Knee Surg Sports Traumatol Arthrosc DOI 10.1007/s00167-014-2943-3
KNEE
Increased medial foot loading during drop jump in subjects with patellofemoral pain Michael S. Rathleff • Camilla Richter • Christoffer Brushøj • Jesper Bencke • Thomas Bandholm • Per Ho¨lmich • Kristian Thorborg
Received: 17 September 2013 / Accepted: 10 March 2014 Ó Springer-Verlag Berlin Heidelberg 2014
Abstract Purpose To compare medial-to-lateral plantar forces during drop jump and single leg squat in individuals with and without patellofemoral pain. Methods This cross-sectional study compared 23 young adults with patellofemoral pain to 20 age- and sex-matched controls without knee pain. The plantar pressure distribution was collected during drop jump and single leg squat using pressure-sensitive Pedar insoles, inserted into a standard flat shoe. The primary outcome was the medial-tolateral force, quantified as the peak force under the medial forefoot as the percentage of force under the total forefoot during drop jump. Secondary outcomes included peak medial-to-lateral force during single leg squat and mean forces during drop jump and single leg squat. Results The primary outcome showed that individuals with patellofemoral pain had a 22 % higher medial-tolateral peak force during drop jump, (p = 0.03). Secondary outcomes showed 32 % higher medial-to-lateral peak force during single leg squat (p = 0.01) and 19–23 % higher M. S. Rathleff (&) Orthopeadic Surgery Research Unit, Research and Innovation Centre, Aalborg University Hospital, 15 Soendre Skovvej, 9000 Aalborg, Denmark e-mail: [email protected]
medial-to-lateral mean force during drop jump and single leg squat (p = 0.02–0.04). Conclusion These findings indicate that individuals with patellofemoral pain display a more medially oriented loading pattern of the forefoot compared to individuals without knee pain. This loading pattern may be associated with the distribution of forces acting on the patellofemoral joint and suggest treatment of PFP should consider interventions that target normalisation of foot loading. Level of evidence III. Keywords Insoles
Patellofemoral pain syndrome Pedar
Introduction Patellofemoral pain is a common knee condition among individuals participating in athletic activities [7, 32, 33] and accounts for up to 25 % of all knee complaints [31]. T. Bandholm K. Thorborg Physical Medicine and Rehabilitation Research – Copenhagen (PMR-C), Department of Physical Therapy, Copenhagen University Hospital, Hvidovre, Copenhagen, Denmark
C. Richter A-Physiotherapy, Solroed Strand, Denmark
T. Bandholm Department of Orthopedic Surgery, Copenhagen University Hospital, Hvidovre, Copenhagen, Denmark
C. Brushøj Department of Orthopaedic Surgery M, Institute of Sports Medicine, Bispebjerg Hospital, Copenhagen, Denmark
T. Bandholm Clinical Research Centre, Copenhagen University Hospital, Hvidovre, Copenhagen, Denmark
J. Bencke The Gait Analysis Laboratory, Department of Orthopaedic Surgery, CopenhagenUniversity Hospital, Hvidovre, Copenhagen, Denmark
P. Ho¨lmich K. Thorborg Sports Orthopedic Research Center - Copenhagen, Arthroscopic Centre Amager, Copenhagen University Hospital, Amager-Hvidovre, Denmark
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Patellofemoral pain is peri-patellar or retro-patellar pain during activities that load the knee extensors and the patellofemoral joint [26]. These activities include jumping and landing, but also squatting on one leg [19, 21]. In patients with patellofemoral pain, clinicians therefore often assess hip, knee and foot kinematics visually during a drop jump or a single leg squat to assess loading patterns. Kinematics and kinetics around the knee joint are affected by distal (foot) [8] and proximal factors (hip) [8, 25, 28]. Individuals with patellofemoral pain display higher degrees of hip adduction, internal rotation of the femur and valgus of the knee during functional activities such as drop jump and stair descent compared to healthy individuals [19, 23, 25, 30, 35]. The medially oriented loading pattern may increase lateral forces acting on the patella and thereby increase compressive forces at the lateral aspect of the patellofemoral joint [9, 15, 23]. The large forces imposed on the knee during these high-load activities are transmitted from the foot and up through the kinetic chain to the knee. Loading of the knee and the patellofemoral joint is therefore dependent on force dissipation and loading patterns of the foot [4, 8, 24]. It has therefore been suggested by several authors that loading of the foot may be involved in the pathogenesis of patellofemoral pain [5, 7, 17]. However, while medial loading of the foot has been suggested as an important factor in patellofemoral pain, limited research on the footloading pattern in patellofemoral pain exists. The existing research in this area has only quantified loading of the foot during gait and stair descent [1, 2], and no studies have measured loading of the foot during dynamic and sportsrelevant tasks, such as drop jump or single leg squat in individuals with patellofemoral pain. The aim of this study was therefore to compare the medial-to-lateral plantar forces of the foot during drop jump and single leg squat in individuals with and without patellofemoral pain. The primary hypothesis was that individuals with patellofemoral pain would have greater peak medial-to-lateral foot loading during a drop jump compared to individuals without knee pain.
Materials and methods This cross-sectional study compared 23 young adults diagnosed with patellofemoral pain to 20 matched, pain-free young adults, of same age and sex. The reporting of the study complies with the ‘‘strengthening the reporting of observational studies in epidemiology’’ (STROBE) statement. Recruitment All 23 individuals with patellofemoral pain were diagnosed by an experienced orthopaedic surgeon at Amager
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Hospital. The individuals without knee pain were recruited through advertisements in schools and local exercise gyms from the same city (Copenhagen) as the individuals with patellofemoral pain. Inclusion criteria for the individuals with patellofemoral pain were anterior or retro-patellar knee pain for more than 6 weeks, which was determined by pain provocation and reproduction of their symptoms (yes or no) in two of the following positions or functions: prolonged sitting, prolonged kneeling, squatting, running, hopping or stair walking. Exclusion criteria were concomitant injury or pain from the hip, lumbar spine or other structures of the knee, previous knee surgery, symptoms of patellofemoral instability or knee joint effusion. Exclusion criteria for the healthy individuals were injury and/or pain in the lower limb in the previous 12 months, or current use of orthotics. Plantar pressure distribution during drop jump and single leg squat The tester who collected in-shoe plantar pressure data was a physiotherapist and not blinded to which of the 43 subjects were diagnosed with patellofemoral pain. In-shoe plantar pressure distribution was collected with Pedar insoles (Novel gmbh, Munich Germany), while individuals were wearing standard shoes (Le Coq Sportif) with flat soles and no medial arch support. Pedar insole system is a valid method for measuring in-shoe foot loading and has a high accuracy with less than 3 % mean difference from day to day during dynamic tasks [3, 11, 12]. Data from the Pedar insoles sensors were sent to a computer through Bluetooth, and data were recorded and analysed using a commercially available software package (Novel). Data were not analysed until data from all subjects had been collected. The insoles had two capacitive sensors per cm2, which measure plantar pressure values from 20 to 600 kPa and sample data at 100 Hz. The data from the Pedar pressure-sensitive insoles were divided into nine zones: the hallux, 2–5 metatarsal bones, the medial forefoot, the central forefoot, the lateral forefoot, the medial midfoot, the lateral midfoot, the medial rearfoot and the lateral rearfoot, using a commercially available mask from Novel (Fig. 1). We quantified the medial-to-lateral force as the peak and mean force under the medial forefoot as the percentage of the total peak and mean, respectively, under the total forefoot. The medial-to-lateral peak force was calculated as the peak force under the medial forefoot divided by the total peak force under medial, central and lateral forefoot, and expressed as a percentage. Likewise, the mean medial-to-lateral force was calculated as the mean force under the medial forefoot divided by the total mean force under medial, central and lateral forefoot, and again
Knee Surg Sports Traumatol Arthrosc
Before individuals performed drop jumps and single leg squat, they were informed that we in this study were interested in measuring how they load their foot during common functional tasks. The tester instructed the individuals to perform a drop jump by dropping off a 20 cm high box and perform a maximum vertical jump upon foot–ground contact (Fig. 2). During the single leg squat, individuals were instructed to stand on one leg and perform a single leg squat to 90 ° knee flexion (Fig. 3). Timing of the single leg squat was controlled using 3 s to go down to 90 ° of knee flexion and 3 s to go up to 0 ° of knee flexion. The knee flexion angle (0 ° and 90 °) was visually determined by the tester. The other leg was kept behind the squatting leg. During both tests, individuals were instructed to keep their arms crossed above their chest. This was chosen to eliminate the influence of arm swing. Individuals performed two familiarisation trials before three successful trials of each test were recorded for analyses. As we were interested in a representative measure of foot-loading patterns, the mean of three trials from each task was used as the data point. Primary and secondary outcome The plantar pressure distribution was collected from the most painful side in individuals with bilateral patellofemoral pain and from an identical proportion of right and left knees among the pain-free individuals. The primary outcome was peak medial-to-lateral force during drop jump, as this task is associated with greater impact forces compared to single leg squat, and has the lowest limits of agreement (LoA) (see next paragraph ‘‘Reliability analysis’’ for further details on the agreement). Secondary outcomes were mean force during drop jump and peak and mean force during single leg squat. The difference between groups is reported as (1) the absolute difference in percentages and (2) the relative difference between groups. The relative difference between groups is calculated as the difference in percentages between groups divided by the average percentage medial-to-lateral foot loading among individuals with patellofemoral pain. Fig. 1 The mask used in Novel software to divide the foot into nine zones
expressed as a percentage. Peak and mean forces were collected during two functional tasks: (1) a drop jump and (2) a single leg squat. Only the loading of the forefoot was analysed, as previous studies have shown that some individuals use a strategy during drop jump in which the rear foot does not touch the floor before take-off [14]. Peak force was chosen as the primary outcome as peak forces during plantar loading of the foot in running have been associated with an increased risk of developing patellofemoral pain [32].
Reliability analysis Twenty young adults were recruited for an intratester, test– retest reliability study. They were not part of the crosssectional study. They were tested twice, with 30 min between the test and retest. Intraclass correlation coefficients (ICC) from a two-way random-effects model (2.1, single measures, consistency definition) were used to express intratester reliability. LoA were used to express the agreement between test and retest. Paired samples t test showed no systematic differences (numerical differences of less than 1 %, p [ 0.46) between test and retest for any of
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Fig. 2 Drop jump divided into four phases: 1 start, 2 dropping off the box with their arms crossed above their chest, 3 landing and 4 a maximum vertical jump right after foot–ground contact. Consent has been obtained for the subject in the picture
and LoA of (-12.6 to 15.0 %) and (-14.7 to 16.6 %), respectively. The study was approved by the local ethics committee of the Capital Region of Denmark (HKF322185). All participants were required to give written informed consent. Statistical analysis
Fig. 3 Single leq squat performed form 0 ° knee flexion to 90 ° knee flexion. Consent has been obtained for the subject in the picture
the investigated variables. ICC and LOA for peak force and mean force during drop jump were ICC 0.77 (95 % CI 0.50–0.91) and ICC: 0.85 (95 % CI 0.64–0.94), and LoA were (-10.2 to 9.6 %) and (-10.4 to 10.2 %), respectively. A slightly lower reliability and agreement were found for peak force and mean force during SLS with ICC of 0.54 (95 % CI 0.14–0.79) and 0.58 (95 % CI 0.20–0.81) Table 1 Demographics
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The sample size was based on the primary outcome and was powered to detect an absolute difference between groups of more than 10 % in the medial-to-lateral foot peak force during the primary outcome (drop jump). Based on our reliability study, a change of more than 10 % can be detected at the individual level and was therefore considered a relevant change in the present study. Using a common standard deviation of 11 %, power of 80 % and an alpha level of 5 % (p \ 0.05), at least 19 individuals were required in each group to detect a 10 % difference in medial-to-lateral loading of the forefoot. Statistical analyses were performed using IBM SPSS Statistics (SPSS for Windows, version 20). All data demonstrated a normal distribution (tested using histograms and QQ-plots and
PFP, n = 23 Mean (±SD)
Pain-free, n = 20 Mean (±SD)
p value
Sex (number of females)
10
10
n.s
Age (years)
25.8 (7.4)
26.6 (3.1)
n.s
Height (cm)
174.4 (10.9)
176.2 (9.2)
n.s
Weight (kg)
71.2 (14.0)
72.1 (14.1)
n.s
Foot length (cm)
25.3 (2.0)
25.5 (1.8)
n.s
Proportion of feet that was tested (% right feet)
88.3
80.0
n.s
Average physical activity per week (minutes)
343.7 (272.4)
536.0 (204.9)
0.02
Knee Surg Sports Traumatol Arthrosc
Primary outcome In the drop jump, individuals with patellofemoral pain had a 10 % higher absolute medial-to-lateral peak force of the forefoot compared to pain-free individuals corresponding to a relative group difference of 22 %, (p = 0.03) (Fig. 4). Secondary outcome
Fig. 4 Medial-to-lateral peak and mean force during drop jump and single leg squat in individuals with patellofemoral pain and individuals without knee pain. Error bars indicate 95 % CI
Table 2 Loading of the forefoot (% of bodyweight) during drop jump and single leg squat PFP, n = 23 Mean (95 % CI)
Pain-free, n = 20 Mean (95 % CI)
Peak force medial forefoot
30.6 (27.8–33.4)
30.1 (25.3–34.9)
Peak force central forefoot
24.0 (21.3–26.7)
25.3 (22.7–27.9)
Peak force lateral forefoot
14.1 (12.0–16.2)
19.6 (16.8–22.4)
Mean force medial forefoot
18.1 (16.2–19.9)
18.5 (15.7–21.4)
Mean force central forefoot
12.9 (11.2–14.6)
15.5 (13.7–17.3)
Mean force lateral forefoot
5.6 (4.6–6.6)
10.0 (7.9–12.0)
Peak force medial forefoot
22.9 (20.6–25.3)
19.3 (16.2–22.4)
Peak force central forefoot
26.3 (24.2–28.5)
24.1 (21.5–26.7)
Peak force lateral forefoot
18.0 (15.6–20.4)
22.2 (19.4–25.0)
Mean force medial forefoot
10.2 (8.8–11.5)
7.2 (5.8–8.7)
Mean force central forefoot Mean force lateral forefoot
11.1 (9.4–12.7) 7.2 (5.8–8.6)
8.6 (7.4–9.8) 8.0 (6.8–9.2)
Drop jump
Single leg squat
The table shows the loading each zone of the forefoot that was used to calculate the medial-to-lateral foot loading
kurtosis and skewness ranged from 0 to 0.70), and parametric tests were applied. Unpaired t-tests were used to compare variables between groups. The level of significance was set to p \ 0.05.
Results Individuals with patellofemoral pain and individuals without knee pain were similar regarding, age, weight, height and foot length (Table 1). Individuals with patellofemoral pain reported less sports activity per week compared to individuals without knee pain (Table 1).
Individuals with patellofemoral pain had a 9 % higher absolute peak force during single leg squat corresponding to a relative group difference of 32 % (p = 0.01). The absolute difference in mean force during drop jump and single leg squat was 8 and 7 %, respectively, corresponding to relative group differences of 19 and 23 %, respectively, (p = 0.02–0.04). The mean forefoot loading across the three zones is shown in Table 2.
Discussion The most important finding of the present study was that individuals with patellofemoral pain display more medially oriented forces under the forefoot during drop jump and single leg squat. These findings are interesting because a more medially oriented foot-loading pattern may increase the lateral forces acting on the patella. Plantar pressure and patellofemoral pain Previous studies investigating plantar pressure among individuals with patellofemoral pain during automated activities such as walking and stair descent have shown contrasting findings [1, 2]. Two cross-sectional studies found a more medially oriented loading of the rearfoot during the initial ground contact during stair descent and gait [1, 2], whereas two prospective studies observed a higher lateral heel loading at heel strike during walking in novice runners who developed patellofemoral pain compared to those who did not develop patellofemoral pain [32, 33]. These two prospective studies observed no trends towards higher medial-to-lateral foot loading. The present study looking at the forefoot during drop jump and single leg squat seems to support the two previous cross-sectional studies [1, 2], who have indicated that a more medial-tolateral oriented loading pattern exists in patients with patellofemoral pain. Such a foot-loading pattern may alter forces acting on the knee and patellofemoral joint. However, the choice of task (drop jump and single leg squat versus walking) and the study sample and design (novice runners and military recruits before they developed patellofemoral pain vs. a mixed population with symptomatic patellofemoral pain subjects engaged in different sports
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Knee Surg Sports Traumatol Arthrosc
activities) may explain some of the differences observed between studies.
Proximal factors Patellofemoral loading may be influenced by both distal and proximal factors [25]. Altered loading of the patellofemoral joint may be caused by the following proximalto-distal mechanism: (1) Increased hip adduction increases knee abduction, which has been shown to increase retropatellar stress [10], and (2) increased hip adduction may tighten the iliotibial band. Tightening of the iliotibial band may then lead to greater lateral forces on the patella through the insertion to the lateral patellar retinaculum [27, 37]. This increase in lateral patellar force may reduce the retropatellar contact area and thereby increase patellofemoral stress. However, the question remains as to whether this mechanism is purely caused by proximal-to-distal factors. Barton et al. [4] showed that greater rearfoot eversion was positively associated with the hip adduction range during gait, which suggests that altered hip kinematics could be influenced by distal factors, or vice versa. This suggests that treatment aiming at decreasing medialisation of the knee should target both the hip and foot. Foot orthotics may be a simple treatment approach that cause an immediate change in foot and knee loading [13]. Joseph et al. [13] demonstrated an immediate decrease in knee valgus and rearfoot eversion during drop jump after inserting a 5 ° medial wedge into the shoes of healthy participants. One of the suspected mechanisms of foot orthotics is that they decrease peak rearfoot eversion, which is associated with less medial loading of the foot [16] and decreased loading at the knee [13, 22]. This could potentially decrease lateral patellofemoral stress and thereby help treat patellofemoral pain [22]. Indeed, previous studies have shown that foot orthotics have a greater clinical effect among individuals with patellofemoral pain who display an increased peak rearfoot eversion during gait [6] and greater midfoot mobility [34]. A change in proximal factors may by be obtained by hip strengthening exercises, which can affect both kinematic at the foot and knee [29]. From the field of ACL research, augmented feedback has been shown to modify key risk factors related to foot, knee and hip kinematics during drop jump [18, 20]. The Pedar insoles may be used as a similar tool with instant feedback on medial-tolateral foot loading during dynamic activities and offer complimentary information in addition to the visual and qualitative assessment of foot-loading patterns. A possible limitation of the study is that individuals with patellofemoral pain were recruited from advertisement, which may decrease the external validity of the sample. This study used shoes without arch support, and it is not
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known whether this may alter the results compared to a design where shoes with arch support are used instead. The assessor was not blinded to subject status which increases the risk of detection bias. However, the tasks were standardised, and objective data were obtained without the possibility to be analysed or manipulated during each assessment. The risk of assessor bias is therefore most likely low [36]. To remove any variability and possible bias due to use of arms, all subjects performed the functional tasks with arms crossed across the chest. It is possible that individuals with patellofemoral pain use their arms to control forefoot-loading patterns, and the difference in medial-to-lateral loading could therefore theoretically be related to removing the influence of the arms during movement. The increased medial-to-lateral foot loading may not be caused by the foot itself. An increased internal rotation at the hip could be associated with increased knee abduction and thereby be responsible for the altered loading pattern of the foot [4, 29]. The interactions and contributions of distal and proximal factors therefore need further investigation. The current finding suggests that treatment of PFP should target the increased medial foot-loading pattern. This may be done through proximal interventions such as hip strengthening exercise or training with feedback, while distal factors may be corrected by foot orthotics.
Conclusion These findings show that individuals with patellofemoral pain display a more medially oriented loading pattern of the forefoot compared to individuals without knee pain. This loading pattern may be associated with the distribution of forces acting on the patellofemoral joint and suggest treatment of PFP should consider interventions that target normalisation of foot loading. Conflict of interest No external funding was received for the current study. There is no conflict of interest.
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Increased medial foot loading during drop jump in subjects with patellofemoral pain Michael S. Rathleff • Camilla Richter • Christoffer Brushøj • Jesper Bencke • Thomas Bandholm • Per Ho¨lmich • Kristian Thorborg
Received: 17 September 2013 / Accepted: 10 March 2014 Ó Springer-Verlag Berlin Heidelberg 2014
Abstract Purpose To compare medial-to-lateral plantar forces during drop jump and single leg squat in individuals with and without patellofemoral pain. Methods This cross-sectional study compared 23 young adults with patellofemoral pain to 20 age- and sex-matched controls without knee pain. The plantar pressure distribution was collected during drop jump and single leg squat using pressure-sensitive Pedar insoles, inserted into a standard flat shoe. The primary outcome was the medial-tolateral force, quantified as the peak force under the medial forefoot as the percentage of force under the total forefoot during drop jump. Secondary outcomes included peak medial-to-lateral force during single leg squat and mean forces during drop jump and single leg squat. Results The primary outcome showed that individuals with patellofemoral pain had a 22 % higher medial-tolateral peak force during drop jump, (p = 0.03). Secondary outcomes showed 32 % higher medial-to-lateral peak force during single leg squat (p = 0.01) and 19–23 % higher M. S. Rathleff (&) Orthopeadic Surgery Research Unit, Research and Innovation Centre, Aalborg University Hospital, 15 Soendre Skovvej, 9000 Aalborg, Denmark e-mail: [email protected]
medial-to-lateral mean force during drop jump and single leg squat (p = 0.02–0.04). Conclusion These findings indicate that individuals with patellofemoral pain display a more medially oriented loading pattern of the forefoot compared to individuals without knee pain. This loading pattern may be associated with the distribution of forces acting on the patellofemoral joint and suggest treatment of PFP should consider interventions that target normalisation of foot loading. Level of evidence III. Keywords Insoles
Patellofemoral pain syndrome Pedar
Introduction Patellofemoral pain is a common knee condition among individuals participating in athletic activities [7, 32, 33] and accounts for up to 25 % of all knee complaints [31]. T. Bandholm K. Thorborg Physical Medicine and Rehabilitation Research – Copenhagen (PMR-C), Department of Physical Therapy, Copenhagen University Hospital, Hvidovre, Copenhagen, Denmark
C. Richter A-Physiotherapy, Solroed Strand, Denmark
T. Bandholm Department of Orthopedic Surgery, Copenhagen University Hospital, Hvidovre, Copenhagen, Denmark
C. Brushøj Department of Orthopaedic Surgery M, Institute of Sports Medicine, Bispebjerg Hospital, Copenhagen, Denmark
T. Bandholm Clinical Research Centre, Copenhagen University Hospital, Hvidovre, Copenhagen, Denmark
J. Bencke The Gait Analysis Laboratory, Department of Orthopaedic Surgery, CopenhagenUniversity Hospital, Hvidovre, Copenhagen, Denmark
P. Ho¨lmich K. Thorborg Sports Orthopedic Research Center - Copenhagen, Arthroscopic Centre Amager, Copenhagen University Hospital, Amager-Hvidovre, Denmark
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Patellofemoral pain is peri-patellar or retro-patellar pain during activities that load the knee extensors and the patellofemoral joint [26]. These activities include jumping and landing, but also squatting on one leg [19, 21]. In patients with patellofemoral pain, clinicians therefore often assess hip, knee and foot kinematics visually during a drop jump or a single leg squat to assess loading patterns. Kinematics and kinetics around the knee joint are affected by distal (foot) [8] and proximal factors (hip) [8, 25, 28]. Individuals with patellofemoral pain display higher degrees of hip adduction, internal rotation of the femur and valgus of the knee during functional activities such as drop jump and stair descent compared to healthy individuals [19, 23, 25, 30, 35]. The medially oriented loading pattern may increase lateral forces acting on the patella and thereby increase compressive forces at the lateral aspect of the patellofemoral joint [9, 15, 23]. The large forces imposed on the knee during these high-load activities are transmitted from the foot and up through the kinetic chain to the knee. Loading of the knee and the patellofemoral joint is therefore dependent on force dissipation and loading patterns of the foot [4, 8, 24]. It has therefore been suggested by several authors that loading of the foot may be involved in the pathogenesis of patellofemoral pain [5, 7, 17]. However, while medial loading of the foot has been suggested as an important factor in patellofemoral pain, limited research on the footloading pattern in patellofemoral pain exists. The existing research in this area has only quantified loading of the foot during gait and stair descent [1, 2], and no studies have measured loading of the foot during dynamic and sportsrelevant tasks, such as drop jump or single leg squat in individuals with patellofemoral pain. The aim of this study was therefore to compare the medial-to-lateral plantar forces of the foot during drop jump and single leg squat in individuals with and without patellofemoral pain. The primary hypothesis was that individuals with patellofemoral pain would have greater peak medial-to-lateral foot loading during a drop jump compared to individuals without knee pain.
Materials and methods This cross-sectional study compared 23 young adults diagnosed with patellofemoral pain to 20 matched, pain-free young adults, of same age and sex. The reporting of the study complies with the ‘‘strengthening the reporting of observational studies in epidemiology’’ (STROBE) statement. Recruitment All 23 individuals with patellofemoral pain were diagnosed by an experienced orthopaedic surgeon at Amager
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Hospital. The individuals without knee pain were recruited through advertisements in schools and local exercise gyms from the same city (Copenhagen) as the individuals with patellofemoral pain. Inclusion criteria for the individuals with patellofemoral pain were anterior or retro-patellar knee pain for more than 6 weeks, which was determined by pain provocation and reproduction of their symptoms (yes or no) in two of the following positions or functions: prolonged sitting, prolonged kneeling, squatting, running, hopping or stair walking. Exclusion criteria were concomitant injury or pain from the hip, lumbar spine or other structures of the knee, previous knee surgery, symptoms of patellofemoral instability or knee joint effusion. Exclusion criteria for the healthy individuals were injury and/or pain in the lower limb in the previous 12 months, or current use of orthotics. Plantar pressure distribution during drop jump and single leg squat The tester who collected in-shoe plantar pressure data was a physiotherapist and not blinded to which of the 43 subjects were diagnosed with patellofemoral pain. In-shoe plantar pressure distribution was collected with Pedar insoles (Novel gmbh, Munich Germany), while individuals were wearing standard shoes (Le Coq Sportif) with flat soles and no medial arch support. Pedar insole system is a valid method for measuring in-shoe foot loading and has a high accuracy with less than 3 % mean difference from day to day during dynamic tasks [3, 11, 12]. Data from the Pedar insoles sensors were sent to a computer through Bluetooth, and data were recorded and analysed using a commercially available software package (Novel). Data were not analysed until data from all subjects had been collected. The insoles had two capacitive sensors per cm2, which measure plantar pressure values from 20 to 600 kPa and sample data at 100 Hz. The data from the Pedar pressure-sensitive insoles were divided into nine zones: the hallux, 2–5 metatarsal bones, the medial forefoot, the central forefoot, the lateral forefoot, the medial midfoot, the lateral midfoot, the medial rearfoot and the lateral rearfoot, using a commercially available mask from Novel (Fig. 1). We quantified the medial-to-lateral force as the peak and mean force under the medial forefoot as the percentage of the total peak and mean, respectively, under the total forefoot. The medial-to-lateral peak force was calculated as the peak force under the medial forefoot divided by the total peak force under medial, central and lateral forefoot, and expressed as a percentage. Likewise, the mean medial-to-lateral force was calculated as the mean force under the medial forefoot divided by the total mean force under medial, central and lateral forefoot, and again
Knee Surg Sports Traumatol Arthrosc
Before individuals performed drop jumps and single leg squat, they were informed that we in this study were interested in measuring how they load their foot during common functional tasks. The tester instructed the individuals to perform a drop jump by dropping off a 20 cm high box and perform a maximum vertical jump upon foot–ground contact (Fig. 2). During the single leg squat, individuals were instructed to stand on one leg and perform a single leg squat to 90 ° knee flexion (Fig. 3). Timing of the single leg squat was controlled using 3 s to go down to 90 ° of knee flexion and 3 s to go up to 0 ° of knee flexion. The knee flexion angle (0 ° and 90 °) was visually determined by the tester. The other leg was kept behind the squatting leg. During both tests, individuals were instructed to keep their arms crossed above their chest. This was chosen to eliminate the influence of arm swing. Individuals performed two familiarisation trials before three successful trials of each test were recorded for analyses. As we were interested in a representative measure of foot-loading patterns, the mean of three trials from each task was used as the data point. Primary and secondary outcome The plantar pressure distribution was collected from the most painful side in individuals with bilateral patellofemoral pain and from an identical proportion of right and left knees among the pain-free individuals. The primary outcome was peak medial-to-lateral force during drop jump, as this task is associated with greater impact forces compared to single leg squat, and has the lowest limits of agreement (LoA) (see next paragraph ‘‘Reliability analysis’’ for further details on the agreement). Secondary outcomes were mean force during drop jump and peak and mean force during single leg squat. The difference between groups is reported as (1) the absolute difference in percentages and (2) the relative difference between groups. The relative difference between groups is calculated as the difference in percentages between groups divided by the average percentage medial-to-lateral foot loading among individuals with patellofemoral pain. Fig. 1 The mask used in Novel software to divide the foot into nine zones
expressed as a percentage. Peak and mean forces were collected during two functional tasks: (1) a drop jump and (2) a single leg squat. Only the loading of the forefoot was analysed, as previous studies have shown that some individuals use a strategy during drop jump in which the rear foot does not touch the floor before take-off [14]. Peak force was chosen as the primary outcome as peak forces during plantar loading of the foot in running have been associated with an increased risk of developing patellofemoral pain [32].
Reliability analysis Twenty young adults were recruited for an intratester, test– retest reliability study. They were not part of the crosssectional study. They were tested twice, with 30 min between the test and retest. Intraclass correlation coefficients (ICC) from a two-way random-effects model (2.1, single measures, consistency definition) were used to express intratester reliability. LoA were used to express the agreement between test and retest. Paired samples t test showed no systematic differences (numerical differences of less than 1 %, p [ 0.46) between test and retest for any of
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Knee Surg Sports Traumatol Arthrosc
Fig. 2 Drop jump divided into four phases: 1 start, 2 dropping off the box with their arms crossed above their chest, 3 landing and 4 a maximum vertical jump right after foot–ground contact. Consent has been obtained for the subject in the picture
and LoA of (-12.6 to 15.0 %) and (-14.7 to 16.6 %), respectively. The study was approved by the local ethics committee of the Capital Region of Denmark (HKF322185). All participants were required to give written informed consent. Statistical analysis
Fig. 3 Single leq squat performed form 0 ° knee flexion to 90 ° knee flexion. Consent has been obtained for the subject in the picture
the investigated variables. ICC and LOA for peak force and mean force during drop jump were ICC 0.77 (95 % CI 0.50–0.91) and ICC: 0.85 (95 % CI 0.64–0.94), and LoA were (-10.2 to 9.6 %) and (-10.4 to 10.2 %), respectively. A slightly lower reliability and agreement were found for peak force and mean force during SLS with ICC of 0.54 (95 % CI 0.14–0.79) and 0.58 (95 % CI 0.20–0.81) Table 1 Demographics
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The sample size was based on the primary outcome and was powered to detect an absolute difference between groups of more than 10 % in the medial-to-lateral foot peak force during the primary outcome (drop jump). Based on our reliability study, a change of more than 10 % can be detected at the individual level and was therefore considered a relevant change in the present study. Using a common standard deviation of 11 %, power of 80 % and an alpha level of 5 % (p \ 0.05), at least 19 individuals were required in each group to detect a 10 % difference in medial-to-lateral loading of the forefoot. Statistical analyses were performed using IBM SPSS Statistics (SPSS for Windows, version 20). All data demonstrated a normal distribution (tested using histograms and QQ-plots and
PFP, n = 23 Mean (±SD)
Pain-free, n = 20 Mean (±SD)
p value
Sex (number of females)
10
10
n.s
Age (years)
25.8 (7.4)
26.6 (3.1)
n.s
Height (cm)
174.4 (10.9)
176.2 (9.2)
n.s
Weight (kg)
71.2 (14.0)
72.1 (14.1)
n.s
Foot length (cm)
25.3 (2.0)
25.5 (1.8)
n.s
Proportion of feet that was tested (% right feet)
88.3
80.0
n.s
Average physical activity per week (minutes)
343.7 (272.4)
536.0 (204.9)
0.02
Knee Surg Sports Traumatol Arthrosc
Primary outcome In the drop jump, individuals with patellofemoral pain had a 10 % higher absolute medial-to-lateral peak force of the forefoot compared to pain-free individuals corresponding to a relative group difference of 22 %, (p = 0.03) (Fig. 4). Secondary outcome
Fig. 4 Medial-to-lateral peak and mean force during drop jump and single leg squat in individuals with patellofemoral pain and individuals without knee pain. Error bars indicate 95 % CI
Table 2 Loading of the forefoot (% of bodyweight) during drop jump and single leg squat PFP, n = 23 Mean (95 % CI)
Pain-free, n = 20 Mean (95 % CI)
Peak force medial forefoot
30.6 (27.8–33.4)
30.1 (25.3–34.9)
Peak force central forefoot
24.0 (21.3–26.7)
25.3 (22.7–27.9)
Peak force lateral forefoot
14.1 (12.0–16.2)
19.6 (16.8–22.4)
Mean force medial forefoot
18.1 (16.2–19.9)
18.5 (15.7–21.4)
Mean force central forefoot
12.9 (11.2–14.6)
15.5 (13.7–17.3)
Mean force lateral forefoot
5.6 (4.6–6.6)
10.0 (7.9–12.0)
Peak force medial forefoot
22.9 (20.6–25.3)
19.3 (16.2–22.4)
Peak force central forefoot
26.3 (24.2–28.5)
24.1 (21.5–26.7)
Peak force lateral forefoot
18.0 (15.6–20.4)
22.2 (19.4–25.0)
Mean force medial forefoot
10.2 (8.8–11.5)
7.2 (5.8–8.7)
Mean force central forefoot Mean force lateral forefoot
11.1 (9.4–12.7) 7.2 (5.8–8.6)
8.6 (7.4–9.8) 8.0 (6.8–9.2)
Drop jump
Single leg squat
The table shows the loading each zone of the forefoot that was used to calculate the medial-to-lateral foot loading
kurtosis and skewness ranged from 0 to 0.70), and parametric tests were applied. Unpaired t-tests were used to compare variables between groups. The level of significance was set to p \ 0.05.
Results Individuals with patellofemoral pain and individuals without knee pain were similar regarding, age, weight, height and foot length (Table 1). Individuals with patellofemoral pain reported less sports activity per week compared to individuals without knee pain (Table 1).
Individuals with patellofemoral pain had a 9 % higher absolute peak force during single leg squat corresponding to a relative group difference of 32 % (p = 0.01). The absolute difference in mean force during drop jump and single leg squat was 8 and 7 %, respectively, corresponding to relative group differences of 19 and 23 %, respectively, (p = 0.02–0.04). The mean forefoot loading across the three zones is shown in Table 2.
Discussion The most important finding of the present study was that individuals with patellofemoral pain display more medially oriented forces under the forefoot during drop jump and single leg squat. These findings are interesting because a more medially oriented foot-loading pattern may increase the lateral forces acting on the patella. Plantar pressure and patellofemoral pain Previous studies investigating plantar pressure among individuals with patellofemoral pain during automated activities such as walking and stair descent have shown contrasting findings [1, 2]. Two cross-sectional studies found a more medially oriented loading of the rearfoot during the initial ground contact during stair descent and gait [1, 2], whereas two prospective studies observed a higher lateral heel loading at heel strike during walking in novice runners who developed patellofemoral pain compared to those who did not develop patellofemoral pain [32, 33]. These two prospective studies observed no trends towards higher medial-to-lateral foot loading. The present study looking at the forefoot during drop jump and single leg squat seems to support the two previous cross-sectional studies [1, 2], who have indicated that a more medial-tolateral oriented loading pattern exists in patients with patellofemoral pain. Such a foot-loading pattern may alter forces acting on the knee and patellofemoral joint. However, the choice of task (drop jump and single leg squat versus walking) and the study sample and design (novice runners and military recruits before they developed patellofemoral pain vs. a mixed population with symptomatic patellofemoral pain subjects engaged in different sports
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Knee Surg Sports Traumatol Arthrosc
activities) may explain some of the differences observed between studies.
Proximal factors Patellofemoral loading may be influenced by both distal and proximal factors [25]. Altered loading of the patellofemoral joint may be caused by the following proximalto-distal mechanism: (1) Increased hip adduction increases knee abduction, which has been shown to increase retropatellar stress [10], and (2) increased hip adduction may tighten the iliotibial band. Tightening of the iliotibial band may then lead to greater lateral forces on the patella through the insertion to the lateral patellar retinaculum [27, 37]. This increase in lateral patellar force may reduce the retropatellar contact area and thereby increase patellofemoral stress. However, the question remains as to whether this mechanism is purely caused by proximal-to-distal factors. Barton et al. [4] showed that greater rearfoot eversion was positively associated with the hip adduction range during gait, which suggests that altered hip kinematics could be influenced by distal factors, or vice versa. This suggests that treatment aiming at decreasing medialisation of the knee should target both the hip and foot. Foot orthotics may be a simple treatment approach that cause an immediate change in foot and knee loading [13]. Joseph et al. [13] demonstrated an immediate decrease in knee valgus and rearfoot eversion during drop jump after inserting a 5 ° medial wedge into the shoes of healthy participants. One of the suspected mechanisms of foot orthotics is that they decrease peak rearfoot eversion, which is associated with less medial loading of the foot [16] and decreased loading at the knee [13, 22]. This could potentially decrease lateral patellofemoral stress and thereby help treat patellofemoral pain [22]. Indeed, previous studies have shown that foot orthotics have a greater clinical effect among individuals with patellofemoral pain who display an increased peak rearfoot eversion during gait [6] and greater midfoot mobility [34]. A change in proximal factors may by be obtained by hip strengthening exercises, which can affect both kinematic at the foot and knee [29]. From the field of ACL research, augmented feedback has been shown to modify key risk factors related to foot, knee and hip kinematics during drop jump [18, 20]. The Pedar insoles may be used as a similar tool with instant feedback on medial-tolateral foot loading during dynamic activities and offer complimentary information in addition to the visual and qualitative assessment of foot-loading patterns. A possible limitation of the study is that individuals with patellofemoral pain were recruited from advertisement, which may decrease the external validity of the sample. This study used shoes without arch support, and it is not
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known whether this may alter the results compared to a design where shoes with arch support are used instead. The assessor was not blinded to subject status which increases the risk of detection bias. However, the tasks were standardised, and objective data were obtained without the possibility to be analysed or manipulated during each assessment. The risk of assessor bias is therefore most likely low [36]. To remove any variability and possible bias due to use of arms, all subjects performed the functional tasks with arms crossed across the chest. It is possible that individuals with patellofemoral pain use their arms to control forefoot-loading patterns, and the difference in medial-to-lateral loading could therefore theoretically be related to removing the influence of the arms during movement. The increased medial-to-lateral foot loading may not be caused by the foot itself. An increased internal rotation at the hip could be associated with increased knee abduction and thereby be responsible for the altered loading pattern of the foot [4, 29]. The interactions and contributions of distal and proximal factors therefore need further investigation. The current finding suggests that treatment of PFP should target the increased medial foot-loading pattern. This may be done through proximal interventions such as hip strengthening exercise or training with feedback, while distal factors may be corrected by foot orthotics.
Conclusion These findings show that individuals with patellofemoral pain display a more medially oriented loading pattern of the forefoot compared to individuals without knee pain. This loading pattern may be associated with the distribution of forces acting on the patellofemoral joint and suggest treatment of PFP should consider interventions that target normalisation of foot loading. Conflict of interest No external funding was received for the current study. There is no conflict of interest.
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