SAN DIEGO – Lateral wedge shoe insoles reduce the gait load on the inside of the knee but fail to lessen pain long term in patients with medial knee osteoarthritis.
A British research team thinks it may have solved the mystery. It seems the inserts work only in the subset of patients who have flatter feet and who plant their heels more squarely when they walk, rather than rolling their foot to the outside, said lead investigator Graham Chapman, Ph.D., at the World Congress on Osteoarthritis.
Previous studies (BMJ 2011;342:d2912) seem to have "grouped everyone together and assumed they are going to respond to wearing a lateral wedge," said Dr. Chapman, a research fellow in biomechanics at the University of Salford (England).
That was not the case when he and his colleagues analyzed 33 patients who had medial knee osteoarthritis (OA) and a Kellgren-Lawrence grade 2 or 3. Their mean age was 59 years, their mean body mass index was 32.2 kg/m2, and 42% (14) were women.
Patients "who walked more on the lateral hindfoot and those with pes cavus [high arch] were unlikely to respond to lateral wedges."
In 13 (39%) of the patients, the inserts did not help. Their use actually increased the load on the inside of the knee, as measured by the external knee adduction moment. In the remaining patients, use of the 5-degree lateral wedge insoles – which look much like any shoe insole except for a "fat bit on outside," Dr. Chapman said – reduced the adduction moment by a mean of 4.1%.
The researchers next looked to see how those who did not benefit from the insoles differed from those who did benefit.
The 13 patients who did not benefit were likely to have more contact on their lateral heel as they walked (about 19 cm2 vs. about 17.5 cm2) and higher medial arches (with mean subarch angles of about 104 degrees vs. about 111 degrees in those who benefited from the inserts). The findings were statistically significant.
Patients "who walked more on the lateral hindfoot and those with pes cavus [high arch] were unlikely to respond to lateral wedges. Excluding persons whose foot dynamics make them unlikely to respond to insoles may leave a large group of patients who can experience their potential therapeutic benefits," the researchers concluded in their abstract.
The next step is a randomized trial to see if people who fit the profile of responders truly do have less pain when they wear shoes with lateral wedges rather than control shoes.
The ultimate goal is to help clinicians predict who will benefit from the insoles, Dr. Chapman said at the meeting, which was sponsored by the Osteoarthritis Research Society International.
Dr. Chapman said he has no disclosures. The work was funded by Arthritis Research UK.