The lateral patellofemoral ligament (LPFL) is a band of fibrous tissue on the outer side of the knee that helps hold the kneecap in its groove on the femur. It has historically received far less attention than its counterpart on the inner side, the medial patellofemoral ligament (MPFL), but surgeons and researchers have come to recognize it as a quiet workhorse of patellar stability. When the LPFL is damaged or inadvertently cut during surgery, the kneecap can shift inward in an abnormal way, creating a painful and disabling condition that can be surprisingly difficult to fix.
Where the LPFL Sits and What It Looks Like
The LPFL runs from the outer edge of the kneecap to the area around the lateral epicondyle of the femur, which is the bony bump you can feel on the outside of your knee. One cadaver study found that its origin on the femur side is variable from person to person. On average, the ligament originated about 2.6 mm below and roughly 10.8 mm in front of the lateral epicondyle, but the range was wide: some specimens had the origin above the epicondyle, others well below it. The overall distance from the epicondyle averaged about 14.5 mm. The insertion on the kneecap was more consistent, landing in the middle third of the lateral patellar edge, at roughly 45% of its lateral articular surface.1PubMed Central. Lateral Patellofemoral Ligament: An Anatmic Study
In terms of size, the LPFL is a modest structure but not an insignificant one. A study measuring its dimensions in cadavers reported a mean width of about 16 mm and a mean length of roughly 42 mm, with individual specimens ranging from 31 to 53 mm long.2Acta Ortopédica Brasileira. Relationship between the lateral patellofemoral ligament and the width of the lateral patellar facet That makes it wider than many people expect for something that was long considered a minor part of knee anatomy. The ligament blends into the lateral retinaculum, the broader sheet of tissue on the outside of the knee, which is one reason it was overlooked for so long. Surgeons cutting through the retinaculum during common procedures were, in many cases, unknowingly severing the LPFL along with it.
What the LPFL Actually Does in the Knee
The kneecap sits in a shallow groove on the front of the femur called the trochlea, and a network of soft tissues on both sides keeps it tracking properly as the knee bends and straightens. The MPFL on the inside resists the kneecap from sliding outward, which is why MPFL tears are the classic finding after a lateral patellar dislocation. The LPFL does the opposite job: it resists the kneecap from sliding or tilting too far inward. Under normal circumstances, these two structures work together as a balanced pair.
When researchers tested LPFL specimens in a laboratory to see how much force the ligament can withstand, they found a mean load to failure of about 90 newtons, though the spread was large. Nine out of ten specimens broke at the midsubstance of the ligament rather than pulling off the bone, suggesting that the tissue itself is the weakest link rather than its attachment sites. The width of the ligament at its midpoint was the factor most closely linked to how much force it could handle before failing.3PubMed. Biomechanical Properties of the Lateral Patellofemoral Ligament: A Cadaveric Analysis By comparison, the MPFL has been reported to withstand roughly double that load in similar testing, which partly explains why the LPFL tears less dramatically in everyday injuries and why it took longer for the orthopedic world to pay attention to it.
Despite its relatively modest strength, the LPFL plays an important stabilizing role. When the knee is nearly straight, the kneecap rides higher in the trochlear groove and is less constrained by bone. In those first 20 to 30 degrees of bending, the soft tissues on both sides of the patella bear the brunt of keeping it centered. Lose the lateral restraint and the kneecap can wobble or shift medially in a way that wears down cartilage and produces pain.
Lateral Patellar Tilt and Long-Term Joint Damage
One of the ways the lateral patellofemoral compartment causes trouble is through something called lateral patellar tilt, where the kneecap is angled so that its outer edge dips into the trochlear groove more than it should. This is not the same as the kneecap shifting entirely out of position; it is a subtler misalignment that puts extra pressure on the cartilage of the outer patellar facet and the matching surface on the femur.
A longitudinal study using data from a large osteoarthritis cohort found that people with greater lateral patellar tilt at baseline had thinner cartilage and more cartilage damage in the lateral trochlear and patellar regions. Over a two-year follow-up, those with higher tilt angles at the start had about 25% higher odds of developing worsening bone marrow lesions in the lateral trochlear area.4The Knee. Lateral patellar tilt and its longitudinal association with patellofemoral osteoarthritis-related structural damage Bone marrow lesions are a sign of stress on the underlying bone and are closely tied to pain and progression of osteoarthritis.
The LPFL is one of several structures that influence how much the kneecap tilts laterally. A tight lateral retinaculum, a shallow trochlear groove, or muscular imbalances can all contribute. But the LPFL’s role as a discrete, identifiable ligament within the retinaculum means that specific damage to it, or specific surgical alteration of it, can shift the balance in ways that show up years later on imaging. The connection between lateral tilt and progressive cartilage loss helps explain why surgeons have become more cautious about releasing lateral structures indiscriminately.
How Lateral Release Surgery Can Backfire
For decades, lateral retinacular release was one of the most commonly performed knee procedures. The idea was straightforward: if a patient had anterior knee pain or the kneecap was tracking too far to the outside, cutting the tight tissue on the lateral side would let it shift inward. The operation is quick and can be done arthroscopically, and in the right patients it genuinely helps. The problem arose when the procedure was applied too broadly or too aggressively.
The lateral retinaculum is not a single uniform sheet. It contains the LPFL along with other reinforcing fibers. When a surgeon cuts deeply enough to release the retinaculum fully, the LPFL is often divided as well. If the patient actually had a tight lateral retinaculum driving their symptoms, releasing it may restore balance. But if the retinaculum was not truly tight to begin with, or if the release went too far and cut into the vastus lateralis tendon, the kneecap can lose its lateral tether entirely. The result is iatrogenic medial patellar instability, meaning the kneecap now shifts or subluxates toward the inside of the knee rather than the outside.
A study examining this complication found that it was more common in patients who had an over-released lateral retinaculum and in patients who did not have clear evidence of a tight lateral retinaculum before the initial surgery. The authors concluded that aggressive surgical corrections and inappropriate indications for the initial release were two key contributing factors.5PubMed. Iatrogenic medial patellar instability following lateral retinacular release of the knee joint In other words, some of these patients should not have had the procedure in the first place, and others had it done too aggressively. Both paths led to the same outcome: a kneecap that now sits too far medially, with pain, catching, and functional limitation that can be worse than the original complaint.
This recognition has shifted surgical thinking considerably. Lateral release is still performed, but the indications have narrowed. Many surgeons now prefer lateral retinacular lengthening over a full release, which loosens the tissue without completely severing it. A review of lateral patellofemoral joint procedures noted that lateral release, lateral retinacular lengthening, and partial lateral patellar facetectomy all have roles in treating patellofemoral problems, but the key is matching the procedure to the specific pathology.6PubMed Central. A Review of the Lateral Patellofemoral Joint: Anatomy, Biomechanics, and Surgical Procedures The era of “release it and see what happens” has, for the most part, ended.
LPFL Reconstruction for Medial Patellar Instability
When a patient develops medial patellar instability after a lateral release and conservative treatment fails, rebuilding the LPFL is one of the few options that addresses the root cause. The concept is essentially the mirror image of MPFL reconstruction, which has become routine for lateral patellar instability. A graft tendon is used to recreate the ligament that was lost.
One described technique uses a gracilis tendon allograft, threaded through drill holes in the lateral patella and fixed to the femur near the lateral epicondyle. The goal is to restore the lateral tether that was removed during the original release, re-centering the kneecap without over-correcting it back to the lateral side. The procedure is minimally invasive and has been reported to successfully treat cases of isolated medial patellar subluxation.7PubMed Central. Lateral Patellofemoral Ligament Reconstruction With a Gracilis Allograft
A retrospective series looked at 17 patients with 19 affected knees who had developed disabling medial patellar instability after prior lateral release surgery and underwent LPFL reconstruction to restore function.8PubMed Central. Lateral patellofemoral ligament reconstruction to restore functional capacity in patients previously undergoing lateral retinacular release The fact that the population studied consisted entirely of patients who had undergone a previous surgery underscores an important point: LPFL reconstruction is almost always a salvage procedure rather than a primary one. Healthy knees rarely need the LPFL rebuilt because it rarely tears in isolation the way the MPFL does during a lateral dislocation. The patients who end up needing this surgery are overwhelmingly those whose lateral structures were cut during a prior operation.
Reconstruction introduces its own challenges. The variable anatomy of the LPFL’s femoral origin, with that wide range of positions relative to the lateral epicondyle, means the surgeon has to decide exactly where to place the graft’s femoral attachment. Too far anterior and the graft will tighten excessively during flexion; too far posterior and it will be slack when it is needed most, at low flexion angles. The relatively consistent patellar insertion in the middle third of the lateral edge provides a more reliable target, but getting both ends right is essential for the graft to function as a natural ligament would.
Why the LPFL Was Overlooked for So Long
The MPFL dominates the patellofemoral instability literature, and for good reason: lateral patellar dislocation is far more common than medial dislocation, and the MPFL is the primary structure that fails in those injuries. Because most surgeons spent their careers treating lateral instability, the lateral structures were studied mainly as things that might be too tight, not as things that might be too weak or absent. The prevailing surgical question for the lateral side was always “should we release it?” rather than “should we preserve it?”
The blending of the LPFL into the lateral retinaculum also made it easy to dismiss as just part of the retinaculum rather than a distinct anatomic entity. It was not until careful cadaver dissections measured its dimensions, attachment sites, and mechanical properties that the LPFL began to be recognized as a proper ligament with its own surgical significance. Even today, not every anatomy textbook gives it a named entry, and many orthopedic residency programs spend minimal time on it compared to the MPFL.
Imaging adds another layer of difficulty. The LPFL is thin enough that it does not always show up clearly on standard MRI sequences, especially if the radiologist is not specifically looking for it. In contrast, MPFL tears produce obvious signal changes because the injury is acute and the torn ends are displaced. The LPFL is more commonly compromised by a slow, surgical insult rather than a sudden traumatic one, which means the imaging findings can be subtle or absent even when the ligament is functionally gone.
How Patellar Instability Gets Assessed
When a patient presents with a kneecap that feels like it is shifting out of place, distinguishing between lateral and medial instability matters enormously because the treatments point in opposite directions. Clinical examination involves pushing the kneecap sideways and gauging how far it moves and whether the patient becomes apprehensive. An experienced examiner can usually tell which direction is the problem, but imaging helps confirm it.
MRI remains the standard tool for evaluating the soft tissues around the patella. It can show whether the MPFL is torn, whether the trochlear groove is abnormally shallow, and whether there is cartilage damage. For the lateral side, MRI can reveal whether the retinaculum has been surgically disrupted and whether there is excessive medial patellar shift or tilt. Dynamic ultrasound has also been explored as a way to assess patellar restraints in real time. Researchers have found that measuring how far the kneecap moves relative to the trochlea at different knee flexion angles under a small applied load can indicate whether the medial restraining structures are intact.9PubMed Central. Utility of dynamic ultrasound in the assessment of patellar instability The same principle could theoretically be applied to the lateral side by pushing the kneecap medially and measuring translation, though the published work so far has focused mainly on medial restraint assessment.
Axial radiographs or CT scans that catch the kneecap in cross-section are useful for measuring patellar tilt and shift quantitatively. A kneecap that tilts medially or sits medial to the midline of the trochlear groove after a lateral release is a strong clue that the lateral restraints were over-released. Comparing imaging before and after surgery, when available, can make the diagnosis straightforward.
Lateral Retinacular Lengthening as a Middle Ground
The recognition that full lateral release carries real risks has pushed many surgeons toward lateral retinacular lengthening as an alternative. Rather than cutting straight through the retinaculum and its embedded LPFL, the surgeon makes a Z-shaped or L-shaped incision that lets the tissue be loosened and then sutured back together at a slightly greater length. The effect is to reduce the tightness pulling the kneecap laterally without eliminating the lateral restraint entirely.
Lengthening preserves some of the LPFL’s mechanical contribution and maintains a continuous tissue envelope around the outside of the kneecap. The trade-off is that it is a slightly more complex procedure than a simple release, and if the retinaculum is not actually tight, lengthening carries the same risk of making things worse. The critical decision remains the same: accurately diagnosing whether lateral tightness is the actual problem before doing anything to the lateral side. Physical examination, tilt testing, and imaging all feed into that decision.
For patients who need both medial reconstruction and some lateral adjustment, surgeons can combine MPFL reconstruction with a measured lateral lengthening. This combination addresses both the medial deficiency that allowed the kneecap to dislocate laterally and any lateral tightness that may have contributed to abnormal patellar tracking. Balancing the two sides is something of an art, and getting it wrong in either direction leaves the patient with ongoing instability.
The Anatomy Problem That Complicates Every Lateral Procedure
One theme that runs through the LPFL literature is variability. The femoral origin of the LPFL can sit above or below the lateral epicondyle and anywhere from slightly behind it to well in front of it. That range, spanning over 20 mm in some dissection series, is enormous relative to the size of the ligament itself.10PubMed Central. Lateral Patellofemoral Ligament: An Anatmic Study Unlike the MPFL, whose femoral origin between the medial epicondyle and the adductor tubercle has been mapped in great detail across hundreds of specimens, the LPFL’s femoral footprint is still being characterized.
This variability has practical consequences. A surgeon performing LPFL reconstruction cannot simply aim for a single anatomic landmark the way MPFL reconstruction protocols direct the graft toward a well-defined point on the medial femur. Instead, the surgeon has to make intraoperative judgments about where the femoral tunnel should go, often relying on the patient’s individual anatomy and the tension behavior of the graft as the knee is moved through its range of motion. Fluoroscopy or navigation may help, but the literature has not yet converged on a standardized femoral fixation point for LPFL grafts the way it has for MPFL grafts.
The patellar side is more forgiving. With the insertion reliably sitting in the middle third of the lateral patellar edge, the surgeon has a smaller target to hit but a more predictable one. Still, the kneecap is thin at its lateral border, so drilling a tunnel there requires care to avoid fracturing through the bone. Some techniques use suture anchors rather than tunnels to reduce that risk.
Who Is Most at Risk for LPFL-Related Problems
The single biggest risk factor for losing LPFL function is a prior lateral retinacular release. Patients who had a lateral release years ago and now have medial-sided knee pain, a sense that the kneecap shifts inward when they bend their knee, or difficulty with stairs and squatting should raise the question of iatrogenic medial instability with their surgeon. The condition can develop months to years after the original procedure, partly because the remaining tissues slowly stretch out once the primary lateral restraint is gone.
People with generalized joint laxity, such as those with hypermobility syndromes, may be at higher risk if they undergo any procedure that loosens the lateral structures. Their baseline tissue stiffness is already low, so removing even a modest restraint can tip the balance. Similarly, patients with a very shallow trochlear groove have less bony support for the kneecap and depend more heavily on soft-tissue restraints on both sides. In these individuals, preserving the LPFL during any lateral-sided surgery is especially important.
Athletes who place high demands on their knees, particularly in sports involving cutting, jumping, and rapid direction changes, may notice LPFL-related problems sooner than sedentary individuals because the forces across the patellofemoral joint are greater during those activities. However, isolated traumatic LPFL tears without prior surgery are rare in the sports medicine literature. The ligament’s low profile in the injury world is precisely why it stayed under the radar for so long: it almost never breaks on its own, but it can cause significant problems when it is inadvertently destroyed.