Patellar dislocation occurs when the patella (kneecap) slips entirely out of its normal anatomical groove in the distal femur, known as the trochlear groove. It typically shifts laterally (toward the outside of the knee). While it frequently affects adolescents, young females, and athletes, repetitive actions like deep squatting and sitting cross-legged can impose significant biomechanical stress on predisposed knees.
Causes and Risk Factors
Patellar instability stems from traumatic events, underlying anatomical variations, or a combination of both:
- Trochlear Dysplasia: A shallow or flattened femoral groove—present in up to 85% of recurrent dislocation cases.
- Patella Alta: An abnormally high-riding kneecap that delays engagement with the femoral groove during flexion.
- Generalized Ligamentous Laxity: Common among adolescents and females, reducing intrinsic joint restraint.
- Muscular Imbalance: Weakness in the vastus medialis obliquus (VMO) combined with a tight lateral retinaculum forces the patella outward.
- Acute Trauma: Sudden twisting with a planted foot, valgus stress, or direct impact to the medial aspect of the knee.
Clinical Symptoms
- Sudden, severe anterior knee pain accompanied by an audible “pop.”
- Visible deformity with the kneecap displaced laterally.
- Rapid joint swelling secondary to hemarthrosis.
- Inability to bear weight or fully extend the knee.
- Recurrent catching, clicking, or apprehension during stair climbing and squatting in chronic instability.
Emergency First Aid
- Do not force reduction: Attempting manual relocation without clinical training risks shearing articular cartilage.
- Immobilize: Keep the limb supported in a comfortable, extended position.
- Apply Cold Therapy: Ice packs wrapped in cloth help manage acute swelling and localized pain.
- Avoid Weight-Bearing: Keep all weight off the injured extremity.
- Seek Urgent Evaluation: Obtain immediate orthopedic assessment and radiographs.
Diagnostic Evaluation
- Plain Radiographs (X-rays): AP, lateral, and axial (sunrise/Merchant) views confirm reduction, evaluate patellar height (Caton-Deschamps index), and identify osteochondral fracture fragments.
- Magnetic Resonance Imaging (MRI): Gold standard for assessing damage to the Medial Patellofemoral Ligament (MPFL)—which tears in over 90% of acute dislocations—as well as chondral shear injuries.
- CT Scan: Utilized in complex cases to calculate tibial tubercle–trochlear groove (TT-TG) distance and coronal alignment.
Treatment Approaches
| Modality | Indication | Success Rate | Recovery Window |
| Conservative Management | First-time dislocation without osteochondral fractures | 55–70% | 2–4 months |
| MPFL Reconstruction | Recurrent instability or high-grade MPFL rupture | 85–95% | 4–6 months |
| Tibial Tubercle Osteotomy (TTO) | Elevated TT-TG distance or patella alta | 80–90% | 4–6 months |
| Trochleoplasty | Severe high-grade trochlear dysplasia | 80–90% | 6–9 months |
Conservative Protocol
Indicated for stable first-time dislocations. The knee is immobilized in an extension brace for 2 to 3 weeks, followed by progressive functional physical therapy focused on swelling reduction, quadriceps activation, and restoring passive range of motion.
Surgical Intervention
Indicated for recurrent episodes, persistent subjective instability, displaced osteochondral loose bodies, or structural malalignment. Procedures often involve arthroscopic debridement paired with MPFL reconstruction using autograft or allograft tendon.
Rehabilitation Milestones
- Weeks 0–6: Protected weight-bearing with crutches, hinged knee brace locked in extension, early isometric quadriceps sets, and gentle passive range of motion.
- Months 2–3: Discontinuation of braces, gait normalization, closed-chain kinetic strengthening, stationary cycling, and proprioceptive retraining.
- Months 4–6: Straight-line running, agility drills, and return-to-sport testing once limb symmetry and dynamic stability are verified.
Frequently Asked Questions About Patellar Dislocation & Instability
What is a patellar dislocation and how does it occur?
A patellar dislocation is an acute orthopedic condition where the kneecap (patella) slips completely out of the femoral trochlear groove, almost invariably shifting toward the lateral (outer) side of the joint. It commonly happens during non-contact twisting movements with the knee slightly flexed and the foot planted, or following direct lateral trauma to the patella.
What is the difference between patellar subluxation and complete dislocation?
Patellar Subluxation: A transient, partial slippage where the kneecap momentarily moves off-center within the trochlear groove but self-reduces back into position.
Complete Dislocation: Total loss of articulation where the patella is entirely displaced over the lateral femoral condyle. This almost always tears the medial patellofemoral ligament (MPFL) and often requires emergency manual reduction.
What are the common anatomical risk factors for patellar instability?
Several anatomical factors increase the likelihood of patellar instability:
- Trochlear Dysplasia: A flattened, shallow, or convex femoral groove that provides inadequate bony constraint.
- Patella Alta: An abnormally high patellar position, delaying its entry into the bony groove during flexion.
- Elevated TT-TG Distance: Increased lateral offset between the tibial tubercle and the trochlear groove (>20 mm), creating a high lateral vector force.
- Ligamentous Hyperlaxity: Generalized joint laxity (e.g., Ehlers-Danlos or benign hypermobility).
- Genu Valgum (Knock-knees): Altered lower extremity alignment that increases lateral tracking pressure.
How is a patellar dislocation diagnosed and evaluated?
A comprehensive clinical and radiological evaluation includes:
- Physical Examination: Testing for hemarthrosis, medial retinacular tenderness, and a positive Patellar Apprehension Test.
- Plain Radiographs (X-rays): Anteroposterior, true lateral, and axial (Merchant/Sunrise) views to evaluate patellar height (Caton-Deschamps index) and trochlear anatomy.
- Magnetic Resonance Imaging (MRI): The standard imaging tool to assess the exact location of MPFL disruption, identify occult osteochondral bone bruises, and check for loose cartilage fragments.
Can a first-time patellar dislocation be treated non-surgically?
Yes. Primary, first-time patellar dislocations without intra-articular loose fragments or high-grade chondral damage are initially managed non-operatively. Treatment consists of R.I.C.E. therapy, short-term immobilization in a stabilizing brace locked in extension, protected weight-bearing, and progressive physical therapy focusing on quadriceps (especially VMO) and hip abductor strengthening.
When is surgical intervention necessary for patellar dislocation?
Surgical management becomes indicated in the following clinical scenarios:
- Recurrent Instability: Repeated episodes of dislocation or severe, debilitating subjective instability.
- Osteochondral Fractures: Displaced loose bone or cartilage fragments from the patella or lateral femoral condyle requiring fixation or debridement.
- Refractory Conservative Failure: Persistent apprehension and inability to return to athletic activities after rehabilitation.
- Severe Structural Dysplasia: High-risk anatomical malalignments that will inevitably cause future dislocations.
What surgical procedures are performed to stabilize the patella?
The surgical strategy is tailored according to patient anatomy:
- MPFL Reconstruction: The gold standard procedure, replacing the torn medial ligament using an autograft (gracilis or semitendinosus) or allograft.
- Knee Arthroscopy: Diagnostic evaluation and removal/refixation of chondral fragments.
- Tibial Tubercle Osteotomy (TTO / Fulkerson procedure): Medialization or distalization of the tibial tubercle to correct excessive lateral pull or patella alta.
- Trochleoplasty: Surgical reshaping and deepening of the femoral trochlear groove in severe dysplasia.
What is the expected recovery timeline after MPFL reconstruction surgery?
The postoperative rehabilitation protocol is staged as follows:
- Weeks 0–6: Weight-bearing with crutches and a hinged knee brace locked in extension; gradual passive flexion exercises (up to 90° by week 6).
- Weeks 6–12: Weaning off crutches and brace, achieving full active range of motion, and low-impact strengthening.
- Months 3–4: Stationary cycling, swimming, and light jogging in a straight line.
- Months 4–6: Sport-specific agility drills and full return to competitive pivoting sports following functional clearance.
What is the success rate of surgical treatment for patellar instability?
MPFL reconstruction yields high clinical success rates between 90% and 95%, with exceptionally low rates of recurrent dislocation (<3%–5%). Patients achieve significant improvements in validated patient-reported outcome measures (Kujala score, KOOS) and high rates of return to pre-injury sports participation.
Does an untreated patellar dislocation increase the risk of knee arthritis?
Yes. Chronic patellar instability and recurrent subluxations lead to repetitive shearing trauma against the patellofemoral articular cartilage. Over time, this accelerates cartilage softening, fibrillation, and premature patellofemoral osteoarthritis, highlighting the importance of achieving definitive joint stability.
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