00005768-900000000-98478

. . . Published ahead of Print

Prospective Evidence for a Hip Etiology in Patellofemoral Pain

Brian Noehren1, Joseph Hamill2, and Irene Davis3

1Division of Physical Therapy, University of Kentucky, Lexington, KY2Department of Exercise Science, University of Massachusetts, Amherst, MA

3Spaulding National Running Center, Harvard University, Cambridge, MA

Accepted for Publication: 7 December 2012

Medicine & Science in Sports & Exercise® Published ahead of Print contains articles in unedited manuscript form that have been peer reviewed and accepted for publication. This manuscript will undergo copyediting, page composition, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered that could affect the content.

Copyright © 2013 American College of Sports Medicine

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Prospective Evidence for a Hip Etiology in Patellofemoral Pain

Brian Noehren1, Joseph Hamill2, and Irene Davis3

1Division of Physical Therapy, University of Kentucky, Lexington, KY; 2Department of Exercise

Science, University of Massachusetts, Amherst, MA; 3Spaulding National Running Center, Harvard

University, Cambridge, MA

Address for correspondence:

Brian Noehren PT, Ph.D.

Assistant Professor

University of Kentucky

Division of Physical Therapy

Wethington Bldg. Rm. 204D

900 S. Limestone

Lexington, KY 40536-0200

Phone: 859-218-0581 Fax:859-323-6003

[email protected] [email protected]

Medicine & Science in Sports & Exercise, Publish Ahead of PrintDOI: 10.1249/MSS.0b013e31828249d2

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Copyright © 2012 by the American College of Sports Medicine. Unauthorized reproduction of this article is prohibited.

The authors do not have any conflict of interests to report.

This study was funded by a Dept of Defense grant DAMD17-00-1-0

Running title: Prospective evaluation of patellofemoral pain

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Abstract:

Purpose: Patellofemoral pain (PFP) is the leading cause of knee pain in runners. Proximal and distal

running mechanics have been linked to the development of PFP. However, the lack of prospective

studies limits establishing a causal relationship of these mechanics to PFP. The purpose of this study

was to prospectively compare running mechanics in a group of female runners who went on to

develop PFP compared to healthy controls. It was hypothesized that runners who go on to develop

PFP would exhibit greater hip adduction, hip internal rotation, and greater rear foot eversion.

Methods: 400 healthy female runners underwent an instrumented gait analysis and were then

tracked for any injuries that they may have developed over a 2 year period. Fifteen cases of PFP

developed which were confirmed by a medical professional. Their initial running mechanics were

compared to an equal number of runners who remained uninjured.

Results: We found that female runners who developed PFP exhibited significantly greater hip

adduction (p=0.007). No statistically significant differences were found for the hip internal rotation

angle (p=0.47) or rearfoot eversion (p=0.1).

Conclusions: The finding of greater hip adduction in female runners who develop PFP is in

agreement with previous cross sectional studies. These results suggest that runners who develop PFP

utilize a different proximal neuromuscular control strategy than those who remain healthy. Injury

prevention and treatment strategies should consider addressing these altered hip mechanics.

Key words: anterior knee pain, running, kinematics, hip

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INTRODUCTION:

Patellofemoral pain (PFP) is the most common running related injury, affecting up to 2.5

million runners in the United States alone (6, 27). It is defined as pain along the retro or peri-patellar

region that is exacerbated by weight bearing activities such as running, squatting, and going up and

down stairs. Patellofemoral pain accounts for 43% of injuries military recruits suffer from, as well as

up to 25% of office visits to primary care sports medicine physicians (7, 28). Patellofemoral pain

also disproportionately affects women over men (4). Unfortunately, even at a 5-20 year follow up,

many of these individuals continue to experience pain, which has been related to a decrease in

physical activity including running (3, 17, 25). Emerging evidence suggests that patellofemoral pain

earlier in life increases the risk of developing patellofemoral osteoarthritis later in life (30). The long

term decrease in physical activity has significant healthcare consequences such as an increased risk

for developing diabetes and cardiovascular disease (2). Thus, identifying the potential underlying

mechanics that result in PFP is critical to effectively treat and prevent this chronic condition.

Patellofemoral pain is largely believed to be a stress injury. Malalignment between the patella

and the femur that reduces the contact area leads to increased patellar contact stress (21). Evidence

from a growing number of cross sectional studies suggests that females with PFP run with greater hip

internal rotation and hip adduction(19, 24, 31). Both motions have been shown in experimental

models to increase the amount of stress on the lateral aspect of the patella, and with repetitive

exposure, may result in pain (11, 14). Rear foot eversion has also been hypothesized to be related to

PFP. Excessive rear foot eversion has been associated with increased knee flexion and a greater

tendency towards knee abduction or genu valgus (16). Increased knee flexion results in greater

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patellofemoral compressive loads which can increase overall contact stress. Genu valgus is

associated with increased Q angle, increasing the lateral component of the quad force and increasing

the tendency for lateral tracking. This results in greater loads on the lateral aspect of the

patellofemoral joint (29). While many potential mechanisms have been proposed there have to date

been few studies that have assessed the proximal and distal contribution of altered mechanics in

female runners with PFP.

In summary, both hip and foot mechanics have been associated with PFP in retrospective studies.

However, these studies cannot discern between cause and effect. Prospective studies are needed to

help further elucidate biomechanical causes of PFP in runners. In fact, the need for such studies has

been strongly advocated by numerous authors (1, 6, 13). There is growing awareness of the

significant healthcare effects that result from chronic pain conditions such as PFP. Prospective

studies of the underlying mechanics that result in PFP will assist in providing the foundation for

interventions for runners with PFP. Thus, the purpose of this study was to assess the gait mechanics

of female runners who go on to develop PFP compared to a healthy control group who did not

develop any injuries. We hypothesized that female runners who go on to develop PFP would have

greater hip adduction, hip internal rotation, and rearfoot eversion. when compared to a healthy

runners who did not develop PFP.ACCEPTED


METHODS:

The participants in this study were part of a larger, prospective investigation of lower

extremity injuries in 400 female runners. All participants were between the ages of 18-45, free from

any current injuries, rear foot strikers, and running a minimum of 20 miles a week. Prior to

participation, each subject signed a consent form approved by the University’s Human Subjects

Compliance Committee. Based upon the hip transverse plane data from a previous cross sectional

study an a apriori power analysis was completed with (α=0.05, β= 0.15) a minimum of 13 subjects

was needed for this study (19). Following the initial screening and consent, an instrumented gait

analysis was conducted on all study participants. Anatomical markers were placed over the iliac

crests, greater trochanters, medial and lateral femoral epicondyle, medial and lateral malleoli, first

and fifth metatarsal heads and the front end of the shoes. The first and fifth metatarsal head markers

as well as medial and lateral malleoli were used to define the foot coordinate system. The coordinate

system of the shank was defined from the medial and lateral malleoli markers as well as medial and

lateral femoral epicondyle markers. For the femur’s coordinate system the markers placed on the

medial and lateral femoral condyle, the greater trochanter as well as from a virtual marker which was

determined as 25% of the distance between the trochanters was used. Lastly the pelvis was defined by

the bilateral greater trochanter markers and the markers placed on the iliac crests. Tracking markers

for the pelvis were placed on the space between the 5 th lumbar vertebrae and the sacrum, and the

anterior superior iliac spines. In addition, a molded thermoplastic shell with four markers was

attached to the proximal thigh and distal shank. Three markers were placed on the heel counter of the

shoe, two markers along the vertical bisection of the heel and one on the lateral side of the heel. We

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have previously shown that the between day reliability as assessed with intraclass correlation

coefficients for the joint angles derived from these markers ranges from fair for hip internal rotation

(0.54-0.58), and rearfoot eversion (0.63-0.71) up to excellent for hip adduction (0.69-0.95) (9, 18).

All participants wore a standard neutral running shoe (Nike, Air Pegasus). Participants then ran along

a 25 meter run way at a speed of 3.7 m/s (± 5%). striking a force plate at its center. Kinematic data

were collected at 120 hz with a 6 camera Vicon 512 motion analysis system (Vicon, Centennial, CO,

USA) and low-pass filtered at 8 Hz with a fourth-order zero lag Butterworth filter. Force data was

sampled at 1080 hz and low-pass filtered at 50 Hz with a fourth-order zero lag Butterworth filter.

Five acceptable trials were collected during the stance phase of running.

Following the biomechanics data collection, a detailed injury history was recorded. For the

next two years, participants reported any running related injuries and their monthly mileage. Only

injuries reported as PFP that were clinically diagnosed by a physician, physical therapist or athletic

trainer were included in the analysis. The diagnosis by the clinician had to include a

determination that the symptoms were related to the patellofemoral joint and not another

structure. Individuals with the diagnosis of patella tendon tendonitis, fat pad syndrome, or

iliotibial band syndrome were excluded from the study. In addition, the PFP group could not have

had a previous episode of PFP. They also had to experience pain for at least 2 months before they

were included in the PFP group. The mechanism of injury had to be related to running, and not

pain do to trauma or that occurred or started in other activities. The control group was age and

mileage matched to the PFP group and consisted of individuals who were free from any previous

episodes of PFP. In addition, those in the PFP group had to be free of any previous history of hip or

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knee injuries as we did not want to be including mechanics that may have resulted from PFP. The

injured leg of the PFP group was compared to the same limb of the control group.

The joint angles were then calculated using Visual3D software (C-motion, Rockville, MD).

Contact was defined as the point when the vertical ground reaction force exceeded 20 Newtons. Toe

off was defined when the force went below 20 Newtons. Discrete variables were extracted from each

individual trial.

Data were statistically analyzed using SPSS (SPSS inc, Chicago, IL). Independent t-tests

were conducted (alpha 0.05, trend = 0.0.5 <alpha < 0.10) to test the hypotheses. The kinematic

variables of interest were peak rear foot eversion, hip adduction and hip internal rotation. All data

were extracted from the individual trials of time series data. Curves were then time normalized and

averaged across 5 trials per subject and then across the subjects in each group. Therefore, the discrete

values reported may not be reflected in the time normalized and averaged data.

RESULTS:

Of the 400 runners followed, 38 reported anterior knee pain. Of these, 34 cases were running-related

with 15 being medically diagnosed and included in the data analysis. The PFP group and control

group were equally matched for age (27 sd 10 years vs, 27 sd 10 years) and monthly mileage (165 sd

53 km vs, 165 sd 43 km). The kinematic curves of the variables of interest are presented in Figure 1.

We found that the PFP group had a significantly greater hip adduction angle (p=0.007). No

significant differences were found though in rear foot eversion (p=0.10) (Table 1). The PFP group

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did have more hip adduction and less rear foot eversion throughout the entire stance period (Figure

1). While the PFP group landed in more hip internal rotation, this difference was non-significant

(p=0.47) (Figure 1).

DISCUSSION:

The purpose of this study was to assess the lower extremity mechanics in runners who go on to

develop PFP. We found that runners who went onto develop PFP exhibited some of the same

mechanics that have been noted in retrospective studies (19, 31). This included a significantly greater

hip adduction angle. We did not find any differences in the hip internal rotation or rearfoot eversion

angle. These results provide the first prospective evidence on the role of gait mechanics in female

runners who develop PFP.

The finding of significantly greater hip adduction in the PFP group further supports that of

other cross sectional studies (19, 31). Increased hip adduction has been shown to concentrate the

contact stress on the lateral aspect of the patella (11). Contract stress on the patella has also recently

been shown to be greater in patients with PFP (8). While the patellar cartilage is aneural, such

repetitive stress can irritate the subchondral bone, which is innervated, and result in pain (10, 21). To

reduce load on the hip abductor muscles as the result of greater hip adduction angle the participants

with PFP may have potentially used compensatory trunk mechanics which may alter the center of

mass and ultimately the loads on the knee (21). In fact, a recent study reported that female runners

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with PFP exhibited a compensatory ipsilateral trunk lean (19). The inclusion of trunk mechanics may

have lent additional insight on the findings of the current study.

Increased femoral rotation has also been shown to increase contact stresses on the lateral facet

of the patella (14). However, the transverse plane findings were not as compelling as those in the

frontal plane. While the PFP group landed with more hip internal rotation on average, this difference

was not statistically significant. There has been some disagreement in the literature regarding hip

rotation in runners with PFP (12,14, 28). This may be due to differences in methods, marker sets and

populations. However, the transverse plane has generally been noted to be sensitive to errors and

tends to be most variable of all planes of motion (22). This increased variability makes it difficult to

detect differences between groups.

We hypothesized that rearfoot eversion would be increased in the PFP group as it has been

associated with genu valgus which can result in misalignment between the patella and the femur,

increasing contact stress (16). It is possible that this was a compensatory mechanism to counter the

medial collapse of the lower extremity associated with increased hip adduction. Interestingly, while

there are many references to the relationship between foot pronation and PFP, there is very little

evidence of this in the literature (1). One recent study found an increase in rear foot motion in a

group of runners with PFP (19). The 2 degree increase was associated with a moderate effect size,

but was not significant. Most studies of foot mechanics have focused on the rear foot. However,

Lundberg et al noted that majority of rear foot eversion occurs at the mid-foot (15). In fact, these

authors note that there is twice as much talonavicular eversion than subtalar eversion (15).

Unfortunately, the difficulty in accurately measuring mid-foot motion has precluded its study in

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relation to PFP. It is interesting to note that foot orthotic devices, designed to minimize pronation,

have been effective in reducing pain in patients with PFP (21). It is entirely possible that they are

having their greatest effect at the mid-foot through their support of the arch. The development of

dynamic imaging techniques, such as biplane fluoroscopy, where joint motions between individual

bones can be assessed, will help to advance our knowledge in this area. The results are also

surprising in light of the studies that have reported significant pain reduction with foot orthotic

devices designed to reduce foot pronation (5, 26).

Based upon the findings of this study, it appears that the largest and most consistent

differences between those who go on to develop PFP and those who do not are in hip adduction.

While we did not assess hip strength in these individuals, weakness of the hip abductors is often

associated with increased hip adduction and PFP (12). However, recent studies have suggested that

strengthening the hip muscles does not lead to improvements in hip mechanics during running (23,

32). However, neuromuscular re-education through gait retraining has been successfully to alter

faulty hip mechanics during running (27). Additionally improvements in pain and function were

reported in these patients PFP, many who have not responded to standard physical therapy (20).

More importantly, these improvements have persisted beyond the intervention, suggesting the

underlying cause was addressed. This current study further highlights the role of increased hip

adduction in the development of PFP.

The current study provides the first prospective evidence of a hip etiology in female runners

who go onto develop PFP. The need for prospective studies assessing gait mechanics in patients with

PFP was advocated in a systematic review of biomechanical risk factors for PFP (2). Additionally,

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this need was highlighted within the published expert consensus statement from the international PFP

conference (1, 6). These prospective data agree with the findings of cross sectional studies, which

also found greater hip adduction (19, 31) in runners with PFP. Similar agreement between

retrospective and prospective data on running mechanics of individuals with iliotibial band syndrome

has been reported (31,32). These results together begin to infer that the mechanics seen following

recovery of an injury are consistent with those seen prior to the injury. While prospective studies are

the gold standard for defining causal relationships, they are costly and difficult to conduct. This

suggests that retrospective studies of mechanics associated with running could be informative of the

cause of the injury.

The study, while compelling, is not without limitations. Our subject numbers were limited by

our purposeful strict inclusion criteria. We only included runners who initially had no history of PFP,

as we did not want a prior injury to possibly influence baseline mechanics. Additionally, we only

included runners whose PFP was diagnosed by a medical professional. This helped to assure this was

a significant problem and helped to increase the validity of the diagnosis. These runners were also

very well matched with the controls in terms of age, as well as mileage run. Because the sample size

estimation was based off of potential differences in hip mechanics we may have been limited in

our ability to detect differences in rearfoot mechanics. Also the use of the greater trochanter

markers to help define the hip joint centers may have resulted in a less accurate positioning of

the hip joint coordinate system and thus increased the variability of the joint angles

particularly in the transverse plane. By comparison a recent cross sectional study using

functional hip joint centers and a different kinematic model was able to show a significant

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difference in transverse plane mechanics between those with and without PFP (19).

Collectively though these studies do indicate that hip mechanics whether they be in the frontal

or transverse plane are altered in female runners with PFP.

In conclusion, the results from the study provide the first prospective evidence of a hip

etiology in females who go onto develop PFP. These results suggest that injury prevention and

rehabilitation programs should address abnormal hip mechanics to prevent the development and/or

recurrence of PFP.

Conflict of interest: The authors have no conflict of interest to report. The results of the present

study do not constitute endorsement by ACSM.

Acknowledgements: This study was funded by a Dept of Defense grant DAMD17-00-1-0

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FIGURE LEGEND

Figure 1: Ensemble curves for the PFP (dotted line) and control group (solid line) for a) hip

adduction angle, b) hip internal rotation angle, and c) rearfoot eversion. Hip adduction, hip internal

rotation angle, and rearfoot eversion angle are positive. Error bars represent half a standard deviation.

A B

C

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Table 1: Kinematic and kinetic variables of interest: mean (standard deviation), as well as the p-value

for the PFP group and control group.

PFP Control P-value Hip Adduction Angle (deg 12.1

(2.8)

8.1 (4.5)

0.007

Hip Internal Rotation (deg) 4.5 (5.2)

3.0 (6.1)

0.47

Rearfoot Eversion Angle (deg)

8.2 (2.39)

10.4 (4.2)

0.1

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