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 peripatellar region that is exacerbated by weight bearing activities such as running, squatting, and going up and down stairs. PFP 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). PFP also disproportionately affects women over men (4). Unfortunately, even at a 5- to 20-yr 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 PFP earlier in life increases the risk of developing patellofemoral osteoarthritis later in life (30). The long-term decrease in physical activity has significant health care 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.
PFP 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 toward knee abduction or genu valgus (16). Increased knee flexion results in greater 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, to date, there have 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 health care 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 rear foot eversion when compared to a healthy runners who did not develop PFP.
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 and 45, free from any current injuries, rear foot strikers, and running a minimum of 20 miles·wk−1. Before participation, each subject signed a consent form approved by the University’s Human Subjects Compliance Committee. Based on the hip transverse plane data from a previous cross-sectional study, an a priori power analysis was completed with α = 0.05, β = 0.15, and 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 fifth 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 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 rear foot 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-m runway at a speed of 3.7 m·s−1 (±5%), striking a force plate at its center. Kinematic data were collected at 120 Hz with a six-camera Vicon 512 motion analysis system (Vicon, Centennial, CO) and low-pass filtered at 8 Hz with a fourth-order zero-lag Butterworth filter. Force data were 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 2 yr, 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 due 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 from any previous history of hip or knee injuries because we did not want to be including mechanics that may have resulted from other injuries. 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 N. Toe-off was defined when the force went below 20 N. Discrete variables were extracted from each individual trial.
Data were statistically analyzed using SPSS (SPSS, Inc., Chicago, IL). Independent t-tests were conducted (α = 0.05, trend = 0.05 < α < 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 five 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.
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 (mean ± SD = 27 ± 10 vs 27 ± 10 yr) and monthly mileage (165 ± 53 vs 165 ± 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, however, in rear foot eversion (P = 0.10; Table 1). The PFP group did have more hip adduction and less rear foot eversion throughout the stance period (Fig. 1). While the PFP group landed in more hip internal rotation, this difference was not significant (P = 0.47; Fig. 1).
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 on to 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 rear foot 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 a 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 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 because of 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 rear foot eversion would be increased in the PFP group because 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, although 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. (15) noted that majority of rear foot eversion occurs at the midfoot. In fact, these authors note that there is twice as much talonavicular eversion than subtalar eversion (15). Unfortunately, the difficulty in accurately measuring midfoot motion has precluded its study in 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 midfoot 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 on 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 reeducation through gait retraining has been successful in altering faulty hip mechanics during running (27). In addition, improvements in pain and function were reported in these patients with PFP, many of whom have not responded to standard physical therapy (20). More importantly, these improvements have persisted beyond the intervention, suggesting that 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 on to 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). In addition, 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 before 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, although compelling, is not without limitations. Our subject numbers were limited by our purposefully strict inclusion criteria. We only included runners who initially had no history of PFP because we did not want a prior injury to possibly influence baseline mechanics. In addition, we only included runners whose PFP was diagnosed by a medical professional. This helped to ensure that 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 on potential differences in hip mechanics, we may have been limited in our ability to detect differences in rear foot 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 difference in transverse plane mechanics between those with and without PFP (19). Collectively, although 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 this study provide the first prospective evidence of a hip etiology in females who go on to 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.
This study was funded by a Department of Defense grant (DAMD17-00-1-0).
The authors do not have any conflict of interests to report.
The results of the present study do not constitute endorsement by the American College of Sports and Medicine.
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