Weight-bearing Musculoskeletal Imaging

Musculoskeletal (MSK) disease comprises a wide range and heterogeneous group of pathologies associated with pain and functional limitation. MSK conditions frequently have significant repercussions on the everyday life of the individuals affected. They lead, in the most serious cases, to appreciable disability and inability to work with consequent economic repercussions. Early diagnosis can lead to patients gaining a better understanding of their condition, less inherent anxiety and unnecessary investigations, and early appropriate treatment that can in turn lead to improvement in symptoms. MSK imaging plays a fundamental role in this diagnostic process, confirming the likely cause of symptoms suspected clinically, establishing the underlying pathomechanical process, and assessing the severity of disease present.

The spine and lower limb joints are those parts of the MSK system most prone to biomechanical alteration as a result of changes in physiologic loading and position. The most common physiologic load applied to the MSK system is weight-bearing. Many MSK pathologies are emphasized when the spine or joints are physiologically loaded, yet for the most part imaging is performed in the unloaded supine position. For many decades, radiography was the sole way to image patients in the upright gravity-loaded position. Radiography is excellent at demonstrating bony alignment, but it is limited in the evaluation of the soft tissues and the field of view that can be examined with a single radiographic projection.

New radiologic devices allow study of the spine and lower limbs in an upright position. These devices use ionizing radiation to obtain digital images similar to conventional radiography but with a greatly reduced radiation dose and a much extended field of view that is amenable to three-dimensional (3D) reconstruction. This technique is particularly useful in the evaluation of scoliosis, spinal curvature, as well as the relationship between spinal, pelvic, and lower limb joint alignment. The radiation dose is 8 to 10 times less than conventional radiology and many hundred times less than a computed tomography (CT) examination. With a field of view of 175 cm and 1:1 scaling, images of either the whole spine and pelvis or the pelvis and both lower limbs can be achieved in entirety and are not subject to photographic enlargement or distortion.

Other new imaging techniques, including CT and magnetic resonance imaging (MRI), have emerged that allow the soft tissues and bony components of the MSK system to be examined more comprehensively in a physiologically relevant upright gravity-dependent position. These techniques have enabled us to understand, for the first time, the effect of weight-bearing on the MSK soft tissues both in health and disease.

One such technique is cone-beam weight-bearing CT that is especially applicable in disorders of the foot, ankle, and knee. It offers several advantages including high spatial resolution, fast time acquisition, low radiation dose, modest cost, and, in particular, imaging in the physiologic standing position. This weight-bearing position allows true 3D alignment to be appreciated. It is particularly relevant to the study of pes planus, diabetic foot disease, and tarsal coalition, as well as operative planning and follow-up.

The technical features of weight-bearing MRI and both its advantages and limitations in diagnosing spinal stenosis and spondylolisthesis are also reported in this issue. Weight-bearing MRI of the spine exaggerates many of the pathologic changes associated with spinal degeneration related to the intervertebral disk, the spinal canal, the neural foramina, segmental instability, and other features. MRI can be acquired in painful body positions so that morphological changes of the intervertebral disk or other spinal structures may be correlated with pain or other symptoms. Weight-bearing MRI of the spine may demonstrate clinically relevant canal compromise or foraminal stenosis that may be underestimated on conventional supine MRI. The usefulness of weight-bearing MRI in patients with adolescent idiopathic scoliosis is discussed, showing how gravity affects the craniovertebral junction, the spinal cord position, disk shape, and spinal alignment. Finally, weight-bearing MRI of the temporomandibular joint (TMJ) that allows a unique orthostatic evaluation of TMJ movement is discussed.

The purpose of this issue is to bring together and discuss the current state of play of new imaging techniques that enable weight-bearing imaging of the MSK system. Weight-bearing is clearly more advantageous in some situations than others. Which conditions and which patients most benefit from weight-bearing imaging is currently being studied. We hope you find this issue helpful as an introduction to this exciting new aspect of MSK imaging.