Most conservative measures to reduce knee adduction moment in patients with knee osteoarthritis involve altering proximal biomechanics, but research suggests that similar kinetic changes and pain relief can be achieved with the use of wedged foot orthoses.
By Emily Delzell
Lateral, or valgus, wedge orthoses have been proposed as a low-cost, low-risk intervention for medial compartment knee osteoarthritis (OA), and in many patients produce immediate relief of knee pain.1 With few conservative strategies for management of knee OA that are effective, affordable, and acceptable to most patients, wedges as an intervention may be attractive to both practitioners and the individuals they treat.
Among other conservative interventions, practitioners say braces are cumbersome and often go unworn, weight loss and regular exercise are difficult for patients to achieve, altered gait strategies can feel unnatural, and chronic use of prescription or over-the-counter analgesics can lead to adverse side effects.
“Lateral wedge insoles offer a good management solution for knee OA, most particularly in the early stage of OA,” said Neil Reeves, PhD, an investigator at the Manchester Metropolitan University Institute for Biomedical Research into Human Movement and Health in the UK, and author of a 2011 review2 on conservative biomechanical strategies for knee OA.
“Perhaps one of the most important features of this counter-measure to OA is that [wedge insoles] provide one of the most minimal forms of intervention for patients,” Reeves said. “Wedges can be worn inside the shoe without the need for the patient to purposely alter their gait strategy, as they would with toe-out gait, for example, and without them having to wear a brace that might be fitted on the outside of clothing.”
The mechanism proposed for the pain relief conferred by the wedges is a reduction in knee adduction moment (KAM), which investigators use as biomechanical surrogate for the compressive forces acting on the medial compartment of the knee.2 High KAM is associated with increased loads across the medial compartment of the knee, and studies have shown that people with knee OA have higher KAM than their healthy peers.3
Numerous studies1,4-7 have shown the application of lateral wedges decreases peak KAM during walking, and the 2011 review by Reeves et al found that lateral wedges with inclinations between 5˚ and 15˚ resulted in peak KAM reductions between 4% and 14%.2
“One of the most important things to understand about the use of lateral wedge insoles is that they cause a shift in the direction of the ground reaction force [GRF] as people walk and it is through this mechanism that they alter the loading on the knee and cause a reduction in loading of the medial knee compartment,” Reeves said.
Although the wedges’ effect on KAM reduction is well-established, a randomized controlled trial (RCT) of the wedges in people with knee OA failed to show improvements in self-reported activity levels and WOMAC (Western Ontario and McMaster Universities Arthritis Index) subscales of pain, stiffness, and function at six months8 and two years.9 These studies, however, did find a statistically significant reduction in nonsteroidal anti-inflammatory drug intake in the wedged-insole group compared with the control group that wore neutral insoles.
A recent RCT conducted in The Netherlands comparing the use of a valgus brace with a full-length 10-mm (6°) lateral wedge insole found that both interventions produced improvements in pain and knee function in individuals with medial compartment knee OA.10 Investigators defined response to treatment as an improvement in pain and knee function of 20% or more.
Of the 45 individuals in the wedged insole group, 20% met the criteria for response, which was statistically equivalent to the number of responders in the braced group (n = 46). Neither intervention achieved correction of frontal plane knee varus misalignment.
The Dutch researchers reported better compliance among participants who wore the wedge compared with braced participants; device wear time in the insole group was a mean of eight hours compared with 5.5 hours for the brace. In addition, subgroup analysis showed a slightly better effect for the insole than for the brace among participants with mild disease.10
In patients with knee OA, the magnitude of reduction in KAM associated with wedge insoles is related to disease severity. A 2006 study by Shimada et al, for example, found that lateral wedge insoles reduced KAM in patients with early stage medial compartment knee OA, but not in those with more advanced disease.11
The investigators classified radiographic disease severity with the Kellgren-Lawrence grading system in which grade I is possible osteophytic lipping; grade II, definite osteophytes and possible joint space narrowing; grade III, moderate and/or multiple osteophytes, definite joint space narrowing, some sclerosis, and possible bony attrition; and grade IV, large osteophytes, marked joint space narrowing, severe sclerosis, and definite bony attrition.
Using this system, Shimada et al categorized 11 knees as grade I, 11 as grade II, 13 as grade III, and 11 as grade IV. Only patients with Kellgren-Lawrence grades I and II experienced significantly decreased peak adduction moments with application of the wedges. The authors concluded that lateral wedge orthoses are appropriate only for patients with early and mild knee OA.
Other research12 suggests, and the clinicians LER spoke with who use lateral wedges in their practice agreed, that the insoles are of no value in patients with severe knee OA. Wedges also need to be reserved for patients with knee problems of mechanical origin.
“Before applying lateral wedges one must also be sure of the diagnosis,” said Paul Scherer, DPM, clinical professor at the College of Podiatric Medicine at Western University of Health Sciences in Pomona, CA, and founder of ProLab Orthotics in Napa, CA.
“Many of my referrals come from primary care physicians, who tend to categorize most knee pain as a mechanical issue. So part of my job is doing a good differential diagnosis, ruling out issues such as autoimmune disease or pseudogout. If you use mechanical solution like an orthotic or a wedge for a problem that doesn’t have a mechanical origin, you’re doomed to failure,” Scherer said.
Once the diagnosis is confirmed, practitioners need to carefully choose patients who will benefit, said Kevin A. Kirby, DPM, MS, who is in private practice in Sacramento, CA. Both Scherer and Kirby noted that though medial compartment OA is by far the most common form of osteoarthritis of the knee, lateral compartment knee OA can also be treated with wedges—in those cases, a varus wedge.
“People who seem to do the best with lateral wedges are those without a large varus deformity,” Kirby said. “If the knee deformity is more than 15 degrees, I don’t think any wedge will produce benefits. I will try wedges in patients with up to 10 degrees of varus to see what kind of relief they experience. Sometimes that reduction is 10 percent, sometimes it’s almost 100 percent, and people call the next day [after fitting] to say the wedge has relieved pain they’ve had for years.”
Recent research suggests that whether a patient experiences immediate pain relief with the application of wedges may be a useful way to predict if he or she will benefit in the longer term.
A 2008 study by Hinman et al1 looked at immediate and three-month clinical outcomes of the use of lateral wedges in people with medial knee OA. Investigators found that participants who reported greater immediate pain relief than their counterparts also reported less pain with walking at three months. Another predictor of better three-month outcomes was an immediate reduction in peak adduction moment, which was experienced to some degree by most, but not all, participants.
In the 40 volunteers in Hinman and colleagues’ study, the magnitude of immediate KAM reduction ranged from 0.1% to 18.2%, while five participants demonstrated an increase. Greater reduction in adduction moment was associated with less functional impairment at three months. Disease severity was also predictive of outcome, and those with more advanced disease tended to report more pain at three months.
Questions of design
In his practice, Scherer said, there is trend toward use of a lateral skive within a custom orthosis. With the skive, the plantar lateral heel area of the positive cast is flattened to create a valgus wedge effect within the heel cup. This causes the center of the force that the orthosis is applying to the heel to shift laterally, increases pronatory torque, and acts to limit inversion or encourage eversion of the rearfoot on the orthosis, Scherer said.
“The lateral skive is an intrinsic method of changing the orthotic itself by raising the lateral side of the heel cup of the orthotic, which puts increased ground reaction force on the lateral side of the foot in various degrees,” Scherer said. “In my lab it is used most commonly for relief of medial knee pain.”
Scherer says adding the skive to a custom orthotic device eliminates some of the difficulties patients may experience while walking with a wedge.
“When you swing your foot forward and the lateral side of your shoe is thicker than the medial side, there is the possibility of catching your foot on the ground. This resolves those problems,” he said.
Kirby, who pioneered the medial heel skive and lateral heel skive techniques in 199013 said he uses the lateral heel skive if he plans to use a custom foot orthosis for the patient, but did not agree that it is overtaking the simpler wedge in use.
“For certain individuals with milder conditions, the valgus rearfoot or valgus forefoot wedge added to the insole of the shoe is all they need,” he said. “For more complicated cases in which the patient may have a medial knee osteoarthritis and a pronated foot, a custom foot orthosis with a lateral heel skive and forefoot valgus extension would be a better treatment than a simple valgus shoe wedge.”
For patients with medial knee OA who can benefit from a custom orthosis, Kirby commonly uses a 3/16-in polypropylene orthosis along with a 2- to 3-mm lateral heel skive, a 3° to 5° everted balancing position, and a flat rearfoot post and a valgus forefoot extension to shift GRF laterally on the plantar aspect of the foot and decrease medial knee compartment pain during daily activities.
Anecdotally, he noted that these custom orthotic devices have effectively reduced pain and increased overall mobility in many of his patients.
Kirby and others2,6 also emphasize the potential importance of using a full-length wedged insole versus one in which the wedge is under the heel alone.
“If you’re treating medial compartment OA, you want the wedge to go from the lateral heel up to the digital sulcus so that the center of pressure underneath both the heel and forefoot can be redirected laterally. In this way, you’re diminishing not only the heel contact but also the maximum forces occurring in the knee,” he said. “I know there are some studies that have looked at heel wedges alone and they were found to be ineffective. In my clinical experience, the full-length insoles seem to work better because I think there is more surface area and that will prolong the shift in center of pressure laterally and help take away the heavy load on the medial compartment.”
RCTs of lateral rearfoot wedge insoles for knee OA have failed to show effects on symptoms or, over time, on joint space narrowing,8,9 and biomechanical studies14-15 of rearfoot wedges have reported nonsignificant effects on KAM. This is in contrast to studies using full-length wedges insoles that found significant reductions in peak KAM.4,5
Researchers have hypothesized that rearfoot wedges may not be adequate to confer the benefits found in the studies that used full-length wedges.2 In 2008 Hinman and colleagues compared two types of customized lateral wedges, full-length wedges and rearfoot wedges, with a no-insole condition in 13 adults with medial compartment OA.6
Compared with no insoles, full-length wedges significantly reduced first and second peak KAM, but rearfoot wedges had insignificant effects. Like Kirby, the investigators concluded that wedging the entire lateral border of the foot rather than the heel alone was the key design feature affecting reduction of KAM.
The lack of significant improvement in RCTs and mixed evidence from other studies has led some groups to recommend against the use of wedges for management of knee OA. The American Academy of Orthopaedic Surgeons (AAOS), for example, in its 2009 guideline on conservative treatment of knee OA, suggested that lateral wedges not be prescribed for patients with symptomatic medial compartmental knee OA. The recommendation was based on limited evidence of benefit and the panel’s finding of the possibility that some patients have better outcomes without the use of wedged insoles.16
“Although we found no risks associated with lateral wedged orthotics, we found that many study patients were better without them,” said John Richmond, MD, who headed the AAOS guideline panel.
“In the grand scheme of managing medial OA of the knee, however, they are a reasonable modality to try, since there is some chance of benefit with little or no risk and reasonable cost,” said Richmond, who is chair of the Department of Orthopedic Surgery at New England Baptist Hospital in Boston, MA. “There was inadequate evidence to recommend them at that time, and in fact, some evidence to the contrary. Since then more data have accumulated showing that roughly 20% of patients have about a 20% reduction in symptoms.”10
The long-term biomechanical effects of wedges are not fully understood, said Kathleen Reilly, PhD, an investigator in the Department of Physiotherapy Research Unit at Nuffield Orthopaedic Centre NHS Trust in Oxford, UK, who has researched the use of lateral wedge orthotics in medial compartment knee OA.
“Lateral wedges open up the medial joint space, and in many patients with medial compartment knee osteoarthritis, this can bring considerable immediate relief,” Reilly said. “For short-term use—for example, a father who wants to walk his daughter down the aisle—these wedges are very useful and probably confer no ill effects, but what is less clear is whether they can lead to musculoskeletal problems in the long term.”
Reilly noted that disruption of interaction between lower limb and foot and ankle function produced by lateral wedges has not been well-considered and her 2006 systematic review of the evidence concluded there was weak evidence of long-term benefit.17
“With longer-term use, the wedges may block or hinder resupination of the foot and may lead to other undesirable musculoskeletal effects that may not be immediately apparent,” she said. “A pronated foot, for example, is one that is stuck in one direction, and that lack of mobility can cause stress further up the limb in a closed chain movement.”
Kirby noted that there is more concern related to introducing pronation than supination with the wedges.
“There are also issues for individuals with existing tibial tendinitis. Putting a valgus wedge under the foot would exacerbate that, and for that reason you may want to use customized orthotics to protect the foot,” he said.
Wedges may also increase the risk of ankle sprain in vulnerable individuals. Reeves noted that the lateral shift in center of pressure associated with the insoles can increase ankle eversion moment by up to 93%,7 which could increase the risk of injury among individuals with a history or ankle sprain or chronic ankle instability.2
“You’ve got to understand the whole machine. You can’t just treat the knee, you’ve got to think about the whole kinematic chain.” Kirby said. “In order to keep everything aligned you must ensure you’re not helping one body part and hurting another.”
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