Need for More Accurate ER Diagnoses of ACL Injuries
It is a generally held belief that pain is the number one reason why patients visit the emergency room. There are many causes of pain. Some causes are:
- Idiopathic pain from prior treatment
In 2006, there were approximately 119.2 million visits to the emergency room in the United States.1 General symptoms such as fever, fatigue and pain accounted for 15.6% of visits, followed by musculoskeletal symptoms (13.8%), digestive symptoms (13.7%), and respiratory symptoms (10.7%).2 According to the CDC, knee pain is the second most common musculoskeletal complaint and the most common sports-related injury (32.3%) seen in the emergency room.3
The anterior cruciate ligament (ACL) is the most commonly injured ligament in the knee. Each year, there are nearly 100,000 to 200,000 ACL tears in the US alone, making its incidence 1:3,500.4 With such prevalent numbers, it could be assumed that emergency room physicians would be proficient at diagnosing ACL tears. However, a recent study showed just the opposite.
Guillodo, et al looked at patients 15-55 years old who presented to the emergency room at a teaching hospital over a 5-month period with a chief complaint of knee pain. The results of a standard physical examination conducted by emergency room physicians were compared to findings by a sports medicine specialist. Of the 79 included patients, 27 (34.2%) had a diagnosis of an ACL rupture established by the specialist and confirmed by MRI.5 Of the 27 ACL ruptures, only seven were correctly diagnosed by the emergency room physicians.
Knee injuries, most notably ACL tears, create a public health burden related to their high incidence, association with disability and considerable cost.6 Early recognition of ACL injury is essential to optimize outcomes through early and appropriate treatment. In this article, ACL injuries will be discussed beginning with the anatomy of the knee, mechanism of injury, signs and symptoms, physical exam and diagnostic testing, risk factors associated with ACL injury and, finally, treatment.
Anatomy of the Knee
The knee is a hinge joint comprised of the femur, tibia and patella. The knee is stabilized by four ligaments (see Figure 1): the medial collateral ligament (MCL), the lateral collateral ligament (LCL), the posterior cruciate ligament (PCL), and the anterior cruciate ligament (ACL). The collateral ligaments provide the knee with lateral stability. The MCL runs along the medial aspect of the knee and prevents the knee from bending inward. The LCL runs along the lateral aspect of the knee and prevents the knee from bending outward. It is important to note that the knee has two menisci (medial and lateral) which act as a cushion to disperse force in the knee. The medial meniscus is contiguous with the MCL while the lateral meniscus is a separate entity from the LCL. This explains why in many MCL injuries there is also damage to the medial meniscus.
While the collateral ligaments stabilize the lateral movements of the knee, the cruciate ligaments stabilize the anterior and posterior glide of the knee. The PCL runs from the posterior intercondylar area of the tibia to the medial condyle of the femur and prevents the posterior glide of the tibia on the femur. Originating at the posteromedial aspect of the femoral condyle, the ACL prevents the anterior glide of the tibia on the femur. The ACL courses distally in an anterior and medial fashion to the anteromedial aspect of the tibia between the condyles. The ACL is comprised of two bundles: an anteromedial bundle that is tight in flexion and a posterolateral bundle that is tight in extension. The blood supply of the ACL is from branches of the middle geniculate artery, while a branch of the tibial nerve—the posterior articular nerve—innervates it. 7
Mechanism of ACL Injury
ACL injuries can occur by a variety of mechanisms including contact and noncontact mechanisms. Noncontact injuries are more common, accounting for 70% of ACL tears.8 The typical mechanism for a noncontact ACL injury involves a running or jumping person who suddenly decelerates and changes directions in a way that involves rotation or lateral bending. When a person moves their leg forcefully into this valgus position with the knee extended and tibia rotated, much stress is put on the ACL. Examples of sports where this pivoting motion is common are alpine skiing, soccer, gymnastics, basketball and tennis. On the other hand, contact-related ACL injuries usually occur from a lateral to medial blow causing hyperextension or valgus stress to the knee. This is commonly seen in football when a player’s foot is planted and an opponent strikes him on the lateral aspect of the leg.
ACL injuries are classified by “grades” with 1 being the least severe and 3 being the most severe. A grade 1 sprain is defined as pain with minimal damage to the ligaments. A grade 2 sprain has more ligamentous damage and mild looseness of the joint. In a grade 3 sprain, the ligament is completely torn and the joint is very loose or unstable.9
Signs and Symptoms
Most patients who sustain an ACL injury complain of feeling or hearing a “pop” in their knee at the time of injury. Many patients then experience acute swelling within 6 hours of injury and a feeling of knee instability. After the initial swelling improves, patients are often able to bear weight but continue to complain of knee instability. Movements such as squatting, pivoting and stepping laterally may cause the knee to give out. In addition, any activity where the entire body weight is transferred to the injured leg, such as walking down stairs, will illicit this instability.
Physical Exam and Diagnostic Tests
A thorough physical exam is essential in the diagnosis of an ACL injury. As with all medical evaluations, a comprehensive history and physical exam is crucial. In patients with a possible ACL injury, the physician should inquire about the timing of the injury, the mechanism, joint swelling, functional ability, joint instability and any associated injuries. The physician must remember the basics and inspect, palpate, test mobility of the knee joint, strength of the quadriceps and hamstrings as well as perform special tests to determine the integrity of the ACL. The physician must not forget to examine the unaffected knee for comparison since many individuals have increased laxity that is not pathologic.
The most sensitive and specific special tests used to exam the integrity of the ACL is the Lachman test. The Lachman test has a sensitivity of 85% and a specificity of 94% for an ACL rupture.10 With the patient supine, the knee is placed in 30 degrees of flexion (see Figure 2). The physician then stabilizes the distal femur with one hand while pulling the proximal tibia anteriorly with the other hand in a scissoring motion. A negative test is one in which there is a distinct endpoint and implies an intact ACL. A positive test has a vague endpoint and implies an ACL injury. Again, it is imperative that both the affected and unaffected knee are examined.