As it turns out, I am prone to the “movie theater sign.” This is a term used to describe pain in the front of the knee after sitting with your knees bent for a while. For example, sitting in a tight space on a long flight or in a movie theater for a few hours. This is one of the hallmark symptoms of anterior knee pain of patellofemoral pain syndrome (PFPS).
The prevalence of PFPS is higher in females compared to males, as concluded by this study by the United States Naval Academy that included 1,525 participants. It found that women were about two times more likely to have PFPS than men.
Anatomy of the kneecap region
The kneecap or patella is a large, triangular, sesamoid bone that can be palpated on the front of the knee joint. The apex of the kneecap points downward, and it has a medial (toward the midline of your body) and lateral border (away from the midline). It articulates with the trochlear groove of the femur, and sits there loosely during complete knee extension. A vertical ridge on the back of the patella divides it into the lateral and medial facets, along with the odd facet on the most medial aspect, that comes in contact with the femur during full knee flexion.
During full knee extension, only the lower pole of the patella is in contact with the femur. The patella is rather unstable in this position, and so, it is more likely to dislocate at this angle. As the knee begins to flex, more of the posterior aspect of the patella begins to come in contact with the femur. At 90 degrees of knee flexion, both the medial and lateral facets are equally in contact with the femur across the width of the patella. Beyond 90 degrees, the loading begins to change. At about 135 degrees, the most lateral aspect of the lateral facet and the odd facet are the only points of contact.
Besides this up and down movements, the patella also has tilts sideways in both directions (towards the inside and the outside) and rotates slightly inwards and outwards. Thus, the motion of the patella is more complex than what we can see on the surface.
The quadriceps muscle, which extends the knee, inserts onto the patella via the quadriceps tendon, goes on to surround the patella as the patellar tendon, which then inserts from the inferior border of the patella to the tibial tuberosity. It is made up of the vastus medialis, vastus lateralis, vastus intermedius, and the rectus femoris.
The tibiofemoral joint comprises of the femoral condyles and the tibial articulating surface. Several muscles play a role in the movement of this joint, including the quadriceps, hamstrings, gracilis, sartorius, tensor fascia lata, popliteus, plantaris, and the gastrocnemius.
The knee ligaments help stabilize and move the joint include the anterior cruciate ligament (ACL), the posterior cruciate ligament (PCL), the medial collateral ligament (MCL), the lateral collateral ligament (LCL), the arcuate ligament, the posterior oblique ligament, and the oblique popliteal ligament.
The medial and lateral menisci are disc-like structures that act as shock absorbers between the tibia and femur. The knee and the attached muscles receive innervation from several nerves including the femoral nerve, the saphenous nerve, the tibial nerve, the common peroneal nerve, and the obturator nerve.
When discussing the biomechanics of the knee joint, it is important to describe the quadriceps force vector, which is the direction in which the patella will move upwards during knee extension, as a net result of various muscle fibers pulling on it in different directions.
The muscle fibers of the vastus lateralis longus and the vastus lateralis obliquus cause an upward and outward pull of 35 degrees. This is only slightly higher than the inward vector generated by the vastus medialis obliquus and the vastus medialis longus, which is 40 degrees.
This means that the inwards force keeps the outward force in check, preventing the kneecap from sliding out during a forceful knee extension movement, thus preventing knee dislocation.
Besides this upwards directed medial and lateral force, the vastii also work together to restrain the patella through a posterior pull in the sagittal plane, keeping the patella pulled in close to the bone. This leads to a stabilizing effect through the compressive force of the patella against the femur. The net effect is a compressive force at an angle of 55 degrees from the horizontal, even at full knee extension. This really helps to provide stability to an otherwise “floating” patella. However, this is also one of the chief pathological processes that ultimately leads to PFPS.
Is patellofemoral pain syndrome the same as runners’ knee?
Runners’ knee is an ambiguous and vague term to describe the presence of PFPS in runners. PFPS can be caused by various reasons, one of which is repeated strain on the kneecap. Quadriceps strength deficit, calf muscle tightness are among the common causes of PFPS in runners, hence the term “runners’ knee.” This study cites the difference in lower limb strength in runners as the cause for PFPS in this population.
What are the main causes of PFPS?
Common causes of PFPS include the following: steep increase in knee loading such as sudden increase in workout rate, volume, or resistance, tightness of structures surrounding the knee including the IT band and muscles, weakness of quadriceps and hip muscles, poor neuromotor control of the hip and knee muscles, and increased mobility of the foot into pronation.
Diagnosis of PFPS
The most common symptoms of PFPS are anterior knee pain (retro-patellar or peripatellar) worse with squatting, stairs, and prolonged sitting. All these activities cause prolonged and/or repeated stress on the patella-femoral joint as they load the joint in a flexed position. Clinical finding may also include a positive patellar tilt test. Differential diagnosis of PFPS is based on the subjective complaints of the patient with specific movements mentioned above, along with a positive patellar tilt test. It is where a physician or physical therapist compares two or more conditions that may cause your symptoms since many types of knee pain share the same features, such as medial meniscus tear and IT band syndrome.
If you see a healthcare provider for anterior knee pain, they may rule out some other causes of this pain before diagnosing the condition as PFPS. Diagnosis includes referral from the lumbar spine or the hip, presence of any systemic or medical conditions, and psychological issues requiring referral to appropriate healthcare practitioners. It is important to note that fear avoidance behavior may be a complication of chronic conditions, including PFPS. This must be recognized and addressed clinically.
The classification of patellofemoral pain syndrome is based on broad categories of findings that may lead to anterior knee pain. This 2005 article describes a classification system, which reflects a consensus reached by the European Rehabilitation Panel for PFPS. The American Physical Therapy Association (APTA) has published clinical practice guidelines (CPG) to classify PFPS based on the findings of the clinical exam, in order to make treatment easy and more efficient. This is known as the impairment-based treatment protocol.
Basically, the classification system guides clinicians to the best course of action based on whether the knee pain exists with the overuse or overload without other issues, movement coordination impairment, reduced knee strength, and reduced knee flexibility.
Each category is defined by the impairments noted that are the chief contributing factors to PFPS.
Overuse or overload is described as an increase in the magnitude of load, frequency of loading, or a rapid increase in the rate of load, which can be described as overuse of the knee. This load can be in the form of everyday activities such as stair climbing or increase in walking distance, gym-based workouts like squats and lunges, or running distance and duration.
Movement coordination impairment of the lower limb resulting in increased or poorly controlled valgus motion at the knee during dynamic tasks is another impairment-based category of PFPS. This results in an increased Q angle, which is the angle formed by an imaginary line connecting the anterior superior iliac spine of the pelvis to the kneecap’s center, which intersects with another imaginary line from kneecap’s center through to the tibial tubercle.
An increased Q angle is caused by increased hip internal rotation and adduction, increased knee abduction (also known as knee valgus, causing the tibial tuberosity to move laterally relative to the patella) causing increased loading or stress on the lateral patella-femoral joint. This theory has been challenged in a 2011 study published in Clinical Biomechanics. The study was performed on male and female recreational runners to study the relationship between Q-angle and the magnitude of knee abduction moment and impulse during running.
Movement coordination impairment described above is studied during single-limb stance and is unrelated to muscle strength of the hip or knee muscles. This is more of a neuromotor control issue rather than a strength issue. The CPG emphasizes that this is not a deficit of strength of any of the proximal muscles, but rather it is a deficit in the timely recruitment of muscles to achieve optimal knee positioning during single-limb stance.
The third subgroup of PFPS with strength deficits includes individuals with PFPS secondary to weakness of hip abductors, external rotators, and extensors, and quadriceps muscles. A systematic review with meta analysis of 14 studies indicated the following: “Strong evidence indicated proximal combined with quadriceps rehabilitation decreased pain and improved function in the short term, with moderate evidence for medium-term outcomes. Moderate evidence indicated that proximal when compared with quadriceps rehabilitation decreased pain in the short-term and medium-term, and improved function in the medium term.”
This study also found that a rehabilitation program focused on closed kinetic chain quadriceps exercises was almost as effective as a proximal rehabilitation program in increasing isometric hip muscle strength. This means that exercises focused on quadriceps strengthening in a closed chain, such as wall squats and lunges, benefit the hip musculature too. This type of training program is useful for patients with PFPS secondary to muscle strength deficits.
PFPS with mobility impairments is further subdivided into hypermobility and hypomobility subgroups. Those in the hypermobility group have an increased navicular drop and the width of the midfoot in standing (weight-bearing) versus sitting (non-weight bearing) positions. They also have a higher Foot Posture Index, which is a graded outcome measure to categorize a pronated versus a supinated foot.
The hypomobility group includes people who have reduced flexibility and shortening of several structures affecting the knee including the lateral patellar retinaculum and muscles, including the hamstrings, calves, quadriceps, and the iliotibial band. Loss of mobility may also be seen at the hip joint, usually seen as loss of hip internal or external rotation.
There are a few things to bear in mind when following the above categories of PFPS. The strength deficits mentioned in the hip are correlated with PFPS, however causation is uncertain. This means that it is not clear whether the anterior knee pain causes the weakness of the hip muscles, or if the hip muscle weakness led to the onset of knee pain.
The other important factor to remember is that for most individuals, the subgrouping is a little more complex than neatly going into one little box or category. In clinical practice, it is not uncommon to find that most individuals straddle more than one category, and the most cases are “gray” rather than black or white.
Finally, psychological factors such as fear avoidance, kinesophobia, pain catastrophizing, anxiety, and depression play a role in individuals with PFPS, just as they do with any other musculoskeletal pain.
Imaging in the form of MRI can offer some insight into PFPS, based on the trochlear sulcus angle, trochlear sulcus depth, the Insall-Salvati ratio (ratio of patellar tendon length to the longest diagonal diameter of the patella), the distance between the tibial tubercle and the trochlear groove in fully extended knee (which is increased in individuals with PFPS), and quadriceps girth measurements. It is safe to summarize that the diagnosis of PFPS is largely a clinical one, and radiological examination should be reserved for those individuals in whom other causes of knee pain such as Osgood-Schlatter and Sinding-Larsen-Johanson diseases, or patellar dislocation are suspected.
If you happen to visit a physician or a physical therapist for anterior knee pain, they will rely on the clinical history as their primary “pain symptom checker” along with the examination findings discussed previously as their primary method of diagnosis of PFPS.
Patellofemoral pain syndrome treatment
This patient-perspective publication by JOSPT is a useful tool to provide to patients to educate them on the presentation and treatment of PFPS.
The importance of subgrouping patients in the four categories discussed above is that these are impairment-based categories, which conclusively guide us into appropriate treatment approaches.
Patients with PFPS related to overuse/overload benefit primarily from patient education to understand activity pacing and activity modification techniques. As mentioned earlier, most individuals straddle a couple categories of PFPS and this category is one of the most likely to demonstrate impairments described in one of the other categories.
Those who fall in the second subgroup of PFPS with movement coordination deficits will benefit from gait retraining, alteration of running mechanics with change in footstrike, and hip and knee neuromuscular reeducation exercises.
The randomized controlled trial of 16 recreational runners in 2016 concluded that besides reduced knee pain, training runners to use a forefoot strike also resulted in decreased knee abduction and increased ankle range of motion. The main limitation of this study is the small sample size and future studies that replicate this design may be required to increase confidence in the conclusions.
Individuals with PFPS with muscle performance deficits should be treated with the primary goal of addressing these deficits. Combined hip and knee strengthening program has been found to improve symptoms of PFPS than hip or knee strengthening alone. Posterolateral hip muscles (hip extensors, hip internal and external rotators) are strengthened in both the open and closed chain.
Knee extension exercises performed in the closed chain target the hip muscles stated above, hence there is significant overlap in hip and knee rehabilitation programs. Closed and open kinetic chain knee exercises have been shown to be effective in almost equal measures. It is interesting to know that hip exercises have been shown to be more effective in the short and medium term compared to knee exercises for PFPS. Hip exercises may also be better tolerated by patients in the initial stages of treatment compared to knee exercises.
The subgroup of PFPS with mobility impairments will benefit from treatment aimed at stretching and subsequent lengthening of the hypomobile structures such as the ITB and various muscle groups, to restore optimal muscle length and function. The stretches described in this clinical commentary published in the International Journal of Sports Physical Therapy in 2016 details these stretches with demonstrative figures.
Some other treatment tools that may benefit patients with PFPS are patellar taping, short term use of prefabricated foot orthoses for individuals to demonstrate hyper-pronation of the foot, gait retraining (forefoot-strike in runners, increasing running cadence, and cueing to reduce peak hip adduction while running), and blood flow restriction (BFR) techniques.
The clinical guidelines also delineate treatments that will not benefit individuals with PFPS. These include biofeedback techniques, dry needling techniques, manual therapy as a stand alone treatment, neuromuscular electrical stimulation, and ultrasound.
The guideline strongly recommends exercise therapy in combination with the other suggested treatments including foot orthoses, patellar taping, patellar mobilizations, and lower-limb stretching. Exercise remains the critical component in the treatment of PFPS.
While this article discusses the classification and treatment of PFPS, it is important to consult a healthcare professional for anterior knee pain to rule out referral from other joints or other sinister pathology that may masquerade as patellofemoral pain.
Patellofemoral pain syndrome exercises
Here are some of the exercises that follow the classification and address the limitations and impairments that define that subgroup. The stretching part includes hip flexor stretch, quad stretch, and calf stretch.
The neuromuscular re-education exercises consist of correction of knee valgus during single limb stance and squat.
Muscle strengthening exercises include knee exercises in the open chain such as knee extension and in the closed chain such as leg press, single leg squats, and wall squats.
Hip exercises include lateral walk with resistance band at knees (this can be progressed by having the band at the ankles and then to the forefeet), lunges, step ups forward and lateral, and standing hip hike. Squats and deadlifts can be progressed according to patient tolerance and capacity.
There is indeed a wide range of exercises for each category and exercises should be tailored to the individual’s needs and goals. Graded exercises along with optimal loading to challenge the target tissue should be the goal with each exercise.
So while the movie theater sign will let you know that your patellofemoral joint is being overloaded/compressed, management is as simple as frequent change of knee positioning, maybe performing some standing marches, or taking a short stroll to minimize symptoms.
PFPS is a common yet complex condition, and a careful examination followed by tailored, customized treatment is the key to management of this condition. Besides the biological and structural causes mentioned here, psychosocial perspectives should not be discounted. The psychological components discussed earlier, in combination with sociocultural elements play a role in most conditions, including PFPS.
When treating anyone, the individual’s psychosocial status matters just as much as the structural impairment. A balanced holistic approach respecting the person as a whole, with their own beliefs, perspectives, and attitudes, shaped by past experience and encounters, will go a long way in better outcomes rather than zooming in on the myopic view of regional conditions.