Disorders of the inert structures

Ligamentous instability

If the knee becomes unstable after a ligamentous injury, permanent problems may result, especially if the patient is an athlete with high functional demand.

If the history indicates that there is possibly instability, an examination to detect the exact location and degree of this is performed. It is important to make this supplementary examination only when symptoms and signs found in the routine clinical assessment indicate ligamentous instability to be the cause of disability. Indeed, it is important to realize that not every lengthened ligament leads to problems.¹ If the athlete’s muscles are strong enough to provide dynamic stability, a slightly unstable knee will not cause trouble. Conversely, it is quite possible that an athlete with slight laxity of one or more ligaments has a chronic painful lesion as well. If the diagnosis of the latter is missed because the routine functional examination is not performed, it may well be that the patient is sent for surgery, which will solve the slight instability but does not cure the main problem.

Straight instability

This is defined as an increased range of angular movement in the frontal plane, i.e. valgus/abduction or varus/adduction movement (Fig. 2), or an increased range of gliding movement in the sagittal plane, i.e. the simultaneous forward or backward gliding movement of the tibial condyles in relation to the femoral condyles (Fig. 3).

Fig 2 Degrees of straight instability in the sagittal plane.

Fig 3 Straight instability: increased range of gliding movement in the sagittal plane.

Rotatory instability

This implies increased rotation movement of the tibia on the femur. The posterior cruciate ligament, located in the centre of the joint, is the fundamental stabilizer and is the axis of the joint, both in flexion–extension and in rotation. Consequently, this ligament is always intact in rotatory instabilities; from the moment the ligament is completely torn, there is no longer a centre of rotation and any kind of straight instability can result.3,4

There are three types of rotatory instability:

• Anteromedial rotatory instability: there is an abnormal forward gliding movement of the medial tibial plateau with respect to the medial femoral condyle, while the lateral tibial plateau retains a relatively normal relationship with the lateral femoral condyle (Fig. 4b).

• Anterolateral rotatory instability: there is an abnormal forward gliding movement of the lateral tibial plateau with respect to the lateral femoral condyle, while the medial tibial plateau retains a relatively normal relationship with the medial femoral condyle (Fig. 4c).

• Posterolateral rotatory instability: there is an abnormal backward gliding movement of the lateral tibial plateau with respect to the lateral femoral condyle. Again, as in anterolateral rotatory instability, the medial tibial plateau remains in normal contact with the corresponding femoral condyle (Fig. 4d).

Fig 4 Rotatory instability: (a) starting position; (b) anteromedial; (c) anterolateral; (d) posterolateral.

Various combinations of these rotatory instabilities can occur. The most commonly encountered are combined anterolateral and anteromedial, and combined anterolateral and posterolateral.

Posteromedial rotatory instability does not exist because an intact posterior cruciate ligament prevents any backward gliding movement of the medial tibial condyle in relation to the medial femoral condyle.

Functional examination

If the history or routine clinical examination indicates the possibility of instability, a supplementary examination is performed to detect the exact localization and degree of ligamentous insufficiency.

Acute stage

In the acute stage, only the first few hours after the accident are suitable for clinical detection of ligamentous laxity. After this, effusion and muscle spasm resulting from the capsulitis will prevent a detailed examination. This is particularly the case in lesions of the medial collateral ligament because of its close relationship with the capsule. So assessment should be made as soon as possible, preferably at the scene of the accident. If intra-articular bleeding is present, which especially accompanies tears of the anterior and posterior cruciate ligaments as well as of the medial collateral ligament, the first few minutes are often the only time available to make a proper clinical assessment of ligamentous laxity. When time has elapsed, examination under general anaesthesia, eventually followed by arthroscopy, is the only course of action. However, if the lesion seems to be less serious, supplementary examination can wait until a proper clinical evaluation can be made.

Chronic stage

In the chronic stage thorough clinical examination suffices to estimate, in almost all instances, the degree of functional instability and to determine the type of treatment required.

Tests

Passive abduction or valgus stress test in 30° of flexion

The patient is placed supine on the couch, the head resting on a pillow to enable observation of the examiner without active raising of the head (which often causes tightening of the hamstrings). The thigh rests on the couch and the lower leg hangs over the side, with the knee in 30° of flexion. One hand is placed about the lateral aspect of the knee, the other grasps the lower leg at the ankle (Fig. 5). Then abduction stress is applied gently and repeatedly, gradually increasing up to the point of pain. The degree of movement and the end-feel are estimated.

Fig 5 Passive abduction or valgus stress test in 30° of flexion.

The normal limb is assessed first, especially in acutely injured patients, because this shows the patient that the examination will not be rough and is unlikely to be painful. The examiner also gets a measure of the stability of the unaffected knee.

Instability indicates a tear limited to one or more of the medial compartment ligaments, i.e. the medial collateral ligament, meniscotibial or meniscofemoral capsular ligaments and the posterior oblique ligament.

If the abduction stress test in full extension also shows instability, the posterior cruciate ligament is probably also ruptured. It should be borne in mind that, in the fully extended position, an intact posterior cruciate ligament still holds the joint surfaces in firm apposition, even in complete tears of the medial compartment ligaments.

Passive adduction or varus stress test in 30° of flexion

This test is performed in the same position as the previous one, but the hands are changed; so one hand is placed about the medial aspect of the knee while the other grasps the lower leg at the outer side of the ankle (Fig. 6). The degree of movement and end-feel are estimated.

Fig 6 Passive adduction or varus stress test in 30° of flexion.

Adduction stress is applied to test the stability of the lateral compartment ligaments: the lateral collateral ligament, meniscotibial or meniscofemoral capsular ligaments and the arcuate ligament.

Instability of the lateral compartment is less common but causes significantly more disability than a comparable amount of instability medially.

Lateral instability cannot be detected when the test is performed with the knee in full extension because the intact tight posterior cruciate ligament precludes any movement in this position. If an adduction stress test in full extension shows lateral instability, rupture of both the lateral compartment ligaments and the posterior cruciate ligament should be feared.

Lachman test

The test is performed with the patient supine and the knee in 20–30° of flexion. The distal femur is grasped from the lateral side to obtain control just above the lateral femoral condyle. The examiner uses the opposite hand to displace the proximal tibia directly anteriorly (Fig. 7).

Fig 7 Lachman test.

In acute rupture of the anterior cruciate ligament this test is preferred to the anterior drawer test in the 90° flexed position because muscle defence by the hamstrings may prevent the forward movement of the tibia and give rise to a false-negative result.⁵ Liljedahl et al⁶ found only 3 of 35 patients to have a positive anterior drawer test in 90° of flexion, whereas with anaesthesia 31 and on arthrography 29 were positive.

Anterior drawer test in external and internal rotation

The patient is positioned as for the abduction–adduction stress tests. The hip is flexed to 45°, the knee to 80–90°. The foot is placed on the couch, well fixed by a portion of the examiner’s buttock resting on the dorsum of the forefoot.

The examiner places both hands around the upper part of the tibia with the index fingers palpating the hamstring tendons to make sure they are relaxed. Both thumbs are placed at the anterior border of the joint so that they can estimate the range of movement. Then the proximal part of the lower leg is pulled forward repeatedly, first gently then with a somewhat stronger pull.

The test is first performed with the lower leg and foot externally rotated beyond the neutral position and as far as is comfortably possible, then internally rotated (Fig. 8).

Fig 8 Anterior drawer test in external (a) and internal rotation (b).

The findings are compared with those of the basic examination, for example in 0° rotation, as well as with the other knee.

With the tibia in external rotation

Anteromedial rotatory instability is indicative of a tear of one or more of the medial compartment ligaments: the medial collateral ligament, meniscotibial or meniscofemoral capsular ligaments and the posterior oblique ligament. An associated anterior cruciate ligament tear increases the amount of subluxation of the medial tibial condyle. This ligament is the second line of defence; in an acute isolated tear of the anterior cruciate the anterior drawer test will be negative.

With the tibia internally rotated

In patients with a medial compartment laxity, the test becomes negative because of the increased tension of the posterior cruciate ligament. Simultaneous forward subluxation of the tibial condyles indicates that the posterior cruciate ligament is torn. In this case, the rotational element is eliminated and straightforward instability exists.

Isolated forward subluxation of the lateral tibial plateau, for example anterolateral rotatory instability, can only be demonstrated if the knee is held in the neutral position, because internal rotation of the tibia will tighten an intact posterior cruciate ligament and prevent any subluxation. This type of instability is indicative of a tear of the middle third of the lateral capsular ligaments.

Prone rotation test ⁷

Sometimes doubt remains as to whether the range of rotation has been increased. A decisive assessment can be made with the patient prone and the knees flexed to a right angle (Fig. 9). The movement of external or internal rotation is performed bilaterally and the range of movement is assessed by the twisted position of the feet.

Fig 9 Prone rotation test.

An increased range of lateral rotation results from a lack of stability of inert structures at the medial compartment (medial collateral ligament, meniscotibial and meniscofemoral capsular ligaments and the posterior oblique ligament) and/or the arcuate complex (arcuate ligament, lateral collateral ligament). An associated tear of the anterior cruciate ligament still increases the amount of rotation.

An increased range of medial rotation is indicative of laxity of the middle third of the lateral capsular ligaments, the anterior and the posterior cruciate ligaments.

Posterior drawer test

The patient is in supine position with the hip flexed to 45° and the knee to 90°. The foot is flat on the couch, in neutral position. The heels of both hands are placed anteriorly against the proximal part of the tibia (Fig. 10). Both thumbs at the anterior border of the joint can estimate the range of movement. The proximal part of the tibia is then pushed backwards repeatedly, first gently then with a strong jerk.

Fig 10 Posterior drawer test.

The posterior drawer test, which includes palpation of the tibia–femur step-off, was recently demonstrated to be the most sensitive and specific clinical test for posterior cruciate ligament deficiency (accuracy 96% with a 90% sensitivity and a 99% specificity).⁸

‘Jerk’ test and pivot-shift test

The patient lies supine, the hip flexed to about 45°, the knee to 90°. The examiner supports the patient’s leg, with one hand at the foot, the other at the proximal end of tibia and fibula. The hand at the foot rotates the tibia slightly internally while the other hand exerts a mild valgus stress at the knee (Fig. 11). Extreme internal rotation may dampen a shift significantly. Maintaining slight internal rotation and mild valgus stress, the examiner gradually extends the knee.

Fig 11 The ‘jerk’ test.

A positive result is indicated if, on attaining about 30° of flexion, relocation occurs with a sudden movement, which is called a ‘jerk’. The forwards shift can be seen and felt by the examiner. At the same moment, the patient will recognize the feeling of instability.

The test demonstrates an anterolateral rotatory instability because of a tear in the middle third of the lateral capsular ligaments. Frequently, the anterior cruciate ligament and/or menisci are also ruptured and increase the instability.⁹

Anterolateral instability can also be demonstrated by carrying out the manūuvre in the reverse direction, starting from a position of a few degrees of flexion: the pivot-shift test.

Bach et al¹⁰ found that the degree of pivot shift, probably because of the role of the iliotibial band, strongly correlates with the position of the hip and knee joint. They advised 30° of hip abduction and 20° of tibial external rotation as the position in which instability is most clearly demonstrated.

The examiner stands at the foot of the couch in order to monitor hip position carefully. The ipsilateral hand cradles and holds the foot in external rotation. The other hand, slightly supinated, holds the proximal and lateral leg. An axial load and valgus movement are applied simultaneously as the knee is slowly flexed passively from the extended position.

Practitioner’s checklist

• An associated acute partial tear of the medial compartment ligamentous structures makes the test too painful to undertake

• A false-positive test may occur in the presence of a torn meniscus

• In children, and in patients who have mild to moderate congenital genu recurvatum without symptoms, it is normal to find the ‘jerk’ and pivot-shift tests, and the anterior drawer test with the tibia in 0° rotation to be mildly positive on both sides

External rotation–recurvatum test

The patient lies supine on the couch. Both legs are lifted simultaneously by grasping the big toes (Fig. 12). The amount of external rotation of the proximal end of the tibia and the degree of recurvatum are observed. In a positive test, unilateral excess of external rotation and recurvatum is obvious, which is seen as increased tibia vara.