Sex differences in passive lateral and medial patellar mobility in healthy young adults

Abstract

BACKGROUND:

Sex differences in passive patellar mobility have not been quantitatively evaluated.

OBJECTIVE:

The present study aimed to: 1) investigate sex differences in absolute and normalized patellar mobility, and 2) verify the relationship between patellar mobility and knee joint laxity.

METHODS:

Two hundred and five pain-free individuals (205 knees, 103 men, 102 women) participated. Passive lateral and medial patellar displacement was measured using a modified patellofemoral arthrometer and manual pushing method at 0 ${}^{\circ}$ and 30 ${}^{\circ}$ knee flexion angles. The absolute values of lateral and medial patellar displacement (LPD and MPD, respectively), and values normalized to patellar width (PW) and body height (HT) were used in the final analysis.

RESULTS:

At a 30 ${}^{\circ}$ knee flexion angle, LPD/PW and LPD/HT were greater in women than in men (LPD/PW: 27.0 $\pm$ 6.7% vs. 22.6 $\pm$ 6.7%, LPD/HT: 0.81 $\pm$ 0.19% vs. 0.70 $\pm$ 0.15%, respectively, $P<$ 0.001, effect size $>$ 0.50). Absolute and normalized lateral and medial patellar mobility correlated positively with the range of the knee extension angle.

CONCLUSIONS:

Sex differences in LPD/PW and LPD/HT at a 30 ${}^{\circ}$ knee flexion were detected in healthy subjects. Although the knee extension angle related to both the lateral and medial direction of the patellar mobility, the relations were only fairly correlated in asymptomatic young subjects.

Keywords

Patellar mobility sex difference displacement knee extension angle

1. Introduction

Table 1
Subject characteristics and gender differences

	Total	Men	Women	$P$ -value
	( $n=$ 205)	( $n=$ 103)	( $n=$ 102)	(Men vs. Women)
Age (y)	20.4 $\pm$ 1.3	20.3 $\pm$ 1.5	20.6 $\pm$ 2.2	NS
Height (cm)	164.5 $\pm$ 8.2	170.8 $\pm$ 5.5	158.2 $\pm$ 5.1	$P=$ 0.001
BMI (kg/m ${}^{2}$ )	21.3 $\pm$ 2.9	22.0 $\pm$ 3.3	20.6 $\pm$ 2.2	$P=$ 0.001
Patella width (mm)	50.4 $\pm$ 4.2	53.1 $\pm$ 3.3	47.7 $\pm$ 3.1	$P=$ 0.001

Patellar mobility is an important clinical assessment in patients with patellofemoral pain [1, 2], and for those with a reduced range of knee motion after knee joint disorders and injuries [3, 4, 5]. Lateral hypermobility of the patella [6] and decreased medial patellar mobility with tightness of the tensor fasciae latae [7] have been described in relation to patellofemoral pain. The so-called stiff knee is considered to be associated with decreased patellar mobility. In clinical settings, however, patellar mobility is assessed using a subjective method, i.e., the patellar glide test [8], because there are few objective methods for determining the relationship between increased or decreased patellar mobility and the symptoms and conditions of stiff knee. One objective method is the use of a patellofemoral arthrometer (PFA) [9, 10]. The intra- and inter-reliability, and validity of the PFA compared to magnetic resonance imaging methods are clinically acceptable [intraclass correlation coefficient $>$ 0.80] [9, 10]. Ota et al. [11] performed an objective patellar mobility assessment using the PFA in patients with total knee arthroplasty, but the longitudinal relationship of medial and lateral patellar mobility with the knee flexion and knee extension angles could not be sufficiently determined after total knee arthroplasty. Objective assessment of patellar mobility using the PFA should therefore be performed in a healthy patient population to clarify the relationship between patellar mobility, and the symptoms and reduced range of motion in the knee joint.

Sex differences in knee joint disorders have not been sufficiently evaluated. Hypothetically, medial and lateral patellar mobility should be greater in women than in men based on sex differences in knee joint laxity [12] and a predisposition to patellar subluxation in women [13, 14]. Additionally, because patellar displacement is considered to be affected by body size, the amount of the displacement is thought to require normalization to size factors, such as patellar width [15] and subject height.

Our study objectives were to: 1) investigate sex differences in absolute and normalized patellar mobility, and 2) verify the relationship between patellar mobility and knee joint laxity (i.e., range of motion of the knee extension angle).

Table 2

Intra-tester reliability on two separate days ( $n=$ 29)

	Knee angle	Day1 mean (SD)	Day2 mean (SD)	ICC (95% Cl)
MPD (mm)	0 ${}^{\circ}$	16.2 (3.2)	16.2 (3.3)	0.994 (0.986–0.998)
	30 ${}^{\circ}$	13.1 (2.6)	13.2 (2.6)	0.989 (0.973–0.996)
LPD (mm)	0 ${}^{\circ}$	16.2 (4.0)	16.5 (4.4)	0.991 (0.977–0.997)
	30 ${}^{\circ}$	14.7 (3.3)	14.7 (3.3)	0.996 (0.991–0.998)

MPD: medial patellar displacement; LPD: lateral patellar displacement; ICC: intra-class correlation.

Table 3

Comparison of patellar mobility between men and women

	Knee angle	Total ( $n=$ 205)	Men ( $n=$ 103)	Women ( $n=$ 102)	$P$ -value	ES
MPD (mm)	0 ${}^{\circ}$	12.1 $\pm$ 3.1	13.0 $\pm$ 2.8	11.3 $\pm$ 3.2	$P=$ 0.001	0.56
	30 ${}^{\circ}$	10.0 $\pm$ 2.7	10.1 $\pm$ 2.6	10.0 $\pm$ 2.8	$P=$ 0.659	0.06
LPD (mm)	0 ${}^{\circ}$	12.9 $\pm$ 3.3	13.2 $\pm$ 3.2	12.7 $\pm$ 3.3	$P=$ 0.349	0.13
	30 ${}^{\circ}$	12.4 $\pm$ 2.9	11.9 $\pm$ 2.7	12.8 $\pm$ 3.1	$P=$ 0.029	0.31
MPD/PW (%)	0 ${}^{\circ}$	24.1 $\pm$ 6.2	24.5 $\pm$ 5.5	23.8 $\pm$ 6.9	$P=$ 0.420	0.11
	30 ${}^{\circ}$	20.0 $\pm$ 5.6	19.1 $\pm$ 5.0	21.0 $\pm$ 6.1	$P=$ 0.020	0.33
LPD/PW (%)	0 ${}^{\circ}$	25.8 $\pm$ 6.7	24.9 $\pm$ 6.1	26.8 $\pm$ 7.2	$P=$ 0.036	0.29
	30 ${}^{\circ}$	24.8 $\pm$ 6.3	22.6 $\pm$ 5.2	27.0 $\pm$ 6.7	$P=$ 0.001	0.74
MPD/HT (%)	0 ${}^{\circ}$	0.74 $\pm$ 0.18	0.76 $\pm$ 0.17	0.71 $\pm$ 0.2	$P=$ 0.720	0.25
	30 ${}^{\circ}$	0.61 $\pm$ 0.16	0.59 $\pm$ 0.15	0.63 $\pm$ 0.18	$P=$ 0.114	0.22
LPD/HT (%)	0 ${}^{\circ}$	0.79 $\pm$ 0.20	0.77 $\pm$ 0.18	0.81 $\pm$ 0.21	$P=$ 0.195	0.18
	30 ${}^{\circ}$	0.75 $\pm$ 0.18	0.70 $\pm$ 0.15	0.81 $\pm$ 0.19	$P=$ 0.001	0.64

MPD: medial patellar displacement; LPD: lateral patellar displacement; PW: patellar width; HT: height; ES: effect size.

2. Methods

2.1 Participants

Two hundred five pain-free individuals volunteered for this study, and 205 extremities were randomly selected (right: 104 limbs; left: 101 limbs) to assess patellar mobility. Mean age, height, and body mass index (BMI) of the participants, and their respective sex differences are shown in Table 1. Subjects were recruited from the student population at the Department of Rehabilitation and Care of Seijoh University using a leaflet that was distributed at a briefing, and a poster that was placed on a bulletin board. Prior to participation, all participants were informed as to the nature of the study, and their informed consent was obtained as approved by the Ethics Committee of Seijoh University (approval number: 201300C23).

Inclusion criteria were no recent knee pain in daily living nor a history of knee pathology (including operation). Exclusion criteria were a positive clinical patellar test (such as Clarke’s test or patellar femoral grinding test).

2.2 Assessment of patellar mobility

To assess the level of passive patellar mobility, we used the same instrument and methods described in a previous report [10]. The PFA (Brace-Fit, LLC, Aichi, Japan) was used to assess medial and lateral passive patellar displacement (MPD and LPD, respectively). The PFA was fixed on the femoral condyles, and the digital caliper was adjusted perpendicular to the line between the center of the patella and the anterior superior iliac spine (Fig. 1).

Figure 1.

Patellar displacement was determined by measuring the difference between the initial position and the displaced position under manual pushing force. The figure shows the assessment of lateral patellar displacement.

Subjects were positioned in the supine position with the leg in a neutral position (0 ${}^{\circ}$ hip rotation). The PFA was then clamped to the femoral condyles aligned parallel to the table. Before each test, the pushing force ( $\sim$ 80 N) was confirmed using a handheld MicroFET2 dynamometer (Hoggan Health Industries Inc., West Jordan, UT, USA). To measure the MPD, the medial side of the patellar border was palpated and located with a laser using the adjustable laser module arm, and the digital caliper was set at zero to establish this point as the initial position. Medial displacement of the patella was then determined by manually pushing the patella medially with $\sim$ 80 N of force, and then that side of the patella was again located by sliding the laser module arm on the caliper and reading the measurement. The measurements were obtained at knee flexion angles of 0 ${}^{\circ}$ and 30 ${}^{\circ}$ . The LPD was assessed in the same manner. The knee extension angle was measured with the subject in a supine position with a rolled towel placed under the heel, and a positive value indicated hyperextension of the knee. The assessments of patellar mobility were evaluated before warm up tasks as described in our previous study [10].

Table 4

Correlation between patellar mobility and knee extension angle

	Men			Women
Patellar	Knee	Knee extension	r	Knee extension	r
mobility	angle	angle ( ${}^{\circ}$ )	( $P$ -values)	angle ( ${}^{\circ}$ )	( $P$ -values)
MPD (mm)	0 ${}^{\circ}$	5.3 $\pm$ 2.2	0.21 (0.034)	5.9 $\pm$ 2.7	0.30 (0.002)
	30 ${}^{\circ}$		0.25 (0.011)		0.37 (0.001)
LPD (mm)	0 ${}^{\circ}$		0.32 (0.001)		0.38 (0.001)
	30 ${}^{\circ}$		0.25 (0.010)		0.26 (0.008)
MPD/PW (%)	0 ${}^{\circ}$		0.21 (0.030)		0.31 (0.002)
	30 ${}^{\circ}$		0.27 (0.012)		0.37 (0.001)
LPD / PW (%)	0 ${}^{\circ}$		0.31 (0.002)		0.38 (0.001)
	30 ${}^{\circ}$		0.25 (0.012)		0.27 (0.007)
MPD / HT (%)	0 ${}^{\circ}$		0.19 (0.049)		0.32 (0.001)
	30 ${}^{\circ}$		0.24 (0.016)		0.38 (0.001)
LPD/HT (%)	0 ${}^{\circ}$		0.31 (0.001)		0.40 (0.001)
	30 ${}^{\circ}$		0.24 (0.014)		0.27 (0.005)

MPD: medial patellar displacement; LPD: lateral patellar displacement; PW: patellar width; HT: height.

2.3 Reliability study

Prior to data collection, the reliability of our measurements obtained with the PFA was assessed by intra-tester reliability of MPD and LPD measurements using the same procedure described in Section 2.2. Twenty-nine healthy adults (15 men and 14 women) participated, with a mean [standard deviation (SD)] age of 22 (0.6) years, height of 166 (5) cm, and BMI of 21.5 (0.6) kg/m ${}^{2}$ . Repeated measurements were obtained on two separate days. For intra-tester reliability, one tester performed the measurements at both testing sessions with at least a 2-day interval. In all cases, measurements were obtained three times, with the final analysis performed using the mean of the three measurements. The intra-tester reliability of both the MPD and LPD measurements obtained with the PFA were assessed using intraclass correlation (Table 2).

2.4 Data analysis

Patellar mobility was measured three times, and the mean of the three values was used in the final analysis. Absolute displacement (MPD and LPD) was normalized by patellar width (PW) measured by a caliper (MPD/PW and LPD/PW) and palpation was used to identify the location of the bony landmarks of the medial and lateral patella at 0 ${}^{\circ}$ of knee flexion. Patellar displacement normalized by subject height (MPD/HT and LPD/HT) was also used in this study.

Differences in patellar mobility values between sexes were analyzed using an unpaired $t$ -test. Effect size (Cohen’s d) was also used for all comparisons. In the present study, a practical difference was set for the two conditions with a significance level of $P<$ 0.05 and an effect size $>$ 0.50. All statistical analyses were performed with SPSS, Version 16.0 (IBM Japan, Tokyo).

3. Results

Table 3 shows the normal variation of the MPD and LPD in all participants. A practical difference existed between sexes only in the MPD at 0 ${}^{\circ}$ knee flexion, based on absolute displacement. The MPD was greater in men than in women. After normalizing using HT, this difference disappeared, however, and LPD/PW and LPD/HT values at 30 ${}^{\circ}$ knee flexion were greater in women (27.0 $\pm$ 6.7 and 0.81 $\pm$ 0.19%, respectively) than in men (22.6 $\pm$ 5.2 and 0.70 $\pm$ 0.15%, respectively) with the two conditions satisfied ( $p<$ 0.001, effect size $>$ 0.50).

Table 4 shows the correlation between patellar displacement and range of motion of the knee extension angle (positive value indicates knee extension). All values of patellar mobility were significantly positively correlated with the range of the knee extension angle.

4. Discussion

The main finding of the present study was that the LPD/PW and LPD/HT values at 30 ${}^{\circ}$ knee flexion were greater in women than in men. Because the participants were asymptomatic, these findings are not likely directly related to knee joint pathology. Previous in vitro studies of LPD [16, 17] and with magnetic resonance imaging [13] reported that LPD increases at 20 $\sim$ 30 ${}^{\circ}$ of knee joint flexion, and that it is associated with a shallow femoral trochlear groove. These tendencies were greater in women than in men at those angles of knee joint flexion [13], and the findings of the present study may well be related to geometric sex features. Additionally, a shallow trochlear depth [18] and patellar alta [19] are reported to be characteristic of the female patellofemoral joint. This relation could not be determined, however, because we did not obtain imaging data of the patellofemoral joint.

Table 3 shows the variation in mean patellar mobility in a healthy adult population. The methods for passive patellar mobility in the present study were similar to the concept of passive joint range of motion with an endpoint, and the displacement indicates the amount of lateral and medial displacement until the manual endpoint. The mean (SD) absolute values of LPD and MPD at 0 ${}^{\circ}$ knee flexion in men and women was 12.9 (3.3) and 12.1 (3.1) mm, respectively. Thus, 95% of LPD and MPD values ranged from 19.5 to 6.3 mm, and from 18.3 to 5.9 mm, respectively. At 30 ${}^{\circ}$ knee flexion, the mean LPD and MPD values were 12.4 (2.9) and 10.0 (2.7), and 95% of the values ranged from 18.2 to 6.6 mm, and from 15.4 to 4.6 mm, respectively. As normal values of patellar mobility, Skalley et al. [20] reported that mean MPD and LPD are 5.3 and 9.3 mm at 0 ${}^{\circ}$ knee flexion using the Patellar pusher with 45 N. The MPD and LPD at 35 ${}^{\circ}$ obtained in the same manner are both 10.4 mm. Fithian et al. [21] reported values of 7.7 and 9.2 mm at 30 ${}^{\circ}$ with 5 pounds of force. Joshi and Heatleyin [22] indicated that the normal range of LPD at 0 ${}^{\circ}$ of knee joint flexion ranged from 8 to 20 mm with 6 pounds of force. These previous studies presented various ranges with various patellar-pushing forces, but gave no consideration to sex differences or normalization to body size. The present study is the first report of a sex difference based on quantitative passive patellar displacement and normalization to PW and HT. Normalization and measurement at the 30 ${}^{\circ}$ flexion position could be a more suitable assessment of lateral subluxation of the patella because of the greater prevalence of patellofemoral pain with subluxation of the patella in women than in men [14].

Both the normalized PW and HT were used to analyze the patellar mobility and patellofemoral alignment. The difference between these values was not remarkable in the present study. PW, however, is reported to be smaller in women than in men [23]. We have to consider using PW for normalization when comparing patellar mobility, especially when comparing sex differences.

As for the relationship between patellar mobility and knee extension angle as an indicator of knee joint laxity, all values of lateral and medial patellar mobility were significantly positively correlated. The correlation coefficients ranged from 0.21 to 0.38. Clinically, women are predisposed to hyperextension of the knee and lateral patellar subluxation, and therefore we expected that LPD would have a larger correlation coefficient than MPD, especially in women. The present findings, however, did not support that assumption. Our results indicated that both lateral and medial patellar mobility were fairly well correlated with knee joint laxity in healthy populations, equally in men and women.

This study has some limitations. The main limitation is that the participants were only from a healthy population, and their patellar mobility was not compared to that of a patient population. We could not define the cut-off line for hypo- or hyper-patellar mobility related to knee joint disorders. Patient populations with patellofemoral pain, subluxation of the patella, and stiff knee [24] after knee joint injuries should have their patellar mobility assessed and compared to that of non-patients. The second limitation is that we did not measure the geometry of the knee. The finding of an increase in the LPD/PW and LPD/HT at 30 ${}^{\circ}$ in women was not analyzed by incorporating geometric factors.

5. Conclusion

The main finding of the present study was that LPD normalized by PW and HT at 30 ${}^{\circ}$ knee flexion was greater in women than in men. Although lateral and medial patellar mobility in both men and women was significantly correlated with the range of the knee extension angle, the correlation was only fair. Patellar mobility varies in healthy adults, and normalizing patellar displacement with PW and HT is considered necessary when patellofemoral joint pathology is investigated in relation to sex differences.

Footnotes

Acknowledgments

The authors are very grateful to the subjects at Seijoh University. This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Conflict of interest

S. Ota has a national patent in Japan for a patellofe-moral arthrometer (patent number 4800016). BraceFit LLC has the exclusive license. BraceFit LLC had no control over the design, analysis, interpretation, writing, or publication of this study.

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