Knee Pain

The knee is a vulnerable and potentially unstable joint. The femur is balanced on the tibia with 2 "shock absorbers" of cartilage (medial and lateral menisci), 4 ligaments and the surrounding muscles to maintain stability. The ligaments are the anterior cruciate ligament (ACL), posterior cruciate ligament (PCL), medial collateral ligament (MCL) and lateral collateral ligament (LCL). In addition the patella slides over the anterior surface in a sesamoid joint. The joint takes the full weight of the body and considerable stress in activities such as running and jumping. It is very susceptible to problems.

Problems with knees are common and there are many causes.

For a description of history, examination and investigations see knee assessment. Eliciting and interpreting some of the signs can be difficult and practice is needed.

Neuropathic joints show considerable destruction but are painless. The commonest cause for a neuropathic knee used to be syphilis but nowadays it is diabetic neuropathy.

• Obesity increases the risk of problems with the knee by increasing the forces on it.
• The female pelvis is wider than the male and so the quadriceps pulls on the knee with a more lateral angle.
• Injuries to the knee, such as torn menisci, predispose to further injuries.
• Good muscles protect the knee.

The causes of pain in the knee can be divided into 6 different aetiologies:

• Primary arthritis in the knee
• Reactive arthritis from other diseases
• Disease of bone around the knee
• Mechanical problems of the knee
• Direct trauma
• Referred pain, usually from the hip

Primary arthritis

• Osteoarthritis tends to be slow and insidious and occurs in the elderly although it can affect younger people, especially if obese. Crepitus is common. X-ray may show reduction of the joint space because of loss of cartilage.
• Rheumatoid arthritis tends to be faster in onset and there will almost certainly be other joints involved, especially the MCP joints. Morning stiffness is marked. Indices of inflammation such as ESR and CRP are often raised but positive rheumatoid factor is a later feature. If in doubt refer, as early treatment with DMARDs reduces long-term joint damage. In childhood, RA can present as Still's disease.

Metabolic Arthritis

A rapid onset is typical of gout and pseudogout. Gout does not often strike the knee but pseudogout does. Diagnosis is by demonstration of bi-refringent crystals in fluid aspirated from the knee and examined under polarised light.

Infection

The patient with septic arthritis will be systemically unwell. Fluid should be aspirated and sent for culture and antibiotics started. A hot, red joint is usually septic arthritis, gout or pseudogout. It may sometimes occur in psoriatic arthropathy and is rare in RA.

Reactive arthritis

Reactive arthritis usually affects younger people and there may be signs or symptoms of those other diseases. They include

• Ankylosing spondylitis
• Psoriasis
• Inflammatory bowel disease
• Reiter's disease
• Behçet's syndrome
• Gonococcal infection

Rarer causes

Other diseases that can cause arthritis that affects the knee include:

• Haemochromatosis in which the knee is the 3rd commonest site of arthritis, after MCP and PIP joints.
• Rheumatic fever is rare in the UK in the 21st century. Arthritis occurs in 80% of patients, usually involves multiple large joints, particularly the knees, ankles, elbows, and wrists and flits from site to site.
• Spontaneous haemarthrosis may occur in coagulation disorders, especially haemophilia and, less frequent and usually less severe, Christmas disease.
• In Familial Mediterranean Fever around 5% develop chronic arthritis that sometimes leads to destructive arthritis of hips or knees and may require joint replacements.

Disease of the bone

• Osteosarcoma usually affects children. The commonest sites are distal femur and proximal tibia. The most frequent presenting symptom of osteosarcoma is pain, especially with activity. There is often delay in making the diagnosis although it seems to have little effect on outcome.¹
• Osteomyelitis can also affect the knee, usually in children.

The complexity and vulnerability of the knee makes it very susceptible to stresses from overuse and malalignment, especially around the time of maximum growth.

• The aetiology of patello-femoral joint problems is multifactorial. It can be difficult to evaluate, diagnose, and treat this condition but conservative treatment is effective in about 80%.² The nomenclature used in diseases of the patello-femoral joint can be very confusing.³
• Chondromalacia patellae tends to affect adolescents, especially female runners. There is pain in the front of the knee, and examination shows crepitus of the knee joint and a positive apprehension test. The usual treatment is NSAIDs and quadriceps exercises but this is not invariably effective.
• Osgood-Schlatter's disease affects adolescents or younger, mostly boys who are keen on sport,⁴ especially football. There is tenderness and swelling over the insertion of the quadriceps tendon into the tibial tuberosity. It may be unilateral or bilateral. Management is to reduce physical activity and they usually grow out of it.
• Baker's cyst is an out-pouching of the synovial membrane into the popliteal fossa. It is uncomfortable but not painful. Management is conservative.
• Popliteus tendonitis causes lateral knee pain and running downhill is a risk factor.
• Iliotibial band syndrome produces lateral knee pain in cyclists, dancers, long-distance runners, football players, and military recruits. They are more likely with poor muscles, lax ligaments and poor training regimes.⁵
• Patellar tendonitis or "jumper's knee" affects young athletes in sports that require repetitive running, kicking, and jumping. There is pain at the inferior pole of the patella during activities such as climbing stairs, running uphill, and jumping. Treatment is rest, NSAIDs, knee bracing and an exercise programme to stretch and strengthen the quadriceps and hamstring muscles. Steroid injection may be helpful but it is essential not to inject into the tendon.
• Recurrent patella subluxation occurs with a tight lateral retinaculum. The patella subluxes laterally to produce medial pain. The knee may give way. It is commoner in girls and with valgus knees. There is increased lateral patellar mobility and a positive apprehension test. Building Vastus Medialis Oblique (VMO) is tried first. Lateral retinacular release may be used or rarely patellar tendon transfer.
• Hoffa's fat pad syndrome can occur when symptoms suggest damage to meniscus or ligament but investigation shows they are intact. MRI shows a hypertrophic pad of fat between the articular surfaces of the joint. There is pain under the patella. There may be effusion. Extending a bent knee with pressure on the patellar tendon elicits a strong pain and a defensive reaction.

Overuse Injuries

Overuse or repetitive trauma can cause bursitis too:

• Prepatellar bursitis or housemaid's knee is caused by recurrent trauma from prolonged kneeling. In chronic cases, a well-circumscribed area of fluctuance is present over the prepatellar area. In acute cases there is warmth, oedema, and erythema over the anterior knee. Fluctuance may be less. Tenderness is maximal over the prepatellar bursa. Knee flexion increases the pain. It may arise from trauma, gout or infection. An acute bursitis requires aspiration to check for infection or crystals. Traumatic bursitis improves with rest and avoidance of kneeling.
• Anserine bursitis causes pain over the medial aspect of the knee, is made worse by climbing stairs, and is often present at night. It is found in overweight women with osteoarthritis of the knees. There is exquisite tenderness over the anserine bursa, over the medial aspect of the knee about 5cms below the joint line. Treatment includes steroid injection into the bursa and an exercise regimen to stretch the adductor and quadriceps muscles.

Trauma is often the result of sports injuries but it is also common in road traffic accidents whether in a car, on a motor cycle or as a pedestrian. A car bumper is about the same height as the knee. The lateral peroneal nerve winds round the head of the fibula and may be damaged on impact, causing foot drop. A careful history is important to note the exact mechanism of an injury. This gives a very good indication of what might be damaged. Fracture of the tibia and fibula are considered in their own article.

• If a footballer or basketball player jumps and lands whilst rotating there is a strong possibility of damage to cartilage. A common mechanism of injury is a varus or valgus force on a flexed knee. A valgus force tends to cause damage to the medial meniscus and a varus force to the lateral meniscus. The medial meniscus is damaged more often. Pain in the line of the joint is common. Effusion develops over a few hours but it may be faster with haemarthrosis. When haemarthrosis develops after trauma about have meniscal tears and have a ruptured ACL. Locking is common, usually at 20 to 45° of joint extension. A click or snap occurs as the joint unlocks. There may be a sensation of giving way.
• Abrupt halting of forward motion may rupture the ACL. Rapid swelling of the joint after injury suggests haemarthrosis. A "popping" sound at the time of impact suggests a ruptured ligament. Physical signs of a deficient ACL may be absent until effusion and muscle spasm have settled.
  A direct blow to the side of the knee may rupture the MCL and damage the lateral meniscus.
• Patellar injury and dislocation are common. Men predominate amongst athletes as they do more contact sports but women more often have non-athletic injuries.

The segmental innervation of the knee is L2,3,4 and so pain can be referred from elsewhere, usually the hip.

• Slipped capital femoral epiphysis (SCFE) is a common adolescent hip disorder that can present as pain in the knee.
• Osteoarthritis of the hip may also be felt in the knee.
• If the cause of pain in the knee is not obvious, examine the hip too.

Non-Drug

• Management must be holistic as simply prescribing analgesics is rarely enough.
• The standard treatment of acute trauma is rest, ice, compression, elevation and rehabilitation (RICER).
• Fluid may reform after aspiration and can be aspirated again. Aspiration of the knee speeds recovery and reduces muscle wasting around the knee. Blood damages joints as is shown by the state of joints in haemophiliacs. Knees that swell are considered in their own article.
• Physiotherapists often used bracing and taping of joints⁶ in conjunction with building VMO.⁷ The way they work is uncertain and may have more to do with improving proprioception than improving inherent stability.
• Sportsmen in particular, should be given advice about rest and rehabilitation. They are impatient to return to activity and do not take kindly to being told simply to rest or to give up their favoured pastime. There is no reason why they should rest those parts of their body that are not affected but the injured parts require a gradual and controlled return to full activity. This is described more fully in the article on sports injuries.

Drugs

• Analgesia may be required, usually in the form of NSAIDs. The usual caveats about at-risk groups apply.
• Specific diseases may require specific therapies such as DMARDs for RA, drugs to lower uric acid after the acute episode of gout has settled or antibiotics to treat gonococcal infection.
• Topical analgesics have no benefit over placebo.⁸ They may possibly give some benefit in OA for the first 2 weeks, but not thereafter.⁹

Surgical

• Arthroscopy is a minimally invasive procedure that can be used for both diagnosis and sometimes treatment too. Torn cartilage can be removed and even ACL reconstruction undertaken.
• Arthritis may eventually require arthroplasty. Total knee replacement has now overtaken total hip replacement as the commonest form of joint replacement surgery.

• Sportsmen need advice such as working on upper body strength whilst the lower part is out of action, what can be done safely to maintain cardio-respiratory fitness and a rehabilitation programme.
• Immobility, whether enforced by an appliance or by pain leads to rapid wasting of muscle. The muscles are crucial for the stability of the joint. Of particular importance is the lower end of the vastus medialis that stabilises the patella, preventing lateral dislocation by the pull of the rest of the quadriceps. It is also called the vastus medialis oblique (VMO) muscle. Even if the knee is very painful and rigid these muscles can be exercised isometrically. Get the patient to lie on his back with a pillow below the knee, to invert the ankle and then to tighten the quadriceps.
• Once some mobility returns there are some simple exercises that can be performed at home without the need for equipment as in gyms. To build the quadriceps stand with feet parallel and a little further apart than the shoulders. Dip down about 20 to 30cms and up again, repeating and building up the number with training. It is better to call them dips than squats to emphasise only a shallow movement as deep flexion of the knee builds up a great pressure inside the joint.
• Building a set of muscles should always be accompanied by building of the antagonists too. Hence the hamstrings must also be exercised. The patient should lie on his back on the floor with his feet up on a chair or low stool. He lifts his buttocks off the floor and back down again. Repetitions are built up as for the quadriceps exercises. Building the muscles around the knee is not just for athletes. The elderly often have very poor muscles making them feel insecure and predisposing to falls.
• The article on physical training gives general advice.

• Good, strong muscles around the knee give good protection. Even with a ruptured ACL, if the muscles are good it is possible to indulge in contact sports such as rugby and judo.
• Physical activity and the avoidance of obesity delay the onset of osteoarthritis.
• Always encourage full warm-up before exercise. The evidence base to support warm up is not strong^10,11 but it is traditional strong advice.