The knee

General

Watch a patient walking first before starting examination of the lower limb.

Look also from in front or behind to see whether the knees are aligned in the sagittal plane. Varus knees (or bow legs) have clear space visible between the knees when the ankles are together. It is not uncommon to see slight varus in males, a normal variant. Severe varus is commonly seen in osteo­arthritis which commonly attacks the medial compartment of the knee first, as the arthritis destroys the joint which then collapses.

Valgus knees tend to brush together as the patient walks even though the ankles may be wide apart. This deformity is commonly seen in rheumatoid arthritis, which attacks the lateral side of the knee first and leads to collapse there.

Look

Expose the legs fully including thighs (rolled up trousers is not enough).

Skin

Check for redness, scars and lacerations. (Do not forget the back of the knee.)

Soft tissue

Swelling. Look for an effusion in the knee. The dimple on the medial side of the knee will be lost compared with the other side if there is an effusion.

Wasting. Vastus medialis can be clearly seen if the patient is asked to force their knees into hyperextension. This muscle wastes within days of a knee injury, and will fail to bulge when contracted, compared with the other side.

Bone

Check for knock-knee, bow legs, fixed flexion and for the position of patella (Fig. 20.9).

Fixed flexion is the position of comfort in the knee and tends to develop secondary to any acute infection or inflammation.

The patella almost always dislocates laterally (Fig. 20.10). If not reduced it may remain jammed outside the lateral femoral condyle.

Feel

Skin

Temperature. Inflammation of the knee will produce a knee hot to the touch compared with the other side.

Sensation. Damage to nerves at or around the knee will produce disturbance of sensation distally. The same exami­nation as described in the section on ‘The foot and ankle’ should therefore be performed.

Soft tissue

Swelling. Check for a knee effusion, using either a patella tap, cross fluctuation or a stroke test.

Stroke test. With the patient lying supine, empty the medial side of the knee joint by stroking any fluid up into the suprapatella pouch. Then watching the medial side of the knee carefully, stroke down the front of the thigh squeezing any fluid lying in the suprapatella pouch back into the medial side of the knee. As the fluid returns the dimple on the medial side of the knee pops out (Fig. 20.11).

The margin of the synovium can most easily be felt on the medial side above the patella. It can be rolled under your fingers, but is only palpable if the synovium is thickened.

Baker’s cyst. This is an outpouching of the synovium through a defect in the capsule posteriorly. It can be difficult to feel. As soon as the knee is flexed the cyst disappears, but reappears in full extension. It is associated with osteoarthritis of the knee. The patient will guide your fingers to the lump if you are having difficulty finding it.

 Wasting. Atrophy of the vastus medialis is more easily seen than felt.

Circulation. The distal pulses and capillary filling should be checked in the same way as examination of the foot.

 Bone

The margins of the patella, the femoral condyles and the margins of the tibial plateau are all easy to feel as they are subcutaneous. The underside of the patella may be tender if there is synovitis of the knee when the inflammed synovium is compressed against the bone.

The inferior pole of the patella is tender if there is tendonitis of the patella tendon origin (jumper’s knee).

The fat pad beneath the patella tendon is tender in Hoffa’s syndrome, an inflammation commonly brought on by sudden forced hyperexten­sion of the knee.

The tibial tubercle is enlarged and tender when the insertion of the patella tendon is inflammed (Osgood—Schlatter’s syndrome), a common condition in athletic adolescents.

A tear in the meniscus may produce tenderness at the joint line margin over the area of the tear. The medial collateral ligament extends from two finger’s breadth above the knee joint to four fingers breadth below the joint. Tenderness over the ligament may indicate trau­matic damage to the ligament.

Move

Active

Flexion. The knee should be able to flex until the heel touch­es the buttock. Loss of flexion can be measured by the number of centimetres that the heel stops short of the buttock, rather than by actually measuring the angle of the knee. Comparison with the other side gives a sensitive guide to loss of range of movement.

Extension. The patient should be asked to force their knee into the bed. Most knees hyperextend at least by a few degrees.

Passive

Flexion. The knee can be bent up passively, but be sure to watch the patient’s face, especially if you push the knee beyond the active range of flexion. It may be limited because of pain.

Extension. With the patient lying supine and relaxed the feet can be raised off the bed by lifting under the heels. Any loss of extension will be visible because one knee will remain higher (in fixed flexion) than the other.

In posterior ligamentous damage to the knee (as may occur in a hyperextension injury) the knee may hyperextend excessively. In this case the abnormal limb will be lower than the other.

Lag test. A subtle test for quadriceps weakness is to ask the patient to lift their leg 10 cm off the bed. Most patients can do this and, indeed, even if the quadriceps mechanism is completely ruptured this manoeuvre is still possible because the patient uses the lateral retinaculum to lock the knee in extension. The patient is then asked to bend the knee 200 and straighten it, again with the leg still in the air (Fig. 20.12). The patient will not be able to return the knee to its original extension if the quadriceps muscle is weak. This loss of flexion is not a fixed flexion deformity (they have already demonstrated that the knee will extend); it is caused by weakness in the quadriceps and is known a ‘quads lag’.

Stability

Collateral ligaments. The integrity of the collateral ligaments can only be tested when the knee is slightly flexed. In full extension the stability of the posterior capsule masks any collateral ligament instability. However, if the knee is flexed more than a few degrees, the knee rotates when stress is put on the knee to test medial and lateral stability, and it is not possible to check integrity of the ligaments. The leg should be rested on the bed with the patient supine. One of your hands should be put behind the knee to lift it slightly into flexion, hold it stable and feel with thumb and fingers over the joint line. Your other hand should grasp the patient’s ankle, and gently stress the lower leg into varus (putting load on the lateral collateral ligament) then into valgus (stressing the medial collateral ligament) while still holding the knee with your other hand (Fig. 20.13). Knee ligaments vary in their laxity between patients, and it is a difference between the two sides which once again gives a clue to instability. Pain over the ligament on stressing without instability suggests a partial tear.

Cruciate ligaments. The anterior is the cruciate ligament most commonly injured.

History. The patient is commonly twisting on a flexed knee and the foot jams on the ground. There is often a loud crack and the knee swells immediately (with blood). If the injury occurs during sport the patient cannot play on and is usually carried off. The injury maybe accompanied by a torn meniscus or, indeed, the instability may subsequently cause a torn meniscus. If the quadriceps are allowed to waste (as they will without treatment) the knee will be unstable giving way on turns and swelling up each time this occurs. If the meniscus is also torn the knee may lock intermittently (become jammed in flexion).

The posterior cruciate has a completely different mechanism of injury. It can occur either as a result of a hyperextension injury, or if the tibia is driven backwards with the knee in flexion (the dashboard injury).

There are several tests for cruciate disruption but one sim­ple method is as follows. The patient lies supine with both knees bent up to a right angle and the feet resting on the bed. The examiner looks from the side to see whether one tibial tubercle is lying further back than the other. If the tubercle on the injured side is lying further back then the knee has a posterior sag (suggestive of a posterior cruciate ligament injury). The examiner then grasps the uninjured knee with fingers meeting in the popliteal fossa and thumbs side by side over the tibial tubercle. Sit on the patient’s foot to keep it still and then rock the upper tibia gently backwards and forwards against the femur, feeling for the amount of laxity in the joint. Now repeat the manoeuvre with the injured limb, comparing the amount of ‘play’ in the injured knee with the normal one. If there is more play and the injured knee had a posterior sag then the posterior cruciate is ruptured. If there was no sag then the problem is rupture of the anterior cruciate (Fig. 20.14).

 

The pivot shift test. This test relies on the fact that an anterior cruciate deficient knee frequently has some rotatory instability in extension. In this position the femoral condyles rolling on the tibia do not control rotation well. With the patient lying supine and the examiner sitting at the patient’s foot facing up the bed, one hand is used to lift the leg off the bed by the ankle and to rotate the tibia inwards on the femur. The examiner’s other hand presses against the lateral side of the knee pushing it into valgus, so that the lateral femoral condyle is engaged firmly with the tibial plateau. This hand now gently pushes the knee into flexion (Fig. 20.15). If there is anterior cruciate instability the knee starts to bend under the guidance of the examiner’s hand, hut then jams at about 100 of flexion. The tibia has rotated so much on the femur (because of the absent anterior cruciate ligament) that the knee will no longer work as a hinge, but jams as soon as it tries to do so. As the examiner’s hand pushes the knee on into flexion, the knee has to come out of internal rotation so that the jammed joint can continue to flex. The jolt as the tibia derotates under the examiner’s hand is clearly palpable to the examiner and to the patient. It is even easier to feel if the examiner’s thumb lies tucked behind the fibula head. It is then forced smartly back when the derotation occurs. This test should be done very gently. If it is not the patient will be hurt and the test will be inconclusive because muscle spasm will mask the pivot shift.

Patella apprehension.

History. Patients who have lax ligaments (are double-jointed) are much more susceptible to dislocation of the patella. The dislocation commonly occurs during a twisting manoeuvre on a flexed knee and if the knee cap relocates immediately the injury can closely mimic the history of an anterior cruciate rupture. The knee swells at once (with blood) and the patient is unable to walk on it. If the patella stays out the patient may claim that they saw a lump on the medial side of the knee. This is in fact the medial femoral condyle (the patient assumes that the patella indicates the position of the knee). Examination of a knee which has recently had a dislocated patella is very difficult as the knee is stiff, swollen and very painful. However, patients who dislocate a patella have frequently previously dislocated the other patella and the patella apprehension test can be performed on the opposite knee.

  The test. If patients have ever dislocated their patella they will be anxious about dislocating it again. If the knee is placed in extension and the patella pushed laterally, flexing the knee will encourage the patella to dislocate over the lateral femoral condyle (Fig. 20.16). As soon as this starts to happen the patient will become very apprehensive. Do not continue — just note the apprehension; you do not want to actually dislocate the patella.