Original Editors - Ajay Upadhyay

Top Contributors - Ajay Upadhyay and Wanda van Niekerk  


The patella is a unique structure that plays a central role in the normal biomechanics of the knee. Unfortunately the patella remains the enigma of sports medicine and sports physical therapy. Across all sports and all age, it is probabily the single most common cause of pain.[1]

Patellofemoral pain syndrome is defined as retropatellar or peripatellar pain, or both, that results from physical and biomechanical changes in the patellofemoral joint.[2]

Patellofemoral pain syndrome is a common source of anterior knee pain in young and active individuals. It accounts for 25% to 40% of all knee pain problems in sports medicine centers. [3][4][5][2][6]


In General Population, females have a higher incidence of patellofemoral pain than male (3:2). There are clear structural, biomechanical, sociological and hormonal difference between women and men that contribute to an increase incidence of PFPS in women. This high incidence in women has been attributed to the gender difference in muscle strength, conditioning and anatomic structure especially increased Q angle.[5] [2] [6]

11% of musculoskeletal complaints in the office setting are caused by anterior knee pain (which most commonly results from PFPS), and PFPS constitutes 16 to 25 percent of all injuries in runners. Patellofemoral pain syndrome (anterior knee pain) is experienced by over 2.5 million Americans while statistics for other countries may be higher.[7][8]This condition is diagnosed at a higher frequency in female atheletes when compared to male athletes.[9]

Despite its frequent occurrence, Patellofemoral pain syndrome remains a difficult and often frustrating condition for all of us to treat, it is not only difficult to treat but also difficult to assess its gravity of pathology. In patellofemoral pain syndrome, identification of the underlying pathophysiology is difficult, though the classic picture in patellofemoral is easily identifiable. The patients complains of retropatellar or peripatellar (mainly medial side) precipitated by prolonged sitting and the pain is proportional to the activity, particularly evident when squatting or descending stairs.

Generally onset is insidious and progression slow. Patellar grind test is positive and the patient complains of discomfort on palpation of medial and lateral borders of patella. Giving way and instability is also common.[7]


Patellofemoral joint is one of the most important joint in lower extremity which plays a major role in weight bearing activities. This joint is made up of by articulation of patella with femur. The knee joint has two part and patellofemoral joint is one of it. The patella and femur forms the patellofemoral joint. The patellofemoral joint is also the least congruent joint of the human body.[10] It is shaped like a shield, with the ape pointing distally and has an anterior and posterior surface and three borders. The articular surface can be divided into medial and lateral facet seperated by central ridge and with an odd medial facet. While the hight and width are nearly constant the thickness of the cartilage and bones varies. The articular cartilage of the patella is thicker than any where else in the body.[11]

The total articular surface is much smaller than the femoral trochlear surface, and the material properties of the patellar surface vary throughout the articular surface as well as from the properties of the opposing trochlear cartilage20. The posterior surface of the patella is covered by articular cartilage and divided by a vertical ridge. The ridge may situated approximately in the centre of patella, dividing the articular surface in to two equal size, medial and lateral patellar facet. Occasionally the ridge may situated slightly towards the medial border of the patella, making the medial facet smaller than lateral.[1]

The patellar surface of the femur is the intercondylar groove or femoral sulcus on the anterior aspect of the distal femur. This groove or sulcus corresponds to the vertical ridge on the patella, dividing the femoral surface in the lateral and medial portion. The femoral surfaces are concave side to side but convex top to bottom[3]

The patella is a common attachment site for vastus intermedius, vastus lateralis, vastus medialis and rectus femoris20,25. The quadriceps tendon fibres, after enveloping the patella, join to become the patella ligament. It is the patella ligament that attaches the patella to the tibial tuberosity.[4]

Soft tissue stabilizers:

Passive stabilizers:

Inferiorly, the patella tendon limits the proximal ascent of the patella from the tibia. It is often slightly oblique laterally from proximal to distal, which adds to the tendency towards lateral displacement of patella. [4]
The lateral peripatellar retinaculum is comprised of two major components, the superficial oblique retinaculum and the deep transverse retinaculum. Superficial oblique retinaculum is rather thin and runs superficially from the iliotibial band to the patella. Deep transverse retinaculum provides superolateral static support for the patella.[1]

Medially, capsular condensations from the tough fibrous layer that that inserts into the superior two-thirds of the posterior part of the medial border of the patella. This medial patellofemoral ligament links the patella to the medial femoral epicondyle and passively limits lateral patella excursion[4].
Above the patella is the central quadriceps tendon expansion of the quadrices muscle.[4]

Active stabilizers:

The four main muscular elements (rectus femoris, vastus medialis and lateralis, and vastus intermedius) of the quadriceps fuse distally into the quadriceps tendon, which can still be identified as three separate layers at their insertion into the patella.[4][3]


Patellofemoral joint reaction forces:

The quadriceps tendon pulls on the patella simultaneously superiorly and by the patella tendon inferiorly. When the pulls of these two structures are verticle or in line with each other, the patella may be suspended between them, making little or no contact with femur.[11] this is in the case when the knee joint is in full extension. Even a strong contraction of the quadriceps in full extension will produce little or no patellofemoral compression. This is the rational for use of straight leg raising exercise with hip in neutral position as a way of improving quadriceps muscle strength without creating or exacerbating patellofemoral problems. As knee flexion proceeds from full extension, the pull of the quadriceps tendon and the pull of the patellar ligament become increasingly oblique, compressing the patella into femur. The contact force on lateral side always exceeds that of the medial side from 00 to 1000 flexion.[1] the peak pressures during passive knee flexion were higher on the lateral facet near full flexion and full extension.[3]

The increase in compression caused by the quadriceps mechanism with increased joint flexion occurs whether the muscle is active or passive. If the quadriceps muscle is inactive, the elastic tension increases with increased knee joint flexion. If the quadriceps muscle is active, both the active tension and passive elastic tension will contribute increasingly to compression as the knee flexion angle increases. The compression creates a joint reaction force across the patellofemoral joint. The total joint reaction force is influenced both the magnitude of active and passive pull of the quadriceps and by the angle of knee flexion. The highest tensile stress is concentrated beneath theinsertion of the patellar ligament.

The patellofemoral joint reaction forces found in gait when the foot first contacts the ground and the knee flexes slightly 100 to 500 is 50% of the body weight. The incresed knee flexion and quadriceps muscle activity seen with stair climbing or with running uphill may increase the patellofemoral joint reaction force 3.3 times body weight at 600 [4][5]. The joint reaction force may reach 7.0 times the body weight at 1300 of knee joint flexion in activities such as jumping when knee flexion is extreme and a strong quadriceps contraction is required.[5]

At the patellofemoral joint, the medial facet bears the brunt of the compressive force and several mechanisms help minimize or dissipate the patellofemoral joint compression on the patella in general and on the medial facet specifically.

Because there is essentially no compressive force on the patella in full extension, no compensatory mechanism is necessary. Conversely the joint reaction force increases as the knee extends from 950 to 450 and then decreases with increases with increasing extension.[5] The adaptive mechanism of the patella in which the high joint reaction forces appears to be fairly successful.
Although some cartilaginous deterioration is commom at both the odd and the medial facet, it bears restritction that these changes rarely causes problems.

During full extension of the knee, the lower border of the patella is in contact with the supra patellar pad of the distal femur and is under little or no load[6][7][12]. If the quadriceps is isometrically in this position, the patella moves proximally with a small lateral shift. The lateral shift is limited by the medial retinaculam, the patellofemoral and mesiscopatellar ligament. With flexion to 200 the tibia derotates(internal rotation)[11] decreasing the lateral vector and in turn the patella is allowed to move into the trochlear groove[11][6]. When the convex medial border of patella comes in contat with the concave femex femoral articular cartilage, it creates a high unit load to further enhance the compressive force causing lateral tethering by patellofemoral ligament. The contact zones widen as it proceeds proximally, facilitating distribution of joint reaction forces. At 900 of flexion patella demonstrate a lateral movement, and the medial femoral condyle is uncovered. As movement continues to 1350 of flexion, the lateral shift continues such that at its completion, the medial patellar surface lies free in the intercondylar notch[6][7] and the odd facet contacts the lateral aspect of medial femoral condyle.

Many contributing factors have been suggested as a possible cause of patellofemoral pain, including an increased Q angle, patella alta, abnormal or excessive foot pronation, quadriceps femoris (vastus medialis) muscle weakness, diminished flexibility of the hamstring and rectus femoris muscles, malalignment of the femur, and weakness of the hip.[4][10][11][1]

The mechanism of patellofemoral pain syndrome is not well understood; however, it has been suggested that the condition may arise from abnormal muscular and biomechanical factors that alter tracking of the patella within the femoral trochlear notch[4][12] contributing to increased patellofemoral contact pressures that result in pain and dysfunction.[7]

It should be distinguished from chondromalacia, which is actual fraying and damage to the underlying patellar cartilage.[5]

Fixed rotation of the femur has a significant influence on the patellofemoral joint contact areas and pressures. This is due to the anatomic asymmetry in the knee with respect to all planes, as well as the laterally directed force vector that naturally exists in bipedal lower-limb biomechanics.[6]
The most commonly accepted hypothesis of the cause of patellofemoral pain syndrome is that abnormal patellar tracking increases patellofemoral joint stress and causes suubsequent wear on the articular cartilage.[12]

Clinical Manifestations

The Classical Symptomes of patellofemoral pain syndromes includes[11] Pain on ascending and descending to stairs, Pain on sitting(Moviegoer’s knee), Pain on squatting, Crepitus, Giving way, Pseudo locking and Swelling.

The patellar glide, patellar tilt, and patellar grind tests, Mc Connell’s test, lateral pull test should be performed as part of the routine assessment of patients with anterior knee pain. Positive results on these tests are consistent with the diagnosis of PFPS.

Abnormal motion of the tibia and femur in the transverse and frontal planes is believed to have an effect on patellofemoral joint mechanics and, therefore, patellofemoral pain.[12] Many research has proposed that lower-extremity torsional and angular malalignment is a major cause of anterior and peripatellar knee pain and it directly influence the patellofemoral joint mechanics[4][2][6][7][10]

Strength of hip abductors and external rotators in individuals with Patellofemoral Pain Syndrome

During ambulation hip abductors and external rotator muscle group act eccentrically to control the motion of hip adduction and hip internal rotation, respectively.[11]

Weakness in the abductors may allow for excessive femoral adduction and this, in turn, may lead to more abducted, or valgus, position of the knee. Knee valgus is believed to increase the lateral force acting on the patella.[1]

Weakness of the hip external rotators may allow for excessive femoral internal rotation and this may lead to increased contact pressure between the lateral femoral condyle and the lateral facet of the patella. [10]

Hip external rotators and abductors are also contributing in pelvic stability and leg alignment by eccentrically controlling femoral internal rotation and influencing hip adduction during weight bearing activities.[7][12]

Weakness of these muscles may increase medial femoral rotation, valgus knee moments, or cause a gluteus medius gait. These deviations may alter the adduction/abduction moments at the hip or lead to an increased Q-angle, which may subsequently alter tracking of the patella, increase compressive forces on the patellofemoral joint, and ultimately lead to knee pain.[10]

Theoretically, weakness of the abductors and external rotators may be associated with poor control of eccentric femoral adduction and internal rotation during weight-bearing activities, leading to misalignment of patellofemoral joint as the femur medially rotates underneath the patella.[6][10][11]


A study shows that patients with patellofemoral pain syndrome showed no significant difference when compared to control group. The difference of isometric strength of hip external rotators in PFPS group and control group are less compared to the isometric strength of hip external rotators. Women are more affected with PFPS compared to men.[13]

An abstract from the above study is:

BACKGROUND: Decreased hip strength may be associated with poor control of lower extremity motion during weight bearing activities, leading to abnormal patellofemoral motions and pain. Previous studies exploring the presence of hip strength impairments in subjects with patellofemoral pain syndrome have reported conflicting results.

OBJECTIVE: To compare the strength of hip abductor and hip external rotators in subjects with and without patellofemoral pain syndrome.

METHOD: 30 subjects, aged 20-60, participated in the study.15 subjects with PFPS were compared with 15 control group with no known history of knee pathology. Hip abductor and external rotator strength were tested using hand-held dynamometer.

Measurement of Hip Abduction strength with dynamometer.png
Measurement of Hip External Rotators with help of Dynamometer.png

RESULTS: Comparison of isometric hip strength between group A and group B is done using unpaired t-test. Patients with patellofemoral pain syndrome showed no significant difference when compared to control group. The difference of isometric strength of hip external rotators in PFPS group and control group are less compared to the isometric strength of hip external rotators. Women are more affected with PFPS compared to men.

CONCLUSION: There is no significant difference of isometric strength of hip abductor and hip ER between patient and control group.

Recent Related Research (from Pubmed)

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  13. Ajay Upadhyay. Lap Lambert Publication. Hip muscles strength and Patellofemoral pain syndrome: Comparison of strength of hip abductors and external rotators in individuals with & without PFPS.