Hip infections are caused by bacteria that reach the hip joint through one of three routes: the bloodstream, direct entry during surgery or injection, or spread from a nearby infected tissue. The most common culprit is Staphylococcus aureus, which along with related staph species accounts for roughly 60% of all hip joint infections. Understanding how bacteria get into the joint, and what makes some people more vulnerable, helps explain why these infections develop and who’s most at risk.
How Bacteria Reach the Hip Joint
The hip is a deep, well-protected joint, so bacteria need a pathway in. There are three main routes. The most common in people who haven’t had surgery is hematogenous spread, meaning bacteria travel through the bloodstream from an infection elsewhere in the body and settle in the joint. A skin infection, urinary tract infection, pneumonia, or even bacteria from the gums or gastrointestinal tract can seed the hip this way. Certain streptococcal species linked to dental and gut sources have been identified in hip infections that started through the bloodstream.
The second route is direct inoculation. This happens when bacteria enter the joint during a procedure, whether that’s hip replacement surgery, a steroid injection, or joint aspiration. Bacteria from the skin or operating environment can be introduced directly into the joint space.
The third route is contiguous spread, where an infection in tissue right next to the hip, such as a deep wound, abscess, or bone infection, extends into the joint itself.
Infections After Hip Replacement
Hip replacement is one of the most common reasons people develop a hip infection. A large meta-analysis covering more than 1 million hip replacement procedures found a surgical site infection rate of about 1.9%. That number may sound small, but given how many hip replacements are performed each year, it represents a significant number of people.
Prosthetic joints are more vulnerable to infection than natural joints. The artificial surface gives bacteria something to cling to, and once they attach, they form a protective layer called a biofilm. This biofilm shields the bacteria from both the immune system and antibiotics, making prosthetic hip infections notoriously difficult to treat.
These infections fall into timing categories that hint at their cause. Early infections, appearing within three months of surgery, typically come on suddenly with swelling, pain, redness, fever, and chills. They’re almost always caused by bacteria introduced during the operation itself. Delayed infections, showing up between three and 12 months after surgery, are also likely acquired during or shortly after the procedure but involve slower-growing organisms. Late infections, occurring more than a year after surgery, are more often caused by bacteria that traveled through the bloodstream from a distant source: a skin infection, a urinary tract infection, or occasionally an invasive medical procedure.
Who Is Most at Risk
Certain health conditions make the hip joint more susceptible to infection. A weakened immune system is the single biggest risk factor. This includes people with diabetes, kidney disease, liver disease, or anyone taking medications that suppress immune function, such as drugs for organ transplants or autoimmune conditions.
Existing joint problems also raise the risk. Osteoarthritis, gout, rheumatoid arthritis, and lupus all make the joint environment more hospitable to bacteria. Rheumatoid arthritis carries a double risk because the disease itself damages joints and the medications used to treat it often dampen the immune response. Older age is an independent risk factor as well, likely because immune function declines and joint wear accumulates over time.
For people with hip replacements, additional risk factors include obesity, prior joint surgery at the same site, prolonged operative time, and having an active infection anywhere in the body around the time of surgery.
The Bacteria Behind Hip Infections
Staphylococcus aureus is the dominant pathogen, responsible for more hip infections than any other single organism. Coagulase-negative staphylococci, a related group commonly found on skin, are also frequent offenders, especially in prosthetic joint infections where they excel at forming biofilms. Together, these staph species cause about 60% of cases.
Streptococcal species are the next most common group. Other gram-negative bacteria can also be involved, particularly when the original infection source is the urinary or gastrointestinal tract.
In rare cases, hip infections are caused by organisms outside the usual bacterial suspects. Mycobacterial infections account for roughly 2% of prosthetic hip infections. Among those, Mycobacterium tuberculosis is the most frequently identified species, found in about 43% of mycobacterial hip infection cases in one systematic review. Non-tuberculosis mycobacteria are considered an emerging cause. Fungal infections of the hip are uncommon but do occur, and both fungi and mycobacteria are responsible for a large share of hip infections where standard cultures come back negative, making diagnosis more challenging.
How Hip Infections Are Identified
Diagnosing a hip infection typically starts with blood tests and joint fluid analysis. Two blood markers are commonly used as initial screening tools. One measures how quickly red blood cells settle in a tube (a general inflammation marker), and the other measures a protein the liver produces during infection. The inflammation marker has about 89% sensitivity and 69% specificity for detecting prosthetic hip infections, while the liver protein marker catches slightly more true infections at 93% sensitivity but is less precise, with only 40% specificity. In practice, this means these blood tests are good at flagging possible infections but not great at ruling out other causes of inflammation.
The more definitive test involves drawing fluid from the hip joint with a needle and counting the white blood cells. A count above 100,000 cells per cubic millimeter strongly suggests infection requiring surgical treatment. Counts above 75,000 have traditionally been considered the threshold for septic arthritis. However, counts between 25,000 and 75,000 fall into a gray zone where the diagnosis is less clear, and infected hips can sometimes produce counts lower than the classic threshold. Doctors weigh these numbers alongside the patient’s symptoms, blood work, and fluid cultures to make a final determination.
What Happens if Infection Takes Hold
A hip infection that isn’t treated quickly can cause permanent cartilage damage within days. The joint lining becomes intensely inflamed, and the enzymes released during the immune response break down cartilage and bone. In a natural hip, this can lead to chronic pain, joint destruction, and eventually the need for a hip replacement. In a prosthetic hip, the implant may loosen as the infection erodes the bone around it.
Treatment for hip infections nearly always involves a combination of antibiotics and some form of surgical drainage or washout to physically remove infected material from the joint. For prosthetic infections, the implant sometimes needs to be removed entirely, replaced with a temporary antibiotic spacer, and then swapped for a new prosthesis weeks or months later once the infection clears. This two-stage process is taxing but often necessary because biofilm on the original implant can harbor bacteria that antibiotics alone cannot reach.
Recovery timelines vary widely. A native hip infection caught early may resolve with a few weeks of antibiotics after drainage. A prosthetic infection requiring implant removal can mean months of limited mobility, extended antibiotic courses, and multiple surgeries before a new joint is in place.