Risk Factors for Patellofemoral Pain in the Military: Systematic Review With Meta-Analysis

Search Strategy

The search words used in this review were based on those used in previous studies, with words related to PFP and risk factors based on those suggested by Neal et al⁹ and words related to military personnel based on those used by Dijksma et al.⁵ The search strategy for all databases is detailed in the Supplemental Table. The databases used in this study were MEDLINE/PubMed, CINAHL, Embase, SPORTDiscus, Web of Science, and Scopus. We also consulted OpenGray, an online database that collects and makes available gray literature from Europe. Hand searching of the reference lists was conducted for potential additional studies that fit the inclusion criteria. The searches were carried out between database inception and January 2023. No language restriction was used.

Study Selection

Search results were exported to Rayyan (Qatar Computing Research Institute) software, and duplicates were removed manually. Two independent evaluators (E.A.B.R. and D.F.M.M.) read the titles and abstracts and determined whether the studies met the inclusion criteria. For this review, the inclusion criteria were as follows: (1) studies with populations composed entirely or in part of military personnel, provided that the latter were in a subgroup so their data could be differentiated from the others, and (2) studies with a prospective cohort design investigating at least 1 variable as a risk factor for PFP.

Data Extraction

The same 2 independent evaluators completed data extraction and separated the data by military personnel who developed PFP and those who did not. Extracted data included the following: descriptive data of the population; author and year of publication; duration of follow-up; definition of PFP; incidence of PFP; and independent variables related to biomechanical, anthropometric, physiological, or psychosocial characteristics or a combination. Disagreements were first resolved by consensus, and, in case of persistence, a third evaluator (S.J.C.A.) was consulted. The Cohen κ index was used to verify the agreement of the evaluators regarding the articles that were included in the review.¹⁵

Risk of Bias

The included studies were assessed for risk of bias using the Newcastle-Ottawa Scale (NOS).^16–18 The NOS is divided into 3 domains: selection, comparison, and outcome.¹⁸ The domains comprise 8 categories relating to methodological quality, and each study was given a score out of a maximum of 9 points (Table 1). This stage was also carried out by the 2 independent evaluators (E.A.B.R. and D.F.M.M.), with disagreements resolved by consensus. In case of persistent divergence, a third evaluator (S.J.C.A.) was consulted.

Table 1.Newcastle-Ottawa Scale¹⁸

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Data Analysis

For the statistical analyses, mean and SD were extracted for each continuous-scale variable. When similar variables were expressed in different scales, standardized mean differences (SMDs) and 95% CIs were determined. This allowed us to have a standardized result of the effect size that the assessed risk factor has on the probability of causing PFP.¹⁹ The effect sizes were categorized as small (≤0.590), moderate (0.60–1.19), or large (≥1.20).²⁰

Using the software package RevMan 5.0 (The Cochrane Collaboration), we tabulated data from similar variables in >1 study and generated meta-analyses with the respective forest plot. High heterogeneity was considered when I² ≥ 50%.^21,22 Given the small number and high heterogeneity of the studies, the random-effects model was used because a fixed-effects model likely would increase the chances of a type 1 error.⁹

Level of Evidence Recommendation

The level of recommendation of the evidence was determined based on the statistical analyses and risk-of-bias analyses. Criteria adapted from van Tulder et al were used (Table 2).²³

Table 2.Level of Recommendation

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Anthropometric Variables

Regarding anthropometric variables, we grouped data for sex,^24–26 height,^26–32 body mass,^26–32 age,^26,27,30,31 and body mass index (BMI).^27,28,32 We found a moderate level of evidence, based on 3 articles, that sex is not a risk factor for PFP in military personnel (risk ratio = 1.12; 95% CI = 0.64, 1.94; Figure 2A).^24–26 Although the studies were of high quality, the heterogeneity was quite high (I² = 83%). In these 3 studies, among those who developed PFP, the percentage of men (62%) was higher.

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We observed that height is not a risk factor for PFP in the military (I² = 57%; SMD = −0.03; 95% CI = −0.27, 0.20) with a moderate level of evidence (Supplemental Figure A).^26–32 A strong level of evidence indicated that body mass is not a risk factor for PFP in this population (I² = 0%; SMD = 0.13; 95% CI = −0.02, 0.28) based on the results of 7 studies (Figure 2B).^26–32 A strong level of evidence, based on 4 articles, indicated that age is not a risk factor for PFP in this population (I² = 1%; SMD = 0.04; 95% CI = −0.18, 0.26; Supplemental Figure B).^26,27,30,31 We observed a strong level of evidence that BMI is not a risk factor for PFP in military personnel (I² = 47%; SMD = 0.20; 95% CI = −0.14, 0.53; Supplemental Figure C).^27,28,32

Strength

Muscle strength was prospectively assessed in military personnel in 5 studies, which allowed the variables to be grouped. In these studies, the strength of the knee flexors, knee extensors, or both were evaluated, with different angles of the knee joint.^{25,27–29,32} In 2 studies,^28,32 isokinetic evaluations were performed, and in the 3 other studies, the evaluations were isometric.^25,27,29 In studies that evaluated knee-extensor strength in isometric contraction, we observed a moderate level of evidence that this variable does not predict PFP in military personnel (I² = 66%; SMD = −0.27; 95% CI = −0.58, 0.04; Supplemental Figure D).^25,27,29

In studies that evaluated knee-flexor strength in concentric isokinetic evaluations, we observed a moderate level of evidence that the variable does not predict PFP in this population at a speed of either 60°/s (I² = 0%; SMD = −0.09; 95% CI = −0.42, 0.24; Supplemental Figure E) or 240°/s (I² = 0%; SMD = −0.10; 95% CI = −0.43, 0.22; Supplemental Figure F).^28,32 In studies that evaluated knee-extensor strength in a concentric isokinetic assessment normalized to body weight, a moderate level of evidence indicated that knee-extensor weakness predicts PFP in military personnel, at speeds of both 60°/s (I² = 0%; SMD = −0.61; 95% CI = −0.94, −0.28; Figure 2C) and 240°/s (I² = 0%; SMD = −0.53; 95% CI = −0.87, −0.20; Figure 2D).^28,32 In studies that evaluated knee-extensor strength in a concentric isokinetic evaluation normalized to BMI, we observed a moderate level of evidence that knee-extensor weakness predicts PFP, at speeds of both 60°/s (I² = 0%; SMD = −0.69; 95% CI = −1.02, −0.35; Supplemental Figure G) and 240°/s (I² = 0%; SMD = −0.51; 95% CI = −0.84, −0.18; Supplemental Figure H).^28,32

Alignment Variables

Alignment variables, such as the frontal-plane knee-projection angle (FPKPA) and the static Q-angle during a single-legged squat, could be grouped from the results of 3 studies.^25,27,30 A moderate level of evidence indicated that a greater FPKPA during the single-legged squat task is a risk factor for PFP in male military personnel (I² = 41%; SMD = 0.55; 95% CI = 0.14, 0.97; Figure 2E).^27,30 A moderate level of evidence, based on the results of 2 studies, indicated that the Q-angle is not a risk factor for PFP in this population (I² = 28%; SMD = 0.20; 95% CI = −0.07, 0.47; Supplemental Figure I).^25,27

Of all studies evaluated, only Hetsroni et al³³ and Rauh et al³⁴ presented variables that could not be grouped with those of any other study. Hetsroni et al assessed variables related to subtalar pronation during walking, not observing any relationship between these variables and the development of PFP in military personnel.³³ Rauh et al classified the foot arch index as normal, low, or high based on criteria that they determined.³⁴ This made the combination of their results with those of other studies unfeasible.

Incidence

The incidence of PFP in this review was 7.61%, demonstrating that this condition affects approximately 8 out of 100 military personnel. This result is similar to that observed in another review, in which an incidence of PFP of 11% was observed in subgroups of individuals from a military population.⁹ However, in a review, Smith et al observed incidence rates of 9.7 to 571.4 cases per 1000 people per year in different military populations.³ This large variability may have resulted from the different characteristics of the military population in different categories of activity. A lower incidence of PFP was observed in countries where good physical fitness is a requirement for military recruitment.³ In military conscripts, Taanila et al found that low levels of physical fitness (determined using a series of physical tests, including the 12-minute run test, push-ups, sit-ups, pull-ups, etc) increased the risk of musculoskeletal injuries by >3 times.³⁵ This finding may be a consequence of the fact that a group with a higher level of physical fitness is probably more adapted to intense periods of physical activity, which would reduce the risk of injury.³⁵ The abrupt increase in training load can be a factor that increases the risk of injury. Already-trained individuals tend to adapt better to the training load imposed by military training compared with individuals with less physical capacity due to previous adaptations associated with muscle memory.³⁶ Discrepancies in the incidence of PFP in military recruits of distinct countries may be due to these groups being submitted to different training methods, but unfortunately the studies provided little detail on that aspect. More detailed information on volume, frequency, and intensity of training is needed to better understand the relationship between military training and the occurrence of injuries such as PFP.

Strength

In our study, decreased concentric isokinetic knee-extensor strength was found to be a predictor of PFP in military personnel. This finding corroborates the results of other systematic reviews with different populations (ie, adolescents, recreational runners, and civilian adults).^9,12 The quadriceps plays a key role in the knee joint because quadriceps activation is important for patellar stabilization during dynamic activities, leading to optimal patellar tracking.³⁷ Greater quadriceps strength also allows more energy dissipation, thus reducing stress on the patellofemoral joint.³⁸ These findings may help explain why rehabilitation programs involving quadriceps strengthening have good results in individuals with PFP.³⁸ Quadriceps muscle strengthening appears to be one of the most important aspects of PFP treatment, with authors of several studies observing beneficial results of interventions including quadriceps strengthening in reducing pain and improving function in individuals with PFP.^38–40 The identification of quadriceps weakness as a risk factor for PFP in military personnel emphasizes quadriceps strengthening as an aspect to be highlighted in prevention programs before the beginning of military training. This preventive strengthening program must fit the initial physical conditions of these soldiers at entry, such as the volume, intensity, and frequency of activities to which individuals were exposed before recruitment. These variables may directly influence quadriceps strength and the muscle’s ability to adapt to new military training due to molecular muscle memory in response to previous training.³⁶ However, this information was rarely presented in detail in the studies.

Frontal-Plane Knee-Projection Angle

The FPKPA during the single-legged squat was identified as a risk factor for PFP in male recruits in our review but with a small effect size despite the statistical homogeneity observed. Previous studies^41,42 identified that individuals with PFP present changes in frontal-plane knee movement; however, in the military population, only 2 studies^27,30 assessed whether the FPKPA is a risk factor for PFP. Nakagawa et al observed that an FPKPA ≥ 4.81° has a sensitivity of 79% and a specificity of 80% for predicting PFP in male military recruits (hazard ratio = 4.6).³⁰ Alrayani et al found that an angle ≥ 5.2° has a sensitivity of 70% and a specificity of 70% for predicting PFP in male recruits (hazard ratio = 2.2).²⁷ These findings indicate that the FPKPA is a relevant variable for male recruits, potentially due to the increase in lateralizing forces on the patella, which may increase the contact pressure between the lateral femoral condyle and the lateral facet of the patella, consequently contributing to PFP.^30,43–45

Anthropometric Variables

The results of this review indicated that sex is not a risk factor for the development of PFP in military personnel. This finding corroborates the findings of a review by Neal et al.⁹ However, it conflicts with the finding of Lankhorst et al, who observed that female military personnel have a higher risk of developing PFP when compared with male military personnel.¹² Their conclusion was based on the results of only one study in which, despite no association between sex and PFP, the incidence of this dysfunction in female military personnel was almost twice as high as that of male military personnel.²⁵ In this review, we identified a small number of studies on the subject, and the results had high heterogeneity. Thus, despite the absence of a causal relationship between female sex and PFP, a larger and more robust investigation of this variable would be important because the total number of female military personnel in studies on the subject is smaller than that of male military personnel. Information about the type, volume, load, and progression of training is also important and should be better detailed in the studies, such as verifying whether both sexes participate in the same training or whether different training courses are used. Generally, military training is of high intensity, with similar activities and objectives and equal exposure for both sexes.⁴⁶ However, Bell et al reported that female soldiers tend to start military training with a lower physical capacity compared with male soldiers and that when both have the same level of physical activity, the risks of musculoskeletal injuries are similar.⁴⁶ Bergman and Miller suggested that training planned by sex can have better results in terms of minimizing injuries.⁴⁷

Height, body mass, age, and BMI were not predictors of PFP in military personnel. These results corroborate the findings of the review by Neal et al.⁹ When analyzing the baseline characteristics of the populations participating in the studies, we found that the variables were very similar because these recruits, who are starting their careers, generally need to meet some anthropometric requirements for admission to the military. In contrast to this result, in their systematic review, Hart et al observed an association between BMI and PFP.⁴⁸ Their finding, however, may have occurred because they also included studies with a cross-sectional design.⁴⁸ Thus, this association may be due to a possible decrease in physical activity levels after the onset of symptoms.

Strengths, Limitations, and Future Studies

Our study had strengths and limitations. The NOS was chosen because it is a valid scale and is widely used in cohort assessments.^49,50 Margulis et al¹⁷ reported that the NOS is recommended by the Cochrane Collaboration in its handbook for systematic reviews of this nature.²¹ The NOS is the most used scale for analyzing the risk of bias in observational studies; however, it is considered a limited tool because it covers few fields and can generate arbitrary results due to warranting the same grade to studies with different qualities.^16,17,51

The search strategy involved 6 databases and 1 gray database, which makes this review more comprehensive when compared with other systematic reviews.^9,12 The great heterogeneity of variables evaluated in the studies was a challenge, and we could group only 14 of 74 variables for meta-analyses. Most variables were evaluated separately, and considering the multifactorial characteristic of PFP, further studies are needed to analyze the interactions between risk factors.⁴⁴ Based on our search strategy, we found no study that assessed whether psychological factors are a risk factor for PFP in military personnel. Given that, in their review, Maclachlan et al emphasized the importance of these aspects, future studies should be done to consider the possibility of psychological aspects, such as pain catastrophizing and anxiety, being potential risk factors for PFP in military personnel.⁵²

Finally, the population in the included studies was restricted to military recruits (ie, soldiers at the beginning of their careers). The generalization of these results to other categories of military personnel, such as elite groups and administrative groups, among others, should be done with caution. In future studies, other categories of the military should be included in the sample, which may generate more comprehensive results for risk factors for musculoskeletal injuries such as PFP in this population. Understanding risk factors is an important step in developing preventive interventions.⁵³ To our knowledge, only one study has investigated the effects of an exercise-based intervention (quadriceps and gluteal strengthening and lower limb stretches) to reduce PFP in the military.³⁸ Although complex, prevention studies are of paramount importance, and future research should be done to investigate whether an intervention addressing known risk factors can effectively decrease the incidence of PFP in the military.