Metatarsal Fracture Imaging
- Author: Prabhakar Rajiah, MD, MBBS, FRCR; Chief Editor: Felix S Chew, MD, MBA, MEd more...
Fractures of the foot are common, and the metatarsals are among the bones most commonly fractured. Acute foot fractures of normal bones are usually caused by the dropping of heavy objects on the foot or by stress associated with abnormal repetitive trauma. In deficient bones, insufficiency fractures may result from normal stress.
The metatarsal bones and tarsal bones are connected by strong ligaments. Soft-tissue support for the joints in the plantar aspect of foot is better than that in the dorsal aspect.
On the anteroposterior view (see the image below), the lateral border of the first metatarsal should be aligned with the lateral border of the medial cuneiform. The medial border of the second metatarsal should be aligned with the medial border of the intermediate cuneiform bone.
On the oblique view (see the image below), the medial and lateral borders of the third metatarsal should be aligned with the medial and lateral borders of lateral cuneiform bone. The medial border of the fourth metatarsal should be aligned with the medial border of the cuboid bone. The fourth and fifth metatarsals are aligned with the cuboid bone, but the lateral part of the fifth metatarsal may project beyond the margin of the cuboid bone by up to 3 mm.
The distance between the base of the first and second metatarsals and the medial and intermediate cuneiform is more than the distance between other corresponding joints.
If a lateral image is obtained, a line drawn through the long axis of talus bone and the long axis of first metatarsal bone should be straight if there is no dislocation.
Many classifications apply to fracture of the fifth metatarsal.
In a simple classification for fractures of the proximal end of the fifth metatarsal, fractures are classified as (1) those of the tuberosity and (2) those of the proximal metatarsal within 1.5 cm of the tuberosity.
Acute fractures, Jones fractures, and stress fractures may be described as (1) early fractures, (2) delayed union fractures, or (3) nonunion fractures.
The Torg classification is used for fractures within 1.5 cm of the metatarsal tuberosity. Type I includes fractures with sharp margins and no widening, sclerosis, periosteal reaction, or cortical hypertrophy. Type II includes fractures with widening, periosteal reaction, sclerosis, or both. Type III fractures involve widening, periosteal reaction, complete sclerosis at the fracture line, or both.
The Stewart classification of fifth metatarsal fractures is as follows: type I, extra-articular fracture between the metatarsal base and diaphysis; type II, intra-articular fracture of the metatarsal base; type III, avulsion fracture of the base; type IV, comminuted fracture with intra-articular extension; and type V, partial avulsion of the metatarsal base with or without a fracture.
The zonal classification, reported by Dameron, Lawrence, and Botte, categorizes metatarsal fractures by the region affected: zone 1 corresponds to the tuberosity, zone 2 corresponds to Jones fractures, and zone 3 is the diaphysis.
Acute fractures may be transverse, oblique, or comminuted (as seen in the images below); they are easily recognized.
Jones and pseudo-Jones, or tennis, fractures
A Jones fracture (see the image below) is caused by inversion of the foot, which produces tension on the peroneus brevis tendon and on the lateral cord of the plantar aponeurosis.[2, 3, 4] In this type of fracture, significant displacement is absent. Jones fractures are more prone to nonunion.
A fracture of the metatarsal tuberosity (see the image below) is an avulsion fracture. This is also called a pseudo-Jones fracture or a tennis fracture. The mechanism of injury is forcible inversion of the foot in plantar flexion, which may occur when one steps on a curb or when one falls while climbing stairs. A direct blow to the tuberosity can cause a comminuted fracture.
Distal, or dancer's, fractures
Distal fractures, also called dancer's fractures, are caused by a rotational force resulting from axial loading with the foot in a plantigrade position.
The Lisfranc joints are the tarsometatarsal joints. A Lisfranc fracture-dislocation (see the image below) is caused by falling from a height, falling down stairs, or stepping off a curb.
Mechanisms of injury are (1) rotation around a fixed forefoot (eg, falling from a horse with the foot caught in the stirrup) or (2) longitudinal compression of the foot. In this second mechanism, the metatarsal head is fixed. The weight of the body is on the hindfoot against the base of the metatarsals during rotation; these forces result in a distal dorsal dislocation of the metatarsal.
Stress fractures are the result of abnormal stress on a normal bone (see the image below). Stress fractures of the foot are also called marcher's foot, because of the high incidence of occurrence in military recruits and in persons who engage in heavy exercise for prolonged periods. This fracture is also common in ballet dancers, gymnasts, and athletes. Other predisposing factors include surgery, stress fractures in adjacent bones, neuropathic disease, and rheumatoid arthritis.
When a normal step is initiated, maximum force is placed on the head of the second or the third metatarsal. With increased activity, microinfarction takes place in the bones, resulting in a fracture.
Stress fractures are difficult to recognize in the early stages (as demonstrated in the image below), when they are manifested only by a periosteal reaction. Bone scans are helpful in this situation. Recognition of fracture is crucial for guiding appropriate management and for preventing complications.
Insufficiency fractures are the result of normal stress on a weakened bone. Such injuries are seen in people with osteoporosis; postmenopausal women are commonly affected.
Radiography is the first and often the only investigation required for the diagnosis of fractures. Radiographs may be used to diagnose all acute fractures, dislocations, and established stress fractures. Small avulsions may be missed on radiographs. As previously mentioned, in the early stages of stress fracture, radiographs may be normal, or they may show only subtle periosteal reaction, which can be easily missed. Radiography cannot be used to assess soft-tissue and ligamentous disruption.[7, 8]
Bone scanning is more sensitive than plain radiography; it is indicated when a stress fracture or an acute fracture is suspected and radiographs are negative. Bone scanning is not a specific investigation. Although bone scanning is sensitive, some stress fractures may go undetected in the early stages of these injuries.
Although magnetic resonance imaging (MRI) is more sensitive than radiography and bone scanning, it is used only for the assessment of soft-tissue structures and ligamentous injuries. MRI is the most sensitive technique for imaging stress fractures of the foot; MRI may depict bone marrow edema even before increased uptake is seen on bone scans. Computed tomography (CT) scanning is useful for finding avulsion fractures and comminuted fractures and in assessing for intra-articular extension. Although CT and MRI are more sensitive than radiography, they are not cost-effective and are not indicated for the diagnosis of fractures.
Radiography is sensitive in the diagnosis of acute fractures. An acute fracture is seen as a linear lucency and a break in the cortical surface. Nondisplaced, impacted fractures may appear as an opaque line; such fractures may be confirmed on a different view.
Fractures may affect any metatarsal, but the fifth metatarsal is most commonly affected (see the images below). The fracture may be transverse, oblique, or comminuted. Longitudinal linear fractures are extremely rare.
The 2 most common fractures in the fifth metatarsal are a fracture at the tip of the tuberosity and a transverse fracture 1.5-2 cm from the tuberosity; the latter is called a Jones fracture. Small avulsions derived from the tip of the base of the fifth metatarsal may be seen only in the oblique projection of the ankle. (See the images below.)
The radiographic findings of a stress fracture depend on the bone involved and the stage of disease. Radiographs are normal in the early stages of the disease (see the first image below); stress fractures appear as well-defined linear lucency or fluffy periosteal reactions by 7-10 days. The periosteal reaction is variable and occasionally florid (as in the second image below).
The head of the second metatarsal and, occasionally, the third metatarsal are commonly affected. The first metatarsal is injured in 10% of metatarsal stress fractures; such fractures involve a different kind of reaction (the endosteal variety), with liner sclerosis. Periosteal reaction is not common in this type of injury. One third of such fractures heal with only an intramedullary callus.
The base of second metatarsals may be affected in ballet dancers. The proximal aspect of the shaft of the fourth and fifth metatarsals is affected; the pattern is that of a linear lucency, which is slow to heal. Fractures in the sesamoid bones are also seen in ballet dancers.
A Lisfranc fracture-dislocation (seen in the images below) is a dislocation of the tarsometatarsal joints. Two types of Lisfranc dislocation have been described: homolateral and divergent.
In the homolateral type, all of the metatarsals are dislocated to one side. Usually, the second to fifth metatarsals are dislocated, but occasionally, all of the metatarsals are affected. Lateral displacement is more common than medial displacement.
A divergent dislocation is medial displacement of the first metatarsal and lateral displacement of the second to fifth metatarsals. A variant of this type is an isolated medial dislocation of the first metatarsal.
Lisfranc dislocations are associated with fractures of the base of the second metatarsal, fractures of the cuboid bone, fractures of the shaft of the other metatarsal bones, dislocations of the middle and medial cuneonavicular joints, and fractures of the navicular bone. The base of the second metatarsal is relatively fixed compared with the other metatarsal bones. Therefore, it is involved in both types.
This dislocation is overlooked in as many as 20% of cases if the alignment is not carefully evaluated. Lisfranc dislocations should be suspected if a gap of more than 5 mm is present between the bases of first and second metatarsals or between the medial and middle cuneiforms.
Radiographs may not show stress fractures in the early stages of these injuries and in as many as 50% of patients. In addition, nondisplaced fractures may be difficult to visualize. Associated ligamentous injuries and soft-tissue changes are not depicted on radiographs.
CT scanning is not essential for diagnosing metatarsal fractures. If CT is planned, it should be performed in at least 2 planes: the coronal plane (perpendicular to the sole of foot) and the axial plane (parallel to the sole). With modern multisection scanners, images may be acquired in 1 plane and reconstructed in other planes with a fairly high degree of resolution.
Images are usually acquired with 5-mm sections, but 3- and 1.5-mm sections may also be acquired; the thinner sections have better resolution. Coronal images are acquired with the patient in the supine position, with knees flexed and feet flat on the table. The heels of both feet are superimposed in the lateral position. Longitudinal images are also acquired with the patient in the supine position, but with knees extended and feet perpendicular to the table.
CT scans often depict fractures that are not visualized on plain radiographs. They are useful in evaluating the following: comminuted fractures; intra-articular extension; soft-tissue trapping, including that of tendons and muscle slips; underlying pathologic lesions, if any; displacement of the fragment in the axial plane; and bony complications.
CT is more sensitive than plain radiography in the detection of stress fractures. CT findings may also show stress fractures and early degenerative changes. MRI is the only modality that is more sensitive than CT, owing to its ability to depict soft tissue and joint structures.
Magnetic Resonance Imaging
Although MRI is sensitive for the diagnosis of fractures, it is not required, because plain radiographic findings are fairly sensitive and specific. MRI is useful in the assessment of fractures and dislocations, soft tissue, the plantar plate, structures of the capsule, the extent of marrow hyperemia, the exact number of bones involved, and small chip fractures.
MRI is more sensitive than radiography and even scintigraphy in the early diagnosis of stress fractures, because it shows bone marrow edema exquisitely. MRI may be used to differentiate stress fractures from early degenerative changes and early stress fractures from synovitis.
MRI scans of the foot should include T1-weighted, T2-weighted, and short-tau inversion recovery (STIR) images in the axial, sagittal, and coronal planes.[10, 11, 12, 13, 14, 15, 16]
The fracture line is visualized as a linear hypointensity in T1- and T2-weighted images, whereas STIR images may show hyperintensity. Edema of the bone has low signal intensity on T1-weighted images and high signal intensity on T2-weighted images. Soft-tissue swelling, ligamentous injuries, and plantar-plate injuries are better visualized with MRI than with other modalities.
MRI is accurate for detecting traumatic injury of the Lisfranc ligament and for predicting Lisfranc joint complex instability when the plantar Lisfranc ligament bundle is used as a predictor, according to a study by Raikin et al. In this study, manual stress radiographic evaluation under anesthesia, along with surgical findings, were used as the reference standard in 21 feet of 20 patients. Intraoperatively, 17 unstable and 4 stable Lisfranc joints were identified. Of the 21 Lisfranc joint complexes, 19 were correctly classified on MRI. In 1 case, a stable Lisfranc joint complex was interpreted as unstable on MRI, and in another, an unstable joint complex was interpreted as stable.
When radiographic findings are normal, ultrasonography is indicated in the diagnosis of metatarsal bone stress fractures, according to a study by Banal et al. They recommended the use of ultrasonography in the diagnosis of metatarsal bone stress fractures because it is a low-cost modality; it is noninvasive, rapid, and easily performed; and it has good sensitivity and specificity[10, 17]
The authors evaluated the sensitivity and specificity of ultrasonography versus those of dedicated MRI (0.2 Tesla) in the early diagnosis of metatarsal bone stress fractures in 37 patients (41 feet). Ultrasonography had a sensitivity of 83%, a specificity of 76%, a positive predictive value of 59%, and a negative predictive value of 92%. The positive likelihood ratio was 3.45, and the negative likelihood ratio was 0.22.
Bone scanning is performed with the use of technetium-99m (99mTc) methylene diphosphonate. Vascular flow and delayed images are obtained. Fractures become evident on bone scans before they become evident on radiographs.
Acute fractures are seen as foci of increased uptake in the affected bone. However, scintigraphy is not routinely indicated for the diagnosis of acute fractures. This study is performed if the clinical findings suggest a fracture but the plain radiographs are negative.
Bone scanning is highly sensitive; its sensitivity is surpassed only by that of MRI in certain instances. For instance, MRI and CT scanning are more sensitive than bone scanning for evaluating stress fractures, because MRI and CT scanning can depict bone marrow edema.
Bone scanning, however, is not specific. Hence, its results should not be reported in isolation. A hot spot may be seen in fractures, degenerative areas, or neoplasms. Nuclear medicine images must be correlated with plain radiographs.
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