Anterior knee pain (AKP), otherwise referred to as patellofemoral (PF) pain, is pain that originates in the anterior knee structures in the absence of an obvious cause. Insidious onset of pain and overuse mechanisms are common presentations, but an acute loading event or a series of incidental events are often noted. Excluded diagnoses of AKP typically relate to the quadriceps and its tendinous insertions around the patella (including Osgood-Schlatter disease, Sinding-Larsen-Johansson syndrome, and patella/quadriceps tendinopathy) (Table 1).
AKP historically has been referred to as “runner’s knee,” PF syndrome (PFS), PF pain syndrome (PFPS), or chondromalacia patellae. The International Patellofemoral Study Group specifically discourages the use of the terms “patellofemoral pain syndrome” and “patellofemoral syndrome” when describing pain presenting in the anterior aspect of the knee (E.A.A., personal communication). Medical syndromes typically are defined by a combination of symptoms, signs, imaging findings, and/or laboratory values (e.g., Marfan syndrome); to date there is no such well-defined combination of consistent signs and symptoms or imaging that exists to define AKP as PFS. Chondromalacia patellae, a pathologic term that refers to softened cartilage, should not be used as a clinical diagnosis since it may or may not be related to clinical symptoms. Therefore AKP is the preferred term for this clinical entity. This review will address AKP presenting in the adolescent athlete.
AKP is one of the most common conditions presenting to clinicians, particularly those managing an active and youthful population. Symptoms can affect nearly 30% of the adolescent population (7,17,29), with adolescent women affected at a rate 2 to 10 times higher than their male equivalents (19,20). Tallay et al. (40) found a prevalence of AKP in greater than 20% of adolescent athletes.
Overuse injury principles can be applied to AKP, and these principles can be useful in directing treatment. This is true even though the exact etiology and nature of the pain continue to be poorly understood. An overuse injury is considered to be a repetitive submaximal or subclinical trauma that results in macro- and/or microscopic trauma. The tissue’s structural unit is damaged, or at least has overexceeded its clinical responsiveness, and this can lead to pain and/or movement dysfunction. The most common form of an overuse injury is from an endogenous source (i.e., mechanical circumstances) in which the musculoskeletal tissue is subjected to greater tensile force or stress than the tissue can fully absorb (37).
The approach to AKP is facilitated by the concept of tissue homeostasis and the envelope of function. Dye et al. (13) described the concept of the envelope of function, referring to the amount of load tissues can tolerate without generating pain. However, when a joint or tissue is subjected to increased load or frequency of load, homeostasis is disrupted, which in turn alters normal function and can be a pain generator. When a patient with AKP is approached and managed from an exclusively structural point of view, one can often be led astray by treating the patient based on imaging. Many patients with AKP have normal imaging, and many patients with abnormal findings on imaging have no AKP. Since AKP has a pathophysiology that is not well-defined or clearly understood, it is important to be aware of the broad differential diagnoses that must be considered when evaluating an adolescent with AKP (Table 1).
Taking a thorough history is paramount to the clinician achieving a correct diagnosis. Specific causes of pain in the anterior aspect of the knee can be indistinct; therefore, the clinician’s primary initial task is to rule out other known causes of pain around the knee (Table 1). Patients present with stereotypical symptoms that involve anterior knee pain which is aggravated both by loaded, flexed-knee activity such as stair climbing and squatting, and also by unloaded, prolonged bent-knee activity such as sitting with the knee flexed (theater sign).
Since the patella retinacular structures extend both medially and laterally from the patella, pain can be associated with the medial side, lateral side, or both. Indeed, despite its common term of “anterior” knee pain, pain in almost every location about the knee can relate to this pain presentation. This can create a very confusing clinical presentation. AKP is rarely associated with swelling and, when present, one must consider intra-articular pathology, most specifically cartilage damage; indeed, the most common form of AKP has minimal cartilage damage. Even in the presence of chondromalacia patellae, this condition is most often asymptomatic, nonprogressive, and metabolically stable and therefore is not a source of pain (13). There is paucity of evidence to suggest that osteoarthritis of the PF compartment is a progression of chondromalacia patellae (2). Radiologic progression from grade I to IV chondromalacia of the patellae was typically associated with trochlear dysplasia and took an average of 18 years to progress (21,42).
Crepitus or grinding noises are often reported by the patient and may or may not be associated with pain. Although the precise cause is unknown, the articular cartilage, synovium, or other soft tissues may be implicated (42).
Maximum PF forces occur in the flexed knee, mid-arc motion, consistent with many activities of daily living (36). Giving-way episodes can be reported; typically the giving-way episode is associated with walking over uneven ground or climbing stairs (42,45). This is particularly common going down stairs, where the quadriceps is firing and is activated over a longer period than going up stairs. This giving-way episode typically occurs when one tries to engage the quadriceps muscle and the quad “fatigues,” and/or there is quad inhibition due to pain, which creates a giving-way episode that can be indicative of poor neuromuscular function. It is important for the treating clinician not to confuse these giving-way episodes with ligamentous instability, which typically occurs with planting, pivoting, or jumping activities (2,20,34).
Another common complaint the patient may report is that the knee infrequently “locks.” The clinician must determine the degree of flexion at which the locking happens. If the knee locks in full extension, this is a manifestation of an extensor mechanism abnormality. The patient does not want to engage the kneecap in the groove because of pain and, therefore, keeps the leg straight. If the knee is locked secondary to a loose body, torn meniscus, or other space-occupying lesion, it is always locked in some degree of knee flexion.
Changes in one’s daily routine also may play a role in changing one’s envelope of function. Training schedule (i.e., type and frequency of sporting activity) may offer important information as to the level of activity and the possibility of overuse; however this change could be more subtle (e.g., changing schools to one that demands more walking or hill climbing). Included in “change” is the importance of identifying the adolescent’s stage of growth and growth history as it relates to muscular control of lengthening lever arms. Factors such as age of menses (and accompanying growth spurt) and menstrual history (both of the patient and female first-degree relatives) are important to document and consider in the context of “change” in one’s envelope of function.
The details of a complete physical exam of the PF joint can be found elsewhere (18,32,34). This review will highlight some specific details.
Body Habitus/BMI/Weight Change
During activities of daily living such as rising from a chair or climbing stairs, the PF compartment is subject to forces that are up to three times that of the patient’s body weight (26,36). If one believes that pain is related to load through the PF compartment, then certainly increased body mass as well as muscle weakness correlates with increased rates of PF load. This load is felt to be, in part, a causative factor of AKP. Conversely, a reduction in weight or an increase in strength can diminish the stresses borne by the PF compartment and alleviate or eliminate the pain. A recent study of female middle school basketball players did not find a significant correlation of BMI or body composition in adolescent female athletes who developed AKP versus those who did not (3), although increased BMI does appear to be a predictor for knee injury in older adolescent athletes (46). Increased BMI may simply be a reflection of decreased lower extremity strength and fitness, which is associated with AKP in active adolescents. As it relates to BMI, there appears to be sex variability in neuromuscular growth spurts relative to postpubertal growth (25), with boys being proportional and girls struggling with disproportionate height and weight increases.
Dynamic Alignment and Muscular Control
Double-leg and single-leg squat mechanics are helpful tools to assess for functional limb alignment and squat mechanics and should be used as part of the initial clinical exam. These simple maneuvers assess function at the low back, pelvis, hip, knee, ankle, and foot (28). When observing both static and dynamic limb alignment, squat mechanics give a snapshot of the forces to which the knee may be subject with increased force and repetition while participating in daily activities as well as sport.
For dynamic balance, direct clinician observation in both the frontal and sagittal planes while performing a simple lower limb activity (e.g., a squat) is helpful to discern poor dynamic limb mechanics. In the frontal plane, observation of valgus knee collapse (“functional” or “dynamic” limb valgus) may contribute to abnormal forces about the PF joint. This functional knee valgus appears as femoral adduction and internal rotation with dynamic activities such as single-leg (Fig. 1A) and double-leg squats (Fig. 1B). It is most often a result of lack of rotational control of the limb under the pelvis, which is dictated largely by core strength including hip, gluteal, back, and abdominal musculature. An outlier may include not only weakness of core musculature but also an exaggeration of underlying excessive limb rotation such as femoral anteversion, tibial external rotation, and pronation at the ankle (14) (Fig. 2). This should be assessed by routine physical exam. If clinically indicated, limb version can be confirmed by either CT assessment of limb rotation or gait analysis by a reputable children’s gait laboratory.
Assessment of quadriceps activation and muscle balance also can be helpful during these activities, as muscle imbalances in quadriceps activation have been associated with AKP. This can be “uncovered” by individual muscle testing of quadriceps and hamstring function, or it can be indirectly discerned by body movement patterns that lack appropriate muscle activation.
In the sagittal plane, load through the PF joint can be assessed. Muscle weakness may lead to increased anterior translation of the knee over the foot, which also can be indicative of up-the-chain weakness, poor body mechanics, and disorganized patterns of neuromuscular recruitment. Decreased ankle dorsiflexion often can be a sign of anterior muscle recruitment and increased load through the anterior knee. Foot pronation has long been associated with AKP (8,41) and can often be corrected with a simple foot orthosis.
Reference to frontal plane and rotational plane abnormalities has been discussed previously. The value of imaging in the initial assessment of AKP can be debated; indeed the clinician must use one’s judgment based on individual history, physical exam, and clinical setting to determine its value. If initial assessment reveals underlying muscle deficits and modifiable body movement habits, one can defer initial radiographs pending initial physical therapy response. If the patient is failing to improve with compliant participation in physical therapy but is still exhibiting pain or has history (i.e., mechanical symptoms) or physical exam (i.e., intra-articular effusion) findings worrisome for other pathology, then plain radiographs can be ordered for review (6,12). We suggest anteroposterior (AP) standing, true lateral, and a patella axial view in low flexion as initial screening radiographs. Magnetic resonance imaging is rarely helpful except in the case of joint swelling, acute trauma, or highly dysplastic anatomy. Upper limits or thresholds for a number of radiographic measurements have been associated with PF instability (6,11). To date, none of these have been shown to be predictive or associated with AKP.
AKP is best diagnosed with a careful history uncovering a pattern of overuse as well as an observant physical exam, discovering muscular deficits and/or alignment issues. The patient typically presents to the sports medicine clinician with exam and imaging as stated previously. With a working diagnosis of AKP, further patient assessment and treatment move to the domain of the physiotherapist. In addressing these guiding principles, the clinician has a framework from which to direct the treatment protocol. Witvrouw et al. (45) have published a classification system to guide nonoperative management of AKP. This system, which has not been validated, does have some clinical utility. It stresses the importance of the multifactorial nature of AKP and the need for individual patient assessment. As such, the assessment is divided into areas of alignment (alignment of the PF joint as well as the lower extremity as a whole) and areas of muscular deficit (subdivided into strength deficits, neuromuscular deficits, and impaired flexibility). What is most evident in the literature, and important for the treating physician, is that no one treatment is effective for AKP; it is a multifactorial problem that requires a multimodal approach. This approach often requires the assistance and expertise of a skilled physical therapist trained in body movement analysis and its treatment.
There are reports that a substantial number of patients have an unfavorable recovery if present >12 months, irrespective of treatment intervention (9,31). Interventions with known efficacy in reducing AKP, which are discussed below, should be utilized with early intervention to maximize prognosis. When applying these interventions, the core belief for the treating physician is that the symptoms of AKP should be modifiable in some predictable way to help direct the treatment program. Systematic reviews on the topics of exercise therapy, bracing, and taping for the treatment of AKP have been conducted. Limited conclusions could be drawn from these reviews (1,16,22,23,43).
Staying Within One’s Envelope
Perhaps the most important goal of the physical therapist is to find exercises that do not create pain. This often requires careful review of the types of exercises or activities that cause pain, the frequency of these exercises, and what makes the symptoms better (13). Painful exercises are to be avoided. Water therapy, which changes load capacity on the knee, may be helpful. The therapeutic approach, based on a tissue homeostasis paradigm, is to decrease loading to a pain-free level first, then incrementally and individually increase muscle strengthening as tolerated within that patient’s unique envelope of function.
Restoring Muscle Balance and Strength
Physical therapy has the highest level of evidence for initial treatment of AKP (12). Quadriceps strength is typically impaired in patients presenting with AKP, and therefore improvement in function with less pain is associated with resolution or at least reduction of symptoms. Many studies to date have tried to focus on an exercise protocol that is best able to improve quadriceps strength in patients with AKP. Quadriceps muscle strengthening plays a role when quadriceps atrophy is noted on exam; however it needs to be done with the whole limb in mind, with emphasis on the hip and core elements as well. Unfortunately, current evidence does not allow for any one method for improving quadriceps strength as superior. Both isometric and isokinetic contractions are used, as well as both concentric and eccentric contractions. All of these focal quadriceps muscle exercises are capable of providing overload to the muscle and therefore improving strength. The key, however, is to do this in a setting that does not provoke pain; this often necessitates trial and error under the watchful eye of a skilled therapist to put together a set of exercises that improves strength, improves body movement patterns, and decreases load in the PF joint. Often a patient’s strength and function targets are aimed at achieving the same levels as the unaffected limb. Indeed, normalization of quadriceps strength has been an important predictor of the long-term success in the rehabilitation of the PF joint (30). Closed chain activity generally is favored over open chain activity as it trains the gluteal and trunk muscles to control imposition and weight bearing, as well as typically decreases stress (and typically pain) across the PF joint. A stable pelvis minimizes unnecessary stress on the knee.
Physical therapy should be customized to the patient’s level of strength and fitness and whenever possible should be made challenging and progressive for the patient. The exercises themselves should be increased in intensity and duration as the patient develops better strength and limb control in their activities. Adequate muscular strength and endurance to perform repetitive activities are necessary; in the absence of such strength, one must reduce activities to achieve an envelope of pain-free function (13). One can begin to expand the envelope of activity after pain control is achieved. This increase in activity must be accompanied by concomitant improvements in strength, coordination, balance, and overall dynamic control of the limb.
For those unable to participate in traditional exercise therapy, water exercises can be beneficial due to both buoyancy and resistance. Buoyancy allows the patient to perform activities that might be otherwise stressful to the PF compartment by diminishing load, and the resistance helps to strengthen the various lower extremity muscles.
Analysis of Body Movement Patterns: Exercise
Analyzing limb strength and extremity kinematics are the main components that contribute to the plan of care for the patient with AKP (10,12,30,45). This approach focuses on movement-centered therapy with a skilled physical therapist. The patient’s response to each intervention will help to guide the next step, as well as define the need for further intervention.
There is increasing evidence in the literature linking weak hip musculature and poor pelvic control with AKP (33,39). Rotational control of the limb underneath the pelvis is critical; this is largely achieved through the roles of gluteus medius and maximus musculature as closed kinetic chain stabilizers (4,35). The strength of these muscle groups is essential, as a lack of rotational control of the limb can result in a valgus collapse pattern that places the knee at risk for acute and overuse knee injuries (24,27,44) (Fig. 1).
Analysis of Body Movement Patterns: Patella Bracing
There is literature support that bracing or taping provides at least short-term pain relief and can be used as an adjunct to treatment in conjunction with muscle retraining (15,22,23). In our personal experience, braces help two different groups of patients: for those with mobile kneecaps, a patella stabilizing sleeve may help patella tracking and thereby reduce abnormal loading; for those without tracking issues, a brace can provide warmth, proprioceptive feedback, and for some, a sense of knee protection.
Analysis of Body Movement Patterns: Orthotics
A pronated foot has implications up the kinetic chain; therefore repositioning the foot to neutral has been favored to help reduce AKP. A recent systematic review of foot orthoses for treatment of AKP saw some evidence indicating that prefabricated foot orthoses may reduce transverse plane knee rotation, although the mechanism behind their effectiveness remains unclear (5). Combining physiotherapy with prefabricated foot orthoses produces results superior to orthoses alone.
Symptoms should be modifiable in some predictable way to help direct a treatment program. If a patient’s symptoms continue to be present despite improvements in strength, correction of muscle imbalances, modification of offending activities, and biomechanical alterations like taping or bracing, then one may consider other less common sources of AKP such as neuropathic pain. The pain must be modifiable to be treatable. In cases where pain has not been sufficiently relieved in some way with the given interventions, it may be appropriate to consider a trial medication directed at neuropathic pain. In rare cases, AKP may present as a form of neurogenic pain excitation (i.e., complex regional pain syndrome (CRPS)). In our experience, this is rare in adolescents. Medications such as gabapentin and pregabapentin can be used as part of a treatment plan. Full discussion of CRPS is beyond the scope of this review (38). Other causes of AKP, listed in Table 1, also should be reviewed in cases of refractory AKP.
Our Approach to Treatment
Rather than being all-inclusive in the examination and treatment of the AKP-challenged adolescent, we hope to establish principles of care that have been successful in our hands for the clinical management of PF disorders.
The physician is important in managing AKP by:
- helping to make an appropriate diagnosis,
- ruling out other sources of AKP,
- validating the patient’s pain pattern as “real,”
- recommending a treatment management plan, and
- educating the patient. Indeed patient education is the cornerstone of managing AKP.
The patient’s role is:
- to understand the causative factors in the creation of their AKP,
- to participate in prescribed physical therapy programs, and
- to understand the progression from pain to wellness. This may include activity modifications and use of the patient’s own insight in modifying these activities.
- Identify and restore muscle imbalances, especially proximal hip and pelvic regions.
- Maximize “modifiable” alignment issues. This specifically refers to pelvic and foot position. A neutral pelvis is favored over positions of excessive pelvic tilt or obliquity, and a neutral foot position is favored over a pronated foot position. In most patients, these are not fixed deformities but are misalignments capable of being repositioned with foot support and/or dynamic muscle strengthening.
- Examine patterns of overuse. This can be a repetitive exercise activity such as running or repetitive work activities that involve lifting, squatting, and/or climbing stairs. One must have adequate muscular strength and endurance to perform repetitive activities. If one part of the kinetic chain is weak or injured, the body often finds ways to accomplish an activity by “working around” the injured body part. This often initiates faulty body mechanics that lead to a painful state that centers on the patella and its associated soft tissue structures. One must reduce activities to remain within an envelope of pain-free function. Once one is within an envelope of pain-free activities, the patient can begin to expand their envelope concomitant with improvement in strength, coordination, balance, and overall dynamic control of the limb.
AKP is often insidious, with multiple confounding variables. An analysis of alignment, strength deficits, and potential inciting overuse factors must be elicited by a careful history and physical exam by the treating clinician. No one treatment is effective for AKP; it is a multifactorial problem that requires a multimodal approach. For many patients (and their parents), ruling out serious disease states and providing reassurance, education, and a structured plan of activity modification and progressive strengthening comprise the ideal starting pathway to pain relief and recovery of function.
The authors declare no conflicts of interest and do not have any financial disclosures.
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