Knee Conditions

Eric W. Edmonds

Benton E. Heyworth

• Introduction to Pediatric Knee Conditions

In the realm of pediatric orthopedics and, in particular, pediatric sports medicine, the knee is by far the most commonly affected joint generating visits to caregivers of children and adolescents. Therefore, pediatric orthopedic specialists must have an intimate knowledge of knee anatomy, its function, and the physical examination maneuvers that allow for the diagnosis of pathologic conditions and injuries that affect the knee.

The most commonly seen musculoskeletal disorders affecting the knee of children can be traced to overuse phenomena, a discreet traumatic event, or an underlying congenital anomaly. Infectious, neoplastic, inflammatory, neurologic, genetic, and syndromic disorders will also affect the knee in childhood. However, the examination and diagnoses of these conditions are discussed in greater detail in previous chapters. The distinction between traumatic and isolated congenital knee issues will be discussed with an understanding that congenital issues can often lead to the traumatic issues via the level of activity that a child participates. One review article recently produced to highlight, “What’s new in pediatric sports conditions of the knee” described tibial spine fractures, osteochondritis dissecans (OCD), and patella instability as the issues with the most recent publications.¹ However, the examination and diagnoses of these young patients would be incomplete without considering other sources of pathology, such as ligament injury, meniscus injury, and atraumatic knee pain associated with growth and the changing biomechanics of the lower limb.

The most common cause of knee pain in children is nontraumatic in nature.² In a Danish study that captured school-age children, the knee and ankle were the most common locations of pain, with the knee affecting over 15% of all children. The pain was found to be recurrent over the 3-year study, with pain lasting on average 8 weeks per episode and each child averaging 2.5 episodes per study year. Thus, familiarity with atraumatic overuse conditions is critical in evaluating a child’s knee and development of a thoughtful differential diagnosis.

Perhaps the most studied conditions of adolescent knee in last 2 decades are ligament injury.³ Injuries to the anterior cruciate ligament (ACL), classically the most studied condition in adult sports medicine, have now been appreciated as a condition primarily affecting the pediatric and adolescent populations and are the fear of most high school athletes. Studies have documented an increased rate of 150% over a recent 10-year period.⁴ The posterior cruciate ligament (PCL) is also a debilitating injury that can affect young athletes, particularly in contact sports. Medial collateral ligament (MCL) and posterolateral corner (PLC) injuries get less literature-based attention but can be the bane of the treating provider if not promptly identified.⁵

Understanding that congenital and developmental risk factors can lead to traumatic knee injuries is underscored by the rising incidence of conditions, such as patellar instability, which can result from a multifactorial etiology that includes genu valgum, femoral anteversion, trochlear dysplasia, patellar dysplasia, and collagenous soft tissue makeup of the knee.⁶,⁷ While the clinical history may clearly point to a diagnosis of patellar dislocation, the examination and diagnosis must include the evaluation of all of these etiologic factors in order to successfully manage the pathology and reduce the risk of recurrence.

The final topic to be discussed in this chapter is the childhood meniscus tear and the evaluation for discoid meniscus. If a clinically significant meniscus tear is identified, most authors will suggest surgical
intervention, and the literature suggests that a much higher rate of successful repair can be obtained, compared with the adult population.⁸ Therefore, identifying these injuries early is paramount to optimal treatment, and vigilance needs to be maintained in the evaluation of the pediatric knee.

• Normal Knee Anatomy

Ligaments

The ACL is a robust ligamentous structure composed of the anteromedial (AM) and posterolateral (PL) bundle. While more confluent than truly discreet substructures, the two bundles have been found to confer differing functional benefits. The AM bundle is tight in flexion and is the primary check against anterior translation of tibia on the femur, while the PL bundle is tight in extension⁹ and is the primary constraint for rotation, thereby providing the rotational stability so essential to cutting and pivoting activities. For the average child, this stability is essential not only for sports but also for simple activities of daily life and free play. Of note, the collateral ligaments and capsule also contribute significantly to rotational stability. The MCL is composed of superficial and deep layers, with the superficial MCL proximal footprint lying just posterior to the medial femoral condyle (MFC) and the distal footprint lying 4-5 cm distal to the joint under the pes anserinus. The deep fibers have a distal footprint directly onto the edge of the tibial plateau near the meniscus. The MCL provides about 80% of valgus constraint at 25° of knee flexion.¹⁰ The PLC is a complex of ligaments and tendons that provide stability to the lateral side of the knee¹¹ and some degree of rotational stability. The primary three constraints to stability include the lateral collateral ligament (LCL), the popliteus tendon, and the popliteofibular ligament. The PCL is approximately twice as strong and twice as thick as the normal ACL, and the tibial attachment is actually extra-articular in location.¹² Much like the ACL, it can be subdivided into two primary bundles, the anterolateral and posteromedial, with the former tight in flexion and the latter tight in extension. Structures about the knee joint can be seen in Figure 14.1.

FIGURE 14.1 Anatomy of the knee joint.

FIGURE 14.2 Muscles about the knee joint.

Muscles

Examining the muscles that affect knee function is important when considering sources of knee pain. The muscles whose tendons cross the knee joint can play a direct role in pain, such as the hamstrings, quadriceps, gastrocnemius, or popliteus. However, the muscles that reach the knee but do not cross the joint can also be a source of pain, such as plantaris, or the muscles that effect knee position during activity, such as the glutei and hip rotators, can have a role in knee pathology¹³ (Figure 14.2).

It’s important to remember that muscle weakness can mimic knee instability from ligamentous injury. The child whose knee buckles may feel unstable because the quadriceps muscle can reflexively stop contracting in the face of severe patella femoral pain.

• Examination of the Knee

Examination

Several overarching principles are critical to physical examination of the knee. While it may seem exhaustive to apply these principles to every patient with knee complaints, having a meticulous, consistent approach gives the examiner the best chance of appreciating subtle or rare findings and appreciating the complex interplay of different factors or underlying risk factors for different causes of knee pain. Such an approach will allow the examiner to developing a differential diagnosis without an excessive reliance upon advanced imaging, which should be a last step in the diagnostic process (Table 14.1).

The first principle for completing a thoughtful knee examination is that the nonpainful or unaffected knee should always be examined before the affected or painful knee. This will allow the examiner to better understand the patient’s normal, baseline anatomy and knee function, as well as establish the patient’s comfort level with basic examination techniques, such as palpation and range of motion. Children, much more than adults, may at least initially guard against performance of simple maneuvers or report discomfort with palpation of certain areas, even if not painful at rest or with activity. Such a comparative approach better allows for detection of true pathology and generally puts a young patient at greater ease with the process. Secondly, the examiner should always let the patient know what steps will be performed in the examination prior to

performing them. This tends to make patients more comfortable and establish an important level of trust in the encounter, which facilitates a meticulous examination. Thirdly, the hips and ankles should be examined for every patient reporting knee pain. This will help the examiner understand the patient’s baseline level of flexibility and appreciate any underlying pathology in the adjacent joints that may be contributing to the patient’s knee condition. Hip conditions not infrequently manifest themselves as nonspecific knee pain that is difficult to reproduce through palpation or provocative testing. This phenomenon is known as “referred pain,” and in patients with hip conditions, this is due to the fact that obturator nerve innervates the hip capsule as well as the medial thigh and knee. Hip pathology, such as slipped femoral capital epiphysis or Perthes disease,
may present with a complaint of knee pain or a slight limp, rather than with hip pain (see Chapter 16). Finally, a gait assessment should be performed on every patient that is able to do so (see Chapter 4). While acute injuries may preclude comfortable weight bearing, the walking and running gait should be assessed for all patients with atraumatic onset of knee pain. What is their foot progression angle? Do their patella point toward each other? Do they have an antalgic gait? Do they demonstrate circumduction?

Table 14.1 A Stepwise Approach to the Physical Examination of the Knee

Step 1: Inspection

Inspection is generally most effectively assessed with the patient in a standing position.

In the setting of an acute injury, supine inspection is performed.

Principles:

For all steps in the examination, the examiner should inform the patient of how they will be examined.
Always begin with contralateral limb if patient has unilateral symptoms.
Examine the painful or injured part last.

Stepwise approach and points to document:

Standing alignment
1. Frontal or coronal plane
  1. Neutral
  2. Clinically significant varus versus valgus
2. Sagittal plane
  1. Full extension versus flexion contracture
  2. Full flexion versus extension contracture
Swelling
1. None
2. Soft tissue/extra-articular swelling
3. Effusion/intra-articular fluid
4. Posterior—popliteal fossa masses/swelling (possible Baker cyst)
Skin integrity
1. Normal
2. Bruising/ecchymosis
3. Erythema/rash
Patellar position
1. Neutral
2. Lateralized
3. Normal height
4. Elevated … patella alta
5. Lowered … patella baja
Localization of pain
1. Patients should be instructed to point to the site of maximal discomfort (if simple provocative maneuver, such as a single leg hop, is needed to incite pain, precede with this step)
2. Patients should be instructed to use one finger, without moving the finger
Gait
1. Walking (see Chapter 4 on Gait)
2. Running
3. Single leg hop
  1. Normal/symmetric versus antalgic or buckling (always begin with contralateral limb if unilateral symptoms)

Step 2: Range of Motion Assessment

Principles:

If any blocks to range of motion or asymmetries are appreciated, attempts should be made to assess the end point and whether they are rigid (boney/mechanical) versus soft (stiffness-based)
Always recognize sources of “referred” pain to the knee from the hips (slipped capital femoral epiphysis or Perthes disease)

Stepwise approach and points to quantitate (degrees):

Hips
1. Supine position … flexion/extension, abduction/adduction
2. Prone position … internal/external rotation
  1. With hip extended, asymmetry in rotation detects underlying hip pathology.
Ankles/feet
1. Dorsiflexion/plantarflexion
2. Inversion/eversion
Knees
1. Extension
  1. Active (can the patient straighten to 0°?)
  2. Passive (how much pain-free hyperextension can be achieved?)
2. Flexion
  1. Active (can the patient pull the calf muscle against the posterior thigh?)
  2. Passive (can the heel be made to touch the posterior thigh?)

Step 3: Effusion Test—The Fluid Wave Sign

Principles:

Some of the most clinically significant and often missed diagnoses (such as a meniscus tear, anterior cruciate ligament (ACL) injury, osteochondritis dissecans (OCD), chondral injuries) have an associated effusion.
Detection of this subtle sign is a critical step in a thoughtful knee assessment and may provide clear evidence of the need for magnetic resonance imaging (MRI).
A lack of knee effusion provides reassurance for proceeding with other measures prior to pursuing an MRI.

Stepwise approach and points to document:

With the knee in full extension and the quadriceps muscle relaxed, the medial capsular fluid should be gently pushed/milked superior to the patella and across the suprapatellar pouch to the lateral side.
The lateral capsular fluid should then be pushed with a thumb at the lateral peripatellar soft spot in a medial direction.
The medial peripatellar soft spot should be observed carefully for the presence of a fluid wave, suggesting abnormal/excessive synovial fluid or hemarthrosis.
In patients with large effusions, it is impossible to confine the fluid to the lateral knee and to generate a fluid wave. The patella will float on the fluid and can be pushed down. The presence of ballotable patella is another sign of a knee effusion.

Step 4: Ligamentous Examination

Principles:

If injury/symptoms are unilateral, always begin with contralateral limb to understand baseline status and normal features, given the spectrum of physiologic variability.
Elimination of guarding is critical to the accuracy of these tests. Patients can be distracted with unrelated conversation or can be instructed to interlock their fingers and focus on pulling their hands apart; sometimes straining their upper body muscles will relax their lower body muscles.
While among the most critical of ligamentous tests, the pivot shift maneuver should be reserved for the end of the ligamentous examination, so as not to create pain that may lead to greater guarding during the other attempted tests. At times, even slight guarding precludes an accurate/unconfounded test.

Stepwise approach and tests to document:

Lachman test for ACL—With the knee flexed to 20°, stabilize the distal femur just above the patella with one hand and anteriorly translate the proximal tibia just below the patella with the other hand. Feel for distance traveled and whether the end point is firm and hard (normal implies the anterior cruciate is intact) or soft and indiscrete (possible ACL disruption).
Valgus stress test for medial collateral ligament—With one hand supporting the leg, the other hand should apply a medially directed force against the lateral aspect of the knee to assess laxity. This is done at full extension and in 30° of flexion.
Varus stress test for lateral collateral ligament—With one hand supporting the leg, the other hand should apply a laterally directed force against the medial aspect of the knee to assess laxity. This is done at full extension and in 30° of flexion.
Posterior sag—With the hip and knee flexed 90°, the amount of posterior displacement of the tibia is compared between the knees. Increased sag implies posterior cruciate ligament (PCL) deficiency.
Drawer test—With the knee flexed 90°, the foot is stabilized by the examiner. The amount of anterior and posterior displacement is quantified. Increased displacement implies a cruciate injury.
Dial test—With the knee flexed (at 30° and 90°), increased external rotation of the tibia at the knee implies posterior/lateral injury of capsule, popliteus, and PCL.
Pivot shift test—With the knee fully extended and the tibia slightly internally rotated, the knee is gradually flexed to 15°-20°. Palpable reduction of the tibia as the knee is flexed implies an ACL-deficient knee.

Step 5: Meniscus Injury Tests

Principles:

Several of the meniscal tests or signs, such as joint line tenderness and pain with hyperextension, are quite sensitive to meniscus injuries, particularly in the acute setting, but not necessarily very specific. In other words, other diagnoses, such as an ACL tear or severe bone bruise, may also be associated with positive joint line tenderness or positive pain with hyperextension.
The overall number of meniscus signs, such as three out of five or four out of five, should be considered, especially as it relates to consideration toward the need for an MRI to definitely diagnose a patient’s knee pain or knee injury.

Stepwise approach and tests to document:

Fluid wave sign for effusion (see above).
Pain with knee hyperextension—With one hand on the anterior aspect of the distal femur, above the level of the suprapatellar pouch, the other hand elevates the tibia from the ankle level, straightening the knee and passively applying slight hyperextension through the knee joint. A meniscus tear will often give pain on the affected side (medial or lateral) or may be described to be felt “in the back” of the knee, in part due to the posterior horn being the most commonly affected segment of the meniscus to sustain a tear/injury.
Pain with knee hyperflexion—With one hand on the anterior aspect of the distal tibia/ankle, the other hand supports the anterior aspect of the midthigh, effectively flexing the knee (and hip) and passively applying hyperflexion through the knee joint. Similar to the hyperextension maneuver, a meniscus tear will often give pain on the affected side (medial or lateral) or may be described to be felt “in the back” of the knee, in part due to the posterior horn being the most commonly affected segment of the meniscus to sustain a tear/injury. Most commonly, with a meniscus tear, there is pain with both hyperextension and hyperflexion, but occasionally one is positive while the other is negative.
Joint line tenderness—With the knee flexed to 70°-90°, the side of the thumb or the side of the index finger is placed along the medial or lateral aspect of the knee, respectively, to apply pressure to the several millimeter gap between the proximal tibial plateau and the femoral condyle. A meticulous, systematic examination will include separate assessments of the anterior horn (anteromedial/anterolateral), meniscal body (directly medial/lateral), and posterior horn (posteromedial/posterolateral).
McMurray test/maneuver—With the knee flexed to 70°-90° and with one hand supporting the knee, the other hand is used to rotate the tibia internally and externally while extending the knee toward 10°-30° of flexion. The tibial plateau is thereby loaded against the corresponding femoral condyle, squeezing the interposed meniscus which, in the face of a tear or contusion, will generate pain. The classic description of a positive test is one of palpable subluxation, or a palpable “pop,” due to an unstable torn meniscus being felt by the examiner’s thumb or index finger, which is placed over the joint line. However, more commonly today, people consider a positive McMurray test to be one of the knee pains generated by the maneuver. For clarity, the authors favor describing either a “pain and a palpable pop or snap with the McMurray test” or “pain, but no palpable pop or snap, with the McMurray test.”

FIGURE 14.3 Inspection of the knee is often most effectively initiated from a squatted position several feet away from the standing patient. Appropriate exposure of the lower extremity should allow the examiner to visualize from the midthigh to the ankle, noting any asymmetry, swelling, and features of standing alignment.

When beginning the examination of the affected knee, the knee should be assessed first with direct visualization or inspection, often most effectively with the examiner squatting several feet away from a standing patient (Figure 14.3).

Is the skin intact? Is there an obvious effusion or hemarthrosis? Is it erythematous? Is the patella centered on the trochlea or is it dislocated (Figure 14.4)?

FIGURE 14.4 This figure demonstrates a laterally dislocated patella (outlined with a skin marker) (green arrow). Radiographic correlation is also provided.

Do their patella point toward each other? The standing coronal plane alignment should be assessed for the presence of genu valgum and genu varum. Next, having the child then point with one finger to the location of their greatest or most common site of pain is helpful in identifying the perceived location of pain. Do they localize the pain, or do they point at the entire anterior knee? Are there any masses on this knee of concern, compared with the contralateral knee?

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