The Knee

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Chapter 669 The Knee

Lawrence Wells, Kriti Sehgal

Normal Development of the Knee

The knee is a major synovial joint and develops between the 3rd and 4th fetal mo. The secondary centers of ossification are formed between the 6th and 9th fetal mo for the distal femur and between the 8th fetal mo and the 1st postnatal mo for the upper tibia. The patellar ossification center appears between the 2nd and 4th yr in girls and the 3rd and 5th yr in boys.

Normal Range of Motion

The fully extended knee is normally in the neutral position. The normal range of motion extends from neutral to about 140 degrees, with most activities performed in the flexion arc of 0-70 degrees. Hyperextension of up to 10-15 degrees is considered normal in a child.

The knee is the largest joint in the body and is a modified hinge type of synovial joint that also permits some element of rotation. It consists of three joints merged into one: an intermediate one between the patella and the femur, and lateral and medial ones between the femoral and tibial condyles. The distal femur is cam shaped, allowing it to have a gliding, hinged motion. The major constraints of the knee are the medial and lateral collateral ligaments, the anterior and posterior cruciate ligaments, and the medial and lateral menisci. There are several bursae about the knee because most tendons around the knee run parallel to the bones and pull lengthwise across the knee joint.

Knee pain is one of the most common presenting complaints in older children and adolescents. This is commonly related to trauma but may also be insidious in onset. Knee effusion may be a common feature associated with knee pain. Depending on the etiology of the intra-articular process, the fluid collected in the knee may be blood (trauma- or hemophilia-induced hemarthrosis), inflammatory fluid (juvenile rheumatoid arthritis), or purulent material (septic arthritis). The presence of fat globules in the blood aspirated from a hemarthrosis suggests an occult fracture. Recurrent effusions can indicate a chronic internal derangement such as a meniscal tear. Aspiration of the joint fluid is often necessary to establish the diagnosis as well as to offer relief of symptoms (Chapter 684).

669.1 Discoid Lateral Meniscus

Lawrence Wells, Kriti Sehgal

Discoid lateral meniscus (DLM) is an anatomic variation of the lateral meniscus that may be asymptomatic or can cause snapping or popping of the knee. There are three types of DLM. The first is the Wrisberg ligament type where the lateral meniscus has no attachment to the tibial plateau posteriorly but has a meniscofemoral ligament or ligament of Wrisberg that connects the posterior horn of the lateral meniscus to the lateral surface of the medial femoral condyle. The ligament prevents the gliding of the meniscus during knee extension, producing a snap with each excursion of the meniscus, leading to hypertrophy and irregularity. The second type is the complete type, which is characterized by a thickened lateral meniscus that does not move in and out of the center of the joint and has normal peripheral attachments. The third type is the incomplete type, which is smaller than the complete type and does not fill the lateral compartment.

Clinical Manifestations and Diagnosis

The cause of discoid meniscus is not defined, but it may be a failure of an embryologic sequence of degeneration of the center of the meniscus. The normal meniscus is attached around its periphery and glides anteriorly and posteriorly with knee motion, but a discoid meniscus is less mobile and may be torn. Occasionally, there is no peripheral attachment around the posterolateral aspect of the meniscus, which can allow it to become displaced anteriorly with knee flexion, producing a loud click or clunk.

The usual presenting complaint is that of a popping or snapping of the knee that is both heard and felt by the child or parent. This is often noted in children >6 yr of age. Most often the snapping is not painful and the child is active. A second type of presentation is of a child who has had no knee symptoms but presents spontaneously or after an injury with pain, snapping, popping, or locking located along the lateral joint line. Physical examination might show a mild effusion and tenderness with fullness over the lateral joint line and crepitation with motion. The typical findings include a palpable snapping as the knee flexes and extends. Along the lateral joint line, the examiner feels a bulge, as the meniscus seems to protrude beyond the margin of the tibia. As the knee moves, the meniscus snaps into the intercondylar notch and the bulge disappears.

Anteroposterior radiography of the knee can show widening of the lateral aspect of the knee joint. Other findings include flattening of the lateral femoral condyle (giving a squared off appearance) and cupping of the lateral aspect of the tibial plateau. MRI or arthroscopy is required for definitive diagnosis.

Treatment

Many children with discoid menisci require no treatment. These children should be followed and treated only if pain or loss of motion occurs. Surgery may be considered when DLM leads to locking, swelling, loss of motion, inability to run, or inability to participate in sports. The treatment is to excise tears and reshape the meniscus arthroscopically (Fig. 669-1). Meniscal instability can occasionally be repaired or reconstructed. Complete excision may be necessary if other procedures are unsuccessful.

image

Figure 669-1 Arthroscopic image of discoid meniscus and arthroscopic shaving with meniscectomy.

Bibliography

Ahn JH, Shim JS, Hwang CH, et al. Discoid lateral meniscus in children: Clinical manifestations and morphology. J Pediatr Orthop. 2001;21:812-816.

669.2 Popliteal Cysts (Baker Cysts)

Lawrence Wells, Kriti Sehgal

Popliteal cysts, or Baker cysts, common in children, are cystic masses filled with gelatinous material that develop in the popliteal fossa, are usually asymptomatic, and are not related to intra-articular pathology. They usually resolve spontaneously, although the process can take several years.

The usual presentation is that of a mass behind the knee that may be fairly large when first noted. There are usually no symptoms of internal derangement of the knee. Physical examination reveals a firm mass in the popliteal fossa, often medially located and usually distal to the popliteal crease. The mass is most prominent when the knee is extended and the patient is lying in prone position.

The most common site of origin is the bursa of the gastrocnemius and semimembranosus. Another common site of origin is a herniation through the posterior joint capsule of the knee. Histologically, the cysts are classified as fibrous, synovial, inflammatory, or transitional. Transillumination of the cyst on physical examination is a simple diagnostic test. Knee radiographs are normal and should be obtained to identify other lesions such as osteochondromas, osteochondritis dissecans, and malignancies. The diagnosis may be confirmed by ultrasonography (to differentiate a solid mass from a cystic lesion) or aspiration. In most cases, these cysts should be left alone, because they often resolve spontaneously. Surgical excision of a popliteal cyst is indicated only when symptoms are severe and limiting and have not resolved after several months. The presence of a solid mass detected on examination or MRI requires exploration.

Bibliography

Van Rhijn LW, Jansen EJ, Pruijs HE. Long-term follow-up of conservatively treated popliteal cysts in children. J Pediatr Orthop. 2000;9:62-64.

669.3 Osteochondritis Dissecans

Lawrence Wells, Kriti Sehgal

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