What Happens When a Hip Replacement Pops Out?

When a hip replacement dislocates, the ball of the prosthetic joint slips out of its socket, causing sudden severe pain, an inability to bear weight, and a visible shortening or rotation of the affected leg. It is a medical emergency that usually requires a trip to the emergency department, where doctors attempt to push the ball back into place under sedation. Dislocation is the most common major complication of hip replacement surgery, but the actual mechanics, treatment, and long-term consequences are more involved than most patients realize before it happens to them.

What It Looks and Feels Like

The moment a prosthetic hip dislocates, the femoral head (the ball component attached to the thighbone) separates from the acetabular cup (the socket component anchored in the pelvis). You feel an immediate, sharp pain deep in the hip or groin. Most people cannot stand or put any weight on the leg. The affected leg often looks noticeably shorter than the other and may be rotated inward or outward, depending on which direction the ball slipped. In a posterior dislocation, which is the more common type, the leg tends to turn inward and appear shortened. In an anterior dislocation, the leg often rotates outward.

The surrounding soft tissues take a hit as well. Muscles around the hip can stretch, partially tear, or get compressed by the displaced femoral head. A cadaver study found that posterior dislocations can cause partial or complete tearing of muscles attached to the greater trochanter, along with overstretching of the adductor muscles.¹ That soft-tissue damage is part of why even a successfully reduced dislocation can leave you sore and limited for weeks afterward.

How Common Is This?

For a first-time hip replacement, the dislocation rate ranges from about 0.2% to 10% per year, a wide range that depends heavily on the type of implant, the surgical technique, and patient characteristics. For revision surgeries (where a previously replaced hip is operated on again), the rate can climb as high as 28%.² The good news is that the overall dislocation risk has dropped substantially over the decades, falling from roughly 3.7% in the 1960s and 1970s down to about 0.7% in the 2010s, driven by improvements in implant design, surgical technique, and component positioning.³

Even with that improvement, the sheer number of hip replacements being performed means the raw count of dislocations is rising. In the United States, emergency department visits for prosthetic hip dislocations grew from roughly 2,400 in 2000 to about 8,100 in 2017, and projections estimate over 10,000 cases per year by 2035. The per-person dislocation rate has not changed significantly; there are just many more people living with hip implants.⁴

What Triggers a Dislocation

A prosthetic hip pops out when the femoral head is levered past the rim of the cup, usually by a combination of body position and force. The most common culprits are movements that push the hip beyond its safe range: deep bending, crossing the legs, twisting the torso while the foot is planted, or sitting on very low surfaces. These are the motions your surgeon warns you about after surgery, and for good reason.

But the triggers go beyond just bending too far. A large meta-analysis covering roughly five million hip replacements identified several risk factors that make dislocation more likely:⁵

• Older age: patients 70 and older had about a 27% higher risk compared to younger patients.
• Neurological conditions: disorders affecting muscle control roughly doubled and a half the risk (relative risk around 2.5).
• Prior hip surgery: having had previous surgery on the same hip more than tripled the risk.
• Spinal fusion: a prior lumbar spine fusion doubled the risk, likely because a fused spine changes how the pelvis tilts when you sit down.

The spinal fusion connection deserves extra attention because it surprises many patients. When the lower spine is fused, the pelvis cannot tilt normally during the transition from standing to sitting. That forces the hip joint into positions where the ball is more likely to lever out of the cup. Surgeons increasingly account for this when planning implant placement, but it remains one of the strongest predictors of instability after hip replacement.⁶

Getting It Back In

The standard first response to a dislocated hip replacement is called closed reduction: a doctor pushes the ball back into the socket without surgery, usually with the patient under sedation or general anesthesia. Various manipulation techniques exist. One described approach allows a single surgeon to perform the maneuver efficiently without an assistant.⁷ After sedation, the doctor applies traction (pulling the leg lengthwise) while rotating it back into alignment. When it works, you hear or feel a clunk as the ball re-seats.

The financial difference between a successful emergency-room reduction and one that fails is striking. When closed reduction works in the ER, median hospital charges run about $6,800. When it fails and the patient needs to go to the operating room, the median jumps to roughly $27,300. Even patients sent directly to the OR without an ER attempt face median charges around $20,600.⁸ The time spent in the hospital also increases dramatically when OR reduction is needed.

About two-thirds of dislocation cases are managed successfully without further surgery. The remaining third eventually need a surgical procedure, such as revision of the implant components.⁶

How Doctors Figure Out What Went Wrong

After reducing the hip, the medical team needs to understand why it dislocated in the first place. A plain X-ray usually confirms that the ball is out of the socket, but it does not tell the full story. Cross-sectional imaging like CT scans is often necessary to measure the exact orientation of the cup and stem, which helps determine whether a component was positioned in a way that made dislocation more likely.⁹ CT scans can measure both the angle of the cup’s tilt in the pelvis and its degree of anteversion (how far forward it faces), both of which affect how secure the joint is in various positions.¹⁰

The diagnostic workup also looks for loosening of the implant, infection, or fractures around the prosthesis, any of which can contribute to instability and would change the treatment plan entirely.

How Implant Design Affects Stability

Not all hip replacements are equally prone to popping out. Two design features have the biggest impact on dislocation risk: head size and cup orientation.

A larger femoral head has to travel farther before it clears the rim of the cup, which means it can tolerate a wider range of motion before dislocating. One study comparing 36-millimeter heads to 28-millimeter heads found a dislocation rate of 0.6% with the larger heads versus 6.5% with the smaller ones.¹¹ Biomechanical testing has confirmed the mechanism: smaller heads generate about 25% less resisting force before the joint impinges and the ball starts to lever out.¹²

Cup positioning is more complicated than surgeons once thought. For decades, the field relied on a “safe zone” of cup angles originally described by Lewinnek, assuming that keeping the cup within certain tilt and rotation ranges would prevent dislocation. But a systematic review found that the majority of studies could not identify a meaningful difference in cup angles between hips that dislocated and those that did not. Most studies that assessed whether cups placed inside the Lewinnek safe zone had lower dislocation rates came up empty.¹³ This finding has pushed surgeons toward more individualized cup positioning, taking into account each patient’s pelvic tilt and spine mobility rather than aiming for a one-size-fits-all target zone.

Does the Surgical Approach Matter?

Hip replacement can be performed from the front (direct anterior approach), the side (lateral approach), or the back (posterior approach). There has been considerable debate about whether any of these approaches is meaningfully safer when it comes to dislocation.

One large study comparing the three approaches found dislocation rates of 1.1% for posterior, 0.7% for direct anterior, and 0.5% for lateral approaches.¹⁴ Another study with five-year follow-up reported zero dislocations in 139 hips done through the anterior approach versus a 4% dislocation rate in 177 hips done from the back.¹⁵ These findings suggest an edge for the anterior approach.

However, a systematic review and meta-analysis comparing the anterior approach to posterior and lateral approaches found no statistically significant difference in dislocation risk overall.¹⁶ The apparent contradiction likely reflects the difficulty of isolating surgical approach from surgeon experience, implant choice, and patient selection. In practice, a surgeon highly skilled in the posterior approach with modern soft-tissue repair techniques may produce dislocation rates just as low as one using the anterior approach.

When Surgery Becomes Necessary

If a hip keeps dislocating despite closed reduction and activity modifications, revision surgery is typically the next step. The goal of revision is to change the implant components so the joint is inherently more stable. Surgeons have several options:

• Larger femoral heads: switching to a head of 36 mm or larger increases the range of motion before impingement.
• Dual-mobility cups: these have a smaller head that moves inside a larger liner, which itself moves inside the cup, creating two levels of articulation and a much wider safe zone.
• Constrained liners: these physically lock the ball inside the socket, preventing dislocation at the cost of limiting motion and transferring more stress to the bone-implant interface.

A study of over 1,275 revision surgeries performed specifically for dislocation found that standard-size heads had a higher rate of needing yet another revision for re-dislocation, while large heads, dual-mobility cups, and constrained liners all performed better.¹⁷ Dual-mobility cups in complex revisions have shown dislocation-free survival of about 96% at five years, though the rate declines to roughly 82% at nine years.¹⁸

For patients who have already failed a constrained liner (which is essentially the last-resort option), switching to a dual-mobility construct has shown promise. One advantage of dual-mobility designs is that if they do dislocate, they can often be reduced with a closed technique, whereas a dislocated constrained liner almost always requires another open surgery. In a small series of these salvage cases, three out of 14 re-dislocated, so the problem is not always solved permanently.¹⁹

The Psychological and Financial Toll

What often catches patients off guard is not just the physical pain of a dislocation but the psychological aftermath. Research has found that repeated instability generates a fear of movement, social withdrawal, and emotional strain. Patients who have experienced dislocation frequently describe it as devastating, reporting restrictions in daily activities and a loss of independence.²⁰ Even patients whose hips have never actually dislocated can develop significant anxiety about it. One study found that patients with more limited hip flexibility before surgery, specifically those who could not flex the hip beyond about 80 degrees, were more likely to report extreme anxiety about dislocation afterward.²¹

The fear is not irrational. Many patients restrict their activities well beyond what is medically necessary, avoiding bending, sitting on normal chairs, or engaging in social outings. This self-imposed limitation can contribute to muscle weakening, which ironically increases dislocation risk by reducing the dynamic stability the muscles provide around the joint.

Financially, a dislocation episode adds substantially to the overall cost of a hip replacement. In the Medicare population, patients who experienced a dislocation incurred about $19,600 more in healthcare costs over one year and about $24,200 more over two years compared to patients whose hips stayed in place. Two-thirds of that extra cost came from additional inpatient admissions, with the rest split among skilled nursing, outpatient care, and rehabilitation.²²

Robotic Surgery and the Push to Reduce Dislocation Rates Further

The latest frontier in dislocation prevention is technology-assisted surgery. Robotic-arm systems allow surgeons to plan component placement on a 3D model of the patient’s anatomy before making a single cut, and the robotic arm helps execute that plan with high precision during the operation. A multivariate analysis found that robotic assistance was associated with a 70% lower odds of dislocation compared to conventional manual technique.²³ Navigation systems (which provide real-time positional feedback without the robotic arm) did not show the same reduction in that particular analysis, suggesting that the physical guidance of the robotic arm during bone preparation and cup placement may be the active ingredient.

Robotic systems are not available everywhere, and they add cost and operating-room time. Whether they become standard will depend on whether the reduction in complications justifies the expense across the full range of patients and hospital settings. For now, they represent one more tool in an evolving effort to make hip replacement dislocation an increasingly rare event. Given that the dislocation rate has already fallen by roughly 80% since the earliest days of the procedure, the trajectory is encouraging, even if zero dislocations remains an unrealistic goal.

Life After Reduction and How to Lower Your Risk

After a successful closed reduction, the focus shifts to keeping the hip in place. Most patients are given movement restrictions for several weeks, typically avoiding deep flexion beyond 90 degrees, crossing the legs, and rotating the hip inward. Some centers have experimented with physical braces designed to limit the specific motions that predispose to re-dislocation: flexion, adduction, and internal rotation.²⁴ Evidence on whether braces actually reduce re-dislocation is still limited, and they can be uncomfortable enough that patients stop wearing them.

What clearly does help is strengthening the muscles around the hip. The gluteal muscles, hip abductors, and external rotators act as dynamic stabilizers that keep the ball centered in the socket during movement. Physical therapy that targets these muscle groups, particularly the gluteus medius, can significantly improve joint stability. Patients who maintain consistent activity levels and muscle strength after their initial recovery period tend to fare better than those who become sedentary out of fear.

If you have a spinal fusion, a neurological condition, or a history of prior hip surgery, it is worth having a frank conversation with your surgeon before the original operation about implant choices that prioritize stability, such as dual-mobility cups or larger head sizes. These decisions are harder to make after the fact than before the first incision.