The femoral neck attaches to the body of the stem through a unique Dual PressTM connection* that is simple, robust, and very stable. This modular design allows a large selection of necks, enabling the proper combination of anteversion angle, lateral offset, and neck length/leg length, to restore proper soft tissue tension and joint biomechanics.
The modular attachment mechanism is new to orthopaedic implants, but the concept was derived from conventional mechanical tool design. The distinguishing feature is that the hole in the stem, and the mating peg on the neck, are cylindrical rather than tapered. To create a mechanical lock, and interference fit is created at the proximal and distal ends of the mating peg. The forces required to seat the neck are relatively small, and easily generated with the stem in situ. This design eliminates the need for tapers, which can be difficult to manufacture and prone to dissociation. For all practical puposes, the stem will perform as a one-piece stem (with a conventional modular head) after attachment of the neck.
The proximal end of each stem includes an alignment pin that engages with the mating hole on the distal surface of each neck. Neutral necks have a single hole; anteversion necks have two holes for 13°. This ability to adjust neck orientation eliminates the need for separate left and right stems, thus reducing inventory requirements, while enabling better restoration of joint biomechanics. The pin and hole also provide additional torsional stability, as well as control of the version angle.
The modular necks are designed so as to provide a wide range of final stem geometries. All necks and stems employ a universal modular junction, allowing any neck to be used with any stem. Extensive mechanical testing has proven the strength of this design.
Clinical experience with this design has shown that small distal canals are often combined with long necks and high offsets. Conversely, large canals occur in older, osteoporotic women with small offsets. The versatility of the APEX MODULARTM design allows the surgeon to create a stable joint with proper leg length for both of these scenarios.
* Patent Pending
Mid-Stem Flutes
Flutes for bone engagement are limited to the mid-stem region*. As in other implant designs, these flutes will provide torsional stability (supplemental to the proximal body); however, because these flutes do not extend to the distal end of the stem, the flutes will not engage until the stem is almost inserted. This alleviates problems related to incorrect stem alignment that can result when the flutes engage too early, including rotation that can occur during stem insertion due to the natural curvature of the femoral canal.
*US Patent 6,702,854; US and International Patents Pending
"V" Slot Distal Stem
The distal stem will have either a single slot (a "clothespin" slot") in the smaller
sizes or a unique "V"-slotTM in the large sizes*. This produces a nearly constant normalized stiffness as a function of stem size, and thus avoids overly flexible (and weak) small stems and overly stiff large stems. Increased flexibility in the larger sizes will make the stem easier to insert, and could, in theory, reduce the occurrence of thigh pain. While distal slots have been used in other designs, the tailoring of the slot number and size as a function of stem size* is new and unique.
*US Patent 6,702,854; US and International Patents Pending
