The Potential Value of Weight Bearing Images Acquired by Cone Beam CT

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Many orthopedic conditions are best evaluated by imaging studies acquired to simulate functional positions and weight bearing activity.  This is especially true of diseases that involve the articular surfaces of the joints, such as fractures, malalignment syndromes, and degenerative diseases.  For example, radiologists and orthopedists both prefer to evaluate patients with knee arthritis by acquiring weight bearing images, as they more accurately reflect simple functions like walking and using stairs.

UBMD Orthopedics and Sports Medicine is a large academic practice plan that provides comprehensive musculoskeletal care to more than 40,000 patients per year.  Our clinical sites offer digital X-ray and MRI imaging, but we do not operate a CT scanner.

We have partnered with Carestream Health to evaluate the ability of a new portable cone beam CT (CBCT) scanner to acquire 2D and 3D images of the extremities.  Of its many capabilities, the CT scanner can acquire images while the patient is weight bearing and with the extremities in various positions while they are under physiological load. A good illustration of the potential utility of this type of imaging is when it is applied to clinical cases of patellofemoral instability.

Two of the challenges of this condition are identifying the cause of patella subluxation or dislocation, and then calculating the amount of correction that would be necessary when tibial tubercle transfer surgery is planned.

In the past several years the tibial tubercle-trochlear groove offset (TT-TG) distance has been relied upon as an objective measure of the Q angle, and when excessive, results in the lateral vector that pulls the patella over the lateral femoral trochlea.  Even though the TT-TG offset is an objective measure, studies have shown that the absolute value can be affected by the method of image acquisition.

Practically all conventional CT scans of the knee are obtained with the patient lying supine on the table with the leg in full extension and the muscles relaxed.  Because of the natural screw home mechanism of the knee, the tibial tubercle rotates externally in relation to the femoral trochear groove during the final few degrees of knee extension.  The active quadriceps exerts a medial vector on the patella. These two factors are reported to falsely elevate the TT-TG offset distance as measured when the CT scan is acquired in the conventional way.

UBMD Orthopaedics and Sports Medicine and Carestream Health are conducting an institutional IRB-approved clinical study to compare measures of TT-TG offset obtained on a conventional CT scanner to those obtained on the prototype portable CBCT scanner – while the patient is standing, the quadriceps is active, and the knee is flexed to 30 degrees.  The hypothesis is that the average TT-TG offset distance measured by CBCT will be less than that measured on regular CT scan.  If the hypothesis is proven, this will have significant implications on defining abnormal TT-TG offset, and on calculating the amount of surgical correction when tibial tubercle transfer is the required procedure for patellar instability.

A second clinical study will evaluate the ability of weight bearing CT scan to more accurately image(2D and 3D) the ankle and tibiofibular syndesmosis.  This type of evaluation may be crucial to determining which ankle injuries are unstable, and allow clinicians to stabilize those that need surgery and avoid surgery on those that do not.

Other researchers at UB and Carestream are looking to validate computational measures of joint surface area of contact and joint surface pressure when obtained while subjects are weight bearing in the cone beam CT scanner.  This has the potential to become a non-invasive tool to evaluate a host of orthopedic conditions that affect the biomechanical behavior of the joints, such as arthritis, meniscus loss, instability, and malalignment syndromes.

Carestream OnSight 3D Extremity System received FDA 510(k) clearance in September 2016.

John M. Marzo, MD, is an associate professor of clinical orthopaedics at the State University of New York at Buffalo’s Jacobs School of Medicine & Biomedical Sciences. His research interests in the basic sciences relate to the biomechanics of the shoulder and the knee, with the most recent project sponsored by Carestream Health to investigate the advantages of a new prototype portable 3D cone beam CT scanner. He is also a member of Carestream’s Advisory Group, a collective of medical professionals that advises the company on healthcare IT trends.