Imaging Studies of the Foot and Ankle



Imaging Studies of the Foot and Ankle


Steven M. Raikin, MD

Daniel Fuchs, MD


Dr. Raikin or an immediate family member has received research or institutional support from Zimmer. Neither Dr. Fuchs nor any immediate family member has received anything of value from or has stock or stock options held in a commercial company or institution related directly or indirectly to the subject of this chapter.


This chapter is adapted from Raikin SM, Winters BS: Imaging Studies in the Foot and Ankle in Chou LB, ed: Orthopaedic Knowledge Update: Foot and Ankle 5. Rosemont, IL, American Academy of Orthopaedic Surgeons, 2014, pp 25-36.





Introduction

Imaging studies are a component of the clinical evaluation of a patient with a condition affecting the foot or ankle. After completion of a careful history and physical examination, imaging studies usually are obtained as the next step in determining a diagnosis and treatment plan. Each of the available imaging modalities has appropriate indications and applications.


Plain Radiography

Plain radiography is the foundation of most diagnostic foot and ankle imaging. Bone anatomy, integrity, and alignment as well as joint congruency can be assessed through a routine set of radiographs, which usually can be obtained during the patient’s initial office visit. Weight-bearing radiographs must be obtained whenever possible. Although a patient with an acute traumatic injury may only tolerate non-weight-bearing radiography to assess structural anatomy and rule out the presence of fracture, these radiographs do not show the foot or ankle in a physiologic position and therefore may not facilitate detection of malalignment or some pathologic conditions.

The popularity of digital radiology has eclipsed that of printed radiographic studies during the past few years. Radiographs are captured digitally in two ways.1 Computed radiography is affordable, offers excellent image quality, and uses existing radiography systems.

True digital radiography requires more expensive technology than traditional radiography or computerized modification but offers greater efficiency and the capacity for higher image quality and a lower radiation dosage. Images are stored in a picture archiving and communication system rather than on hard copy radiograph. The most commonly used format for medical images is Digital Imaging and Communication in Medicine (DICOM), which allows safe, high-quality off-site viewing and analysis of radiographs. Digital radiography also allows computer-assisted measurements of distances or angles to be generated for use in diagnosing pathologic foot and ankle conditions.


The Foot

Routine radiography of the foot includes the AP, lateral, and oblique views. The internal (medial) oblique view usually is part of the routine three-view set and is useful for
showing the lateral tarsometatarsal joints and detecting a possible calcaneonavicular coalition. The external (lateral) oblique view should be ordered to more clearly show perinavicular joints or identify an accessory navicular bone.

In specific circumstances, other views can be used to obtain additional information about the bony anatomy. The sesamoid axial projection can reveal sesamoid-metatarsal arthritis, fracture or osteonecrosis of the hallux sesamoids, or sesamoid alignment relative to the crista on the plantar aspect of the metatarsal head. The calcaneal axial (Harris-Beath) view is obtained at a 45° angle from posterior to proximal. This view allows analysis of the posterior and middle facet of the subtalar joint to detect arthritis or a tarsal coalition. The Broden views are a sequence of angled radiographs centered over the sinus tarsi and taken at a 10° to 40° cephalic tilt. These views reliably show the posterior facet of the subtalar joint in calcaneal fracture management or subtalar fusion. The talar neck view provides a true AP view and is useful in assessing talar neck fractures. This view is obtained with the foot internally rotated 15° and the beam angled 15° cephalad while centered over the talar neck.


The Ankle

A three-view weight-bearing set of radiographs is routinely recommended for analysis of the ankle, including the AP, lateral, and mortise views. For the AP view, the ankle, mortise is aligned in approximately 20° of external rotation relative to the sagittal plane. A true mortise view, in which the x-ray beam is oriented perpendicular to the intermalleolar axis, is therefore obtained with the leg internally rotated. This view allows the best assessment of mortise congruity and the talar dome. Several ankle-specific views also are used to evaluate the ankle joint. The 50° external rotation view is optimal for the detection of posterior malleolar fracture fragments.

The hindfoot alignment view is obtained with the patient standing on a raised platform and the x-ray beam directed from behind and angled 20° caudally, with the cassette placed perpendicular to the beam.

Stress views are used to assess ankle instability. While a lateral radiograph is being obtained, an anterior drawer test is performed (the evaluator’s hands are protected by lead gloves, or a mechanical jig is used). Anterior displacement of the talus of more than 10 mm (or more than 5 mm than that of the stressed uninvolved ankle) is consistent with lateral ankle ligament dysfunction. Similarly, varus or valgus stress radiographs are used to assess the integrity of the lateral ligament complex or deltoid ligament, respectively. A 10° difference in the talar tilt of the injured and normal extremities is considered pathologic. However, the reliability and reproducibility of stress testing is questionable because of the wide variation in test findings in normal ankles and healthy patients, tester-dependent variation in forces applied against the ankle, and variation in patients’ resistance to the force. Therefore, the results of stress testing alone should not determine the treatment of ankle instability.

Fluoroscopy has become a vital tool for assessing joint or bone alignment and positioning internal hardware during surgery. Technical advances have resulted in great improvement in fluoroscopic image quality as well as the size and ease of handling of the machine. The small C-arm units used by many foot and ankle surgeons expose the surgical team and patient to minimal radiation (outside of the direct path of the beam).2


Radiographic Measurements

Many radiographic measurements have been described for evaluating the alignment of the foot and ankle, several of which are particularly important. Computer-assisted measurement of digital radiographs has resulted in improved accuracy. A smartphone goniometer application is extremely accurate for measuring certain angles.3 All measurements are most accurate if based on weight-bearing radiographs.

The hallux valgus angle and the first-second intermetatarsal angle are the mainstay measurements for assessing the severity of hallux valgus deformity. These measurements have good intraobserver and interobserver reliability when standard methods are used. The first and second metatarsal axes are each drawn by connecting the midpoint of the shaft of the metatarsal 2 cm from the proximal and distal articular surfaces. The axis of the phalanx is drawn through the shaft midpoints 0.5 cm from each of its articulations. The hallux valgus interphalangeal angle is drawn between the phalangeal axis of the proximal and distal phalanges of the hallux. The distal metatarsal articular angle is the angle subtended by a line drawn perpendicular to the long axis of the first metatarsal and a line corresponding to the distal articular surface of the first metatarsal. Compared with the hallux valgus and first-second intermetatarsal angle, the distal metatarsal articular angle is less reliable and reproducible for determining joint congruency.4 These measurements can be computer enhanced, but this step usually is not necessary.

The talus-first metatarsal angle, also called the Meary angle, is drawn through the long axes of the talus and the first metatarsal on the lateral radiograph. This angle is useful for assessing arch height and recently was found to be reliable for measuring adult flatfoot deformity.5 The talonavicular coverage angle measures the lateral subluxation of the navicular at the talar head or the ex-tent to which the talar head is uncovered, and it is useful for evaluating forefoot abduction in pes planovalgus deformities. On a weight-bearing AP radiograph of the foot, a line is created joining the two ends of the articular surface of the talar head, and another line joins the matching two ends of the joint surface of the navicular. The angle
created by perpendicular lines drawn from the midpoints of each of these two lines is the talonavicular coverage angle; a normal value is less than 7°.


Tenography

Plain radiographs can be used to observe secondary changes caused by tendon dysfunction but cannot be used to evaluate the tendons themselves. Tenography involves the injection of contrast material into the tendon sheath under fluoroscopy, often followed by a steroid injection. Tenosynovitis, stenosing tenosynovitis, and a tendon tear or rupture can be seen. Tenography can be particularly useful in a posttraumatic setting in which the value of other diagnostic modalities is limited by the presence of anatomic deformity or associated hardware (eg, in peroneal impingement after calcaneal fracture).6 In the absence of a tendon tear, intrasheath steroid injection with tenography relieves symptoms in some patients with tenosynovitis around the foot and ankle.7 This cost-effective but invasive technique is associated with a small risk of tendon rupture. Tenography has been less frequently used for diagnostic purposes since the widespread adoption of MRI.


Ultrasonography

Ultrasonographic evaluation of tendons around the foot and ankle offers several advantages over other modalities. Ultrasonography is cost-effective, safe, noninvasive, and not based on radiation. Structures are evaluated in real time. Dynamic evaluation of tendon function allows conditions such as peroneal subluxation or dislocation to be directly visualized. The transducer can be manipulated to avoid interference from any metallic implant in the ankle. This ability is useful in evaluating tendon injury after hardware placement in the foot or ankle, which would create interference during MRI. Ultrasonography also avoids the possibility of the so-called magic angle phenomenon on MRI (artifact seen in tendons oriented 54.7° to the magnetic field), which can inaccurately suggest the presence of intratendinous pathology. Ultrasonography is slightly less sensitive than MRI for use in diagnosing tibialis posterior tendon pathology, but a recent study found that discrepancies did not result in altered clinical management.8 Ultrasonography has high sensitivity and specificity for detecting peroneal tendon tears and 100% positive predictive value for detecting peroneal subluxation, as correlated with intraoperative findings.9

For evaluating tears in the lateral ankle ligament, ultrasonography has sensitivity and specificity comparable to that of MRI. Morton neuroma, recurrent interdigital neuroma, and other soft-tissue masses or cysts can be detected on ultrasonography after an equivocal clinical evaluation.

Most modern ultrasonography machines are able to evaluate regional blood flow. The movement of blood cells within the vessels causes a change in the pitch of reflected sound waves called the Doppler effect. A computer converts the Doppler sounds into colors that are overlaid on the musculoskeletal ultrasonographic image. The presence of local hyperemia secondary to inflammation can confirm that the ultrasonographic findings are consistent with the pathology or symptoms.

The primary disadvantage of ultrasonography is that it is a technician-dependent modality. Many medical centers lack a radiologist trained and experienced in interpreting the results of musculoskeletal ultrasonographic studies. Ultrasonography offers only poor visualization of bone. Because direct skin contact and the application of gel are required, ultrasonography cannot be used through a cast or splint.

Recent advances in technology have led to improvements in the quality, portability, and cost of ultrasonography machines, and office-based ultrasonography is now feasible. To complement the clinical examination and standard radiographic evaluation, a diagnostic ultrasonographic examination can be done during the patient’s initial visit to the foot and ankle surgeon. Ultrasonography can be particularly useful in making a subtle diagnosis and planning treatment; for example, a determination that the patient has a Morton neuroma in the second web space rather than early plantar plate dysfunction precludes the use of a cortisone injection, which can cause metatarsophalangeal instability.10 In-office ultrasonography also can be used to identify tendinosis, a tendon injury around the foot and ankle, a mass or tumor, or a nonradiopaque foreign body such as a wood splinter. Ultrasonography can be used to improve the accuracy of diagnostic injections in the foot or ankle, guide injections for a condition such as Morton neuroma or plantar fasciitis, and guide the aspiration of cysts. The use of ultrasonography was found to improve the accuracy and outcome of therapeutic injections and thereby reduce the need for additional procedures.11


Computed Tomography

CT provides high-resolution thin-slice studies of the foot and ankle. The development and implementation of multidetector-row slip-ring technology has resulted in improved imaging of osseous structures. Source images acquired in the axial, coronal, or sagittal plane allow three-dimensional views of the bony anatomy for diagnosis and therapeutic planning. Relatively new computer software creates multiplane reconstructions without increasing the patient’s exposure to ionizing radiation; for example, coronal images can be constructed from axial images, and three-dimensional reconstruction models can be created from the initial source images (Figure 1).
Around the foot and ankle, studies with 3-mm or thinner slices are routinely recommended. The foot position in the scanner is important because the source image planes are obtained parallel to the osseous structures of the foot in each plane using a tomographic scout localizer image. The presence of cast or splinting material around the region does not interfere with CT but may create difficulty in positioning the foot. The advent of multidetector-row systems has permitted rapid image acquisition and high-quality three-dimensional reconstruction to be achieved with decreased radiation dosages.






FIGURE 1 Three-dimensional CT showing a calcaneus fracture. Three-dimensional images are particularly useful in preoperative planning.

In the foot and ankle, CT is most commonly used to assess bony abnormalities and is particularly useful in providing fine osseous detail not obtainable with plain radiography. Fracture, infection, osteochondral injury (requiring accurate assessment of the bony anatomy and lesion size), arthritis, osteonecrosis, and osseous tumor or congenital abnormality can be diagnosed using CT. This is particularly useful in suspected subtle fracture, joint diastasis, or articular incongruity not detectable on plain radiography, as in suspected Lisfranc injuries, syndesmotic injuries, and tibial plafond fractures. In addition, CT can be used to assess the extent of joint comminution in calcaneal or plafond fractures and to assist in preoperative planning (Figure 2).

CT has high accuracy and sensitivity for the detection of fracture nonunion and is more reliable than serial radiographic studies for assessing the extent of union after surgical arthrodesis12 (Figure 3). The use of micro-CT and nano-CT technology is likely to further enhance the accuracy of this modality. The interpretation of postoperative CT can be negatively influenced by metallic artifact or scatter from internal hardware. Relatively new software uses metal deletion techniques to limit this effect and improve the accuracy of study interpretation.

Current CT scanners use multidetector configurations in which a narrow, fan-shaped x-ray beam and detector rotates around the patient to quickly acquire multiple image sections. This technology cannot be used with the patient in a weight-bearing stance, as is needed to evaluate extremity alignment and deformity during physiologic loading. Axial loads can be applied to joints to simulate weight bearing when the patient is supine, but the resulting studies have questionable accuracy. The recently developed cone-beam CT technology uses a pyramid-shaped x-ray beam and a large-area detector to obtain volumetric data from multiple projections through a single rotation around the patient, who is in a standing, weight-bearing position.13 The technique for image reconstruction is similar to that used for multidetector CT. The three-dimensional data obtained from cone-beam CT allow a much improved quantitative analysis of many pathologies. Other advantages of cone-beam CT over multidetector CT include more rapid acquisition of images, superior image quality, and lower radiation dosages (approximately 9 mGy compared with 27-40 mGy). The compact, portable design of the cone-beam CT machine has positive implications for workflow and storage. Cone-beam CT technology has the potential
to be extremely valuable in musculoskeletal extremity imaging from both clinical and economic viewpoints.






FIGURE 2 Coronal CT showing a comminuted intra-articular calcaneus fracture.






FIGURE 3 Axial CT showing nonunion after an attempted talonavicular arthrodesis.

Weight-bearing CT (WBCT) has recently become available for clinical use. This technology allows for three-dimensional imaging of the foot and ankle under the axial load of the patient’s body weight. Various potential advantages of WBCT over non-weight-bearing CT have been proposed including the improved ability to evaluate deformity throughout the foot and ankle, determine the extent of joint space narrowing of arthritic joints, and assess for bony impingement that may only be radiographic apparent with weight bearing.14 WBCT has been used as a research tool to define normal functional anatomic and physiologic relationships including the orientation of the subtalar joint, fibular rotation, and translation at the distal tibiofibular joint with tibiotalar motion and talus rotation within the ankle mortise with weight bearing.15,16

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Feb 27, 2020 | Posted by in ORTHOPEDIC | Comments Off on Imaging Studies of the Foot and Ankle
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