What are the differential diagnoses for a 65‑year‑old diabetic man with a six‑month history of a pruritic, non‑progressive lesion near the surgical scar of a total knee replacement performed ten years ago, who has been using clobetasol (clobetasol propionate), mupirocin (mupirocin) and neomycin (neomycin) ointments?

Differential Diagnoses for Pruritic Lesion Near TKR Scar

The most likely differential diagnoses in this diabetic patient with a chronic, pruritic, non-progressive lesion near a 10-year-old TKR scar include: allergic contact dermatitis (particularly to topical antibiotics like neomycin), late-onset prosthetic joint infection with cutaneous manifestations, diabetic dermopathy, and less likely Charcot-like inflammatory changes or localized bullous pemphigoid.

Primary Differential Considerations

Allergic Contact Dermatitis (Most Likely)

• The prolonged use of neomycin-containing ointments is a critical red flag, as neomycin is one of the most common causes of allergic contact dermatitis, particularly in patients with chronic wounds or ulcers 1.
• The 6-month duration with lack of improvement despite multiple topical treatments strongly suggests iatrogenic sensitization rather than therapeutic benefit 1.
• Clobetasol propionate, while highly potent and effective for inflammatory dermatoses, can cause local side effects including skin atrophy with prolonged use, potentially complicating the clinical picture 1, 2.
• The itchy, non-progressive nature is consistent with a chronic allergic reaction rather than an infectious or metabolic process 1.

Late Prosthetic Joint Infection with Cutaneous Manifestations

• Even 10 years post-TKR, late prosthetic joint infection remains a critical consideration, particularly in diabetic patients who have increased infection risk 1.
• The International Diabetes Federation emphasizes that accurate differential diagnosis between osteomyelitis, soft tissue infection, and other conditions is crucial because these require very different treatments 1.
• Cutaneous manifestations near the surgical site may represent a sinus tract or superficial extension of deeper infection, even without obvious purulent drainage 1.
• The Infectious Diseases Society of America notes that diabetic complications may mask systemic manifestations of infection, making osteomyelitis tolerable and potentially unrecognized 1.
• Preoperative hyperglycemia and diabetes significantly increase the risk of infected knee replacement, with diabetic patients showing 4-fold higher infection rates 3.

Diabetic Dermopathy

• Diabetic dermopathy is among the most common cutaneous manifestations in diabetes mellitus patients, occurring in approximately one-third of all diabetic patients 4.
• The chronicity (6 months), non-progressive nature, and location near a site of previous trauma (surgical scar) are consistent with diabetic dermopathy 4.
• However, diabetic dermopathy typically presents as atrophic, hyperpigmented patches rather than pruritic lesions, making this less likely as the primary diagnosis 4.

Localized Bullous Pemphigoid

• The British Association of Dermatologists guidelines note that bullous pemphigoid can present with localized disease and is associated with diabetes as a comorbidity 1.
• The pruritic nature and chronicity could be consistent with early or localized bullous pemphigoid 1.
• However, the lack of blistering and the specific location near a surgical scar make this less likely 1.

Critical Diagnostic Workup Required

Immediate Assessment

• Discontinue all topical medications immediately, particularly neomycin-containing products, to assess for contact dermatitis 1.
• Examine for secondary findings that raise suspicion of infection: non-purulent secretions, friable granulation tissue, wound undermining, and foul odor 1, 5.
• Assess for sinus tract formation, which may indicate underlying prosthetic joint infection even without obvious purulent discharge 1.

Laboratory and Imaging Studies

• Obtain plain radiographs of the knee in two planes as the first investigation to assess for osteomyelitis, soft tissue gas, foreign bodies, and loosening of prosthetic components 1, 5.
• Check inflammatory markers: CBC with differential, ESR, and CRP, though note that elevations are absent in up to one-half of patients even with severe diabetic foot infection 5.
• Evaluate glycemic control with HbA1c and fasting glucose, as poor control increases infection risk and affects wound healing 1, 5, 3.
• If radiographs show lucency around prosthetic components or other concerning findings, consider advanced imaging with MRI or nuclear medicine studies 1.

Specialized Testing if Needed

• Consider patch testing for neomycin and other topical antibiotic components if contact dermatitis is strongly suspected after medication discontinuation 1.
• If infection cannot be excluded clinically, obtain tissue samples via image-guided aspiration for culture rather than superficial swabs 1, 5.
• Skin biopsy may be necessary if the diagnosis remains unclear after initial workup, particularly to evaluate for bullous pemphigoid or other inflammatory dermatoses 1.

Critical Pitfalls to Avoid

• Do not assume that the absence of systemic symptoms excludes prosthetic joint infection, as diabetic neuropathy and complications may mask classical inflammatory signs 1.
• Do not continue topical antibiotic therapy without clear indication, as prolonged use increases sensitization risk and may obscure the underlying diagnosis 1, 6.
• Do not delay evaluation for undiagnosed or poorly controlled diabetes, as this presentation may represent diabetic complications requiring optimization of glycemic control 1, 5.
• Do not overlook the significance of the lesion's location near the surgical scar, which should trigger evaluation for both contact dermatitis and late prosthetic joint infection 1.
• Do not rely solely on inflammatory markers to exclude infection, as they may be normal even in severe cases 5.