Knee Q Angle Measurement

The Knee Q angle (also known as Quadriceps Angle) is defined as the angle between the quadriceps muscle (primarily the rectus femoris) and the patellar tendon. It sometimes called quadriceps pull angle.

It represents the dynamic “instability” of patella, the greater it is the more unstable patella will be.

The Knee Q Angle was first described by Brattstrom in 1964, although many authors had previously described the importance of genu valgus and its relationship to patellofemoral instability. Brattstrom described the Q-angle as the angle formed by the resultant vector of the quadriceps force and the patellar tendon with the knee in an “extended, endrotated” position

See Also: Knee Range Of Motion

How to Measure Knee Q Angle?

A line is drawn from the anterior superior iliac spine to the midpoint of the patella, corresponding to the quadriceps’ tensile direction.

Another line is drawn from the tibial tubercle to the midpoint of the patella, corresponding to the patellar tendon.

The angle formed by the crossing of these two lines is called the Q-angle of the knee.

The hip and the foot should be placed in a neutral position, because significant internal rotation of the leg and pronation of the foot alter the Quadriceps angle.

Quadriceps Angle Assessment

Normal Q angle is 13° for men and 18° for women when the knee is straight.

Any angle less than 13° may be associated with patellofemoral dysfunction or patella alta.

An angle greater than 18° is often associated with subluxing patella, increased femoral anteversion, genu valgum , or increased lateral tibial torsion.

The Q angle of the knee can be influenced distally through motions of the tibia. For example, external rotation of the tibia moves the tibial tuberosity laterally, thereby increasing the angle, whereas tibial internal rotation decreases the Q-angle by moving the tibial tuberosity medially.

As with tibial rotation, the Q angle can be influenced through motions of the femur. For example: increased femoral internal rotation results in a larger angle, whereas femoral external rotation minimizes the angle.

The Q-angle increased as the foot moved from external to internal rotation. While it’s decreased as the foot shifted from pronation to supination.

Q Angle & Total Knee Arthroplasty

An increased Q-angle should be avoided in total Knee Arthroplasty because:

1. An increased Q-angle increases the resultant lateral subluxation force (i.e., lateral pull effect).
2. Prosthetic patellar replacements (by design) are less restrained than native patella.
3. Prosthetic patellar replacements are therefore more likely to sublux with increased Q-angle forces.

The Q-angle goal in TKA is to restore proper angle with techniques that do not compromise mechanical alignment or stability of the knee.

Q Angle Reliability

Although this measurement has been used to evaluate and treat patellofemoral joint pathology, few studies have examined its reliability.

Greene et al. evaluated the interobserver and intraobserver reliability of the Q-angle measurement comparing clinically derived Q-angle measurements with radiographically derived measurements. A reliability analysis was performed using intraclass correlation coefficients (ICCs). 7

For interobserver measurements: the ICCs ranged from 0.17 to 0.29 for the four variables evaluated (right and left, extension and flexion).

For intraobserver measurements: the ICCs ranged from 0.14 to 0.37.

The average ICC between the clinically and radiographically derived measurements ranged from 0.13 to 0.32, which demonstrates poor interobserver and intraobserver reliability of Q-angle measurement and poor correlation between clinically and radiographically derived Q-angles.

Notes

Remeasure the Q-angle with the quadriceps isometrically contracted. Differences between the two measures may provide insight to patellar tracking abnormalities.

The Q-angle may be measured with the knee in 30° of flexion, centering the patella within the femoral trochlea. Measurement of the Q-angle with the patient standing better replicates the functional alignment of the lower extremity. The Q-angle measured with the patient short-sitting should be smaller than measures obtained with the patient standing or long-sitting.

To remove the effect of femoral rotation tubercle-sulcus angle can be measured. In sitting position with knees flexed 90° intersection of line joining tibial tuberosity and center of patella and perpendicular to patellar center gives tubercle-sulcus angle. Normal value is <5° in males and <8° in females. An increase in this angle also signifies lateral shift of tibial tuberosity.

This angle corresponds to the physiologic valgus angle of the femoral shaft, this creates a lateral pull on the patella.

This tendency is a factor in habitual patellar dislocation and in the patellofemoral pain syndrome (anterior knee pain). It also creates problems in total knee arthroplasty.

The significance of a normal Q-angle was highlighted in a study by Huberti and Hayes who concluded that, with a normal physiologic Q-angle, the pressure distributions on the patella were remarkably uniform at each of the angles of knee flexion tested (20, 30, 60, 90, and 120 degrees) between the medial and lateral facets. In contrast, when the physiologic Q angle was decreased by 10 degrees, the patellar contact pressure increased by 53% at 20 degrees of knee flexion. However, when the knee flexion angle increased beyond 20 degrees, the contact pressure actually decreased, with the lowest value at 90 degrees.

References

1. Khasawneh RR, Allouh MZ, Abu-El-Rub E. Measurement of the quadriceps (Q) angle with respect to various body parameters in young Arab population. PLoS One. 2019 Jun 13;14(6):e0218387. doi: 10.1371/journal.pone.0218387. PMID: 31194851; PMCID: PMC6564690.
2. BRATTSTROEM H. SHAPE OF THE INTERCONDYLAR GROOVE NORMALLY AND IN RECURRENT DISLOCATION OF PATELLA. A CLINICAL AND X-RAY-ANATOMICAL INVESTIGATION. Acta Orthop Scand Suppl. 1964;68:SUPPL 68:1-148. PMID: 14171734.
3. Biedert RM, Warnke K. Correlation between the Q angle and the patella position: a clinical and axial computed tomography evaluation. Arch Orthop Traum a Surg 2001;121(6):346–349.
4. Livingston LA. The accuracy of Q angle values. Clin Biomech (Bristol, Avon). 2002 May;17(4):322-3; author reply 323-4. doi: 10.1016/s0268-0033(02)00014-1. PMID: 12034129.
5. C C Greene, T B Edwards, M R Wade, E W Carson. Reliability of the quadriceps angle measurement. C C Greene et al. Am J Knee Surg. Spring 2001. PMID: 11401177.
6. Olerud C, Berg P. The variation of the Q angle with different positions of the foot. Clin Orthop Relat Res. 1984 Dec;(191):162-5. PMID: 6499307.
7. Huberti HH, HayesWC: Patellofemoral contact pressures. The influence of Q-angle and tendofemoral contact. J Bone and Joint Surg 66-A:715– 724, 1984.
8. Greene CC, Edwards TB, Wade MR, et al: Reliability of the quadriceps angle measurement. Am J Knee Surg 14:97–103, 2001.