A SUPPLEMENT TO THE SEPTEMBER 2000 ISSUE OF DIAGNOSTIC IMAGING
 

September 2000

MRI finds runners’ overuse injuries

Early diagnosis and intervention head off serious injuries to bone, soft tissue in pelvis and lower limbs

By Gabrielle Bergman, M.D., and Michael Fredericson, M.D.

Running is an increasingly popular form of exercise and competition in our society. Running-related injuries are most often due to overuse, and stress fractures account for most of these overuse injuries. Reported studies have found that distance runners and joggers have the highest incidence of stress fractures among different groups of athletes.1

Repetitive submaximal stress creates a region of accelerated bone remodeling termed a bony stress lesion, which may progress to a stress fracture if the stress continues. Overuse injuries to soft-tissue structures such as tendons, muscles, and ligaments are also common.

Stress injuries in runners occur almost uniquely in the pelvis and lower extremities. These injuries are generally secondary to an overload of forces on a specific region of the body, with the local forces or stresses exceeding critical limits. Overuse injuries are further related to an individual runner’s biomechanical predisposition, training methods (volume and intensity of activity), and other factors such as diet, muscle strength, and flexibility.

Prevention of stress fracture or early intervention is the preferable course of action. Many of these overuse injuries present with vague symptoms, however, such as the groin pain that may be seen with sacral, femoral neck, or femoral diaphysis stress fractures. MR imaging is a valuable diagnostic tool to identify, confirm, or grade subtle overuse injuries in runners and other athletes.2

It is important to recognize that certain stress fractures, in particular locations, represent high-risk injuries that require prompt diagnosis, aggressive treatment, and occasionally even internal fixation. These stress fractures are known to be associated with increased risks such as delayed union or nonunion, bony fragmentation, further displacement, and later development of osteoarthritis.

Sacral Stress Fracture

Stress fractures of the sacrum predominantly affect young female runners.3 The presenting symptoms, with posterolateral pelvic dull pain, often mimic those of sacroiliitis. The fracture is unilateral and involves the anteroinferior aspect of the sacral wing, distinguishing it from the more frequent H-shaped configuration seen with sacral stress fractures in older osteoporotic individuals.

MR imaging is required for diagnosis, as x-rays are usually negative at presentation and may remain negative. Intermediate signal on T1-weighted and high signal on T2-weighted images are seen in the anteroinferior aspect of the affected sacral wing, indicating bone marrow edema that extends to the sacroiliac joint. Often a discrete fracture line is present as well.

If these fractures go undetected, there is potential for a displaced fracture involving the sacroiliac joint, leading to serious disability. Treatment requires an initial period of rest and temporary use of crutches if any pain occurs during ambulation, with gradual return to full running activity in six to eight weeks.

Proximal Hamstring Tendinitis

During the late swing phase of the running gait, the hamstring muscles contract eccentrically to prevent knee hyperextension. Repetitive overload can place undue stress on the hamstrings during running. Chronic inflammation and fibrosis may also involve the sciatic nerve coursing nearby. Patients complain of deep, vague buttock pain brought on by running.

T1-weighted and T2-weighted images with fat saturation in the axial and coronal planes show soft-tissue edema surrounding a sometimes-thickened hamstring tendon. Intermediate signal may appear within the tendon, indicating tendinopathy, and fluid in the bursa between the ischial tuberosity and the tendon is often seen as well.4

These findings may be relatively subtle, and clinical information regarding the site of the symptoms and the type of activity causing them is important for scan interpretation. Treatment involves a hamstring flexibility and eccentric strengthening program, while injection of corticosteroid into the tendon sheath just below the ischial attachment may be helpful in more severe cases. Persistent cases may require surgery.

Femoral Neck Stress Fracture

Stress fracture of the femoral neck should be considered in a distance runner or jogger who presents with hip, thigh, or groin pain.5 If initial hip x-rays are normal, MR imaging with T1-weighted and fat-saturated T2-weighted scans in the coronal and axial planes should be performed. These may demonstrate the presence of a hip joint effusion, bone marrow edema in the femoral neck, and sometimes a low-signal fracture line in the medioinferior aspect of the femoral neck. The fracture line is often best seen on T2-weighted images, outlined by the high signal of the bone marrow edema. As with most fracture lines associated with stress fractures, it does not traverse the entire femoral neck.

Early detection of femoral neck stress fracture is crucial, as continued stress may lead to a complete fracture with displacement and its associated risks of avascular necrosis and osteoarthritis. If a stress reaction without a fracture line is detected, treatment can be conservative, with an expected healing period of two to three months before a gradual return to running.

Stress On Femoral Diaphysis

Stress fractures that involve the femoral diaphysis typically cause vague, poorly localized thigh pain that is activity-related and has an insidious onset.6 If running continues, the pain progresses to the point where it is experienced with normal daily ambulation and even during rest at night.

The greatest tension strain during running occurs at the posteromedial cortex of the proximal femoral diaphysis, where an anterolateral bow is normally present. This region is particularly susceptible to repetitive submaximal stresses. Significant muscular forces related to the origin of the vastus medialis and the adductor insertion likely contribute as well to development of localized stress lesions in this location. It has been suggested that this injury is relatively common but frequently misdiagnosed as a muscle injury and therefore not evaluated by any imaging methods.7

MR imaging shows periosteal edema as well as bone marrow edema that involves the posteromedial aspect of the femur near the junction of the proximal and middle thirds of the femoral diaphysis (Figure 1). Axial plane T2-weighted imaging usually shows the pathology best. The edema is typically well delineated, but a discrete fracture line is uncommon in this location, and the lesion is therefore categorized as a bony stress reaction. Provided no evidence indicates a cortical fracture, a gradual return to running is possible once the athlete can ambulate and perform normal weight-bearing activities without pain, usually within eight to 12 weeks.

Iliotibial Band Syndrome

Iliotibial band syndrome is a common cause of lateral knee pain in runners.8 Symptoms usually come on after running a reproducible time or distance and consist of a sharp pain or burning on the lateral aspect of the knee. The iliotibial band, also known as the iliotibial tract, is formed by the tensor fascia lata and a contribution from the gluteus maximus. It inserts distally on Gerdy’s tubercle on the tibia, acts as an anterolateral stabilizer of the knee, and functions to decelerate adduction of the thigh. The iliotibial band remains under tension in both extension and flexion of the knee. Recurrent friction occurs when it slides over the lateral femoral epicondyle.

Axial and coronal plane T1-weighted and T2-weighted MR images with fat saturation show soft-tissue edema along the iliotibial band, usually on the deep aspect, at the level of or near the lateral femoral epicondyle.9 Sometimes a well-defined dumbbell-shaped bursa with high signal on T2-weighted images is seen, indicating that fluid or hypertrophic synovium is filling the bursa (Figure 2).

Therapy involves a comprehensive stretching program for the iliotibial band and the proximal hip musculature, with surgery required only rarely if this soft-tissue mobilization program is unsuccessful.

Symptomatic Synovial Plica

A symptomatic synovial plica is a well-recognized cause of anteromedial knee pain in runners. The medial plica is a normally asymptomatic vestigial fold at the inner synovial surface of the knee joint thought to be present in up to 30% of the population. Distally, it enters into Hoffa’s fat pad. If a normal medial plica is traumatized and becomes fibrotic, it may bowstring over the medial femoral condyle during knee motion, leading to local inflammatory changes. Even chondral fissuring and thinning has been reported to be caused by a large fibrotic plica.

Medial plicas are well seen on MR images as linear low-signal structures on both T1-weighted and T2-weighted images in the axial as well as sagittal planes. When a small joint effusion is present, this outlines the margins of the medial plica, making it even clearer. If the MR exam is performed specifically to evaluate for the presence of a plica, injection of saline or dilute gadolinium into the joint is helpful. The presence of a medial plica on MR images should be reported and may be graded, but MR images cannot reliably determine if a plica is likely to be symptomatic.

The plica itself never becomes edematous and does not vary significantly in its thickness. Instead, the medial plica can be graded as to how far it extends toward the femoropatellar joint: Type A plica is a cord along the medial knee capsule; type B is a flat band extending to the apex of the medial femoral condyle; type C is a flat band inserting itself between the patella and the femoral condyle; and type D is any plica that shows fenestration.

In the proper clinical setting, a type B or C medial plica detected on MR exam, in the absence of other local pathology, may lead an orthopedist to perform arthroscopy with excision or laser ablation of the plica.

Stress On Tibial Diaphysis

Many athletes, particularly runners, commonly experience pain along the medial border of the tibia. The pain is most often a dull, aching discomfort. Distinguishing a tibial stress reaction or fracture from the more common shin splint syndrome is often difficult clinically. It has been suggested that the periostitis seen with shin splint syndrome represents an early injury in a spectrum that also includes bony stress reaction and stress fracture.

An MR exam need not scan the entire leg. A marker should be placed over the region of pain, and axial images performed of that leg only, from about 10 cm proximal to the marker to 10 cm distal to the marker. The periosteal and marrow edema usually extends for less than 10 cm and is centered at the point of pain and tenderness.

Axial T1-weighted and fat-saturated T2-weighted images can demonstrate subtle bone marrow edema, periosteal edema, edema in adjacent muscle, and sometimes a fracture line (Figure 3). The examiner can grade these stress reactions by severity to predict the time required to allow healing: A grade I injury shows periosteal edema only on the fat-supressed T2; grade II shows abnormal increased signal both at the periosteum and bone marrow on fat-suppressed T2-weighted imaging only; grade III injuries show marrow edema on T1-weighted imaging as well; and grade IV shows presence of a fracture line.10

The abnormal signal is thought to represent edema or hemorrhage related to accumulated tissue microdamage and the associated reparative response. Therapy includes temporary cessation of running, which is essential to allow for bony remodeling and repair. This period can range from three weeks for a minor injury to 12 weeks for a severe injury with frank cortical fracture.

Achilles Tendinitis

The Achilles tendon is another common site for overuse injuries in runners. The affected athlete feels pain localized to the Achilles tendon, and the foot is often described as being stiff in the early morning and at the start of a run.

Two distinct sites are prone to injury: Intrasubstance tendinitis occurs about 2 to 6 cm proximal to the tendon insertion on the calcaneus; and insertional tendinitis occurs at the tendon-bone interface and may be associated with a bony protuberance of the calcaneus, the Haglund deformity.11 Runners can also develop mild but chronic inflammatory changes that involve the vascular and fibrous tissue that surrounds the Achilles tendon. This condition, termed peritendinitis, is less likely to progress to tendon tear.

Axial and sagittal T1-weighted and fat-saturated T2-weighted images can be used to differentiate peritendinitis—with edema along the Achilles tendon but normal thickness and signal of the tendon—from intratendinous degeneration and partial- or full-thickness tears at any level. Patients with posterior heel pain may also show superficial bursitis, or “pump bump,” as a result of friction from the heel of a shoe. This is limited to the superficial Achilles tendon bursa and well seen on T2-weighted imaging.

If conservative treatment is unsuccessful after about six months, surgery is an option for athletes unable to train at their full capacity. Surgery typically entails release of the peritenon, excision of inflamed or scarred paratenon, and, when necessary, debridement of degenerative tendon tissue.

NAVICULAR STRESS FRACTURE

For detection of navicular stress fracture, a high-risk stress injury to bone, MR imaging is almost always required, as x-rays are less sensitive. Early diagnosis is crucial for early treatment to avoid significant disability from this injury. Runners usually present with insidious onset of vague, activity-related midfoot pain, often accompanied by tenderness over the dorsal border of the navicular.

MR examination is challenging, as the bone marrow edema can be subtle and the fracture line unimpressive and easy to overlook, short and incomplete. Images must be obtained in all three planes, and use of both T1-weighted and fat-saturated T2-weighted imaging is crucial. Thin-section CT may also be necessary in the follow-up of these injuries, to detect subtle, early fragmentation if symptoms do not improve while the patient avoids weight-bearing activity or if the patient tries to return to activity before healing has occurred. If early fragmentation is seen, this necessitates a more prolonged period of immobilization or early internal fixation treatment.

Metatarsal Stress Fracture

Stress fractures of the metatarsal necks or distal diaphyses, sometimes still referred to as “march fractures,” also occur in runners and typically involve the second, third, or fourth metatarsal. These stress fractures tend to heal well, and running can be resumed gradually as soon as ambulation no longer causes pain, and radiographs show early callus formation. A period of four to six weeks is usually required to permit full healing.

Patients present with localized pain, and an x-ray exam at presentation may be negative or show very subtle periosteal bone formation. T1-weighted and T2-weighted MR imaging in the oblique coronal and sagittal planes of the foot, however, may show quite pronounced periosteal and marrow edema of the involved bone and surrounding soft-tissue structures (Figure 4). Often a well-defined fracture line appears as well. To see the full extent of each metatarsal bone on one image, the axial images of the foot should be obliqued into the plane of the metatarsal bones.

Plantar Fasciitis

Plantar fasciitis symptoms are common in runners. They are characterized by insidious onset of sharp pain at the plantar-medial aspect of the calcaneus and are often most severe during the first few steps in the morning. The plantar fascia is a thick aponeurosis that arises from the medial calcaneal tuberosity and spans the bottom of the foot, inserting onto the base of each proximal phalanx.

As the metatarsal joints extend passively during running, with weight shifting to the ball of the foot, the plantar fascia experiences traction distally, which increases the arching of the midfoot, enhancing its stability. This process, in turn, creates a stiffer lever for efficient toe-off during running. Microtears occur with overuse, however, initially leading to a reparative inflammatory response. Chronic changes progress to collagen necrosis, angiofibroblastic hyperplasia, and matrix calcification.

Sagittal and coronal T1-weighted and T2-weighted MR images of the foot will show edema around the plantar fascia, with focal thickening of the plantar fascia, usually at the calcaneal insertion.13 Sometimes subtle bone marrow edema also appears near the calcaneal insertion of the plantar fascia. Less frequently, increased signal is seen within the fascia on T2-weighted images, indicating a plantar fascia tear.

Bone spurs may be seen on hindfoot x-rays in as many as 50% of adult feet with plantar heel pain, but in the absence of significant fat pad atrophy, they are less likely to be the cause of pain. Most spurs arise just deep to the plantar fascia, in the non-weight-bearing substance of the flexor brevis muscle. In those few patients who fail to respond to prolonged conservative treatment, surgical release of the plantar fascia may be considered.


Dr. Bergman is section chief of musculoskeletal radiology and Dr. Fredericson is director of sports medicine clinics, division of physical medicine and rehabilitation, department of functional restoration, both at Stanford University.

References

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