A subluxation of the knee is a partial dislocation, meaning the bones that form the joint shift out of their normal alignment but don’t separate completely. In most cases, the term refers to the kneecap (patella) sliding too far to the outer side of its groove on the thighbone, though the shinbone can also subluxate relative to the thighbone after severe ligament injuries. The experience is distinct from a full dislocation and often more confusing for the person it happens to, because the joint may pop back into place on its own, leaving behind pain and swelling but no obvious deformity. Understanding what actually happened inside the knee, and why it tends to happen again, matters for choosing the right treatment.
How the Kneecap Normally Stays in Place
Your kneecap sits in a shallow channel on the front of the thighbone called the trochlear groove. As you bend and straighten the knee, the kneecap glides up and down within that groove. Two systems keep it centered. The first is the shape of the groove itself: a deeper groove cradles the kneecap more securely. The second is a web of soft tissue, especially a band called the medial patellofemoral ligament (MPFL), which runs from the inner edge of the kneecap to a specific point on the thighbone. The MPFL acts as the primary restraint against the kneecap drifting outward. A systematic review of its properties found that when the MPFL is absent or damaged, the kneecap shifts toward the outer side and engages the groove later during bending, raising the risk of dislocation and subluxation.1PubMed. Properties and Function of the Medial Patellofemoral Ligament: A Systematic Review A muscle on the inner thigh, the vastus medialis obliquus, works alongside the MPFL to provide a dynamic pull that fine-tunes kneecap tracking.2PubMed. Functional bundles of the medial patellofemoral ligament
When either the groove’s shape or the soft tissue restraints are insufficient, the kneecap can slide partially out of its track. That partial slide is a patellar subluxation. If it goes far enough that the kneecap clears the edge of the groove entirely, it becomes a full patellar dislocation. Many people experience episodes that fall somewhere in between, and the distinction between a “bad subluxation” and a “dislocation that spontaneously reduced” can be hard to draw after the fact.
Tibiofemoral Subluxation Is a Different Injury
Less commonly, “knee subluxation” refers to the tibiofemoral joint, where the shinbone shifts relative to the thighbone. This type typically follows high-energy trauma such as a car accident or a severe sports collision, and it involves damage to multiple ligaments at once. A multicenter case series of patients with multiligament knee injuries found that the most common direction of tibiofemoral subluxation was lateral, occurring in about 80 percent of cases, with the remaining 20 percent shifting medially.3PubMed Central. Uniplanar Coronal Tibiofemoral Subluxation in Patients After Multiligament Knee Injuries: A Multicenter Retrospective Case Series Tibiofemoral subluxation is a surgical emergency or near-emergency because of the risk to the blood vessels and nerves behind the knee. The rest of this article focuses on patellar subluxation, which is far more common and the condition most people mean when they search for information on knee subluxation.
What a Subluxation Episode Feels Like
The hallmark sensation is the kneecap “giving way” or feeling like it shifted to one side, sometimes accompanied by a pop. Many people describe the knee buckling underneath them during a cutting or pivoting movement. After the kneecap slides back, the knee swells, often within hours. In more severe episodes, blood can accumulate inside the joint, a finding called hemarthrosis. One review of acute knee hemarthrosis in adolescents identified patellar dislocation and subluxation as among the most frequent causes, alongside cruciate ligament tears and cartilage fractures.4PubMed. Acute Hemarthrosis of the Adolescent Knee
After the acute episode fades, people commonly notice:
- Tenderness along the inner kneecap: This is where the MPFL attaches, and it is almost always strained or torn during a subluxation event.
- A feeling of instability: The knee feels unreliable, especially when changing direction or walking on uneven surfaces.
- Apprehension: A strong, involuntary sense of anxiety when the kneecap is pushed outward during an exam or when performing certain movements. This is one of the most telling clinical signs.
- Difficulty straightening the leg: Swelling and pain can limit full extension, and occasionally a loose fragment of cartilage can physically block motion.
Repeated subluxation episodes can blur together over time. Some people with chronic instability describe a “catching” or “clicking” sensation rather than a dramatic give-way, because the kneecap shifts only slightly before returning. The damage still accumulates, though, and the knee may worsen gradually rather than in a single dramatic event.
Who Gets Patellar Subluxation and Why
Patellar instability has a strong demographic skew. A population-based study spanning 21 years found that the peak incidence of first-time lateral patellar dislocation or subluxation occurs between ages 14 and 18 in both girls and boys. Among children 13 and younger, girls were affected at nearly twice the rate of boys.5PubMed Central. Incidence of First-Time Lateral Patellar Dislocation: A 21-Year Population-Based Study In high school athletes, an epidemiological study found that subluxations were roughly twice as common as full dislocations. The highest rates appeared in girls’ gymnastics, boys’ football, and boys’ wrestling. When comparing boys and girls in the same sports, girls had a higher injury rate.6PubMed. Epidemiology of Patellofemoral Instability Injuries Among High School Athletes in the United States
Anatomy plays the biggest role in determining who is vulnerable. Several structural features stack the odds against a person’s kneecap staying centered:
- Trochlear dysplasia: A shallow or flat groove on the thighbone. This is the single most common anatomical risk factor.
- Patella alta: A kneecap that sits higher than normal, so it enters the groove later during bending and spends more time in an unprotected position.
- Increased tibial tuberosity–trochlear groove distance: The point where the patellar tendon attaches on the shinbone is positioned too far to the outside, pulling the kneecap laterally.
- Abnormal lateral patellar tilt: The kneecap is tilted outward at rest, predisposing it to slide in that direction.
These risk factors were identified in MRI studies comparing patients who had experienced a dislocation with healthy controls.7PubMed Central. Anatomic Risk Factors for Patellofemoral Joint Instability: An Infographic as a Visual Learning Tool Importantly, they tend to cluster. In one large case-control study, only about 15 percent of patients who had dislocated their kneecap had no identifiable anatomical risk factor at all. Trochlear dysplasia appeared in about two-thirds of the dislocation group, and when it combined with patella alta or an increased tibial tuberosity–trochlear groove distance, the risk increased dramatically, on the order of 37- to 41-fold compared to controls.8PubMed. Prevalence and patterns of anatomical risk factors in patients after patellar dislocation: a case control study using MRI
People with connective tissue disorders are also disproportionately affected. In a cohort of patients with hypermobile Ehlers-Danlos syndrome, roughly 55 percent reported knee or patellar subluxations at least once a week.9PubMed Central. Assessment of gait mechanics and muscle strength in hypermobile Ehlers Danlos Syndrome For these individuals, joint laxity is systemic, and the kneecap is just one of many joints that subluxates regularly.
How It Is Diagnosed
A doctor’s clinical examination is the starting point. The most useful bedside test is the moving patellar apprehension test, in which the examiner pushes the kneecap outward while the patient slowly bends the knee from full extension. If the patient involuntarily tenses up or reports fear that the kneecap is about to dislocate, the test is positive. Studies have found this test to be highly reliable, with reported accuracy around 94 percent, near-perfect sensitivity, and strong specificity for identifying patellar instability.10PubMed. The moving patellar apprehension test for lateral patellar instability11Journal of Orthopaedics. Is the patella apprehension test a valid diagnostic test for patellar instability? A systematic review
MRI is the imaging workhorse. It reveals the full picture: whether the MPFL is torn and where, whether cartilage was damaged, whether loose fragments are floating in the joint, and the anatomical shape of the trochlear groove. In one MRI study of 195 knees after patellar dislocation, MPFL tears at the kneecap attachment appeared in about half of cases, while tears at the thighbone attachment occurred in about a quarter. Osteochondral lesions, which are cartilage or bone-chip injuries, showed up in nearly half the knees, and loose bodies were found in about 13 percent.12PubMed Central. Medial patellofemoral ligament injury patterns and associated pathology in lateral patella dislocation: an MRI study A separate study similarly found MPFL tears in about 95 percent of first-time traumatic patellar dislocations, with osteochondral injuries and loose bodies in roughly 28 percent of cases.13PubMed Central. Patterns of Associated Knee Ligament and Chondral Injuries in First-Time Traumatic Patellar Dislocation: A Retrospective Magnetic Resonance Imaging (MRI) -Based Study
In children and adolescents, imaging interpretation requires extra care. Standard measurements used to assess kneecap alignment in adults can vary with skeletal maturity, and recent literature advocates adjusting certain measurements or using alternative ratios for younger patients.14PubMed Central. Patellofemoral Instability in the Pediatric Population15PubMed Central. Radiographic Evaluation of Pediatric Patients with Patellofemoral Instability
Non-Surgical Treatment
After a first-time subluxation or dislocation, most doctors start with non-surgical management. The initial phase involves reducing swelling with ice, compression, and a short period of immobilization in a brace or sleeve that keeps the kneecap from drifting laterally. Once the acute inflammation settles, rehabilitation begins.
The centerpiece of non-surgical rehab is strengthening the muscles that stabilize the kneecap, particularly the quadriceps and the inner thigh portion of that muscle group, the vastus medialis obliquus. The logic is straightforward: if the passive restraints (the MPFL and groove shape) are suboptimal, building the dynamic restraints can partially compensate. That said, research has long noted a disconnect between the emphasis clinicians place on this muscle and what studies have been able to prove about selectively training it.16Journal of Orthopaedic & Sports Physical Therapy (JOSPT). Rehabilitation of patellofemoral joint disorders: a critical review In practice, broader lower-limb strengthening and exercises that improve hip control and landing mechanics are now standard parts of the program.
Patellar taping and bracing are also commonly used during recovery and return to activity. The idea is to provide an external force that nudges the kneecap inward. MRI studies show that both taping and bracing can shift the kneecap’s position, though bracing appears more consistent at centering it, particularly at low degrees of knee bend.17PubMed Central. Effect of patellar taping and bracing on patellar position as determined by MRI in patients with patellofemoral pain One case study using MRI found that bracing improved patellar centering by several measurements, while taping actually worsened patellar position in some positions, though both reduced pain during functional tasks like stepping down.18Journal of Sport Rehabilitation. Effect of Patellar Taping and Bracing on Patellar Position: An MRI Case Study Some clinicians have described taping techniques that, combined with a guided exercise program, allowed patients to return to sports, though they recommend continued taping for athletics to reduce recurrence.19PubMed Central. Taping for an Acute and Subacute Patellofemoral Dislocation, Recurrent Subluxation
A Cochrane review pooling data from four studies found that surgery reduced subsequent subluxation compared to non-surgical management, but the difference was modest, and the confidence interval did not rule out the possibility that surgery offered no benefit at all.20Cochrane Library. Surgical versus non‐surgical interventions for treating patellar dislocation This reflects the genuinely difficult judgment call clinicians face: for a first-time subluxation without major anatomical risk factors, non-surgical treatment is reasonable and often successful. But for patients with strong anatomical predispositions or repeated episodes, conservative care alone may not be enough.
When Surgery Is Considered
Surgery enters the conversation when subluxations keep happening despite adequate rehabilitation, when imaging shows significant anatomical abnormalities that conservative treatment cannot correct, or when a cartilage injury from the initial event needs to be addressed. The specific procedure depends on which anatomical problems are contributing to the instability.
MPFL Reconstruction
The most widely performed procedure for recurrent patellar instability is reconstruction of the medial patellofemoral ligament using a tendon graft. A graft, taken either from the patient’s own body or from a donor, replaces the torn MPFL to restore the restraint that keeps the kneecap from sliding outward. A systematic review found that MPFL reconstruction has a high success rate for resolving instability, though the overall complication rate of about 26 percent is not negligible, encompassing issues such as stiffness, persistent pain, and occasional graft failure.21PubMed Central. A systematic review of complications and failures associated with medial patellofemoral ligament reconstruction for recurrent patellar dislocation
Surgeons can also choose to repair the existing MPFL rather than reconstruct it with a graft. A long-term comparison found that repair carried a substantially higher redislocation rate — about 41 percent versus 14 percent for reconstruction — though patient-reported functional scores and return-to-sport rates were similar between the two approaches.22PubMed Central. Comparison of Failure Rates at Long-term Follow-up Between MPFL Repair and Reconstruction for Recurrent Lateral Patellar Instability This finding has pushed many surgeons toward reconstruction as the preferred option for recurrent instability.
Graft choice and precise tunnel placement are the technical details that most affect outcomes. Research has found no significant difference in pain reduction, return to activity, or failure rates between grafts taken from the patient (autografts) and donor grafts (allografts), though some evidence suggests allografts may carry a slightly lower failure rate.23PubMed Central. A Landscape of Recent Literature on the Predictors of Success and Failure in Medial Patellofemoral Ligament Reconstruction Where the surgeon places the tunnel on the thighbone is critical. Too high, too low, or too far in any direction changes the tension on the graft throughout the range of motion, potentially creating new problems.
Procedures for Underlying Bone Problems
When the instability is driven by the shape of the bones rather than just damaged soft tissue, soft tissue surgery alone may not solve the problem. Two bone procedures are commonly performed alongside or instead of MPFL reconstruction. A tibial tubercle osteotomy involves cutting and repositioning the bump on the shinbone where the patellar tendon attaches, effectively shifting the pull on the kneecap to a more favorable alignment. A trochleoplasty reshapes the trochlear groove itself, deepening it so the kneecap has a more secure channel to ride in. These are used for patients with trochlear dysplasia, patella alta, or an increased distance between the tibial tuberosity and the trochlear groove.24Arthroscopy Techniques. Patellofemoral Joint Reconstruction for Patellar Instability: Medial Patellofemoral Ligament Reconstruction, Trochleoplasty, and Tibial Tubercle Osteotomy A systematic review comparing the two found that both provide satisfactory functional improvement, but trochleoplasty may carry a lower risk of recurrent instability than tibial tubercle osteotomy alone.25PubMed. Outcomes of Trochleoplasty versus Tibial Tubercle Osteotomy for Treatment of Patellar Instability Associated with Trochlear Dysplasia: A Systematic Review and Meta-analysis In severe cases, all three procedures can be combined in a single operation.26Arthroscopy Techniques. Technical Note Medial Patellofemoral Reconstruction Using Quadriceps Tendon Autograft, Tibial Tubercle Osteotomy, and Sulcus-Deepening Trochleoplasty for Patellar Instability
One long-term concern with bone-realignment surgery is the potential for osteoarthritis down the road. A retrospective follow-up of patients who underwent a tibial tubercle transfer procedure found an association with advanced arthritis in both the kneecap joint and the main knee joint over time, especially when the procedure was performed late in the course of the disease.27PubMed Central. Prevalence of osteoarthritis and clinical results after the Elmslie-Trillat procedure: a retrospective long-term follow-up
Surgery in Young Patients With Open Growth Plates
Because patellar instability peaks in adolescence, surgeons regularly face a population whose bones are still growing. Standard MPFL reconstruction techniques place a tunnel or screw near the growth plate on the thighbone, which could disturb growth if done carelessly. To get around this, physeal-sparing techniques have been developed. These approaches use modified fixation methods, such as wrapping the graft around the bone rather than drilling through the growth plate, or placing hardware in a direction that avoids the physis.28PubMed Central. Arthroscopic reconstruction of the medial patellofemoral ligament in skeletally immature patients using the modified sling procedure: a novel technique for MPFL reconstruction Survey data from surgeons treating skeletally immature patients show roughly even use of autograft and allograft, with suture anchors being the most popular fixation at the kneecap and interference screws the most common at the thighbone.29Orthopaedic Journal of Sports Medicine. Technique Variation for MPFL Reconstruction in Skeletally Immature Patients: Data from the Jupiter Cohort
Getting Back to Activity After Treatment
Whether you had surgery or not, the path back to full activity after a knee subluxation follows a sequence of milestones. Early on, the priority is reducing swelling and restoring range of motion. Next comes strength work, particularly rebuilding the quadriceps, which weaken rapidly after any knee injury. Safe return to sport requires meeting specific benchmarks: no pain, no swelling, full range of motion, near-symmetrical strength compared to the uninjured leg, and good dynamic stability during functional movements.30PubMed Central. Return to sport after patellar dislocation or following surgery for patellofemoral instability
Increasingly, clinicians use objective testing rather than just time-based guidelines to determine readiness. A criterion-based approach might include quadriceps strength testing, single-leg hop tests, lateral step-down tests, agility assessments, and balance measures, alongside an evaluation of psychological readiness.31PubMed Central. Return to Play Considerations After Patellar Instability The psychological piece matters more than people expect. After experiencing a kneecap sliding out of place, the fear of it happening again can be more limiting than the physical recovery, and addressing that fear is a legitimate part of rehabilitation.
Timelines vary widely. After non-surgical treatment of a first-time subluxation, some people return to unrestricted activity in six to eight weeks. After MPFL reconstruction, the typical range is four to six months for non-contact activities and six to nine months for full contact sports. Bone procedures like a tibial tubercle osteotomy add time because the bone itself needs to heal. Rather than fixating on calendar dates, tracking the functional benchmarks above gives a more reliable picture of whether your knee is actually ready.