Patellar Instability & Recurrent Kneecap Dislocation

If your kneecap has dislocated — once, or repeatedly — the question worth asking is not "do I need surgery." It is: what is the underlying anatomy that let it dislocate, and what does the cartilage look like now? Patellar instability is one of the few orthopedic problems where the same diagnosis can lead to five different operations depending on the answers. A first-time dislocator with a normal-shaped trochlea, a normal patellar height, no cartilage damage, and an isolated MPFL injury is a very different patient than a 19-year-old with three dislocations, a shallow trochlear groove, a high-riding patella, and a free cartilage fragment in the joint.

This page is the sub-specialty hub for patellofemoral instability. It covers what patellar instability is and how it feels, the anatomical risk factors that drive recurrence, who develops the condition, what to do after a first-time dislocation, the full treatment ladder (physical therapy → MPFL → TTO → cartilage repair, with the trochleoplasty and lateral retinacular options where they apply), MPFL-alone vs. MPFL+TTO decision-making, pediatric considerations, what to expect on surgery day, recovery, risks, insurance reality, and when to seek a sub-specialty second opinion. For the dedicated procedure walk-through, see the MPFL reconstruction surgery hub; for the realignment side, see joint preservation and osteotomy; for cartilage repair after dislocation, see MACI cartilage repair and cartilage transplantation (OATS & allograft).

The Patellar Instability Spectrum: Subluxation → Dislocation → Recurrence

The kneecap (patella) sits in front of the knee and slides up and down in a groove on the front of the thigh bone — the trochlear groove. As you bend and straighten the knee, the patella tracks through the groove and acts as a lever for the quadriceps. Patellar instability describes a continuum: at one end, the kneecap shifts partially out of the groove and returns on its own (a subluxation); at the other end, the kneecap fully leaves the groove and either reduces with leg extension or has to be reduced manually, sometimes in the emergency room (a dislocation, sometimes called a kneecap dislocation or patellar dislocation). Both ends of the spectrum can damage the medial patellofemoral ligament (MPFL) — the primary soft-tissue restraint to lateral patellar translation — and both can damage the cartilage on the back of the patella or the lateral side of the trochlea.

The reason the distinction matters clinically is not just the severity of the event itself. It is what the event tells you about the underlying anatomy. A patient who can subluxate their patella with simple twisting motions, or whose patella has slipped repeatedly without a fall or contact event, almost always has anatomical risk factors that need to be measured before deciding on treatment. A first-time traumatic dislocation in an athlete with otherwise normal anatomy is a different problem from a fifth atraumatic subluxation in a loose-jointed adolescent.

Repeated episodes — whether full dislocations or recurrent subluxations — progressively wear down the patellar and trochlear cartilage and contribute to early patellofemoral arthritis. This is the most under-appreciated long-term consequence of recurrent patellar instability and the reason aggressive evaluation after the second dislocation is usually warranted. Dr. Strickland's writing on what happens when patellar dislocation is untreated covers this in more detail.

How an Episode Feels

It is typically painful, and in severe cases it can be difficult to fully extend the knee. After a true dislocation, the kneecap may be visibly out of place — sometimes patients have to go to the emergency room to have the patella reduced back into the groove. Often the injury is accompanied by significant swelling (hemarthrosis) that takes days to weeks to resolve, and weeks to months before the knee feels normal again. For the immediate steps after an episode, see how serious is a patella dislocation and what to do after a kneecap dislocation.

Daily activities like going up and down stairs, squatting, kneeling, or getting out of a chair may feel painful or unsteady. Many patients describe a sense of "giving way" or feeling like they cannot trust the knee during sports or even routine movements. Because of this, patients often start avoiding the activities they enjoy, which leads to quadriceps weakness, hip-stabilizer weakness, and paradoxically more instability over time.

Common symptoms after a dislocation event include:

• Audible pop or popping sensation when the kneecap moves out of place
• Visible deformity — the kneecap may sit visibly off to the side until it reduces
• Immediate pain and difficulty walking after the event
• Rapid swelling within minutes to hours, often a tense effusion (hemarthrosis)
• Bruising along the medial side of the kneecap (where the MPFL tore)
• Apprehension — a learned protective response when the knee is in positions where the kneecap previously slipped
• Mechanical symptoms — catching, locking, or grinding — which can suggest a loose cartilage or osteochondral fragment in the joint

For early-stage subluxations that don't fully dislocate, see how to tell if your kneecap slightly dislocated. For first-time dislocations in the urgent care setting, see how urgent care treats a dislocated kneecap.

Anatomical Risk Factors

Why does the same first-time dislocation lead to one recurrence in some patients and ten in others? The answer is anatomy. A set of anatomical features, measured on imaging, drives most of the recurrence risk and most of the treatment decisions. The classical computer-modeling work (a study by Lutul Farrow, a fellow member of the International Patellofemoral Study Group) identified three anatomical factors linked to higher risk of recurring patellar dislocation; in clinical practice, Dr. Strickland evaluates several additional features that further refine the risk profile. Dr. Strickland's posts on patellar tracking abnormalities and why patellofemoral height matters ground the clinical framing.

Patellofemoral height in particular is one of the under-appreciated outcome predictors — see Dr. Strickland's writing in "It's the patellofemoral height". Other refinements that may apply to your case include the femoral anteversion angle (see evaluating patellar dislocation via the femoral anteversion angle), the lateral trochlear inclination angle (see measuring the lateral trochlear inclination angle), and Wiberg patellar shape type (see Wiberg patellar type impact on outcomes and survival). It's worth knowing that some assessments of patellofemoral instability have been shown to have poor interrater reliability — which is why high-volume sub-specialty evaluation matters more for this diagnosis than for many others.

The number of risk factors a patient has — and which ones — is what determines the operation. A patient with patella alta and an elevated TT-TG distance needs a tibial tubercle osteotomy in addition to MPFL reconstruction; a patient with a normal trochlea, normal patellar height, and isolated MPFL injury usually does well with MPFL reconstruction alone. Skipping the imaging-based assessment is the most common reason patellar stabilization surgery fails.

Who Develops Patellar Instability

Loose-jointed young women and female athletes who experience a traumatic dislocation while playing their sport are at the highest risk for recurrent subluxation and patellar dislocation. Soccer, basketball, dance, gymnastics, cheerleading, and skiing are common backstories. Patellar instability does occur in men and boys — particularly with high-energy contact sports — but recurrence is less common in that population, and the underlying anatomy is more often normal.

The age at first dislocation is the single biggest predictor of recurrence. Patients whose first dislocation happens at a young age have a substantially higher recurrence rate than those whose first dislocation happens later in life. This is partly because young patients are still active and exposed to risk, and partly because younger patients more often have the underlying anatomical risk factors that produced the first dislocation in the first place.

In some patients, patellar instability happens without any clear trauma — the kneecap slips during a normal activity like turning, descending stairs, or even just standing up. This is often a sign of significant anatomical risk — particularly trochlear dysplasia and ligamentous laxity — and these patients usually warrant earlier surgical evaluation. A separate subset of patients develops medial patellar facet lesions from the friction of repeated subluxation, and these patients often require concurrent cartilage repair.

First-Time Dislocations: What to Do and When to Operate

If you have had a first patellar or kneecap dislocation, the first step is a careful evaluation that includes:

• MRI to evaluate the MPFL, the cartilage on the back of the patella and the lateral trochlea, and to look for loose osteochondral fragments in the joint
• Standing alignment films to assess overall lower-extremity alignment
• Lateral X-ray with patellar height measurement (Caton-Deschamps or Insall-Salvati index) to identify patella alta
• Axial imaging with TT-TG distance when bony realignment may be considered

Most first-time dislocations without cartilage damage and without major anatomical risk factors are treated non-operatively: a brief period of bracing, structured physical therapy focused on quadriceps and hip-stabilizer strengthening, and a graduated return to activity. Many of these patients do well long-term and never require surgery. Dr. Strickland's writing on whether you can fully recover from a dislocated patella covers the realistic expectations.

Surgery is considered after a first dislocation when:

• There is a loose cartilage or bone fragment in the joint that needs to be removed or fixed
• There is a large MPFL avulsion off the patella that may benefit from acute repair or reconstruction
• There are multiple anatomical risk factors that make recurrence likely — trochlear dysplasia plus alta plus an elevated TT-TG is a much higher-risk profile than any one of those alone
• The patient is a young athlete returning to high-risk sport (soccer, basketball, dance, skiing) where the consequences of a re-dislocation in season are significant

After a second dislocation, the picture changes. Recurrence rates without surgery rise sharply, and the cartilage on the back of the patella has often taken progressive damage from each subluxation event. MPFL reconstruction is usually recommended after a second dislocation, with the operation chosen based on the anatomical risk profile.

The Treatment Ladder: PT → MPFL → TTO → Cartilage Repair

Patellar stabilization for recurrent dislocations is one of the few orthopedic problems where the surgical decision is not "operate or don't" — it is "which operation, in which combination." The treatment ladder progresses from least invasive to most:

The ladder is conceptual — in practice, the decision is often made up front based on the imaging-based risk profile rather than progressing rung by rung after recurrent failures. A patient with patella alta and a chondral defect almost certainly needs all three additions (MPFL + TTO + cartilage repair) in the same operation, not three sequential surgeries spaced years apart. A study reviewed by Dr. Strickland supports the more anatomic combined-reconstruction approach.

MPFL Alone vs. MPFL+TTO — The Decision Framework

This is the most important sub-specialty decision on the page. MPFL reconstruction alone is sufficient for many patients with recurrent dislocations. A subset have bony anatomy that means the soft-tissue reconstruction alone will be biomechanically asked to do more than it should — in those patients, combining MPFL reconstruction with a tibial tubercle osteotomy (TTO) addresses the bony alignment that is contributing to the instability. The decision is based on imaging measurements, not symptoms alone. Dr. Strickland has written specifically on the most common MPFL + TTO patient questions.

When bony factors are missed at the index surgery, the MPFL graft is asked to substitute for both the soft-tissue restraint and the missing bony containment — a setup for recurrent instability or graft stretching. Unaddressed bony risk factors are one of the most common causes of failed MPFL reconstruction. Dr. Strickland's cadaveric study on anteromedialization (AMZ) TTO demonstrated improved patellar contact forces in dysplastic knees. For revision-TTO discussion specifically, see case report on arthroscopy at revision TTO and HSS Grand Rounds on revision TTO. For the distalization technique used in patella alta, see TTO distalization for patella alta. For when the surgery is described as a "transfer" rather than an "osteotomy," see tibial tubercle osteotomy or transfer. The international Modified Delphi Consensus on patellar instability (which Dr. Strickland contributed to) provides the multi-surgeon expert reference framing for these decisions.

Trochleoplasty (For Severe Dysplasia)

When the trochlear groove itself is severely dysplastic — flat, convex, or with a high-grade Dejour-classified deformity — soft-tissue reconstruction and bony realignment alone may not be sufficient. Trochleoplasty is a procedure that reshapes the bony trochlear groove itself to provide a containing track for the kneecap. It is reserved for the most anatomically severe cases and adds significant complexity to the operation and the recovery. Whether trochleoplasty is appropriate is a sub-specialty judgment based on the trochlear morphology on imaging, prior treatment history, and patient factors. For most dysplastic knees, MPFL + TTO without trochleoplasty is sufficient; trochleoplasty enters the conversation for the most extreme dysplasia or for revision settings where prior soft-tissue and bony procedures have failed.

Concurrent Cartilage Repair

A patellar dislocation is rarely just a soft-tissue event. As the patella shears off the trochlear groove and snaps back into place, the cartilage on the back of the patella and along the lateral wall of the trochlea can be damaged. This shows up on MRI as a bone bruise pattern at minimum, and in some cases as a frank osteochondral lesion or a loose fragment in the joint. Patients with patella alta in particular have a higher rate of cartilage damage at the time of evaluation. Dr. Strickland's work on treatments of patellar chondral lesions, the podcase on patellofemoral instability and cartilage lesions, and understanding patellofemoral instability cartilage lesions all discuss this combined-pathology population.

When focal cartilage damage is present, MPFL reconstruction is often combined with a cartilage procedure in the same operation:

• Loose fragment fixation or removal — for displaced osteochondral fragments after acute dislocation
• MACI (matrix-induced autologous chondrocyte implantation) — a two-stage procedure where cartilage cells are harvested from the patient at the index surgery, grown in a lab, and implanted at a second surgery onto a focal patellar or trochlear cartilage defect. See MACI cartilage repair for the full walk-through
• Osteochondral allograft (OCA) — for larger lesions that include subchondral bone. See cartilage transplantation
• Chondroplasty — to clean up smaller loose flaps of cartilage

One specific technical consideration when patellar cartilage repair is being performed via a lateral approach: preserving patellar vascularity. Dr. Strickland's study on patellar vascularity after lateral parapatellar approach addresses the technical detail that matters here. When chronic patellofemoral cartilage wear has progressed beyond focal repair territory, the conversation shifts to patellar arthritis management and possible patellofemoral replacement.

Emerging Implant-Based Options

For carefully selected patients, newer joint-preserving implants are being studied alongside the established procedures. The FDA trial of the Patella LIFT procedure evaluates an implant designed to offload the patellofemoral joint. Dr. Strickland's team performed the first East Coast Patella LIFT surgeries at HSS as part of FDA PELICAN trial enrollment. Whether you are a candidate is determined by trial inclusion criteria, anatomy, cartilage status, and prior treatment history. This is not currently available for persistent patellar instability.

Adjacent technologies are also part of the patellofemoral toolkit. Dr. Strickland's writing on 3D imaging of the patellofemoral joint and 3D printing for patellofemoral instability rehabilitation describes how patient-specific models inform planning and rehab. These options are appropriate for a small subset of patients and are discussed at consultation when the imaging and history fit.

Pediatric Patellar Instability

Pediatric and adolescent patellar instability is a distinct clinical scenario because the growth plates are still open. Surgical technique must avoid the distal femoral physis, and the bony procedures available to adults (TTO, trochleoplasty) are usually not options until skeletal maturity. Dr. Strickland is a co-author on the multicenter JUPITER pediatric MPFL reconstruction study, which documented the high variation in pediatric MPFL technique across high-volume centers.

Most adolescents with recurrent dislocation are surgical candidates — the recurrence rate without surgery is high in this group, and continued dislocations cause cumulative cartilage damage during a phase of life when the patient is most active. The decision-making about timing, graft choice, and physeal-respecting fixation is sub-specialty work, and adolescent patients benefit from a surgeon who treats this population in volume.

For young patients whose growth plates are still open, the standard bony procedure to bring a high-riding kneecap down (a tibial tubercle osteotomy) isn't safe — it would damage the growth plate. Dr. Strickland published a way around this: a stitch-based technique that shortens the patellar tendon without cutting it. It can be done in the same operation as MPFL reconstruction. The technique was published in Arthroscopy Techniques (2022), with follow-up outcomes data published in the Orthopaedic Journal of Sports Medicine (2025).

Prehabilitation

For elective MPFL reconstruction (with or without TTO or cartilage repair), prehabilitation matters. Dr. Strickland recommends starting prehab at least 4 to 6 weeks before surgery. The goals are full pain-free range of motion, strong quadriceps activation, and good hip and core control. Patients who arrive at surgery with a quiet, mobile knee and active quads regain motion and strength faster after the operation than patients who arrive with a stiff, swollen, quad-inhibited knee.

A productive prehab phase typically includes:

• Working with a physical therapist who has knee or sports medicine expertise
• Familiarizing yourself with the planned post-operative rehab protocol
• A home exercise program built around quad sets, straight-leg raises, hip and glute work, and gentle range of motion within pain-free limits
• An up-to-date medication and supplement list reviewed with your surgical team
• A plan for transportation home, post-op support at home, and time off from work or school

Acute dislocations with a locked knee or a loose osteochondral fragment do not get this 4-to-6-week window — those patients need to go to the operating room sooner. For more, see Dr. Strickland's how to prepare for an MPFL reconstruction guide.

What to Expect on Surgery Day

Patellar stabilization surgery is performed as an outpatient procedure in most cases — you go home the same day. Combined cases involving TTO and concurrent cartilage repair may have additional recovery considerations that are reviewed at consultation. Typical patient experience:

• Arrival and pre-op — you arrive a couple of hours before the procedure for intake, change into a gown, and meet the anesthesia team
• Anesthesia consultation — the anesthesiologist reviews your history and discusses the anesthesia plan. For Dr. Strickland's patellar stabilization surgeries, this is typically regional anesthesia (a spinal block with sedation) rather than general anesthesia. During surgery, a periarticular block is also placed around the knee joint to help reduce pain during the first day after surgery as part of an opioid-sparing pain protocol.
• Surgery — isolated MPFL reconstruction typically takes 1 to 2 hours; combined MPFL + TTO is longer; combined MPFL + TTO + cartilage repair is the longest of the three
• Recovery room — you wake up in the recovery area; your knee is in a brace locked in extension. Physical therapy starts the same day or the next morning with crutch training and basic motion within the safe limits of the brace
• Going home — you go home with crutches, the brace, ice, prescribed pain medication, and clear written instructions. A responsible adult must drive you home
• First 48 hours — ice, elevation, prescribed medications, gentle motion as instructed. The first week's focus is effusion control, quadriceps activation, and walking with weight in the brace

Cold-compression devices and quadriceps muscle stimulators (when prescribed) can help reduce swelling and support muscle reactivation in the first weeks. The block typically wears off over the first 12 to 24 hours, during which the leg is numb and weight-bearing requires the brace and crutches.

Recovery Timeline

Recovery is a structured, milestone-based progression. The dates below are general guidelines; your specific plan depends on which procedures were performed.

The two factors that most consistently slow recovery are inconsistent physical therapy attendance and rushing back to high-impact activity before clearance. For the full week-by-week walk-through, see Dr. Strickland's how long does MPFL recovery really take.

Risks of Surgery

Patellar stabilization surgery is well-established with predictable outcomes for most well-selected patients, but no surgery is risk-free. The risks reviewed at consultation include:

• Recurrent instability or graft failure — the most clinically relevant long-term risk. Most often associated with unaddressed bony risk factors, non-anatomic tunnel placement, or returning to high-demand activity before milestones are met. Imaging-guided patient selection and an honest discussion of whether TTO needs to be added are the main mitigations
• Persistent stiffness or loss of motion — reduced by prehabilitation, early focus on regaining motion, and structured PT
• Patellar fracture — rare with all-suture anchor fixation, more often associated with patellar bone tunnels
• Articular cartilage damage from patellar fixation — minimized by all-suture anchors compared with larger metallic implants
• Anterior knee pain — can persist after surgery, particularly when there is unaddressed concurrent patellofemoral cartilage damage
• Tibial tubercle nonunion or hardware irritation — specific to TTO; rare but can require hardware removal
• Infection — uncommon but a serious complication if it occurs
• Blood clot (DVT or pulmonary embolism) — risk is mitigated by early mobilization and individualized prophylaxis
• Anesthesia-related risks — including nausea, sore throat, rare allergic reactions, transient nerve irritation from the regional block
• Cartilage repair-specific risks — for MACI/OATS additions, including incomplete cartilage incorporation and the need for revision cartilage work
• Iatrogenic patella baja — rare complication of distalization-TTO; see Dr. Strickland's writing on surgical treatment of iatrogenic patella baja
• Incomplete return to prior level of sport — some patients do not return to their prior level even with technically excellent surgery

Outcomes are generally good in well-selected patients, but results depend on individual anatomy, adherence to rehab, and overall health. Specific risks for your case depend on imaging, prior surgeries, concurrent procedures, and goals — reviewed at consultation.

Common Patient Concerns

The three concerns we hear most often before patellar stabilization surgery, with honest answers.

Insurance and Cost

MPFL reconstruction, tibial tubercle osteotomy, and concurrent cartilage repair are covered by all major commercial insurance plans, Medicare, and most self-funded and union plans when the diagnosis and indication for surgery meet medical-necessity criteria. The variables that drive your specific out-of-pocket cost are:

• Your plan's deductible and coinsurance
• In-network vs. out-of-network status — for the surgeon, the facility (Hospital for Special Surgery or affiliated outpatient surgery center), and the anesthesia group
• Bundled vs. separate billing for the surgeon, facility, anesthesia, imaging, physical therapy, and concurrent procedures (TTO, cartilage repair)
• Out-of-network benefits — if you have them and choose to use them; we are happy to provide the codes you need to verify your benefits in advance

The Patella LIFT procedure, when performed under the FDA PELICAN trial, has separate considerations because it is investigational. Trial-related costs and the standard-of-care portions of the operation are handled differently; this is reviewed in detail when trial eligibility is being discussed.

Before surgery, our office verifies your benefits, obtains pre-authorization where required, and reviews the estimated out-of-pocket cost with you. For benefits verification or to discuss self-pay arrangements, call us at (646) 960-7227 or contact the office.

Patient Outcomes

Outcomes of patellar stabilization surgery vary by anatomy, adherence to rehab, and the specific procedures performed. A few patient stories from Dr. Strickland's practice illustrate the range of presentations and recoveries:

• Paulette Gangemi — back in the game after patellar dislocation
• Liam's story — MPFL reconstruction surgery
• Peyton's tibial tubercle osteotomy — recovery after combined patellar stabilization with TTO

Each story highlights individual factors and outcomes — results are not predictive of other patients' results. The point is that combined patellar stabilization with the right procedure for the right anatomy can return active patients to their lives.

When to Seek a Sub-Specialty Second Opinion

Patellofemoral instability is one of Dr. Strickland's primary areas of clinical and academic focus. She is a member of the International Patellofemoral Study Group, a co-author of the multicenter JUPITER pediatric MPFL study, and a contributor to the Modified Delphi consensus on patellar instability. She is frequently asked for second opinions on first-time and recurrent dislocations — especially when cartilage injury, osteotomy, or revision MPFL reconstruction may be part of the plan.

A sub-specialty second opinion is particularly worth seeking when:

• You have had a second dislocation and are trying to decide between continued non-operative care and MPFL reconstruction
• You have already had MPFL reconstruction that failed — either re-dislocation or persistent pain — and are facing revision
• Your imaging shows multiple anatomical risk factors and the question is whether to add a TTO to the MPFL reconstruction
• You have cartilage damage on your post-dislocation MRI and want a clear answer on whether to combine cartilage repair with stabilization
• You are an adolescent or young athlete and the goal is return to high-level sport, not just stopping the dislocations
• Your imaging shows severe trochlear dysplasia and you want to understand whether trochleoplasty is on the table
• You are interested in the Patella LIFT FDA trial and want to know whether you meet trial inclusion criteria
• You have generalized ligamentous laxity or a connective tissue disorder (Ehlers-Danlos, Marfan, hypermobility spectrum)

Access & Office Locations

Dr. Strickland sees patellar instability patients at two offices, both of which work with patients traveling in from outside the immediate area:

• New York City (primary): Hospital for Special Surgery, East River Professional Building, 523 East 72nd Street, 2nd Floor, New York, NY 10021. Upper East Side, accessible from Manhattan, the outer boroughs, Long Island, Westchester, and northern New Jersey via the Queensboro and Triboro bridges and the FDR Drive. Phone: (646) 960-7227.
• Stamford, CT (satellite): Stamford Chelsea Piers, 1 Blachley Road, Stamford, CT 06902 — convenient for patients in Fairfield County, lower Connecticut, and Westchester.

Many patients travel to New York for sub-specialty patellofemoral care, particularly for revision MPFL reconstruction, combined MPFL+TTO planning, concurrent cartilage repair, and Patella LIFT trial eligibility evaluation. We coordinate consultation, imaging review, and surgery scheduling to minimize travel for out-of-state patients.

Frequently Asked Questions

Related Specialty Care

For the dedicated procedure walk-through of MPFL reconstruction itself — surgical technique, graft choices, two-point patellar fixation, recovery timeline — see the MPFL reconstruction surgery page. For the realignment side of patellar stabilization (TTO, HTO, DFO), see joint preservation and osteotomy. For cartilage repair after dislocation-related cartilage damage, see MACI cartilage repair and cartilage transplantation (OATS & allograft). For chronic patellofemoral cartilage wear, see patellar pain and patellar arthritis. For complex anatomy where computer-guided planning is helpful, see Mako robotic-assisted surgery. When ACL injury has occurred alongside patellar dislocation, see ACL tear surgery.

For Dr. Strickland's published research and editorial commentary on patellofemoral instability, see her cadaveric AMZ-TTO study, the Modified Delphi consensus on patellar instability, the JUPITER pediatric MPFL study, the two-point patellar fixation editorial, and her commentary on patellofemoral height as an outcome predictor. For practical patient-facing guides, see MPFL reconstruction questions, MPFL + TTO questions, how to prepare for MPFL reconstruction, and how long does MPFL recovery really take. For patellar dislocation patient-education posts, see my kneecap dislocated — what should I do?, how serious is a patella dislocation?, and what happens with untreated patellar dislocation.