What Is the Hip Labrum?

If you’ve been told you have a hip labral tear or femoroacetabular impingement, you probably have a lot of questions — and you’ve probably encountered a lot of confusing terminology. What is the labrum? What does it actually do? What do “cam” and “pincer” mean? And why does a torn labrum hurt?

This article is meant to give you a clear understanding of the anatomy and biomechanics behind these terms. I’ll cover treatment options — both nonoperative and surgical — in a separate article.

The Hip Joint: A Ball and Socket

The hip is one of the largest and most stable joints in the body. It’s a ball-and-socket joint: the ball is the femoral head (the top of your thighbone), and the socket is the acetabulum (a cup-shaped part of your pelvis). Both surfaces are covered with articular cartilage — a smooth, slippery lining that allows the bones to glide against each other with very little friction. Damage to this cartilage is what we call arthritis.

Unlike the shoulder — which sacrifices stability for an enormous range of motion — the hip is built for both stability and weight-bearing. The socket is deep, and the ball fits snugly inside it. But the bony socket alone doesn’t cover enough of the femoral head to fully secure it. That’s where the labrum comes in.

What Is the Labrum?

The labrum is a ring of tough fibrocartilage that lines the rim of the acetabulum, essentially extending the edge of the socket. The word itself comes from the Latin for “lip,” and that’s a good way to picture it — a lip of cartilage that deepens the cup around the femoral head.

The labrum increases the depth of the socket and the surface area of the joint by roughly 10%, allowing the acetabulum to cover more than half of the femoral head (Tan et al., Am J Orthop, 2001). It’s triangular in cross-section, about 2 to 3 millimeters thick, and it runs almost the entire way around the rim of the socket. At the bottom of the socket, where there’s a small notch, the labrum blends into a structure called the transverse acetabular ligament, forming a continuous ring.

It’s wider and thinner at the front of the hip, and thicker at the back — a detail that turns out to matter, because the front (anterior) portion is the area most vulnerable to tearing (Groh & Herrera, Curr Rev Musculoskelet Med, 2009).

What Does the Labrum Actually Do?

The labrum does more than just make the socket deeper. It serves several critical functions that are essential to keeping the hip healthy.

It creates a suction seal. The labrum forms a tight seal around the femoral head, trapping a thin layer of pressurized synovial fluid (joint fluid) inside the joint. This fluid layer is what keeps the cartilage surfaces lubricated and helps distribute forces evenly across the joint. A cadaveric study by Ferguson et al. demonstrated that removing the labrum resulted in a 22% increase in the rate of cartilage consolidation and significantly reduced the fluid pressure inside the joint (Ferguson et al., J Biomech, 2003). Without the labrum’s seal, the cartilage loses the protective fluid layer it depends on.

It stabilizes the hip. That same suction seal creates a negative pressure inside the joint, which acts like a vacuum holding the femoral head in the socket. A cadaveric study by Nepple et al. showed that the labrum is responsible for approximately 70–77% of the hip’s resistance to distractive forces at small displacements (Nepple et al., Arthroscopy, 2014).

It protects the cartilage. By maintaining fluid pressurization and distributing loads more evenly, the labrum reduces the stress on the articular cartilage. When the labrum is damaged, those forces become concentrated on a smaller area, which accelerates wear and can eventually lead to arthritis.

It provides proprioception and detects pain. The labrum contains nerve endings — both free nerve endings that transmit pain signals (nociception) and specialized nerve end organs that detect pressure, position, and movement (proprioception). These nerve fibers are most concentrated in the front and top of the labrum — the same area most commonly torn — which helps explain why labral tears can be so painful (Alzaharani et al., BMC Musculoskelet Disord, 2014).

What Is Femoroacetabular Impingement (FAI)?

Femoroacetabular impingement — or FAI — is a condition where the shape of the bones around the hip causes abnormal contact between the femoral head and the acetabular rim during movement. Over time, this repeated abnormal contact pinches and damages the labrum, which is caught between the two bony surfaces.

It’s important to understand that FAI is a clinical syndrome, not just an X-ray finding. An international consensus statement defines it as requiring a triad of symptoms (typically groin pain), positive findings on physical exam, and characteristic imaging features (Griffin et al., Br J Sports Med, 2016). Many people have the bony shape of FAI on imaging without any symptoms at all — and those people don’t have FAI syndrome, they simply have cam or pincer morphology.

There are two types of bony abnormality, and they can occur alone or together.

Cam Impingement

In cam impingement, the problem is on the femoral (ball) side. Instead of the femoral head being perfectly round, there’s extra bone at the junction between the head and the neck — essentially a bump that reduces the normal concavity between the ball and the neck of the femur. When the hip flexes and rotates, this bump gets jammed into the socket. Because the ball isn’t round, it doesn’t rotate smoothly — instead, the aspherical portion shears against the cartilage inside the socket, grinding it away from the outside in.

Cam morphology is more common in young, athletic men and may develop during adolescence when the growth plate at the top of the femur is still open. High-impact sports during this period are thought to contribute to its development (Chaudhry & Ayeni, Sports Health, 2014). The damage it causes to cartilage tends to be larger and more severe than with pincer impingement, typically affecting the front and top of the acetabulum.

Pincer Impingement

In pincer impingement, the problem is on the acetabular (socket) side. The rim of the socket extends too far over the femoral head — either in one area (focal overcoverage) or all the way around (global overcoverage). This excessive coverage means the femoral neck hits the labrum earlier in the range of motion than it should.

With repeated contact, the labrum gets crushed between the rim and the femoral neck, leading to labral degeneration, tearing, and eventually ossification (the labrum hardens into bone). Pincer morphology is more common in middle-aged women (Tannast et al., AJR, 2007). The cartilage damage it causes tends to be narrower and more circumferential — a thin strip right at the edge of the socket — compared to the broader damage seen with cam impingement.

Combined Impingement

Many patients have features of both cam and pincer morphology, although there is some debate in the literature about whether these truly coexist or whether they represent distinct patterns. Regardless, the clinical result is the same: the labrum is caught in a vice between abnormally shaped bones, and over time it tears.

How Does a Torn Labrum Cause Pain?

Understanding the labrum’s functions makes it easier to understand why a tear causes problems. When the labrum tears, several things can happen.

Direct pain from the tear itself. The labrum is richly innervated with free nerve endings, particularly in the front and top portions — exactly where most tears occur. The collagen fibers at the junction between the labrum and the cartilage run parallel (rather than perpendicular) in the anterosuperior region, creating a structural weak point where tears most commonly initiate (Peelle et al., Clin Orthop Relat Res, 2005). When the labrum tears through this nerve-rich zone, it can generate sharp, deep pain in the groin. This pain is often provoked by hip flexion and internal rotation.

Loss of the fluid seal. A torn labrum disrupts the suction seal. In a cadaveric study, a labral tear reduced intra-articular fluid pressurization by approximately 25%, and a partial resection reduced it by nearly 50% (Nepple et al., Knee Surg Sports Traumatol Arthrosc, 2014). With less fluid pressure, the cartilage surfaces lose their protective lubrication layer and are exposed to higher contact stresses. This is one of the mechanisms by which untreated labral tears are thought to accelerate the progression toward arthritis.

Mechanical symptoms. A torn or frayed labrum can catch between the femoral head and the socket during hip movement, causing clicking, catching, locking, or a sensation of the hip “giving way.” These mechanical symptoms can be intermittent and unpredictable, and they can be just as disruptive as the pain itself.

Inflammation. Labral tears can trigger a local inflammatory response within the hip capsule, with elevated levels of inflammatory markers in the joint fluid. This inflammation can irritate the surrounding structures and contribute to pain even beyond what the mechanical tear alone would cause.

Altered biomechanics and muscle guarding. When the labrum isn’t functioning properly, the hip loses some of its normal stability and proprioceptive feedback. The surrounding muscles — particularly the hip flexors and deep external rotators — may tighten or spasm in an attempt to compensate, which can cause secondary pain around the hip, groin, and even the buttock. Patients often develop an altered gait or movement patterns without realizing it.

The Typical Presentation

Patients with symptomatic labral tears most commonly describe a deep, aching pain in the groin. Many demonstrate what’s called the “C-sign” — gripping the side of the hip between the thumb and index finger, indicating that the pain is deep and hard to pinpoint. Pain is usually worse with prolonged sitting, walking, and activities that involve hip flexion and rotation. Athletes who play sports involving pivoting, cutting, or deep squatting are particularly susceptible.

That said, it’s also worth noting that many labral tears are completely painless. MRI studies of people with no hip symptoms at all have found labral tears in a significant number of them, and the prevalence increases with age. Not every tear needs treatment, and not every tear is the source of a patient’s pain. I discuss this in detail in a separate article on whether you need surgery for a hip labral tear.

Looking Ahead

Now that you have a foundation for understanding the labrum, FAI, and how tears cause symptoms, the natural next question is: what do we do about it? I’ll cover that in an upcoming article on treatment — including physical therapy, injections, and when arthroscopic surgery makes sense.