Uncemented Cylindrical Stems
Tad M. Mabry
The surgeon must take great care to obtain excellent femoral exposure, as cylindrical stems require both preparation and implantation in line with the femoral canal.
Initial axial and rotational stability is achieved by way of a diaphyseal “scratch fit” over a distance of 5 to 7 cm.
Although these stems have been used successfully across a very wide spectrum of femoral morphologies, the surgeon should consider alternative means of stem fixation at the extremes of femoral canal dimensions, and in the setting of coxa vara with significant trochanteric overhang (Figure 8.1).
Sterile Instruments and Implants
Surgical approach-specific hip retractors
Large-diameter high-speed burr and/or lateralizing reamer
Instruments specific to the selected cementless, cylindrical component
Cylindrical stems may be implanted via any of the hip arthroplasty approaches.
However, these straight, rigid stems are not forgiving of suboptimal femoral exposure. To gain appropriate fixation, the femur must be prepared and the implant inserted in line with the diaphysis.
Given the excellent femoral exposure offered, the posterolateral approach and the direct lateral approach are typically favored when using this style of femoral component.
Before detailed planning for a cylindrical stem, the surgeon should assess for any femoral deformity that might lend itself better to alternative methods of reconstruction.
Compromised bone quality (significant osteoporosis, thin cortices, excessively narrow or wide femoral canal dimensions)
The intended hip center is first determined from templating the acetabular side of the hip reconstruction.
The femoral reconstruction is then planned in such a way as to effect the necessary change (or maintenance) in leg length. The following 3 key features of the intended stem are then determined (Figure 8.2):
Stem diameter—the goal is diaphyseal contact over 5 to 7 cm.
Figure 8.1 ▪ Attempting to use a cylindrical stem in the setting of coxa vara with significant trochanteric overhang risks trochanteric fracture and/or stem malposition. In a case such as this, one should consider alternative stem designs.
Figure 8.2 ▪ Templated cylindrical stem noting ideal stem diameter (6.5 cm of cortical contact), metaphyseal fill, and restoration of offset.
Metaphyseal fill—most cylindrical stems have a roughly triangular metaphyseal segment of varying sizes. The goal is to maximize metaphyseal fill without excess bone removal and without impeding rigid diaphyseal fixation.
Stem offset—the goal is to maintain, or slightly increase, femoral offset, given the beneficial effects on hip stability and wear mechanics.
Note, the options for stem offset may relate to the size of the metaphyseal segment in some systems.