Anterior knee pain is one of the most common injuries affecting runners, accounting for 25% of all running injuries.¹ The etiology of anterior knee pain is multifactorial in nature,² but one of the most commonly cited biomechanical risk factors is excessive rearfoot pronation. In this biomechanical injury paradigm, excessive rearfoot pronation is thought to induce excessive tibial internal rotation, which in turn disrupts the screw-home mechanism of the tibiofemoral component of the knee joint.^3-5 However, the lower extremity is believed to compensate for this increase in tibial internal rotation by also increasing the amount of femoral internal rotation, preserving the screw-home mechanism. While these compensations are thought to maintain the normal mechanics of the tibiofemoral joint, they are believed to negatively alter those of the patellofemoral joint and, through repetition, cause injury.^3,4 This biomechanical sequence of events has been the rationale of the widely accepted link between pronation and injury as well as the foundation for many treatment strategies.

Clinical orthotic studies have supported this injury paradigm. When focusing on short-term clinical outcomes, studies have shown consistently that foot orthoses reduce pain and improve the function of approximately 70% of individuals experiencing anterior knee pain.^6-9 The same, however, cannot be said of biomechanical and epidemiological studies, which have consistently failed to provide evidence of a link between pronation and anterior knee pain.^10-17 Although the cause of this discrepancy is effectively unknown, we believe one reason may be that excessive pronation is not being evaluated in the correct context.

Pronation is typically defined as the frontal plane angle formed between the tibia and calcaneus. The dynamic peak pronation angle (i.e., while walking or running) is then used to classify a runner as having normal or excessive pronation. Clinically, the dynamic peak pronation angle that separates normal pronation from excessive pronation is based on a practitioner’s experience and judgment. Similarly, biomechanical studies create a threshold value by using the means and standard deviations reported in the literature. However, this technique is limited in that it evaluates pronation without considering an individual’s joint anatomy, flexibility, or proximal response to pronation, and therefore is not patient specific.

In other words, should a healthy runner exhibiting 18° of dynamic peak pronation be considered an excessive pronator and at risk of injury? Conventional wisdom would say yes. However, what if this runner has 25° of pronation range of motion (ROM) available? In this scenario, despite having 18° of dynamic peak pronation, the joint still has 7° of additional ROM available. Using this frame of reference, should a clinician still consider this runner an excessive pronator? And should a runner with only 5° of dynamic peak pronation be classified as having normal pronation if she has only 6° of passive pronation ROM available, and therefore is coming within 1° of her available ROM? Using this conceptual framework, we feel that there may be a more patient-specific and optimal way of defining excessive pronation.

Pronation buffer

We recently tested the concept of evaluating pronation in the context of a joint’s available passive pronation ROM in 17 runners with anterior knee pain and 19 without.¹⁸ We first measured runners’ available passive pronation ROM using a custom-built device and a motion-capture system (Figure 1). Participants’ peak dynamic pronation when running was then captured using the same motion-capture system. We then calculated a pronation buffer from these two measurements to represent the angular distance a runner maintained from his or her end-range (pronation buffer = passive pronation ROM – peak dynamic pronation).

Using this technique, we found that runners with anterior knee pain came significantly closer to their passive pronation ROM boundary and maintained, on average, only a 4.2° pronation buffer, while asymptomatic runners had a greater pronation buffer of 7.3° (Figure 2). This difference came from injured runners having, on average, 3° less passive pronation ROM as measured with the custom-built device. However, similar to previous research, we found no differences in peak dynamic pronation between asymptomatic and injured runners (injured = 7.7°, asymptomatic = 7.57°). Nevertheless, it is important to note that, on an individual level, this was not always the case. There were several runners who had small pronation buffers as a result of having the classic excessive dynamic peak pronation and not limited passive pronation ROM. In summary, this research supports the concept that pronation needs to be evaluated in a patient-specific and more anatomically relevant context.

A small pronation buffer could increase a runner’s risk of injury at the foot and knee for a variety of reasons. First, a small pronation buffer limits the foot’s ability to absorb or adapt to environmental perturbations from the running surface, such as cambered or rough terrain. As a result, more proximal joints such as the knee may be required to compensate and move unnaturally, increasing their risk of injury. Secondly, as a result of both the compensations and functioning near end-range, increased stress and strain could be placed on the joints’ surrounding tissue. Lastly, by limiting the rearfoot’s ability to absorb impact forces through joint rotation, greater demands again may be placed on more proximal joints. While the potential effect of a small pronation buffer on injury requires further investigation, we believe this concept could be clinically valuable in the interim.

Clinically, the pronation buffer can be calculated by evaluating a patient’s dynamic peak pronation relative to their measured passive pronation ROM. Through this procedure, a clinician can determine three things: first, whether a patient has a small pronation buffer; second, whether the small buffer is a result of large amounts of dynamic peak pronation or a lack of passive pronation ROM; and third, through the end feel, the type of tissue (bony, ligamentous, or muscular) that is limiting the rearfoot’s passive pronation ROM. Clinicians can then use this information as part of comprehensive clinical examination to direct a treatment approach.

Tailoring treatment

In cases in which a patient presents with a small pronation buffer as a result of having excessive dynamic peak pronation but a normal amount of passive pronation ROM, treatment should focus on controlling pronation. Ideally, this would be accomplished by strengthening the musculature of the lower extremity and foot,^19-21 but sufficient strength gains would require a certain amount of time to accrue and individuals will likely want to continue participating in their chosen activity during this period. Therefore, we feel motion-controlling devices such as foot orthoses and stability footwear should be used as to tools to control foot motion while the strength of the foot is developed. However, as the foot strength increases, we believe these motion-controlling devices should be weaned systematically and as tolerated by the patient to continue developing the foot’s strength.

The findings from our research suggest that patients will commonly have small pronation buffers as a result of having limited passive pronation ROM. In these cases, the type of tissue-limiting passive pronation ROM should be noted, as it will help direct a clinician’s treatment strategy. For example, if there is a hard end feel into pronation and the limited motion is thought to be due to the joint’s osseous (bony) geometry, then the chance of increasing the passive pronation ROM is unlikely. In such cases a clinician should utilize motion-controlling devices such as foot orthoses or stability footwear to resist the motion of the joint as it approaches end range;²² these devices, however, should still allow enough motion for the foot to adapt to environmental perturbations. In addition, these runners should seek out relatively smooth and flat terrain to limit the motion required for the foot to adapt to running surface variations. Together, motion-controlling devices and selecting appropriate running terrain should limit dynamic peak pronation and, in turn, increase the pronation buffer a runner is working with.

When limited passive pronation ROM is the result of a soft tissue limitation, a clinician should still consider using motion-controlling devices and limiting the running terrain during the early stages of rehabilitation. However, it is our belief that treatment should focus on increasing the joint’s passive pronation ROM through stretching or joint mobilizations.²³ Additionally, concurrent focus should be placed on improving the strength and coordination of the musculature of the foot so it is prepared to control motion in this new range. Ultimately, as an individual’s passive pronation ROM increases and they are able to maintain their pronation buffer, a clinician should again start to systematically wean these individuals from motion-controlling devices.

In conclusion, we feel that evaluating excessive pronation in a patient-specific and anatomically based context, for example, determining the size and structural nature of the pronation buffer, may identify those at risk of injury more effectively than the more traditional definition of excessive pronation. In addition, we believe evaluating pronation in this context in conjunction with a thorough clinical exam will inform clinicians about the root causes of a small pronation buffer and help identify specific treatment strategies.

This novel way of evaluating pronation shows promise, but more work is necessary to continue understanding the effects of a small pronation buffer. However, in the interim, this evaluation technique may be clinically valuable.

Pedro Rodrigues, MS, PT, PhD, and Trampas TenBroek, PhD, are researchers in the New Balance Sports Research Lab in Lawrence, MA. Joseph Hamill, PhD, is professor of biomechanics in the Department of Kinesiology at the University of Massachusetts Amherst.