What Is the Dorr Classification of Femoral Bone?

The Dorr classification is a system that sorts the upper thighbone into three shapes, called Type A, Type B, and Type C, based on how thick the outer bone wall is and how wide the inner canal is. Orthopedic surgeons developed it specifically to help plan hip replacement surgery, because the shape and quality of the bone around the hip joint directly affects which implant will hold best and how much risk a patient faces during and after the operation. Though it sounds like a narrow surgical tool, the classification touches on bone density, aging, fracture risk, and the long-term performance of artificial joints.

Where the Classification Came From

The system was introduced by Lawrence Dorr, an orthopedic surgeon, based on observations of 52 consecutive patients who were undergoing primary total hip arthroplasty (hip replacement). By examining the shape of the proximal femur, the upper portion of the thighbone near the hip joint, Dorr identified three distinct patterns of bone geometry that recurred across patients. Those patterns turned out to predict how the bone would behave during and after surgery well enough that the classification became a standard part of preoperative planning worldwide.¹

The Three Types

Each Dorr type describes a combination of cortical thickness (the hard outer shell of the bone) and canal width (the hollow interior where marrow sits). The three types sit on a spectrum from dense and narrow to thin and wide.

Type A: The cortex is thick and the canal is narrow, creating a funnel-like shape that tapers noticeably from top to bottom. This bone is strong and dense, typically seen in younger or more active patients. Surgeons sometimes call it a “champagne flute” shape because the canal flares significantly at the top and narrows quickly below.
Type B: The cortex has moderate thickness and the canal is intermediate in width. This is the most common shape in patients presenting for hip replacement. The bone has lost some of its youthful density but still provides a solid foundation for an implant.
Type C: The cortex is thin and the canal is wide, giving the bone a straight, tube-like appearance often called a “stovepipe” shape. There is little difference in canal width between the top and the bottom of the femur. The bone stock is significantly reduced, which creates more challenges for getting an implant to grip securely.

A cementless hip stem that fits perfectly in a Type A femur may rattle around inside a Type C femur. That mismatch is exactly what the classification was designed to prevent.

How Surgeons Measure It

In everyday practice, many surgeons eyeball the Dorr type on a standard hip X-ray, looking at the overall canal shape and cortical thickness. But when a more precise assessment is needed, several radiographic measurements can help. The most commonly used indices include the cortical index (a ratio comparing the outer bone diameter to the inner canal diameter), the canal-to-calcar ratio, and the canal flare index, which captures how much the canal widens from below the hip joint to further down the shaft.

Research into which measurement best separates one Dorr type from another has found that the cortical index on a lateral (side-view) X-ray is most useful for distinguishing Type A from Type B, while the cortical index on a front-to-back X-ray works best for separating Type B from Type C. Specific cutoff values have been proposed: on the front-to-back view, a cortical index above about 0.58 suggests Type A, between 0.49 and 0.58 suggests Type B, and below 0.49 suggests Type C.²

The canal flare index, which compares the canal diameter at the level of the hip joint to the diameter further down the shaft, can also be used to categorize femoral shape, and it forms the basis of classification in some CT-based studies as well.³

How Reliable Is the Classification in Practice

One concern with any visual classification system is whether different doctors looking at the same X-ray actually agree on the type. Studies on this question have found that agreement is moderate at best. In one study that tested orthopedic residents, general orthopedic surgeons, and hip fellowship-trained specialists, the agreement scores (measured on a statistical scale where 1.0 means perfect agreement) hovered in the range of roughly 0.48 to 0.59, regardless of experience level.⁴ That is considered moderate agreement, meaning surgeons will sometimes disagree about whether a borderline case is a Type B or a Type C. Fellowship training in hip surgery did not significantly improve reliability, which suggests the categories have genuinely blurry boundaries rather than being hard to learn.

This is worth keeping in mind: the Dorr classification is a useful guide, not a precision instrument. A femur that one surgeon calls a “high B” might be called a “low C” by another. What matters is that both surgeons recognize the bone is weaker than average and plan accordingly. The quantitative indices described above can add objectivity, but in most operating rooms, the assessment still starts with a practiced eye on an X-ray.

The Link to Bone Mineral Density

The Dorr classification was built around bone shape, not bone density directly. But shape and density turn out to be related. Research comparing Dorr types with DEXA scan results (the standard bone-density test) has found that patients with Type C femurs have significantly lower bone density at the femoral neck than patients with Type A femurs.⁵ In other words, a thin-cortex, wide-canal femur almost always comes with weaker bone overall, not just a different geometry.

The correlation is not absolute, though. Some patients with Type C geometry on X-ray still have reasonably normal density readings, and some with Type B geometry have surprisingly low density. Sex also affects the relationship. Research has shown that certain radiographic indices correlate better with femoral neck bone density in women than in men. In male patients, the canal flare index was only poorly correlated with actual density, while in female patients the correlation was stronger.⁶ This means a Type B assignment on X-ray may carry slightly different implications for underlying bone quality depending on the patient’s sex.

Why It Matters for Hip Replacement

The whole reason the Dorr classification exists is to guide implant selection and surgical technique during total hip arthroplasty. The decision that hinges most directly on Dorr type is whether to use a cementless stem (one that relies on bone growing into its textured surface for stability) or a cemented stem (one that is grouted into place with bone cement).

Type A and B femurs generally provide enough bone stock for a cementless stem to grip well. The cortex is thick enough and the canal tapered enough that a press-fit stem achieves immediate stability. Type C femurs are a different challenge. The wide, straight canal and thin cortex make it harder for a standard cementless stem to find a snug fit, which is why some surgeons favor cemented stems for these patients.

Even within the cementless category, stem design matters significantly in Type C bone. Research comparing tapered wedge stems (which grip by wedging into the canal) with fit-and-fill stems (which are designed to match the canal’s shape as closely as possible) has found that the tapered wedge design carries a lower risk of stem-related complications specifically in Dorr Type C femurs. In Types A and B, the two designs performed comparably.⁷

Long, straight cylindrical stems have also shown good results in Type C bone. One study on elderly patients with Type C femurs found that the cementless Wagner SL stem, which has a long tapered cylindrical design, achieved reliable stability by fitting closely against the wide stovepipe canal from the upper femur down into the shaft.⁸

Periprosthetic Fracture Risk

One of the most clinically significant findings related to Dorr type is its influence on the risk of periprosthetic fracture, a break in the bone around the implant that can happen during surgery or in the weeks and months afterward. In a study of patients receiving hip replacements for acute femoral neck fractures, each step up the Dorr scale came with a markedly higher fracture rate when cementless stems were used: roughly 2% in Type A, about 4% in Type B, and nearly 16% in Type C.⁹ That jump from Type B to Type C is dramatic and helps explain why surgeons are particularly cautious with these patients.

The same study found that neither collared stem designs nor prophylactic cables (wires wrapped around the femur to reinforce it) provided statistically significant protection against fracture in cementless fixation. That finding pushes the decision back toward implant choice and fixation method rather than add-on protective measures.

Biomechanical research has explored which specific stem designs hold up best in simulated Type C bone. In one computer modeling study, the Corail and Omnifit stems demonstrated the highest fracture resistance, while the Excia and Profemur stems showed the lowest. Although even the weakest-performing stems could still withstand forces above seven times body weight, they fell below the threshold considered safe for higher-demand activities, suggesting that patients with Type C bone may need to be counseled about activity levels even after a successful surgery.¹⁰

Stem Subsidence and Early Stability

Even when the bone does not fracture, a hip stem can sink slightly into the canal during the first weeks after surgery, a phenomenon called subsidence. A small amount of settling is normal and expected, but more than about 3 millimeters raises concern about instability and potential failure. In Type C femurs, stem design makes a real difference here. A comparison of fit-and-fill stems versus tapered wedge stems in patients with Type C bone found that by six weeks after surgery, the fit-and-fill group had sunk an average of about 0.65 mm while the tapered wedge group had sunk only about 0.24 mm. More strikingly, roughly 18% of the fit-and-fill stems exceeded the 3 mm threshold that signals instability, compared to just 2% of tapered wedge stems.¹¹ Statistical analysis confirmed that using a fit-and-fill stem was an independent risk factor for excessive subsidence in Type C bone.

Stress Shielding After Surgery

Once a hip stem is in place and bearing load, the bone around it begins to remodel. If the implant is stiffer than the surrounding bone, it absorbs more of the mechanical load, and the bone that is no longer being stressed can thin out over time. This phenomenon, called stress shielding, is more pronounced in Type C femurs. Studies of various cementless stems have consistently found that a low cortical index and Dorr Type C morphology are associated with more severe stress shielding after surgery.¹²

The pattern makes intuitive sense: bone that was already thin before surgery has less reserve to lose. Research on the Summit cementless stem found that Type C femurs developed more severe stress shielding and formed fewer “spot welds” (areas where bone grows directly onto the implant surface, a sign of good fixation) compared to Types A and B.¹³ Stem design also plays a role: in one comparison of two short-stem designs in Type B femurs, the Tri-Lock stem produced significantly more stress shielding than the ACTIS stem.¹⁴ Bone-preserving implant surfaces, such as hydroxyapatite coatings, have shown promise in maintaining bone density in the critical zones around the implant, particularly in Type A and B femurs where bone quality still offers something to preserve.¹⁵

Long-Term Results Across Dorr Types

The Dorr classification often enters conversations about whether a particular surgery will last. Long-term follow-up data offer some reassurance. A study tracking the Taperloc femoral component in patients with Type C femurs out to 20 years found a survival rate of 98%, with no stems loose by radiographic criteria and bone loss (osteolysis) identified around only two stems. The comparison group of Type A and B patients did slightly better numerically, with a low revision rate and radiographic loosening in just 1% of hips.¹⁶ These results suggest that with appropriate stem selection, even Type C bone can support a durable hip replacement.

The 98% survival figure for Type C femurs at 20 years is particularly encouraging because Type C patients tend to be older and have weaker bone, two characteristics that might be expected to predict worse outcomes. It reflects both the quality of modern implant design and the value of matching the implant to the bone.

Uses Beyond Primary Hip Replacement

Although the Dorr classification was developed for hip replacement planning, surgeons have started applying it in other contexts. In trauma surgery, where patients with hip fractures may receive internal fixation rather than a full joint replacement, the Dorr index has shown value in predicting whether the hardware is likely to fail. Research on proximal femoral nails, a common fixation device for hip fractures, found that preoperative Dorr assessment was effective in predicting postoperative outcomes, helping surgeons anticipate which patients might need more aggressive fixation strategies.¹⁷

Artificial intelligence is also entering the picture. AI-assisted planning tools for total hip arthroplasty have begun incorporating Dorr classification and bone density data into their prediction models. However, because Type B femurs make up the largest proportion of the surgical population, training datasets tend to be skewed toward that type, which can affect how accurately the algorithms perform for the less common Type A and Type C cases.¹⁸ As these tools improve, better representation of all three Dorr types in training data will be important for reliable preoperative planning across the full range of bone quality.