Rectangular Stem: Stability is Overwhelming!

The rectangular stem has a rectangular cross-section and a 3° taper on the coronal and sagittal planes. The design concept is that the stem body has a double-sided taper, the three-dimensional dimension gradually decreases from the proximal end to the distal end, and the stress transmission is continuous and natural, with no stress concentration point. The longitudinal taper is beneficial to the axial stability of the prosthesis. The rectangle is conducive to close contact with the bone bed, anti-rotation, and anti-settling. The rectangular stem is "press-fit without filling" and preserves a large amount of endosteum, providing adequate blood supply to the medullary cavity and promoting bone growth [1,2].

So, how does the design of a rectangular femoral stem affect its initial and long-term stability?

Filling condition of the femoral medullary cavity with a rectangular stem.

Some studies have shown [3] that the average filling rate of the rectangular stem on the frontal X-ray film is 82.72%, and it may be firmly embedded in the sudden narrowing of the medullary cavity at the femoral metaphysis or 20 mm below the lesser trochanter or in the femoral isthmus. At the same time, the rectangular stem is used for reaming during implantation, compressing the cancellous bone, so that the stem body is embedded in the cortical bone of the medullary cavity, and a firm initial fixation is obtained so that the patient can get out of bed and load bearing in the early postoperative period. The sinking rate of the handle body is low.

Stress shielding condition of rectangular stem

The front-to-back diameter of the rectangular stem is smaller than that of the tapered round stem. According to the elastic modulus theory, the axial stiffness of the prosthesis increases with the square of its cross-section or stem diameter, and the buckling stiffness is the fourth power of the stem diameter. The stress shielding of the proximal femur and the absorption of the calcar is also smaller than those of the tapered stem. From a clinical point of view, it is difficult to determine how much stress shielding is tolerable, but there is no need to worry about mild stress shielding. Some studies have suggested that [4,5] stress shielding does not significantly affect the treatment effect. With adaptive reconstruction, bone density can be gradually recovered postoperatively, so the side effects of stress shielding effects may have been exaggerated by previous literature.

In conclusion, the rectangular stem has a reasonable filling rate of the pulp cavity and a stable initial fixation by press-fitting, and the stem shape design reduces the stress shielding of the stem body as much as possible, laying a foundation for the long-term stability of the stem body.

Just Medical continues the classic design of the rectangular stem and provides the DELTA rectangular cementless femoral stem with HA+Ti double coating, which can achieve good bone ingrowth and further increase the long-term stability of the prosthesis.