The femur, or thigh bone, is the body’s longest and strongest single bone, acting as a main structural pillar. Its strength comes not only from the thick outer layer, or cortex, but also from specialized internal structures that manage movement forces. Deep within the proximal, or upper, part of the femur lies the calcar femorale. This internal bony buttress plays a role in the health and function of the hip joint, allowing the upper femur to withstand tremendous loads. Understanding this anatomy is important for comprehending hip biomechanics and orthopedic challenges.
Defining the Calcar Femorale: Location and Structure
The calcar femorale is a dense, specialized internal structure located in the upper portion of the femur, near where the bone joins the hip socket. It is not a separate bone but rather a thickened, vertical plate of concentrated cancellous bone tissue within the marrow cavity. The plate runs vertically upward and projects laterally, extending into the base of the femoral neck.
The calcar forms a septum-like structure, creating a boundary between the femoral neck and the shaft. It is composed of multilayer compact bone, giving it a higher density than the surrounding cancellous bone. While the calcar is present in adults, it is not visible in infant femurs, developing fully in early adulthood. Its density can be comparable to that of the diaphyseal cortex.
Biomechanical Role: The Strut of the Proximal Femur
The head and neck of the femur are subjected to a combination of forces, including compression, tension, and shear forces. The calcar acts as a compressive strut, designed to resist the enormous compressive loads transmitted from the femoral head down the medial side of the neck. The primary function of the calcar femorale is to manage and redistribute the significant mechanical stress placed on the hip joint.
This dense internal plate is a component of the internal truss system of the proximal femur. The entire system allows the femur to effectively transfer body weight from the hip joint to the main femoral shaft. The calcar helps redistribute stress by decreasing the load on the posterior and medial aspects of the proximal femur. Simultaneously, it increases the load on the anterior and lateral aspects, making the stress transmission more uniform. When the calcar is compromised, strain values in the posterior and medial aspects increase significantly, demonstrating its importance in maintaining load equilibrium.
Calcar and Femoral Neck Fractures
The integrity of the calcar femorale is a determining factor in the stability and treatment of fractures affecting the proximal femur. A fracture that disrupts the calcar compromises the main compressive support structure of the femoral neck. This disruption leads to mechanical instability, significantly increasing the risk of fixation failure.
Surgeons must carefully assess the calcar region when planning treatment for femoral neck fractures. For internal fixation methods, securing the posterior medial fracture mass, including the calcar femorale, is essential for achieving mechanical stability. Placing internal fixation devices, like cannulated screws, close to or buttressed against the calcar strengthens this area. Biomechanical data suggests that calcar-buttressed screw fixation enhances the strength of the fixation and reduces the chance of implant failure.
Importance in Total Hip Replacement Surgery
The calcar femorale is a significant anatomical landmark and structural consideration during total hip arthroplasty (THA). Surgeons rely on the calcar’s position to guide the sizing and placement of the prosthetic femoral stem within the shaft. Proper alignment of the stem with the medial calcar is important for optimizing the fit of the implant.
Calcar loading ensures that the prosthetic stem rests securely against the remaining bone of the calcar. This contact promotes optimal load transfer through the femur. By maintaining this proper load transfer, calcar-guided placement can reduce stress shielding, where the implant absorbs too much load, leading to the resorption and weakening of the surrounding bone. For patients with pre-existing bone loss or where the calcar region is missing, specialized calcar-replacement femoral components are sometimes necessary. These specialized stems provide the necessary support when the natural buttress is absent, ensuring the prosthesis has adequate stability.