Osteoarthritis of the knee

Description

Osteoarthritis (OA) of the knee is a complex disorder resulting from biomechanical and biochemical changes in the joint leading to progressive loss of articular hyaline cartilage, as well as alterations of the synovium, joint capsule and other connective tissue and underlying subchondral bone.  OA leads to clinical manifestations of joint stiffness, pain, limitation in the range of motion and functional alteration.  It is the most common form of arthritis affecting the knee and is not associated with any systemic symptoms.

It is divided in two types:

         - primary OA (related to aging, genetic predisposition)

         - secondary OA (as a result of an injury, overuse or inflammatory                                                      conditions).

Clinical Manifestations

Older age and abnormally elevated bone mass index are risk factors for the development of OA of the knee. 

 The major symptom of OA is pain which is localized to the knee. It is exacerbated by activity, and is relieved by rest.  Patient experiences pain when rising from a seated position, which may be aggravated with climbing stairs or bending the knee. As the disease progresses, pain may occur at rest, and during the night. Some patients can experience pain with weather changes.

Stiffness upon awakening or after a prolonged period of rest is a common complaint (gelling phenomenon). It usually lasts for less than half an hour and is relieved with activity. Patients can sometimes experience an acute flare of symptoms, manifested by worsening pain and joint swelling.  However, an abrupt onset of warmth, erythema or swelling of the joint should prompt a search for additional causes of symptoms (gout, pseudogout, trauma, infection) and should warrant consideration of an arthrocentesis.

Progressive loss of motion can occur. Patient may report a sensation of sudden giving way of the knee.  Such symptoms may arise as a result of progression of articular cartilage loss but can also be related to quadriceps weakness, which is associated with OA of the knee and can lead to falls.  Symptoms of buckling or catching could indicate meniscus tears and need to be evaluated further with MRI since patients with these symptoms may require surgical intervention.

On physical examination, there is bony enlargement and localized joint tenderness. Pain with passive range of motion is a common finding. Crepitus or a grating sound or feel can occur as the joint is moved. A cool effusion may be present.  Limitation of knee extension on both active and passive range of motion may be noted. Muscle atrophy can occur rapidly and is most often related to disuse. Disproportionate involvement of the medial or lateral compartment may lead to genu varus (bowed legs) or genu valgus (knocked knees), respectively.

Red flags

     Don’t miss this:

1-    The presence of fever, knee swelling, warmth and overlying erythema, should prompt a work-up for an associated inflammatory conditions, such as infection, gout or calcium pyrophosphate deposition (CPPD) disease.  PERFORM ARTHROCENTESIS. 

2-    Posterior knee pain and swelling radiating to the calf may be due to a ruptured Baker cyst.

Epidemiology

The National Arthritis Data Workgroup estimated that in 2005, 9.3 million US adults’ ages 26 years or older had symptomatic knee OA.

The prevalence rises with age, and is more common in women than men, with a female-to male incidence ratio of 1.7: 1.  The prevalence of knee OA may be higher in blacks than in whites.

Knee OA has a significant economic impact.  OA of the knee is a common reason for ambulatory care visits and hospitalization for costly total knee replacement surgery. Pain and functional limitation affect quality of life, leading to disability, lost earnings, and the need for chronic pain management. Patients with OA are at higher risk of developing peptic ulcer and renal disease due to their frequent use of NSAIDS.

Pathology and pathophysiology

Osteoarthritis has traditionally been thought of as affecting primarily the cartilage, but changes also occur in the subchondral bone and synovial fluid.

The surface of the joints is covered by a thin layer of cartilage. The cartilage is comprised of chondrocytes and a matrix, rich in collagen and proteoglycans. The cartilage distributes the load applied to the joint, reduces friction within the joint, and protects the underlying bone.

In early stages of OA, there is a gradual degradation of the matrix, leading to softening, fissuring and eventually loss of the cartilage.  As the cartilage wears, it exposes the underlying subchondral bone. The subchondral bone responds to the increased loading stress. It becomes thickened and denser and subchondral cysts can develop.

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Osteophytes develop at the joint margins. Fragmentation of the osteophytes results in the presence of intra-articular loose bodies.

Although OA is considered a non-inflammatory disease, there is increasing evidence of an upregulation of inflammatory pathways throughout the pathogenesis of OA. Cytokines and matrix metalloproteinases are released in the joint and contribute to matrix degradation.

Differential diagnosis

Rheumatoid arthritis (RA): The knee can be swollen, warm, and tender in RA. Stiffness is worse after resting the knee. Rheumatoid factor (RF) and anti-cyclic citrullinated peptide (anti-CCP) antibodies are usually present. Radiographs reveal periarticular demineralization, erosions and symmetric joint space narrowing.

CPPD: Calcification of the articular cartilage (chondrocalcinosis) on plain radiographs may be seen, particularly at the wrists and knees. Synovial fluid analysis reveals the presence of positively birefringent crystals on polarizing light microscopy.

Septic arthritis: The knee is often abruptly warm, swollen, and tender. There may be an antecedent history of infection elsewhere (sepsis, pneumonia, endocarditis, cellulitis) or, far less commonly, penetrating trauma with direct inoculation.  Fever can be present.  CRP and ESR are often increased.  The synovial fluid is inflammatory, usually with WBC>10,000 cells/mm3 on light microscopy.  

  Etiology

Predisposing factors in addition to age and weight:

1-                  Congenital or developmental (leg length inequality) 

2-                  Trauma  

3-                  Inflammation from conditions such as rheumatoid arthritis, gout, infection

4-                  Endocrine: diabetes, acromegaly

5-                  Metabolic: hemosiderosis, ochronosis

6-                  Avascular necrosis

7-                  Neuropathic (syphilis, diabetes)

Radiographic and laboratory findings

There are no specific laboratory abnormalities associated with osteoarthritis. Complete blood count (CBC), serum chemistry and ESR are normal unless there is an associated disease.

Synovial fluid is non-inflammatory with WBC count less than 2000/mL.

The diagnosis of OA relies heavily on the history and is confirmed bye the physical examination and radiographic findings. 

Radiographs of the knees are usually the first and the most important step in the diagnostic evaluation. The most specific sign is the presence of osteophytes. Other early findings include: joint space narrowing, subchondral sclerosis, and subchondral cyst formation (geodes). In advanced stages, there can be complete loss of the joint space.

MRI of the knees can be used in early OA evaluation but is not required.  It can be most useful when trying to assess for the presence of meniscal or ligaments tears, loose bodies or a Baker’s cyst.

Risk factors and prevention

Risk factors:

Age > 50 years

Female gender

Family history

Previous knee injury

Obesity

Knee malalignment

Joint laxity

Occupational or recreational overuse: heavy lifting, excessive kneeling,      squatting

Inflammatory disease: rheumatoid arthritis, gout, pseudogout Hemochromatosis

Prevention:

Weight loss

Avoid injury in occupational and recreational activities

Treatment options

1. Weight loss recommended for obese patients
2. Exercise regimen:

1. a.   Low impact aerobic exercise
2. b.   Quadriceps strengthening
3. c.   Aquatic therapy

1.  Analgesics:

1.  
   1. Topical NSAIDS or capsaicin
   2. Acetaminophen (< 3 g/d)
   3. Non-steroidal anti-inflammatory drugs
   4. Opioids if acetaminophen and NSAIDS are ineffective or are      contraindicated
      
2. Intra-articular treatment

1. a.   Intra-articular corticosteroid injection for temporary relief
2. b.   Intra-articular hyaluronic acid injection, also called viscosupplementation (conflicting evidence for efficacy)

1. 5.           Surgery:   
   1. a.   Total knee replacement for disabling pain
   2. b.   Arthroscopy for removal of loose bodies and partial meniscectomy
   3. c.       Realignment osteotomy for patients who are active and have unicompartmental OA

Outcomes

 
Weight loss, avoidance of excessive use of the joint, and physical therapy can halt or delay the progression of OA in certain patients. NSAIDS alleviate pain, but frequent use increases the risks of gastrointestinal bleed, hypertension, and renal insufficiency.

Total knee arthroplasty is associated with reduction in pain and improvement in function in the majority of patients. The longevity of the prosthesis varies according to comorbid features and the patient’s activity level.  Although estimates are often around 10-15 years, majority prosthetic joints can function well substantially longer. Young and active patients may require more frequent revisions of the prosthesis compared to elderly patients.

Holistic medicine

There is considerable interest in the use of complementary and alternative medicine in the treatment of osteoarthritis.  A variety of interventions may decrease pain and improve function.  Though the strength of the evidence remains controversial, therapies for which there is evidence from randomized, controlled trials to support use include:

• §  Glucosamine/chondroitin
• §  Acupuncture
• §  Knee braces and wedge inserts
• §  Tai chi
• §  Spa therapy and mineral baths
• §  Neutraceuticals: avocado soybean unsaponifiables, methylsulfonylmethane, Harpagophytum procumbens extracts
  (devil’s claw).

Miscellany

Osteoarthritis is the most common form of arthritis affecting the knee.  It has been found in skeletal remains, dating back to the Neolithic times. It is an established public health problem, as it causes significant disability. In the past, many viewed osteoarthritis as an inevitable consequence of aging (it was previously sometimes referred to as degenerative joint disease) with little physicians could offer until patients were ready for knee replacement.

The situation is now changing. Knowledge regarding matrix composition, cell biology, cartilage degradation and repair, as well as the role of obesity and injury in its pathogenesis, is likely to lead to discoveries of biologic markers and disease-modifying drugs.

Key terms

 
Osteoarthritis

Clinical manifestations

Differential diagnosis

Epidemiology

Etiology

Exercise

Knee

Laboratory findings

Obesity

Pain

Pathogenesis

Radiographic findings

Risk factors

Treatment

Skills and competencies  Student must recognize the symptoms of osteoarthritis of the knee: pain, stiffness, swelling, limited function, weakness, deformity, grinding and instability.

Student must learn how to examine a knee, and recognize abnormal signs pertaining to osteoarthritis of the knee: Bony enlargement, tenderness, joint effusion, limitation of motion, crepitus, deformity and malaligment.