What Is a Hip Stem in Hip Replacement Surgery?

A hip stem is the metal shaft that anchors into your thighbone during a hip replacement, serving as the foundation for the entire artificial joint. It replaces the upper portion of the femur that has been removed during surgery, and the choice of stem design, material, and fixation method affects everything from how the bone heals around it to how long the implant lasts. Though the ball-and-socket bearing surface gets most of the public attention, surgeons often consider the stem the most consequential decision in the operation, because it determines how force travels through your leg for decades to come.

What the Stem Actually Does

In a healthy hip, forces from walking, climbing stairs, and standing travel through the ball-shaped femoral head, down through the femoral neck, and into the dense bone of the upper thigh. When a surgeon removes the damaged femoral head and neck, the stem takes over that job. It sits inside the hollowed-out canal of the femur, and a new ball (the femoral head component) attaches to the top of the stem via a tapered connection called the trunnion. Every step you take sends roughly two to three times your body weight through that stem and into the surrounding bone.

The critical engineering challenge is making the stem stiff enough to bear those loads without breaking, while flexible enough that the bone around it still “feels” the force it needs. Bone is living tissue that constantly remodels in response to the loads placed on it. When a very rigid metal stem absorbs most of the force and shields the surrounding bone from its normal workload, the bone gradually thins. This phenomenon, called stress shielding, is the central trade-off in stem design and has driven much of the innovation in hip replacement over the past several decades.

How Stem Design Influences Bone Health

Standard-length stems extend well down into the femoral shaft, and they anchor primarily through contact along that shaft. This works well mechanically, but it tends to redirect load away from the upper femur, right where you most want the bone to stay strong. Finite element modeling and lab studies consistently show that both standard and short stems reduce the strain on the proximal femur compared to the intact bone, but short stems that anchor higher up in the metaphysis (the flared region just below the neck) produce a more natural loading pattern.

One comparative study found that a short, calcar-loading stem reduced surface strain in the uppermost femoral region by about 40%, while a standard tapered stem reduced it by roughly 87% in the same zone. That difference matters over time: less strain reduction means the bone retains more of its natural stimulus to stay dense.

¹ A finite element study comparing a metaphyseal-anchoring short stem to a standard-length stem reached a similar conclusion: the short stem caused less unloading in the proximal femur, while the standard stem shifted its highest stresses further down the shaft.²

Even more conservatively designed femoral neck implants, which sit almost entirely in the neck region rather than extending into the canal, have been shown to produce medial strains closer to what the intact bone experiences. The trade-off is that strains on the lateral side of the greater trochanter increase, raising questions about long-term fracture risk in that area.³

A systematic review of short stem literature confirmed that these designs preserve proximal bone stock and show slightly less bone loss in the early postoperative years compared to anatomic-length stems.⁴ A separate trial comparing an ultra-short uncemented stem with a conventional tapered stem found lower periprosthetic bone loss and equally excellent clinical outcomes at two years.⁵

Cemented Versus Cementless Fixation

The two broad families of hip stems are cemented and cementless. A cemented stem is held in place by a layer of polymethylmethacrylate bone cement that fills the gap between the metal and the bone. A cementless stem relies on a tight press-fit during surgery, followed by bone growing directly into or onto the implant surface over subsequent weeks. Both methods have track records spanning decades.

A long-term study comparing cemented and cementless stems in the same patients, all younger than 50 at the time of surgery, found remarkably similar femoral component survival rates at roughly 26 years: about 96% for the cemented side and 95% for the cementless side.⁶ A meta-analysis pooling data from randomized trials and national registries found a pooled relative risk of revision for aseptic loosening of 0.90 favoring cementless implants, though there was substantial disagreement between the trial data and the registry data.⁷

In practice, surgeon preference, patient age, and bone quality tend to guide the choice. Cementless stems dominate in younger, more active patients because the biological fixation can potentially last longer and the bone-preserving properties of press-fit designs are appealing. Cemented stems remain common in older patients with weaker bone, where achieving a reliable press-fit is harder and the rapid stability that cement provides helps people get moving sooner.

Tapered Versus Cylindrical Geometry

Among cementless stems, the two main geometries are tapered (wedge-shaped) and cylindrical (straight-sided). Tapered stems achieve stability by wedging into the canal, and they tend to concentrate loading more proximally. Cylindrical stems rely on a long stretch of contact with the inner wall of the bone for their grip.

One study of revision hip replacements found that patients who received cylindrical stems reported thigh pain at a rate of about 13%, compared to roughly 5% in those who got tapered stems. Two of the cylindrical-stem patients developed persistent pain, while thigh pain in the tapered group generally resolved within a year.⁸ A separate analysis found more radiolucent lines (a sign of incomplete bone-implant contact), more subsidence, and more stress shielding in cylindrical stems compared to tapered ones.⁹ Thigh pain with cylindrical stems is thought to arise from the rigid distal tip pressing against the inner cortex of the bone, especially during high-impact activities.

Why Material Stiffness Matters

The metals used in hip stems each have different stiffness, and that stiffness has a direct effect on how much bone thinning occurs around the implant. Simulation models have predicted bone loss ranging from about 23% in the upper medial cortex around a cemented titanium stem to as high as 76% around an uncemented cobalt-chromium stem. Very flexible experimental stems produced almost no bone loss, but the forces at the bone-implant interface rose sharply, creating a different kind of failure risk.¹⁰

A clinical study confirmed that stress shielding was strongly related to the ratio of stem stiffness to bone stiffness, particularly in torsion. Among cemented stems, moderate-to-severe stress shielding appeared in about 20% of hips when the stiffness ratio was low, and jumped to 70% when it was high.¹¹ This is one reason titanium alloy has largely displaced cobalt-chromium for cementless stems: titanium is roughly half as stiff, producing a better mechanical match with the surrounding bone.

Research into 3D-printed porous titanium stems has pushed this concept further. By engineering a lattice micro-structure throughout the stem, researchers have shown it is possible to reduce stress-shielding-related bone loss by about 75% compared to a solid titanium stem.¹² These lattice designs mimic the porosity of natural bone, and early results are promising, though long-term clinical data is still accumulating.

Surface Coatings and Bone Ingrowth

For cementless stems, the surface finish is nearly as important as the shape. Most modern cementless stems have a roughened or porous-coated proximal section that encourages bone to grow into the surface. Some also include a layer of hydroxyapatite, a calcium phosphate ceramic that closely resembles the mineral component of natural bone. A randomized trial comparing porous-coated stems with and without a hydroxyapatite-tricalcium phosphate coating found that the coated stems had significantly less femoral bone loss at two years.¹³ The coating appears to accelerate the bone-bonding process and may provide a more durable long-term seal against micromotion and particle migration.

Getting the Offset Right

A hip stem does not just anchor the new joint; it also determines how far out from the pelvis the center of the hip sits. This distance, the femoral offset, affects the tension of the muscles that stabilize the hip, particularly the abductors on the outer side. If offset is not restored adequately, the hip can feel unstable, the patient may limp, and the risk of dislocation rises.

Extended-offset stems are specifically designed with a greater lateral offset to accommodate patients whose anatomy demands it. A study of elderly patients with femoral neck fractures found that those who received an extended-offset stem showed better recovery of abductor muscle strength, which the researchers linked to improved joint stability.¹⁴ Stem manufacturers typically offer a range of offset options for each stem family, allowing the surgeon to fine-tune the reconstruction to each patient’s anatomy.

Stem Alignment and Surgical Approach

Even the best-designed stem can underperform if it is not placed accurately. Malalignment, where the stem sits in the canal at an angle that deviates from the ideal position, can lead to uneven load distribution, thigh pain, and increased wear. One study focused on the direct anterior approach (a popular minimally invasive technique) found that without a specialized curved rasp to prepare the canal, the rate of stem malalignment was close to 49%. With the curved rasp, that dropped to 27%, and severe malalignment was essentially eliminated.¹⁵ Surgical approach, patient positioning, and the instruments used all influence how straight the stem ends up.

When a Stem Loosens

The most common long-term reason a hip stem fails is aseptic loosening, meaning the bond between the implant and the bone breaks down without any infection involved. The main driver is tiny particles of wear debris, shed from the bearing surfaces over years of use. These microscopic particles trigger a response from immune cells called macrophages, which ramp up signals that recruit bone-dissolving cells and suppress the body’s normal bone-protective mechanisms.¹⁶ Research has also identified a stress response inside macrophages triggered by wear particles, and blocking that stress pathway in animal models dramatically reduced bone destruction around the implant.¹⁷ Advances in bearing materials, particularly highly cross-linked polyethylene and ceramic-on-ceramic pairings, have significantly reduced wear particle generation, which should translate into lower loosening rates over time.

Surgeons can get an early warning of potential trouble by tracking how much a stem moves in the first couple of years. A meta-analysis of radiostereometric analysis studies found that for cemented stems with a shape-closed design, every 0.1 mm of subsidence measured at two years corresponded to a 4% increase in the long-term revision rate. Stems that sank less than 0.15 mm by two years had revision rates below 5% at ten years, while those exceeding that threshold fared significantly worse.¹⁸ A separate study of a cemented stem with minor design changes confirmed that two-year subsidence was a strong independent predictor of later failure.¹⁹

Periprosthetic Fractures and Risk Factors

A fracture of the thighbone around a hip stem is one of the more serious complications after hip replacement. A retrospective analysis identified six independent risk factors: older age, a femoral neck fracture as the original diagnosis, osteoporosis, having a partial rather than total replacement, bone resorption around the upper femur, and having a standard- or long-stem prosthesis rather than a short one.²⁰

Stem geometry also influences the type of fracture that occurs. Anatomic stem shapes have been associated with a higher incidence of periprosthetic fracture than straight stems, with a particular predisposition to clamshell-type fracture patterns.²¹ Cemented stems tend to produce spiral and comminuted burst fracture patterns, while cementless stems more often produce clamshell fractures. Polished taper-slip cemented stems carry a unique risk of metaphyseal split fractures, reported at around 10% of fractures in that stem type.²²

Trunnionosis and the Head-Neck Junction

Where the femoral head attaches to the stem via a taper (the trunnion), micro-motion and corrosion can gradually damage the metal surfaces. This process, loosely called trunnionosis, releases metal ions and debris into the surrounding tissue, sometimes causing pain, swelling, and tissue damage that mimics infection. The exact cause remains debated, but implant design and the geometry of the taper connection are thought to play a role.²³ Larger femoral heads appear to produce greater fretting damage at the taper, likely because the longer lever arm amplifies the forces at the junction. And the longer an implant stays in the body, the greater the cumulative corrosion damage.²⁴ This is relevant because larger heads were widely adopted to reduce dislocation risk; the benefit is real, but it comes with a trade-off at the taper.

Revision Stems and Dealing with Bone Loss

When a hip stem fails and needs to be replaced, the revision surgery is considerably more complex than the original procedure. The first stem, along with any cement, has to be removed, and the bone left behind often has defects ranging from mild thinning to large cavities. Revision stems are typically longer and may be modular, meaning the surgeon assembles the stem from separate components to customize the fit.

A study of modular, tapered, fluted titanium revision stems followed for an average of ten years found that all stems achieved distal fixation, and about 68% of hips showed evidence of new bone forming around the implant. Roughly 79% of the hips had severe bone loss before surgery. The mechanical failure rate was 1.4%.²⁵ Even in the most severe bone deficiency categories, modular tapered stems have provided predictable clinical outcomes and strong intermediate-term survival.²⁶

Special Situations and Unusual Anatomy

Not every thighbone has a textbook shape. Patients with developmental dysplasia of the hip often have a narrow, misshapen femoral canal with an unusual neck angle. Standard stems may not fit well or may sit at an angle that poorly restores the hip’s mechanics. Short stems with metaphyseal anchorage have shown good early results in this population, with significant improvements in hip scores and successful restoration of femoral offset even in dysplastic anatomy.²⁷ Tapered conical stems, like the Wagner cone, have also proven useful in young dysplasia patients with small frames, because the cone shape allows the surgeon to set the rotation of the stem independently and the smaller diameters fit narrow canals.²⁸

At the other end of the spectrum, hip replacement in patients under 21 remains rare and carries particular concerns about implant longevity and the near-certainty that these patients will outlive even the best implant and need at least one revision.²⁹ Bone-preserving short stems are particularly attractive in this group, since leaving more intact bone in place gives the revision surgeon more to work with decades later.

3D-Printed and Custom Stems

Additive manufacturing has begun to change what is possible when off-the-shelf implants do not fit. For patients with severe proximal femoral bone defects or deformities, a traditionally shaped stem may leave gaps, sit at a poor angle, or fail to achieve adequate contact with the inner canal wall. 3D-printed custom stems, designed from the patient’s own CT scan, can match the canal geometry closely and improve initial stability.³⁰ Beyond fit, the same printing technology enables the lattice micro-structures mentioned earlier, allowing engineers to tune the implant’s stiffness region by region. A stem could, in theory, be stiff at the tip where it bears load against cortical bone and compliant near the top where preserving bone density matters most. This kind of graded design is difficult or impossible with conventional manufacturing and represents one of the more genuinely exciting frontiers in implant engineering.