APEX K2 Modular Hip System

By: Omni Life Science  09-12-2011
Keywords: implant, Surgical Technique

Inspired by the clinically proven design profile of the original dual-tapered stem, the APEX K2 MODULAR™ Hip System builds on the philosophy of a dual-tapered trapezoidal stem geometry that facilitates primary fixation and rotational stability. A straightforward and efficient broach-only surgical technique is intended to preserve endosteal bone and intramedullary vascularization.

Unlike traditional dual-tapered stem designs, the APEX K2 MODULAR™ stem allows the surgeon complete intra-operative versatility with the ability to independently select the appropriate stem, neck and head configuration based on individual patient anatomy. The patented Dual Press™ technology allows the modular neck to fully seat against the proximal surface of the stem, without compromising final implant strength or structural integrity.

With Standard and Mid Length Stem options, modular proximal components and simple unified surgical instrumentation, the

APEX K2

MODULAR™ Hip System is engineered to provide the optimal solution when restoring joint mechanics in hip arthroplasty.

Innovative Concepts:


The APEX K2™ MODULAR stem design is based on the fundamental goals of total hip replacement. With the ability to independently select lateral offset, vertical height, and version, surgeons are given complete flexibility to successfully recreate individual hip biomechanics. The modular stem’s versatility allows customization of final implant dimensions that maximize anatomical restoration and minimize joint reaction forces.


Modular stem designs have often been scrutinized for compromises in fatigue strength and reproducible surgical technique. Additionally, modularity has been accused of increased generation of particulate debris, leading to third body wear. The patented Dual Press™ modular connection mechanism allows the neck to fully seat against the proximal surface of the stem utilizing a simple assembly device. Unlike traditional taper-style modular junctions, this eliminates gapping and evenly distributes forces throughout the entire stem. Clinically, this corresponds to reproducible neck positioning in relation to the stem and the ability to confidently pair relatively small diameter stems with significant lateral offsets.

Functionally, the Dual Press™ junction allows a modular stem design to maintain similar structural properties to many monoblock designs. High cycle fatigue testing results confirmed the integrity of the robust Dual Press™ design1. Titanium particle debris generation was less than .004mg after 48.5 million loading cycles. (As a point of comparison, this is approximately 1000x below the reported yearly volumetric wear of metal-on-metal articulations2).

The development of a proximally modular femoral stem has enabled the study of actual head positions which result when surgeons are free to restore leg length and offset independent of distal stem size. In a retrospective, clinical analysis, the modular components of 1482 cases were sorted by head center location and distal stem size3. Head position was depicted as the "vertical drop" to the neck resection level and the lateral offset from the stem’s midline axis.

Results:

  • Average lateral offset increases with stem diameter
  • Patients with small canals required high offsets and/or long neck lengths
  • A significant number of patients with large canals have short neck lengths
  • For all stem sizes, head positions are quite lateralized

Most contemporary THA femoral components offer one or two neck offsets (or angles) for each distal stem diameter. This limits final head position to adjustments in neck length via the modular head. Often, this requires compromises between leg length and joint stability. The APEX K2™ MODULAR Hip System provides a range of 96 neck and head combinations for each stem size, allowing surgeons to select the optimal size for each individual patient.

Despite evolutions in implant design, achieving proper joint stability remains complex in total hip arthroplasty. Surgical navigation and advanced instrument design are the most recent pursuits aimed at making the procedure more reproducible. With the introduction of earlier modular hip systems, surgeons gained obvious advantages over traditional monoblock stems.

The APEX K2™ MODULAR hip stem is engineered to provide surgeons an optimal solution in achieving joint stability without compromises in implant integrity or surgical technique.

Independent modularity of neck, stem, and head parameters givesurgeons intra-operative flexibility to assemble an implant according to individual patient anatomy and biomechanical requirements. Upon final seating of the distal stem within the canal, the multiple neck and head configurations can easily be manipulated to achieve proper anatomical positioning.

Independent adjustment of lateral offset and vertical height allows correction for leg length discrepancy and appropriate soft tissue tensioning.

Accurate positioning of the implant creates the foundation necessary to establish clinical success and implant longevity. The APEX K2™ MODULAR stem’s dual-tapered trapezoidal geometry provides intrinsic axial and rotational stability upon final seating in the metaphysis.

The enhanced metaphyseal dimensions of the stem are designed to provide additional resistance to torsional loads. The trapezoidal cross- sectional profile inherently establishes solid purchase in cortical bone to facilitate primary fixation. Combined with the broach-only surgical technique, the stem’s geometry is also intended to preserve intramedullary vascularization.

A CP-Titanium plasma spray surface treatment is applied to the proximal third of the stem with a specific porosity to encourage bony ingrowth, providing a circumferential seal against distal particulate migration. Distally, Corundum blasting creates a roughened macrostructure to encourage bony ongrowth.

The APEX K2™ MODULAR Hip System is implanted via an efficient, broach-only surgical technique. The metaphysis is prepared in accordance to the templated stem size by sequential broaches. The surgical technique allows for the option of either trialing head and neck combinations off the final broach or the actual femoral implant. Depending on surgical philosophy, the modular stem can easily be assembled on the back table prior to implantation or in-situ in accordance to minimally invasive techniques.


Footnotes:
  • 1. Data on file OMNI life science™
  • 2. McKellop H, Park SH, Chiesa R, Doom P, Lu B, Normand P, Grigoris P, Amstutz H.: “In Vivo Wear of Three Types of Metalon-Metal Hip Prostheses During Two Decades of use”, Clin. Orthop., S128:140, 1996.
  • 3. Noble, P.C.; Alexander, J.W.; et al.:" The Anatomic Basis of Femoral Component Design." Clin. Orthop., 235:148, 1988.
  • 4. Sakalkale, D.P.; Sharkey, P.F.; Eng, K. et al.:"Effect of Femoral Component Offset On Polyethylene Wear In Total Hip Arthroplasty." Clin. Orthop., 388:125, 2001.

Keywords: implant, Surgical Technique

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