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Optimizing Polyethylene Materials to the Application: When it Comes to Manufacturing Methods, Hips are Not Knees

Gary Miller, PhD
Exactech, Inc.

Polyethylene for joint arthroplasty inserts is manufactured in one of three ways. One very common method is to extrude ultra-high molecular weight polyethylene (UHMWPE) powder under pressure and heat, creating a long cylinder of “extruded bar” material that can be cut into sections and machined to its final dimensions (Figure 1).

Figure 1. The ram extruded polyethylene process draws resin into a cylindrical heated chamber and compresses the mass under high temperature and pressure.

The historically popular extrusion consolidation process is inexpensive and fast, but non-consolidated areas of UHMWPE powder and large variability in material properties are often observed. This variability can lead to premature wear and failure of the insert and is not used in Exactech’s hip and knee inserts.

A second method is to place the UHMWPE powder on a larger flat molding form and apply pressure and heat to consolidate a large slab that is cut into smaller pieces and machined into its final shape (Figure 2).

Figure 2. The sheet-molded polyethylene process results in a large slab that is cut into blocks or cylinders and machined to final shape.

The third method, net compression molding, creates specific precision molds for each part to fully mold them one at a time using specialized, computer-controlled equipment that applies implant-specific optimized pressure and temperature and time profiles to both heat and cool the parts using a proprietary formula (Figure 3).

Exactech Polyethlene Process

Figure 3. The net compression molded polyethylene process yields the exact shape of the articular surface.

There are advantages and disadvantages to each manufacturing method. The most important factor to consider, however, is: which method is best for each application?

Wear in hips vs wear in knees

The mechanisms for wear and failure for hips and for knees are quite different due to the differences in congruency of the articulation and the loads applied during activities of daily living. The hip is a congruent “ball and socket” joint with an orbital reciprocating motion. This leads to a dominance of abrasive wear with secondary additional damage caused by femoral neck impingement should it occur. Knees, with their less congruent articulations and rolling and sliding motion, exhibit delamination and pitting as the dominant wear mechanisms with abrasive wear as the tertiary mechanism. Because of this difference in mechanisms, hip and knee inserts benefit from different mechanical properties for their poly inserts. For hips, we look for improved abrasive wear resistance through moderate cross linking of the polymer while maintaining fracture toughness so that locking mechanism failure and cracking from impingment are avoided. In knees, fracture toughness, which is key to reducing or eliminating delamination and pitting, is a key property.

How do manufacturing processes affect wear rate?

To address the wear performance of extruded and sheet molded materials, many manufacturers apply radiation cross linking technologies that increase the cross linking density to decrease the wear rate in hip and knee articulations. While the application of radiation cross linking does improve abrasive wear behavior, it also makes the resulting materials more susceptible to oxidation and other mechanical property degradation, most notably fracture toughness. In an attempt to mitigate these undesirable effects on mechanical properties, post processing heat treatments or antioxidants are used.

There are two general types of highly cross linked UHMWPE with post processing regimens used to address the oxidation and mechanical property degradation. Both have advantages and disadvantages, depending on the application being considered. Annealed highly cross linked UHMWPE still contains residual free radicals making it susceptible to continued oxidation. Re-melted highly cross linked poly has fewer retained free radicals, however, its mechanical and fatigue/fracture toughness properties are compromised with documented potential for structural problems. More recently, antioxidant-treated polymers (vitamin E) have also been introduced, however, the treatment does not fully eliminate oxidation potential, and the long-term effects on the body are unknown.

Does higher cross linking really lead to a better acetabular insert?

Many manufacturers use a high dose of irradiation (up to 100kGy/10MRad) for enhancing cross linking to reduce the amount of volumetric wear. This, however, comes at a cost to some of the mechanical properties of this bearing surface – mainly fracture toughness, as discussed above. Exactech manufactures Connexion GXL® polyethylene components with sheet -molded UHMWPE using two precision split-doses of 25kGy each in vacuum packaging for a total of approximately 50kGy (Figure 4) to create improved cross-link density.

Figure 4. UHMWPE Fracture Toughness measured using 3-point Bend “J-Integral” Testing.6

This process provides a 59 % reduction in gravimetric abrasive wear over the clinically successful standard Exactech polyethylene while maintaining an acceptable level of fracture toughness to mitigate potential edge/impingement and locking mechanism problems (Figure 5).

Figure 5. Wear rates determined by independent lab testing at the J. Vernon Luck Orthopaedic Research Center (McKellop)7. Lysis data determined by Dowd et. al.8

What if you didn’t have to sacrifice fracture toughness to get excellent tibial insert wear rates?

Recognizing that the abrasive wear mechanism for hip implants is quite different from knee components whose dominant wear mechanisms are delamination and pitting (highly affected by fracture toughness), knee insert components made with Exactech’s proprietary net compression molding technology do not require high levels of radiation cross linking and the subsequent post-processing treatments to create the preferred performance properties for knee applications.

All of the articular surfaces of Exactech tibial polyethylene inserts are carefully molded into the part and not machined as in other processes. By the nature of this proprietary, net compression molding consolidation process, the inserts have high fatigue strength, high fracture toughness, low wear rates and are much less sensitive to oxidation after sterilization.

Comparative laboratory testing published by various manufacturers and researchers shows that Exactech’s net compression molded polyethylene has demonstrated approximately 6X less wear than extruded UHMWPE. This is achieved without sacrificing other important mechanical properties. (Figure 6)

Figure 6. Comparative wear rates for several styles of implants and materials show the net compression molded Opterak articulation being among the lowest wear rates when compared to competitive implants.

Conclusion

The longevity and clinical results of total knee and total hip replacement components depend upon many factors. Exactech, using methods that optimize both mechanical and material properties to match the implant design and application, offers significant improvements in implant performance.


References
1. Data on file at Exactech, Inc.
2. McKellop H, Shen FW, Lu B, Campbell P, Salovey R. Development of an extremely wear-resistant ultra high molecular weight polyethylene for total hip replacements. J Orthop Res. 1999 Mar;17(2):157-67.
3. Dowd JE, Sychterz CJ, Young AM, Engh CA. Characterization of Long-Term Femoral-Head-Penetration Rates. Association with and Prediction of Osteolysis. J Bone Joint Surg Am. 2000 Aug;82-A(8):1102-7.
4. Furman BD, Lai S, Stephen Li S. A comparison of knee simulator wear rates between directly molded and extruded UHMWPE. Presented at Society for Biomaterials, 2001.
5. Herrera L, Sweetgall J, Essner, A, Wang A. Evaluation of sequentially cross linked and annealed wear debris. World Biomaster Cong., Amsterdam, May 28-Jun 1, 2008, 583
6. Papannagari R, Hines G, Sprague J, Morrison M. Long-term wear performance of an advanced bearing knee technology. ISTA, Dubai, UAE, Oct 6-9, 2010.
7. Ezzet KA, Hermida JC, Collwell CW, D’Lima DD. Oxidized zirconium femoral components reduce polyethylene wear in a knee wear simulator. Clin Orhtop 428:120-124, 2004
8. https://www.stryker.com/jointreplacements/sites/triathlon/implants.php 

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