Diagnosing Joint Space Involvement in Patellar Laceration
Direct visualization through the wound is the primary method to diagnose joint space involvement in a patellar laceration, supplemented by saline load test when the wound depth and trajectory suggest possible joint penetration.
Clinical Examination Approach
Initial Wound Assessment
- Inspect the wound depth and trajectory to determine if it extends through the subcutaneous tissue toward the joint capsule
- Palpate for exposed bone or cartilage through the laceration, which indicates deep penetration
- Assess for fat globules in the wound or bleeding, which may suggest joint capsule violation
Saline Load Test (Gold Standard Bedside Test)
- Inject 30-60 mL of sterile saline into the knee joint through a separate site (typically suprapatellar approach) with the knee in extension
- Observe for fluid extravasation through the patellar laceration
- Positive test (fluid leaking from wound) confirms intra-articular communication and joint space involvement
- This test has high sensitivity and specificity for detecting joint capsule penetration in traumatic knee wounds
Physical Examination Findings Suggesting Joint Involvement
- Hemarthrosis or large joint effusion on ballottement suggests intra-articular injury 1
- Palpable bone fragments at the patellar surface may indicate osteochondral fracture with joint involvement 2
- Limited or painful range of motion with crepitus suggests intra-articular pathology 3
Imaging Evaluation
Plain Radiography (First-Line)
- Obtain anteroposterior and lateral radiographs of the affected knee as initial imaging 1
- Add axial (patellofemoral) view to evaluate for patellar fractures and assess joint space 1
- Look for indirect signs including soft tissue opacity at the cranial aspect of the joint, bone fragments at patellar insertion, or air in the joint space 4
- Radiographs are usually satisfactory for assessment of patellar complications and helpful in guiding treatment 5
Advanced Imaging When Indicated
- MRI is the most useful modality for characterizing bone and soft-tissue injuries, detecting osteochondral fractures, and assessing cartilage damage 1, 6
- Ultrasound provides detailed information regarding intra-ligamentous damage and soft tissue integrity, and may be combined with MRI for comprehensive assessment 4
- CT with metal artifact reduction can detect patellar complications when radiographs are inconclusive, particularly for evaluating bone detail 5
Key Diagnostic Pitfalls
- Do not rely solely on radiographs for soft tissue and cartilage assessment, as they have major limitations in detecting capsular injuries 6
- Hemarthrosis alone is not specific for joint space involvement, as it can occur with periarticular injuries 6
- Small joint penetrations may be missed on clinical examination alone without saline load testing
- Associated injuries (meniscal tears, ligamentous damage) occur in up to 40% of cases and require MRI for detection 6