A bipolar hemiarthroplasty is a partial hip replacement that uses a specially designed two-part head to replace the ball of the hip joint while leaving the natural socket intact. It’s most commonly performed after a displaced femoral neck fracture, the type of broken hip where the ball of the thighbone loses its blood supply and can no longer heal on its own. The “bipolar” name refers to the implant’s two separate points of movement, which distinguish it from simpler, single-piece designs.
How the Implant Works
A normal hip joint is a ball-and-socket: the round head of your femur sits inside a cup-shaped socket (the acetabulum) lined with cartilage. In a bipolar hemiarthroplasty, only the ball side is replaced. The implant has three main parts: a metal stem that fits down into the hollow center of the thighbone, a smaller metal ball that attaches to the top of that stem, and a larger outer shell with a plastic lining inside it.
The smaller ball rotates freely inside the plastic-lined outer shell, and the outer shell also moves against your natural hip socket. This gives the implant two separate surfaces of motion. The inner joint handles most of the everyday movement, while the outer shell glides against your cartilage only during bigger motions. The idea behind this design is to reduce the grinding force on your natural socket cartilage, since the metal shell isn’t the only thing absorbing movement.
Bipolar vs. Unipolar: Why the Extra Joint Matters
Older, simpler hemiarthroplasty implants are “unipolar,” meaning a single solid metal ball sits directly against the cartilage of your hip socket. Over time, that constant metal-on-cartilage contact wears the socket down, a problem called acetabular erosion. In severe cases, the metal head can actually push through the thinned cartilage and migrate into the pelvis.
Bipolar implants were developed specifically to address this. In a large review of over 1,000 hemiarthroplasty cases, patients with bipolar implants had higher hip function scores and lower rates of socket erosion compared to those with unipolar designs. Bipolar heads also have a self-centering quality: the outer shell naturally finds its correct position within the socket, which lowers the risk of the implant slipping out of place. Overall dislocation rates for hemiarthroplasty using a standard surgical approach run around 2.5%, and dislocations tend to occur less frequently with the bipolar design.
Who Gets This Surgery
The primary candidates are older adults with displaced femoral neck fractures. In younger, more active patients with hip fractures, surgeons typically prefer a total hip replacement, which resurfaces both the ball and the socket. But for elderly patients, especially those with lower activity levels or significant medical conditions, a bipolar hemiarthroplasty offers a shorter, less complex operation with a faster path to getting back on their feet.
The surgery is also occasionally used for older patients with hip arthritis or rheumatoid arthritis who may not tolerate the longer procedure of a total hip replacement. In these cases, 10-year implant survival rates range from about 82% for osteoarthritis patients to 90% for those with rheumatoid arthritis.
What Happens During Surgery
The surgeon accesses the hip joint through either the front (anterior) or back (posterior) of the hip. Both approaches involve a skin incision on the outer side of the hip, then working through layers of muscle and tissue to reach the joint capsule. The posterior approach gives the surgeon better access to the inside of the thighbone for fitting the stem, while the anterior approach may carry a lower risk of post-surgical dislocation because it avoids cutting through the muscles at the back of the hip.
Once the joint is exposed, the surgeon removes the damaged femoral head by cutting through the neck of the thighbone. The hollow canal inside the femur is then shaped to accept the metal stem, which may be press-fit or cemented into place. A trial head is placed on the stem first to confirm the right size and neck length, ensuring the leg length and joint tension feel correct. Once confirmed, the final bipolar head is attached, the joint is put back together, and the incision is closed.
Recovery and Weight Bearing
One of the key advantages of this surgery for elderly patients is that it allows full weight bearing right away. International guidelines recommend getting patients up and moving on the first day after surgery, and most protocols allow the operated leg to bear full body weight immediately with no restrictions. In practice, a physiotherapist guides the first steps, focusing on correct movement patterns and fall prevention rather than speed. Adequate pain management before each session helps patients participate in mobilization with minimal discomfort.
Early mobility matters enormously for older patients. Prolonged bed rest after a hip fracture increases the risk of blood clots, pneumonia, pressure sores, and muscle loss. The relatively quick return to walking is one reason hemiarthroplasty remains a preferred option over internal fixation (pinning the bone back together), which often requires a period of limited weight bearing while the fracture heals.
Long-Term Socket Wear
Despite its design advantages, the bipolar implant still rests against natural cartilage, and that cartilage does wear down over time. Research measuring the rate of socket cartilage loss after bipolar hemiarthroplasty found an average thinning of about 0.23 millimeters per year. Given that normal hip socket cartilage is only 1 to 3.6 millimeters thick, this means significant cartilage loss could develop roughly 7 to 8 years after surgery, at which point the underlying bone becomes exposed and the risk of the implant pushing into the pelvis increases.
Activity level plays a direct role. More active patients experience faster cartilage degeneration because the metal outer shell delivers more stress to the socket surface with each step. This is part of why surgeons reserve total hip replacement for younger, more active patients, who would wear through that cartilage much sooner, and choose bipolar hemiarthroplasty for those whose lower activity levels give the cartilage a longer functional life.
When a Second Surgery Is Needed
Some bipolar hemiarthroplasties eventually need to be converted to a total hip replacement. In studies of failed hemiarthroplasties that required conversion, the most common reasons were loosening of the stem inside the thighbone (34% of cases), socket erosion causing groin pain (32%), infection (12%), implant breakage (9%), dislocation (7%), and fracture of the bone around the implant (5%).
Pain after hemiarthroplasty generally falls into two categories. Groin pain typically signals that the socket cartilage has worn through and the metal is pressing against bare bone. Thigh pain, especially a “start-up” pain when first standing from a chair, usually means the stem has loosened inside the femur. Groin pain is the more common trigger for conversion surgery, though it’s worth noting that converting to a total hip replacement doesn’t always eliminate groin pain completely.
For many elderly patients, the implant outlasts them comfortably. The decision to use a bipolar hemiarthroplasty factors in life expectancy, activity level, and the balance between a simpler initial surgery and the possibility of needing a more complex revision years down the road.