Patellar Instability: From Dislocation to Surgery

Patellar instability is a condition in which the kneecap (patella) slides partially or fully out of the groove at the front of the thighbone where it normally sits. Most often the kneecap shifts outward, toward the lateral side of the knee. First-time dislocations tend to strike young, active people, and the combination of anatomy, soft-tissue laxity, and loading demands determines who is vulnerable and how likely the problem is to come back. The condition ranges from a single traumatic episode that heals on its own to a recurrent pattern that gradually damages cartilage and erodes knee function over years.

What Keeps the Kneecap in Place

Your kneecap rides in a shallow channel on the front of the femur called the trochlear groove. During the first roughly 30 degrees of bending, before the kneecap drops fully into that groove, the main thing preventing it from sliding sideways is a band of tissue on the inner side of the knee called the medial patellofemoral ligament, or MPFL. Research confirms the MPFL provides roughly half to 60 percent of the force resisting lateral patellar shift during that early range of motion.¹ Once the knee bends deeper, the bony walls of the trochlear groove take over most of the stabilizing work. This two-phase system means that problems with either the soft tissue or the bony groove can lead to instability, and many patients have issues with both.

Anatomical Risk Factors

Not everyone who takes a hard pivot on the basketball court dislocates a kneecap. The people who do tend to share certain structural features, and a systematic review confirmed that three stand out as statistically significant risk factors for both first-time and recurrent patellar dislocation: trochlear dysplasia, an increased tibial tuberosity–trochlear groove (TT-TG) distance, and patella alta.²

Trochlear dysplasia means the groove itself is abnormally shallow or even flat. Without adequate bony walls, the kneecap has nothing to settle into, so lateral forces can push it out far more easily. TT-TG distance describes how far the attachment point of the patellar tendon on the shinbone sits to the outside of the deepest point of the trochlear groove. A larger offset creates a more outward-pulling force on the kneecap every time the quadriceps fires. Patella alta, or a high-riding kneecap, means the patella sits higher than normal relative to the groove. Modeling studies show that patella alta decreases the contact area between the kneecap and the groove and reduces the lateral restraining forces that keep the patella centered, even after surgical ligament reconstruction.³ Each of these factors can exist in isolation, but they frequently overlap in the same knee, compounding the risk.

Why Women Are Affected More Often

Patellar instability is more common in women than in men, and the difference is not just a matter of activity patterns. Women tend to have higher rates of trochlear dysplasia, patella alta, a wider Q angle (the angle at which the quadriceps pulls on the kneecap), and soft-tissue imbalances that collectively shift the odds.⁴ Research comparing men and women with patellar dislocation found that the bony groove measurements, including sulcus angle, trochlear asymmetry, and trochlear depth, differed significantly by sex, though patellar height did not.⁵ In practical terms, a woman presenting with a first-time dislocation is somewhat more likely to have underlying bony anatomy that predisposes her to recurrence.

Joint Hypermobility and Connective-Tissue Disorders

Anatomy is not the only driver. People whose joints are generally loose, whether from a formal hypermobility syndrome or simply being “double-jointed,” face a substantially higher risk. A large matched-cohort study of over 90,000 individuals found that those with joint hypermobility syndromes experienced patellofemoral instability events at roughly 11 times the rate of matched controls at one year, and the highest risk was in patients with Ehlers-Danlos syndrome, who had about 16 times the odds of an instability event at one year.⁶ Even outside formal diagnoses, studies consistently find that generalized joint laxity is far more common among people who have dislocated a kneecap than among those who have not.⁷

This matters clinically because hypermobile patients often respond differently to treatment. Their soft tissues are inherently more lax, so a reconstructed ligament may stretch sooner, and the return-to-sport timeline may need adjustment. Surgeons treating these patients frequently build additional stabilizing procedures into the surgical plan.

What Happens When the Kneecap Dislocates

A patellar dislocation is not just a kneecap “popping out.” The event typically tears the MPFL and causes bruising to the bone on the undersurface of the patella and on the outer edge of the femur. MRI studies show this pattern consistently.⁸ In one retrospective imaging study, a torn or ruptured MPFL was found in 95 percent of first-time dislocations.⁹

The more serious hidden injury is an osteochondral fracture, in which a chunk of cartilage and underlying bone breaks off, usually from the kneecap itself or the lateral femoral condyle. This is especially common in adolescents. One study of adolescent patients who had surgery for these fractures found that the fragments came from the patella about twice as often as from the femoral condyle, and even after surgical fixation, about 28 percent of patients had recurrent instability.¹⁰ If these fragments are not detected and treated, they can become loose bodies inside the joint, grinding against cartilage and accelerating damage. That is one reason why MRI after a first dislocation is considered important: it reveals injuries that a plain X-ray misses.

Diagnosing Patellar Instability in the Clinic

Beyond imaging, a hands-on examination remains central to diagnosis. The classic test involves pushing the kneecap laterally while the knee is extended; if the patient flinches, tenses, or grabs your hand in fear that the kneecap is about to slide out, that apprehension response is considered a positive finding. A refined version called the moving patellar apprehension test, in which the kneecap is translated while the knee is moved from flexion into extension, has demonstrated very high diagnostic accuracy, with 100 percent sensitivity and about 88 percent specificity when compared against the ability to dislocate the kneecap under anesthesia.¹¹

That said, the reliability of common patellar instability tests between different examiners is not as solid as those accuracy numbers might suggest. A study assessing inter-rater agreement found only fair to moderate reliability for the J-sign and fair to substantial variability for apprehension testing, depending on the limb examined.¹² In other words, two experienced clinicians examining the same knee can disagree on the findings. This is one reason imaging plays a complementary role rather than being optional.

Nonoperative Treatment After a First Dislocation

After a first-time patellar dislocation, the initial approach is usually conservative. The evidence supports getting the knee moving early with active range-of-motion exercises and progressive strengthening, rather than locking the knee in a stiff brace for weeks. Systematic reviews have found no meaningful difference in redislocation rates between protocols that limit weight-bearing and those that allow full weight-bearing, which is one reason clinicians now lean toward avoiding strict immobilization.¹³

A randomized controlled trial comparing a rigid, motion-restricting brace with a simple neoprene sleeve after a first dislocation found essentially identical redislocation rates at three years, around 34 to 38 percent in both groups. The rigid brace group actually had worse early outcomes: less knee motion at four weeks, more quadriceps wasting, and lower functional scores at six months. By three years, the functional gap had closed, but the rigid brace offered no protective advantage at any time point.¹⁴ The practical takeaway: an expensive, bulky brace is probably not worth it after a straightforward first dislocation. A comfortable knee sleeve, paired with structured rehabilitation, tends to achieve the same stability with less muscle loss.

One honest caveat: surgery does appear to produce a lower rate of re-dislocation compared to conservative management, but whether it produces better overall knee function is still unclear.¹⁵ For a first-time dislocator without major structural risk factors or loose fragments in the joint, rehabilitation remains a reasonable first step.

When Surgery Becomes the Answer

Surgery enters the conversation when dislocations keep recurring, when significant anatomical risk factors make recurrence highly likely, or when an osteochondral fracture needs repair. The most common procedure is reconstruction of the MPFL using a tendon graft. At long-term follow-up averaging 12 years, MPFL reconstruction had a recurrent dislocation rate of about 14 percent, compared to 41 percent with simple repair of the torn ligament.¹⁶ Reconstruction also maintained favorable functional scores over a decade of follow-up.¹⁷

Technical details matter more than you might expect. The precise placement of where the graft attaches to the femur is critical; being off by even a few millimeters changes the tension pattern through the range of motion and can cause the reconstruction to fail or create new pain. Research into graft choices, whether autograft (from the patient’s own body) or allograft (from a donor), shows no significant differences in pain reduction, return to activity, or failure rates overall, though some data suggest allografts may have a slightly lower recurrence rate.¹⁸

Bony Procedures for Structural Problems

When the kneecap is high-riding or the tibial attachment is too far lateral, soft-tissue reconstruction alone may not be enough. A tibial tubercle osteotomy (TTO) involves cutting the bony bump where the patellar tendon attaches and moving it to a better position, often inward and sometimes slightly downward. This is a customizable procedure tailored to the specific malalignment present.¹⁹ Published series report excellent stability outcomes in roughly 77 percent of cases treated with medial transfer, with or without a downward shift when patella alta is present.²⁰

Trochleoplasty for a Shallow Groove

For patients with severe trochlear dysplasia, essentially a flat or convex groove, some surgeons offer trochleoplasty, which reshapes the groove itself to create a deeper channel. This is a more invasive and technically demanding procedure, and it reliably eliminates instability. In one series combining trochleoplasty with MPFL reconstruction, there were no episodes of redislocation, and knee function scores improved significantly.²¹ A more recent study in adolescents and young adults with severe dysplasia also found favorable outcomes and no recurrent instability at a minimum two-year follow-up, regardless of whether TTO was added.²²

The trade-off is pain. Even in small series, a substantial proportion of patients report anterior knee pain after trochleoplasty.²³ This is an operation typically reserved for people whose groove anatomy is so abnormal that nothing less invasive has a realistic chance of working.

Special Considerations in Children and Adolescents

Patellar instability peaks in adolescence, which creates a surgical dilemma: the growth plates around the knee are still open. Standard adult techniques for tibial tubercle osteotomy or femoral tunnel drilling can damage these growth centers and potentially cause limb-length differences or angular deformities. Surgeons have responded by developing physeal-sparing approaches, meaning techniques that stabilize the kneecap without crossing or disrupting the growth plate.²⁴

Results with physeal-sparing MPFL reconstruction are encouraging. Studies with at least two years of follow-up have reported no cases of growth arrest, limb-length discrepancy, or angular deformity in skeletally immature patients.²⁵ These modified techniques have expanded the surgical options for young patients who might previously have been told to wait until skeletal maturity, often enduring repeated dislocations and accumulating cartilage damage in the meantime.

Getting Back to Sport

Return to sport after patellar instability treatment is more complicated than clearing a timeline. Current thinking emphasizes a combination of objective physical benchmarks, such as quadriceps strength symmetry and single-leg hop performance, alongside psychological readiness and confidence in the knee during sport-specific movements.²⁶ The psychological dimension is not trivial: fear of redislocation is one of the main reasons athletes do not return to their pre-injury level, and it tends to be undertreated compared to the physical rehabilitation.

After isolated MPFL reconstruction, return-to-sport rates are generally favorable, with studies reporting rates in the range of 75 to 81 percent.²⁷ When more complex bony procedures are added, though, recovery takes longer and the overall return-to-sport level tends to be lower.²⁸ Athletes considering combined soft-tissue and bone surgery should understand that the trade-off for better long-term stability is a harder road back to competition.

Long-Term Cartilage Damage and Arthritis

This is the part of the patellar instability story that does not get enough attention. Every time the kneecap slides out, cartilage takes a hit. MRI studies show that cartilage lesions are present in about 71 percent of patients after a single dislocation, rising to 97 percent in those with multiple dislocations. The prevalence of early osteoarthritis follows the same pattern, correlating with the number of dislocations.²⁹

A population-based study following patients for over a decade after a first dislocation painted a stark picture of cumulative risk. About 10 percent of those who had dislocated a kneecap were diagnosed with patellofemoral arthritis during the study period. The cumulative incidence climbed steadily over time, reaching roughly 15 percent by 20 years and nearly 49 percent by 25 years, compared to under 9 percent in a matched group without dislocation history. The hazard ratio was about 7.8, meaning dislocation patients developed arthritis at nearly eight times the rate of controls.³⁰

One piece of somewhat reassuring news: arthritis driven by instability, dysplasia, or malalignment appears less likely to spread beyond the kneecap compartment to the rest of the knee than arthritis from other causes like obesity or inflammatory disease.³¹ Still, the arthritis numbers underscore why preventing recurrent dislocations matters so much, particularly in teenagers whose joints still have decades of use ahead.

Measuring How Patients Actually Feel

Surgeons track stability and range of motion, but what patients care about most is whether they can trust their knee during daily life and sports. Generic knee scoring systems often miss the unique anxiety that comes with patellar instability, the fear of a sudden, painful event that could happen while stepping off a curb. Two disease-specific questionnaires have been developed and validated for this population: the Banff Patella Instability Instrument, which captures quality-of-life impacts, and the Norwich Patellar Instability Score, which focuses on the situations that provoke symptoms and fear of dislocation.³² These two tools correlate well with each other and with the more commonly used Kujala score.³³ Their existence matters because they push clinicians and researchers to evaluate treatment success from the patient’s perspective, including the psychological burden, not just whether the kneecap stayed in the groove on an exam table.