Surgery STAT: Common complications after intra-articular injections in horses


Three concerns to be on the look out for in your veterinary patients and how to treat them.

Before you inject, reflect. (Getty Images)

Administration of intra-articular medications or local anesthetics is commonly performed in lameness diagnosis and treatment. The most common complications encountered after joint injections are septic arthritis, flare reaction (aseptic acute synovitis) and periarticular cellulitis.


A quick guide to differentiating among the complications of intra-articular injections in horses

Septic arthritis

  • Clinical signs are typically delayed two to three days after injection, especially if corticosteroids were injected.

  • Synovial fluid with TNCC greater than 30 x 109/L, with more than 80% of neutrophils on a differential count or a total protein concentration greater than 4 mg/dl, should be considered infected until proven otherwise.

  • Even in infected joints, synovial fluid culture results are negative in 50% of cases without enrichment media and in 27% of cases in which enrichment media are used.

Flare reaction (acute aseptic synovitis)

  • Clinical signs are usually present 24 hours after injection and, with appropriate treatment, usually improve in one to three days.

  • Because of the overlap in changes in synovial fluid between septic arthritis and flare reaction, it is recommended to submit culture samples in all cases.

Periarticular cellulitis

  • Clinical signs usually appear in first 24 to 48 hours.

  • The swelling is usually diffuse and affects only subcutaneous tissue without joint effusion in most cases.

  • Synovial fluid is typically normal or has changes consistent with mild inflammation.

  • Culture samples from subcutaneous pockets can be obtained with ultrasonographic guidance.


Septic arthritis

The overall incidence of post-injection septic arthritis has recently been reported to be 0.092% based on a large sample size of cases, which includes a 0.091% incidence for local anesthetic, 0.161% for corticosteroids, 0.159% for hyaluronic acid (HA) and 0.394% for polysulfated glycosaminoglycans (e.g. Adequan-Luitpold Pharmaceuticals).1 Despite the low incidence, septic arthritis can have potentially devastating effects on the patient, making early recognition and treatment of great importance.

Post-injection septic arthritis is diagnosed based on clinical signs, radiography and ultrasonography (if indicated), and cytology and culture of the synovial fluid. Clinical signs (lameness, joint effusion) may be delayed if the joint was injected with corticosteroids, although the changes in synovial fluid are already present.2 The gold standard for diagnosing septic synovitis is a positive bacterial culture result; however, culture results from infected joints are negative in almost 50% of clinical cases when enrichment media are not routinely used and in 27% when they are.3 Because of the limitations of bacterial isolation, cytologic examination of synovial fluid is the single most useful means of diagnosis.4

With synovial sepsis, the total nucleated cell count (TNCC) is usually greater than 10 x 109/L and often much higher.5 Most cells are neutrophils, yet degenerative changes are often not present, possibly because bacterial toxins do not occur in synovial fluid to the same extent that they do in other septic bodily fluids.5 As a general rule, synovial fluid with a TNCC greater than 30 x 109/L, more than 80% neutrophils, or a total protein concentration greater than 4 mg/L should be considered infected, particularly if these findings correlate with clinical signs and predisposing circumstances.6

The serum amyloid A (SAA) concentration in synovial fluid is significantly higher in horses with septic synovitis than in normal horses and those with low inflammation joint conditions.7 Furthermore, since synovial fluid SAA concentration does not change in response to repeated arthrocentesis, it has potential as a useful marker of synovial sepsis (although further validation is required).7

As previously mentioned, corticosteroid injection can delay the onset of clinical signs and the increase in synovial fluid total protein concentration and TNCC in horses with septic synovitis, particularly during the initial few days. Neutrophils greater than 90% in a differential count and a pH of less than 6.9 may be the most reliable parameters for diagnosing septic arthritis after corticosteroid injection.2

Flare reactions

Chemical synovitis, often referred to as joint flare, can occur in response to the injection of corticosteroids, local anesthetics, HA and polysulfated glycosaminoglycans.

After serial arthrocentesis and injection of local anesthetic, gentamicin sulfate, ceftiofur sodium or amikacin, clinicopathologic values (TNCC, percentage of neutrophils and total protein concentration) in the synovial fluid of equine joints increase to greater than reference ranges and can approach values suggestive of septic arthritis.5 Thus, prior sampling and treatment may complicate interpretation of synovial fluid analysis.

Early onset of clinical signs (around 24 hours), a synovial TNCC less than 30 x 109/L, and resolution within one to three days have been suggested to be helpful in differentiating chemical synovitis from septic arthritis.8 Clearly, overlap with the findings expected with septic synovitis may still occur, and repeated lameness examinations and follow-up sampling are appropriate. Marked lameness and synovial effusion can occur with a nonseptic flare, so acute synovitis after arthrocentesis is best treated as septic arthritis if doubt exists.

The overlap among clinicopathologic values in septic synovitis with a low TNCC (as may occur in early septic arthritis), septic arthritis resulting from corticosteroid injection into a synovial structure or infection with an organism of low virulence, and nonseptic arthritis that can occur secondary to trauma or therapeutic intervention (e.g. synoviocentesis alone, injection of medications) can make differentiating among these diagnoses impossible.

Lameness associated with flare reactions ranges from mild to severe. In some horses, distinguishing reactive arthritis from infectious arthritis may be difficult, and prompt management with intra-articular lavage, systemic and local antimicrobial drug administration and anti-inflammatory therapy (systemic) should be instituted. Bacterial culture and susceptibility testing should be performed if any suspicion exists that a bacterial infection is present or if synovitis does not resolve quickly as well as in horses in which lameness persists beyond the one to three days that typically are required for resolution of a flare. My and others' clinical experience suggests that concurrent systemic NSAID administration helps reduce the risk of flare reactions after joint injections.

Periarticular cellulitis

Occurrence of periarticular cellulitis is rare after joint injection, but it should not be overlooked when diffuse swelling of the area surrounding the injection site develops. Typically horses with cellulitis have marked swelling around the area of injection that may spread to surrounding regions and lameness of the affected limb. The swelling is normally hot and painful on palpation, and sometimes pitting occurs when digital pressure is applied to the affected area.

Diagnosis is based on clinical signs, radiography, ultrasonography and blood work. On ultrasonographic examination there is marked thickening and increased echogenicity of the subcutaneous tissue, sometimes accompanied by fluid pockets associated with the injection sites.

Fluid aspiration of the pockets for culture is recommended to direct the antimicrobial therapy. Conservative treatment consisting of supportive local treatment (bandages, cold water therapy) and systemic NSAIDs and antibiotics (selected based on bacterial culture and susceptibility testing of the fluid obtained from the fluid pockets identified with ultrasound) should result in gradual improvement over five to seven days.


The three most common problems associated with intra-articular injection in horses are septic arthritis, flare reactions and periarticular cellulitis. These conditions can be difficult to differentiate from one another, but early recognition, use of the appropriate diagnostic aids and prompt treatment will result in an increased likelihood of successful outcome.

Dr. Abuja is an equine surgeon with Rhinebeck Equine in New York. Dr. Abuja's main areas of research interest are minimally invasive surgical techniques and surgical implants. In addition to surgery, he also enjoys the challenges of lameness diagnosis, sports medicine and multiple types of imaging. Dr. Abuja is married to Dr. Leah Steinberg, who is also a veterinarian, and they have three children. Dr. Abuja enjoys include spending time with family and friends, traveling, and music.



1. Bohlin AM. Infectious arthritis following intra-articular injection in horses not receiving prophylactic antibiotics: A retrospective cohort study of 2833 medical records, in Proceedings. Am Assoc Equine Pract 2014;60:255-257.

2. Tulamo RM, Bramlage LR, Gabel AA The influence of corticosteroids on sequential clinical and synovial fluid parameters in joints with acute infectious arthritis in the horse. Equine Vet J 1989;21(5):332-337.

3. Schneider RK, Bramlage LR, Moore RM, et al. A retrospective study of 192 horses affected with septic arthritis/tenosynovitis. Equine Vet J 1992;24(6):436-446.

4. Morton AJ. Diagnosis and treatment of septic arthritis. Vet Clin North Am Equine Pract 2005;21(3):627-649.

5. Steel CM. Equine synovial fluid analysis. Vet Clin North AmEquinePract 2008;24(2):437-454.

6. Bertone AL. Infectious arthritis. In: McIlwraith C, Trotter G, eds. Joint disease in the horse. Philadelphia: WB Saunders, 1996;397-409.

7. Jacobsen S, Thomsen MH, Nanni S. Concentrations of serum amyloid A in serum and synovial fluid from healthy horses and horses with joint disease. Am J Vet Res 2006;67(10):1738-1742.

8. Bertone AL. Non-infectious arthritis. In: Ross MW, Dyson SJ, eds. Diagnosis and management of lameness in the horse. Philadelphia: WB Saunders, 2003;606-610.

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