Podiatric Diagnostic Ultrasound

Introduction

Diagnostic ultrasound is a medical technique developed in the late 1970s for examination and evaluation of various muscuoskeletal disorders. Simply stated, it allows the physician to view underlying soft tissue and bony structures and pathology in a definition similar to an MRI. Ultrasound, however, incorporates the use of high frequency sound waves instead of a magnetic field to stimulate tissue and produce an image. The vibrations produced by the ultrasound unit in tissue are a series of rhythmic mechanical compressions and expansions that are interpreted by the computer to produce images. A secondary advantage of this modality is the ability to perform active/passive range of motion studies of a particular part to visualize more accurately partial or complete tendon, muscle, or ligament tears. The test is painless and noninvasive.

Ultrasound Specifics for Common Pathologies

Plantar fasciitis:

Inflammation or injury to the plantarfascia, which is a structure originating on the bottom surface of the heel and traveling forward to insert in the toe joints at the ball of the foot, can be readily identified with an ultrasound examination. A primary reason for this abnormality is mechanical strain and repetitive microscopic trauma produced from excessive elongation of the cord. This structure consists of three primary components commonly referred to as the medial, intermediate, and lateral slips

The plantar fascia is typically examined/visualized from the side of the heel as well as the bottom. A measurement is obtained demonstrating the thickness of the plantar fascia slightly distal to its origin and areas that appear thickened as the ligament travels forward. A comparison with the normal foot/plantar fascia will produce a calculation of the percentage of the enlargement due to the inflammation present in the area. This calculation can be utilized to track the progress achieved with NSAIDs, steroid injections, and physical therapy.

Morton's neuroma:

A Morton's neuroma is a painful thickening of the insulating sheath surrounding the nerve in the ball of the foot just before the toe joints. It most commonly affects the area between the third and fourth metatarsals, but is also found between the second and third metatarsals as well. Pain will generally be present in the ball of the foot and symptoms will often affect the associated toes. Nerves carry many different messages for our bodies and therefore the discomfort associated with a neuroma can produce a variety of symptoms. Most commonly the pain is described as a sharp, instantaneous pain in the ball of the foot radiating to the toes. It may also be described as a burning or stinging sensation.

The diagnostic ultrasound will identify the area of enlargement which can be quantified through measurement. A maneuver referred to as Mulder's sign is performed to further enhance identification of the soft tissue mass. This involves a mechanical compression of the ball of the foot which will clearly demonstrate a "popping" of the neuroma on the ultrasound scan. Identification of the nerve is enhanced by altering the angle of the probe and demonstrating a consistent echo appearance. Tendon, muscle, and ligaments vary their signal based upon the angle of the ultrasound beam.

Ultrasonically guided intralesional injection of the Morton's neuroma is extremely beneficial for accurate deposition of any steroid or other neurolytic agents that may be employed in the nonsurgical treatment of this pathology.

Heel spur:

A planter calcaneal exostosis, or heel spur, is characterized by an enlargement or growth of bone on the inferior surface of the heel. This proliferation of bone is caused by a continual strain of the origin site of the plantarfascia on the heel bone. It is characterized by particularly severe pain with the first few steps in the mornings and is also present to a significant degree after periods of rest.

Ultrasound imaging in most cases primarily visualizes soft tissue structures. The heel spur is an example of a bony abnormality that presents well with this modality. The spurring itself is clearly depicted at the plantarfascia will attachment and additional swelling, enlargement or thickening, and fluid accumulations are easily identifiable.

Heel spurs may also be found on the rear of the heel bone in the area of the attachment of the Achilles tendon. In this location it is referred to as a Haglund's deformity. In some instances the proliferation of bone may extend into the substance of the Achilles tendon itself. A bursa may also be identified with ultrasound imaging in the area between the heel bone and the Achilles tendon insertion. Retrocalcaneal bursitis appears as a dark pouch.

Achilles tendonitis/tear

Pain due to inflammation or injury in the area of the Achilles tendon is most often associated with overuse exercise, direct trauma, or a genetically short structure. An inflammatory process will present clinically as a painful enlargement in the area in question. Injury represented by a tear or rupture will present clinically with a depression at the site of trauma. This defect is referred to as a tendon dell.

Ultrasound imaging in either case easily identifies the specific pathology present. Inflammatory processes most often demonstrate a thickening of the affected tissue, swelling or fluid accumulation in the area of question, or other soft tissue abnormalities. Tears and/or ruptures are noted as interruptions of the continuity of the linear fiber arrangement within a tendon. The overall extent of the physical damage can be viewed in multiple planes for a more accurate determination of the severity of the condition.

Ankle sprains and other Ligament Injuries:

Common areas subject to ligament injuries that are conducive to ultrasound imaging are those found with ankle sprains, midfoot strains, or injuries to the forefoot. There are three ligament structures found on the lateral aspect (outside) of the ankle that are the most commonly torn or strained. They are the anterior talo-fibular, calcaneal fibular, and posterior talo-fibular ligaments. These represent the primary stability for the outside of the ankle. Most ankle sprains are of the inversion type. This is a motion in which the leg and body roll over the outside of the ankle with the foot turned under. When all three ligaments are ruptured a fracture of the outside leg bone, fibula, will then follow.

Mid tarsal joint and metatarsal joint strains typically associated with active injuries will demonstrate interruption of fibers when torn, thickening of ligament structures with inflammation, fluid accumulations, and hematoma (tissue bleeding) formation in the area of acute damage.

Tarsal tunnel syndrome:

The tarsal tunnel is a structure behind the inside ankle bone that occupies a large vein, artery, and nerve. It is similar to the more publicized carpal tunnel located in the wrist. Patients that tend to roll inward with their ankle and foot structure (collapse/pronate), are more susceptible to this condition. Excessive strain, blunt trauma, strangulating scar tissue formation from long-term flat footed position entraps the posterior tibial nerve producing enlargement, pain, and inflammation.

Sonography is a viable adjunctive tool for the examination, diagnosis, and treatment of this condition. In the event steroid injections are a considered treatment alternative, the use of ultrasound guidance for proper placement of medication is a significant benefit.

Plantar fibromas/tendon lesions/bone cysts/ganglion:

The vast majority of soft tissue masses found in the foot and ankle are benign. This does not discount the possibility for a malignancy to be present. Pathological instantaneous are usually locally invasive and well demarcated. They may be dense, nodular, fibrotic tissue as in the plantar fibromas or soft, pliable, gelatinous structures like the ganglion cyst. Bone lesions are generally areas that resemble circular defects simulating an absence of osseous material. Tendon lesions are generally external growths that protrude from the body of the tendon itself. The primary thought process involving all these pathologies simply stated is, most are obvious abnormalities of the affected tissue. They are easily identified with ultrasound imaging.

Fractures:

Stress fractures (hairline) and toe fractures are among the most common seen by a Podiatrist. The second, third, and fifth metatarsals are the most frequent long bones fractured in the human foot. Fractures of the second and third metatarsals have been commonly referred to as "march" fractures since they are often associated with excessive standing, walking, and hiking. Brittle bone diseases will also predispose a patient to these types of fractures. Digital fractures occur frequently with barefootedness at home. A dislocation in addition to the fracture is common.

Hairline fractures are extremely difficult to identify on standard x-rays when seen within the first 10 days after the injury. Ultrasound imaging will detect the stress fracture Winn guided by palpation with the probe earlier than that possible with conventional x-rays. A disruption of the hard outer cortex of the bone and frequent hemorraging are readily identifiable.

Wounds and diabetic foot ulcers:

Ultrasound imaging provides a noninvasive means of viewing the wound/ulcer defect both superficially and deep. Chronic wounds and surrounding tissue can be evaluated for abnormalities, foreign bodies, and any extent of injury. If the invasive depth of the ulceration or wound extends beyond the visible perimeter it will be identified. This sideways invasive characteristic is called undermining. When the destructive track of the wound/ulcer progresses deeper into the tissue nearing or at bone level, these paths must be detected early. Bone infections are among the most difficult and challenging to treat. This type of information has a direct impact on the selection of appropriate treatment recommendations.

Sophisticated diagnostic ultrasound equipment will also include a noninvasive color vascular Doppler capability. A thorough evaluation of a patient's circulation is invaluable.

Great toe joint pain:

A bunion (deviated joint) and Hallux limitus (stiff joint) are two of the most common problems that produce significant pain in the great toe joint. The bunion is represented by a large bony protrusion on the side of the big toe joint and a movement of the great toe toward the second toe. The Hallux limitus is recognized by the severe restriction of motion in the up and down direction of the great toe joint. This is often associated with a large spur formation on both the long bone as well as the first toe bone that form the great toe joint. The tendon structures superficial to this spur formation or frequently inflamed contributing to the pain patients will experience with this pathology.

Tendonitis, capsulitis, bursitis, and joint cartilage narrowing as well as the bony abnormalities, are readily identifiable through ultrasound imaging. The inflammatory process produces distinct thickening and/or fluid accumulations at the sites of the specific pathology. Although ultrasound imaging is typically not relied upon for diagnosing this particular osseous problem, it will readily demonstrate a clear picture of the enlargements and the resulting soft tissue effects. Similarly, the information obtained with the ultrasound imaging provides the surgeon a greater degree of accuracy in designing necessary treatment protocols.

Foot & Ankle Images

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