Achieving medial-lateral (M-L) stability for people with amputations who use a prosthesis is important. Common causes for M-L instability include improper prosthetic alignment; improper socket fit; uneven wear on the sole of the shoe; a loose foot in a shoe that is too big; or too much side-to-side motion, exhibited by the carbon fiber foot inside its foot shell. The first four of these causes of M-L instability can be addressed fairly directly. The last one is more difficult to correct.
As a prosthetist, I feel there is a design shortcoming in some carbon fiber feet with removable rubber foot shells that needs to be addressed by the manufacturers. There is far too much side-to-side motion in the M-L (coronal) plane between the foot and its shell, especially for feet that have been used extensively, and this motion extends proximally to the socket.
Whether our patients have transfemoral or transtibial amputations, their overall stability while using their prostheses starts from the ground up, similar to a foundation supporting a house. If the prosthetic foot is loose in the shoe, we can tighten the laces, and if that doesn’t work, we can place a foam filler in the shoe to tighten the fit.
But there are times when these interventions aren’t enough to stabilize the prosthesis in the coronal plane, and if you hold down the foot shell, you can still move the prosthesis side-to-side with ease—often to quite an alarming degree. When this occurs among my patients, in an attempt to reduce the amount of side-to-side pylon movement, I will place one or more pieces of 1/8-in to 1/4-in. rigid foam, either crepe or puff, between one or both sides of the carbon fiber foot and the inside of its rubber foot shell.
However, there must be a better way to address this issue. With that goal in mind, I extend a challenge to manufacturers of carbon fiber feet to develop an insert or filler that can be placed over or around the carbon fiber foot. With the insert strategically positioned inside the rubber foot shell, it would significantly reduce the ease with which many carbon fiber feet can rock side-to-side inside their foot shells during standing or ambulation.
I don’t know what materials would be most effective for such an insert; I don’t know what shape it should take; and I don’t know how it should be secured in place around the foot and inside the shell so that it doesn’t interfere with accessibility to setscrews. But I do know that whatever you—the manufacturers of prosthetic feet— develop, it will be a major improvement over wedging pieces of rigid foam into one or both sides of a foot shell that eventually shift out of place or work their way completely out of the foot shell. Having access to inserts specifically designed for such a purpose would benefit prosthetists and people who use prostheses.
If other prosthetists or foot manufacturers think I’m missing an obvious fix, I encourage you to contact me with your solutions.
Peter Seaman, CLP, provides compassionate patient care at Master’s Orthotics & Prosthetics, Silverdale, Washington. He can be reached at .
