This is a previously healthy 4-year old female who is 
brought to the ED by her mother because of fever and 
right leg pain since the previous night.  The patient 
stated that she had been swimming the previous day, 
and had slipped in the pool, twisting her right leg.  
Immediately after that, however, she was able to walk 
around easily without pain.  Later that night she 
developed a fever (temperature not measured), and her 
right leg began to hurt.  She was taken to an ED 
because she had difficulty walking.  She was diagnosed 
with a hip sprain and was sent home on ibuprofen.  Her 
fever and leg pain worsened to the point that she was 
no longer able to ambulate, prompting a return to the 
ED.
     Exam  VS:  T40.7, P164, R40, BP 139/84.  She is 
anxious and in obvious discomfort, although she 
appears alert and non-toxic.  She prefers to keep her 
right hip abducted and externally rotated, with the right 
knee flexed.  She refuses to bear any weight on the 
right leg.  Range of motion of the right hip is severely 
limited, especially internal rotation, adduction, and 
extension.  The overlying skin is warm but not 
erythematous. Examination of the remainder of the right 
lower extremity, the left lower extremity, and spine are 
within normal limits.

View the patient's general appearance.

     Laboratory Results: CBC WBC 31,700 with 4% 
bands, 85% segs, 6% monos, and 5% lymphs.  Hgb 
12.4, Hct 35.4.  Platelet count 394,000.  CRP 8.2.  ESR 
39.  Hip radiographs were obtained.

View hip radiographs.

     Think about the most likely diagnosis at this time.  
The photo of her general appearance shows her 
preferred position of comfort with her hip in external 
rotation.  What radiographic abnormalities would you 
expect to see?
     Her hip radiographs show a widened joint space in 
her right hip.  Compare the joint space to the left hip 
and it is rather obvious here.  [Use the "tear drop" 
distance method as described in Case 15 of Volume 4 
(Osteoid Osteoma), to measure the joint space.]  This 
finding is seen in the minority of patients with septic 
arthritis; thus, the absence of joint space widening 
cannot be relied upon to rule out septic arthritis. 
     An orthopedic consultation is obtained.  Under 
fluoroscopic guidance, the right hip is needle-aspirated, 
revealing grossly purulent fluid.  Laboratory analysis:  
80,000 WBC's with 88% segs, 1% monos, and 11% 
lymphs.  16,000 RBC's.  Gram stain shows many gram 
positive cocci in chains.
     What organisms are the most common causes of 
septic arthritis ?  Given the laboratory results, what 
organism is likely in our patient ?  What antibiotic 
regimen would be appropriate for patients with septic 
arthritis ?
     The patient is placed on IV cefuroxime and 
hospitalized for operative drainage of the right hip. 
Cultures of the aspirate grow out group A 
beta-hemolytic strep.

Teaching Points and Discussion:

Diagnosis
     1.  Early diagnosis and treatment of septic arthritis of 
the hip cannot be overemphasized.  Fibrous deposition 
and lysosomal enzymes from polymorphonuclear 
leukocytes in synovial fluid can rapidly damage 
cartilage.  Permanent damage will occur in only a few 
days if the diagnosis and treatment are delayed.

View later changes. 

     This radiograph of a different patient at follow-up 
shows the result of a poorly treated septic hip.  The 
patient's right hip shows degenerative changes.  There 
is flattening of the femoral epiphysis with an irregular 
articular surface.  Early effective treatment can 
minimize such subsequent complications.
     2.  The diagnosis is particularly difficult to make in 
infants, who cannot voice their complaints, and 
especially in neonates who may not have the typical 
findings of fever, chills and leukocytosis.  Don't be 
fooled by a history of trauma.  Parents of toddlers will 
almost always recall a recent "injury" prior to the onset 
of symptoms.  Such a history is not sufficient to rule out 
septic arthritis.
     3.  The affected extremity will typically be held in a 
position of slight flexion, abduction and external 
rotation. This maximizes the intraarticular space, thus 
decreasing the tension of the joint exudate.  Refer to 
the photo of our patient in this position of comfort.  Click 
on [Patient]
     4.  Laboratory findings include an elevated 
erythrocyte sedimentation rate and/or C-reactive 
protein, as well as a leukocytosis, often with a "left 
shift".  The left shift (a high percentage of immature 
granulocytes) is not reliable and is noticeably absent in 
our patient.  Blood cultures may be positive in up to 
50% of cases.  The most important procedure in 
making the diagnosis, however, is hip aspiration 
(arthrocentesis).  If the aspirated synovial fluid contains 
more than 20,000 white blood cells and has greater 
than 75% polymorphonuclear leukocytes, infection 
should be strongly suspected.  Synovial glucose 
content less than 1/2 of serum and/or protein greater 
than 1/3 of serum are suggestive of septic arthritis.  
Positive results of direct smear and cultures of synovial 
fluid are diagnostic.  Joint fluid cultures are positive in 
about 60-80% of cases.
     5.  Don't expect plain radiographs to establish the 
early diagnosis of septic arthritis.  Plain films may 
demonstrate a widened hip space, but the absence of 
this does not rule out septic arthritis.  Lateral 
displacement of the proximal femur due to synovial 
exudate and inflammation can also be seen in the early 
stages.  Late findings include periostitis, periarticular 
osteoporosis, and dislocation of the femoral head.  
Ultrasound is the most effective study in demonstrating 
the presence of a joint effusion acutely.  Radionuclide 
bone scanning can be helpful in the early stages, but 
the definitive diagnostic tool remains isolation and 
identification of the organism by hip aspiration.

Pathogenesis
     1.  The most common etiologic organism is 
Staphylococcus aureus.  Other organisms include 
Haemophilus influenzae type B (more common in 
younger patients), Streptococcus pyogenes (group A 
beta-hemolytic strep), Neisseria gonorrhea (more 
common in young adults), pneumococcus, and 
salmonella.
     2.  The infection may reach the hip joint by either 
hematogenous spread, or from a focus of osteomyelitis 
in the femoral neck or (less commonly) the ilium.  
Hematogenous spread is more common in children 
than in adults.

Treatment
     1.  The treatment for septic arthritis of the hip 
includes intravenous antibiotics and immediate surgical 
incision and drainage of the hip, followed by a period of 
immobilization.  Antibiotics should be continued for at 
least 4-6 weeks, depending on the sensitivities of the 
organism and the patient's clinical status.
     2.  The initial choice of antibiotics should take into 
account the most common etiologies of septic arthritis.  
In the case presented above, cefuroxime was used to 
cover streptococci and staph aureus.  Cefuroxime 
(similarly cefotaxime and ceftriaxone) would also cover 
H. influenzae type B.  An anti-staphyloccocal penicillin 
(nafcillin, oxacillin) could also have been selected to 
cover streptococci and staph aureus.  However, given 
the growing frequency of methicillin resistant staph 
aureus, empirically starting vancomycin pending culture 
results and antibiotic sensitivities may be justified as 
well.

Differential Diagnosis
     1.  Transient ("toxic") synovitis can present with 
intense pain, refusal to walk, and limited and/or painful 
range of motion.  A bone scan may help to rule out a 
septic hip, but if septic arthritis is suspected, the hip 
should be tapped and the patient should be given IV 
antibiotics.  A brief period of observation can also be 
helpful.  If there is dramatic improvement in 24 hours, 
the diagnosis is most likely to be transient synovitis.
     2.  Osteomyelitis can sometimes cause a 
sympathetic effusion of the joint, without an actual 
infection of the joint itself.
     3.  Acute rheumatic fever can also present with 
acute arthritis of the hip.  However, there is usually a 
migrating pattern to the arthritis.  Other findings 
consistent with rheumatic fever may be present, such 
as a heart murmur suggestive of acute carditis, 
subcutaneous nodules, erythema marginatum, or 
chorea.  The Jones' criteria may be helpful in making 
this diagnosis.
     4.  Hemophilia with bleeding into the joint space may 
be difficult to distinguish from septic arthritis.  However, 
the patient is usually a known hemophiliac.  This 
condition requires urgent decompression of the joint 
space to prevent permanent damage to the femoral 
head.
     5.  Other causes of refusal to walk associated with 
fever include diskitis, new-onset leukemia, peri-rectal 
abscess, and osteomyelitis.             

References:
     1.  Salvati E. Neonatal and Infantile Septic Arthritis.  
In:  Tronzo R (ed).  Surgery of the Hip Joint.  New York, 
Springer-Verlag, 1984, pp. 387-403.
     2.  Simon RR, Koenigsknecht SJ (eds).  Emergency 
Orthopedics, third edition.  Norwalk, CN, Appleton & 
Lange, 1995, pp. 421-423.
     3.  Sponseller PD.  Bone, Joint and Muscle 
Problems.  In:  Oski F, et al (eds).  Principles and 
Practice of Pediatrics.  Philadelphia, PA, Lippincott, 
1994, pp.1042-1043. 
     4.  Paterson D.  Septic arthritis of the hip joint.  
Orthopedic Clinics of North America 1978;9(1):135.