A Closer Look At Gait Analysis In Patients With Diabetes
Plantar shear can cause calluses and recent studies have implicated it in the development of foot ulcers. Yavuz and coworkers found plantar shear values in patients with diabetes were significantly higher than in healthy controls.17 Also, for the majority of patients with diabetes, the sites of peak shear did not match the sites of peak plantar pressure. Lott and colleagues also reported increased maximal shear stress for patients with diabetic neuropathy and a history of ulceration.18
Elevated plantar pressure is a moderate risk factor for ulceration in patients with diabetic neuropathy yet abnormal foot pressures alone do not seem to cause ulceration.11 A study by Masson and colleagues comparing the frequency of elevated plantar pressures in patients with diabetes and patients with rheumatoid arthritis found a similar frequency of elevated plantar pressures for both groups.19 About one-third of the patients in the diabetes group had a history of ulceration in comparison with none in the rheumatoid group. Furthermore, I have personally observed that patients with normal plantar pressures can still get an ulcer while patients with elevated plantar pressures may not ulcerate.
Attempts to identify a peak pressure threshold to predict diabetic ulcers based solely on plantar pressure have been disappointing. There is no agreed threshold value at this time.20 Armstrong and colleagues did not find an optimal cutoff for peak pressure in patients with diabetic ulceration and another study by Lavery and colleagues observed a sensitivity of 64 percent and specificity of 46 percent using an 87.5 N/cm2 optimal cutoff value.21,22 Furthermore, Veves and coworkers reported that only 38 percent of peak pressure locations matched the area of ulceration for their patients with diabetes.23 They also observed that the location of peak pressure changed in 59 percent of patients over 30 months’ follow-up.
Historically speaking, we tend to view any treatment that lowers peak plantar pressures as good and increased plantar pressures as bad. For example, we commonly accept that diabetic shoe insoles can lower peak pressure by distributing pressure over an increased area. However, peak pressures might not be that cut and dry. What about patients with diabetes who increase their walking speed after a treatment intervention like an ankle foot orthotic (AFO) or Charcot foot reconstruction? As their speed increases, forces on the foot increase and so should peak plantar pressures. Is peak pressure as detrimental as we once thought or should we be looking at other gait parameters?
Although other gait parameters like pressure-time integrals and pressure gradients have also received study in patients with diabetes, perhaps we should also consider parameters independent of walking speed. Najafi and coworkers explored the use of an alternative walking parameter independent of gait speed called the regression factor.20 The regression factor tries to represent the similarity of the actual pressure distribution with a normal distribution using a timescale normalization scheme. Regression factor values range from -1.0 to +1.0 and as the value increases positively, the patient is walking more normally. In their study of post-foot reconstruction Charcot patients, Najafi and coworkers found regression factors increased postoperatively, suggesting a transition to more normal plantar pressure distribution. Interestingly, the regression factor did not change significantly when the study participants increased their average speed by 29 percent but peak plantar pressure increased by 8 percent.