The Dorr Classification is a system used by orthopedic surgeons to assess the quality and structure of the proximal femur (upper thigh bone) before a total hip replacement (THA). This classification helps surgeons predict how well a hip implant will attach to the bone, which is crucial for the long-term success and stability of the new hip joint. The system categorizes the femur’s internal structure and cortical bone thickness into three distinct types, labeled A, B, and C.
Categorizing Femoral Bone Quality
The Dorr classification separates patients into three groups based on the geometry of the intramedullary canal and the thickness of the cortical bone. These anatomical differences reflect the underlying bone density and overall health of the femur. Dorr Type A femurs are often described with the visual analogy of a “champagne flute.” This type is characterized by a thick, distinct cortical bone layer and a narrow intramedullary canal, indicating dense and robust bone stock.
The next category, Dorr Type B, represents an intermediate or “normal” morphology. In this type, the cortical bone is moderately thick, but there is evidence of bone loss in the proximal area compared to Type A. This leads to a slightly wider intramedullary canal, sometimes described as having a residual funnel shape. Type B femurs are considered to have a mixed bone quality.
Finally, Dorr Type C femurs are characterized by substantial thinning of the cortical walls and a wide intramedullary canal, giving it a “stovepipe” or cylindrical appearance. This morphology is associated with the poorest bone density and a “fuzzy” or indistinct look to the bone on an X-ray. Type C bone is more frequently observed in elderly, thinner patients, particularly women, and those with conditions like osteoporosis.
Determining the Dorr Classification
Orthopedic surgeons assign a Dorr classification primarily through the analysis of standard preoperative radiographic imaging, such as X-rays. While the classification can be determined qualitatively by visual assessment, quantitative measurements provide a more objective method. Two primary measurements derived from these images are used to help classify the femoral morphology: the Cortical Thickness Index (CTI) and the Canal Flare Index (CFI).
The Cortical Thickness Index measures the ratio of the cortical bone thickness to the total diameter of the femur at a specific point, typically 10 centimeters below the lesser trochanter. A higher CTI value, such as the 0.58 often seen in Type A bone, indicates a thick cortex and a strong bone structure. Conversely, lower values, like the 0.42 often seen in Type C bone, confirm a thin cortex and poor bone density.
The Canal Flare Index (CFI) is also used to assess the funnel shape of the proximal femur. This index is calculated by dividing the diameter of the femoral canal at a point just above the lesser trochanter by the diameter of the canal at the isthmus, which is the narrowest point. A high CFI suggests a “champagne flute” shape (Type A), while a low CFI, such as a ratio less than 3, points toward the “stovepipe” appearance of a Type C femur.
Impact on Total Hip Replacement
The Dorr classification significantly influences the surgeon’s choice of implant type and surgical technique during a total hip replacement. The primary decision is whether to use a cemented or a cementless femoral stem. Type A femurs, with their dense bone and narrow canal, are generally ideal for cementless, or press-fit, stems. The thick cortical bone provides excellent immediate stability, allowing the bone to grow directly onto the implant surface for long-term fixation.
For Dorr Type C bone, the decision becomes more complex due to the thin cortex and wide canal. This poor bone stock creates a higher risk of complications, such as intraoperative fracture when attempting to insert a press-fit stem. Historically, Type C bone often necessitated the use of cemented stems, where an acrylic bone cement is used to secure the implant within the wide canal, providing immediate and reliable fixation.
Dorr Type B femurs accommodate the widest variety of implant designs, including both cemented and uncemented options. Modern cementless tapered wedge stems, which rely on achieving stability in the more proximal part of the bone, are increasingly used successfully even in Dorr C patients.