Chapter 6 Diagnosis of Anterior Cruciate Ligament Tear

Introduction

The overwhelming majority of orthopaedists are very skilled in the diagnosis of anterior cruciate ligament (ACL) tears. However, acute ACL tear is perhaps the most underdiagnosed orthopaedic condition that usually requires surgery because most tears present to emergency room or primary care providers who cannot necessarily be expected to make the diagnosis. The history and exam and diagnostic tests are less reliable than commonly thought, and the presentation is often not “classic.” Failure to refer to an orthopaedist in these cases, or failure of the patient to actually see the referred-to orthopaedist, results in underdiagnosis and delays in diagnosis that can extend over months or years.

Diagnosis in the Acute Versus the Chronic Setting

Diagnosis of complete ACL tears differs in some respects in the acute versus the chronic state regarding the history, physical exam, and diagnostic tests. This chapter will discuss the acute versus chronic diagnostic dichotomy for each of these diagnostic modalities. In the acute setting the diagnosis is primarily of the ACL tear itself, whereas in the chronic setting the diagnosis more often includes the signs and symptoms of secondary damage. Because the most important aspect of ACL reconstruction is the prevention or mitigation of subsequent meniscal and articular damage to the knee, it is paramount that ACL tears are diagnosed and treated acutely before such further damage occurs.

Partial Tears

This chapter deals primarily with complete ACL tears. Traditionally, partial tears have been found to produce a smaller degree of anteroposterior (AP) laxity than complete tears on Lachman or instrumented Lachman testing, as described later. Until the present time, the only alternatives have been nonoperative treatment or complete reconstruction, which would necessitate ablation of the remaining ligament. Given these alternatives, nonsurgical treatment has been the usual alternative if less than 50% of the ligament was torn.¹ With more awareness of ACL double-bundle anatomy, single-bundle repairs that preserve the remaining ligament have been developed.² These repairs have been used in some cases of single-bundle partial ACL tear. Lachman testing and arthrometer testing in these cases appear to show 2- to 3-mm asymmetry in anteromedial (AM) bundle tears and 1- to 2-mm asymmetry in posterolateral (PL) bundle tears.³ Arthroscopy is required for definite anatomical diagnosis. The pivot shift is of much greater value in the anesthetized versus the awake patient. Diagnostic criteria as well as surgical indications and techniques in these cases continue to evolve.

History

Acute

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