UHMWPE: Ultra-high molecular Weight Polyethylene

XLPE: Cross-Linked Polyethylene

Both of these materials are widely used in Joint Replacement implants as friction interfaces. Today, I'll take you deeper into their material properties, advantages, and disadvantages. Let's see if you've made the right choice in clinical use.

• What is UHMWPE and XLPE?

Traditional Ultra-High Molecular Weight Polyethylene (UHMWPE) is produced by polymerizing ethylene into fine powder and then using processes like compression molding or ram extrusion at high pressure and melting temperatures. UHMWPE used in joint implants often undergoes low-dose irradiation for sterilization.

Cross-Linked Polyethylene (XLPE) is a modified form of UHMWPE. The radiation dose for high cross-linked polyethylene liner materials generally ranges from 50-100 kGy. After irradiation, the material is either heated above its melting temperature (135°C) or annealed below its melting temperature. In this article, high cross-linked polyethylene refers to XLPE produced under these specific processing conditions.

• Mechanical Performance: UHMWPE vs XLPE

Cross-linking increases PE density, thus improving its wear resistance. However, the mechanical properties of highly cross-linked polyethylene may be affected to some extent due to increased cross-linking density, and post-processing may further affect mechanical properties/oxidative performance.

Quasi-static tensile performance

Gomoll et al.'s research found that, for a given degree of crystallinity, high cross-linking in polyethylene leads to a decrease in modulus, fracture strength, and ductility (ductility is related to fracture strain) (Table 1, blue highlights).

Table 1: Physical Properties of Ultra-High Molecular Weight Polyethylene at Different Radiation Doses

Mechanical properties are derived from real stress-strain curves, while fracture toughness is determined using the Rice and Sorenson J-integral method.

Fracture Toughness

Research by Gencur et al. [3] demonstrates a linear correlation between radiation dose and fracture toughness (Figure 2), with high doses of gamma radiation significantly reducing the fracture toughness of highly cross-linked polyethylene.

Gomoll and colleagues[2] found that at low doses (≤50 kGy), cross-linking increases fracture toughness, while highly cross-linked polyethylene exhibits spontaneous, unstable fracture after reaching JIC (Table 1, highlighted in red).

Fatigue Crack Growth Resistance

Baker et al.[4,5] found that the initial crack propagation value, ΔKincep, decreases with increasing cross-linking dose. In XLPE at a dose of 200 kGy, ΔKincep decreases by 61% compared to UHMWPE, indicating that high cross-linking has an adverse effect on fatigue crack growth resistance (Table 2, highlighted in blue).

Table 2：Fatigue Crack Growth Data for High Molecular Weight Polyethylene and Cross-Linked Polyethylene

• How to Choose Between UHMWPE and XLPE？

Cross-linked polyethylene (XLPE) has demonstrated excellent wear resistance in total hip replacement (THR) surgery, thanks to its exceptional resistance to abrasive and adhesive wear. This has led to the recent adoption of highly cross-linked polyethylene (XLPE) in total Knee Replacement (TKR) surgery.

But does the effectiveness of cross-linking carry over to TKR? Research [6] suggests that this largely depends on the types of motion and stresses involved in the polyethylene bearing.

Unlike the high conformity ball-and-socket joint seen in THR (with its unique abrasive and adhesive wear), the congruency of the femoral-tibial joint surfaces in TKR is relatively lower, involving more sliding and rolling motion, as illustrated in Figure 3.

A) Contact area within the joint generates compressive stress along the surface of the insert, perpendicular to the femoral condyle implant surface.

B) As the contact point moves, a new contact area is formed, creating a corresponding compressive stress zone.

C) Alternating motion of the contact points generates cyclic tensile stress beneath the insert joint surface.

Based on the biomechanical principles mentioned above and considering the material characteristics, it can be observed that if high cross-linked polyethylene (XLPE) is used as the insert material in total knee arthroplasty (TKA), the potential issues may include pitting, delamination, and stress concentration leading to insert fractures.

Scenario 1:

The tensile stress at the edge of the polyethylene insert and the surface shear stress in the joint contact area can lead to pitting and delamination of the insert. Cross-linking, however, can reduce the resistance to crack propagation, thereby increasing the risk of fatigue wear failure in the insert. This situation is illustrated in Figure 4.

The left arrow indicates cracking and early delamination beneath the surface of the insert. The right arrow shows an area of delamination in the center of the insert.

Scenario 2:

As the load on the tibial post and around the edge of the insert increases, wear occurs in that region. The reduced fatigue strength and ductility of XLPE increase its sensitivity to this type of damage in the insert. In extreme cases, this can lead to severe consequences such as fracture of the tibial post and edge of the insert, as illustrated in Figure 5.

Cross-linking reduces abrasive and adhesive wear in high cross-linked polyethylene (XLPE) materials. However, it appears that it cannot reduce the risks of crack propagation, deformation, pitting, and delamination commonly seen in TKR. The reduced toughness, ductility, and fatigue resistance of high cross-linked polyethylene (XLPE) may lead to the eventual failure of fixed-bearing TKR inserts. Therefore, the use of high cross-linked polyethylene in TKA remains questionable.

So, how should one choose between these two materials? See the table below for details.

Table 3: Recommended Optimal Insert Material Selection for Different Scenarios

The Gass Knee Joint System offers a variety of polyethylene inserts, which are compression-molded from imported UHMWPE material from Germany. This ensures excellent resistance to abrasive wear and adhesive wear while providing superior fatigue wear resistance.

References