The knee

The knee does not feature prominently in children’s orthopaedics, but commonly poses problems in adolescents with anterior knee pain otherwise known as chondromalacia patellae. Subluxation and dislocation of the patella may also start to cause problems at this age. In the young adult prob­lems with the knee are exceptionally important. All contact sports and high demand sports, such as skiing, produce a rich crop of sports injuries. Rheumatoid arthritis affects the knee in the later stages just as it affects the hip, and in the elderly osteoarthritis of the knee is as common as osteoarthritis of the hip, and within the next few years total knee replacement may become as common an operation as total hip replacement. Certainly, it is already quite as successful.

History of knee injury

The history of an accident can be divided into three types:

acute extrinsic, the patient was struck on the knee; acute intrinsic, something snapped or gave way in the knee; and chronic, where the onset was gradual and not related to any specific event. Each of these kinds of history is related to a completely different diagnosis (Table 23.8).

  Acute extrinsic  Tibial plateau fracture
  Acute intrinsic  Torn mensiscus, dislocated patella, torn ligaments
  Chronic  Anterior knee pain, Osgood—Schlatter disease,
    jumpers knee, plica, arthritis

Investigation for torn meniscus

A torn meniscus only shows up on X-ray if dye is used (an arthrogram — see Fig. 23.27). It also shows up very well on MRI. The investigation of choice is therefore MRI. If, however, the history is absolutely clear of mechanical locking, then arthroscopy is the investigation of choice, as it is going to be necessary anyway and it allows treatment to be undertaken at the same time. If, however, the diagnosis is in doubt than a noninvasive investigation such as MRI is the first line of investigation, the only problem being that interpretation of the MRI can be difficult and there is an incidence of false positives.

Osteochrondritis of the knee

A small flake fracture of the articular surface of the knee is also possible after a twisting injury. The presentation is very similar to a torn meniscus and plain X-rays may miss the diagnosis unless the defect is silhouetted in one of the views. Both MRI and CT scan are used in making the diagnosis, but arthroscopy can miss it because unless the whole of the articular surface is carefully probed the actual defect may be invisible from the surface and systematic probing of the whole articular surface will be needed to find the actual defect. If the fragment is small and breaks away into the joint it may cause mechanical locking and needs to he removed. If it is large then an attempt should be made to replace it using a recessed screw, buried pins or some kind of bone glue. If it is not felt to be possible to replace the fragment some people drill the base of the defect to stimulate healing by providing a blood supply to the damaged area, but the cartilage will only be replaced by fibrocartilage which has poor wear properties.

Ruptured anterior cruciate ligament

This is an epidemic in modern sport. It occurs when a very high twisting force is applied to the bent knee. If contact sports did not allow the use of studs on boots the injury would probably almost disappear. It is only when the foot is fixed firmly to the ground and the body continues to twist fast on it that the injury is inevitable. The patient often hears a loud crack in the knee and collapses. They cannot play on both because the pain is severe and because the knee swells rapidly and is unstable. The other common cause for an acute haemarthrosis following an acute intrinsic injury to the knee is dislocation of the patella. A careful history and examination will distinguish the two conditions. In patella dislocation the patient may have noticed something out of place immediately after the accident or have had the problem before. Sometimes the patella remains dislocated.

Aspiration of the joint will produce a dramatic reduction of pain. If there are fat globules in the aspirate then a fracture must be suspected as marrow has escaped into the joint (Fig. 23.28).

Treatment of the ruptured anterior cruciate ligament

In the child the anterior cruciate avulses with a fragment of bone. It is relatively simple to fix this fragment back into the tibia either with a bone screw or with sutures passed through to the front of the tibia. In the adult the anterior cruciate tears its central part, where there is no capacity to heal. This does not necessarily mean that surgery to substitute the ligament is required. In a significant number of patients intense physiotherapy rebuilds the control of the knee to levels where the absence of an anterior cruciate does not hamper performance. In other patients whose performance is compromised by an anterior cruciate deficient knee it is a perfectly valid option for the patient decide to modify their lifestyle to cope with their disability. For example, they may decide that the time has come to give up contact sport. It is only after these first two options have been exhausted that the possibility of performing surgery to substitute for the anterior cruciate ligament should even be considered. Even then the surgery is complex, the rehabilitation is difficult and there is no evidence in the long term that functionally the knee maintains any greater stability, or that reconstruction protects the knee from osteoarthritis. There is also now good evidence that some of the synthetic ligaments used are not strong enough for the loads exerted on them and break up in the knee. If they do so the debris may cause an inflammatory arthritis. Finally, it is very difficult to make sure that the substitute ligament has the right biomechanical properties and is correctly positioned to give a stable knee without causing stiffness. A substitute ligament cannot have a proprioceptive function in the knee, a role that some people feel is one important facet of the anterior cruciate ligament’s role.

As the ligament does not heal, the alternatives are either to realign the structures outside the knee to prevent the instability caused by anterior cruciate deficiency or a second possibility is to take a ligament from elsewhere in the patient and route it through the track of the anterior cruciate to re­create the function of the ligament. A third option is to put in a synthetic ligament. A ligamentous augmentation device (LAD) is a synthetic ligament which in itself is not strong or elastic enough to undertake the function of an anterior cruciate but combined with tendon material from the patient is supposed to perform this function.

Extra-articular repair

The Macintosh operation involves reefing the lateral dynamic structures of the knee. It pulls back on the lateral tibial plateau in flexion in an attempt to stop it sliding forward in a uncontrolled way (the pivot shift), which makes the knee so unstable in twisting and turning.

lntra-articular repair

Intra-articular repairs can be performed either through the arthroscope or open. The Jone’s repair involves freeing the proximal end of the middle third of the patella tendon and rerouting this tendon through the knee along the line of the anterior cruciate. Semitendinosus tendon can be used in a similar way or a synthetic tendon can be inserted. The key issue surgically is to make sure that the origin and insertion of this new ligament are in the correct place to allow free movement of the knee without undue laxity.

Posterior cruciate

A posterior cruciate tear is commonly associated with a hyperextension injury and disruption of the posterior structures of the knee (Fig. 23.29). Treatment of the knee in a plaster usually allows the posterior capsule to heal up. There is then little functional disability despite the fact that the posterior cruciate does not heal.

Rupture of the medial collateral ligament

Ruptures of the medial collateral ligament occur when the leg is forced into valgus usually by a blow on the outside of the leg. With moderate trauma there is usually only a tear of the short cruciate ligament. If the force is greater then the medial collateral ligament disrupts. The knee is grossly unstable when stressed into valgus. If on ray a flake of bone can be seen avulsed from the femur then reattachment using a screw is relatively straightforward. The medial collateral ligament inserts into the tibia well below the knee joint and avulsion in this area can also be repaired surgically by suturing the tendon back. Midsubstance tears at the level of a joint are more difficult to repair, and surgery is not necessary. A cast brace for 6 weeks allowing for flexion of the knee but protecting it from valgus forces will encourage good healing.

Anterior knee pain chondromalacia patellae

This condition tends to start in the teens and is more common in girls than in boys. It may follow an injury to the knee, particularly a dislocation of the kneecap. The patient gives a history that they have pain on stairs, particularly going down stairs when the knee may actually feel unsafe. They also experience severe pain when sitting for any length of time. When they try to move after the knee has been still for a while the pain may be so severe that the knee feels locked. This is not true mechanical locking. It seems to be more to do with synovial inflammation. On examination, there may be some quadriceps wasting, there is not usually an effusion but there is tenderness if the synovium around the patella is pressed against the patella itself. This is sometimes confused with tenderness beneath the patella; it is the synovium caught between your finger and the patella that is causing the pain as there are no nerve fibres under the patella itself.

Management

There is no proven treatment for this condition, but because its severity fluctuates it is a diagnosis which has attracted all sorts of unusual therapies. There is little doubt that pain killers and nonsteroidal anti-inflammatory drugs make little or no difference to the pain. Strapping and tubigrip can actually make the pain worse, and there is no evidence that avoiding activities which make the pain worse makes any difference to the natural history of the condition. The use of crutches, plaster cylinders and even wheelchairs merely seems to confirm the child in the role of an invalid. There is always quadriceps inhibition and eventually wasting, so exer­cises to build up the quadriceps, particularly the vastus medialis, should be encouraged. Unfortunately, these exer­cises can be quite painful, but if the child is encouraged to contract the muscles isometrically (without moving the knee) the pain can be minimised. There are several surgical

operations described for this condition but there is no evidence that any of them do any good and some of them certainly make the condition worse, so they should be avoided.

Dislocating kneecap

It is most common to see a patient who has had a previous dislocation. The kneecap normally relocates itself, but if it has not the patient can be given nitrous oxide. When the knee is straightened the patella will relocate. Immediately after a dislocation the patient will have a quadriceps lag and should therefore be protected with a back slab and crutches. Physiotherapy should be started at once. If there have been recurrent dislocations, then swelling may be minimal but quadriceps wasting may be marked. It is then even more important to arrange physiotherapy to build up the muscles around the knee if further dislocations are to be avoided.

Surgery for the dislocated patella (Fig. 23.30) should only be undertaken as a last resort. If the patient has built up the quadriceps well but the kneecap is still unstable then initially a soft-tissue realignment of the patella should be considered by performing a lateral release. This involves dividing the lateral retinaculum of the patella so that the pull on the patella is more medial.

Dislocation of the knee

In high-energy accidents the knee can be dislocated. The tibia is driven backwards and dislocates posteriorly. The key feature in this injury is that there is a high risk of neurovascular injury. The pulses to the foot must be intact before and after relocation, and even if they return great care must be taken that a compartment syndrome does not develop. The knee can then be treated in a plaster for 6 weeks with active quadriceps exercises.

Fractured patella

There are two types of fractured patella. The first is a direct blow to the patella, which tends to shatter the patella into many fragments which do not displace as the patella tendon and the fibres which run over the patella itself are usually intact. The second type of fracture is an avulsion fracture where the patella snaps in half under the load of the quadriceps muscle trying to prevent the knee from being forcibly flexed. In the latter case the fracture is usually transverse and displaced.

Management of the communited undisplaced fracture

There is often an open wound over the patella and this needs cleaning. If it is contaminated it should be left open for delayed primary closure. It is unlikely that the patella can be reconstructed when it is in a large number of fragments, and therefore the choice is either to treat the patella as a ‘bag of bones’ (simply mobilised without any attempt at reduction) or to excise it. The patella acts as a fulcrum for the quadriceps tendon and a stabiliser, so if it is excised the knee may subsequently be weaker and less stable. However, if it is moblised early as a ‘bag of bones’ the articular surface may be smoothed but will never be truly congruent and early oesteoarthritis in the patellofemoral joint is inevitable.

Management of the displaced transverse fracture of the patella (avulsion)

The fracture can be managed in exactly the same way as an olecranon fracture with wires passed through the patella and a figure-of-eight tension band passed around the outside of the patella. The load of the quadriceps passing through the patella and down the patella tendon into the tibial tubercle serves to compress the fracture provided that the outer cortex is held together with a tension band (Fig. 23.31). The patient is mobilised with partial weight-bearing until the fracture is healed. The pins and wires are then removed as otherwise they cause skin irritation.

Tibial plateau fractures

Tibial plateau fractures occur in the elderly when the knee is forced into varus or valgus and the osteoporotic bone fails before the ligaments. Fracture can also occur in the young adult but are then associated with very high-energy injuries which may also have damaged the ligaments and caused fractures elsewhere. The pattern of fracture of the plateau depends on the direction of the force exerted. If the knee was forced into valgus then there tends to be a depressed fracture of the lateral plateau; if the knee is forced into varus then, similarly, the medial plateau will be fractured. If there is an axial force then both plateaux may be damaged. A check should always be made on the contralateral side of the fracture for ligamentous disruption as the compressed side will have acted as a fulcrum for the load to come on to the opposite ligament. There are two main types of tibial plateau fracture. The first is a vertical cleft fracture where a whole fragment is displaced downwards. The second is an egg-shell-type fracture where the plateau is buckled in by the femoral condyle. Most fractures are a mixture of these two types. The buckle ‘egg-shell’ fracture can be quite difficult to see on an X-ray, as it is often only part of the plateau (usually the posterior part) that is depressed. Unless a careful check is made the intact part of the plateau will give the appearance that the knee is normal in both the AP and the lateral directions. Fractures in which there is more than 2 mm displacement in the articular surface or in which the joint is unstable need open reduction and internal fixation in all but the most elderly and frail patient. The investigation of choice is therefore tomography or a CT with reconstruction of the tibial plateau in the vertical axis to show the size and depth of the depressed section. The final decision on whether the knee needs reconstructing may have to made under anaesthetic when the stability can be tested.

Management of vertical shear fractures

If the fracture is a pure vertical shear fracture with displacement of more than 2 mm then the fragment needs to be exposed, elevated, and fixed back with a buttress plate and lag screws across the knee. Early partial weight-bearing and mobilisation can then be started.

Management of depressed fractures

In these cases the joint may need to be opened so that the fragments can be elevated under direct vision. The incision should be extended down beside the tibia and a small window opened in the cortex into which a punch can be introduced. The fragments can then be elevated from inside the bone and the cavity packed with bone graft. If there is combined vertical shear fracture then this can be fixed with a buttress plate (Fig. 23.32). Most of the load to the knee goes through the medial side so a fracture of the lateral plateau may only need protecting with a cast brace to prevent the knee going into valgus, but a fracture of the medial plateau should be nonweight-bearing for some weeks.

Rheumatoid arthritis of the knee

Rheumatoid arthritis commonly affects the knee, but the diagnosis is simple as it is one of the later joints to be affected. In the first instance the problem is pain and swelling secondary to an aggressive synovitis. At this stage medical treatment, nonsteroidal anti-inflammatory drugs, pain killers, splints and physiotherapy are the best choice. Synovectomy can be performed chemically using short acting radioisotopes. Surgical synovectomy can also be performed through an arthroscope. This is technically difficult because of the florid synovitis and should only be undertaken if there is still a good range of movement and the articular surfaces are well preserved. In the later stages of rheumatoid arthritis there is severe arthritis in the lateral compartment with a contracture of the lateral structures of the knee, producing external rota­tion of the tibia and a fixed flexion deformity. Total knee replacement is now the operation of choice but this may need to be combined with a careful soft-tissue release to obtain a stable straight knee.

Management of osteoarthritis of the knee

Osteoarthritis of the knee may be idiopathic or secondary to trauma. The disease may affect all three compartments of the knee: the patellofemoral joint as well as both the medial and lateral compartments of the knee, but its main effect appears to be on the medial compartment. Initially the patient may present with pain on walking. Weight-bearing X-rays may show slight narrowing of the medial joint space. If medical treatment and physiotherapy are no longer controlling the symptoms and there appears to be a major inflammatory ele­ment to the disease then an arthroscopic washout should be considered. It is not known how this operation works but it can give considerable relief of pain in the short term. A valgus osteotomy may help to transfer the weight-bearing away from the medial compartment and more equally into the medial lateral compartment. This is only suitable if the lateral compartment shows no sign of arthritis on X-ray, the knee is stable and there is a good range of movement (at least 900).

  Operative technique for high tibial osteotomy for osteoarthritis

The degree of yams (Fig. 23.33) should be measured carefully on long leg X-rays. The size of wedge which needs to be removed from the tibia should then be calculated with the aim of putting the knee in neutral or very slightly into valgus. At operation a laterally based wedge is removed from tibia just below the joint surface leaving the medial cortex intact to stabilise the osteotomy. An alternative procedure is the ‘dome osteoto­my’, where a series of drill holes in the shape of an arch is drilled through from the front of the tibia to the back and then joined using a sharp osteotome. The distal tibia can then be rotated in the dome to correct the alignment of the knee. As with the wedge osteotomy, the new position can then be held using a long leg plaster. Care must be taken in either operation to avoid damaging the vessels immediately behind the knee, which lie very close to the bone especially if the knee is extended. These procedures are therefore best all performed with the knee well flexed.

Knee replacement

Most knee replacements are total condylar knee replacements (Fig. 23.34). The femoral component replaces both femoral condyles and there is a tibial plateau component which covers the tibia. An optional extra is a resurfacing component for the back of the patella. Unicompartmental knee replacements are available, but unicompartmental arthritis is not that common, and technically these implants can be difficult to insert.

Operative technique for total condylar knee replacement

The patient is placed supine on the table. It is not necessary to use a tourniquet although many people do. The risk of causing vascular damage, especially in the elderly, outweighs any possible advantages in obtaining a blood-free field. A long midline anterior incision should be made from 10 cm above the patella to 3 cm below the tibial tubercle. This incision can be curved slightly medially. The incision is carried down to the patella and patella tendon without undercutting the edges. The medial margin of the patella tendon and of the patella is now exposed and the incision carried on through this tissue to the knee joint. Above the patella the incision is brought round to the midline of the patella and then extended vertically up the quadriceps muscle in the midline for 10 cm above the knee. The patella is turned upside down and dislocated laterally and the knee flexed up to 90 degree .Any bleeding in the wound edges should now stop so that a tourniquet would now serve no further useful function. The fat pad and menisci are excised and a jig is placed on the femur. The jig should be sized so that the femoral component will be the same size as the condyles it is replacing.

Having checked that rotation is correct the anterior and posterior cuts should then be made on the femur. Very little of the anterior condyles should be removed. Each fragment of the posterior condyle should be about the size of a thumb nail and no thicker than the thickness of the replacement metal component. The tibial jig should now be applied. Two types of jig are available. One is fitted on a rod passed down the medulla of the tibia and relies on the tibial shaft anatomy being normal and the entry hole being in the centre of the tibia. The second type of jig (extramedullary jigging) relies on the surgeon clipping the jig to the tibia and achieving the correct alignment by eye. This second system relies more on the surgeon’s skill and expertise, and tends to be used more by experienced knee surgeons who may wish to make allowances for abnormal anatomy. The cut on the tibia should only remove as much tibia as is needed to make room for the thickness of the thinnest of the tibial components. If there is a defect in either the medial or the lateral plateau no attempt should be made to remove the defect by cutting lower, as the defect can be filled with cement or, indeed, with special components that can be fitted to the implant. The gap between the cut top of the tibia and the posterior cut of the femur should be measured with the knee in 90 degree of flexion using the spacers provided on the instrument tray. This gap measured will include the thickness of the tibial component and of the femoral component.

With the knee now extended, and using the same spacer, a mark should be made on the femur so that the distal cut of the femur is made at a height that will keep the components in the same amount of tension through flexion and extension. If, however, it is found that it is difficult to get the distal tibia in the correct alignment to the femur in extension (70 degreeof valgus) then a soft tissue release may need to be carried out on either the medial or lateral collateral ligaments. The lateral ligament is best released from the femur; the medial side can be released from both femur and tibia until the knee is in correct alignment in extension. A similar amount of bone should then be cut from the femoral condyles as the thickness of the metal of the components, allowing for any defects in the femur caused by osteoarthritis. The distal cut in the femur should now be carried out using a jig to ensure that it is at the correct angle to the anterior and posterior cuts. The corners are trimmed where necessary, and a trial femoral component can be fitted to test against a trial tibial component. Iris now possible to see whether the cuts have been put in the right place and whether the tension of the knee in both flexion and extension is correct. On good designs the tension in flexion should be slightly less than in extension to allow rotation of the tibia on the femur in flexion. If the alignment and tension are good then the components can be cemented into place. There is no evidence that resurfacing the back of the patella makes any difference to the long term outcome, and the complications of this thin implant are well described. Nevertheless, the patellofemoral joint is clearly a potential source of pain and some surgeons argue that it should be replaced.

The patella is reduced and a single tacking suture put in to hold the medial retinaculum to the patella. A check is then made that the patella tracks normally in the new intercondylar groove. If it does not, osteophytes may need to be trimmed from the edge of the patella. If the lateral retinaculum is too tight then a lateral retinacular release may need to be performed. The wound is closed in layers and the knee placed in a back slab splint. There is no evidence that continuous passive movement after surgery either reduces blood loss, decreases pain or improves the overall outcome, hut it is commonly used. The key to success is good postoperative physiotherapy aimed at building up the power of the quadriceps then regaining full extension and at least 90degreeof flexion. Total knee replacement does not give immediate pain relief in the same way as a total hip replacement. The patient also needs to do a great deal of work over the next 6 months to regain strength and range of movement in the knee. This needs to be clearly explained to the patient before embarking on the surgery.

Complications of total knee replacement

Total knee replacement is subject to wound breakdown if care is not taken with the anterior incision. If the wound does break down then the knee replacement is likely to become infected.

Tourniquet damage is well described, especially in the elderly. The tourniquet may fracture atherosclerotic plaques in the femoral vessels and, if left on too long, may cause nerve damage. If tourniquet time is more than 1 hour in the elderly, the tourniquet should probably be released and then reinflated after 10 minutes.

Tourniquets can also cause burns if alcohol-based skin preparation solution is allowed to seep down into the tourniquet padding (Fig. 23.35).

Damage to the vessels behind the knee can be avoided by making the cuts on the tibia with the knee flexed and being very careful not to let the saw cut too deep.

The retractor on the lateral side of the knee must be used with care if damage to the lateral popliteal nerve is to be avoided.

If the knee is put in too loose it may be unstable and require bracing.

The infection risk of total knee replacements is around the same as total hip replacements, somewhere below 2 per cent. The risk of infection can be reduced using three doses of prophylactic antibiotics specifically to cover Staphylococcus.

The incidence of deep vein thrombosis after total knee replacement appears to be high, but few of these cases produce any clinical symptoms. The overall death rate after total knee replacement is low (less than I per cent, and the death rate from pulmonary embolus even lower). There is, therefore, no evidence to support the use of thromboprophylaxis in total knee replacement, either chemical or mechanical.

Some total knee replacements fail to mobilise quickly par­ticularly if physiotherapy resources are not good or the patient lacks motivation. After 2 weeks a manipulation under anaesthetic may need to be performed to release any adhe­sions. This must be done with great care to avoid breaking the femur.

Total knee replacement does not produce immediate relief of pain but if severe pain persists the possibility of aseptic or septic loosening must be considered. The joint should be aspirated and fluid sent for culture. If a trial of antibiotics does produce relief of pain then the diagnosis of septic loosening is most likely and a revision should be considered.

Aseptic loosening of total knee replacements probably occurs in a similar way to the hip, with particulate wear stimulating an inflammatory response. Special implants are available for revision but tissue should be sent at the time of surgery for histology and bacteriology. The problems of wound healing after a revision are considerably increased, as is the risk of infection and stiffness.