INTRODUCTION
Many pathologic conditions that affect joints are dependent upon the specific position of the joint, related to reactions to stress, or occur in response to a "loaded" condition. Static-view MRI examinations may miss the pathological findings that are present because the joint is not assessed through a range of motion. The functional information obtained using kinematic MRI often serves to definitively identify the underlying abnormality or to supplement the information acquired with standard, static-view MRI techniques (Shellock 1996, 1997).

A functional evaluation of the joints can be accomplished using kinematic MRI. "Kinematic" is a biomechanical term that describes the movement of a body without reference to force or mass. Kinematic MRI involves the evaluation of interactions of the soft tissues and bony anatomical features that comprise the joint. Typically, the relative alignment of the anatomical structures is studied through a specific range of motion for a given joint.

Kinematic MRI techniques have been developed and applied to assess the ankle, cervical spine, patellofemoral joint, shoulder, wrist, and temporomandibular joint. This article will discuss the kinematic MRI protocols for functional imaging of specific joints (with the exception of the temporomandibular joint), describe the various clinical applications of kinematic MRI, and illustrate the usefulness of this technique for diagnosis of pathological conditions.

GENERAL ASPECTS OF KINEMATIC MRI
Protocols and Techniques. Protocols for kinematic MRI have used a variety of pulse sequences and joint positioning strategies. In general, kinematic MRI methods are divided into three primary types: (1) incremental/passive positioning, (2) active movement, and (3) active movement/against resistance.

The incremental/passive positioning technique involves the gradual movement of the joint through a specific range of motion. MR images are obtained at each position using a T1-weighted spin echo, fast spin echo, or gradient echo pulse sequence. The active movement kinematic MRI technique uses fast gradient echo pulse sequences or echo planar imaging to rapidly obtain MR images of the joint during active (i.e., dynamic) motion initiated and controlled by the patient. A temporal resolution of one image per second or less is typically required for this form of kinematic MRI procedure. The active movement/against resistance method is similar to the active movement technique with respect to the MRI requirements. However, this form of kinematic MRI also imposes a resistive load to stress the joint during dynamic motion of the anatomy of interest.

Positioning Devices. Positioning devices are typically necessary for the performance of kinematic MRI examinations. Basically, these devices are used to guide the joint in a specific plane of imaging and through a specific range of motion. Obviously, positioning devices must be constructed of components that are compatible with the electromagnetic fields used for MRI. Most importantly, they need to be designed with a thorough understanding of the biomechanical aspects of the joint.

Positioning devices may also incorporate surface coils or be used to apply resistance or stress to the joint during movement to facilitate imaging. There are several commercially-available positioning devices (e.g., CHAMCO, Cocoa, FL; General Electric Medical Systems, Milwaukee, WI; MEDRAD, Pittsburgh, PA) that have been developed in consideration of the above-mentioned factors.

KINEMATIC MRI OF THE ANKLE
Using a combination of standard, high-resolution static MRI methods and kinematic MRI, a variety of pathological conditions that affect the ankle may be studied. Investigations have reported that kinematic MRI helps to provide a more thorough examination of the ankle, particularly in cases of functional abnormalities associated with osseous subluxations and bony or soft-tissue impingement (Shellock 1997). Clinical applications include assessment of tibio-talar rotation, evaluation of partial tears of the tendons and ligaments, determination of loading areas of the talar dome, and assessment of subtalar instability. More recently, a kinematic MRI application has been developed for assessment of peroneal tendon subluxation.

The relatively small size and complex anatomy of the ankle make high-resolution kinematic MR views essential for optimal examination. A positioning device that incorporates a surface coil is critical for the proper performance of kinematic MRI of the ankle. Typically, kinematic MRI studies are performed using the incremental/passive positioning technique with the ankle positioned in dorsiflexion and then progressively moved through a range of motion to plantarflexion (Figure 1). Sagittal or axial T1-weighted, spin-echo or fast spin echo MR images are acquired at each increment. For assessment of subtalar instability, coronal plane MR imaging is performed while the ankle is incrementally positioned into eversion and inversion positions.

Abnormalities of the osseous anatomy and alterations of the structural soft-tissue restraints markedly affect the stability of the ankle and cause significant malalignment of joint surfaces. These changes are typically manifested as a loss of range of motion, particularly in dorsiflexion. Furthermore, slight displacements of the tibiotalar or fibulotalar articulations can produce substantial changes in loading stress on the ankle and subsequent, severe pathological conditions.

Osseous or soft tissue impingement syndromes that affect the ankle typically cause pain and inhibit motion during dorsiflexion (Figure 2). It was previously thought that impingement syndromes of the ankle were caused by osseous structures, such as osteophytes, located on the anterior aspect of the tibia that impacted with the talus during dorsiflexion. However, soft-tissue impingement syndromes associated with plantar flexion inversion injuries of the ankle have since been described (Farooki et al. 1998). Following tears of the lateral ankle ligaments, the healing process may be accompanied by scarring and capsular hypertrophy in the anterolateral space. This can produce a soft-tissue impingement during dorsiflexion of the ankle. In these cases, kinematic MRI of the ankle demonstrates impingement of the soft-tissue mass during movement.

Subluxation of Peroneal Tendons. Ankle injuries may result in subluxation of the peroneal tendons; this is often unrecognized and frequently misdiagnosed as an ankle sprain, such that the appropriate initial management of the patient is delayed. Chronic subluxation of the peroneal tendons can be disabling, particularly if tendon pathology is also present.

In the acute injury, clinical evaluation of the peroneal tendons may be hindered by associated swelling and tenderness, which can be partially attributed to disruption of neighboring ligaments or osseous structures. Additionally, there may be pain over the calcaneofibular ligament with peroneal tendon subluxation, which may confuse the presenting symptomatology. Chronic, recurrent subluxation is also difficult to detect clinically, especially if the displacement is subtle or if the tendons spontaneously move back into position. Therefore, the identification of peroneal tendon subluxation by physical examination is frequently a difficult task because the clinical signs may mimic other forms of ankle injuries and co-existing abnormalities may be present. The presence of peroneal tendon subluxation may have associated serious sequellae. A patient that has subluxation of the peroneal tendons that is not repaired or treated in the early stages faces a poor prognosis because inflamed tendons are more likely to tear. Therefore, a rapid and accurate diagnosis of this abnormality is critical for appropriate patient management.

Kinematic MRI of the ankle in the axial plane using incremental, passive positioning has recently been described by Shellock et al. as a useful means of identifying subluxation of the peroneal tendons. Subluxation of the peroneal tendons occurs when the tendons move laterally over the lateral malleolus. This may occur with varying degrees of severity, from subtle displacements relative to the retromalleolar groove only evident in certain positions of the ankle, to constant displacement of the peroneal tendons unrelated to the position of the ankle.

When an abnormality is seen using kinematic MR images obtained in the axial plane, the peroneal tendons are displaced relative to the retromalleolar groove and/or are displaced relative to the lateral aspect of the calcaneus in association with changes in the position of the ankle. In rare cases, medial subluxation of the peroneal tendons may also occur.

Evaluation of patients with chronic ankle pain is difficult because there are a variety of possible mechanisms potentially responsible for the symptoms, including impingement of soft-tissue structures, impingements of bony anatomy, osteochondritis dissecans, avascular necrosis, or loose bodies. Kinematic MRI, combined with high-resolution static-view MR imaging of the ankle, provides information that permits differentiation among these various conditions (Muhle C et al. 1997, Shellock 1997).

KINEMATIC MRI OF THE CERVICAL SPINE
Kinematic MRI of the cervical spine may be performed using a positioning device that incorporates a flexible or rigid surface coil to facilitate imaging and an incremental, passive positioning technique (Figure 3). Sagittal plane MR images are typically obtained with the cervical spine in flexion, neutral, and extension positions. The sagittal plane images obtained through the cervical spine are assessed qualitatively. Attention is directed towards evaluation of position dependent occipital-cervical changes, cord narrowing, occult subluxation, or other forms of functional pathology.

Since there may be a significant change in spinal canal and cord compression during flexion and extension of the cervical spine, kinematic MRI may be especially useful for further assessement of the cervical spine during these movements (Figure 4). Indications include evaluation of spinal stenosis, postoperative cases, and suspected instability.

KINEMATIC MRI OF THE PATELLOFEMORAL JOINT
Disorders of the patellofemoral joint are a primary source of anterior knee pain and occur with a frequency comparable to that of meniscal lesions. Patellar malalignment and abnormal tracking are typically produced by incongruency between the patella and femoral trochlear groove and result in instability of the patellofemoral joint and painful symptoms. Malalignment and maltracking of the patella are believed to produce significant shearing forces and excessive contact stresses that develop lesions and eventually degenerate the articular cartilage. A chronically malaligned patella may also change the load distribution in the patellofemoral joint and cause clinical symptoms in the absence of a detectable cartilage defect.

The detection of patellofemoral joint abnormalities solely by physical examination is often a formidable task because the clinical signs may mimic other forms of internal derangements of the knee and co-existing abnormalities are common. In addition, patients with persistent symptoms following patellar realignment surgery present a particular diagnostic challenge. Proper classification of patellar alignment and tracking abnormalities is crucial for an optimal decision to be made concerning the most appropriate treatment for these conditions.

Since patellofemoral incongruency is most likely to occur during the initial degrees of patellofemoral joint flexion, diagnostic imaging techniques that show the joint during this portion of the range of motion are required for the identification of abnormalities. Imaging the patellofemoral joint at 45° or more of flexion frequently causes clinically important information to be missed. Studies have shown that patellar malalignment and abnormal tracking are not consistently or reliably identified by imaging techniques that image the joint at flexion angles greater than 30 degrees (i.e., most plain radiographic methods).

Abnormal conditions of patellar alignment and tracking exist during the earliest portion of the range of motion of this joint, as the patella enters and articulates with the femoral trochlear groove. As flexion of the joint increases, the patella moves deeper into the femoral trochlear groove. At this point, patellar displacement is less likely to occur, because the femoral trochlear groove functions to buttress and stabilize the patella.

While routine MRI examinations of the knee are useful for identifying various forms of pathology, these images often cannot demonstrate the presence of patellar malalignment (Shellock, 1996). The best technique to diagnose patellar subluxation is to image the patellofemoral joint during the earliest increments of joint flexion (Brossman et al. 1993, 1994, Shellock 1996, 1997).

The state-of-the-art method for performing kinematic MRI of the patellofemoral joint employs the active movement against resistance technique. This procedure is conducted by obtaining MR images during active movement of the joint against an externally applied load. This particular form of kinematic MRI was developed to stess the quadriceps and other associated soft tissues in order to displace the patella in the presence of any imbalances in these anatomic structures. The application of resistance to stress the patellofemoral joint during kinematic MR examination has been shown to elicit patellar malalignment and tracking abnormalities that may not be observed during unloaded examinations.

The active movement against resistance kinematic MRI examination is performed using a nonferromagnetic positioning device that incorporates a mechanism that has a resistance applied to the patellofemoral joint in the sagittal plane (Figure 5). With the patient in a prone position, the movement against this resistance primarily requires activation of the extensor mechanism, such that imbalances and the resultant effect on the patellofemoral joint may be appreciated.

When the patient is placed prone on the positioning device, special care should be taken to position the extremities so that the individual's lower extremity alignment (observed while in an upright position) is maintained. This patient positioning scheme is unique because it allows rotational movements of lower extremities to occur during flexion of the patellofemoral joints, which is important because excessive internal or external rotation may be partially responsible for abnormal patellar alignment and tracking. Of note is that the kinematics of the patellofemoral joint are the same with prone and supine positioning of the patient.

To effectively determine patellar alignment and tracking, three to four different transaxial sections through the femoral trochlear groove or femoral trochlea (depending on the position of the patella) should be evaluated. These images must be obtained during the initial increments of flexion for thorough and proper assessment of this joint. Qualitative criteria are used to describe the patella relative to the femoral trochlear groove or femoral trochlea during joint flexion. Normal patellar alignment and tracking is displayed when the ridge of the patella is positioned directly in the center of the femoral trochlear groove and this orientation is maintained throughout the early and later increments of joint flexion, as the patella moves in a vertical plane.

Abnormalities of patellar alignment and tracking are apparent on kinematic MRI studies when there is any deviation of this normal pattern of patellar movement exhibited on one or more slice locations at five degrees of joint flexion or greater (Figure 6). The severity of the problem is displayed by the movements of the patella over the range of motion. Thus, it is relatively easy to determine if the abnormal patellar movement patterns are improving, staying the same, or worsening during flexion of the joint.

Kinematic MRI of the patellofemoral joint has been utilized to determine the effects of bracing on patellofemoral joint abnormalities by performing kinematic MRI without and with the brace applied. This is often crucial for proper management of the patient because certain braces may not improve patellofemoral conditions or may even increase subluxation of the patella. Additionally, rehabilitation methods may be monitored to rapidly assess the beneficial aspects of this therapy.

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ANKLE
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