Etiology of Anterior Knee Pain in the Adult

Ronak M. Patel, MD

Robert H. Brophy, MD

Patellar pain, also known as anterior knee pain, patellofemoral syndrome, or even chondromalacia, refers to a number of conditions. The differential diagnosis of anterior knee pain can be divided into conditions extrinsic or intrinsic to the patellofemoral joint. Some of these conditions are readily apparent on the physical examination, whereas the diagnosis of others requires varying degrees of sophistication interpreting the history, physical examination, and imaging.

BACKGROUND

During the last 100 years, patellar instability has received greater attention from the orthopedic community than patellar pain. In 1959, Cotta counted more than 100 patellar realignment operations in the literature.¹ These were all designed to address instability (see Chapter 19). The causes of instability are indeed more obvious; they tend to be more mechanical in nature than pain-producing conditions, and consequently they give the appearance of being more amenable to a surgical solution. There are numerous structural and anatomic factors leading to mechanical instability such as ligamentous laxity and/or incompetence, muscle imbalance, patella alta, trochlear and femoral condyle dysplasia, and an increased tibial tubercle to trochlear groove distance. Surgical options to address these factors are described which include but are not limited to medial patellofemoral ligament (MPFL) repair and reconstruction, lateral release, tibial tubercle osteotomy, and trochleoplasty.

A misleading breakthrough occurred in 1928, when Aleman identified areas of softening and blistering on the patellar cartilage. He termed these “chondromalacia,” from the Greek words for “cartilage” and “softening,” and he understandably attributed patellar pain to these lesions.² However, this premise was not borne out. With the exception of deep, bone-exposing lesions, chondral lesions about the patella are not currently believed to correlate well with the presence or absence of pain. Therefore, the term “chondromalacia” in association with patellar pain is unfortunate, and its use should be discouraged, if not abandoned.³

“Patellar pain” is not a monolithic entity but rather a constellation of conditions subsequently discussed in this chapter. When tending to a patient, it behooves the physician to identify the condition by name rather than treat a nonspecific “patellofemoral syndrome.”

ANATOMY

The extensor mechanism consists of the four quadriceps muscles (rectus femoris, vastus intermedius, vastus lateralis, and vastus medialis), the patella, the femoral trochlea, and the tibial tuberosity. But all structures from the pelvis down can indirectly be responsible for patellar pain.⁴,⁵ The anterior knee fat pad, synovium, and capsule have some of the most sensitive afferent nerve fibers while, interestingly, patellar cartilage has nearly no pain response.⁶ The lower portion of the vastus medialis is called the vastus medialis obliquus (VMO) and may have its own nerve supply.⁷ It normally reaches at least as far down as the junction of the proximal and middle thirds of the patella, and its fibers normally lie at an angle of 50° to 65° from the long axis of the femur.⁸ The lateral border of the trochlea is more prominent than its medial counterpart.

The patella features articular cartilage that is the thickest in the human body (up to 7 mm thick near the center of the patella), a testament to the stresses imposed on it. The cartilage is also peculiar in that it does not follow the contour of its bony bed. Thus, the apex of the articular cartilage can be either medial or lateral to the bony apex of the patella.⁹ This anatomic quirk can be important when one assesses the congruence (fit) of the patellofemoral joint. The patellar cartilage is more permeable and more compressible than its trochlear counterpart.¹⁰,¹¹ This is postulated to explain the higher prevalence of chondrosis on the patella compared to the trochlea. The patellar cartilage offers many facets, the two most obvious being medial and lateral. However, approximately one-half of the time the median ridge separating these facets veers medially over the distal half of the patella.¹²

The resultant vector of the quadriceps muscles is not colinear with the patellar tendon. The two form an angle the complement of which is called the Q angle (see Physical Examination).

BIOMECHANICS

The patella is a complex lever that diminishes the effort required of the extensor mechanism by transmitting tensile forces of the quad tendon to the patellar tendon.¹³ The patella descends 7 cm caudally by full flexion and has
full patellofemoral contact at 45° of flexion. With increasing knee flexion, the patellofemoral joint reactive forces increase substantially.¹⁴ Conversely, the absence of a patella causes the quadriceps to work harder during knee extension and during the heel-strike phase of gait.¹⁵ The patella lies above the femoral trochlear groove (“trochlea”) when the knee is fully extended, and it engages the trochlea in the first 10° to 20° of knee flexion. It is very slightly lateralized at full extension and is rapidly centered over the patella as the knee flexes.¹³

HISTORY

The clinician must inquire about the nature of the pain and its specific location; about factors that aggravate and quiet the pain; about the presence of skin sensitivity, numbness, tingling, or burning; about the presence or absence of swelling or redness; and about the factors that seem to have brought about the symptoms.³,¹⁶,¹⁷ Pain typically occurs insidiously, but any history of an injury or prior surgery must be elicited. While difficult to sometimes elicit, overuse is a common theme in anterior knee pain. They should be asked about new activities or a recent increase in activities. Patients typically describe a vague anterior or retropatellar pain, but certain forms of patellar malalignment such as tilt and lateral displacement can cause medial or even popliteal pain.¹⁸ Having the patient fill out a pain diagram to specifically localize the area of the pain is helpful.¹⁹ Ask the patient what positions, movements, and activities make the pain worse, such as squatting, kneeling, sitting, stairs, running, etc. to understand the position of patellofemoral contact inciting pain.¹⁷ A complaint of crepitus, locking, or catching may suggest a structural issue such as instability, osteochondral defect, or arthritis.

Burning, unrelenting pain should raise the suspicion of a radiculopathy or a complex regional pain syndrome-like condition. The onset of symptoms after an unusually stressful activity suggests overuse or even a stress fracture. A fall directly onto the knee may have caused a neuroma-like condition. Recurrent swelling may reflect a synovitis. The history must also inquire about other etiologies such as back, hip, or infectious etiologies of knee pain. Pain at night or rest should raise suspicion about a tumor.

PHYSICAL EXAMINATION

The physical examination begins with the patient standing. Limb alignment is evaluated as the examiner looks for signs of genu varum or valgum, or patellar squinting, whereby the patellae point inward (see “Miserable Malalignment” in this chapter and Fig. 19.1 in Chapter 19). As the patient walks, the feet are checked for pes planovalgus or cavovarus deformities. While standing, the patient’s hip external rotator muscles should be evaluated.¹⁷ As the patient does a single-legged squat, the presence of valgus collapse at the knee or internal rotation at the hip should be observed. Weak external rotator muscles cause functional internal rotation at the hip which can lead to lateral patellar tracking and patellofemoral pain. Anterior knee pain with dynamic tests such as a deep squat or duck walk is another finding consistent with patellofemoral pathology.

One can check for joint laxity which is most commonly done using Beighton’s criteria.²⁰ Does the patient stand with the knees in recurvatum? Can the thumb be pushed down far enough to touch the forearm? The patient is asked to sit at the edge of the table and to extend the knee. Sudden lateral to medial relocation of the patella as the knee nears extension is called the J sign. The knee is examined for any obvious skin discolorations or incisions, effusion, or deformity.

The Q angle assesses the patella’s tendency to displace laterally when the quadriceps contract. It is subtended by two imaginary lines, one spanning the thigh from the anterior superior iliac spine to the center of the patella and a second going from the center of the patella to the tibial tuberosity. Historically, angles of less than 15° are normal, those more than 20° are abnormal, and those in the 15° to 20° range were judged on a case-by-case basis. However, the Q angle does not reliably predict diagnosis, treatment, or outcomes of the patellofemoral-related pain.²¹ Its clinical utility remains in question.

The peripatellar tissues are palpated, including over the quadriceps and patellar tendons, patella, lateral and medial retinaculum, and iliotibial band (ITB). The patient is asked to tense his or her quadriceps muscles. The VMO and quadriceps muscle is examined for bulk and tone.

The patella proper is first examined by gently palpating its medial and lateral borders. An imaginary line between these two landmarks should be parallel to the floor when the limb is in neutral rotation. When one side is lower (more posterior) than its counterpart, the patella is said to be tilted.²²,²³ In a patient with no history of previous surgery, tilt is always lateral—that is, the lateral side is down. Tilt is said to be reducible when the lateral border can be readily lifted out of its abnormal position. The examiner displaces the patella medially and laterally. Some patellae display abnormal amounts of play, and this can be quantified by quadrants of translation. The examiner’s fingers are curled around the medial and lateral borders of the patella as the facets (and intervening soft tissues) are palpated for tenderness (Fig. 18-1).²³,²⁴,²⁵ Downward pressure is applied to the patella as the knee is put through a range of motion. If instability is suspected (see Chapter 19), a laterally directed force is applied to the patella as the knee extends from a flexed position. If the patient experiences pain or suddenly becomes apprehensive as the knee approaches extension, a positive apprehension sign is said to be present. The examiner can place their palm on the patella while the patient actively ranges the knee to feel for crepitus. The presence of crepitus near or at full
extension suggests a distal patellar chondral lesion, while crepitus at other flexion angles suggests a more proximal lesion.¹⁷ The presence of a synovial plica should be considered, but this may be difficult given the lack of specific physical exam findings²⁶ (see Plica). There may be palpable and painful soft tissue band over the medial femoral condyle. Provocative maneuvers may elicit pain and/or a click with the knee ranged from extension to flexion. The popliteal space is examined, and if any mass is suspected, the patient is examined prone. The subject can now be placed in the lateral decubitus position, and the Ober test is performed to assess the tightness of the ITB (Fig. 18-2).

FIGURE 18-1 A and B: The patellar “facets” and intervening soft tissues are gently squeezed. This maneuver is normally painless.

The remainder of the knee examination is carried out to look for nonpatellar sources of knee pain. The hip and back should be evaluated as well as potential sources of referred knee pain.

IMAGING

Plain X-Rays

Imaging confirms and refines the diagnosis suggested by the history and the physical examination; it does not supplant them. For the large majority of patients the radiographic examination begins and ends with plain x-rays.³ A standing anteroposterior (AP) and even a standing tunnel (also known as the “schuss” view, Rosenberg view, standing flexion view) are taken to evaluate the femorotibial joint. The lateral and Merchant (“axial,” “sunrise”) views are most pertinent to the patella.

FIGURE 18-2 The Ober test assesses the tightness of the iliotibial band. With the hip extended to neutral and the knee flexed 90°, the knee should drop down to the examining table.

The lateral x-ray is taken with the knee at 30° of flexion and in such a way as to obtain near overlap of the posterior femoral condyles (Fig. 18-3).²⁷,²⁸ Patellar height, patellar tilt, and trochlear dysplasia are thus assessed. Patellar height is defined as the position of the patella relative to the trochlear groove and to the tibia. A patella that is too high is called “alta,” and one that is too low is called “infera” or “baja.” Parameters used to calculate patellar height include the Insall-Salvati and Blackburne-Peel ratios, both of which normally equal approximately 1 (see Fig. 19.11 in Chapter 19).²⁹ The Insall-Salvati ratio is adversely affected by unusual shapes of the patella.³⁰ The Blackburne-Peel ratio has been shown to be more reliable and have less interobserver variability.³¹ On the lateral radiograph the ventral aspect of the normal patella reveals two lines, the condensations of the bony median ridge and the lateral border of the patella. As the patella tilts, these lines become confluent and the patella becomes more globular in appearance (Fig. 18-3B).³,²⁷ Note that tilt varies with the degree of knee flexion, often becoming less pronounced as the knee flexes (in patients with malalignment). When seen on a lateral radiograph, the trochlea appears as a white line that normally remains parallel to the outline of the lateral femoral condyle (Fig. 18-4). Intersection of these two lines therefore signifies an absence of trochlear depth—that is, the bony aspect of the trochlea is flat. The trochlea is said to be dysplastic. This radiographic sign has been called the “crossing sign” and the “lateral trochlear sign.”³²,³³ The cartilage may actually be convex. The more distal the intersection of the lines, the more extensive the dysplasia.

The axial radiograph provides information on the medial-lateral position, tilt, and shape of the patella, and it provides a snapshot of the trochlear sulcus. Axial views obtained in the earlier degrees of flexion show the more
proximal portion of the trochlea (where trochlear dysplasias begin), whereas views taken in deeper degrees of flexion reveal the more distal trochlea. Moreover, in the typical patient with patellar subluxation, lateral displacement of the patella is most apparent in the early degrees of flexion, the patella reducing itself into the sulcus as the knee flexes. Axial views should therefore be obtained with the knee as close to extension as possible with 30° being a reasonable goal. By and large, this requires a simple leg rest placed at the end of the x-ray table as described by
Merchant et al.³⁴ Without such a leg rest, axial views are taken with the knee flexed 60° and are nearly useless for the purposes listed here. Note that if the knee appears in the middle of the x-ray film, one can deduce that the knee was flexed more than 45° when the x-ray was taken. If, on the contrary, the trochlea just barely appears over the bottom of the film, the knee was flexed closer to 30°. On a 30° axial view, the normal trochlear sulcus angle is approximately 140°. An angle of more than 145° signifies trochlear dysplasia.³⁵

FIGURE 18-3 A: On an optimal lateral x-ray, the posterior condyles are superimposed. The ventral surface of the patella features two parallel lines that are clearly separate (arrows). These represent the bony median ridge and the lateral border of the patella. B: As the patella tilts laterally, the two lines become confluent.

FIGURE 18-4 The crossing sign. A: The white lines, representing the trochlea (black arrow) and the subchondral bone of the lateral femoral condyle (white arrow), are normally parallel. The distance between the two is a measure of trochlear depth. B: Intersection and blending of the two lines indicate an absence of depth over the distance that they overlap. The more distal the point of intersection, the more extensive the dysplasia (white arrow, lateral femoral condyle; gray arrow, blending of the two lines—crossing sign; black arrow, trochlea).

If trochlear dysplasia is present only at the proximal-most portion of the trochlea, it may not be detected on the best of axial views but may instead be appreciated on a good lateral view.

Merchant et al described a congruence angle to judge the medial-lateral position of the patella.³⁴ A normal angle is negative, whereas a positive value indicates lateral displacement. The congruence angle is relatively independent of leg rotation. When the axial view is obtained with the patient’s leg in neutral rotation, tilt can be assessed relative to any horizontal line drawn on the film. Angles of less than 5° are normal, whereas tilt angles of more than 10° are frankly abnormal (see Fig. 19.10 in Chapter 19).³⁶ Many other parameters of tilt and displacement appear in the literature.³⁷

Magnetic Resonance Imaging

Magnetic resonance imaging (MRI) is rarely required in the setting of patellofemoral pain and should not be a substitute for adequate x-ray films. In the preoperative patient, an MRI can help delineate chondral lesions and even differentiate between trochlear and/or patellar lesions.³⁸ If tumors, fractures, or osteochondritis dissecans are insufficiently delineated on x-rays, MRI can provide finer detail.

On routine MRI, the patella is centered over the femur. Lateral displacement, although not necessarily symptomatic, is abnormal.³⁹ As with the physical and roentgenographic examination, tilt is abnormal. The angle formed by the plane of the posterior femoral condyles and the slope of the lateral (bony) facet should be more than 7°.⁴⁰ Alternatively, the angle formed by the plane of the posterior femoral condyles and a line connecting the medial and lateral borders of the patella should be less than 10°.

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