# Clinical Cases: Musculoskeletal Infections

## Case 1: Septic Arthritis of the Knee

### Patient Presentation
**Demographics:** 58-year-old man

**Chief Complaint:** "My right knee is extremely swollen and painful, and I've had fevers."

**History of Present Illness:**
The patient presents with 3 days of progressive right knee pain and swelling. He initially thought he had tweaked his knee while walking but noted rapid worsening with development of fever and chills. He now cannot bear weight on the leg. He has difficulty moving the knee in any direction due to severe pain. He denies any penetrating trauma but reports he had a skin boil on his leg drained about 2 weeks ago.

**Past Medical History:**
- Type 2 diabetes mellitus (poorly controlled, HbA1c 9.2%)
- Rheumatoid arthritis (on methotrexate)
- Hypertension

**Medications:**
- Methotrexate 15 mg weekly
- Folic acid 1 mg daily
- Metformin 1000 mg twice daily
- Glipizide 10 mg twice daily
- Lisinopril 20 mg daily

**Social History:**
- Retired truck driver
- Former smoker
- Occasional alcohol

### Physical Examination
- **Vital Signs:** BP 108/68 mmHg, HR 110 bpm, Temp 39.2C, RR 20
- **General:** Ill-appearing, diaphoretic
- **Right Knee:**
  - Large tense effusion
  - Marked erythema and warmth
  - Exquisite tenderness - patient guards against any movement
  - Held in 20-30 degrees of flexion
  - Unable to actively extend or flex
- **Skin:** Healing wound on right lower leg (site of prior furuncle drainage)
- **Other joints:** Chronic MCP changes from RA, no acute inflammation

### Workup and Results

**Laboratory Studies:**
| Test | Result | Reference Range |
|------|--------|-----------------|
| WBC | 18,200/mm3 | 4,500-11,000/mm3 |
| Neutrophils | 88% | 40-70% |
| ESR | 98 mm/hr | 0-15 mm/hr |
| CRP | 24.6 mg/dL | <0.5 mg/dL |
| Blood Glucose | 312 mg/dL | 70-100 mg/dL |
| Creatinine | 1.4 mg/dL | 0.7-1.3 mg/dL |
| Blood Cultures | Pending | - |

**Synovial Fluid Analysis:**
| Test | Result | Interpretation |
|------|--------|----------------|
| Appearance | Purulent, opaque | Septic |
| WBC | 92,000/mm3 | Highly inflammatory (>50,000 suggests septic) |
| PMN | 96% | Neutrophil predominant |
| Crystals | None | Excludes gout/pseudogout |
| Gram Stain | Gram-positive cocci in clusters | Likely Staphylococcus aureus |
| Culture | Pending | - |

**Imaging:**
- **Knee X-ray:** Large joint effusion, soft tissue swelling, no fracture or erosions

### Clinical Image

![Knee effusion septic arthritis](case_01_image.jpg)

*Clinical photograph and diagram demonstrating a large knee joint effusion with visible swelling and loss of normal joint contour, characteristic presentation of septic arthritis. Source: [Wikimedia Commons](https://commons.wikimedia.org/wiki/File:Knee_effusion.PNG), CC BY-SA 3.0.*

### Diagnosis
**Septic Arthritis of the Right Knee - Presumed Staphylococcus aureus**

**Diagnostic Features:**
- Acute monoarticular arthritis with systemic signs
- Synovial WBC >50,000/mm3 with >90% PMNs
- Gram-positive cocci on Gram stain (presumed S. aureus)
- Risk factors: diabetes, immunosuppression (methotrexate), prior skin infection

### Treatment Plan

**Emergent Management:**
1. **Joint drainage:** Orthopedic surgery consulted for arthroscopic lavage vs. daily needle aspirations
2. **Hold methotrexate** until infection cleared
3. **IV antibiotics:**
   - **Vancomycin 1.5g IV loading, then 1g IV q12h** (MRSA coverage given community onset, prior skin infection)
   - Adjust based on culture and sensitivity
4. **Blood cultures** x 2 sets drawn before antibiotics

**Supportive Care:**
1. IV fluid resuscitation
2. Insulin sliding scale for glucose control
3. Pain management
4. DVT prophylaxis

**Culture Results (48 hours):**
- Synovial fluid and blood cultures: Methicillin-sensitive Staphylococcus aureus (MSSA)
- **De-escalate to nafcillin 2g IV q4h** (or cefazolin 2g IV q8h)

**Duration:**
- Minimum 4 weeks IV antibiotics for native joint septic arthritis
- Consider oral step-down to ciprofloxacin + rifampin or high-dose oral beta-lactam if organism susceptible and good clinical response

**Follow-up:**
- Repeat aspiration to confirm sterilization
- Monitor inflammatory markers (CRP, ESR) for response
- Physical therapy once infection controlled
- Discuss restarting RA therapy (wait 2-4 weeks post-antibiotics)

### Teaching Points
1. **Septic arthritis is a medical emergency:** Untreated leads to rapid cartilage destruction within days; prompt diagnosis and drainage are critical
2. **Synovial WBC >50,000 with >90% PMN:** Strongly suggests septic arthritis; Gram stain is positive in only 50-75% of cases
3. **Staphylococcus aureus:** Most common cause across all age groups; always suspect MRSA in community-acquired cases
4. **Risk factors:** Diabetes, immunosuppression, pre-existing joint disease (RA, OA), skin infections, and prosthetic joints increase susceptibility
5. **Drainage is essential:** Antibiotics alone insufficient; surgical or repeated needle drainage required

---

## Case 2: Vertebral Osteomyelitis

### Patient Presentation
**Demographics:** 64-year-old man

**Chief Complaint:** "I've had terrible back pain for 6 weeks that's getting worse."

**History of Present Illness:**
The patient describes progressive lower back pain that began insidiously approximately 6 weeks ago without any clear precipitant. The pain is constant, worse at night, and disturbs his sleep. It is not relieved by rest and is aggravated by movement. He has lost 12 pounds over this period. He had a urinary tract infection treated with oral antibiotics about 2 months ago. Over the past week, he has developed subjective fevers and night sweats.

**Past Medical History:**
- Type 2 diabetes mellitus
- Benign prostatic hyperplasia
- Recent urinary tract infection (E. coli, treated with ciprofloxacin x 7 days)

**Medications:**
- Metformin 1000 mg twice daily
- Tamsulosin 0.4 mg daily
- Ibuprofen 400 mg TID (self-prescribed for back pain)

**Social History:**
- Retired plumber
- Non-smoker
- Occasional alcohol

### Physical Examination
- **Vital Signs:** BP 128/78 mmHg, HR 88 bpm, Temp 38.1C, BMI 28 kg/m2
- **General:** Uncomfortable, difficulty finding comfortable position
- **Spine:**
  - Paraspinal muscle tenderness L3-L5 region
  - Percussion tenderness over L4 spinous process
  - Limited range of motion due to pain
  - No midline step-off
- **Neurologic:**
  - Strength 5/5 bilateral lower extremities
  - Sensation intact
  - Reflexes 2+ and symmetric
  - Negative straight leg raise
- **Rectal exam:** Prostate mildly enlarged, non-tender

### Workup and Results

**Laboratory Studies:**
| Test | Result | Reference Range |
|------|--------|-----------------|
| WBC | 11,800/mm3 | 4,500-11,000/mm3 |
| ESR | 88 mm/hr | 0-15 mm/hr |
| CRP | 8.4 mg/dL | <0.5 mg/dL |
| Blood Cultures | Pending | - |
| Urinalysis | 5-10 WBC/hpf | <5 WBC/hpf |
| Urine Culture | No growth | - |
| HbA1c | 8.1% | <5.7% |

**Imaging:**
- **Lumbar spine X-ray:** Subtle endplate irregularity at L4-L5, disc space narrowing
- **MRI lumbar spine (with contrast):**
  - L4-L5 discitis with involvement of adjacent vertebral endplates
  - Enhancing inflammatory tissue in disc space
  - Pre-vertebral and epidural phlegmon without frank abscess
  - No spinal cord compression or significant canal stenosis

**CT-guided Biopsy:**
- L4-L5 disc space biopsy performed
- Culture: Escherichia coli (same susceptibility pattern as prior UTI)
- Histopathology: Acute and chronic inflammation consistent with osteomyelitis

### Clinical Image

![Osteomyelitis tibia](case_02_image.jpg)

*Radiograph demonstrating chronic osteomyelitis with cortical destruction, periosteal reaction, and sequestrum formation. While this image shows extremity osteomyelitis, similar destructive changes occur in vertebral osteomyelitis. Source: [Wikimedia Commons](https://commons.wikimedia.org/wiki/File:Osteomyelitis_Tibia.jpg), Public Domain.*

### Diagnosis
**Vertebral Osteomyelitis (L4-L5 Discitis) with Epidural Phlegmon - E. coli**

**Pathogenesis:**
- Hematogenous seeding from urinary tract infection 2 months prior
- E. coli from UTI established vertebral infection
- Diabetes as predisposing factor

### Treatment Plan

**Antibiotic Therapy:**
1. **IV ceftriaxone 2g daily** based on E. coli susceptibilities
2. Duration: 6 weeks IV therapy for vertebral osteomyelitis
3. May consider oral step-down to fluoroquinolone after 2-4 weeks if good response

**Monitoring:**
- Weekly CRP to track response (should decline within 1-2 weeks)
- Repeat MRI at 4-6 weeks if inadequate clinical response
- Serial neurologic exams for any new deficits

**Activity:**
- Bed rest initially, then mobilization with lumbar brace
- Avoid heavy lifting and bending

**Surgical Considerations:**
- Currently no surgery indicated (no abscess, no neurologic deficit, no instability)
- Would require surgery if:
  - Epidural abscess with compression
  - Neurologic deterioration
  - Failed medical therapy
  - Spinal instability

**Follow-up:**
- Infectious disease consultation for antibiotic management
- Repeat imaging if not improving
- Address diabetes control (uncontrolled diabetes impairs infection response)

### Teaching Points
1. **Vertebral osteomyelitis presentation:** Insidious back pain, worse at night, with systemic symptoms; average delay to diagnosis is 6-12 weeks
2. **Common sources:** Urinary tract (especially gram-negatives), skin infections, endocarditis, dental procedures
3. **MRI is imaging of choice:** Shows disc space infection, vertebral endplate involvement, and epidural extension before radiographic changes appear
4. **Tissue diagnosis important:** Blood cultures positive in ~50%; CT-guided biopsy provides organism identification if cultures negative
5. **Duration:** 6 weeks of appropriate antibiotic therapy; epidural abscess or other complications may require longer courses or surgery

---

## Case 3: Diabetic Foot Osteomyelitis

### Patient Presentation
**Demographics:** 62-year-old man

**Chief Complaint:** "The wound on my foot won't heal and it smells bad."

**History of Present Illness:**
The patient has had a wound on the bottom of his right foot for approximately 3 months. It started as a small blister under the first metatarsal head that progressively enlarged. He has had intermittent drainage and recently noticed a foul odor. He has limited sensation in his feet and did not notice the wound until it was already significant. He has tried wound care at home with limited success.

**Past Medical History:**
- Type 2 diabetes mellitus for 20 years (poorly controlled, HbA1c 10.2%)
- Diabetic peripheral neuropathy
- Peripheral arterial disease
- Hypertension
- Chronic kidney disease stage 3

**Medications:**
- Insulin glargine 40 units at bedtime
- Insulin lispro sliding scale
- Lisinopril 40 mg daily
- Atorvastatin 40 mg daily
- Aspirin 81 mg daily

**Social History:**
- Former smoker (quit 2 years ago, 40 pack-year history)
- Works from home

### Physical Examination
- **Vital Signs:** BP 148/88 mmHg, HR 84 bpm, Temp 37.4C
- **General:** Overweight, appears chronically ill
- **Right Foot:**
  - 3 cm x 2 cm ulcer at first metatarsal head plantar surface
  - Ulcer base with necrotic tissue and purulent drainage
  - Foul odor
  - Surrounding cellulitis extending 3 cm
  - Probe-to-bone test: Positive (metal probe reaches bone through ulcer)
  - Absent dorsalis pedis and posterior tibial pulses
  - Monofilament sensation absent
- **Left Foot:** Calluses, no open wounds, absent pulses

### Workup and Results

**Laboratory Studies:**
| Test | Result | Reference Range |
|------|--------|-----------------|
| WBC | 14,200/mm3 | 4,500-11,000/mm3 |
| ESR | 92 mm/hr | 0-15 mm/hr |
| CRP | 6.8 mg/dL | <0.5 mg/dL |
| Creatinine | 1.8 mg/dL | 0.7-1.3 mg/dL |
| HbA1c | 10.2% | <5.7% |
| Blood Glucose | 268 mg/dL | 70-100 mg/dL |

**Wound Culture (deep tissue at time of debridement):**
- Staphylococcus aureus (MRSA)
- Enterococcus faecalis
- Pseudomonas aeruginosa
- Bacteroides fragilis
- Polymicrobial infection

**Imaging:**
- **Foot X-ray:**
  - Cortical erosion of first metatarsal head
  - Periosteal reaction
  - Soft tissue gas
- **MRI Foot:**
  - First metatarsal osteomyelitis with marrow edema
  - Surrounding soft tissue abscess
  - Extent of infection mapped for surgical planning
- **Ankle-Brachial Index:** 0.5 right, 0.6 left (severe PAD)

### Diagnosis
**Diabetic Foot Osteomyelitis (First Metatarsal) with Polymicrobial Infection and Severe Peripheral Arterial Disease**

**Key Diagnostic Features:**
- Positive probe-to-bone test (PPV >85% when pre-test probability high)
- Radiographic bone destruction
- MRI confirmation
- Polymicrobial wound culture typical of diabetic foot infection

### Treatment Plan

**Multidisciplinary Approach:**
1. **Vascular surgery consultation:** Assess revascularization options (severe PAD limits wound healing and antibiotic delivery)
2. **Infectious disease consultation:** Antibiotic management
3. **Podiatry/Orthopedic surgery:** Surgical debridement

**Vascular Assessment:**
- Arterial duplex and possible angiography
- Consider revascularization (angioplasty, bypass) before definitive bone surgery

**Antibiotic Therapy:**
1. **IV vancomycin 1g q12h** (MRSA coverage, adjust for CKD)
2. **IV piperacillin-tazobactam 3.375g q6h** (gram-negative and anaerobic coverage, adjust for CKD)
3. Duration: 4-6 weeks after adequate surgical debridement
4. Narrow based on culture sensitivities when possible

**Surgical Management:**
1. Debridement of necrotic and infected tissue
2. Bone biopsy for culture confirmation
3. Possible partial first ray amputation if osteomyelitis cannot be adequately debrided
4. Wound VAC (vacuum-assisted closure) for wound management

**Glycemic Control:**
- Insulin drip or intensive insulin regimen while hospitalized
- Goal glucose 140-180 mg/dL

**Offloading:**
- Non-weight-bearing on affected foot
- Total contact cast or offloading device once wound healing

### Teaching Points
1. **Probe-to-bone test:** If a sterile metal probe can reach bone through a diabetic foot ulcer, osteomyelitis is highly likely (sensitivity 66%, specificity 85%)
2. **Polymicrobial infections:** Diabetic foot osteomyelitis typically involves multiple organisms including S. aureus (often MRSA), gram-negatives, and anaerobes
3. **Vascular assessment critical:** Peripheral arterial disease must be addressed for wounds to heal; revascularization may be necessary before or concurrent with bone surgery
4. **MRI vs. X-ray:** MRI is more sensitive for early osteomyelitis; X-ray changes take 2-3 weeks to appear
5. **Amputation consideration:** Severe infection with inadequate blood supply may require amputation to save life; early aggressive management aims to avoid this outcome
6. **Multidisciplinary team:** Optimal outcomes require coordination between infectious disease, vascular surgery, podiatry/orthopedics, endocrinology, and wound care