May 2015

Foot orthoses and patellofemoral pain: frontal plane effects during running

shutterstock_100144160Evidence supports the use of foot orthoses for PFP, but their mechanism of action is not well understood. New research suggests the effect of orthoses on timing of frontal plane moments may be an important variable.

By Thomas Gus Almonroeder, DPT, and Kristian O’Connor, PhD

Running is a common mode of physical activity due to its convenience, relatively low cost, and positive impact on health.1 Unfortunately, the incidence of lower extremity running-related injury has been reported to be as high as 79%, with the majority of these events occurring at the knee joint.2

Patellofemoral pain (PFP) is the most common of these running-related knee injuries.

Although the etiology of PFP is considered multifactorial, it appears to be related to abnormal alignment of the patella within the trochlear groove of the femur, which leads to an increase in patellofemoral joint stress.3,4 Excessive pronation of the foot is thought to contribute to this increase in patellofemoral joint stress by promoting compensatory alterations in lower extremity mechanics—such as increased internal rotation of the tibia and femur, adduction of the hip, and abduction of the knee—that can disrupt normal mechanics of the patellofemoral joint.3,5 The concept of a link between the mechanics of the foot and PFP is also supported by the finding that clinical measures of increased foot mobility are risk factors for the development of PFP.6,7

As a re­sult of the possible link between foot mechanics and injury risk, clinicians often use foot orthoses designed to limit excessive pronation in the management of PFP.8,9 Clinically, foot orthoses have shown some effectiveness at reducing pain10,11 and improving function11 in patients with PFP, and they may hasten recovery in the early phase of the condition.12 However, clinical and biomechanical studies investi­- gating the effects of foot orthoses have used a variety of custom and prefab­ricated foot orthoses with varying degrees and locations of wedging.13-16 This makes providing clear recommendations regarding orthotic prescription and design challenging.

The lack of consistency is likely the result of an incomplete understanding of the key variables that need to be considered when prescribing or designing a foot orthosis. The challenge in determining these variables is most likely due to the fact that the mechanism of action by which foot orthoses may facilitate improved clinical outcomes for patients with PFP is largely unknown.15,17 Ultimately, the key to optimizing orthotic design and prescription lies in understanding their underlying mechanism of action.

Mechanical and clinical effects

5coverstory-iStock16532037Based on the theoretical relationship between foot pronation and frontal plane knee mechanics that may contribute to the etiology of PFP,3 it would seem intuitive that in this patient population foot orthoses would significantly affect the kinematics of the knee during dynamic activities such as running. Multiple studies have investigated these effects using a combination of custom, semicustom, and prefabricated orthotic devices with a relatively conservative degree of wedging (< 6°) and reported insignificant results on kinematic variables in the frontal plane.14-16,18

The angle-time series from data collected in our lab is presented in Figure 1. The orthotic device used in this study was prefabricated and had 5° of medial rearfoot wedging.15 From this figure it is apparent that the effect of the orthosis across the entire phase of stance is consistently less than 1°. These data, along with the findings of previous studies, indicate the effects of a foot orthosis on frontal plane knee kinematics appear to be small, and likely both statistically and clinically insignificant.

From a mechanical standpoint, interventions to treat or prevent PFP are often assessed in the context of their influence on frontal plane moments.19,20 This is likely because a 2006 study by Stefanyshyn et al found that increased knee abduction moments are risk factors for PFP in both retro­spective and prospective analy­ses.21 It would seem logical that the clinical effectiveness of a foot orthosis could also be defined in terms of its ability to decrease these net joint moments.

Similar to studies investigating knee kinematics, studies investi- gating the effects of foot orthoses on frontal plane moments have also included prefabricated, semicustom, and custom devices with varying degrees and locations of posting and wedging. Inter­est- ­ingly, multiple studies have found that foot orthoses were associated with increased peak knee abduction moments,14,15,18 which does not support their use in a patient population with PFP. A recent study published by researchers from our lab15 was consistent with such previous reports, as participants demonstrated an increase in peak knee abduction moment with the application of a prefabricated foot orthosis with 5º of medial rearfoot wedging.

5coverstory-iStock_000021000034_Large-copyHowever, such seemingly negative mechanical effects have not necessarily been associated with negative clinical effects—a conflict that may be best exemplified by a 2008 study by MacLean et al that investigated the short- and long-term effects of a custom foot orthosis in patients with overuse running-related injuries of the knee.14 Researchers did a biomechanical analysis for these individuals with and without a custom foot orthosis at a baseline session as well as after six weeks of orthotic use. The patients reported a significant
reduction in their symptoms during running, indicating the device had a positive clinical effect. During both the baseline and follow-up sessions, however, participants demonstrated a slight increase in the peak knee abduction moment in the orthotic condition compared with the control condition. From a mechanical standpoint, these findings seem to be inconsistent.

As previously mentioned, most studies investigating the effects of foot orthoses on frontal plane mechanics have used a fairly conservative degree of wedging (< 6º). Only one study has investigated a more prominent inverted orthotic device that applied either 15° or 25° of medial rearfoot wedging depending on the participant’s standing foot posture. That study found a statistically significant, though relatively small (< 4°), increase in knee adduction with the application of the orthosis.16 These authors also reported an increase in the knee abduction moment in the inverted orthotic condition, a finding consistent with the kinematic results. Unlike the results reported with more conservative orthotic interventions, this aggressive intervention does appear to influence frontal plane kinematics, though the finding of an increase in the peak knee abduction moment still does not support their use in patients with PFP.

The importance of timing

Figure 1. Knee frontal plane angle-time series for 31 recreational runners during running under baseline and orthotic conditions. (Adapted from reference 15.)

Figure 1. Knee frontal plane angle-time series for 31 recreational runners during running under baseline and orthotic conditions. (Adapted from reference 15.)

Whereas most investigators have chosen to study the effects of foot orthoses on the magnitude of frontal plane moments, we recently reported that the application of a prefabricated foot orthosis with a 5° medial rearfoot wedge was associated with a significant delay in the timing of the peak knee abduction moment during the stance phase of running.15 The effects of foot orthoses on the peak knee abduction moment, as well as the timing of the peak knee abduction moment collected in our lab, are shown in Figure 2. It is apparent in this figure that the peak knee abduction moment is greater in the orthotic conditions, but also shifted to later in stance.

Our results are consistent with a 2003 study by Mundermann et al that compared the effects of custom orthoses (with posting, molding, or a combination of both) to flat inserts.13 For each orthotic condition, these authors reported a significant delay in the timing of the peak knee abduction moment.13 This finding may be related to the aforementioned clinical effects, as delaying the peak knee abduction moment would effectively decrease the rate of loading at the knee joint. The rate of loading has been previously implicated as a possible contributing factor in running-related overuse injuries,22,23 as runners with a history of injury have demonstrated a higher rate of loading of the vertical ground reaction force than runners with no history of running-related injury.23

Figure 2. Knee frontal plane moment-time series for 31 recreational runners during running under baseline and orthotic conditions. (Adapted from reference 15.)

Figure 2. Knee frontal plane moment-time series for 31 recreational runners during running under baseline and orthotic conditions. (Adapted from reference 15.)

Although the findings of our study,15 as well as those of Mundermann et al,13 are certainly interesting, it is important to note that neither included an injured population. As a result, the relevance of these findings in relation to the mechanism of action responsible for positive clinical effects of orthoses in patients with PFP will need to be established.

Most investigators studying the mechanical effects of foot orthoses on PFP have chosen to focus on the knee. However, one study also looked at the effects of orthoses on the frontal plane mechanics of the hip.18 This was an important contribution as increased hip adduction has been proposed to alter patellofemoral joint mechanics,3 and it has also been reported that patients with PFP demonstrate significantly greater hip adduction angles across a variety of dynamic tasks.24 These authors reported that the orthosis did not significantly affect peak hip adduction angle, but they did report a significant reduction in hip adduction range of motion with application of a prefabricated foot orthosis with a 6° medial rearfoot wedge.

Theoretically, this reduction in hip adduction would reduce patellofemoral joint stress;3 however, though this finding was statistically significant, the decrease in hip adduction range of motion associated with the orthosis was less than 1°.18 It is possible but unlikely that an effect of this magnitude would explain the reported clinical improvement associated with foot orthoses in patients with PFP. This study also reported that the same orthotic device had no effect on peak frontal plane hip moments. Together, these results indicate that a relatively conservative orthotic intervention has minimal effects on hip mechanics.

Other possible mechanisms

5coverstory-iStock11923686-copyAlthough the idea that foot orthoses have significant effects on frontal plane mechanics seems logical, there appears to be little evidence to establish the relationship between these mechanical effects and the clinical benefits reported previously by individuals with PFP. This has led some authors to propose that the benefits of foot orthoses may be unrelated to their mechanical effects.17,18 A 1999 review paper by Nigg et al highlighted this theory, with the authors proposing that the key function of a foot orthosis may be related to variables such as improved sensory feedback and altered muscle activity.17 This is an interesting concept and is supported by studies that have used electromyography (EMG) to analyze muscle activity of the lower extremity with the application of foot orthoses.25,26

For example, a 2005 study by Hertel et al found increased activity of the vastus medialis and gluteus medius during a single-leg squat and lateral step down task for an orthotic condition compared with baseline.26 Both of these muscles are thought to be important in PFP. The gluteus medius is an abductor of the hip, and, as a result, may contribute to the reductions in hip adduction associated with foot orthoses in patients with PFP reported by Boldt et al.18 The vastus medialis is also important in PFP, as it prevents lateral tracking of the patella within the femoral groove.

Perhaps the most interesting aspect of the study by Hertel et al is their finding that this increase in muscle activity was associated with both medially and laterally wedged foot orthoses.26 Although medial wedges are often prescribed for patients with PFP, if the true mechanism behind the beneficial effects of a foot orthosis is to alter muscle activity, the location of the wedging may be less important than providing key sensory input to the plantar surface of the foot. This idea does have some biologic plausibility, but other studies using EMG to investigate the effects of foot orthoses on similar muscles have reported no significant orthotic effects during more dynamic tasks such as running.27,28 A systematic review has highlighted the need for future studies to investigate any possible changes in muscle activity in groups with lower extremity pathology.29

5coverstory-iStock_000004361753_Large-copyAnother theory suggests that there are subgroups of patients with PFP who would benefit from an orthotic intervention.30,31 Barton et al, for example, developed a preliminary clinical prediction rule (CPR) to identify patients with PFP who have a greater likelihood of a successful outcome when using a prefabricated foot orthosis.31 Fourteen of the 60 patients included in their single-arm study reported feeling “markedly better” after 12 weeks of orthotic use, and the researchers included four predictors in the final CPR.

Barton et al’s CPR certainly requires validation before it is used clinically, but it does lend support to the notion that there may be a subpopulation of patients with PFP that may specifically benefit from orthoses. It is certainly possible that orthoses may have a different mechanical effect in this subgroup that leads to them experiencing clinical benefits. This has not been addressed in previous studies analyzing the mechanical effects of foot orthoses and should be
investigated.

Conclusion

There is evidence to support the use of foot orthoses in the management of PFP, especially in the short term. However, providing clear guidelines on the prescription and design of these devices is difficult due to a poor understanding of their mechanism of action. From a mechanical perspective, foot orthoses appear to significantly increase peak knee abduction moment, which does not support their use in the management of PFP. However, there is also evidence that they shift the timing of this peak to later in stance, though the clinical implications of this finding have not been established.

It is also possible that other mechanisms, such as the effect of orthoses on lower extremity muscle activity, may contribute to their clinical benefits. An important next step for orthotic research may be to compare the biomechanics of individuals who benefit clinically from foot orthoses with those who do not to identify key characteristics associated with improved clinical outcomes.

Thomas Gus Almonroeder, DPT, is a physical therapist and a PhD student in kinesiology at the University of Wisconsin-Milwaukee. Kristian O’Connor, PhD, is associate professor and chair of the Depart­ment of Kinesiology at the University of Wisconsin-Milwaukee.

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