Right Knee Instability: Evaluation and Management
Begin with plain radiographs (AP and lateral views) if the patient meets Ottawa Knee Rule criteria (age >55, isolated patellar tenderness, fibular head tenderness, inability to flex to 90°, or inability to bear weight), then proceed to MRI if internal derangement or ligamentous injury is suspected based on clinical examination. 1
Initial Clinical Evaluation
Perform a focused physical examination targeting specific instability patterns:
Key Physical Examination Maneuvers
- Lachman test - most sensitive for ACL injury, performed at 20-30° flexion
- Pivot shift test - assesses rotational instability from ACL deficiency
- Varus/valgus stress testing at 0° and 30° - evaluates collateral ligament integrity
- Posterior drawer test - identifies PCL injury
- Patellar apprehension test - detects patellofemoral instability 2, 3
Critical point: Examine the uninjured knee first to establish baseline laxity, as significant inter-examiner variability exists in instability testing 4.
Imaging Algorithm
Step 1: Plain Radiographs
Obtain AP and lateral views as initial imaging for patients meeting clinical criteria. Add patellofemoral views if patellar instability is suspected 1.
Step 2: MRI for Soft Tissue Assessment
MRI is the next appropriate study when radiographs are negative but clinical examination suggests:
- Ligamentous injury (positive Lachman, pivot shift, or collateral ligament laxity)
- Meniscal pathology (joint line tenderness, mechanical symptoms)
- Occult fracture despite negative radiographs 1
Do not obtain MRI as initial imaging - it is not appropriate for first-line evaluation of acute knee trauma 1.
Management Based on Pathology
ACL Instability
For young, active patients with ACL tears and no prior conservative treatment, ACL reconstruction with autograft is the most appropriate initial treatment (rated 8/9 on AAOS appropriateness scale) 5.
Treatment Options by Appropriateness:
Appropriate (score 7-9):
- ACL reconstruction with autograft (score 8)
- ACL reconstruction with allograft (score 7)
- Activity modification without reconstruction (score 7)
May Be Appropriate (score 4-6):
- Supervised rehabilitation without reconstruction (score 6)
- Self-directed exercise without reconstruction (score 6)
- ACL functional brace without reconstruction (score 6)
Key Decision Factors:
Favor surgical reconstruction when:
- Patient participates in cutting/pivoting sports
- Recurrent instability episodes occur
- Patient is young and active
- Goal is return to high-level athletics
Rationale: Nonsurgical management risks future meniscal and articular cartilage damage from recurrent instability episodes 5.
Consider conservative management when:
- Patient has low activity demands
- Willing to modify activities
- Older age with degenerative changes present
- Medical comorbidities increase surgical risk
Medial/Collateral Ligament Instability
Most isolated medial collateral ligament injuries heal with conservative management (bracing and physical therapy). Surgical repair or reconstruction is indicated for patients with persistent instability after conservative treatment 6.
Patellofemoral Instability
Management requires identification of anatomic risk factors:
- Trochlear dysplasia
- Elevated tibial tubercle-trochlear groove (TT-TG) distance
- Patella alta
- Increased femoral anteversion or tibial torsion 7, 8, 9
First-time dislocators without anatomic predisposition: Conservative management with physical therapy focusing on quadriceps strengthening
Recurrent instability or high-risk anatomy: Surgical stabilization with medial patellofemoral ligament (MPFL) reconstruction, potentially combined with tibial tubercle osteotomy or trochleoplasty based on specific anatomic abnormalities 10, 8
Common Pitfalls
- Obtaining MRI before radiographs - violates appropriate imaging sequence and increases costs unnecessarily
- Applying Ottawa rules to children <5 years - these rules are not validated in this age group 1
- Delaying ACL reconstruction in young athletes - increases risk of secondary meniscal and chondral injury 5
- Treating all instability patterns the same - requires pattern-specific diagnosis (ACL vs. MCL vs. patellofemoral) 2, 11