The anterior cruciate ligament or ACL is a structure in the knee that serves to control both anterior translation of the tibia on the femur and rotation of the knee joint. ACL injuries are fairly common with an estimated 80,000–250,000 incidences a year with 0.5–3% of those occurring in children and adolescents [1–3]. It was previously thought that ACL tears were predominantly found only in older populations, with pediatric and adolescent patients suffering more from tibial avulsion injuries rather than true midsubstance tears of the ligament. However, recent studies have shown that midsubstance ACL tears in pediatric patients have in fact slowly become more common [4]. A recent study by Ganley et al. looked at high school athletes and found that on average if they play a sport all four seasons for 4 years, they have an 11.2% and 6.4% risk of tearing their ACL for girls and boys, respectively [5]. This injury rate goes up even higher for certain sports such as girls’ soccer, girls’ basketball, and girls’ football. While some believe these rising incidences of ACL tears are due to increasing numbers of young athletes focusing more and more on a single sport while doing so year round, others believe that it is due to increased awareness and more accurate diagnoses of pediatric ACL tears [6].

The types of functional ACL injuries can include tibial spine avulsion fractures to partial ACL tears to complete ACL tears, with partial tears ranging from 10 to 50% of all ACL injuries [7]. And although ACL injuries are one of the most well-researched orthopedic injuries to date, there are still many debates about the best treatment options for these injuries, including nonoperative vs. operative, surgical technique and graft choice [8]. In this chapter we will limit discussion to the clinical evaluation of diagnosing an ACL tear.

Many factors can lead to challenges in making an accurate ACL injury diagnosis in a pediatric patient. A range of ACL injury types as mentioned above coupled with the difficulty in effectively gathering information about injury conditions due to the age of patients along with fear of pain or difficulty in cooperating during physical exam can all lead to making an accurate diagnosis challenging [3]. And while diagnosing complete ACL tears can be relatively more straightforward, diagnosing partial ACL tears can prove especially challenging in pediatric patients. The injury history of a partial ACL tear may not be terribly revealing or suggestive; patients do not always report the classic symptoms or injury mechanism seen often in full ACL tears, and clinical exam, arthrometry (KT-1000), and radiographic imaging results may be subtle or limited in their utility. As with any diagnosis, an accurate ACL tear diagnosis is best determined after correlating all the available information: the patient history, physical examination, imaging studies, and any arthrometric measurements that may be available [1]. The following pages outline each of these aspects of the clinical evaluation in further detail.

A physician should always elicit and review previous medical history when evaluating a patient to help determine the correct diagnosis. It should be noted whether there have been any prior injuries to the knee, previous treatments, and all outcomes of those treatments if received. Building an understanding of the previous history gives a better idea of what may be causing or contributing to the current patient condition and aids accurate diagnosis. The next step should be to obtain the history of current injury from the patient, including the mechanism and circumstances surrounding the injury. For older patients, obtaining an accurate history and mechanism of injury may be more reliable, but for younger pediatric and adolescent patients, correlation with a parent or other witness can prove invaluable in building an accurate understanding of the injury circumstances [3]. Nowadays especially at many sporting events, video footage is available, and the videos that patients bring in can be particularly elucidating. Even videos on cellphones can be helpful in determining the mechanism of injury as most phones these days have high-quality cameras.

The most common injury mechanism for an ACL tear is a noncontact pivoting movement, while the foot is planted with the knee in slight flexion [1, 3, 8]. Other possible but less common mechanisms of ACL injury include hyperextension with valgus or rotational force and, far less commonly, extreme hyperflexion or hyperextension of the knee [4, 8]. Often seen during sports such as football, soccer, or basketball, patients will describe how they tried to change speed or direction quickly and felt their knee give away [4]. Common history elements for ACL tears include running, jumping, cutting, “snapping,” “buckling,” “giving out,” or “popping” sensations in the knee [1, 3, 8]. Inability to bear weight, inability to return to play, swelling, and hemarthrosis are usually strong markers of a significant knee injury, and it should be noted whether the patient fell at the time of their injury, whether they were able to get back up on their own or required assistance, and whether they were able to return to sport activities at that time [3, 8]. While pain, swelling, and instability after an ACL injury often prevent weight bearing, it is not unheard of that patients with tears, especially partial tears, are able to return to competitive play within a week or sometimes even the same game. In fact some studies have found that upwards of 15% of patients with ACL tears, whether partial or complete, were able to return immediately to their sport after injury [9].

After history has been obtained, a physical examination should be performed. The patient should be wearing shorts during examination rather than long pants, which even when rolled or held up obscure the knees, thighs, and entire lower extremity from being evaluated all at once. The physical exam should begin with a general standing alignment evaluation. Any underlying leg length inequalities and/or anatomic genu varum or genu valgum should be noted at this time. The contralateral side should be evaluated first, followed by the affected side in order to establish baseline for the patient.

Inspection for any ecchymosis, swelling, and/or knee effusion of the affected side may then begin. If there is traumatic effusion and radiographs have not been performed, yet they should be performed prior to physical exam to prevent potentially displacing a nondisplaced tibial spine fracture [8]. Once radiographs have been reviewed, the patient may be safely examined. As mentioned above, acute hemarthrosis of the knee is usually a strong indication of a serious knee injury. In adults around 67% of those with acute hemarthrosis have some degree of ACL tear whether partial (26%) or full (41%) [10]. Studies in children have found that rate is slightly lower around 47% for ages 7–12 but more similar around 65% for adolescents 13–18 [11]. Still others have found these rates to range vastly from as low as 29% in younger populations 18 or younger to over 72% when a wider range of 16- to 44-year-olds are included [9, 12].

Once inspection for ecchymosis, swelling, and/or knee effusion has been noted, a palpation exam may be performed [1, 3, 13]. In cases where the exam is limited due to excessive effusion, the knee can be aspirated first [1]. While palpation of the ACL is not possible, palpation to other structures can provide important information about possible concomitant injuries. In fact around 16% of patients with ACL tears have associated meniscal tears, 30% have medial collateral ligament tears, and 2% have lateral collateral ligament tears [10, 14]. Palpating along the medial and lateral joint line can assess for possible meniscal injury, and palpation at the medial and lateral ligament insertion sites can assess for MCL and LCL injuries. The MCL and LCL can also be assessed further with valgus and varus stress tests, respectively. These tests should both be performed with the knee in extension and once at 30° of flexion. Opening of the joint in extension and at 30° flexion implies posterior capsular injury, whereas isolated collateral ligament injury would lead to joint opening at 30° of flexion. Palpation to the medial and lateral borders of the patella along with a patellar translation test can also be performed on patients with a suspected patellar dislocation [3]. Patellar dislocations can be fairly similar in presentation to the ACL tear, with some studies showing almost identical incidences of these injuries in patients who present with acute hemarthrosis (29% ACL tears and 25% patellar dislocations) [12].

Following palpation exam, basic range of motion of the knee should be examined. Limited flexion or extension ability as well as complaints of “locking” or “catching” can suggest concomitant meniscal injuries or chondral loose bodies. These are more commonly seen in full ACL tears than in partial tears [1]. If locking or catching is found on range of motion exam, further meniscal pathology should be examined with tests such as the McMurray’s or Thessaly test. As with any diagnosis, diagnosis for concomitant injuries should be diagnosed only after correlating all available information.

After preliminary inspection, palpation, and range of motion exams have been performed, the physician should perform the specific cruciate ligament tests. The anterior drawer, Lachman (a modified anterior drawer test), and pivot shift test are the three major physical exam tests performed on patients with suspected ACL tears [4].

A brief look through history shows that the Lachman test, named after Dr. John W. Lachman, has actually been repeatedly described by various physicians over the years. As early as 1875, Greek physician George Noulis wrote:

When the leg was then moved forward and backward, it was found that the tibia will slide anteriorly and posteriorly. When only the ACL was severed, movement of the tibia could be shown when the knee was “barely flexed.”

However, it wasn’t until 1976 that this universal test became more established after being given its name by Dr. Lachman’s student and mentee, Dr. Joseph Torg [15]. In a clinical setting, the Lachman test is the most sensitive of the three tests [16]. With the knee at 30 degrees of flexion and the leg in slight external rotation to relax the hamstring, the lower leg is translated anteriorly. A positive test is when the tibia moves forward and the concavity of the patellar tendon becomes convex [4]. Severity is graded on a three-point scale I, II, or III with a categorical grade of A (firm) or B (soft, absent, or ill defined) [17]. Grade I mild instability is defined as 3–5 mm of translation, grade II moderate instability is defined as 5–10 mm of translation, and grade III severe instability is defined as greater than 10 mm of translation [18]. The Lachman test (as well as the anterior drawer and pivot shift tests) should always be compared to the contralateral side for comparison. Some patients such as those with ligamentous laxity will translate considerably on exam per baseline, and so comparison with the contralateral side will establish this baseline and prevent false positives.

The anterior drawer test , the second most sensitive in a clinical setting, is similar to the Lachman test but performed at 90° instead of 30 [16]. The origin of the test is somewhat unclear, but Malanga et al. note that the same George Noulis who first described what is now called the Lachman test also described the anterior drawer test in his same 1875 doctoral thesis:

With the patient’s leg flexed [in large degrees of flexion], the thigh can be grasped with one hand at the lower leg with the other hand keeping the thumbs to the front and fingers to the back. If the lower leg is held in this grip and then moved backwards and forwards, it will be seen that the tibia can be moved directly backwards and forwards [19].

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