A 58-year-old woman comes to the clinic due to a 6-month history of left knee pain that is worse with activity. She initially had pain only with ambulation but has progressively developed rest pain at night. In addition, the patient has had intermittent morning stiffness of 10-15 minutes duration. There is no associated fever or weight loss. Medical history is significant for hypertension, type 2 diabetes mellitus, gastroesophageal reflux disease, and obstructive sleep apnea. Current medications include hydrochlorothiazide, lisinopril, omeprazole, atorvastatin, insulin glargine, metformin, and aspirin. Temperature is 37.2 C (98.9 F) and blood pressure is 146/86 mm Hg. Examination shows a mild effusion, tenderness, and decreased range of motion of the left knee. A fluctuant swelling along the posterior aspect of the knee is palpable. Arthrocentesis with synovial fluid analysis reveals the following:
| White blood cells | 1100/mm3 |
| Gram stain | no organisms |
| Crystals | none |
Plain film x-ray of this patient's knee joint would most likely reveal which of the following?
Joint fluid characteristics | ||||
Normal | Noninflammatory (eg, OA) | Inflammatory (eg, crystals, RA) | Septic joint | |
Appearance | Clear | Clear | Translucent or opaque | Opaque |
WBCs (mm3) | <200 | 200-2,000 | 2,000-100,000 | 50,000-150,000 |
PMNs | <25% | 25% | Often >50% | >80%-90% |
OA = osteoarthritis; PMNs = polymorphonuclear leukocytes; RA = rheumatoid arthritis; WBCs = white blood cells. | ||||
This patient with chronic knee pain and a bland synovial effusion (no organisms, <2,000 white blood cells/mm3) has osteoarthritis (OA), a progressive disorder that commonly affects the hands and weight-bearing joints. Major risk factors include obesity, advancing age, diabetes, and prior joint injury. Most patients have pain with weight bearing and few associated symptoms, although a subset can develop swelling, rest pain, and brief (<30 minutes) morning stiffness. Examination shows crepitus, periarticular bony enlargement, and painful or decreased range of motion. X-ray findings include narrowed joint space, osteophytes, and subchondral sclerosis/cysts.
Synovial effusions are common in older patients with OA; communication of the joint space with the gastrocnemius or semimembranosus bursa allows the synovial fluid to flow posteriorly into the bursa, forming a popliteal (Baker) cyst. Effusions can also form in a variety of other arthritic conditions (eg, rheumatoid arthritis [RA], crystalline arthritis) or knee injury. Arthrocentesis can be used to distinguish the underlying etiology; joint fluid cell counts in osteoarthritis are usually significantly lower than in RA or crystal-induced arthritis.
(Choices A and E) Calcification of joint cartilage (chondrocalcinosis) is seen in calcium pyrophosphate dihydrate deposition (pseudogout); synovial fluid shows an inflammatory effusion with rhomboid, positively birefringent crystals. Gout causes an inflammatory effusion and needle-shaped, negatively birefringent crystals; x-rays characteristically show punched-out erosions with a rim of cortical bone. Both gout and pseudogout are characterized by recurrent flares of pain rather than chronic, progressive pain.
(Choice C) X-ray in septic arthritis will show a normal joint space with swelling of adjacent soft tissues. Synovial fluid characteristically has a very high cell count with visible organisms on Gram stain.
(Choice D) RA can cause popliteal cyst formation, but the synovial fluid in RA shows an inflammatory effusion. In addition, it typically also involves the small, nonweightbearing joints of the hands and is associated with systemic symptoms (eg, fever, fatigue), prolonged (vs brief) morning stiffness (eg, ≥30 minutes), and synovitis (warm, spongy joints). X-rays reveal periarticular osteopenia with erosions of the joint margin.
Educational objective:
Physical examination in osteoarthritis shows crepitus, periarticular bony enlargement, and painful or decreased range of motion. X-ray findings include a narrowed joint space, osteophytes, and subchondral sclerosis/cysts. Patients can also have a bland synovial effusion (no organisms, <2,000 white blood cells/mm3).