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A Locked Knee in an Adolescent Boy

Resolving Meniscal Pathology in Young Athletes

H. Wesley Dykes, Jr, DO; James Tytko, MD


In Brief: A young athlete presenting with a locked knee is an uncommon occurrence in most outpatient clinics. Locked knees in children are primarily caused by meniscal tears, a discoid meniscus, or osteochondritis dissecans. When diagnosing meniscal tears, the history, physical examination, MRI, and/or arthroscopy are used to support a definitive diagnosis. Arthroscopic surgery is performed with the intention to preserve as much of the nontraumatized meniscus as possible, as in this case of an adolescent whose knee required surgical repair. However, in some cases, a total menisectomy is inevitable.

When a physician encounters a young patient who reports episodes of knee locking, the conditions that first spring to mind are congenital meniscal abnormalities and osteochondritis dissecans (OCD). The possibility of a meniscal tear, however, is important to consider. Young knees aren't immune to the sometimes harmful twisting and pivoting maneuvers that are common in many sports that are popular with kids.

Case Report

A 13-year-old boy came to our primary care sports clinic after his right knee had been locked in 90° of flexion for approximately an hour. The onset occurred while he attempted to tie his shoes in a somewhat twisted position. He reported experiencing similar locking episodes four or five times in the past 2 or 3 years, but he was always able to reduce the locking himself, and he had no episodes of swelling or giving way. The patient visited two local orthopedic surgeons prior to this visit, but not with his knee acutely locked. Results from a previous magnetic resonance imaging (MRI) study were read as normal. He was diagnosed with patellofemoral syndrome and given exercises for therapy. His medical history was unremarkable.

On physical exam, his knee was flexed to 90°, and he was unable to extend or flex beyond that point. No appreciable effusion was present, but his leg was exquisitely tender along the lateral joint line. His quadriceps and patellar tendons were grossly intact, and the patella appeared in good alignment. Ligamentous stability and hip range of motion could not be adequately assessed given the locked position of his right knee. Radiographic findings were unremarkable.

The patient was placed supine, and slow, gradual traction was applied manually to the knee until a large, palpable pop occurred over the posterolateral joint line. He was then able to fully extend the leg. Ligamentous stability was normal, and he was immediately placed in a straight-leg brace. A repeat MRI was ordered, and the patient left with instructions to ice, elevate, and keep the leg in extension until the next visit.

The MRI revealed an increased vertical signal in the body of the lateral meniscus, suggesting a tear (figure 1). When our orthopedic surgeon performed arthroscopy, he found a bucket-handle tear of the posterior horn of the lateral meniscus that was amenable to repair. The patient's postoperative course was uneventful, and he is currently doing well.

Unlocking the Diagnosis

A locked knee in a child can be caused by a meniscal tear, a discoid meniscus, or OCD. The discoid meniscus is a congenital anomaly wherein the meniscus is an abnormally thickened, disk-shaped structure instead of semilunar. Lateral discoid menisci are more prevalent than medial ones.1 The discoid meniscus extends farther centrally to cover more of the tibial plateau, thereby making it more prone to tearing because of abnormal shearing forces across the tibiofemoral articulation.2 Once torn, the meniscus can become lodged in the joint, resulting in locking of the knee.

The three categories of discoid menisci (complete, incomplete, and Wrisberg's ligament-deficient) are defined by the amount of tibial articular surface covered by the meniscus. Wrisberg's ligament-deficient menisci are unstable and prone to tearing, because they lack the posterior stabilization that prevents excess motion. Therefore, discoid menisci are inherently at risk for tearing and consequent locking caused by an abnormally thickened configuration or posterior instability.

OCD may lead to osteochondral fragmentation. Avascular necrosis of subchondral bone, which results in articular cartilage separation, has been described from histologic studies and is one theory for the development of OCD. In the knee, the medial femoral condyle is most affected.

Patients may or may not recall a precipitating traumatic event and often report diffuse achiness, swelling, crepitus, and locking. Isolated meniscal injuries in children with congenitally normal menisci were once considered uncommon. However, a growing number of meniscal injuries in the pediatric population may be attributed to the general increased interest and participation in athletics.3 Sports such as basketball, soccer, wrestling, and football appear to have a higher incidence of meniscal injuries compared with other sports. Meniscal tears increase in frequency with age; therefore, they are more prevalent in adolescent patients than in young children. Meniscal tears can occur with ligamentous injuries in youths or as isolated incidences in children younger than 14 years old who have congenital anomalies.4

Anatomy and Function

The menisci are crescent-shaped, fibrocartilagenous structures that: (1) absorb energy from compressive loads placed across the knee, thus protecting the femoral and tibial articular surfaces; (2) assist in joint stabilization by decreasing the amount of anterior and posterior translation of the femur across the tibia; and (3) aid in joint lubrication by distributing synovial fluid.3,4 The semilunar-shaped medial meniscus is stabilized by its attachments to the anterior and posterior intercondylar fossae, the articular capsule, and the transverse ligament anteriorly, and coronary ligaments posteriorly. The lateral meniscus is semicircular and is normally stabilized by attachments to the anterior and posterior intercondylar fossa and Humphrey's ligaments, and Wrisberg's ligaments posteriorly. A Wrisberg ligament-deficient meniscus, however, lacks posterior stabilization because of insufficient attachment to the posterior capsule.

The menisci receive their blood supply via the genicular arteries. In the young athlete, the outer one third of the menisci is well vascularized and therefore has been termed the red-red zone. The middle one- third is less vascularized and is known as the red-white zone. The inner one-third is poorly vascularized and is referred to as the white-white zone. These zones are important in determining the method of treatment for meniscal injuries. When surgery is indicated, tears that occur in the red-red zone are more amenable to repair rather than partial removal of the meniscus because of the increased vascularity. Tears occurring in the red-white and white-white zones are less favorable for repair and more amenable to partial resection because of inadequate vascularization. The most frequent type of meniscal tear encountered in children is longitudinal.4

History and Physical Exam

In a child, the presenting history is paramount when attempting to make an accurate diagnosis of a meniscal tear. Meniscal injuries can be easily mistaken for patellofemoral pathology or other knee conditions if the history is poorly communicated, inadequately comprehended, or misinterpreted. In many instances, it may be difficult to ascertain the exact mechanism of injury because of the inability of young patients to effectively articulate their symptoms.3 However, a parent, relative, or caregiver may help provide pertinent information in these cases.

Meniscal injuries in young patients usually occur as a result of some type of twisting or pivoting injury that produces shearing forces and compressive loads across the meniscus. Patients usually report medial or lateral joint-line discomfort that is worse with activities (eg, squatting, stair climbing, running, and jumping) and relieved with rest. Mild swelling will often be noticed, and episodes of giving way and/or locking may have been experienced.

A true locking episode occurs when the knee is hyperflexed with weight distributed across the knee as the patient attempts to extend the leg. Before full extension is achieved, a popping sensation is often felt when the meniscal flap wedges into the joint, rendering it immoveable. Our patient experienced this locking mechanism when bending over to tie his shoes with his knee hyperflexed and his weight loaded across the knee. Patients, on occasion, can reduce their locked knees without any assistance by internally and externally rotating their legs in a twisting motion until a pop or clunk is felt, signaling reduction of the meniscal flap. However, most locked knees will require arthroscopic surgery. Gradual manual traction to reduce the knee is often unsuccessful when attempted by a medical practitioner, but it worked for this patient.

A mild effusion may or may not be apparent, depending on the interval between presentation and the time of injury. Flexion or hyperflexion with rotation of the knee will often elicit discomfort. When a patient has a locked knee, the knee will be flexed to a degree, unable to fully extend. Discomfort may be elicited with forced extension when the patient is supine. McMurray's click and Apley's grind tests may elicit positive findings; however, they have a sensitivity of only 58%,4 and patients are usually too uncomfortable to permit these tests. Additionally, evaluation for OCD (eg, performing the Wilson test) should be considered if locking or catching episodes are present. Lastly, a thorough hip exam is essential in all pediatric patients to rule out hip pathology that may refer pain to the knee.

Diagnostic Imaging

Standard plain radiographs consisting of anteroposterior, lateral, skyline, and tunnel views should be ordered initially to rule out OCD, loose bodies, or tumors and to evaluate the patella. MRI is considered the imaging study of choice for the detection of meniscal injuries; however, its utility in the role of detecting intra-articular pathology in the pediatric population has been questioned in the orthopedic literature.5-7

Kocher et al5 studied the diagnostic performances of clinical exam and selective MRI, in comparison to arthroscopy, in the evaluation of intra-articular knee disorders in children and adolescents. Five common intra-articular knee disorders (ie, anterior cruciate ligament [ACL] tears, medial and lateral meniscal tears, lateral discoid tears, and OCD) were evaluated during a 7-year period in 113 pediatric patients age 16 or younger (118 knees). No significant difference was found between clinical exam and MRI when compared with arthroscopic findings. Stratification by diagnosis revealed significant differences for sensitivity of lateral discoid meniscus (clinical exam, 88.9%; MRI, 38.9%) and specificity for medial meniscal tears (clinical exam, 80.7%; MRI, 92.0%). MRI in children younger than 12 had a significantly lower overall sensitivity (61.7% vs 78.2%) and specificity (90.2% vs 95%) compared with children 12 to 16 years old.

Stanitski6 performed a similar study comparing the correlation of arthroscopy and clinical exam to MRI findings of injured knees in children and adolescents. He evaluated meniscal, ACL, and articular surface injuries in 28 patients who were 8 to 17 years old. He noted a highly positive correlation (78.5%) between clinical and arthroscopic findings and highly negative correlations between arthroscopic findings and MRI (78.5%) and between clinical exam and MRI (75%). He concluded that MRI diagnoses added little to patient management and at times provided spurious information.

McDermott et al7 studied the correlation of MRI and arthroscopic diagnosis of knee pathology in the meniscus and ACL in children and adolescents. Between 1992 and 1996, 108 consecutive arthroscopies were performed in patients 4 to 17 years old, and these were retrospectively reviewed. Fifty patients had MRI preoperatively. Comparisons between MRI and intraoperative findings were performed. Patients between the ages of 4 and 14 had a significant decrease in sensitivity, specificity, positive predictive value, and accuracy compared with patients between 15 and 17 years old. However, negative predictive values for the 4-to-14 age-group exceeded those in the 15-to-17 age-group. Thus, they concluded that predicting intra-articular knee pathology with MRI is much less accurate in children and youth, whereas in adolescents, it is comparable to adults. This finding may explain why our patient's meniscal tear went undetected by the first MRI. Arthroscopy provides the most definitive evaluation of meniscal integrity.


Meniscal tears in children, as in adults, can be repaired, excised, or left untouched. Meniscal preservation in the pediatric population is currently the preferred option in the orthopedic community.8-10 Children generally have a more robust blood supply to the peripheral third of the meniscus (red-red zone) in comparison with adults, thus improving the potential for healing. Peripheral tears, assessed during arthroscopy, that are less than 1 cm long and have less than 3 mm of motion when probed are deemed stable and do not require surgical repair.11 Rehabilitation can begin when pain becomes tolerable. Nondegenerative, noncomplex peripheral tears 1 cm or longer with greater than 3 mm of motion are suitable for repair. The orthopedic literature is replete with favorable outcomes from peripheral meniscal repair.9,10,12-14

Mintzer et al9 reported a 100% healing rate in 26 athletes who were age 17 or younger when they underwent meniscal repair. A rim width less than 3 mm and ACL reconstruction at the time of repair significantly improves the rate of healing.8,10,12 Tears that extend more centrally beyond the peripheral third are more probable candidates for partial excision.

A Wider View

Meniscal tears in children and youth are still fairly rare; however, as more children and adolescents participate in sports, we may see more of these injuries. To reduce the likelihood of misdiagnosis, the impression that children do not tear their menisci should be discarded by clinicians who care for pediatric patients. Young patients who present with knee pathology should be thoroughly evaluated. If locking occurs, the differential diagnosis should include OCD, meniscal tear, and discoid meniscus until they are ruled out. Patients with discoid menisci are usually asymptomatic unless they have Wrisberg's ligament pathology that typically causes snapping knee syndrome. Meniscal preservation is the general treatment of choice when surgery is indicated. Meniscal tears involving the peripheral third have the best prognoses for healing after surgical repair or if treated conservatively because of the rich blood supply to this region.


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Dr Dykes is a family practice physician at Mechanix Sports and Occupational Medicine, a division of The Family Physicians Group in Memphis, Tennessee, and Dr Tytko is a family practice physician and the sports medicine fellowship director at Kettering Sports Medicine Center in Kettering, Ohio. Address correspondence to James Tytko, MD, Kettering Sports Medicine Center, 3490 Far Hills Ave, Kettering, OH 45429; address e-mail correspondence to [email protected].

Disclosure information: Drs Dykes and Tytko disclose no significant relationship with any manufacturer of any commercial product mentioned in this article. No drug is mentioned in this article for an unlabeled use.