Study Probes the Effect of CAM Boots on Ankle and Hindfoot Joint Mechanics

Editor’s note: The following content was originally scheduled for the AAOS Now Daily Edition, which publishes each year onsite at the AAOS Annual Meeting, but this year’s meeting in March was canceled due to COVID-19. Despite the cancellation, members can access virtual content from the Annual Meeting by visiting the Academy’s Annual Meeting Virtual Experience webpage at aaos.org/VirtualAAOS2020.

A study assessing the effect of the tall controlled ankle motion (CAM) boot and short CAM boot on contact pressures of the ankle, subtalar, talonavicular, and calcaneocuboid joints found that immobilization in a tall CAM boot significantly reduced the peak contact pressures of the ankle and hindfoot to a greater degree than the short CAM boot. In addition, the greatest decrease in peak contact pressures was seen in the subtalar and talonavicular joints. The study was presented as part of the Annual Meeting Virtual Experience.

Eight lower-extremity cadaver specimens were mounted on a servohydraulic test frame. The specimens were loaded to 700 N at a cyclical frequency of 1 Hz with the posterior tibial, peroneus longus, peroneus brevis, flexor hallucis longus, flexor digitorum longus, and Achilles tendon physiologically tensioned. Pressure sensors were placed in the ankle, subtalar, talonavicular, and calcaneocuboid joints. In the sagittal plane, the specimens were loaded on a neutral surface, followed by 20 degrees of dorsiflexion. Each specimen served as its own control, with contact pressures measured with no immobilization (control), followed by placement in a short CAM boot and tall CAM boot. In addition, contact pressures in the immobilized limbs were measured at muscle loads both equal to and half of the load applied to the control to account for decreased muscle activation during immobilization.

There was no difference in the average and peak contact pressures of the ankle, subtalar, talonavicular, and calcaneocuboid joints when the researchers compared the short CAM boot to no immobilization at equal tendon loads. The tall CAM boot significantly decreased average and peak contact pressures of the ankle, subtalar, and talonavicular joints compared to no immobilization. The reduction in contact pressures was accentuated when the load applied to the tendons was decreased in accordance with diminished muscle activation during immobilization. Neither immobilization device decreased the contact pressures of the calcaneocuboid joint at equal tendon loads. Neither CAM boot changed the location of the center of pressure in any joint.

The lead author of the study, Niall A. Smyth, MD, an orthopaedic surgeon specializing in foot and ankle disorders at the Cleveland Clinic in Florida, he said he was inspired to undertake the research after a conversation with Helene Simpson, a physiotherapist in Cape Town, South Africa. “She brought up a significant deficiency in the orthopaedic literature—that we have no idea how CAM boots affect the intra-articular pressures of the foot and ankle,” Dr. Smyth said. “This is despite their widespread use as tools for protection and rehabilitation.”

As expected, the tall CAM boot significantly reduced the peak contact pressures of the ankle and hindfoot to a greater degree than the short CAM boot, he said. However, the CAM boots offloaded the ankle joint less than he and his colleagues expected. The short CAM boot minimally affected the peak contact pressures of the ankle, whereas the tall CAM boot allowed for an approximate 10 percent to 15 percent decrease in contact pressures of the ankle joint. In addition, neither the tall nor short CAM boot affected the contact pressures of the calcaneocuboid joint appreciably. The other surprising finding, he said, was that the CAM boots did not shift the center of pressure of any joint studied.

The clinical takeaway, Dr. Smyth said, is, “If the purpose of using a CAM boot is to offload the ankle or hindfoot joints, then a tall CAM boot should be used preferentially over a short CAM boot. Furthermore, with regard to the ankle joint, there is not a clinically drastic decrease in contact pressure with the use of a CAM boot. Therefore, clinical relief of pain may be more attributable to restricting range of motion rather than offloading the joint itself. The opposite may be true of the subtalar and talonavicular joints, which experience a significant decrease in contact pressure when loaded in a CAM boot.”

The next step in exploration to determine the effect of CAM boots on foot and ankle joint mechanics is to perform the study using a robotic gait simulator. “This would allow for the assessment of contact pressures in a dynamic setting with greater clinical application,” he said.

A limitation of the study is that it was performed with axial loaded specimens with the foot in neutral and 20 degrees of dorsiflexion. He said, “This does not mimic true gait and therefore may not directly reflect the contact pressures of patients who use the CAM boot while ambulating.”

Dr. Smyth’s coauthors of “The Effect of Controlled Ankle Motion Boots on Contact Pressures of the Ankle and Hindfoot,” are Pooyan Abbasi, MSc; Cesar De Cesar Netto, MD, PhD; Nicholas Casscells, MD; Stuart M. Michnick, MD; Brent G. Parks, PhD; and Lew C. Schon, MD. The poster and abstract for this study (PO666) can be viewed at https://aaos.scientificposters.com/index.cfm?k=7gsrp544tf.

Terry Stanton is the senior medical writer for AAOS Now. He can be reached at tstanton@aaos.org.