July 2009

OA research: It’s all about the shoes

Laterally wedged orthoses are effective to a point, but the next big thing in knee osteoarthritis treatment just might be a prototype shoe with a sole that notable for its variable stiffness rather than its slope.

grandfather and grandaughter walking

iStockphoto.com/Gary Sludden

Osteoarthritis (OA) of the knee is a degenerative joint disease that afflicts approximately 30% of Americans over the age of 65.1,2 The medial compartment of the knee the most commonly affected area of the joint.3,4 The high prevalence of medial compartment knee OA is due in part to the high percentage of loading transmitted across the medial aspect of the knee, approximately 60% to 80% of the total load.5-7 Knee OA is associated with pain and major limitations in a patient’s mobility, and thus may not only restrict a patient’s activities of daily living but may also have a major impact on his or her social life and psychological health.8 The prevalence of the disease is only expected to increase in the coming years as the baby-boomer generation ages, and it is anticipated that by 2030 more than 20% of the American population will be over the age of 65 and at risk of developing this debilitating disease.9

Current therapies for knee OA aim to attenuate pain, improve function, and reduce the risk of further joint deterioration, while minimizing adverse effects of treatment.10 Surgical interventions, including high tibial osteotomy and total knee arthroplasty, are often reserved for advanced cases. High tibial osteotomy realigns the static position of the knee to alter load distribution,11,12 and, although often effective in reducing loads on the medial compartment, is a highly invasive procedure with a long recovery period and associated complications including deep vein thrombosis, infection, technical complications, and nonunion requiring further surgery.13 Total knee replacement is an effective medical intervention that can relieve pain and disability of end-stage osteoarthritis. However, this procedure is highly invasive, is reserved for severe cases, and has limited durability in patients who are likely to outlive their implants (which last approximately 20 years) or those who wish to resume demanding activities.10 In earlier stages of the disease when non-surgical treatments are desirable, drug therapies may be used. Pharmacologic treatment, though effective in reducing pain, does not stop the degenerative disease process of OA, and is also associated with adverse effects including gastrointestinal or renal toxicity.14 There is a clear need for effective non-invasive treatment of knee OA without the potential harmful side-effects of drug therapies.

Figure 1: Intervention shoe used in the study. The shoe sole was approximately 1.3-1.5 times stiffer on the lateral aspect than the medial aspect, with the medial aspect of the shoe similar to the control (C) shoe sole stiffness.

Figure 1: Intervention shoe used in the study. The shoe sole was approximately 1.3-1.5 times stiffer on the lateral aspect than the medial aspect, with the medial aspect of the shoe similar to the control (C) shoe sole stiffness.

An important parameter to consider in joint deterioration in knee OA is the knee adduction moment.7 In healthy individuals, the load transferred through the medial compartment of the knee during walking is nearly 2.5 times greater than the load transferred through the lateral compartment.7 Analytical methods have shown that an increase in the medial compartment joint loading is related to an increase in the peak external adduction moment about the knee.7,15,16 This peak knee adduction moment has been associated with the severity, rate of progression, and treatment outcome of medial compartment knee osteoarthritis.17-19 Reducing the knee adduction moment, therefore, is a key objective of new treatments designed to slow the progression of knee OA.

Offloading options
Conservative treatments studied for knee OA have included unloader braces, laterally-wedged insoles, and footwear. Valgus bracing is used to reduce the load on the medial compartment of the knee by application of an opposing external valgus moment about the knee joint.20 Valgus bracing has been found to reduce the knee adduction moment in patients with medial compartment knee OA by approximately 12%.20,21 Several studies have also investigated clinical results, and found improvements in both pain relief and physical function with the use of bracing.22,23 However, the studies also found that many patients do not adhere to bracing treatment in the long term. In one study, more than 40% of patients assigned to the bracing intervention discontinued brace use after one year, with the main reasons being no effect of treatment, skin irritation, and poor fit.22 A review of custom-fitted load-shifting braces also found a 24% failure rate after one year of treatment, increasing to nearly a 40% failure rate after three years of treatment, with failures including reluctance to use the brace due to bulk and inconvenience, leg swelling, skin irritation, and pulmonary embolus.24 Thus, the side effects of braces may limit their applicability and must be weighed when considering long-term patient use.

Sasaki and Yasuda first reported the potential of a laterally-wedged insole to reduce medial compartment knee joint loading.25,26 More recent studies of lateral wedging in both healthy15,27 and osteoarthritic28-32 subjects have reported varying results in adduction moment reduction. In healthy subjects, a study of 5o wedges found reductions in average peak knee adduction moment of approximately 7%.15 However, another study in a healthy population found reductions in the adduction moment only when using a rigid ankle-foot-orthosis in combination with a lateral wedge, and not with a lateral wedge alone.27 In an osteoarthritic population28 a 5o lateral wedge failed to produce reductions in peak knee adduction moment. Another study reported reductions in the adduction moment of 6% with a 5o lateral wedge and 8% with a more severe 10o lateral wedge.29 However, the more severe lateral wedges were associated with discomfort, which may limit their long-term clinical applicability.29,33

Figure 2: Depiction of the difference in stiffness between the lateral (stiffer) and medial (less stiff) aspects of the variable-stiffness intervention shoe sole.

Figure 2: Depiction of the difference in stiffness between the lateral (stiffer) and medial (less stiff) aspects of the variable-stiffness intervention shoe sole.

The clinical efficacy of lateral wedges also remains unclear. There have been mixed reports on improvement in pain and function with use of lateral wedges. Several studies have found improvements when using wedges and analgesics compared analgesics alone25,34 or when only investigating the effect of the lateral wedge in osteoarthritic subjects.33,35 However, several more recent studies comparing lateral-wedged insoles with neutral insoles failed to find significant differences between the two  conditions with regard to reduced pain.36-38

Gotta be the shoes
The limited clinical success of wedges and braces led to the investigation of a novel load-modifying shoe intervention for treatment of medial compartment OA of the knee. The variable-stiffness shoe is a normally-appearing athletic shoe (Figures 1,2) with a sole that is not wedged but has been custom-designed so that the lateral aspect has greater stiffness than the medial aspect. By comparison, conventional motion-control athletic shoes may have soles of varying stiffness, but such that the medial aspect is stiffer than the lateral aspect; neutral-cushion athletic shoes and conventional dress shoes typically have constant-stiffness soles.

The variable-stiffness sole of the test shoe is made of compression molded ethylene vinyl acetate, and the lateral sole (Asker C durometer 55 ± 2) is 1.3-1.5 times stiffer than the medial sole (Asker C durometer 70-76 ± 2). The shoe design was first tested in a healthy cohort.39 Results from the study were promising, reporting an average reduction in the peak knee adduction moment during gait of approximately 7% without reports of subject discomfort.39 The findings in the healthy population spurred investigation of the shoe design in the target population, individuals with medial compartment knee osteoarthritis.

A one-year prospective randomized control study was used to compare the effects of the variable-stiffness intervention footwear with constant-stiffness control footwear in an osteoarthritic population. Seventy-nine individuals (42 male and 37 female; average age: 60 ± 10 years) with symptoms of medial compartment knee OA participated in the study after giving written consent in accordance with the Institutional Review Board. Subject enrollment criteria are given in Table 1. At a baseline visit, subjects were randomly and blindly assigned to either the variable-stiffness intervention shoe or the constant-stiffness placebo control shoe to wear as their main walking shoe for 12 months (defined as at least four hours of wear per day).  The subjects were encouraged to wear the shoe as much as possible.

Figure 3: Absolute reductions in peak knee adduction moment with the intervention shoe versus the control shoe at slow, normal, and fast walking speeds for all subjects (n=79) at the baseline gait test. * = significant reduction at p<0.01.

Figure 3: Absolute reductions in peak knee adduction moment with the intervention shoe versus the control shoe at slow, normal, and fast walking speeds for all subjects (n=79) at the baseline gait test. * = significant reduction at p<0.01.

The overall goal of the study was to evaluate the effectiveness of variable-stiffness shoes in treatment of subjects with symptoms of medial compartment knee osteoarthritis. Specifically, the study aimed to understand (1) the effects of the variable-stiffness shoes on immediate and long-term changes in peak knee adduction moment compared to a constant-stiffness control shoe and (2) the effects of the variable-stiffness shoes on pain and function after long-term wear.

Knee loading results
Gait analysis was performed at the baseline test using a method previously described41 to assess joint loading of the subjects’ more symptomatic leg. As reported earlier,40 at the baseline gait test the peak knee adduction moment was significantly lower in the variable-stiffness intervention shoe than in the constant-stiffness control shoe at slow, normal, and fast walking speeds (Figure 3; p<0.01). The average between-condition difference in knee adduction moment increased significantly (p<0.001) with increasing walking speed, from 2.4% (p<0.01) at the slow speed, to 5.7% at normal speed (p<0.001), and 6.2% at the fast speed (p<0.001).

The magnitude of change in the peak knee adduction moment with the intervention shoe, however, varied among patients. While the majority of patients (more than 80%) had reductions in adduction moment with the intervention shoe compared to the control shoe at the baseline gait test, the change in peak knee adduction moment  ranged from more than a 20% reduction to a 7% increase at the normal walking speed.40 Importantly, reductions in the knee adduction moment at this baseline test occurred without overloading of other lower extremity joints, with secondary changes in frontal plane moments at the hip, knee, and ankle primarily showing reductions.40

The reductions in knee adduction moment seen in this study are in good agreement with reductions of approximately 5% to 6% reported using 5o and 6o laterally-wedged insoles.29,31 The reductions are less than those reported following high tibial osteotomy, where reductions in the knee adduction moment have been reported to range from approximately 19%42 to 30%11. However, given the invasiveness of high tibial osteotomy, as well as the associated complications,13 the variable-stiffness shoe may provide a suitable early intervention for patients with earlier-stage medial compartment knee OA, before surgical procedures are necessary. A 2002 study by Miyazaki et al.17 found that the risk of progression of medial compartment knee osteoarthritis increases more than six-fold with a one-unit increase in the knee adduction moment. Thus, reductions found in this study may be large enough to slow the progression of the disease.

The increase in reduction in the knee adduction moment with increasing speed may suggest that at faster speeds, greater forces are placed upon the variable-stiffness shoe, causing a greater differential displacement in the shoe sole. The intervention shoe may therefore be thought of as a dynamic lateral wedge.40 The magnitude of the change in adduction moment with the intervention shoe at the baseline gait test did vary substantially among subjects, and future research is needed to examine subject specific characteristics, such as disease severity, which may affect patient response to the shoe.42 Subject specific shoe stiffness variations may improve reductions in adduction moment, and could be considered in a future design iteration.

The baseline testing results are promising, suggesting that the intervention shoe may be an effective non-invasive treatment for medial compartment knee OA. Testing after six and 12 months of shoe wear also demonstrates that the reduction in adduction moment with the intervention shoe is maintained.42-45 These results are important, as they demonstrate that the intervention shoe is suitable for long-term use to reduce loading on the affected compartment of the knee joint.

Pain and function outcomes
To assess pain and physical function at the baseline test and after six and 12 months of shoe wear, the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) was used.46 The WOMAC consists of five questions on pain, two questions about joint stiffness, and 17 questions about function. A low score indicates little pain/best function; a high score indicates more pain/worse function.

At the six month time point, significant reductions in total WOMAC score (indicating improvements in pain and function) were seen for the intervention group, but not for the group wearing the control shoe.44 At the 12 month time point, reductions in total WOMAC score for the intervention group continued, indicating sustained improvements in pain and function, while the control group had no significant improvements in pain or function.43 Furthermore, analysis by disease severity suggested that the intervention shoe may have the greatest effect on pain, function and knee adduction moment in patients with early medial compartment knee OA.42

Conclusion

The results of this study suggest that alterations in the properties of the sole material of a shoe can reduce a specific aspect of joint loading related to increased risk of OA progression. The improvements in pain and function, in combination with the reduction in knee adduction moment, suggest the variable-stiffness intervention shoes may be an inexpensive, low-risk treatment for medial compartment knee osteoarthritis.  This intervention may have broad applicability to delay the onset or slow the progression of this debilitating disease, allowing osteoarthritis patients to remain active even as they age

Jennifer C. Erhart, PhD, is a post-doctoral fellow at the Stanford University Biomotion Laboratory in the department of mechanical engineering.

Nicholas J. Giori, MD, PhD, is chief of orthopedic surgery and acting co-director of the Bone and Joint Rehabilitation Research and Development Center at the VA Palo Alto Healthcare System, and an assistant professor of orthopedic surgery and mechanical engineering at Stanford University.

Thomas P. Andriacchi, PhD is a professor of mechanical engineering and orthopedic surgery at Stanford University and acting co-director of the Bone and Joint Rehabilitation Research and Development Center at the VA Palo Alto Healthcare System.

None of the authors had any conflicts of interest regarding this manuscript.

Acknowledgements

This work was supported by the Veterans Administration (VA A02-2577R).  The authors would like to thank Chris Dyrby, Anne Mündermann, and Barbara Elspas for assistance in data collection and valuable feedback on this project, and Nike Inc. for providing the intervention shoes used in this study.

References

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Table 1:  Subject inclusion and exclusion criteria

Inclusion Criteria

  • Persistent medial compartment knee joint pain
  • Ambulatory without aids. Between 40 and 80 years of age
  • Ability to give informed consent

Exclusion Criteria

  • Nerve or muscle disease associated with walking difficulty
  • Serious injury to foot, ankle, back, or hips. Use of shoe insert or hinged knee brace
  • Gout or recurrent pseudogout
  • Total knee replacement
  • Narcotic pain medication use
  • Intraarticular joint injection in previous 2 months
  • Body Mass Index (BMI) > 35 kg/m2
  • Diagnosed or symptomatic osteoarthritis in other lower extremity joints
  • Inability to have an MRI scan

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