Bone and joint sepsis is frequently seen first by the primary care physician. It's manifestations are variable and the diagnosis may be unclear. The findings vary according to age, location, organism, and the general condition of the host. Laboratory and x-ray studies are not consistent. Treatment options include antibiotics alone or combined with surgical drainage. The choice of appropriate antibiotics and length of treatment must be individualized.

Septic Arthritis

This condition typically involves major weight bearing joints, primarily the hip and knee. Ninety five per cent of cases occur in a single site, most commonly the hip in an infant and the knee in a child. Infection reaches the joint either from bacteremic seeding or from extension of metaphyseal osteomyelitis into the joint space. Rarely, the joint is contaminated from a penetrating wound.

The clinical presentation varies with age. In an infant, it is often difficult to localize the symptoms to the joint. The infant will appear listless, irritable, feed poorly, and may have an elevated temperature. There is almost always protective muscle spasm and the infant does not voluntarily move the joint (pseudo paralysis). The hip usually assumes a position of flexion, abduction, and external rotation and there may be local signs of swelling, asymmetrical skin creases, and pain with motion.

In the child, the onset is acute with high fever and systemic illness. There is localized swelling and pain around the joint with limited motion and cessation of weight bearing. Laboratory workup includes a complete blood count, erythrocyte sedimentation rate, blood culture, and x-ray of the involved joint. The white count is often elevated. The sedimentation rate is significantly increased (>50 mm./hr.) in ninety per cent of cases. Blood culture is positive in fifty per cent and the x ray may show soft tissue swelling and joint distention.

All children with joint pain, swelling, and limited motion in the presence of clinical signs of infection or systemic toxicity should have a joint aspiration. The joint fluid analysis will show marked elevation of white blood cells (>50,000) and polymorphonuclear leukocytes (>95%) with a marked decrease in glucose compared to the serum level. A gram stain will provide a presumptive diagnosis in thirty per cent of cases. In one third of cases, no organism will be isolated.

The most commonly identified organisms and the appropriate antibiotics for treatment are shown in Table 1. Staph. aureus is the most common pathogen in neonates and children over age two while H. flu is most frequently found in those between age one month and two years.

The differential diagnosis is especially important as it dictates treatment modalities. Juvenile rheumatoid arthritis often involves multiple joints and the synovial fluid analysis shows fewer white blood cells and polys. Acute rheumatic fever presents with migratory joint involvement, EKG changes, and elevated ASO titer. Hemophilia will present with an inflamed joint but a bloody effusion. The most difficult differential involves non specific or transient synovitis of the hip. Usually, the onset of symptoms is less acute, there is no systemic illness, it doesn't occur in non walkers, the child is limping but walking, and hip motion is less painful. Elevated temperature is rare and the sedimentation rate is lower.

Early intervention in septic arthritis is critical not only to relieve the acute symptoms but to preserve joint function. It is well recognized that cartilage can be destroyed by enzymes released by bacteria and also is adversely affected by the products of the inflammatory reaction that occurs to combat the infection. The joint effusion in an infant's hip may mechanically impede the blood supply and cause avascular necrosis of the femoral head if not expeditiously decompressed.

Hence the treatment of septic arthritis requires sterilization and decompression of the joint as well as removal of the degradation products and debris of the inflammatory process to halt tissue destruction and preserve the integrity of the joint. Aspiration and irrigation may be used in early infection in an accessible joint that is easily monitored locally and in cases that improve rapidly with such treatment. The knee can often be treated arthroscopically by this means. The hip must always be surgically debrided as it is not accessible to external irrigation techniques and is not easily monitored. Furthermore, delayed diagnosis is common in the infant's hip and the possibility of avascular necrosis from swelling necessitates an immediate incision and drainage. The duration of antibiotic treatment relates to the adequacy of joint cleansing and the clinical response to the medication. Usually a course of parenteral antibiotic for one week followed by at least two weeks of oral dosage is adequate.

Acute Hematogenous Osteomyelitis

All ages are susceptible to osteomyelitis but the majority of cases occur between ages three to twelve. Involvement of long bones is characteristic with sixty five per cent of cases occurring in the femur, tibia, or humerus. Trauma and recent illness or infection often are precursors in some non specific fashion. Most cases occur in the metaphysis of rapidly growing bones where there is a relative paucity of phagocytic cells in the compact cancellous structure. This area is also rich in terminal arteriolar loops where the blood flow is slower, venous lakes abound, and the oxygen tension is low. These factors produce an environment conducive to bacterial proliferation.

The specific cause of osteomyelitis is probably multifactorial. Inflammation can be rapidly followed by abscess formation and intramedullary bone destruction. Decompression of the abscess may occur spontaneously through the thin cortex of the metaphysis, elevating the loose periosteum and allowing sub periosteal spread of the infection. A child's periosteum is very thick and strong and contains the abscess beneath it. This leads to impairment of the vascular supply to the cortical bone, cortical ischemia, necrosis, and sequestrum formation.

However, the periosteal blood supply from muscle remains intact and new bone, known as involucrum, is laid down. Pus usually does not spread down the medullary cavity because it is blocked by the inflammatory response m that area. It may spread to an adjacent joint through the growth plate of an infant (prior to the formation of the secondary ossification center) or through the metaphyseal cortex if it is intracapsular such as in the proximal humerus or femur.

The usual clinical presentation is the sudden onset of deep, poorly localized metaphyseal pain in a child who shows few signs of systemic illness. Point tenderness can usually be elicited over the involved area. At this point, the diagnosis of osteomyelitis must be considered and a rapid diagnostic workup carried out. The sedimentation rate is elevated in ninety per cent of cases, but may be normal on the first day of symptoms. The white blood cell count is normal in forty to seventy five per cent of patients and the initial blood culture is rarely positive. X ray changes in bone are usually not seen for ten to fourteen days after the onset of symptoms although some soft tissue swelling or obliteration of fascial planes may be appreciated at two to four days. Bone scans are positive in one to two days and are ninety to ninety five per cent accurate. Subperiosteal or metaphyseal aspiration is the definitive diagnostic procedure. This allows confirmation of the diagnosis, decompression of the abscess, and provides a specimen to determine appropriate antibiotic therapy.

The culture is positive in seventy per cent of cases. Staph aureus is the pathogen in seventy to seventy five per cent of all cases. Under age two, H. flu and strep must be considered. There may be a concomitant meningitis present with H. flu. In sickle cell disease, the involvement is usually diaphyseal and salmonella is often isolated. Osteomyelitis resulting from puncture wounds of the foot frequently grow out pseudomonas. Bone infection in the newborn is often related to invasive procedures such as arterial or venous catheterizations. In the presence of an immature immune system, infection is poorly controlled and may spread to multiple bone and joint sites. Group B strep is the most common pathogen in neonates, but staph aureus and gram negative coliforrns also occur.

Treatment principles are based upon delivering a sufficient level of antibiotic to the involved area to halt the destructive process. Aspiration to obtain the organism and identify the optimal bactericidal agent is essential. If pus is obtained, surgical drainage is indicated since purulent material diminishes antibiotic effectiveness either through direct reaction or by production of B lactamase.

If there is no abscess, antibiotics alone may be used provided there is a distinct clinical improvement in twenty four to thirty six hours. If not, surgical drainage is indicated.

Appropriate initial antibiotic therapy is outlined in Table 2. There is no difference in the effectiveness of oral and parenteral antibiotics provided that an adequate serum concentration is achieved. Oral antibiotics are not recommended for initial treatment of bone infection because it is critical to achieve an adequate serum level promptly. With the oral route, there is the risk of inadequate absorption of the drug. Antibiotics in the blood enter bone at levels equal to those in the serum, except for dead bone. It also appears that there is adequate diffusion into abscesses and pus (although pus will deactivate the drug).

The usual clinical presentation is the sudden onset of deep, poorly localized metaphyseal pain in a child who shows few signs of systemic illness.

Table 1: Initial Antibiotic Therapy in Septic Arthritis

Table 2: Initial Antibiotic Therapy in Osteomyelitis

All ages are susceptible to osteomyelitis but the majority of cases occur between the ages of three to twelve.

The usual regimen of antibiotic use for osteomyelitis calls for parenteral administration for five to seven days. If there is a significant clinical response, oral antibiotics are started and adequacy of serum concentration is determined. Dosage is adjusted to obtain a peak bactericidal titer of at least 1:8. Treatment is continued for at least four weeks. If clinical evolution is slow and the sedimentation rate is still elevated, antibiotics should be continued until these parameters are normal. Complications may result from inadequate treatment. These include chronic osteomyelitis, growth disturbance, septic arthritis, and pathologic fracture.

Summary

Infection involving bones and joints is relatively common in children and can result in a serious, permanent disability. It is imperative to be suspicious of this entity in any child with sudden onset of localized metaphyseal or joint pain and loss of function in that region, especially in the presence of mild or severe systemic symptoms. Aspiration of the bone or joint is essential to make the diagnosis.

Treatment modalities must effect an end to tissue destruction as rapidly as possible to prevent complications. A combination of antibiotics and surgical drainage is usually required.