The Biomechanical Effects of Diabetic Neuropathy
The number of patients with diabetes continues to rise, as does the incidence of diabetic neuropathy. Diabetic peripheral neuropathy affects approximately 50% of all adult patients with diabetes and is associated with significant morbidity.1 Some of the effects of neuropathy include pain, foot ulceration, and increased risk of fall. All these morbidities are prominent issues within the podiatric field and topics that are frequently discussed due to the detrimental consequences they can have for our patients.
However, one aspect of diabetic neuropathy that is not frequently addressed directly is the biomechanical repercussions of neuropathy. The goal of this article is to delve into the literature regarding the biomechanical ramifications of neuropathy to better understand how to assess it, address it directly, and potentially prevent the more obvious sequelae of diabetic neuropathy.
Diabetic neuropathy affects the sensory, motor, and autonomic systems. Walking is a culmination of the sensory and motor components of the nervous system. The motor system’s role is apparent, and the sensory system allows for adjustment of the motor system to generate safe and effective movement. With a breakdown in both the motor and sensory aspects of walking, gait is greatly affected. Patients with diabetic neuropathy tended to walk slower than the comparison population in one study.2
Additionally, patients with diabetic neuropathy have greater variations in their gait. Specifically, Lalli et al found that patients with painful diabetic neuropathy had more varied step length and step velocity.3 This variation could lead to a potential increase in falls.3,4 The risk of falls is a major concern for patients and is something podiatric physicians are always considering when evaluating patients. Because of the potential detrimental effects of a fall, recognizing risk factors and educating the patient on limitations and safe ambulation are common practices.
What Is the Connection Between Diabetic Neuropathy and Joints?
Another area of the lower extremity greatly affected by diabetic neuropathy is the joints. Patients with diabetic neuropathy are typically found to have reduced joint movements. Knee and ankle joint movements are noted to be reduced in patients with neuropathy compared to patients without diabetes.5 At least 10–20 degrees of dorsiflexion and 40–55 degrees of plantarflexion are needed for normal ambulation.6 With a reduction in ankle range of motion, the plantar pressures on the foot increase and gait becomes altered.7 Two substantial concerns with increased pressures to the plantar aspect of the foot are ulceration and shearing forces. Being aware of the risks associated with limited motion in the ankle joint and subsequently addressing any identified abnormality could prevent serious diabetic neuropathy–related consequences.
To a similar point, the metatarsophalangeal joints (MTPJs) of the foot are also significantly affected by diabetic neuropathy. There is a substantial reduction in the range of motion at the MTPJs in patients with diabetes.8 The first MTPJ is especially reduced in patients who have a history of ulceration.9 Joint reduction and motor neuropathy can also cause deformity at the joints through muscle imbalance. The imbalance and deformity can lead to increased pressures in certain areas of the foot. When these deformities are combined with joint motion restriction throughout the foot, the concern for ulceration heightens.
Insights on Foot Structure, Strength, and Gait
Foot structure overall also is affected with neuropathy. Sacco et al noted a “crumbling” of the medial arch of the foot associated with diabetic neuropathy when compared to a control group.10 With a collapse of the medial arch, there can be devastating consequences on the gait and biomechanics in a patient with diabetes. Hastings et al noted that the medial arch collapse led to an increase in foot projection angle.11 This increase in the foot projection angle and medial column loading in patients with diabetes affects the soft tissue structures of the foot. The medial support structures of the foot are at higher risk for injury.11 With increased insult to these supporting structures, the deformity can increase, causing a repeating cycle of increasing foot deformity and pressures.
With regards to strength, patients with diabetes were noted to have weaker muscle strength than their counterparts without diabetes.11 Muscle atrophy due to lack of neural stimulation is typically first noted in the feet and small intrinsic muscles. As the disease progresses, the atrophy becomes notable in the larger muscles of the legs. The muscle fibers slowly begin to be infiltrated with fat on a histologic level and these infiltrations further affect muscle function.11
First, the patient’s endurance is affected. Patients with diabetes were shown by Allen et al to have less endurance than patients without diabetes.12 The theory behind this is that the lack of neuromuscular transmission may have caused this decrease in stamina. Patients with diabetes were also not able to contract their muscles as well, either evoked or voluntarily, with ankle dorsiflexion in a study by Allen et al.13 These changes reduce the function of the involved muscles and continue to affect gait and stability in this patient population.
In patients with neuropathy, it has been shown that the gait cycle itself changes. In one study, maximum support moment and midstance support moments were higher in the patients with diabetic neuropathy compared to controls.14 Additionally, the braking and propulsion forces were reduced in the group with diabetic neuropathy.15 Authors of this study suggest that the lack of muscle strength in these patient causes this redistribution of forces through the gait cycle, leading to an inability to control their motions through the gait cycle.
In Conclusion
Overall, the effects of diabetic neuropathy in patients can be devastating. While the ulceration, falls, and pain are often the most discussed effects, the biomechanical consequences of neuropathy alone cause altered gait and steadiness, affect foot function through joint and muscle deterioration, and transform the structure of the foot.
Often these are thought to just be a consequence of diabetic neuropathy. However, clinicians are encouraged to take a more active look at the biomechanical aspects of diabetic neuropathy. Instead of watching the foot change slowly over the years, take note of the changes and adjust treatment plans accordingly. Clinicians can educate the patient on their increased fall risk. Prescribing appropriate footwear to help support the muscles and structure of the foot is another consideration. Encouraging exercise, physical therapy, and other means of maintaining muscle strength and joint mobility is also wise. Taking an active role in the care of patients with diabetic neuropathy may help prevent some of the morbidities associated with neuropathy and diabetes.
Dr. Ansert is a Fellow at the University of Texas Southwestern Diabetic Limb Salvage Fellowship in Dallas, TX.
References
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