Polyethylene in Total Knee Arthroplasty

Pakdee Rojanasopondist, BA

Orhun Muratoglu, PhD

Ultrahigh-molecular-weight polyethylene (UHMWPE or polyethylene) is the material of choice as for the fabrication of tibial knee inserts and patellae components used in total knee arthroplasty. Owing to the relatively wide range of articular geometries, kinematics, and loading conditions, the analysis of the material properties and clinical performance of polyethylene in knee arthroplasty is more complex than in hip arthroplasty. The challenge in producing polyethylene for a prosthetic knee is to optimize the interplay of the basic material properties to allow a practically unlimited number of loading cycles without material failure—with the generation of a minimum of biologically active particles—for that specific design.

MEDICAL GRADE POLYETHYLENE

Ethylene is a gaseous hydrocarbon composed of two carbon atoms and four hydrogen atoms, C₂H₄. Polyethylene is a long-chain polymer of ethylene molecules in which all of the carbon atoms are linked, each of them holding its two hydrogen atoms¹ (Fig. 12-1). The mechanical properties of UHMWPE are strongly related to its chemical structure, molecular weight, crystalline organization, and thermal history.²

UHMWPE microstructure is a two-phase viscoplastic solid consisting of crystalline domains embedded within an amorphous matrix²,³ (Fig. 12-2). Connecting the crystalline domains are bridging tie molecules.³ UHMWPE is defined as polyethylene with an average molecular weight of more than 3 million g per mol.² The UHMWPE currently used in orthopedic applications has a molecular weight of 3 to 6 million g per mol, a melting point of 125°C-145°C, and a density of 0.930 to 0.945 g per mL.²,⁴ UHMWPE has been miscalled high-density polyethylene in the orthopedic literature, and it is important to distinguish between the two. High-density polyethylene is defined as polyethylene with a density greater than 0.940 g per mL and a molecular weight below 200,000 g per mol.² UHMWPE has a lower density than high-density polyethylene and has a much higher molecular weight, higher impact strength and toughness, and better wear characteristics under abrasive conditions.³,⁴

FIGURE 12-1 Linear chemical structure of polyethylene.

The nomenclature for UHMWPE resins has evolved. The current nomenclature is outlined in Table 12-1.² Ticona (Summit, NJ) is the sole supplier of medical grade UHMWPE resins to orthopedic manufacturers.

Ticona, previously Hoechst Celanese Corporation, produces GUR resins. GUR is an acronym as follows: The first letter represented the Hoechst nomenclature for polyethylene resins; in this case, G corresponds to “granular.” The second letter represents the molecular weight in the old Hoechst system, with the higher letters equivalent to very high molecular weights; conveniently, U was selected for use with UHMWPE. The third letter, R, stands for Ruhrchemie AG (Oberhausen, Germany), the plant (which was partially owned by Hoechst) where this product was first produced.

For Ticona resins, the first digit of the grade was used to describe the loose bulk density of the resin; however, in 1998, the nomenclature was consolidated with the first digit uniformly “1.” The second digit indicates the absence (“0”) or presence (“1”) of calcium stearate in
the resin. Calcium stearate is an additive in the manufacturing process of many polyethylene resins, which acts as a corrosion inhibitor,²,⁵ whitening agent,⁴ and lubricant to facilitate the extrusion process.²,⁵,⁶ The third digit is related to the average molecular weight of the resin; the number “2” is used for resins that have an average molecular weight of 3.5 millions g per mol, whereas the number “5” is used for resins with an average molecular weight between 5.5 and 6.0 million g per mol. The fourth digit is an internal code designation. The biggest difference between GUR resins is in impact strength and abrasion resistance. GUR 1020 and 1120 have higher levels of impact strength, whereas GUR 1050 and 1150 have higher abrasion resistance. The most commonly supplied UHMWPEs to orthopedic manufacturers in the United States are GUR 1050 and GUR 1020.²

FIGURE 12-2 Molecular structure of ultrahigh-molecular-weight polyethylene. (From Spector M, Bellare A. Implant materials: metals, polyethylene, polymethylmethacrylate. In: Pellicci PM, Tria AJ Jr, Garvin KL, eds. OKU-2—Hip and Knee Reconstruction. Rosemont, IL: American Academy of Orthopedic Surgeons; 2000, with permission.)

TABLE 12-1 Nomenclature of Medical Grades of Ultrahigh-Molecular-Weight Polyethylene. Note That GUR 1020 and GUR 1050 Are the Only Ones Currently in Clinical Use

Resin Designation	Actual Producer	Previous Designation	Previous Producer
GUR 1150	Ticona (Summit, NJ)	GUR 4150	Hoechst
GUR 1050	Ticona (Summit, NJ)	GUR 4050	Hoechst
GUR 1120	Ticona (Summit, NJ)	GUR 4120	Hoechst
GUR 1020	Ticona (Summit, NJ)	GUR 4020	Hoechst
1900	Basell (Wilmington, DE)	–	Hercules-Himont-Montell
1900H	Basell (Wilmington, DE)	–	Hercules-Himont-Montell

In general, UHMWPE resin powders consist of numerous fused, spheroidal UHMWPE particles with a fine network of submicron-sized fibrils that interconnect the microscopic spheroids. Ticona resins have a mean particle size of approximately 140 µm.² Physical properties of Ticona resins are outlined in Table 12-2.

TABLE 12-2 Physical Properties of Ticona and Himont Resins

	Average Molecular Weight	Density	Tensile Modulus	Yield Stress	Impact Strength	Particle Size
Resin	(10⁶ g/mol)^a	(g/mL)^b	(MPa)^b	(MPa)^b	(kJ/m²)^b	(µm)^a	Calcium stearate^a
GUR 1150	5.5-6.0	0.93	680^c	≥17^c	≥130^d	140	Yes
GUR 1050	5.5-6.0	0.93	680^c	≥17^c	≥130^d	140	No
GUR 1120	3.5	0.93	720^c	≥17^c	≥210^d	140	Yes
GUR 1020	3.5	0.93	720^c	≥17^c	≥210^d	140	No
1900	4.4-4.9	0.93	750^e	19^f	65^g	300	No
1900H	>4.95	0.93	750^e	19^f	65^g	300	No
MPa, megapascal. Note that the GUR 1020 and GUR 1050 are the only polyethylene resins currently in clinical use.
^aAdapted from Kurtz SM, Muratoglu OK, Evans M, et al. Advances in the processing, sterilization, and crosslinking of ultrahigh molecular weight polyethylene for total joint arthroplasty. Biomaterials. 1999;20:1659-1688. ^bData from Ticona product data sheet and Basell product data sheet. ^cInternational Standards Organization (ISO) 527 test method. ^dISO DIS 11542 test method.^eAmerican Society for Testing and Materials (ASTM) D 790B test method.^fASTM D 638 test method.^gMontell P 116 test method.

There are two methods to produce orthopedic devices with UHMWPE (Fig. 12-3). The first method is machining of components from stock polyethylene material. Stock material is available as cylindrical ram-extruded bars or large molded sheets, from which the implant is machined into its final shape.²,³,⁴

To produce ram-extruded bars, polyethylene powder is introduced into a cylinder that contains a reciprocating ram extruder. The powder is then compacted and heated at temperatures between 180°C and 200°C to become consolidated. Extruded bars are available in many diameters and lengths. The most commonly used are 2.0, 2.5, and 3 inch wide and 5 and 10 feet long.

To produce molded sheets, polyethylene powder is placed in a mold, usually with a size of 4 × 8 feet. Once in the mold, the powder is cold pressed under pressures between 5 and 10 megapascal (MPa) to reduce the amount of air trapped. The compressed powder is then heated at approximately 200°C until the powder is completely
fused. After that, the mold is cooled under approximately 7 to 10 MPa.⁷ Most common molded sheet thicknesses are 60 mm, 2.25, 2.50, and 3.0 inch.

FIGURE 12-3 Manufacturing process of surgical implants from ultrahigh-molecular-weight polyethylene (UHMWPE). In components made by direct compression molding, the obtained surface finish is smooth and without machining marks.

Although any type of resin can be used with either ram extrusion or compression molding, GUR 1050 is more commonly used for ram extrusion, whereas GUR 1020 is more commonly used for compression molding.²

Polyethylene components can also be made by direct compression molding. In this process, the manufacturer effectively converts the resin powder into the final component by placing it into a mold for the finished component, which is compressed and heated.⁴ The obtained surface finish is smooth and without machining marks. Typically the direct compression molded parts are molded to a near net shape, where the final articular surface is formed during molding and the back side of the component is machined after molding.

Long-term performance of TKA implants can be compromised by adhesive/abrasive wear of the articular surface and the backside surface of the UHMWPE tibial insert as well as by articular surface wear of the patellar component. More importantly, pitting and delamination wear damage on the articular surface of the tibial insert and patellar components can adversely affect outcomes and, in catastrophic cases, lead to a revision surgery. There are many factors that may affect adhesive/abrasive wear and delamination damage of UHMWPE components such as articular surface geometry, metal backing, motion patterns, sterilization methods, level of cross-linking, and oxidative stability.

ARTICULAR SURFACE GEOMETRY

The geometry of an articulation can be generally described as a convex surface on a concave surface. In total knee replacement (TKR), the degree of conformity between tibial and femoral articulating surfaces can be described as the ratio of the radius of curvature of the tibial component (R₂) to the radius of curvature of the femoral component (R₁): R₂/R₁. Such an analysis can be done for the sagittal and coronal geometries. As the ratio approaches 1, the articular conformity increases. Thus, the most conforming articulation would have matched radii and a ratio of 1 (ie, a total hip replacement [THR] or a flat-on-flat surface). Because the sagittal radius of curvature of a femoral component may not be constant over the entire sagittal profile, conformity may vary throughout knee range of motion.⁸ Constraint—the restriction of motion—is an independent result from increased conformity: A flat-on-flat articulation is completely conforming and has no constraint to motion, whereas a dished articulation of matched radii is completely conforming, yet motion is constrained to one plane.⁸

Condylar designs with conforming tibiofemoral articulations have large contact areas and lower contact stresses⁹,¹⁰,¹¹ but may not allow physiologic translational and rotational movements. Relatively flat tibial articulations can accommodate such motions but have smaller contact areas and higher contact stresses.¹²,¹³,¹⁴,¹⁵,¹⁶

Polyethylene contact stresses are a function of the load, the contact area, and component thickness.¹⁷ The maximum contact stress on tibial components increases as the polyethylene thickness decreases.¹⁶,¹⁸ The location of the maximum shear stress has implications for wear when conventional UHMWPE is used, owing to the fact that shear stresses are associated with propagation of subsurface cracks when the fatigue crack propagation resistance of UHMWPE is compromised by oxidation. In this regard, conformity is an important factor. In contrast to THRs in which the components are highly conforming and the maximum shear stress is at the surface, in TKRs, the maximum shear stress is located between 1 and 2 mm beneath the surface.¹⁸ Unfortunately, this is also the location of maximum oxidation and the so-called white band, which develops in certain components after gamma radiation in air or in inert gas or after radiation cross-linking. The
maximum stresses, occurring in the weakest zone of these components, cause subsurface fatigue, leading to aggressive delamination, high wear rates, and clinical failure.

METAL BACKING

Metal backing of the tibial component was introduced to improve the load distribution at the bone-implant interface, reducing the stress in the supporting bone.¹⁸,¹⁹ Given sufficient polyethylene thickness, there is no apparent detrimental effect on the wear of the primary articulating surface as a result of metal backing.

The polyethylene insert-metal tibial base interface is, however, a potential additional source of wear debris (Fig. 12-4). Analysis of motion between the polyethylene tibial insert and metal baseplate of nine modular total knee implant designs, including five snap-fit (Miller-Galante II, Press-Fit Condylar, Duracon, Genesis, Ortholoc) and four tongue and groove designs (anatomic modular knee, IB II, Axiom, Maxim), showed that in every implant the polyethylene insert-metal base interface was subject to micromotion. No significant differences were observed in the motion allowed by each one of those mechanisms.²⁰ Because a significant correlation between severe wear on the back surface of polyethylene inserts and tibial osteolysis has been established,²¹,²² locking mechanisms that minimize relative motion are desirable.

MOTION PATTERN

Polyethylene wear is a function of the motion pattern. In wear tests that use a linear motion path, such as a reciprocating pin on disc, the rate of polyethylene wear for a given set of test conditions is 10 to 100 times lower than that of wear tests that have crossing motion paths, such as in a hip simulator.²³ In theory, a total knee with simple linear motion pattern(s) would exhibit very low wear.

FIGURE 12-4 Backside wear on modular tibial polyethylene insert. Large gouges and gross deformation are due to implant extraction. Note, however, the diffuse burnishing and small scratches from relative motion against the metallic baseplate.

Yet, in vivo cine-fluoroscopic studies of clinically well-functioning total knees have demonstrated more complex motions that include variable degrees of rolling, sliding, and rotation on the same surface. In contrast to normal knees, the kinematics of posterior cruciate-retaining total knees is characterized by anterior femoral translation or “skidding” of between 3 and 9 mm.²⁴,²⁵ Posterior cruciate-substituting knee systems demonstrate more posterior femoral rollback with flexion, although some anterior femoral translation can occur.²⁴ Additionally, low-conformity, cruciate-retaining designs can demonstrate paradoxic rotation or negative screw home around a lateral axis.²⁵ Condylar lift-off is a frequent occurrence in cruciate-retaining and cruciate-substituting designs. These motion patterns result in high polyethylene stresses and material failure, such as pitting and delamination.²⁶,²⁷

WEAR PARTICLES

The number, shape, and size of polyethylene wear particles are multifactorial: They are a function of the modes and mechanisms of wear that produce them, the stresses on the bearing surface, the motions, and the polyethylene molecular orientation. Most of the polyethylene wear particles produced in a prosthetic joint are micron to submicron in size and are produced in mode 1 in very large quantities by well-functioning joints.²⁸ Techniques have been developed to isolate and analyze wear particles generated in vivo by retrieving them from periprosthetic tissues.²⁸,²⁹,³⁰,³¹,³²,³³,³⁴ The concentration of debris particles from prosthetic joints is directly correlated to the duration of implantation³⁵ and can extend into the billions per gram of tissue.³⁰,³¹,³²,³⁶

Significant differences in polyethylene wear particles from THRs and TKRs have been found. THRs release a relatively high number of submicron polyethylene particles and relatively few particles several microns in dimension, whereas TKRs release a broader range of particles that include some very large flakes measuring hundreds of microns across but relatively fewer submicron particles.³⁷,³⁸,³⁹ Although some studies reported up to 71% submicron particles in TKR cases, compared to 85% in THR cases,³⁹ others have reported only 36% of submicron particles in such cases.⁴⁰ The overall average area of particles from total knees has been reported to be approximately twice that of total hips, owing to large flake-shaped particles, relatively common in knee specimens, measuring several microns in length and width.³⁹

Such differences in polyethylene particles from TKRs and THRs can be explained by differences in the articulating surfaces, stresses, and motion patterns between them. Increased contact stresses resulting from a decreased conformity, as occurs in TKRs, can exceed the yield strength of polyethylene.¹²,¹⁸,⁴¹ Furthermore, in a TKR with relative low conformity, the motion pattern can include
rolling, sliding, and rotation on the same surface; rotation with anteroposterior sliding has been associated with high wear.²⁷ The combination of these factors results in differences in the balances of the wear mechanisms in THRs and TKRs. In THR, the predominant wear mechanisms appear to involve microadhesion and microabrasion with the generation of many polyethylene particles less than 1 µm in length. The resultant wear damage is predominately burnishing and scratching.²⁸ In contrast, pitting and subsurface delamination have been commonly identified as wear damages in TKRs because the wear mechanisms involve a greater amount of abrasion and fatigue. These mechanisms result in visually striking surface damage seen on some retrieved polyethylene tibial bearings.⁴²,⁴³

STERILIZATION METHODS

Clinical and laboratory research have revealed that sterilization methods can dramatically affect the in vivo performance of a polyethylene component. UHMWPE components for total joint arthroplasty can be sterilized using gamma irradiation, gas plasma, or ethylene oxide (EtO). Gamma irradiation in an air environment was the industry standard since the early 1970s until about the mid-1990s, using doses between 2.5 and 4.0 megarads (Mrad), most commonly between 3.0 and 3.5 Mrad.

In addition to sterilization, gamma radiation breaks covalent bonds in the polyethylene molecule. This produces free radicals—unpaired electrons from the broken covalent bonds—that can combine with oxygen (if present) during the irradiation process, during shelf-storage, and in vivo. Oxidation of the polyethylene molecule is a chemical reaction that results in chain scission (fragmentation and shortening of the large polymer chains) and introduction of oxygen moieties into the polymer molecules.⁴ Such oxidation lowers the molecular weight of the polymer (which reduces its toughness), increases the density through increased crystallization, and results in a reduction in fracture strength, increase in modulus, and a decrease in elongation to break.⁴⁴,⁴⁵,⁴⁶,⁴⁷

Peak levels of oxidation typically occur approximately 0.5 to 2.0 mm below the surface of a polyethylene component, forming the so-called white bands seen on microtomed sections of components sterilized by gamma radiation (Fig. 12-5).⁴⁶ As the degree of oxidation increases, so does the occurrence of fatigue cracking and delamination, as is observed in retrieved tibial components (Fig. 12-6).¹²,⁴⁷,⁴⁸,⁴⁹,⁵⁰ Why the peak level of oxidation is subsurfaced remains a topic of debate.

Oxygen can diffuse into the components during shelf-storage and in vivo. Components with less than 1 year from the time of sterilization to the time of implantation exhibit lower in vivo oxidation and better in vivo performance than components with a longer so-called shelf-life before implantation.⁵¹ A survival analysis performed on 108 TKRs sterilized by gamma irradiation in air showed that after 5 years of implantation, tibial-bearing surfaces that had shelf-lives of less than 4 years had a 100% survival rate, whereas those that had shelf-lives of 4 to 8 years and 8 to 11 years had survival rates of 88.6% and 79.2%, respectively.⁵² In laboratory wear tests, polyethylene that had been gamma irradiated in air and aged exhibited a higher wear rate than nonirradiated material.⁵³ However, polyethylene components that have been irradiated in air and tested within months exhibit lower wear rates than identical components that have not been irradiated, owing to a favorable amount of cross-linking compared to the amount of oxidation.

FIGURE 12-5 Microtome section of a polyethylene tibial component sterilized by gamma radiation. Peak levels of oxidation occur approximately 0.5 to 2.0 mm below the articular surface, forming the so-called white band. (Photo courtesy of Dr. John P Collier.)

Irradiation can produce a beneficial effect on polyethylene wear properties as a result of cross-linking. Cross-linking occurs when free radicals, located on the amorphous regions of polyethylene molecules, react to form a covalent bond between adjacent polyethylene molecules. Cross-linking can be accomplished using peroxide chemistry, variable dose ionizing radiation, or electron beam irradiation.

FIGURE 12-6 Retrieved tibial component that had been sterilized by gamma radiation in air with subsequent shelf-storage for 9 months. Note the gross material failure secondary to subsurface fatigue. This affects the tibiofemoral articulation.

It is believed that cross-linking of the polyethylene molecules resists intermolecular mobility, making it more resistant to deformation and wear in the plane perpendicular to the primary molecular axis. This has been demonstrated to dramatically reduce wear from crossing path motion, as it occurs in an acetabular cup.⁵⁴,⁵⁵ Cross-linking has a detrimental effect on some fundamental material properties, including yield strength, ultimate tensile strength, and elongation to break.⁵⁴ The decrease in these properties is proportional to the degree of cross-linking.

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