knee

The Zimmer NexGen Trabecular Metal Material Primary Patella—a porous patella. A biomaterial structurally similar to cancellous bone, Trabecular Metal Material has up to 80% porosity and physical and mechanical properties very similar to bone. No competitive product on the market today offers a material that compares to it in these respects.

The Zimmer NexGen Trabecular Metal Material Primary Patella mimics the shape of the NexGen Patellar Groove in mid-to-deep flexion. This optimizes the patello-femoral contact area, where stresses on the patella are the highest. In addition, the NexGen Primary Porous Patella possesses a metal backing with low stiffness, which may further minimize stresses.

The Monoblock Trabecular Metal Patella is a one-piece construct. It retains the articular geometry of the current NexGen All-Poly Component and therefore is compatible with all NexGen Femoral Components. The base is manufactured from Trabecular Metal Material and then polyethylene is directly compression molded from powder into the pores of the Trabecular Metal, resulting in a 1-piece monoblock construct.

The direct compression molding of the poly into the pores of the Trabecular Metal Material base minimizes the potential for the poly disassociating from the Trabecular Metal Material base. The weakest link of this construct is the actual shear strength of the poly itself.

Trabecular Metal Material simulates bone like no other prosthetic material. That's because it is designed to replicate the shape, the cellular structure, the elasticity, and the weight-bearing characteristics of bone. It's a unique, highly porous, three-dimensional biomaterial, not a coating. Trabecular Metal Material unites strength with excellent biocompatibility. It has up to 80% porosity, but retains physical and mechanical properties that resemble bone more closely than other porous materials.  Thus, it allows significant bone and soft tissue ingrowth.

NexGen Trabecular Metal Primary Patella

The Trabecular Metal Material base is shaped by a machining process. The base is inserted into a mold and then polyethylene is directly compression molded into the pores of the Trabecular Metal Material. GUR 1020 calcium stearate-free UHMWPE resin is utilized. Compression molding is performed through a proprietary process via computer-controlled time, temperature, and pressure to ensure maximum consolidation. This construct eliminates any possibility of micromotion between the poly and the base.

Fixation is enhanced by cement penetration into the pores of the Trabecular Metal Material. This provides a true 3-D interlock to maximize stability. Stability is also offered by the central hexagonal post implanted into the cancellous bone of the resected patella. The drill prepares the patella with a hole that is approximately the same size as an inscribed circle within the flats of the hexagonal post. Interference is then provided by the corners of the hexagon post.

Poly thickness in the areas of loading were carefully considered during design of the Trabecular Metal Patella and were maximized to ensure physiological load-carrying capability. The Trabecular Metal Material base has been chamfered to avoid creating a stress riser and to provide a surface under the poly that is perpendicular to the applied load from the femur. Minimum poly thickness is 3.0 mm. However, this thickness is, in effect, increased by 1.5 mm through interdigitation into the pores during the molding process. In addition, Trabecular Metal Material possesses a modulus of elasticity similar to bone and cement, thereby reducing stress in the poly articular surface. A magnified cross section of the Trabecular Metal Patella shows 1.5 mm of penetration of direct compression-molded poly into the Trabecular Metal Material core.  

Dedicated trials, drill guides, and drills are available to properly size, prepare, and evaluate articulation. Three instrument options are available to prepare the patella and implant the component. Resection instruments and techniques are the same as for existing NexGen patella implants. Since the Trabecular Metal Patella has a central hex peg, the drill, drill guides, and provisionals are distinct. The provisionals have a round peg that is slightly smaller than the hole made by the drill. This will allow the trial fitting of the provisional to be accomplished without affecting the interference fit with the implant. Consult the NexGen Trabecular Metal Primary Porous Patella Surgical Technique for detailed information.

The Zimmer NexGen Trabecular Metal Primary Patella was tested using the Zimmer MATCO Polyethylene Deformation Test, which is the testing standard that Zimmer uses for all patella testing. The test performance criteria states that each component should maintain sufficient polyethylene thickness, show no evidence of cracks or fracture of the polyethylene, and maintain good contact with the bone cement substrate.

These standards have been established to determine the likelihood of clinical failure of the design due to "cold flow," "delamination," or "wear through" of the polyethylene material under severe loading conditions.

The method of revision must always be a consideration in design. To break the bond between the anterior face of the patella and fixation material, a saw blade can perform a bone-conserving cut while cutting through the central post. A trephine can then core out the remaining hex post, minimizing the removal of bone. To further minimize the bone loss, a poly "cap" is provided on the bottom of the hex post, which provides a barrier between the cement and Trabecular Metal Material. This feature avoids fixation at the inaccessible part of the post. The Trabecular Metal Augmentation Patella or an all-poly, three-peg primary patella can then be used for the revision.

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