The history and mechanism of ACL tear are familiar to all orthopaedists.⁴^–¹⁰ The history most commonly entails twisting, landing, or a valgus blow to the knee. However, almost any history of knee trauma can be associated with ACL tear. These atypical histories may represent unusual mechanisms or inaccurate remembrances by the patient. The important point is never to eliminate ACL tear from the differential diagnosis based on the history. Classically, swelling is marked within a few hours. However, some ACL tears never produce more than minimal swelling, even acutely. Patients often hear or feel a “pop,” but many do not. Similarly, patients may have felt the knee “go out of place,” or felt their “leg go one way and the body another” but often they have not felt these sensations. Pain may be severe and persisting or may be mild and transient.

Nonorthopaedists are aware that ACL tear is a serious injury and are often misled into thinking that the injury “is only a sprain” because the history and exam are much less dramatic than they are expecting for such a serious injury. Team physicians should therefore perform a Lachman test on any knee injury during a game because ACL tears in the heat of competition are often not obvious by the athlete’s historical account and sometimes produce little pain initially before swelling sets in. This underdiagnosis by history is particularly true in emergency rooms (ERs), where the diagnosis of ACL tear may not be made by the emergency physician. Patients will often feel that the injury is not serious, especially if they do not have a concomitant meniscal tear, which would have produced its own set of symptoms. This is particularly true if the injury is called a “sprain,” such that the patient in many cases feels that there is no need for orthopaedic follow-up. Because magnetic resonance imaging (MRI) will usually not be ordered at this time, the diagnosis is easily missed.

Patients with meniscal or articular cartilage damage will usually have continued symptomatology from their cartilage damage and are more likely to follow-up. Patients with bucket-handle tears and locked knees will virtually always seek further care and be diagnosed accurately by the exam or MRI, or at arthroscopy.

Chronic

Chronic ACL tears often present because of pain from a meniscal tear or articular cartilage damage. Patients may or may not give a history of instability. Classically instability will occur during pivoting, but the symptoms can take almost any form. It can be confused with patellar instability, particularly in adolescents, as well as meniscal tear. Any symptom of instability should cause the orthopaedist to rule in or rule out ACL tear.

Physical Exam

Pivot Shift

The pivot shift is a specific but very insensitive test for ACL tear in the nonanesthetized patient.^11,¹² It is also subject to great interobserver error. Because the pivot shift is often quite painful when positive, has low sensitivity, and usually adds nothing beyond the Lachman test, I (C. Prodromos) use it only rarely for the diagnosis of ACL tear in the office. I do use it routinely in the 1- and 2-year follow-up exams, where its negativity confirms that ACL reconstruction has been successful.

Lachman Test

The Lachman test,¹³ the anterior drawer test in approximately 20 degrees of flexion, is the most reliable exam test for ACL tear¹¹ but is far more reliable in the chronic case, when secondary restraints have stretched and there is less hamstring spasm, than in the acute case. After 21 years of sports medicine practice, I still find the Lachman inconclusive with some frequency in the acute setting, particular in regards to the differential between partial and complete tear, because of persisting hamstring spasm. The firmness of the endpoint may be particularly hard to evaluate. The examiner may or may not be successful in relaxing the hamstrings. Palpating them posteriorly and simultaneously while asking the patient to relax them is often effective. It is important that the patient is in the supine, not sitting, position, and he or she should be instructed to relax the entire body to help relax the knee. The Lachman test should be considered definitive only if it is clearly negative with a firm endpoint. It is important that the examiner be able to differentiate between a negative Lachman test and a false negative caused by this hamstring spasm to avoid missing a torn ACL.

Anterior Cruciate Ligament Versus Posterior Cruciate Ligament Tear

A posterior cruciate ligament (PCL) tear produces increased AP laxity and can mimic an ACL tear. Classically there will be increased AP laxity, but with a firm anterior endpoint, with a PCL tear. However, this can also be seen with a healed partial ACL tear. If there is a question of ACL versus PCL tear, then MRI or the quadriceps active test ¹⁴ should be used to differentiate the two. In addition, it is wise to always arthroscopically inspect the knee before any graft harvesting takes place to make certain that the ACL is in fact completely torn.

Valgus Laxity

In patients with coexisting medial collateral ligament insufficiency, and hence valgus laxity, the Lachman test can be false positive. Rotation of the lax proximal medial tibial plateau can mimic translation of the entire proximal tibia if rotation is not carefully controlled by the examiner during the exam. Thus, when the examiner is aware that valgus laxity exists, he or she should pay particular attention to controlling tibial rotation during the test to minimize this possibility. This can be challenging in patients with large-girth lower extremities.

Locking

“Pseudolocking” may be seen classically with partial tears.^15,¹⁶ However, a knee with a 20-degree or so persisting flexion contracture (i.e., pseudolocking) can occasionally be seen with isolated complete ACL tear from hamstring spasm alone. True locking is seen with ACL tear in combination with displaced bucket-handle meniscal tears. In these cases the “locking” is actually reflex hamstring spasm in response to extension in the presence of the displaced meniscal tear. Thus, the Lachman test is always difficult to perform and frequently false negative because of the hamstring spasm.¹⁷

Hemarthrosis

The presence of a large hemarthrosis is much more highly associated with ACL tear in adults ¹⁸^–²¹ than in children.²² Patellar dislocation and fracture are other leading causes of hemarthrosis. The former can usually be accurately diagnosed by physical exam, the latter by radiography. Arthrocentesis is usually not indicated. Its only diagnostic value is in determining whether a large effusion is a hemarthrosis. In most of these cases, an MRI will be ordered, which will provide much more information and spare the patient the pain of the arthrocentesis. If the effusion is sufficiently tense, hemarthrosis may be indicated for pain relief. If MRI is unavailable and the exam is equivocal, then arthrocentesis may be useful. A 16-gauge needle is preferable, but an 18-gauge needle may be used.

Patellofemoral Injury

Although concomitant ACL tear and patellar dislocation or injury is unusual, it does occur.²³ The presence of physical exam signs of acute patellar instability should not cause the examiner to fail to test for ACL instability.

KT-1000 or Other Instrumented Lachman Test

The KT-1000 ²⁴^–²⁹ (Figs. 6-1 and 6-2) maximum manual examination is a highly accurate method for definitive diagnosis of ACL tear that is heavily relied on in our clinic. When it indicates a complete ACL tear, we generally do not order an MRI scan. A side-to-side difference of more than 4 mm, particularly with an absolute value of 10 or more, is nearly 100% specific for complete ACL tear³⁰ if the examiner is experienced in its use. The more difficult differential may be between complete and partial ACL tear. We have found partial ACL tears to usually have a laxity of 2 or 3 mm. When it is greater, a complete tear has almost always existed. Others have found a slightly larger range.³¹ Larger differences, up to 4 and perhaps 5 mm, can be seen after ACL reconstruction without graft discontinuity. It is important to point out that the maximum manual test is more reliable than other methods. A 20-lb pull in particular will understate the amount of laxity. The 30-lb pull will as well, but to a lesser extent.³² Other arthrometers are in use, particularly in Europe, with reportedly good results.³³ We have no experience with them.

Fig. 6-1 Maximum manual examination is the most accurate KT-1000 testing mode.

Fig. 6-2 Most complete tears will have a reading of 10 mm or more as well as a side-to-side difference of 4 mm or more on maximum manual testing.

As described earlier, PCL tears can mimic ACL tears. The “quadriceps active test”¹⁴ performed with the KT-1000 has been shown to reliably differentiate the two.

Examination Under Anesthesia

The examination under anesthesia (EUA) dramatically increases the sensitivity of the pivot shift test.¹² The accuracy of the KT-1000 is also improved. We may perform both just prior to arthroscopy when the diagnosis is in doubt. The differential in question is usually between a partial and complete ACL tear. EUA may appear to be unnecessary because arthroscopic examination can seemingly determine whether a complete tear exists. However, with partial tears the EUA is a valuable supplement to the arthroscopic findings in determining whether reconstruction is needed. The difference between a partially torn but substantially intact ACL that would do well with conservative treatment versus a completely torn ACL that has scarred in with fibrofatty tissue and is essentially functionless is not always obvious arthroscopically. In these circumstances the EUA is very helpful in helping to determine proper treatment.

Radiographs

Radiographs are typically negative; however, certain radiographic signs may be present. These include the lateral tibial rim or “segond” fracture and posterior lateral tibial plateau fracture or lateral femoral condyle impaction fracture.³⁴ Tibial spine peaking is common in chronic tears but is a nonspecific sign. Tibial eminence fracture is seen occasionally in the skeletally immature and rarely in the skeletally mature (Figs. 6-3 and 6-4). Radiographic signs of a hemarthrosis are usually present.

Fig. 6-3 A rare tibial eminence avulsion (arrow) in an adult, producing instability equivalent to interstitial anterior cruciate ligament (ACL) tear.

Fig. 6-4 A rare tibial eminence avulsion (arrow) in an adult, producing instability equivalent to interstitial anterior cruciate ligament (ACL) tear.

Magnetic Resonance Imaging

Sensitivity rates of 80% to 81% for arthroscopically proven complete ACL tears have been reported using MRI.^30,³⁵ Others have reported accuracy rates of more than 90%^36,³⁷ and sensitivity and specificity over 95%.³⁸ However, Tsai et al found only a 67% specificity rate for complete tear.³⁹ The MRI was very sensitive for detecting some ACL injury, but it was much less specific for differentiating the complete from the partial tear. This is an important distinction because the former is usually a surgical lesion, whereas the latter is usually not.

Although MRI is a useful test, a negative MRI should not rule out an ACL tear that otherwise seems present clinically. The best course of action in such circumstances is to either obtain a KT-1000 exam by a reliable operator and/or to proceed to examination under anesthesia using the pivot shift and Lachman tests and to direct arthroscopic examination if necessary.

The normal ACL is both distinctly seen and appears taut (Fig. 6-5). The torn ACL is indistinct and appears lax (Fig. 6-6). Bone bruises (Fig. 6-7) in the lateral compartment are seen in roughly half of acute ACL tears.^40,⁴¹ Their absence should thus not be relied on to rule out ACL tear. A fracture of the posterior lip of the tibia is another characteristic finding (Fig. 6-8). Transchondral fracture with intact articular cartilage is sometimes also seen (Fig. 6-9).