Musculoskeletal Clinics: Acute Knee Pain in a Teenaged Football Player
PATIENT PROFILE: A 17-year-old girl presents, on crutches, with left knee pain from an injury she sustained during a flag football game the previous night. While running down the field, she attempted to remove the flag from another player. When this player cut away from her, the patient made a quick stop and rotated her body to the right in an effort to reach the flag. Immediately, she felt pain in her left knee, which seemed to collapse as she fell to the ground. She did not hear a “pop.” She attempted to get up but was unable to support any weight on the leg and had to be helped off the field by 2 other players. There was immediate swelling; this was treated with ice, and the patient was given crutches and told to see a physician in the morning. Overnight the swelling increased, and she now has difficulty in extending and flexing the knee. Her general health is good, and she has no history of knee injury. FOR WHICH TEST WOULD YOU EXPECT A POSITIVE RESULT AT THIS POINT? A. Lachman test. B. McMurray test. C. Valgus stress to detect opening of the medial joint line. D. Varus stress to detect opening of the lateral joint line. E. None of the above. THE CONSULTANT’S CHOICE Lateral joint line opening with varus stress (D) indicates a tear of the lateral collateral ligament, and medial joint line opening with valgus stress (C) indicates a tear of the medial collateral ligament. These types of injuries occur when there is a direct blow to the knee or force is applied to the knee’s lateral or medial side. This patient’s injury did not involve any contact. The mechanism of this patient’s injury, which consisted of sudden deceleration and twisting, is the classic mechanism for an anterior cruciate ligament (ACL) tear and a possible mechanism for a meniscal tear. However, the Lachman test (A) for ACL sprains and tears and the McMurray test (B) for meniscal tears are difficult to perform and the results difficult to interpret 24 hours after an injury because of swelling, pain with any movement,and reflex hamstring spasm. Results of the Lachman test may be reliable in the first 30 minutes or so after an injury. After this initial period, however, it takes up to 10 days for the swelling, pain, and spasm to decrease to the point that the results are again accurate. The same is true for the McMurray test. These tests may still be attempted if the patient can tolerate them, but they must be repeated in 7 to 10 days to be assured of the results. Thus, at this point, none of the tests (E) would be expected to yield a positive result. Examination reveals significant swelling of the left knee, inability to flex the knee beyond 70 degrees, and an extension lag of 10 degrees. Flexion in the right knee is normal (to 145 degrees) (Figure 1), as is extension (to 0 degrees) (Figue 2). Results of a Lachman test (Figure 3) and McMurray test (Figure 4) in the right knee are negative, and there is no laxity in that knee with either varus stress (Figure 5) or valgus stress (Figure 6). THE CONSULTANT’S CHOICE The lack of a pop (C ) is not typical of an ACL tear. An audible pop accompanies up to 50% of ACL injuries. Acute knee swelling after an injury (B) indicates immediate bleeding into the knee joint. More than 65% to 70% of knee injuries with hemarthrosis are associated with ACL sprains or tears. Inability to bear weight (A) is characteristic of a fracture or significant ligamentous injury to the knee and requires evaluation for a significant injury. Between 60% and 70% of ACL injuries are associated with noncontact mechanisms such as sudden deceleration and rotational forces (D). Meniscal injuries are usually associated with rotation or twisting. The swelling noted with meniscal injury usually starts 8 to 12 hours after the injury. Contact usually causes injury to the collateral ligaments. More powerful contact may produce ACL injury in association with collateral ligament and meniscal injury. Injury to all 3 is sometimes referred to as the “terrible triad.” Although this patient did not hear a pop when she was injured, the acute knee swelling and the mechanism of action (noncontact, involving sudden deceleration) point to an ACL injury as the most likely diagnosis. Figure 3 – The Lachman test assesses the degree of anterior translation in the knee and whether the end point is firm, soft, or nonexistent. The test is 85% sensitive and 95% specific for ACL injury. To perform a Lachman test, have the patient lie on his or her back with the knee in 20 to 30 degrees of flexion and the leg relaxed. Relaxation ensures accurate results; palpate the hamstrings to check for adequate relaxation. Stabilize the femur with your nondominant hand, grasp the tibia with your dominant hand, and pull the lower leg anteriorly (A). An alternative method can be used if the patient’s leg is too large or your hand is too small (or both): have an assistant stabilize the femur while you place both thumbs in the joint lines, grasp the calf muscles with your fingers, and apply force in an anterior direction (B). Pathophysiology of ACL injuries. The ACL, one of the static stabilizers of the knee, is the primary restraint on anterior translation of the tibia with respect to the femur and the secondary restraint on internal rotation of the tibia. The ACL ascends from the anterior intercondylar area of the tibia and extends superiorly, posteriorly, and laterally to attach to a facet at the back of the lateral wall of the intercondylar fossa of the femur. The attachment to the tibia can sometimes be avulsed; the resulting fracture is called a Segond fracture. The classic history of an ACL injury is one of noncontact injury that involves sudden deceleration, hyperextension, or twisting, accompanied by a pop—and that produces significant pain and immediate swelling. Such an injury may occur when a patient “plants and cuts” or executes a straight-leg landing or a one-step stop with the knee hyperextended. ACL injuries are common in basketball, football, and soccer players. About one third occur after deceleration and one third occur on landing. Risk factors. Potential risk factors for noncontact ACL injury include lower levels of conditioning, strength, coordination, or skill, and elevated levels of friction between athletic shoes and the playing surface. Women are at greater risk for ACL injury than men. Certain typical features of female anatomy are potential risk factors. These include greater joint laxity, a wider femoral notch, and smaller ACL size than are usually seen in men. Women also have greater hip varus and knee valgus than do men; these features result in a larger Q angle and greater foot pronation (eversion)—a pattern of lower extremity alignment that predisposes one to ACL injury. Women further increase the risk of injury by their tendency to land with less knee flexion and greater angles of valgus than men do. Fatigue increases knee valgus and decreases knee flexion in both men and women performing stop-jump tasks; however, the effect is greater in women. Female sex hormones may also increase the risk of ACL injury. There are estrogen and progesterone receptors in the ACL, and high estrogen levels correlate with ACL laxity. WHICH 2 OF THE FOLLOWING WOULD YOU DO NOW? A. Obtain a plain radiograph of the left knee. B. Obtain an MRI scan of the left knee. C. Drain the fluid from the left knee and inject cortisone into the joint. D. Obtain an emergency orthopedic consult for immediate surgery. E. Have the patient continue to use crutches and return to see you in 1 week. THE CONSULTANT’S CHOICE Plain radiography (A ) is indicated to rule out fractures of the tibial plateau, epiphysis, or patella and an avulsion fracture of the lateral tibia (Segond fracture associated with ACL tear). MRI (B) is not as reliable in the acute setting as it is once the swelling and limited motion are resolved. Draining the fluid (C), which is probably blood, is not indicated except to relieve pain. Injection of cortisone into the joint (also C) is of no value in this setting. Moreover, injecting the patient with a needle may increase the risk of infection. An emergency orthopedic consultation for immediate surgery (D) is not necessary. If surgery is required for an ACL tear, it is best delayed for a month or so. Thus, evaluation by an orthopedic surgeon—if indicated—need not be done on an emergent basis. Patients with ACL tears should use crutches and should not bear weight (E ). Some clinicians also use a knee stabilizer to keep the knee from overextending or flexing. However, avoid using a knee stabilizer for longer than a few days; keeping the knee immobilized promotes quadriceps atrophy, which can increase rehabilitation time. See the patient again in a week; the passage of a week’s time increases the likelihood that the results of specific tests, such as the Lachman test, will be accurate. The patient was monitored for the next 3 weeks. During that time, results of a Lachman test became positive, showing a 6- to 8-mm increase in anterior tibial translation of the left knee compared with the right knee, with a softer but definite end point. These findings suggest a partial ACL tear. In a normal knee, there is less than 4 mm of tibial translation; between 5 and 10 mm of translation is seen with partial tears. Full tears result in 11 mm or more of translation and a softer or indefinite end point.