December 2011

PET/CT for PFP: Not ready for prime time

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By Shalmali Pal

The latest research on patellofemoral pain (PFP) syndrome suggests that different mechanisms are responsible and treatments must be tailored accordingly. A new imaging study has drilled down into the source of PFP, offering evidence of a link between the syndrome, bone metabolic activity, and increased mechanical stress.

Christine Draper, MD, and colleagues from Stanford University performed imaging studies in 20 patients with chronic PFP using 18F-fluoride (18F-NaF) PET/CT (posi­tron em­ission tomography/computed tom­­o­graphy), a technique that can localize regions of elevated bone metabolic activity.

Many of the patients took part in athletic activities and some exercised at least five hours a week, Draper said. Some biomechanical issues were diagnosed, including patellar maltracking.

The research group reported increased radiotracer uptake indi­cative of heightened bone metabolic activity in 17 of the 20 painful knees analyzed. These regions of increased uptake corresponded to 15 patellae and four femoral trochleae; the average volume of the regions was 1 cm3.

Draper pointed out that the researchers did not evaluate other mechanisms of pain in the three knees that did not show radiotracer uptake. But some potential causes of pain that may not be revealed by a PET/CT scan include soft tissue abnormalities, such as retinacular overuse, and inflammation.

Overall, the normalized mean and peak standard uptake values (SUV) for the region of tracer uptake were greater in the 20 painful knees than the pain-free knees. The mean normalized SUV was 36% lower (p = 0.01) and the peak normalized SUV was 59% lower (p = 0.0006) in the pain-free knees than the painful knees. In 10 of 15 patients, the region of pain during palpation corresponded to the region of increased tracer uptake.

The authors concluded that increased metabolic activity may correlate with mechanical stress on sensory nerve fibers in the patella, particularly in the posterior patella, and could be the source of patellofemoral pain. The findings were e-published on August 2 by the Journal of Orthopedic Research (JOS).

Asked about the therapeutic implications of their results, Draper said treatments aimed at reducing mechanical stress at the joint may be most effective in this subset of PFP patients, but she stressed that these imaging results are preliminary.

“We have not yet studied the effects of treatments on bone metabolic activity,” she said.

Christopher Powers, PT, PhD, called the imaging results noteworthy for providing an explanation for why PFP occurs, but noted that the study doesn’t address the question of what causes the increase in metabolic activity.

“It’s a nice study that shows an objective measure of activity,” said Powers, director of the program in biokinesiology at the Los Angeles-based University of Southern Califor­nia. “[The researchers] did not try to relate these results to some type of physical issue; for example, mech­anical loading or a motion pattern. So we really don’t know what is causing metabolic activity and pain. As a treating clinician, I need to know that.”

But Draper said the current results do, to some degree, dovetail with a May 2009 JOS study that her group did on altered joint kinematics in PFP with and without a patellar brace or sleeve. Would the degree of metabolic activity in particular regions—the lateral patellar facet versus the medial patellar facet—help narrow down how the brace or sleeve could be used most effectively?

“Having a more complete understanding of the specific regions of the joint that are most likely to be affected could help dictate treatment options,” Draper said.

For instance, elevated bone activity on the lateral side of the joint could indicate excessive stress on the lateral side, perhaps due to a muscle imbalance or maltracking, she explained.

“The chosen treatments [in such cases] may differ from an individual with elevated bone activity that is more centrally located. In these patients a realignment brace may not provide as much benefit,” she said.

But whether all PFP patients are candidates for PET/CT imaging is still an open question. If other imaging modalities are unable to help diagnose the source of pain, PET/CT may be a viable option, Draper said.

Sponsored by an educational grant from medi USA

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