Knee Problems: Diagnostic Tests for Ligament Injuries

As one of the joints most commonly injured, and also highly susceptible to arthritis, the knee is a frequent source of complaint. In the first of three articles (Consultant, May 2013), we discussed the anatomy of the knee, vital historical information, and the initial observation of the joint. The second article (Consultant, July 2013) detailed information gained from palpation and tests that evaluate patellofemoral and tibiofemoral articulation. In this article, we will describe the tests used to examine the medial and collateral ligaments and the anterior and posterior cruciate ligaments (Table).

The presentation in these three articles is by no means all-inclusive. Nevertheless, skilled performance and interpretation of the history and physical examination will enable you to diagnose the vast majority of knee injuries, both acute and chronic.

A description of the entire range of knee tests in the orthopedic literature risks obscuring the important principles and losing sight of the forest for the trees. Neither is it necessary to routinely perform every test included in this and the two preceding articles for every knee that you examine. The medical history should help you determine a working diagnosis, which will then guide the physical examination.

Ligament examinations are most productive when there is no interference caused by pain, spasm, swelling, or patient anxiety. Before these impediments can manifest, a “golden period” of approximately 30 minutes exists after acute injury. Make use of this time, if possible. If these hindrances have already developed, it may be necessary to eliminate them before examination by means of medication, physical therapy modalities, joint aspiration, an injection of local anesthetic, or the passage of time.

COLLATERAL LIGAMENTS

Check laxity to varus and valgus stress, first with the knee at 30 degrees of flexion and then at full extension (including the patient’s normal amount of recurvatum). Try to support the leg; this will make the patient more comfortable, and it will render the examination more accurate.

Valgus stress test. For this measurement of medial collateral ligament (MCL) laxity, have the patient lie supine, with the lower thigh resting on the table’s edge and the leg hanging off the table. This position extends the hip, relaxes the hamstrings, and stabilizes the thigh.

Place your proximal hand on the lower thigh to stabilize it further. With your distal hand, apply gentle valgus stress on the patient’s ankle or foot, first at 30 degrees of flexion, then at full extension. Palpate any opening of the medial joint line with the fingers of your proximal hand. Repeating the test by moving the leg with a gentle rocking motion often helps to relax the patient and may elicit positive results more easily. Remember, patient relaxation is the key to an accurate instability examination.

Varus stress test. The varus examination measures fibular collateral ligament (FCL) instability. It is more difficult to perform, because the femur tends to roll externally unless it is well supported, and this adds a rotational element to the examination. If this is accompanied by slight flexion of the knee, the result can mimic or accentuate varus instability.¹ As with the valgus stress test, you may try to stabilize the patient’s lower thigh on the edge of the examination table and apply a varus movement instead, but this can be difficult to reproduce reliably without any unwanted rotational motion.

One way to stabilize the knee consistently for this test is to sit on the edge of the table with the patient’s ipsilateral ankle resting on the upper portion of your knee. The patient’s thigh is supported by the table, and the ankle by your knee. A varus stress can then be imparted directly to the knee, and any true varus laxity can be palpated at the lateral joint line. Again, perform this test at 30 degrees and then at full extension.

Injury to the primary stabilizers will lead to laxity at 30 degrees but not at complete extension, because the secondary stabilizers limit instability at full extension. Laxity at full extension indicates injury to both primary and secondary stabilizers.

The superficial MCL (known also as the tibial collateral ligament) is the primary stabilizer against valgus stress. Secondary stabilizers include the anterior and posterior cruciate ligaments, the posterior medial capsule/posterior oblique ligament, and the deep MCL.^2-4

The FCL is probably the primary stabilizer against varus stress.⁴ Secondary stabilizers include the mid-third lateral capsular ligaments (although many consider this a primary stabilizer^2,3,5), the arcuate-popliteus complex, the cruciate ligaments, the iliotibial band, and the biceps femoris.^2-5

According to the Committee on the Medical Aspects of Sports, less than 5 mm of instability for any primary stabilizer is a grade 1 sprain; 6 to 10 mm constitutes a grade 2 sprain; and 11 mm or more, a grade 3 sprain. In clinical practice, however, the joint-line gapping is often difficult to measure accurately.

A grade 1 sprain has no increased instability, and there is a firm end point. The diagnosis is made by locating palpable tenderness and by eliciting pain with the appropriate stress test.

A grade 2 sprain is manifested by increased instability, but there is still a firm end point. Although grades 1 and 2 sprains may be painless initially, provocative stress testing should produce pain throughout the ligament after 12 to 24 hours.²

A grade 3 sprain (complete ligament tear) produces gross laxity with either a mushy end point or none at all—except, perhaps, the bony block caused by contact of the opposite tibial plateau and femoral condyle.² Occasionally, complete tears are not painful because there are no intact fibers left to stress. Thus, you can diagnose a collateral ligament injury if the patient’s history is appropriate, the ligament is tender to palpation, and provocative stress testing produces ligamentous pain or instability.

ANTERIOR CRUCIATE LIGAMENT (ACL)

Anterior drawer test. Have the patient lie supine, flexing the injured knee at 90 degrees. You might help stabilize the joint by sitting lightly on the ipsilateral foot.

The most important part of this examination is to palpate the anterior tibial plateau first and determine its relation to the femoral condyles. This is the only way to avoid a misdiagnosis in the presence of a posterior cruciate ligament (PCL) deficiency. The easiest method is to place one hand around each side of the knee and palpate the relationship with your thumbs.

The tibial plateau should protrude approximately 1 cm in front of the femoral condyles. If it does not, but obvious laxity is demonstrated, then the PCL is injured. The tibia has translated posteriorly, demonstrating the posterior sag sign (which is discussed later). In this presentation, any apparent anterior instability may actually be the relocation of a posterior instability—an unusual, but nonetheless classic, clinical situation. If there is any question, reexamine the other knee for comparison.

After establishing that the normal starting point exists, palpate the hamstrings to be sure they are relaxed. (Have patients rest their head on a pillow; the hamstrings often tighten if patients actively raise their head to watch you.⁶)

Apply a slow but firm anterior pull to the proximal tibia. When the force is applied slowly, it generally elicits pathologic translation more accurately than the same force applied quickly. You may strengthen this maneuver by pressing your thumbs back against the femoral condyles. Two criteria are being evaluated: the amount of anterior translation and the firmness of the end point. When compared with the contralateral knee, increased anterior translation of the proximal tibia is consistent with ACL deficiency. An intact ACL gives a very firm end point that has been compared to a running dog who is tied to a pole and has run out of slack. If the ACL is torn, you feel a soft, mushy end point—like landing on a thick foam pad.

The anterior drawer test is not as sensitive as the Lachman test (79.6% versus 98.6%⁷), because at 90 degrees of flexion, as the former test is performed, the menisci (which are attached to the tibia by the coronary ligaments) tightly cup the femoral condyles. This mechanism may inhibit anterior tibial translation at this angle, even in the presence of a complete ACL tear. For this reason, the Lachman test (performed at a lesser degree of knee flexion) is the most sensitive measure of ACL insufficiency.⁷ Nevertheless, we always perform the anterior drawer test because it is simple and lends further support to the diagnosis.

Lachman test. The patient lies supine, with the knee flexed at 20 to 30 degrees. It may help the patient relax and make the examination easier if you support the lower thigh by placing your knee under it. Again, palpate the hamstrings to ensure that they are relaxed and not preventing anterior tibial translation.

Next, steady the lower thigh with one hand and slowly pull the proximal tibia forward. When compared with the uninjured knee, increased excursion suggests an ACL tear. The quality of the end point is extremely important. The normal ACL serves as a checkrein to anterior tibial translation, and its end point is firm. A soft, mushy, or indefinite end point is abnormal.

As an alternative method of performing the Lachman test, rest the palm of your proximal hand on the patient’s patella and your fingers on the upper tibia while you apply anterior translation. The proximal hand becomes a “human KT-1000,” an instrument that measures anterior tibial excursion to the nearest millimeter. This allows more sensitivity for subtle degrees of translation.

Nirschl modified Lachman test. It may be difficult to carry out the standard Lachman test if you have small hands (especially if the patient is particularly large) or if the patient is unable to relax in the supine position. For this reason, we present this modification.

Seat the patient with the knee flexed at 30 degrees over the edge of the examination table. After checking to make sure the hamstrings are relaxed, hold the anterior portion of the leg just above the ankle. Place your other hand behind the tibia, just below the knee. Using gentle motion, rock the upper part of the tibia forward while you push the lower end backward. The distal motion acts as a lever to aid in forward translation of the upper tibia. The end points of the examination are the same as for the standard Lachman test.

Pivot shift test. Originally described by MacIntosh,^8-10 this is actually a test for anterolateral rotational instability.¹¹ Whether an isolated ACL tear can lead to a positive pivot shift test or whether secondary stabilizers must also be lax or torn has been a controversial issue within the orthopedic literature. The secondary stabilizer for anterolateral rotational instability is the middle third of the lateral capsular ligament.^5,6,12 Regardless, injury to the middle third of the capsular ligament accentuates the pivot shift phenomenon. This test is also fairly sensitive (89.9%) when performed on patients who are not anxious or who are under general anesthesia.⁷

To perform the pivot shift test, grasp the supine patient’s extended leg. The exact position of your hands may be unimportant, as long as both are below the knee and the phenomenon can be elicited reliably. MacIntosh⁹ recommends grasping the foot in one hand and placing the other hand below the knee joint, fingers forward and palm behind the fibular head. Apply compression, valgus, and internal rotation as you flex the knee.

The knee is subluxed in extension, and the pivot shift actually represents a relocation phenomenon. At approximately 30 to 40 degrees, the iliotibial band changes from a knee extensor to a knee flexor and relocates the lateral tibial plateau by pulling it posteriorly. In a positive test, this can be felt as a clunk and seen as a jump. A normal knee has a smooth arc, without jumps or shifts.

It may not be possible to elicit a positive test unless the patient is completely relaxed. If a patient was not apprehensive during the earlier part of the knee examination, including the McMurray test (discussed in part 2 of this series) and others, but shows anxiety during the pivot shift test, a strong indication of ACL insufficiency may be inferred.

There are several grading systems for the pivot shift test, and all are somewhat subjective. We believe the easiest and most reproducible is: 0 (absent), 1+ (pivot glide), 2+ (pivot shift), and 3+ (momentary locking).¹³

Flexion rotation drawer test. This is another test for anterolateral rotational instability, but it is less painful than the pivot shift test. With the patient relaxed and supine, hold the tibia with the knee extended in neutral position. In the presence of anterolateral rotational instability, the weight of the thigh causes external femoral rotation (the same as tibial internal rotation). Use a gentle, smooth motion to flex the tibia approximately 20 degrees while you push posteriorly on the proximal tibia and apply light axial pressure to the knee.

This flexion allows the lateral side of the knee to relocate, resulting in a palpable shift. As the patella rotates externally and internally with the femur (remember, the tibia is being held in neutral rotation while the femur rotates), you can easily observe the relocation by watching patellar rotation. The patella moves externally with the femur during extension (and subluxation) and internally with the femur during flexion (and relocation).⁴

In performing all these tests of ACL integrity, you may want to abduct the leg slightly to relax the iliotibial band. This increases the magnitude of anterior translation and the intensity of the pivot shift. For teaching purposes, all the ACL tests are grouped together in this article. Nevertheless, it is helpful to perform the anterior and posterior drawer tests (described later) before the Lachman test, to rule out a PCL injury and to be sure that the starting point for the Lachman examination is the correct neutral position.

Up to 50% of the ACL (either of its two bundles) can be torn, and the physical examination results will still be normal. It takes a 75% tear of the ACL (regardless of which part is torn) before clinical examination can detect a difference.¹⁴ Moreover, some tests may have negative results or may be subtly positive if the torn ACL stump heals to the proximal portion of the PCL.⁷ This results in some resistance to anterior tibial translation.

Crossover test. In this functional test for ACL deficiency, the patient stands with both feet pointed straight ahead. Stand on the side of the injured knee, and step on the outside of the ipsilateral foot to plant it firmly. Ask the patient to turn approximately 90 degrees toward you, crossing the unaffected leg over the planted foot. If necessary, hold the patient’s arms or shoulders to help balance. A feeling of apprehension or instability constitutes a positive result.¹⁵

POSTERIOR CRUCIATE LIGAMENT

Posterior drawer test. This is performed much like the anterior drawer test, with the patient’s knee bent at 90 degrees and the foot stabilized as you gently sit on it. In the absence of a functional PCL, the proximal tibia often sags posterior to its normal neutral position. This is the posterior sag sign (see below).

Again, palpate the anterior tibial plateau with your thumbs. Press down against the proximal tibia. If the PCL is injured, you can feel abnormal posterior translation of the upper tibia against the lower femur as well as the lack of a firm end point.

Although the anterior drawer test is not as sensitive as the Lachman test for ACL deficiency, the posterior drawer test is the best test for PCL deficiency. And although grading systems for the anterior drawer test are not of great clinical relevance and rarely affect treatment decisions, grading the posterior drawer test is important. Partial tears (grades 1 and 2) may be managed differently than complete tears (grade 3), especially in patients with multiple ligament injuries.

Grade 1 sprain. A posterior drawer test positive for a grade 1 sprain shows abnormal posterior translation compared with the other knee and a somewhat softer end point, but the anterior tibial plateau remains anterior to the distal femoral condyles.

Grade 2 sprain. Here, the anterior tibial plateau is even with the distal femoral condyles.

Grade 3 sprain. In this injury, the anterior tibial plateau is displaced posterior to the anterior distal femoral condyles, and you are unable to feel a firm end point.

A positive posterior drawer test without evidence of other ligamentous instabilities (especially posterolateral rotational instability, as described later) indicates an isolated PCL injury with, perhaps, a posterior capsule injury. Clarification of this point is clinically significant, because isolated PCL injuries often may be managed conservatively. In contrast, PCL injuries that are associated with other ligament injuries often require surgery to obtain optimum results.

Posterior sag test. As previously described, the posterior sag sign indicates abnormal posterior displacement of the upper tibia, with PCL insufficiency. It can best be seen by supporting the distal tibia while the patient’s hip and knee are both flexed at 90 degrees. With the patient’s quadriceps thus relaxed, the tibial tubercle will be less prominent than that on the uninjured side. You can facilitate comparison by using the inferior pole of the patella as a reference point.

Quadriceps contraction test. Also known as the active drawer test, this is performed with the patient supine, knee bent at 90 degrees, and foot flat on the table. Stabilize the foot by sitting on it, and ask the patient to try to slide it slowly down the table (ie, to extend the knee). You do not actually want the foot to move. The idea is for the patient to begin a gentle, isometric contraction of the quadriceps. As this muscle is activated, you can observe the posteriorly displaced tibia being drawn forward by the extensor mechanism.

External rotation test. This determines posterolateral rotational instability (PLRI), or external rotational instability, in which the lateral tibial plateau translates posteriorly. PLRI is caused by injury to the posterolateral corner of the knee (lateral collateral ligament, the arcuate-popliteus complex, and, occasionally, biceps femoris or lateral gastrocnemius head^3,16).

The presence of PLRI is best identified with the patient lying prone. Grasp both ankles and externally rotate both legs, first at 30 degrees and then at 90 degrees. Compare the amount of external rotation and the quality of the end point.

Judge external rotation by comparing the angle between the medial border of the foot and the axis of the femur on both sides. The test result is considered significant if external rotation on the affected side is 10 degrees or more than that on the normal side. External rotational instability at 30 degrees, which decreases in magnitude at 90 degrees, indicates PLRI with an intact PCL. External rotational instability at both 30 and 90 degrees indicates PLRI with a deficient PCL.

External rotation may also be caused by anteromedial rotational instability; in this case, the medial tibial plateau translates anteriorly. Anteromedial rotational instability results from injury to the MCL, medial capsule, and posterior oblique ligament, and it is exaggerated by a concomitant ACL tear. Valgus testing and careful palpation of the proximal tibia for areas of tenderness can help you differentiate these two entities.^6,17,18

Reverse pivot shift test. This tests for posterolateral rotational instability in much the same way that the MacIntosh pivot shift tests for anterolateral rotational instability. Grip the leg as for the standard pivot shift test and exert valgus stress, but rotate the tibia externally, rather than internally. The leg is extended from a flexed position, instead of flexed from the extended position as in the standard pivot shift test. The lateral tibial plateau relocates with extension, producing a clunk or shift.

If there is any question as to whether the shift is due to an ACL deficiency or injury to the posterolateral corner, look for increased external rotational laxity. This is seen only in the latter condition. ■

REFERENCES:

1. Kelly MA, Insall JN. Clinical examination. In: Insall JN, ed. Surgery of the Knee. 2nd ed. New York: Churchill Livingstone Inc; 1993:63-82.

2. Marshall JL, Baugher WH. Stability examination of the knee: a simple anatomic approach. Clin Orthop. 1980;146:78-83.

3. Fowler PJ. The classification and early diagnosis of knee joint instability. Clin Orthop. 1980;147:15-21.

4. Noyes FR, Grood ES, Butler DL, Paulos LE. Clinical biomechanics of the knee: ligament restraints and functional stability. In: Funk FJ, ed. Symposium on the Athlete’s Knee: Surgical Repair and Reconstruction. St Louis: Mosby; 1980:1-35.

5. Hughston JC, Andrews JR, Cross MJ, Moschi A. Classification of knee ligament instabilities: the lateral compartment. J Bone Joint Surg (Am). 1976;58:173-179.

6. Hughston JC, Andrews JR, Cross MJ, Moschi A. Classification of knee ligament instabilities: the medial compartment and cruciate ligaments. J Bone Joint Surg (Am). 1976;58:159-172.

7. Kim SJ, Kim HK. Reliability of the anterior drawer test, the pivot shift test, and the Lachman test: an arthroscopic investigation. Clin Orthop. 1995;317:237-242.

8. MacIntosh DL, Galway HR. The lateral pivot shift: a symptomatic and clinical sign of anterior cruciate insufficiency. Presented at American Orthopaedic Association Annual Meeting; 1972.

9. MacIntosh DL. The lateral pivot shift. Symposium on Treatment of Injuries of the Knee. Chicago. American College of Surgery, 1973.

10. Galway HR, Beaupre A, MacIntosh DL. Pivot shift: a clinical sign of symptomatic anterior cruciate insufficiency. J Bone Joint Surg. 1972;54-B:763-764.

11.Larson RL. Physical examination in the diagnosis of rotatory instability. Clin Orthop. 1983;172:38-44.

12. Jakob RP, Hassler H, Staubli HU. Observations on rotatory instability of the lateral compartment of the knee. Acta Orthop Scand Suppl. 1981;52:1-32.

13. Larson RL, Taillon M. Anterior cruciate ligament insufficiency. J Am Acad Orthop Surg. 1994;2:26-43.

14. Hole RL, Lintner DM, Kamaric E, Moseley JB. Increased tibial translation after partial sectioning of the anterior cruciate ligament. Am J Sports Med. 1996;24:556-560.

15. Arnold JA, Coker TP, Heaton LM, et al. Natural history of anterior cruciate tears. Am J Sports Med. 1979;7:305.

16. Hughston JC, Norwood LA. The posterolateral drawer test and external rotational recurvatum test for posterolateral rotatory instability of the knee. Clin Orthop. 1980;147:82-87.

17. Veltri DM, Warren RF. Posterior cruciate ligament injuries. J Am Acad Orthop Surg. 1993;1:67-75.

18. Marshall JL, Rubin RM. Knee ligament injuries: a diagnostic and therapeutic approach. Orthop Clin North Am. 1977;8:641-667.

Jeffrey E. Budoff, MD, is a board-certified orthopedic surgeon specializing in hand, wrist, elbow and shoulder. He has completed fellowships in both hand surgery and sports medicine.

Robert P. Nirschl, MD,MS, is the director of the Nirschl Clinic and of the Arlington Hospital Orthopedic and Family Practice Sports Medicine Fellowship programs. He is also associate professor of orthopedic surgery at Georgetown University Medical Center, Washington, DC.