Quadriceps femoris muscle angle: normal values and relationships with gender and selected skeletal measures.

Quadriceps Femoris Muscle Angle: Normal Values and Relationships with Gender and Selected Skeletal Measures An understanding of the normal anatomical and biomechanical features of the patellofemoral joint is essential to any evaluation of knee function. One important concept in patellofemoral joint function is the quadriceps femoris muscle angle, better known as the "Q angle." This angle, which has been defined by Hungerford and Barry as the acute angle formed by the vector for the combined pull of the quadriceps femoris muscle and the patellar tendon, [1] is important because of the lateral pull it exerts on the patella. Theoretically, higher Q angles increase the lateral pull of the quadriceps femoris muscle on the patella and potentiate such disorders as chondromalacia patellae or recurrent lateral subluxation of the patella.

The method of measuring the Q angle is easy to perform. Clinicians have estimated the Q angle as the acute angle between 1) the line connecting the anterior superior iliac spine (ASIS) and the midpoint of the patella (representing the line of pull of the combined quadriceps femoris musculature) and 2) the line connecting the tibial tubercle with the same reference point on the patella. The knee is fully extended, but not hyper-extended, during measurements. [2-5] For this measurement to be meaningful, however, clinicians must first have established normal values.

We undertook this study for the following purposes: 1) to describe and document normal values for Q angles in a young, adult population and 2) to identify relationships between Q-angle values, gender, hip width, and femur length. We expected to find Q-angle values consistent with those cited in the literature and to find higher Q angles among women than men. We hypothesized that the Q angles would correlate with gender as well as with measurable anatomical differences in body proportions.

Review of the Literature

Although normal values for the Q angle are cited in the literature, references to objective research studies and normal ranges are difficult to find. Insall cites James (personal communication) as a source for a normal value of 15 degrees, with 20 degrees and greater being abnormal for both men and women. [6] Hvid et al cite James (personal communication, 1979) as a source for normal limits of 15 degrees in men and 20 degrees in women. [7] No documentation or explanation is offered for this 5-degree difference between sexes. Davies and Larson do not state a range for normal values, but they do describe Q angles greater than 20 degrees as excessive. [3] The American Orthopaedic Association considers 10 degrees to be normal and 15 to 20 degrees to be abnormal. [2]

In a study on patellar realignment of patients suffering from chondromalacia patellae, Insall et al suggested that an increased Q angle is indicative of pathological lateral forces on the patella. [4] In a preliminary study, they measured the Q angles of 50 hospital personnel with "normal" knees and reported an average Q angle of 14 degrees. They did not indicate any difference between the Q angles of male and female subjects or discuss their distribution. After establishing 20 degrees as an upper limit for normal Q angles, Insall et al found that of the 83 symptomatic knees measured, 40 had abnormally high Q angles. [4]

As part of a larger study, Aglietti et al measured the Q angles of 150 healthy subjects (75 men, 75 women) and 90 subjects (34 men, 56 women) with chondromalacia or recurrent subluxation. [8] They found average values for healthy men and women as 14 [+ or -] 3 and 17 [+ or -] 3 degrees, respectively. Among the pathologic group, average values were 16 degrees for men and 19 degrees for women. The differences between the sex-matched groups were significant (p [is less than] .0001). No healthy men had Q angles of 20 degrees or greater, but 11 (15%) of the women did. In contrast, 40% of the subjects (10 men, 26 women) in the pathologic group had Q angles of 20 degrees or more. Insall and associates found a similar percentage of symptomatic knees with high Q angles in an associated study on proximal realignment for patients with patellar pain or instability. [9] Forty-eight percent of these painful knees had Q angles of 20 degrees or more.

If increased Q angles indicate the presence of pathological lateral forces on the patella and if women do have greater Q angles than men, women could be at greater risk than men for developing patellofemoral joint problems. These theories are supported by the findings of Hvid et al [7] and Yates and Grana. [10] Hvid et al measured the Q angles of 12 women and 10 men who were treated nonoperatively for chondromalacia. [7] Their data showed that 11 of the 12 women had Q angles of at least 15 degrees and 7 women had angles greater than 20 degrees. Only one male subject had a Q angle greater than 15 degrees. In a prospective study on patellofemoral pain, Yates and Grana found that patellofemoral problems are most common among young women. [10] Fifty-one (76%) of the painful knees in their study belonged to women. Thus, the research suggests that women have larger Q angles and a greater incidence of patellofemoral joint pain than do men.

The reason that women have larger Q angles than men is not discussed in the literature. Outerbridge proposed that women may have more lateral shift of the patella during quadriceps femoris muscle contraction secondary to more widely spaced hips. [11] Simmons stated that women have shorter femurs than men. [12] Theoretically, the combination of wider hips and shorter femurs could increase the valgus of the lower limbs and thus increase the Q angle.

Method

Subjects

The subjects in this study were 50 male and 50 female volunteers from The University of North Carolina at Chapel Hill (Chapel Hill, NC). Their ages ranged from 18 to 33 years (X = 22.6 years). Potential subjects were gathered from fraternities and sororities, physical therapy classes, and cheerleading tryouts. All subjects signed informed consent forms, and the protocol was approved by the Committee on the Protection of the Rights of Human Subjects, School of Medicine, The University of North Carolina at Chapel Hill. All volunteers were given a questionnaire and unilateral patellar apprehension tests [13] to screen for patellofemoral joint pathologies. The questionnaire consisted of the following questions:

1. Have you had a diagnosed knee disorder?

2. Have you ever injured your knee?

3. Are you bothered by knee pain?

Volunteers answering "yes" to any of these three questions and those with positive apprehension test results were excluded from participation in this study.

Procedure

All measurements were taken with the subjects in a standing position and the knees in full extension. Calipers were used to measure the hip width (distance in centimeters between the greater trochanters) and femur length (distance in centimeters from the most lateral point of the greater trochanter to the lateral joint space of the knee) of all subjects. A limitation of this study was our inability to use roentgenograms for more precise measurement of hip width and femur length; however, we did find the chosen landmarks to be easily palpable with minimal overlying soft tissues. A universal (standard) goniometer was used to measure the Q angle in the manner suggested by the literature with the ASIS, midpoint of the patella, and tibial tubercle as landmarks. [2-5] A string was stretched between the ASIS and midpoint of the patella to ensure accurate alignment of the goniometer. All measurements for a given subject were taken by the same investigator (MGH or TLH). Interrater and intrarater reliabilities for all anatomical measurements were established in a preliminary study using a subgroup of seven subjects. Repeated measures were compared using Pearson product-moment correlation coefficients (r). All intrarater values were greater than .92. All interrater values were greater than .78 (Q angle, r = .87; hip width, r = .94; femur length, r = .78).

Data Analysis

Normal Q-angle values and ranges were established by calculating the mean and standard deviation for each group: men, women, and the entire sample. The Regress II software system wasused to calculate a Pearson product-moment correlation matrix between all measured variables and to calculate partial correlations between them. [14 A point biserial correlation [15] was used to analyze the relationships of gender with the other variables. The level of significance selected was .01 (two-tailed test).

Results

Descriptive statistics for the Q angles of all subjects are shown in Table 1, and the statistics for the other skeletal measures are shown in Table 2. Table 3 presents a Pearson product-moment correlation matrix for all measured variables.

Tables 1 through 3 suggest significant differences between the Q angles of the male and female subjects. The mean Q angle for the female subjects is 4.6 degrees greater than that for the male subjects. This finding is consistent with the negative point biserial correlation found between gender and Q angle, which indicates greater Q angles in women when men are coded as 2 and women coded as 1. Positive and significant correlations are also demonstrated between Q angles and the skeletal measurements (hip width and femur length) and between gender and these measurements. To determine the critical relationships, the effects of hip width and femur length were controlled (partialled out). When these measures were controlled, the relationship between Q angle and gender remained significant (r [is greater than] -.445, p [is less than] .001). When the effect of gender was controlled, however, the relationships between Q angles and these anatomical measures (r = -.128 - .104) were no longer significant.

Discussion

The results of this investigation establish 13.5 [+ or -] 4.5 degrees as the mean Q angle for the population sample. Our mean differs 1.5 degrees from the normal values attributed to James [6] and by 0.5 degree from the average found among hospital personnel by Insall et al. [4] It differs by 3.5 degrees from the value accepted by the American Orthopaedic Association. [2] All of these values are within one standard deviation from the 13.5-degree mean found in our study. Thus, the normal value established in this study is consistent with the values previously considered normal. The American Orthopaedic Association's description of Q angles between 15 and 20 degrees as abnormal, however, does appear to be inaccurate for women.

In addition to establishing this value for normal Q angles, the results of this study establish an average of 11.2 [+ or -] 3.0 degrees for young men and 15.8 [+ or -] 4.5 degrees for young women. These values are lower than those found by Aglietti and associates by 2.8 degrees and 1.2 degrees, respectively. [8] Our lower values may be an effect of weight bearing because Aglietti and associates measured Q angles in a supine rather than a standing position. Consistent with their results, none of our male subjects had values of 20 degrees or greater, but we did have a larger proportion of female subjects with values in that range (25% as compared with 15%). Our value also support the 15- and 20-degree upper limits Hvid et al used for men and women, respectively. [7] Our actual range of asymptomatic knees, however, did extend to 25.5 degrees for women and 18 degrees for men.

Although these results generally support commonly accepted Q-angle ranges, they contradict the general belief that women have wider hips than men. As Table 2 shows, the mean hip width for the male subjects, as measured from greater trochanter to greater trochanter, is actually slightly larger than the mean for women. Hip width was not standardized to height, but the results do suggest that men have longer femurs and therefore smaller ratios of hip width to femur length than women. One-tailed Student's t tests for independent means confirm a significant difference between these ratios for men and women (t = 6.87, p [is less than] .0005). The t-test results also demonstrate a significant difference between male and female subjects' femur lengths (t = 3.72, p [is less than] .0005) and fail to show a significant difference between hip width measurements (t = .52).

Consistent with our finding that hip widths are not actually larger in women than in men in this sample, we found that hip width does not relate significantly to Q-angle value. Although the ratio of hip width to femur length was greater for women than for men, it did not correspond with increasing or decreasing Q angles. By controlling the influence of one variable on the others, we determined that the relationship that remained significant was the relationship between gender and Q angle. When the effect of gender was eliminated, the relationships between the other anatomical measures and Q angle were no longer significant. The only variable identified as an enduring preictor of Q angle was gender.

Our inability to find a relationship between the anatomical measures and Q angle may possible have been affected by the bony landmarks we chose for our measurements. Differences in hip width may be obscured by differences in thickness of overlying soft tissue or by the angle of the femoral neck. We chose Pearson product-moment correlation coefficients to establish the reliabilities of our measurements; however, this may not have been the most appropriate statistic. [16] Roentgenograms could allow more valid measurements of all variables studies, but they may not be easily reproduced by the clinician.

This study identifies only one reliable predictor of Q angle--gender. The reason women have larger Q angles than men remains unclear. Further research is needed to determine why this difference exists. Future studies may investigate relationship between Q angle and rotational factors such as femoral anteversion and retroversion or tibial torsion.

Despite the failure of this study to explain differences in Q angles or to identify predictors other than gender, the results do indicate differences between the Q angles of men and women. By knowing normal values and ranges, clinicians can legitimately define and recognize abnormal values. Although a Q angle of 20 degrees can be identified as within normal limits for women, this value is abnormally high for men.

Clinically, the measurement of Q angle merits inclusion in the screenings of athletes and the evaluations of patients with patellofemoral pathologies. Such measurement seems particularly important for women because their Q angles are normally higher than those of men and because Q angles greater than 20 degrees are predisposing factors to patellofemoral pain and instability. [9] The clinician must remember, however, that Q angle alone does not result in these pathologies. Twenty-two percent of our healthy female subjects had Q angles in the range of 20 to 25.5 degrees without any symptoms of pathology.

Conclusion

This study established 13.5 [+ or -] 4.5 degrees as the mean Q angle for healthy subjects between the ages of 18 and 35 years, regardless of gender. The range established is consistent with previously accepted Q-angle values. The mean Q angle for women (X = 15.8[degrees [+ or -] 4.5[degrees]) is 4.6 degrees higher than that for men (X = 11.2[degrees' [+ or -] 3.0[degrees]). The data collected are important because they provide clinicians with better definitions of normal and abnormal and because they define the difference between the Q angles of men and women. Unfortunately, the data do not identify an anatomical explanation or correlate. None of the anatomical measures investigated were significantly related to Q-angle values when the effect of gender was eliminated.

References

[1] Hungerford DS, Barry M: Biomechanics of the patellofemoral joint. Clin Orthop 144:9-15, 1979

[2] Manual of Orthopaedic Surgery. Park Ridge, IL, American Orthopaedic Association, 1972

[3] Davies GJ, Larson R: Examining the knee. The Physician and Sportsmedicine 6(4):49-67, 1978

[4] Insall J, Falvo DA, Wise DW: Chondromalacia patellae: A prospective study. J Bone Joint Surg [Am] 58:1-8, 1976

[5] Percy EC, Strother RT: Patellalgia. The Physician and Sportsmedicine 13(7):43-58, 1985

[6] Insall J: Chondromalacia patellae: Patellar malalignment syndrome. Orthop Clin 10:117-122, 1979

[7] Hvid I, Anderson IB, Schmidt H: Chondromalacia patellae: The relation of abnormal joint mechanics. Acta Orthop Scand 52:661-666, 1981

[8] Aglietti P, Insall J, Cerulli G: Patellar pain and incongruence: Part I. Clin Orthop 176:217-224, 1983

[9] Insall J, Aglietti P, Tria AJ: Patellar pain and incongruence: Part II. Clin Orthop 176:225-232, 1983

[10] Yates C, grana W: Patellofemoral pain: A prospective study. Orthopedics 9:663-667, 1986

[11] Outerbridge RE: Further studies on the etiology of chondromalacia patellae. J Bone Join Surg [Br] 46:179-190, 1964

[12] Simmons K: The Bush Foundation study of child growth and development. Monogr Soc Res Child Dev 9(1):1-87, 1944

[13] Hoppenfeld S: Physical Examination of the Spine and the Extremities. New York, NY, Appletone-Century-Crofts, 1976, p 194

[14] Madigan S, Lawrence L: A Multiple-Regression Program for Apple II/2e/2c. Northridge, CA, Human Systems Dynamics, 1983

[15] Edwards AL: An Introduction to Linear Regression and Correlation. New York, NY, W H Freeman & Co Publishers, 1984, pp 54-58

[16] Shrout PE, Fleiss JL: Intraclass correlations: Uses in assessing rater reliability. Psychol Bull 86:420-428, 1979

M Horton, BS, PT, is Staff Physical Therapist, Forsyth Memorial Hospital, 3333 Silas Creek Pkwy, Winston-Salem, NC 27103 (USA). She was Acute Physical Therapy Supervisor, Southeastern Regional Rehabilitation Center, Cape Fear Valley Medical Center, PO Box 2000, Fayetteville, NC 28302-2000, when this article was written.

T Hall, BS, PT, is Staff Physical Therapist, Moses H Cone Memorial Hospital, 1200 N Elm St, Greensboro, NC 27401.