Understanding How The Achilles Tendon Affects Plantar Pressure
Approximately 23.6 million people in the United States have diabetes, according to the 2007 statistics from the American Diabetes Association.1 Many of these patients have an associated comorbidity of obesity and, all too often, Achilles tendon contracture.
As we age, the tendon naturally tightens. However, diabetes exacerbates this tightening process as increased blood sugar levels deposit glucose in the collagen of the tendon, greatly reducing its elasticity. Obesity can also contribute to the tightening of the posterior muscle group, which includes the Achilles tendon. The glycation effect with the Achilles tendon also occurs in the plantar fascia and other tendinous tissues.
In one study, researchers in Brazil assessed ultrasound studies of the Achilles tendon of patients with diabetes in comparison to those of patients without diabetes. They found disorganized tendon fibers in 89 percent of patients with diabetes (62 of 70) and calcification of the tendon in 76 percent (53 of 70) of the patients with diabetes.2
The study also indicates a correlation between the duration of diabetes and tendon disorganization. The oldest patients in the study also appeared to have a greater prevalence of Achilles tendinopathy.2
In a similar study in 2006, Orendurff, et al., revealed via electron micrographs alterations in the collagen tissue in tendons that are likely related to the non-enzymatic glycolation of the tissue by advanced glycolytic end products.3
Both of these studies demonstrate the biologic effects in the tendon, including changes in the inherent stiffness in the tendon, which subsequently leads to increased forefoot pressures.
A Closer Look At Pressure Distribution
Increased stiffness in the tissue of the Achilles tendon may result in joint contracture, causing an earlier heel off and increased pressure to the forefoot for a longer period of time.
Accordingly, the Achilles tendon plays a significant role in the formation of calluses and ulcers in the forefoot. The Achilles in some cases can act as an impediment to healing ulcers with increased forefoot load and decreased ankle dorsiflexion.
In a large multicenter prospective study involving 248 individuals with diabetes, Caselli, et al., specifically looked at the forefoot to rearfoot ratio of plantar pressures and found that both of these pressures are increased in diabetic neuropathic feet.4
Caselli claims that while other studies had looked at forefoot pressures, this study was the first to address the increased peak pressure present at the rearfoot in patients with diabetic neuropathy in comparison to non-diabetic patients. The authors concluded that the forefoot to rearfoot peak pressure ratio is increased only in patients with severe diabetic neuropathy.4
This imbalance in the pressure distribution (forefoot to rearfoot peak pressure ratio) occurs with increasing degrees of neuropathy. Equinus and muscle weakness develop in later stages of peripheral neuropathy and forefoot ulceration may result.5
Abnormal gait biomechanics with neuropathy and intrinsic muscle loss also contribute to increased plantar forefoot pressures. When shear and vertical forces exert pressure on the skin overlying bony prominences, callus formation occurs. This callus increases pressure on underlying tissues. Removing callus under bony prominences in the forefoot can reduce plantar pressure by 26 percent.6
What The Literature Reveals About Achilles Tendon Lengthening
Achilles tendon lengthening, which is aimed at reducing mechanical loads to the plantar surface of the foot, can be beneficial for patients with diabetes. Researchers have found this procedure to be effective in reducing forefoot pressure by 27 percent.3