Abstract
Knee pain is a common presenting symptom in primary care. Around 4.5 million people in the UK, over 100 patients per GP, have severe or disabling knee pain. About 25% of people over the age of 55 years complain of chronic knee pain which is usually diagnosed as osteoarthritis. At the other end of the age spectrum, knee pain affects as many as one in three teenagers and young adults at some time or other.
The GP curriculum and knee pain
The GP curriculum (section 15.9 Rheumatology, Musculoskeletal & Trauma) outlines the skills and attitudes needed to diagnose and treat knee pain in primary care. It covers primary care management, person-centred care, problem-solving skills, community and holistic approaches to knee pain and the specific knowledge base required. This includes knee pain as a common and important condition. GPs in training should be able to
Assess the patient with knee pain Describe the indications for further investigation including the Ottawa rules for X-ray and indications for blood tests Explain the aetiology and natural history of knee pain Distinguish inflammatory from non-inflammatory conditions Describe the key national guidelines that influence health care provision for musculoskeletal problems (e.g. NICE guidelines) Understand the principles of treatment for common conditions managed largely in primary care including the use and monitoring of non-steroidal anti-inflammatory drugs (NSAIDs) Advise patients regarding what they are physically able to do, according to their level of disability Facilitate self-help strategies to empower the patient Know when joint injections and aspirations are appropriate in general practice and be able to perform knee joint injections Understand the roles of allied health professionals such as physiotherapists Describe the indications for referral within a suitable time frame to the most appropriate health care practitioner Communicate truthfully and sensitively to patients for whom therapeutic options have been exhausted Recognize the emotional impact dealing with disability can have on the GP
The nature and role of the knee joint render it particularly susceptible to local injury and it is also affected in several systemic diseases. The bony structure, comprising the lower end of the femur essentially resting on the almost flat upper end of the tibia, unlike in the hip, is inherently unstable. Four ligaments and the surrounding muscles thus confer stability to this hinge joint. A thorough history enquiring into trauma, swelling, mobility, locking, clicking and giving way is vital to forming a differential diagnosis and planning investigation and management. Referred pain from the hip should always form part of that differential.
Arthritis
Arthritis of any joint is characterized by swelling, warmth, pain and restricted movement. The cause of the inflammation can be infective or non-infective.
Septic arthritis
Septic arthritis is the most urgent cause of knee pain to recognize. The suppurative inflammation within the joint is usually due to haematogenous spread although it can originate from osteomyelitis in the neighbouring bone or from a penetrating injury or procedure such as joint injection or aspiration or post-arthroscopy. The process is usually confined to a single joint and the knee and the hip are the most common sites. General features include pyrexia and tachycardia with local signs of effusion, warmth and redness of overlying skin. More importantly, there is often a pseudoparalysis of the joint with diminished movement due to significant pain and spasm. Recognizing pseudoparalysis is of particular importance in the paediatric age group.
Osteoarthritis
Osteoarthritis is an active disease which usually begins in middle age although symptoms may not manifest until later in life. The cause is multifactorial and there is an important genetic predisposition. There is characteristic overgrowth of underlying bone and loss of articular cartilage. The resultant radiograph changes demonstrate loss of joint space, subchondral bony sclerosis, marginal osteophyte and subchondral cyst formation. Primary osteoarthritis of the knee particularly affects the medial compartment of the knee and the posterior surface of the patella. Patients may also develop a varus deformity.
Rheumatoid arthritis
Rheumatoid arthritis (RA) is a systemic condition which typically involves an insidious onset polyarthritis. The presence of ongoing synovial inflammation causes destruction of articular cartilage and erosion of the underlying bone with resulting joint deformity. It can affect the knee joint, but is likely to do so in association with other joints particularly the metacarpophalangeal joints and other small joints of the hand. There is morning stiffness lasting more than an hour and inflammatory markers such as the erythrocyte sedimentation rate and C-reactive protein are raised. Other tests include raised anti-citrulline antibody which is associated with erosive RA. Suspicion of RA should prompt early referral as early treatment with disease modifying agents reduces long-term joint damage and disability.
Juvenile idiopathic arthritis is an inflammatory arthritis which occurs in patients less than 16 years old. It can manifest in many ways, but generally affects the larger joints including the knee. Systemic features include fever and rash. The eyes may also be affected by anterior uveitis. Any patient with anterior uveitis should be followed up by an ophthalmologist.
Seronegative arthritis
Seronegative arthritides may manifest with arthritis of the knee. These conditions are all associated with HLA-B27 antigen and display negative serum rheumatoid factor. They are all characterized by sacroiliitis and spondylitis. Conditions in this group include
Ankylosing spondylitis Psoriatic arthritis Behçets disease Arthritis associated with inflammatory bowel disease Whipple's disease Reactive arthritis
Reactive arthritis
In the case of reactive arthritis, exposure to an infectious agent causes an immune response which has a predetermined genetic basis. The seronegative arthritis usually affects large joints such as the knee. Subtypes include a genital form (due to Chlamydia or gonorrhoea) and an enteric form (due to Salmonella, Shigella, Campylobacter, Yersinia, etc.). Reiter syndrome is a form of reactive arthritis characterized by the triad of conjunctivitis, urethritis and a seronegative arthritis. Knee pain in reactive arthritis is managed with analgesia and rest. In cases of Reiter syndrome, topical and oral antibiotics may also be required to manage the conjunctivitis and urethritis, respectively.
Crystal arthropathies
The two most common metabolic conditions to affect the knee are the crystal arthropathies: gout and pseudogout. Gout should be considered in any acute monoarthritis, not just arthritis involving the great toe. Pseudogout is part of a spectrum of arthritides associated with calcium pyrophosphate microcrystal deposition and should be considered in any elderly patient with acute monoarthritis of the knee, shoulder, elbow or wrist. Pseudogout is more likely to affect the knee than gout. Synovial fluid analysis under polarized light is the key to diagnosis of either and enables the exclusion of septic arthritis. Light microscopy of synovial fluid in gout demonstrates needleshaped crystals which are negatively birefringent under polarized light. Synovial fluid in pseudogout, on the other hand, shows brick-shaped crystals which are positively birefringent. X-rays can be helpful in identifying chondrocalcinosis.
Once diagnosed, both gout and pseudogout are treated almost identically in the acute phase with NSAIDs or oral, intramuscular or intra-articular steroids. Colchicine, a drug that inhibits neutrophil microtubule formation and is a useful prophylactic agent in both crystal arthropathies, but can also be used in the acute phase particularly in patients with heart failure or other contraindications to treatment with NSAIDs. When prescribing colchicines, it is advisable to carefully follow the recommended dosage in the British National Formulary.
Chronic gout can be managed with dietary purine reduction and the pharmacological disruption of purine metabolism with allopurinol (a xanthine oxidase inhibitor that reduces endogenous uric acid production) and probenecid (an uricosuric agent). When instituting prophylactic therapy, the following precautions should be taken:
Starting prophylactic treatment may induce an acute attack of gout, so a NSAID or colchicine should be given concomitantly for at least the first 3 months Prophylactic treatment must not be started during an acute attack as this may prolong symptoms If an acute attack occurs during prophylactic treatment, the prophylaxis should be continued at the same dose and the acute attack treated with a NSAID or colchicine
Bone disease around the knee
Osteomyelitis
Osteomyelitis is more common in children and is usually due to haematogenous spread from a remote source. The most common site of infection is the distal femur and the proximal tibia. Contiguous osteomyelitis, seen more commonly in adolescents and adults associated, is associated with trauma. A prosthetic knee joint, sickle-cell disease, immunocompromise and diabetes predispose to osteomyelitis. The presentation can range from insidious onset with localized signs and symptoms to acute systemic toxicity.
Patients presenting with a history of trauma may pose a diagnostic difficulty as the initial symptoms may be similar to early osteomyelitis. It is therefore important to tell patients with a history of minor trauma to return if their symptoms do not settle or if they become systemically unwell.
Osteosarcoma
Osteosarcoma is rare and also primarily a disease of childhood (most commonly the second decade of life). It principally affects the distal femur and proximal tibia. Pain on exertion is the most frequent presenting symptom. History of an injury should not be assumed to exclude the possibility of a bone sarcoma. NICE recommends referral of children or young people with
rest pain, back pain and/or unexplained limp (a discussion with a paediatrician or X-ray should be considered before or as well as referral) persistent localized bone pain and/or swelling and X-ray showing signs of cancer. In this case refer urgently.
Mechanical abnormalities
The knee joint by its very nature is particularly susceptible to mechanical stresses from overuse or malalignment. Such disorders commonly present during periods of maximal growth, i.e. during the teenage years, and usually present with anterior knee pain. The group of conditions is somewhat confusing as they are often eponymous and idiosyncratic (Table 1). Reassuringly, conservative treatment is effective in about 80% of cases.
Common mechanical abnormalities responsible for knee pain
Trauma
Traumatic causes of knee pain are often the result of sporting injuries or due to road traffic accidents. A careful history allows the clinician to identify the mechanism of injury and therefore determine which structures may be affected. Trauma can lead to both soft tissue and bony damage.
Ligament injuries
Depending on the mechanism of trauma, the medial and lateral collateral and anterior and posterior cruciate ligaments may be injured. Injuries to the ligaments are usually felt as immediate pain, but can be hard to localize. Associated features include joint swelling, warmth and pain on movement and/or weight bearing. Damage to the medial and lateral collateral ligaments is felt as pain on the inner or outer aspects of the knee whereas involvement of the cruciate ligaments is felt as pain deep within the joint. Patients with cruciate ligament rupture may describe a ‘popping’ sensation at the time of impact.
Anterior cruciate ligament
Anterior cruciate ligament (ACL) injury should be suspected in any patient presenting with acute haemarthrosis of the knee following trauma. The commonest mechanism of injury to the ACL is due to valgus stress and external rotation of the tibia on the femur. This mechanism may also injure the medial meniscus and medial collateral ligament. The triad of ACL, medial meniscus and medial collateral ligament injury is known as the triad of O'Donoghue. The other common mechanism of injury is due to forced hyperextension of the knee as is seen in dashboard injuries. This may also be associated with rupture of the posterior cruciate ligament.
Meniscal tears
Meniscal tears are usually seen in young adults and are predominantly sustained during sporting activities which involve shearing forces of rotation applied to a flexed, weight-bearing limb. There is a higher incidence with increasing age and degeneration of the underlying cartilage. Spontaneous repair of a torn meniscus is unlikely as it is an avascular structure. Clinical features of a meniscal tear include joint line tenderness, locking of the knee and the sensation of ‘giving way’. The knee may be held slightly flexed and an effusion may develop over a few hours. Clinical tests such as McMurray's and Apley's grinding test may be positive, but may cause pain and discomfort to patients.
Fractures
Serious trauma can cause fractures to any of the bones which contribute to the knee joint. These include condylar and supracondylar fractures of the distal femur, fractures of the fibula and tibial plateau fractures. Fracture of the head of the fibula may result in damage to the lateral peroneal nerve and cause foot drop. Radiographs of the knee are useful in determining fracture patterns (Box 1).
Patella
Dislocation of the patella may occur from a twisting injury or due to direct impact. Displacement is most commonly lateral. The affected knee is swollen and movement markedly limited. There are three types of fracture of the patella:
Undisplaced fracture: the result of direct impact Comminuted fracture: due to direct impact to the front of the knee during a fall Transverse fracture: due to forced extension of knee while the quadriceps is contracted. Transverse fractures result in complete disruption of the extensor mechanism of the knee
Ottawa rules for knee X-ray
The following are indications for an X-ray following knee injury:
age 55 years or over tenderness over the head of the fibula isolated tenderness of the patella inability to flex the knee to 90 degrees inability to walk four weight-bearing steps immediately after the injury and in the emergency department/surgery
Referred pain
Knee pain is not always due to a primary cause within the knee itself. Pain may be referred from elsewhere due to the segmental innervations of the knee (L2, 3 and 4). The assessment of any joint should always involve assessment of the joint proximal and distal to it, in this case the hip and ankle. This is particularly relevant in cases where no obvious cause is found in the knee itself. The hip is usually the culprit in referred knee pain and causes include osteoarthritis of the hip and slipped capital femoral epiphysis in the adolescent age group.
Investigation
Further investigation of knee pain is often unnecessary. The most commonly requested investigations in primary care are X-rays to assess bony integrity and inflammatory markers to find the cause of local inflammation or to exclude systemic disease. It should be remembered to request weight-bearing radiographs of the knee joint when assessing osteoarthritic change to give a true functional representation of joint space. Aspiration of the potentially septic joint as well as MRI and CT scanning of the knee joint are usually within the domain of secondary care. A large multicentre randomized trial is currently underway investigating whether GPs should have direct access to MRI for patients with suspected internal derangement of the knee (the Direct Access to Magnetic resonance imaging: Assessment for Suspect Knees (DAMASK) study).
Management
Conservative
Weight loss, exercise and physiotherapy form the core approach to treating the vast majority of knee pain. The loss of 1 lb of body weight can correlate to the loss of 3 lb of weight at the knee itself. Non-impact exercises such as swimming can be considered when the knee needs to be ‘off-loaded’. The acronym RICE (Rest Ice Compression and Elevation) is invaluable in managing minor trauma to the knee.
In the case of osteoarthritis, some of the conservative treatments recommended by NICE include
Advice on footwear, e.g. insoles and shock absorbing properties Bracing/joint supports for patients with biomechanical joint pain and instability Assistive devices, e.g. walking sticks
GPs can make a significant difference to patients by advising them on the use of conservative measures. A typical scenario involves a patient who has recently been advised to use a walking stick. The GP should ensure that the walking stick is the correct size, i.e. when the patient is standing, the length of the stick should be the same as the vertical distance between the patient's wrist crease and the floor. The patient should also be advised to hold the walking stick in the contralateral hand to the affected knee.
Medical
Analgesia is the main stay of medical management for knee pain particularly with respect to osteoarthritis. The first-line treatments recommended by NICE include regular paracetamol supplemented with topical NSAIDs. If these fail to control pain, then step up to oral NSAIDs. Cyclooxygenase 2 inhibitors and opioids can also be considered. There is also mention of the use of topical capsaicin for knee osteoarthritis. In cases of moderate to severe pain, intra-articular corticosteroid injections may be appropriate.
Glucosamine is claimed to have chondroprotective properties and felt to have a role by some in the management of osteoarthritis. Different trials have come to different conclusions. A Cochrane review highlighted a potential role in osteoarthritis of the knee with a limited benefit and favourable risk profile compared to NSAIDs.
Other medical treatments of knee pain will depend on the cause. Therapies range from disease-modifying agents for RA to NSAIDS for acute gout and prophylactic allopurinol and uricosurics for chronic gout.
Physiotherapy
Physiotherapy can be particularly useful for patients with minor trauma, those recovering from more major knee injury or knee surgery and those with mechanical knee pain. Physiotherapists are often very experienced in assessing patients with knee pain and a physiotherapist's opinion can be invaluable if you are unsure of the cause of a patient's knee pain and have excluded serious causes. Physiotherapists are also very helpful in recommending aids and appliances that might benefit a patient.
Surgical
When conservative approaches to the management of knee pain are unsuccessful, surgery may be indicated. Surgical approaches range from minimally invasive arthroscopy to total replacement of the joint.
Arthroscopy
Arthroscopy can be a therapeutic as well as a diagnostic procedure. During the procedure, irregular bony surfaces can be smoothed, damaged cartilage and meniscus can be trimmed and loose debris removed. In the case of osteoarthritis, NICE does not recommend arthroscopic debridement and lavage unless there is a clear history of mechanical locking.
ACL repair
Conservative approaches involving the strengthening of surrounding muscles must always be considered prior to ACL repair. Primary repair of the ACL is not recommended because it leads to persistent laxity and instability of the knee. The ACL should instead be reconstructed using autografts. These may be derived from the patella tendon, hamstring tendon, quadriceps tendon or fascia lata. The advantages of using autografts include minimal host reaction, decreased chances of infection and better function.
Arthroplasty
Indications for total knee replacement include pain, deformity and instability. Total knee replacement surgery is now the most common form of joint replacement undertaken in the UK, overtaking total hip replacement. The most common indications are osteoarthritis followed by RA. Following arthroplasty, the prosthetic joint itself may be a cause of pain in the knee. Pain in a previously painless prosthetic joint may indicate infection (pain typically worse at night) or mechanical loosening (pain worse first thing in the morning).
Referral
Many patients benefit with knee pain that is not settling with simple measures in primary care benefit from physiotherapy assessment, advice and/or treatment. Table 2 outlines a framework for referral of patients with knee pain to secondary care.
A referral framework for knee pain
Key points
An awareness of the anatomy and function of the knee joint is crucial to determining the aetiology of knee pain The knee is not only susceptible to localized injury but is also a site of manifestation of systemic disease Hip pathology can often manifest as knee pain X-rays are a common first-line investigation in primary care. Weight-bearing films should be specifically requested if suspected osteoarthritis. The Ottawa rules for knee X-ray should be borne in mind for traumatic injury to the knee. Conservative and medical measures are all that is needed for treating the vast majority of knee pain in primary care