Atrophy of the Quadriceps Is Not Isolated to the Vastus Medialis Oblique in Individuals With Patellofemoral Pain
Abstract
Study Design
Cross-sectional.
Objectives
To determine if quadriceps atrophy was present in people with patellofemoral pain (PFP), and whether the vastus medialis oblique (VMO) was selectively involved.
Background
Despite the lack of research investigating individual quadriceps muscle size in individuals with PFP, it has been suggested that selective atrophy of the VMO relative to the vastus lateralis could be associated with PFP.
Methods
The quadriceps muscle sizes of 35 participants with PFP (22 with unilateral and 13 with bilateral symptoms) and 35 asymptomatic control participants matched for age and sex were measured using real-time ultrasound. The thicknesses of the VMO, vastus lateralis, vastus medialis, rectus femoris, and vastus intermedius were measured. Paired-samples t tests were used to compare muscle thickness between limbs in those with unilateral PFP, and independent t tests were used to compare muscle thickness between groups with and without PFP.
Results
In those with unilateral PFP, the thickness of all portions of the quadriceps muscle was statistically smaller in the symptomatic compared to the asymptomatic limb: VMO (P = .038), vastus medialis (P<.001), vastus lateralis (P = .005), vastus intermedius (P = .013), and rectus femoris (P = .045). No difference was found in thickness of any of the portions of the quadriceps on the affected side of people with PFP compared to asymptomatic controls: VMO (P = .148), vastus medialis (P = .474), vastus lateralis (P = .122), vastus intermedius (P = .466), and rectus femoris (P = .508).
Conclusion
Atrophy of all portions of the quadriceps muscles is present in the affected limb of people with unilateral PFP. There was no atrophy of the quadriceps in individuals with PFP compared to those without pathology. Selective atrophy of the VMO relative to the vastus lateralis was not identified in people with PFP. J Orthop Sports Phys Ther 2015;45(8):613–619. Epub 25 Jun 2015. doi:10.2519/jospt.2015.5852
Patellofemoral pain (PFP) is a common source of knee pain in active adolescents28 and adults,15 particularly those involved in running and jumping activities.6,8,24 Exercises to selectively improve the strength or contraction timing of the vastus medialis oblique (VMO) relative to the vastus lateralis (VL) are commonly used in PFP rehabilitation.13,31
These exercises are typically provided under the assumption that there is dysfunction of the VMO, presenting as selective weakness, atrophy, or inhibition.2 It is not clear if these exercises are appropriate in PFP rehabilitation, as there is no consensus on whether VMO dysfunction is present in individuals with PFP, and how to assess this potential dysfunction.
Parameters suggestive of muscle force production may be assessed with measures of muscle strength, activation (electromyography), or size. It is impossible to investigate whether selective dysfunction of the VMO relative to the VL exists in people with PFP using strength measurements, as the force contribution of the individual quadriceps heads cannot be measured in vivo. Electromyography has been used in research and clinical practice to assess for delayed activation of the VMO in individuals with PFP. However, there are inconsistent results from these studies as to whether VMO activation is delayed,4 and it is difficult for clinicians to discern what constitutes a clinically relevant delay in VMO contraction.17 Quadriceps muscle thickness (a parameter of muscle size) has been strongly correlated to quadriceps maximum voluntary contraction,30 which suggests that muscle thickness may be indicative of muscle force. The measurement and comparison of muscle size between individual portions of the quadriceps muscles may be a valid alternative to distinguishing between selective VMO dysfunction and whole-quadriceps dysfunction.
Muscle size is a surrogate of muscle power,26 and muscle atrophy (reduced muscle size) is suggestive of muscle weakness.25 Selective atrophy of the VMO has been speculated to occur in individuals with PFP14,32 and is often used as a justification for targeted VMO exercises.14 However, there is a paucity of literature comparing the size of the VMO relative to the VL between PFP and asymptomatic limbs. Only 1 study has measured VMO and VL size in individuals with PFP in comparison to a control group,27 and there are no published data comparing differences between limbs in individuals with unilateral PFP.12 Further investigation into the relative sizes of the VMO and the VL in individuals with PFP is required to determine if it is appropriate to use selective VMO atrophy to justify targeted VMO strengthening.
Real-time ultrasound can be used to measure parameters of muscle size and is a suitable tool to investigate muscle atrophy through measures of muscle thickness.16 The availability of real-time ultrasound in many physical therapy clinics could allow clinicians to differentiate between selective VMO and whole-quadriceps atrophy in individuals with PFP.
The aims of this study were to measure the individual portions of the quadriceps muscles (vastus medialis [VM], VMO, VL, rectus femoris [RF], and vastus intermedius [VI]) with real-time ultrasound to determine which portions of the quadriceps were potentially atrophied in individuals with PFP, and to investigate if there was selective atrophy of the VMO relative to the VL. A secondary aim of the study was to determine the prevalence of quadriceps atrophy in individuals with PFP.
Methods
Participants
Posters were placed in physical therapy and medical clinics to recruit individuals with PFP and asymptomatic controls. Participants with PFP were matched to controls by age, sex, and lower-limb dominance. A power calculation, performed in G*Power 3.1.3 (Heinrich-Heine-Universität, Düsseldorf, Germany), was used a priori to determine that a sample of 52 participants was required to detect a large difference (effect size, ≥0.8) between limbs. Procedures were approved by the La Trobe University Faculty of Health Sciences Human Ethics Committee. Participants in this study provided written informed consent, and their rights were protected.
Inclusion Criteria
Participants were eligible for inclusion if they had symptoms for 6 weeks or more, were between 18 and 40 years of age, and were diagnosed with PFP. Diagnosis of patellofemoral pain was based on subjective and objective criteria similar to those used in previous studies.1,7 Potential participants were included if they had all of the following: pain in the anterior knee region; atraumatic onset of symptoms; anterior knee pain with 2 or more of the following: running, hopping, squatting, stair negotiation, kneeling, or prolonged sitting; anterior knee pain with at least 2 of the following: palpation of the peripatellar region, compression of the patellofemoral joint, resisted isometric contraction at 0°, 30°, 60°, and 90° of knee flexion; and maximal pain during the past week of at least 3/10 on a numeric pain-rating scale.
Exclusion Criteria
Potential controls and participants with PFP were excluded if they had any history of surgery or significant injury to either lower extremity that resulted in a period of non-weight bearing, back pain, or any internal knee derangement.
In addition, potential participants with PFP were excluded if other potential sources of anterior knee pain (patellar tendinopathy, Hoffa fat-pad syndrome, infrapatellar or suprapatellar bursitis, patellar dislocation/subluxation) were identified. Potential participants with pain isolated to the inferior pole of the patella on palpation were also excluded.
Demographic Data
Data on demographics and factors that could be associated with quadriceps muscle size were obtained for all participants. These data included age, sex, height, weight, body mass index, thigh girth, limb dominance, and activity level (Marx activity rating scale).23 In the PFP group, symptom duration, symptom severity measured on a 0-to-10 visual analog scale,10 and the self-reported Kujala scale for anterior knee pain21 were also recorded (TABLE 1).
| PFP (n = 35) | Control (n = 35) | P Value | |
|---|---|---|---|
| Age, y | 28.2 ± 6.0 | 28.3 ± 5.8 | .968 |
| Sex, n | … | ||
| Female | 20 | 20 | |
| Male | 15 | 15 | |
| Height, cm | 172.5 ± 8.8 | 171.2 ± 9.6 | .552 |
| Weight, kg | 72.6 ± 12.4 | 72.6 ± 12.7 | .992 |
| Thigh girth, cm | 56.5 ± 3.8 | 55.6 ± 5.1 | .413 |
| Marx activity rating scale (0–16)† | 3.7 ± 4.5 | 2.8 ± 4.0 | .381 |
| Kujala scale (0–100)‡ | 77.6 ± 10.5 | NA | NA |
| Pain severity (0–10)§ | 5.4 ± 1.7 | NA | NA |
Procedures
Ultrasound assessment of both quadriceps was performed on all consenting participants. To determine the location at which to perform the ultrasound measurements of the individual portions of the quadriceps muscle, an anthropometric tape measure was used to measure the distance from the superior tip of the patella to the anterior superior iliac spine. The thickness of the VM was measured at 20% of this distance; the thicknesses of the VI, VL, and RF were measured at 50% of this distance; and the thickness of the VMO was measured at 2 cm above the patella. These locations were based on data from the work of Kawakami et al20 and were intended to correspond to the maximum cross-sectional area (CSA) of each individual section of the quadriceps muscle. To account for the medial location of the VM and VMO, measurements were taken at 12.5% of thigh circumference in the medial direction; to account for the lateral location of the VL, measurements were taken at 10% of thigh circumference in the lateral direction. These measurement points were marked with a pen to allow identification during the ultrasound measurements (FIGURE 1).
Ultrasound
All measurements were performed by a physical therapist with specific training in the use of real-time ultrasound. With the participant lying supine, a strap was placed around both feet to prevent hip external rotation. Ultrasound measurements (HDI 3000; Koninklijke Philips NV, Amsterdam, the Netherlands) were made by placing a 38-mm, 13- to 18-Hz linear transducer over each of the previously identified locations. The images were taken with the probe angled so that the femur was visible and centered in the screen. The depth of the image was adjusted until the femur was visible on screen, and the gain was adjusted until muscle boundaries were also visible on screen. Three images were taken of each muscle and saved in a deidentified format for subsequent analysis. The average of the measures from the 3 images was used for analysis. Sufficient ultrasound gel was used to reduce muscle compression with the transducer head. It was not possible to blind the researcher who captured the images to all of the participants' symptoms; therefore, the images were stored in a deidentified format to blind the researcher to group allocation.
Data Analysis
The images were retrieved from the ultrasound unit in Digital Imaging and Communications in Medicine (DICOM) format, and the measurements of muscle thickness were performed in ImageJ software (National Institutes of Health, Bethesda, MD). Muscle thickness of the VMO, VM, and VI was defined as the distance between the superficial border of the muscle and the most superficial aspect of the femur. Thickness of the VL and RF was defined as the distance between the superficial border and the deep border of the muscle in the direction of the most superficial aspect of the femur (FIGURE 2).
Validity of Real-Time Ultrasound Muscle Thickness Measurements
Measurements of muscle thickness were used because it is not feasible to measure CSA with ultrasound in a clinical setting. A previous investigation11 reported the correlations (Pearson correlation coefficient [r]) between ultrasound and magnetic resonance imaging values of muscle thickness to be VMO = 0.86, VM = 0.86, VL = 0.94, VI = 0.37, and RF = 0.86; and the correlations (Spearman correlation coefficient [rho]) between ultrasound muscle thickness and magnetic resonance imaging CSA measures to be VMO = 0.20, VM = 0.73, VL = 0.83, VI = 0.31, and RF = 0.88. The intrarater reliability of individual ultrasound measures was assessed using the intraclass correlation coefficient, by comparing the measures taken from 3 separate images for each muscle in 68 participants. The reliability of the measurements was excellent, with intraclass correlation coefficient values ranging from 0.96 to 0.98.
Statistical Analysis
Statistical Package for the Social Sciences software (SPSS Inc, Chicago, IL) was used for all data analyses.
For those with unilateral PFP, paired-samples t tests were used to compare the thickness of each portion of the quadriceps muscle, and the proportion in thickness of the VM and VMO relative to the VL, between the symptomatic limbs and the contralateral asymptomatic limbs. Statistical significance was set at P≤.05 for all analyses.
To determine how common quadriceps atrophy is in individuals with PFP, the proportion of participants with unilateral PFP who were classified as having quadriceps atrophy was calculated. To determine what could be considered atrophy, the 95% confidence interval of the mean for the absolute difference in muscle thickness between limbs for the control group was calculated. The lower bound of the 95% confidence interval was 8.5%, establishing a deficit of 8.5% or greater in the sum of thickness values in the affected limb compared to the unaffected limb as the basis for determining quadriceps atrophy.
To identify a net trend in quadriceps atrophy, the sum of the thickness measurements from each individual portion of the quadriceps muscle was calculated. This total quadriceps thickness value was compared between symptomatic limbs and the matched limbs of the control group, and between symptomatic and asymptomatic limbs in those with unilateral PFP.
To evaluate differences in muscle thickness between PFP limbs (affected limb of those with unilateral symptoms and the most painful limb of those with bilateral symptoms) and the limbs of the control group, independent-samples t tests were performed. For data analysis, limbs for the PFP and control groups were matched based on dominance. This analysis was performed for the muscle thicknesses of the VM, VL, VMO, VI, and RF.
To determine whether differences were present in the relative size of the medial and lateral quadriceps in individuals with PFP, differences in the proportion of muscle thickness of the VM relative to the VL, and the VMO relative to the VL, were investigated and compared to individuals in the control group.
Results
A total of 70 participants were recruited. Of the 35 participants with PFP, 22 had unilateral symptoms and 13 had bilateral symptoms. Seven (32%) of the 22 participants with unilateral PFP had symptoms in their dominant (kicking) limb. There was no statistically significant difference between groups in height, weight, or activity level (TABLE 1).
Individuals With Unilateral PFP
Significantly smaller muscle thickness was identified in all of the portions of the quadriceps (VMO, VM, VL, VI, and RF), as well as in the whole quadriceps (sum of all the portions), in symptomatic compared to asymptomatic limbs of individuals with unilateral PFP (TABLE 2). No significant difference between limbs was found for the thickness ratio of the VMO relative to the VL and of the VM relative to the VL.
TABLE 2
Comparison of Quadriceps Muscle Thickness Between Symptomatic and Asymptomatic Limbs of 22 Participants With Unilateral PFP
| PFP Limb | Asymptomatic Limb | Mean Difference† | P Value | |
|---|---|---|---|---|
| VMO | 2.37 ± 0.37 | 2.51 ± 0.46 | −0.15 (−0.29, −0.01) | .038 |
| VM | 2.95 ± 0.53 | 3.24 ± 0.52 | −0.29 (−0.43, −0.15) | <.001 |
| VL | 2.38 ± 0.32 | 2.52 ± 0.36 | −0.14 (−0.23, −0.05) | .005 |
| VI | 1.98 ± 0.51 | 2.16 ± 0.47 | −0.18 (−0.31, −0.04) | .013 |
| RF | 2.27 ± 0.36 | 2.35 ± 0.34 | −0.08 (−0.16, 0.00) | .045 |
| Quadriceps‡ | 11.95 ± 1.63 | 12.79 ± 1.75 | −0.84 (−1.21, −0.47) | <.001 |
| VMO/VL§ | 1.02 ± 0.17 | 1.02 ± 0.18 | −0.01 (−0.07, 0.06) | .833 |
| VM/VL§ | 1.25 ± 0.21 | 1.30 ± 0.22 | −0.05 (−0.14, 0.03) | .209 |
Smaller total quadriceps muscle thickness (greater than 8.5% deficit) was found in the symptomatic limb of 10 (45%) of the 22 participants with unilateral symptoms compared to the asymptomatic limb.
Between-Group Comparisons
The muscle thicknesses of the VMO, VM, VL, VI, and RF in the limb of those with PFP were not significantly different from those of the same muscles in the matched limb of those in the control group (TABLE 3). Similarly, the ratio in muscle thickness of the VMO relative to the VL and of the VM relative to the VL was no different between PFP and control limbs.
TABLE 3
Comparison of Quadriceps Muscle Thickness Between Those With PFP and a Matched Limb of Those in the Control Group
| PFP Group (n = 35) | Control Group (n = 35) | Mean Difference† | P Value | |
|---|---|---|---|---|
| VMO | 2.37 ± 0.38 | 2.53 ± 0.49 | −0.15 (−0.36, 0.06) | .148 |
| VM | 2.93 ± 0.61 | 3.04 ± 0.67 | −0.11 (−0.42, 0.20) | .474 |
| VL | 2.33 ± 0.33 | 2.47 ± 0.41 | −0.14 (−0.32, 0.04) | .122 |
| VI | 1.88 ± 0.46 | 1.80 ± 0.48 | 0.08 (−0.14, 0.31) | .466 |
| RF | 2.30 ± 0.37 | 2.25 ± 0.30 | 0.05 (−0.11, 0.21) | .508 |
| Quadriceps‡ | 11.81 ± 1.73 | 12.08 ± 2.03 | −0.27 (−1.17, 0.63) | .554 |
| VMO/VL§ | 1.03 ± 0.17 | 1.03 ± 0.17 | 0.00 (−0.08, 0.08) | .930 |
| VM/VL§ | 1.26 ± 0.25 | 1.24 ± 0.24 | 0.02 (−0.09, 0.14) | .677 |
Discussion
This study showed that each portion of the quadriceps muscle was smaller in the affected limb of individuals with unilateral PFP compared to the unaffected limb, and that in the affected limb, the VMO was not selectively smaller than the VL. No significant difference was found in quadriceps muscle thickness between the symptomatic limbs of those with PFP and the matched limbs of asymptomatic participants. In combination, these results suggest that atrophy of each individual quadriceps head is present in individuals with unilateral PFP, and that there is no difference in the amount of atrophy of the VMO relative to the VL in those with PFP.
A systematic literature review with meta-analysis, published in 2013, concluded that there was quadriceps atrophy between limbs in individuals with unilateral PFP.12 However, there was a lack of data on whether the atrophy was present throughout the quadriceps or isolated to a specific section of the quadriceps (eg, the VMO). To our knowledge, the current study is the first to investigate and identify atrophy of each portion of the quadriceps muscle (VMO, VM, VL, VI, and RF) between limbs of people with unilateral PFP,27 and the results showed that atrophy of the quadriceps, as defined by a difference between limbs of at least 8.5%, is common, with a prevalence of 45% in those with unilateral PFP.
A previously published systematic review12 also found quadriceps atrophy in people with PFP compared to an asymptomatic population. One previous study27 measured each individual section of the quadriceps and found atrophy of the VL at the mid thigh and of the VMO at the distal thigh between symptomatic and asymptomatic participants. Another study19 found VMO atrophy in those with PFP but did not measure the other portions of the quadriceps. The finding of this study that there was no significant difference in the thicknesses of any of the portions of the quadriceps between people with and without PFP was inconsistent with the conclusion of quadriceps atrophy in the systematic review.12 This could be explained by an unknown difference in baseline characteristics between groups beyond age, height, weight, limb dominance, or sex, which were all similar.
Selective atrophy of the VMO relative to the VL is often suggested to be present in individuals with PFP, despite a lack of supporting data.5,14,18 One previous study investigated the size of the VMO relative to the VL and found no difference between people with and without PFP.27 The findings of the current study are consistent with those of Pattyn et al27 and suggest that there is no selective atrophy of the VMO in people with PFP compared to asymptomatic control participants. Additionally, to our knowledge, this is the first study to investigate the size of the VMO relative to the VL in individuals with unilateral PFP, and the findings suggest that there is also no selective atrophy of the VMO of the affected limb compared to the unaffected limb in this population.
It is impossible from these data to determine whether smaller quadriceps size is a predisposing factor to PFP, or whether it occurs after the onset of pain. Lesser quadriceps strength has been identified as a risk factor for PFP,22 and it is reasonable to expect that lesser strength is associated with smaller quadriceps size.
Clinical Relevance
Selective atrophy of the VMO is often inferred from visual inspection of the quadriceps, especially the VMO bulk, in the clinical examination of individuals with PFP.3,14,29 The medial location and the teardrop shape of the VMO may make it the easiest portion of the quadriceps to visually compare muscle bulk between limbs. The finding that atrophy is present throughout the quadriceps in individuals with unilateral PFP suggests that caution should be applied when interpreting reduced VMO bulk on visual inspection as selective atrophy of the VMO.
Exercises targeting the VMO are typically prescribed on the premise that there is selective dysfunction of the VMO relative to the VL.9 The results from the current study, combined with results from previous research,27 suggest that there is no difference in the size of the VMO relative to the VL in individuals with PFP. This, combined with a systematic review that did not identify a significant delay in VMO contraction in individuals with PFP,4 questions the justification of selective VMO dysfunction and the attempts to selectively activate the VMO in PFP rehabilitation. It is also unclear that any exercises can preferentially activate and strengthen the VMO. If selective VMO dysfunction does exist in individuals with PFP, future research investigating alternative methods of identifying and rehabilitating selective VMO dysfunction is required.
The findings of this study suggest that real-time ultrasound can detect quadriceps atrophy in individuals with unilateral PFP. Quadriceps muscle thickness measurements can be performed relatively quickly with ultrasound, and it is feasible for clinicians to use these measurements in clinical assessment. However, it is difficult for clinicians to assess for quadriceps atrophy in people with bilateral symptoms. The data from unilateral PFP suggest that quadriceps atrophy is common in people with PFP (45% of our participants), and when extrapolating these data to those with bilateral symptoms, the high likelihood that quadriceps atrophy is present should be taken into consideration when deciding if quadriceps-strengthening exercises are appropriate.
Limitations
Caution is advised when extrapolating the results from between-group analysis to suggest that there is no quadriceps atrophy in individuals with PFP, as the results are in contrast to findings of a recent meta-analysis that found quadriceps atrophy in those with PFP.12 The validity of ultrasound thickness measurements of the VI was poor.11 The VMO measurement was strongly correlated to magnetic resonance imaging measurement of muscle thickness and poorly correlated to muscle CSA; caution should be applied in extrapolating the findings of VMO thickness to gross VMO morphology. The assessor was blind to group allocation when measuring muscle thickness, but not to the images captured. The muscle thickness measurements were performed at rest; the relationship between VMO and VL morphology during the contracted state could be different.
Conclusion
In individuals with unilateral PFP, smaller muscle thickness of the VM, VMO, VL, RF, and VI was found in their affected limb when compared to their contralateral side. In this same group, no difference was found in the proportional thickness of the VMO relative to the VL between limbs, suggesting that atrophy is not specific to the VMO. These findings suggest that atrophy is present in all portions of the quadriceps in individuals with PFP, and that exercises with the goal to preferentially activate and strengthen the VMO are not justified in this population.
Key Points
Findings
The thickness of each portion of the quadriceps muscle is smaller in the affected limb of people with unilateral PFP.
Implications
Strengthening interventions should target the entire quadriceps, and exercises with the goal to preferentially activate and strengthen the VMO are not justified in this population.
Caution
This study does not establish whether the quadriceps atrophy is a cause or effect of PFP.
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