People With Patellofemoral Pain Have Bilateral Deficits in Physical Performance Regardless of Pain Laterality

METHODS

We conducted a cross-sectional study adhering to the Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) guideline recommendations.¹⁸ All procedures were approved by the University Ethics Committee, and all participants provided written informed consent before enrollment.

RESULTS

None of the tests used in this study violated the statistical assumption of sphericity based on the Mauchly test. We observed no significant differences among groups for age, body mass, height, and body mass index, nor were sport and leisure physical activity levels different (Table 1). We observed lower self-reported function in people with unilateral and bilateral PFP than pain-free controls but no significant differences between the PFP groups (d = 0.23–0.26, 95% CI = −3.42, −2.30; Table 1). People with bilateral PFP had greater symptom duration but not pain severity than those with unilateral PFP (d = −0.35, 95% CI = −0.71, −0.01; Table 1).

Table 1. Mean ± SD for Demographics and Self-Reported Measures^a

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Forward Step-Down Test

We observed a significant interaction for FSDT (η_p² = 0.060; Table 2). People with unilateral and bilateral PFP demonstrated lower performance with both painful or most painful (d = −0.65 to −0.76, 95% CI = −1.13, −0.32) and pain-free or less painful limbs (d = −0.47 to −0.59, 95% CI = −0.92, −0.12) than pain-free controls, based on small to medium effects (Table 3). We observed no significant differences in FSDT performance between PFP groups (d = −0.11 to 0.12, 95% CI = −0.47, 0.48; Table 3). Also, no significant differences were found in the proportion of symmetrical/asymmetrical participants across groups (P = .058; Figure A).

Table 2. Mean of the Performance During Forward Step-Down and Single-Leg Hop Tests Using Both Limbs of People With Unilateral and Bilateral Patellofemoral Pain and Controls^a

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Table 3. Pairwise Comparisons Among Groups for the Performance During the Forward Step-Down Test^a

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Single-Leg Hop Test

We observed no significant interaction or limb main effect for SLHT (η_p² = 0.009–0.016), but we found a significant group main effect (η_p² = 0.034; Table 2). People with unilateral and bilateral PFP demonstrated lower performance with both painful or most painful (d = −0.33 to −0.46, 95% CI = −0.81, −0.02) and pain-free or less painful limbs (d = −0.35 to −0.39, 95% CI = −0.75, −0.03) than pain-free controls (Table 4). We observed no significant differences in SLHT performance between PFP groups (d = −0.12 to −0.04, 95% CI = −0.48, 0.31; Table 4). Also, significant differences were found in the proportion of symmetrical/asymmetrical participants across groups (P = .817; Figure B).

Table 4. Pairwise Comparisons Among Groups for the Performance During the Single-Leg Hop Test^a

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DISCUSSION

Our results showed lower performance of both limbs of people with PFP when performing the FSDT and SLHT than pain-free controls. This is partially in agreement with our initial hypotheses, as we expected worse performance in both limbs of people with bilateral PFP but not bilaterally in those with unilateral PFP. The lack of differences for the proportion of limb asymmetries among groups further supports the impaired performance of the pain-free and less painful limbs of people with PFP. These results highlight the limitations of using the contralateral limb as the reference standard when assessing performance-based measures of function such as FSDT and SLHT in people with PFP, regardless of PFP laterality.

We observed that both limbs of people with PFP, regardless of the presence of unilateral or bilateral complaints, have lower FSDT (15%–25%) and SLHT (10%–12%) performance. These results support previous reports of impaired performance of the painful or most painful limbs of people with PFP as compared with pain-free controls during both tests and reinforce the impact of PFP on step-down and hop performance.^9,16 Even though between-groups differences had small to medium effect sizes, physical performance of people with PFP can become worse over time as an effect of pain chronicity and age.⁸ As FSDT and SLHT are capable of differentiating people with and without PFP, clinicians may benefit from such performance-based measures to complement their assessment of physical function of patients with PFP.

A novel finding from our results is that not only the performance of the painful or most painful limbs but also of the contralateral limbs (ie, pain-free and less painful) of people with PFP are reduced when compared with pain-free controls. Our results are in accordance with recent reports that women with both unilateral and bilateral PFP displayed lower muscle volume of the bilateral hips and knees than pain-free women.²⁹ Together, these findings suggest that the pain-free and less painful limbs may not be equivalent controls for clinical or field-based assessments. We recommend clinicians assess and prioritize rehabilitation of both painful or most painful and pain-free or less painful limbs when targeting performance-based measures of function using FSDT or SLHT in people with PFP. In the absence of reference values for FSDT and SLHT performance in young adults with PFP, preintervention and postintervention differences (ie, within-limbs comparisons) may be used to track progress or estimate gains.³⁰

Limb symmetry during physical performance tests has been primarily recommended to interpret and benchmark injured limb deficits or performance recovery in people after anterior cruciate ligament (ACL) injuries, but its practical use has been questioned, first, because LSI values tend to fluctuate slightly from person to person, and asymmetries observed in those without pain is not surprising, and secondly, because it has been reported that LSI may overestimate functional improvements due to worsening contralateral limb performance over time of those with ACL reconstruction.^10,31–33 A similar interpretation can be used for the results found for our sample of young adults with PFP in which the nondifferences in proportion of symmetries across groups may indicate that this method of measurement may overstate patients’ physical performance. In the PFP population, a worsening performance of the pain-free and less painful limbs may reflect bilateral pain-related movement adaptations, deconditioning, lower hip and knee muscle strength or muscle volume, as well as an overall perception of incapacity.^{4,8,16,29,34–36} Although more research is needed to confirm or refute this, we recommend caution when interpreting LSI as a benchmark of FSDT and SLHT performance in people with unilateral and bilateral PFP.

The proportion of asymmetry between limbs during the FSDT was 12% higher for participants with unilateral PFP (40% of asymmetry) than participants with bilateral PFP (28% of asymmetry), despite the lack of significant differences between groups. However, it is important to acknowledge that not all participants with unilateral PFP presented an impaired performance of the pain-free limb. Still, most of them (60%) were symmetrical (ie, painful and pain-free limbs had similar outcomes), which reinforces that researchers and clinicians should avoid using the pain-free limb of people with unilateral PFP as a reference standard of their physical performance during the FSDT. Future studies are needed to investigate why some people with unilateral PFP have both limbs affected and others not.

Research in performance-based measures is still precarious in PFP compared with the literature of other knee disorders. Authors of recent systematic reviews including 30 to 42 studies and less than 13 000 participants have recognized the importance and appropriateness of using clinical tests to assess performance recovery and to predict future self-reported or knee-related outcomes after ACL or meniscal injuries.^6,31 On the other hand, even recognizing the importance of measures of function in the PFP population, no recommendations have been made from the International Patellofemoral Research Network for the inclusion of performance-based measures of function on the essential items that should be reported in PFP studies according to the most recent REPORT-PFP.²⁰ The lack of in-depth investigations into the physical performance of people with PFP may have played a role in this lack of recommendations. Further evidence on reference values or cutoff points, measurement properties (eg, responsiveness), and interventions addressing improvements in performance-based outcomes are encouraged to strengthen future recommendations.

CONCLUSIONS

People with PFP, regardless of the presence of unilateral or bilateral pain, present with lower FSDT and SLHT performance on both painful or most painful and pain-free or less painful limbs than pain-free controls. Also, no differences were found in the proportion of symmetrical/asymmetrical (ie, LSI ≥ 90% or < 90%, respectively) participants across groups and tests. Our results suggest that the pain-free or less painful limb of people with PFP should not be used as a reference standard during clinical or field-based assessments of performance-based measure of function using FSDT and SLHT. Side-to-side symmetries should also be interpreted with caution, as they may overstate the physical performance of people with PFP by not assuming bilateral deficits during FSDT and SLHT. Determining reference values or cutoff points may be helpful to estimate deficits and rehabilitation progresses on performance-based measures of limbs in young adults with both unilateral and bilateral PFP.