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 on to develop PFP exhibited some of the same mechanics that have been noted in retrospective studies. 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. Increased hip adduction has been shown to concentrate the contact stress on the lateral aspect of the patella. Contract stress on the patella has also recently been shown to be greater in patients with PFP. While the patellar cartilage is aneural, such repetitive stress can irritate the subchondral bone, which is innervated, and result in pain. 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. In fact, a recent study reported that female runners with PFP exhibited a compensatory ipsilateral trunk lean. 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. 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. 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. 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. 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. One recent study found an increase in rear foot motion in a group of runners with PFP. 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 midfoot. In fact, these authors note that there is twice as much talonavicular eversion than subtalar eversion. 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. 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.
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. However, recent studies have suggested that strengthening the hip muscles does not lead to improvements in hip mechanics during running. However, neuromuscular reeducation through gait retraining has been successful in altering faulty hip mechanics during running. In addition, improvements in pain and function were reported in these patients with PFP, many of whom have not responded to standard physical therapy. 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. In addition, this need was highlighted within the published expert consensus statement from the international PFP conference. These prospective data agree with the findings of cross-sectional studies, which also found greater hip adduction in runners with PFP. Similar agreement between retrospective and prospective data on running mechanics of individuals with iliotibial band syndrome has been reported. 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. 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.