ZIMMER 5-POINT P O LY E T H Y L E N E. One Source Addresses All The Poly Wear Issues.

Transcription

1 ZIMMER 5-POINT P O LY E T H Y L E N E One Source Addresses All The Poly Wear Issues.

2 Performance Proven By M M a t e r i a l The polyethylene used in orthopaedic implants today is not all of equal quality. Diff e rences in molecular weight, d e n s i t y, cry s t a l l i n i t y, and other pro p e rties can substantially a ffect the perf o rmance of a fabricated polyethylene bearing surf a c e. An independent study across all grades of ram-extruded polyethylene showed that yield s t rengths varied by 25%, elastic modulus varied by 35%, and creep varied by 400%. 2,3 Another study demonstrated a highly variable defect density from one resin lot to another. 4 These studies underscore the importance of carefully selecting and thoro u g h l y inspecting raw materials. T h e re are also diff e rences caused by additives to the bulk resin, such as calcium stearate, which is routinely added to polyethylene that is to be ram extruded. Recent studies show that not adding calcium stearate can improve material morphology 5 and can result in an increase in thin film tensile test mechanical pro p e rt i e s. 6 ZIMMER CERTIFIED RAW MAT E R I A L S Zimmer compression-molded polyethylene is made f rom resins which have no added calcium stearate. Ultrasound and microscopic inspection is perf o rmed on all raw and finished material lots to help ensure that polyethylene articulating components will be virtually free fro m voids, inclusions, and unconsolidated are a s. Zimmer maintains standards that meet or exceed ASTM re q u i rements. In addition, Zimmer specifies that all comp ression-molded polyethylene be free from fusion defects. The resulting material is Zimmer Certified Raw Material w h e reby Zimmer certifies that its polyethylene raw materials and finished articulating surface implants conform to stringent Zimmer specifications. P ro c e s s i ng The fabricating method can have a profound impact on the quality of the final polyethylene component. The p roper balance and control of pre s s u re, temperature, and time in the processing technique can affect many i m p o rtant pro p e rties of the material. Investigators have shown that superior manufacturing methods can result in more consistent consolidation and more favorable morphological characteristics (Fig. 1). E x t ruded bar stock with calcium stearate Molded bar stock with no calcium stearate added Fig. 1 Extent of consolidation varies significantly in orthopaedic polyethylene. A P P L I C ATION-SPECIFIC, COMPRESSION- MOLDED POLY E T H Y L E N E All polyethylene hip and knee articulating components currently manufactured by Zimmer a re compression-molded in either a sheet-molded f o rm followed by machining to final component g e o m e t ry, or net shape compre s s i o n - m o l d e d. Tibial components are net shape molded to create the articulating surf a c e, then machined on the bottom surface. Acetabular cups are machined f rom compression-molded sheets. To obtain a high degree of polymer fusion, optimized processing techniques are utilized with rigorous contro l s. C o m p ression molding precisely controls the pro c e s s i n g parameters of pre s s u re, temperature, and time. Te s t i n g conducted at Zimmer has shown that compre s s i o n - molded components maintain their original mechanical p ro p e rties better than extruded rod components when both are subjected to gamma sterilization and accelerated aging. 7

3 By More Than 20 Years Of Suc D e s i g n Another significant factor in ensuring the long-term durability of polyethylene components is the design of the implant itself. Some design factors are i m p o rtant to both hip and knee implants, while others are joint-specific. Joint-specific design considerations re s u l t f rom the diff e rent mechanical loading environments pre s e n t, as well as diff e rences in kinematics. The design re q u i rements are especially challenging for modular knee inserts, where the competing criteria for c o n f o rmity and constraint must be adequately balanced. The primary challenges for acetabular components are to minimize wear at the articular surface in addition to backside surface wear (liner/shell interf a c e ). W E A R - FACTORED DESIGN Design features have been optimized by Zimmer engineers in the process of designing contemporary articular surf a c e s within the limitations of polyethylene material pro p e rt i e s. Fig to 1.00 ratio of radii results in a large contact area that reduces stre s s. K N E E S High Conformity between Art i c u l a t i n g S u rf a c e s The ratio of radii between all N e x G en Total Knee Solution a rticular surface components and femoral components is 1.07 to 1.00 (Fig. 2) as has been used in other highly successful knee systems. P recise Finishing Techniques for Femoral C o n d y l e s Zimmer employs robotic polishing techniques to optimize conformity by producing consistently uniform articular surfaces. Containment Rail that Controls Cold Flow N e x G e n tibial base plates help control cold flow with a containment rail that surrounds the art i c u l a r s u rface component. H I P S Multi-Faceted Locking Mechanism This mechanism on the Tr i l o gy Acetabular Cup is specifically designed to address overall polyethylene wear via the following feature s : A n t i - rotational tabs secure the liner in place. Full congruency inhibits micromotion as it maintains integrity under load and stre s s. Bottoming out feature prevents rim loading by ensuring uniform metal shell support of the polyethylene liner. Polar boss minimizes transverse forces by pro - viding an additional stabilization point. Optimized Thickness The Tr i l o g y A c e t a b u l a r System is designed to maximize polyethylene thickness without compromising necessary rangeof-motion and metal shell thickness. P recise Finishing Techniques for Femoral H e a d s S t a t e - o f - t h e - a rt technology is employed to produce super-finished femoral heads which exceed ISO and ANSI standards for sphericity and s u rface finish.

4 Of Success. S t e r i l i z a t i o n Gamma irradiation is the most common method of sterilization for polyethylene implants. But with oxygen p resent, this method may accelerate oxidation, which can potentially limit the useful p ro p e rties of the material. 8,9 When polyethylene components a re irradiated in a near oxygenf ree environment, however, the material is enhanced t h rough cro s s - l i n k i n g. G A M M A - I R R A D I ATION STERILIZAT I O N All Zimmer polyethylene components a re sterilized by gamma irradiation in a nitrogen environment. This pro c e s s induces cross-linking, which has been shown to improve the wear resistance of the material in labora t o ry testing. 1 0, 1 1 F u rt h e rm o re, irr a d i - ating the component in a nitrogen e n v i ronment minimizes the oxygen p resent and significantly reduces oxidation. The result is a more durable material that is more resistant to abrasive wear. P a c k a g i n g The storage enviro n m e n t can also affect the mechanical perf o rmance of a polyethylene implant. Over time, polyethylene can experience significant oxidative degradation when oxygen is p resent during shelf storage. NITROGEN PA C K A G I N G Zimmer polyethylene implants are packaged in a n i t rogen environment using high oxygen barr i e r materials to help prevent oxidation during sterilization and storage. The innermost barrier is subjected to an evacuation of air followed by the infusion of nitrogen gas, lowering the internal oxygen level to less than.25%. The oxygen permeation rate is further reduced by a foil-backed lid that is securely sealed. This unique packaging pro c e s s helps ensure that the oxygen level will remain below ambient conditions for at least five years (Fig. 3). 1 Fig. 3 Based on oxygen permeability of packaging materials and average size polyethylene implant. The Complete Story F a i l u re to consider any one of these five basic perf o r- mance factors can have a substantial impact on the perf o rmance of the polyethylene component. Zimmer has given careful consideration to all. For the complete story on polyethylene and the outstanding clinical success of Zimmer polyethylene implants, ask your Zimmer re p resentative for our Design Rationale on polyethylene, or call

5 5-Point Polyethylene Addressing All The Factors. C reating polyethylene bearing surfaces that perf o rm well in human joints has long been a focus of ort h o p a e d i c re s e a rch and development. To meet this challenge, eff o rts have been made by implant manufacturers to address five key factors: material, processing, design, sterilization, and packaging. These eff o rts, however, typically involve only one or two of the factors. Although addressing any one of the factors can potentially enhance the perf o rmance of a polyethylene component, optimizing the implant involves a c a reful balance that reflects the delicate interplay of all five factors. Zimmer optimizes polyethylene perf o rmance by addressing all five factors for each specific implant. This careful attention to every important factor continues to build Zimmer s outstanding reputation for polyethylene perf o rmance. And our measure of success is not based on lab studies alone. It is clearly evident in the results of more than 20 years of actual clinical experience with compre s- sion-molded polyethylene articular surfaces in more than 170,000 TKAs, and more than 210,000 THAs. 1 M AT E R I A L Highest quality ultra-high molecular weight polyethylene with no calcium stearate added. D E S I G N Documented history of more than 20 years of clinical success. PA C K A G I N G N i t rogen packed in a protective cavity to minimize shelf oxidation. P R O C E S S I N G C o m p u t e r- c o n t rolled compressionmolded polyethylene. S T E R I L I Z AT I O N Gamma irradiation in an inert (N 2 ) a t m o s p h e re promotes cro s s - l i n k i n g.

6 1. Zimmer data on file. 2. Li S, Howard EG. Characterization & description of enhanced UHMWPE for o rthopaedic bearing surfaces. Presented at the 16th Annual Meeting of the Society for Biomaterials; May 20-23, 1990; Charleston, SC. 3. Li S. Special workshop on wear in joint replacement. Orthopaedic Researc h Society; 1990; New Orleans, LA. 4. Tanner M, Hoernschemeyer D, Whiteside L. Polyethylene quality variations in c u rrently available bar stock. Presented at the 64th Annual Meeting of the American Academy of Orthopaedic Surgeons; Febru a ry 1997; San Francisco, CA. 5. Li S, Callaghan JJ, Rosenberg AG, et al. The Adult Hip. Philadephia, PA ; Lippincott-Raven Publishers; 7:4. 6. Lin S, Gsell R, Rohr W, et al. The effect of manufacturing variables on mechanical pro p e rties of UHMWPE. Proceedings from Symposium on Modern Trends and Perspectives in Total Hip Implants; revisions 1996; Rome, Italy. 7. Pletcher D, Swarts D. Compression molding of UHMWPE: a reliable process for implantable devices. Zimmer Technical Paper; Nagy EV, Li S. Fourier transform infrared spectroscopy techniques for evaluation of polyethylene orthopaedic bearing surfaces. Trans. Soc. For Biomaterials ; 1 3 : Rimnac CM, Klein RW, Betts F, et al. Post irradiation aging of ultra high molecular weight polyethylene. J Bone Joint Surg. In pre s s. 1 0.G robbelaar CJ, Du Plessis TA, Marais F. The radiation improvement of polyethylene prosthesis. J Bone Joint Surg ; B ( 3 ) : Oonishi H, Kuno M, Ikada Y, et al. Super low wear cross-linked UHMWPE by heavy high-dose gamma-radiation. Proceedings from the 2nd Congress of Hip Section of We s t e rn Pacific Orthopaedic Assn; 1996:4.