Orthopaedics - Knee

Question 1

What bones articulate to form the knee? Where are the anterior and posterior cruciate ligaments located?

Answer 1

The knee is the articulation between the femur, tibia and patella. The two femoral condyles articulate with the tibial plateau. The space between the femoral condyles is known as the “notch” and houses the anterior and posterior cruciate ligaments. These prevent anterior and posterior subluxation of the tibia respectively. Injury to the anterior cruciate ligament is common in footballers and skiers, and causes instability when attempting twisting movements.

Question 2

What type of bone is the patella?

Answer 2

The patella is a sesamoid bone within the tendon of quadriceps. It helps the quadriceps to function efficiently and glide smoothly across the front of the femur. It sits in a groove called the trochlea.

Question 3

Where are the collateral ligaments located?

Answer 3

Either side of the knee are the medial and lateral collateral ligaments. These can be sprained or torn if violent varus or valgus force is applied to the knee.

Question 4

What are the menisci?

Answer 4

These are fibrocartilagenous structures that lie within the joint space between the femur and tibia. There is a medial and lateral menisci. They act as shock absorbers and help distribute force across the joint surfaces. The lateral meniscus is larger and more mobile, whereas the medial meniscus is fixed and smaller. The menisci can be torn resulting in painful clicking and sometimes locking of the knee.

Question 5

What are the three compartments of the knee?

Answer 5

The knee has only one joint cavity anatomically, but it can be thought of as three compartments:

1) patellofemoral compartment - between the trochlea and patella
2) medial compartment - between the medial femoral condyle and medial tibial plateau
3) lateral compartment - between the lateral femoral condyle and lateral tibial plateau

Question 6

What are the causes of knee osteoarthritis?

Answer 6

As in other synovial joints, OA may be idiopathic (wear and tear) or secondary to trauma or infection.

Question 7

What are the symptoms of knee osteoarthritis? What form of knee malalignment is most commonly seen?

Answer 7

Symptoms are pain, stiffness, crepitus, and loss of movement. X ray findings include loss of joint space, formation of oesteophytes, subchrondral sclerosis and cysts (remembered by the mnemonic LOSS), and in severe cases, loss of normal anatomical alignment of the limb.

Clinically, the patient may have swelling, stiffness - especially loss of extension - and deformity in the form of malalignment. Malalignment may be varus (bow-legged) or valgus (knock kneed) depending on whether OA affects either the medial or lateral compartments respectively. Varus OA is the commonest form.

Question 8

What are the treatment options for knee OA?

Answer 8

Conservative - weight loss, activity modification and physiotherapy (improves strength, range of movement and proprioception)

Medical - NSAIDs

Surgical - number of options available:

• knee arthroscopy
• patellofemoral joint replacement
• high tibial osteotomy
• unicompartmental knee replacement
• TKR

Question 9

What is knee arthroscopy?

Answer 9

Although traditionally it was thought that washing the knee out by means of an arthroscopy improved symptoms, this has not been borne out by clinical trials. Any improvement in symptoms is short‐lived, and this procedure is not recommended by the National Institute for Health and Care Excellence (NICE) as a treatment for OA. It may have a role in the debridement of degen- erate meniscal tears, which cause mechanical symptoms in selected young patients.

Question 10

What is a patellofemoral joint replacement? What patients why receive this?

Answer 10

Osteoarthritis of the patellofemoral joint (PFJ) causes anterior knee pain, which is worse coming down stairs: loading the knee in the flexed position generates high forces through the PFJ. If arthri- tis involves only the PFJ, selective replacement of this part of the joint may be indicated. The procedure is usually performed in younger patients, in whom a total knee replacement would not be expected to last their lifetime. Failure and revision rates of PFJ replacements are high because large areas of the knee are left untreated, which allows OA to progress.

Question 11

Which patients might be suitable for a high tibial osteotomy?

Answer 11

Some young, active patients have OA limited to the medial compartment resulting in varus malalignment of the limb. The malalignment may be corrected by cutting and realigning the tibia. Correcting the axis of the limb in this way reduces force transmis- sion through the diseased part of the knee and reduces pain. It delays the need for a total knee replacement by up to 10 years in 50% of cases.

Question 12

What is a unicompartmental knee replacement?

Answer 12

If OA affects just the medial or lateral compartment, half the knee can be replaced. This is known as a unicompartmental knee replacement. The patient must not have significant stiffness or deformity and the cruciate ligaments must be intact. It is a difficult procedure to get right, and only a few patients fit the criteria. Revision rates are high.

Question 13

What patients will receive a TKR? What is important to preserve in the operation in order to maintain joint stability?

Answer 13

Replacement of the whole joint is the most commonly performed surgical procedure for tricompartmental OA of the knee. The knee is opened anteriorly and the patella flipped over in a lateral direction to allow access to the joint. The end of the femur is cut with a cutting‐block and a metal prosthesis cemented into position. The tibia is cut using a jig to realign the axis of the limb. A metal and plastic prosthesis is cemented in place. The underside of the patella may also be resurfaced with a plastic button.

A TKR should improve range of movement, treat pain and restore the normal axis of the limb. It is vital that the collateral ligaments are preserved in order to maintain stability. The thickness of the plastic tibial component and the amount of bone resected from the femur and tibia may be varied in order to achieve stability of the knee in both flexion and extension. This process is known as balancing the flexion and extension gaps.

Question 14

What complications are associated with a total knee replacement?

Answer 14

1) Infection
2) Damage to nerves and blood vessels (e.g. popliteal artery, tibial nerve, common peroneal nerve)
3) Dislocation of the patella
4) Instability
5) Wear and loosening

Question 15

How does a prosthetic joint infection present? How should it be managed?

Answer 15

Infection may be introduced at the time of surgery, develop in the wound in the immediate post operative period, or be seeded years later by bacteraemia from another source.

Presentation is pain, swelling, warmth, and signs of sepsis. Aspiration of the joint allows and organism to be identified. But, in contrast to a native joint, if an arthroplasty is in situ DO NOT aspirate in A&E or the ward. It must be done in theatre under aseptic conditions.

If infection occurs within a few weeks of surgery, the joint may be saved by rapid return to theatre for debridement, washout and change of the plastic liner. If this fails or the patient presents late, the joint must be revised in two stages with a prolonged course of antibiotics.

Question 16

What is the pathogenesis of TKR loosening?

Answer 16

1) Wears of the plastic bearing of the TKR result in the production of wear particles which are roughly the same size as bacteria
2) macrophages phagocytose the wear particles
3) Macrophages produce cytokines and intracellular signalling molecules. This results in recruitment of more macrophages and lymphocytes which causes swelling and synovitis
4) Cytokines activate osteoclast precursors which differentiate into active osteoclasts
5) Osteoclasts resorb bone around the implant, resulting in a lucent line on x-rays, cyst and loosening of the component

Question 17

How long does a TKR usually last?

Answer 17

More than 90% of TKRs last 15 years or more. A lucent line on X ray, accompanied by pain and loss of stability is an indication for revision although infection must be excluded first.

Question 18

What is the main function of the menisci?

Answer 18

The most important function of the menisci is to bear load across the joint. Load bearing is greatest with the knee in the flexed position, with 90% of the load passing through the menisci in 90° flexion.

Question 19

What is the most common cause of meniscal tears and what are the clinical features?

Answer 19

In a twisting injury the menisci may become torn, most commonly the medial meniscus. Adolescents or young adults are most commonly affected. Pain is localised to the joint line, worse when loading the knee in flexion, such as coming down stairs. Mechanical symptoms such as locking or catching are classic. There is delayed knee swelling.

Question 20

What are the examination findings in a meniscal tear?

Answer 20

Examination findings include joint line tenderness and a small effusion. McMurray’s test is a provocative test in which the knee is loaded and twisted in flexion producing a painful click.

In contrast to acute tears, chronic tears occur in older patients without a history of trauma. They are due to degeneration and osteoarthritis (OA).

Question 21

How should a suspected meniscal tear be investigated?

Answer 21

Imaging should include an X‐ray to exclude arthritis or fracture. MRI is the best modality to demonstrate a tear. There are various descriptive terms to describe tear morphology including bucket handle, parrot beak, longitudinal and radial.

Question 22

How is a meniscal tear treated? Under what circumstances can a repair be considered?

Answer 22

Treatment is usually via arthroscopy:

• Debridement: trimming the torn edge back to a stable base. If large amounts of meniscus are removed, more force is transmitted to the cartilage, and the risk of OA in the future is increased
• Repair: only the peripheral third of the meniscus has a blood supply. Certain tears in this region can heal if repaired. Indications for repair are non‐degenerative longitudinal or bucket‐handle tears within 5mm of the periphery of the meniscus. Tears extending into the avascular central portion of the meniscus will not heal.

Question 23

What mechanism of injury can cause an anterior cruciate ligament tear? What are the typical clinical features?

Answer 23

These occur with the twisting forces experienced by skiers and footballers. As the ligament ruptures, patients may hear a pop and the knee gives way. The ligament contains an artery, which bleeds into the knee when torn. This results in a large effusion developing within minutes. ACL ruptures may be associated with meniscal tears and medial collateral ligament (MCL) rupture, a combination of injuries known as the ‘terrible triad’.

Question 24

How is an ACL tear diagnosed?

Answer 24

Diagnosis is based on the history and clinical examination. Disruption of the ACL results in a positive anterior drawer (excessive anterior translation of the tibia in 90° flexion), Lachman’s test (excessive AP sloppiness in 30° flexion) and pivot‐shift test (subluxation of the tibia with external rotation and valgus that reduces with a clunk on flexion). MRI is the best imaging modality.

Question 25

How is an ACL tear treated?

Answer 25

Treatment of isolated ACL injury is initially conservative with specialist physiotherapy to train the quadriceps to stabilise the knee. If this fails and instability remains, ACL reconstruction may be carried out using a graft obtained from the patient’s hamstrings or patellar tendon.

Question 26

What injuries cause a posterior cruciate ligament injury?

Answer 26

These occur when the tibia is forced backwards with the knee flexed. This occurs when a car occupant strikes the dashboard in a collision. PCL injuries are rare in isolation and may be associated with multiligamentous injury (see below). Treatment of isolated PCL injury is initially physiotherapy and bracing, but surgical reconstruction may be carried out.

Question 27

What are the features of a collateral ligament tear?

Answer 27

These occur when the knee is subjected to severe valgus or varus injury, such as being tackled from the side. Isolated injuries are commonly treated conservatively in a hinged brace, which stabilises the knee against varus/valgus stress.

Question 28

What is a multiligamentous knee injury?

Answer 28

This is a serious limb‐threatening injury. It takes a large amount of energy to dislocate the knee, and associated life‐threatening injuries may coexist. The danger of a dislocated knee is that the popliteal artery may be damaged, resulting in limb ischaemia. Unstable knee dislocations should be stabilised initially with an external fixator and an arteriogram performed urgently. Reconstruction may be performed in stages and is technically demanding. Note that knee dislocation where the femur and tibia dislocate is not the same as the (much more benign) patellar dislocation, where the patella slips out of the trochlea.

Question 29

What causes dislocation of the patella?

Answer 29

Most commonly occurs as a traumatic primary event, either through direct trauma or through severe contraction of quadriceps with knee stretched in valgus and external rotation.

Genu valgum, tibial torsion and high riding patella are risk factors.
Skyline x-ray views of patella are required, although displaced patella may be clinically obvious.
An osteochondral fracture is present in 5%.
The condition has a 20% recurrence rate.

Question 30

What is chondromalacia patellae

Answer 30

Teenage girls, following an injury to knee e.g. Dislocation patella.
Typical history of pain on going downstairs or at rest.
Tenderness, quadriceps wasting.

Question 31

What are the two types of patellar fracture?

Answer 31

i) Direct blow to the patella causing undisplaced fragments

ii) Avulsion fracture

Question 32

What differential diagnoses should be considered in cases of a swollen knee?

Answer 32

1) Septic arthritis
2) Crystal arthopathy
3) Haemarthrosis
4) Inflammatory arthropathy
5) Bursitis

Question 33

How does septic knee arthritis present and how should it be managed?

Answer 33

Bacteria may enter the joint by direct inoculation, from adjacent osteomyelitis or via haematogenous seeding. The joint is hot, swollen and erythematous and any movement is very painful. Aspirate may show pus or turbid fluid. Microscopy shows neutrophils and organisms may also be seen. Treatment is urgent wash‐out of the joint in theatre. Antibiotics should be given only after an aspirate has been obtained.

Question 34

What are the features of crystal arthropathy?

Answer 34

Gout is crystals of uric acid precipitating within the joint. Crystals are thin, needle‐shaped and negatively birefringent on polarised light microscopy. The disease often also affects other joints, especially the first metatarsophalangeal joint (MTP) joint. It may be due to an idiopathic error in purine metabolism or to increased protein turnover in haemolytic anaemia or after chemotherapy. Pseudogout is a disorder in which calcium pyrophosphate crystals form. They are rhomboid and positively birefringent, and the condition tends to affect older people. X‐rays of the knee may show calcification of the menisci known as chondrocalcinosis.

Question 35

How should gout be treated? When should allopurinol prophylaxis be considered?

Answer 35

Acute management:

• NSAIDs
• intra-articular steroid injection
• colchicine* has a slower onset of action. The main side-effect is diarrhoea
• oral steroids may be considered if NSAIDs and colchicine are contraindicated. A dose of prednisolone 15mg/day is usually used
• if the patient is already taking allopurinol it should be continued

Prophylaxis:

• allopurinol should not be started until 2 weeks after an acute attack has settled as it may precipitate a further attack if started too early
• initial dose of 100 mg od, with the dose titrated every few weeks to aim for a serum uric acid of < 300 µmol/l
• NSAID or colchicine cover should be used when starting allopurinol

Indications for allopurinol:

• recurrent attacks - the British Society for Rheumatology recommend ‘In uncomplicated gout uric acid lowering drug therapy should be started if a second attack, or further attacks occur within 1 year’
• tophi
• renal disease
• uric acid renal stones
• prophylaxis if on cytotoxics or diuretics

Question 36

What can cause a haemarthrosis?

Answer 36

Haemarthrosis – blood in the joint. Associated with trauma or spontaneous. May be due to coagulopathy or warfarin. If associated with trauma, consider ligamentous injury (especially ACL) or fracture. Fractured bone results in a mixture of blood and fat in the joint – a lipohaemarthrosis. Fat floats – it is less dense than blood – and this may be seen on X‐ray as two distinct fluid levels. Aspirate reveals fat globules in the syringe.

Question 37

What is bursitis?

Answer 37

Swelling outside the knee joint: prepatellar bursitis (housemaid’s knee) or infrapatellar bursitis (parson’s knee) are diagnosed by careful examination.