Abstract
Introduction
Moderately crosslinked, thermally treated ultrahigh molecular weight polyethylene (UHMWPE) has to date demonstrated a good balance of wear resistance and mechanical properties. MARATHON™ Polyethylene (DePuySynthes Joint Reconstruction, Warsaw, IN) is made from polyethylene resin GUR 1050, gamma-irradiated at a dose of 5.0 Mrads to create crosslinking of polyethylene, and followed by a remelting process to eliminate free radicals for oxidative stability. 10-year clinical study [1] and laboratory wear simulation tests [2–3] have reported excellent wear performance of the MARATHON poly.
There continues to be demand for improved head-to-shell ratio acetabular systems because larger head sizes have the benefits of increased stability and range of motion. The increased head-to-shell ratio is often times achieved by using a reduced liner thickness. One of the clinical concerns of thinner poly liners is the potential for rim fracture, particularly in the occurrences of rim loading or impingement at high cup angles [4–7].
This study investigated the performance of thinner poly liners to the challenge of high angle rim loading and neck-to-liner impingement.
Materials and Methods
Three groups of ETO sterilized MARATHON polyethylene liners (ID/OD: 28/44, 32/48, and 36/52 mm) were paired with matching CoCrMo heads (n=6 each group). To simulate rim loading, liners were assembled in the metal shells tilted at 64° (Figure 1) with sinusoidal loading (0 to 5000N at 3Hz) in a 37°C water bath for 5-million cycles or until component failure, whichever occurred first.
For neck-liner impingement testing, metal shells were potted at 54º (in the abduction/adduction plane with a ±10° of motion per ISO 14242–1 [8]) on a hip simulator (n=4 each group) using a physiological loading (max 3000N at 1Hz) for 3-million cycles (Figure 2). The impingement occurred at 64º during the simulated gait cycle (Figure 3).
The liners were inspected every million cycles, using a high intensity light to search for signs of crack initiation and/or fractures. Both test methods were validated to be able to replicate liner fractures.
Results
All MARATHON ETO liners passed 5-million cycles of high angle fatigue testing. All liners passed 3-million cycles of impingement testing.
Discussion
Causes of liner fractures can be multi-factorial. Examples of influencing factors are: poly material, component designs, patient activity level, implant orientations, and neck-liner impingement. Improved head-to-shell ratio design requires reduction of the liner thickness, which raises the concern of liner fracture. In this study, we investigated adverse testing conditions of rim-loading and neck-liner impingement, in order to evaluate the mechanical performance of thin polyethylene liner.
