Introduction

The patellofemoral compartment of the knee is the most frequently affected by osteoarthritis (OA) [1,2], which is associated with considerable pain and functional limitations during activities of daily living [3–6]. However, studies on OA of the knee focus mainly on individuals with OA in the tibiofemoral compartment or both compartments, whereas isolated patellofemoral osteoarthritis (PFOA) is studied little.

Consequently, the literature on biomechanics associated with tibiofemoral OA is vast, especially OA in the medial tibiofemoral compartment. This preference likely reflects the greater incidence of the condition, as activities of daily living often involve weight-bearing loads, as occurs during gait. Throughout the gait cycle, an external adduction moment tends to rotate the tibia medially in relation to the femur [7–10]. Such mechanics lead to the compression of the tibiofemoral compartments, especially the medial compartment [11,12]. Thus, a high knee adduction moment reflects an increase in compressive forces on the medial face of the knee [13,14], which may be one of the predisposing factors for the emergence of OA in these compartments. However, data on biomechanical characteristics in individuals with isolated PFOA are scarce.

As patellofemoral pain (PFP) is speculated to be a precursor of PFOA [6,15,16], altered biomechanics may be similar between the two conditions. Studies have reported an increase in hip adduction, knee abduction, contralateral pelvic drop and ipsilateral trunk lean in individuals with PFP during functional activities involving body weight bearing compared to healthy individuals [17–19]. The increase in these movements on the frontal plane can result in an increase in patellofemoral stress [20–22].

As occurs in PFP, some studies have found an increase in hip adduction during gait and knee abduction during the task of sit to stand in individuals with PFOA compared to healthy controls [23,24]. On the other hand, a recent study found no difference in the kinematics of the pelvis, hip or knee during the single-leg squat at 45° of knee flexion between individuals with and without PFOA [25]. However, the authors of the study evaluated the kinematics of the pelvis and lower limb only at this specific angle of knee flexion. As the movement pattern can change throughout the knee flexion range, the kinematics of the pelvis and lower limb should also be evaluated at other points of this range.

An important limitation of previous studies on segment/joint kinematics in individuals with PFOA is the failure to evaluate trunk movement. As stated above, previous studies found that individuals with PFP have excessive ipsilateral lean of the trunk during functional tasks involving weight bearing, such as the single-leg squat and the stepping task [17,18]. This excessive ipsilateral trunk lean may occur as compensation for contralateral pelvic drop due to weakness of the hip abductors [21]. In turn, excessive ipsilateral trunk lean results in an external abductor moment at the knee, increasing the patellofemoral load [17,18].

The strength of the hip abductors is lower in individuals with isolated PFOA [26–28]. It is known that the hip abductors are involved in the control of the movements of the pelvis, hip and knee on the frontal plane [17,18,29]. Thus, it is important to determine whether the kinematics of these segments/joints on the frontal plane are altered in this population at different points of the knee flexion range during the single-leg squat. A better understanding of this aspect could assist in the planning of rehabilitation programs for individuals with PFOA.

Therefore, the aim of the present study was to compare frontal plane trunk, pelvis, hip and knee motion at 30°, 45° and 60° of knee flexion during the descending and ascending of the single-leg squat, and isometric hip abductor strength between individuals with and without isolated PFOA. We hypothesized that individuals with isolated PFOA would exhibit greater ipsilateral trunk lean, contralateral pelvic drop, hip adduction and knee abduction and would have weaker isometric hip abductor strength.

Materials and methods

Results

From a list of 108 individuals, 88 were excluded based on the exclusion criteria or failed to return for the subsequent assessments. Of these, 44 individuals had mixed OA (in patellofemoral and tibiofemoral [grade ≥ 2] compartments), 12 had isolated PFOA grade 1, and three had isolated tibiofemoral OA. Twenty participants matched the eligibility criteria. The demographic and anthropometric characteristics of the groups are displayed in Table 1. Nine of the ten participants in the PFOA group had isolated PFOA. Only one participant in the group had doubtful OA (KL grade 1) in the tibiofemoral compartment. No significant differences between groups were found regarding age or body mass index (p > 0.05). The two groups were similar in terms of physical activity level. In comparison to the control group, the PFOA group had higher scores on all WOMAC domains (p ≤ 0.002) (Table 1).

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Table 1. Demographic and clinical characteristics of patellofemoral osteoarthritis group and control group.

https://doi.org/10.1371/journal.pone.0267446.t001

The results of the kinematic analysis are presented in Table 2. The individuals with isolated PFOA had a significantly larger hip adduction angle during the single-leg squat at 45° (mean difference [95% CI] = 6.44° [0.39–12.48°]) and 60° (mean difference [95% CI] = 8.44° [2.15–14.73°]) of knee flexion in the descending phase as well as at 60° (mean difference [95% CI] = 7.58° [2.1–13.06°]) and 45° (mean difference [95% CI] = 5.33° [0.24–10.42°]) of knee flexion in the ascending phase. No significant differences between groups were found regarding trunk lean, pelvic elevation or knee abduction at 30°, 45° and 60° of knee flexion in the descending or ascending phases (p > 0.05). The PFOA group presented lower isometric hip abductor torque compared to the control group (mean difference [95% CI] = -0.34 Nm/kg [-0.67 to -0.01 Nm/kg]), exhibiting an average of 21.9% less isometric hip abductor torque.

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Table 2. Between-group comparisons of frontal plane joint angles during single-leg squat (in degrees) and normalized isometric hip abductor torque (Nm/kg).

https://doi.org/10.1371/journal.pone.0267446.t002

Discussion

The aim of the present study was to compare the frontal plane kinematics of the trunk, pelvis, hip and knee at 30°, 45° and 60° of knee flexion during the descending and ascending phases of the single-leg squat and isometric hip abductor torque between individuals with and without isolated PFOA. The results of this study partially confirmed the hypotheses, demonstrating that individuals with symptomatic radiographic PFOA present larger hip adduction angles at 45° and 60° of knee flexion in both the descending and ascending phases of the single-leg squat and have less capacity to generate isometric hip abductor torque in comparison to individually matched controls. Nakagawa et al. [18] reported similar results for individuals with PFP during the descending and ascending phases of the stepping task compared to control subjects. This finding is important, as excessive hip adduction and knee abduction are the main components of dynamic knee valgus in the frontal plane [22] and an increase in dynamic knee valgus results in an increase in the quadriceps angle (Q angle), with a consequent increase in lateralizing forces that act on the patella, leading to greater stress on the lateral patellofemoral joint [21]. Larger hip adduction angles in individuals with PFOA have also been found in the late support phase of the gait cycle [24].

In contrast, Macri et al. [25] found no significant differences in hip adduction angles during the single-leg squat at 45° of knee flexion between individuals with and without PFOA. This divergence may be due to differences in the evaluation systems used. A three-dimensional motion analysis system was used in the present investigation, whereas Macri et al. [25] used a two-dimensional system to estimate the alignment of the pelvis, hip and knee on the frontal plane. The eligibility criteria also differed between the two studies. Macri et al. [25] included individuals with Grade ≥ 1 PFOA according to the KL classification (individuals with radiographic proof of at least doubtful narrowing of the joint space with the possible formation of osteophytes), whereas only individuals with Grade 2 or 3 PFOA were included in the present study (radiographic proof of definite osteophyte formation and possible or definite narrowing of the joint space with some sclerosis and possible deformity of the bone extremities). Moreover, Macri et al. [25] did not mention the relative and absolute frequency of the degrees of PFOA among the individuals included in the study. Finally, the individuals with PFOA in the study by Macri et al. [25] did not present a reduction in isometric hip abduction strength compared to the control subjects. As the hip abductor muscles act eccentrically to control adduction of this joint during the single-leg squat, the strength deficit found in the present study may be associated with the greater hip adduction in the PFOA group. Such factors may explain the divergence in the findings between the two studies.

Contrary to our hypothesis, no significant differences between groups were found regarding the kinematics of the trunk and pelvis. We had hypothesized that individuals with isolated PFOA would present greater contralateral pelvic drop and greater ipsilateral trunk lean, as found in a previous study involving individuals with PFP during the single-leg squat and the stepping task [17,18]. Excessive ipsilateral trunk lean may be compensation for the weakness of the hip abductor muscles [21]. However, although the PFOA group in the present study exhibited a 21.9% deficit in isometric hip abductors torque, this deficit did not result in changes in the kinematics of the pelvis or trunk. It is possible that greater deficits in hip abduction strength are necessary before the occurrence of changes in trunk and pelvis kinematics during the single-leg squat. Pohl et al. [28] also found no difference between groups with and without PFOA regarding peak contralateral pelvic drop during gait on a treadmill. In contrast, Crossley et al. [24] found that individuals with PFOA exhibited greater contralateral pelvic drop during the late support phase of the gait cycle. As the authors did not evaluate hip abductor strength, it is not possible to determine whether the individuals with PFOA in the study by Crossley et al. [24] exhibited greater strength deficit compared to those in the present study, which could result in excessive contralateral pelvic drop.

Also contrary to our hypotheses, no difference between groups was found regarding the movement of the knee in the frontal plane. In contrast, Hoglund et al. [23] found greater knee abduction in individuals with PFOA during the tasks of sitting and standing. The movement of the trunk on the frontal plane can alter the load and position of the knee. Excessive ipsilateral trunk lean displaces the vector of the ground reaction force laterally to the knee, producing an external knee abduction moment [21], which can contribute to an increase in abduction in this joint [51,52]. Therefore, the lack of a difference in the trunk movement on the frontal plane may explain the lack of a difference between groups regarding the knee movement found in the present study.

To the best of our knowledge, this is the first study to evaluate frontal plane trunk, pelvis, hip and knee motion at different angles of knee flexion during the single-leg squat in individuals with and without PFOA. It is important to recognize kinematic changes in the trunk, pelvis, hip and knee at different angles of knee flexion during the descending and ascending phases of a given functional task, such as the single-leg squat, to design more specific and effective treatment protocols for individuals with PFOA. For instance, the present study showed that individuals with isolated PFOA have lower isometric hip abductor torque and an increase in adduction in this joint, suggesting that it would be potentially beneficial to include strengthening exercises targeting the hip abductors in the treatment of such individuals. A recent viability study found that a six-week program of core and hip muscle strengthening for individuals with PFOA can reduce pain in the short term and this improvement can be maintained for at least six months [53]. However, large randomized controlled trials are needed for a better understanding of the effects of hip muscle strengthening programs on pain and function in the long term for this population.

This study has limitations that should be considered. The sample size may have led to the lack of differences between groups regarding the other kinematic variables of the trunk, pelvis and knee (type II error). Thus, future studies with a larger sample size may find differences between groups for the other variables. The cross-sectional design limits the ability to establish cause-and-effect relationships. Therefore, prospective studies are needed to enable drawing definitive conclusions regarding the role of the kinematics of the trunk, pelvis, hip and knee as well as hip muscle strength in individuals with PFOA. Previous studies have shown that individuals with PFP have altered hip and knee kinematics in the transverse plane as well as strength deficits regarding hip extension and external rotation. The present study only evaluated the hip and knee kinematics on the frontal plane and hip abduction strength. Thus, future studies should evaluate the kinematics of the hip and knee on the transverse plane at different angles of knee flexion during the single-leg squat as well as hip extension and external rotation strength. Moreover, the kinematics of the trunk, pelvis, hip and knee in individuals with isolated PFOA should be evaluated during the execution of other functional tasks.

Conclusions

The individuals with isolated PFOA presented increased hip adduction at 45° and 60° of knee flexion in the both descending and ascending phases of the single-leg squat in comparison to healthy controls. The individuals with isolated PFOA also exhibited less capacity to generate hip isometric abductor torque. The results of this study suggest that hip abductor strengthening and motor control training with a focus on frontal plane hip alignment may be appropriate in the management of PFOA.