstrated in the coronal plane [Figure 6].
VOLAR AND DORSAL PLATES
Volar and dorsal plates [Figures 8a and b] are fibro- cartilaginous structures which surround the first MCP and IP joints and strengthen the joint capsule. Volar plates are more important than dorsal plates for providing joint stability and hence are of greater concern during injury.
TENDONS AND APONEUROSIS
The abductor pollicis brevis tendon overlies the radial collateral ligament and inserts at the radial base of the proximal phalanx of the thumb [Figures 7a-d]. Some adductor pollicis fibers insert on the ulnar sesamoid bone, while others form the adductor aponeurosis and continue distally superficial to the UCL and then insert on the proximal phalanx [Figures 7a-d].
Dorsally, the extensor pollicis brevis (EPB) tendon blends with the dorsal plate of the first MCP joint and inserts on the dorsal proximal phalanx base [Figures 9a and d]. The EPB tendon lies on the radial side of the extensor pollicis longus (EPL) tendon proximal to the first MCP joint. The EPL tendon continues distally and inserts onto the dorsal base of the distal phalanx [Figures 9 a-d].
COMMON INJURIES AND THEIR BIOMECHANICS ALONG WITH IMAGING FINDINGS AND IMPLICATION ON TREATMENT
Injury patterns to the thumb can be divided into those of the (a) CMC joint ligaments (commonly injured-anterior oblique and dorsoradial ligaments), MCP, and IP joint collateral ligaments, (b) flexor or extensor tendons, (c) pulleys, and (d) volar/palmar plates.
Biomechanical patterns include varus, valgus, hyperextension, strong flexion, and forced flexion during active extension and axial loading with flexion of thumb metacarpal base. Table 1 shows the structures which are injured on the basis of the biomechanics.
|Biomechanical patterns→Structures injured ↓||Varus force||Valgus force||Hyperextension||Strong flexion||Forced flexion during active extension||Axial loading with flexionof thumb metacarpal base|
|At the carpometacarpal joint level||Anterior oblique ligament
|At the metacarpophalangeal joint level||Radial collateral ligament||Ulnar collateral ligament||Volar plate
Flexor pollicis longus tendon
|Pulleys||Extensor pollicis brevis tendon
Extensor pollicis longus tendon
|At the interphalangeal joint level||Radial collateral ligament||Ulnar collateral ligament||Volar plate
Flexor pollicis longus tendon
|Pulleys||Extensor pollicis longus tendon|
UCL injury at the MCP joint of the thumb is very common and occurs due to valgus stress on an abducted thumb. A Stener lesion occurs when the UCL is completely torn from its proximal phalangeal base attachment, is retracted, and lies superficial and proximal to the adductor pollicis aponeurosis as seen in the coronal T2 fat saturated and coronal PD FSE images at the MCP joint [Figures 11a and b]. Non Stener lesion is when the torn UCL maintains its position deep to the adductor pollicis aponeurosis as seen in the coronal PD FSE image at the MCP joint [Figure 12]. Stener lesion requires operative management as the interposition of the adductor aponeurosis prevents healing whereas non Stener lesion is managed conservatively with immobilization in a short arm-thumb spica cast for 4 weeks.
Volar plate injuries occur due to hyperextension force [Figures 13a-c] and are avulsion fractures usually accompanied by phalangeal base fractures. Pulley injury of thumb is an uncommon injury and occurs during strong flexion.[6,7] Sagittal PD FSE and sagittal T2 fat saturated images show volar plate injury suggested by altered signal intensity along with proximal phalangeal base edema suggestive of fracture in the setting of trauma [Figures 13a and b]. When the sagittal image [Figure 13b] shows that the FPL tendon is not closely apposed to the underlying phalanx(bowstringing), then it suggests injury to the pulley which is confirmed on axial image [Figures 13c]. Volar plate injury is reported as per the Eaton classification based on the size and the degree of comminution of the intra articular fragments and the percentage of articular surface involved, as it helps in deciding the management.[8,9] Based on Eaton classification, stable closed reduction is performed for small intra-articular fragments involving <25–40% of the articular surface and with greater involvement of the articular surface, the treatment depends on the degree of comminution of the intra-articular fragment.[8,9]
Radial collateral ligament injury occurs due to forced adduction on a flexed MCP joint. Grades 1 and 2 injuries with maintained ligament continuity are usually managed conservatively while Grade 3 injuries with discontinuity may require surgical management. Coronal PD FSE image at the MCP joint [Figure 14] shows Grade 3 injury to the radial collateral ligament.
EPL and brevis tendon injuries occur due to forced flexion during active extension or penetrating trauma and lacerations. Extensor tendon injuries are divided into various zones. A zone wise description of the extensor tendon injury is useful in surgical planning. Flexor tendon injuries which occur due to hyperextension forces or penetrating injuries are also divided into various zones and managed as per the zone wise location of the injured tendon. Figure 15 depicts the various zones of extensor and flexor tendon injuries schematically. The zones of extensor and flexor tendon injuries are depicted on sagittal images. Figures 16a and b shows complete rupture of EPL tendon with the tendon retracted to the level of proximal phalanx in keeping with zone II injury.
Figure 17a and b shows completely ruptured FPL tendon with the tendon retracted to the level of MCP joint in keeping with zone T II injury.
The most common TMC joint ligament injury mechanism is axial loading with flexion of the thumb metacarpal base.
Bennett fracture, which is a non-comminuted intra- articular fracture of the first metacarpal base, is the most common injury of the thumb CMC joint and is usually associated with first metacarpal dorsal subluxation or dislocation. In Bennett fracture-dislocation injuries, the deep AOL attachment to the proximal ulnar metacarpal fragment remains intact, leading to an osseous fragment from the volar aspect of the metacarpal base [Figure 18], while the rest of the metacarpal is pulled radially by the APL. If there is no dislocation or any signs of clinical or radiological instability, then, conservative management with thumb spica cast for 4–6 weeks is recommended in case of acute post-traumatic painful joint. Figure 18 shows Bennett fracture with stretch injury of the anterior oblique (beak) ligament.
DRL injury is best depicted in the sagittal images of the TMC joint and this ligament is usually torn from its trapezoid attachment [Figure 19].
Appropriate high resolution imaging, an understanding of the normal and abnormal MRI appearance of the structures along with an understanding of the biomechanics and implications in the management of thumb injuries is important in clinically relevant interpretation of these scans.
First author, who is also the corresponding author, has presented this article as an e-poster and won the third prize in the National Musculoskeletal Society (MSS) conference held in Pune in August 2019.
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