Farzana Ansari

Effect of processing, sterilization and crosslinking on UHMWPE fatigue fracture and fatigue wear mechanisms in joint arthroplasty

Ultra high molecular weight polyethylene (UHMWPE) has been used as a bearing surface in total joint replacements (TJR) for nearly five decades. This semi-crystalline polymer has extraordinary energetic toughness owing to its high molecular weight and entanglement density. However, it is challenged by a need to offer a combined resistance to fatigue, wear and oxidation in vivo. The processing, sterilization treatment, and microstructural tailoring of UHMWPE has evolved considerably in the past 50 years but an optimized microstructure remains elusive. This review seeks to provide an overview of this processing history to address two primary questions: First, how does microstructure affect fatigue fracture and fatigue wear mechanisms in UHMWPE? And second, can microstructure be optimized to provide resistance to fatigue, oxidation and wear in vivo? Previous literature demonstrates that while crosslinking improves resistance to adhesive/abrasive wear, it also reduces resistance to fatigue crack propagation and fatigue wear by restricting molecular mobility and rendering the polymer more brittle. Crystallinity improves fatigue resistance but generally increases elastic modulus and concomitant contact stresses in vivo. The presence of fusion defects or oxidation reduces further fatigue resistance and enhances fatigue wear. Thus, UHMWPE microstructural evolution comes with trade-offs. Currently there is no singular formulation of UHMWPE that is ideal for all TJR applications.

Clinical trade-offs in cross-linked ultrahigh-molecular-weight polyethylene used in total joint arthroplasty.

Highly cross-linked formulations of ultrahigh-molecular-weight polyethylene (XLPE) offer exceptional wear resistance for total joint arthroplasty but are offset with a reduction in postyield and fatigue fracture properties in comparison to conventional ultrahigh-molecular-weight polyethylene (UHMWPE). Oxidation resistance is also an important property for the longevity of total joint replacements (TJRs) as formulations of UHMWPE or XLPE utilizing radiation methods are susceptible to free radical generation and subsequent embrittlement. The balance of oxidation, wear, and fracture properties is an enduring concern for orthopedic polymers used as the bearing surface in total joint arthroplasty. Optimization of material properties is further challenged in designs that make use of locking mechanisms, notches, or other stress concentrations that can render the polymer susceptible to fracture due to elevated local stresses. Clinical complications involving impingements, dislocations, or other biomechanical overloads can exacerbate stresses and negate benefits of improved wear resistance provided by XLPE. This work examines trade-offs that factor into the use of XLPE in TJR implants.

Fractography and oxidative analysis of gamma inert sterilized posterior-stabilized tibial insert post fractures: Report of two cases

BACKGROUND Highly crosslinked ultra-high molecular weight polyethylene (UHMWPE) has shown success in reducing wear in hip arthroplasty but there remains skepticism about its use in Total Knee Replacement (TKR) inserts that are known to experience fatigue loading and higher local cyclic contact stresses. METHODS Two Legacy Posterior-Stabilized (LPS) Zimmer NexGen tibial implants sterilized by gamma irradiation in an inert environment with posts that fractured in vivo were analyzed. Failure mechanisms were determined using optical and scanning electron microscopy along with oxidative analysis via Fourier Transform Infra-Red (FTIR) spectroscopy. RESULTS Micrographs of one retrieval revealed fatigue crack initiation on opposite sides of the post and quasi-brittle micromechanisms of crack propagation. FTIR of this retrieval revealed no oxidation. The fracture surface image of the second retrieval indicated a brittle fracture process and FTIR revealed oxidation in the explant. CONCLUSIONS These two cases suggest that crosslinking of UHMWPE as a manufacturing process or sterilization method in conjunction with designs that incorporate high stress concentrations, such as the tibial post, may reduce material strength. Moreover, free radicals generated from ionizing radiation can render the polymer susceptible to oxidative embrittlement. CLINICAL RELEVANCE Our findings suggest that tibial post fractures may be the results of in vivo oxidation and low level crosslinking. These and previous reports of fractured crosslinked UHMWPE devices implores caution when used with high stress concentrations, particularly when considering the potential for in vivo oxidation in TKR.

Notch fatigue of ultrahigh molecular weight polyethylene (UHMWPE) used in total joint replacements

Ultrahigh molecular weight polyethylene (UHMWPE) has remained the primary polymer used in hip, knee and shoulder replacements for over 50 years. Recent case studies have demonstrated that catastrophic fatigue fracture of the polymer can severely limit device lifetime and are often associated with stress concentration (notches) integrated into the design. This study evaluates the influence of notch geometry on the fatigue of three formulations of UHMWPE that are in use today. A linear-elastic fracture mechanics approach is adopted to evaluate crack propagation as a function of notch root radius, heat treatment and Vitamin E additions. Specifically, a modified stress-intensity factor that accounts for notch geometry was utilized to model the crack driving force. The degree of notch plasticity for each material/notch combination was further evaluated using finite element methods. Experimental evaluation of crack speed as a function of stress intensity was conducted under cyclic tensile loading, taking crack length and notch plasticity into consideration. Results demonstrated that crack propagation in UHMWPE emanating from a notch was primarily affected by microstructural influences (cross-linking) rather than differences in notch geometry.

Unscrewing instability of modular reverse shoulder prosthesis increases propensity for in vivo fracture: a report of two cases.

A modular cemented Tornier Aequalis RSA (Tornier,Bloomington, MN, USA) was retrieved from the leftshoulder of a 79-year-old male patient after 9 years2 months in vivo (Fig. 1, A). The patient’s proximal hu-merus initially fractured during a ground-level fall. Surgerywas then performed with open reduction and internal fix-ation with a proximal humeral locking plate. This fixationfailed, and a hemiarthroplasty surgery was then performed.Because the tuberosity fixation failed, revision hemi-arthroplasty surgery was performed. After the third surgi-cal procedure, the tuberosities resorbed with subsequentanterior-superior escape of the prosthesis. There was also a5-cm area of proximal humeral bone loss. Another revisionsurgery was then performed with conversion to a reverseprosthesis. Preoperative radiographs showed partial disas-sembly of the screw joint between the metaphysis anddiaphysis on the humeral stem (Fig. 1, B). The implant hadnot yet fractured or dissociated completely (Fig. 1, C).Extensive metallosis was observed in the retrieved peri-prosthetic tissue (Fig. 1, D).Retrieval analysis including optical and metric evalua-tion showed a gap at the junction of the metaphysis anddiaphysis. The 2 components were disassembled ex vivo,

Analysis of severely fractured glenoid components: clinical consequences of biomechanics, design, and materials selection on implant performance

BACKGROUND The longevity of total shoulder replacement is primarily limited by the performance of the ultrahigh-molecular-weight polyethylene (UHMWPE) glenoid component in vivo. Variations in glenoid design (conformity, thickness), biomechanics (joint kinematics), and UHMWPE material selection (sterilization, cross-linking) distinguish total shoulder replacements from hip and knee arthroplasty devices. These variables can lead to severe mechanical failures, including gross fracture. METHODS Sixteen retrieved glenoids with severe fracture were analyzed. The explant cohort included 3 material groups (gamma-sterilized Hylamer; gamma-sterilized UHMWPE; and gas plasma-sterilized, remelted, highly cross-linked UHMWPE [HXL]) and a range of conformities (0- to 10-mm radial mismatch). Analysis included fractography (optical and scanning electron microscopy) and Fourier transform infrared spectroscopy for oxidative analysis. RESULTS Fracture primarily occurred along the exterior rim for all 16 explants. Fourier transform infrared analysis and fractography revealed significant oxidative embrittlement for all gamma-sterilized glenoids. Fatigue striations and internal flaws were evident on the fracture surface of the HXL glenoid, with little oxidation detected. CONCLUSIONS Fracture initiated at the external rim of all devices. Elevated oxidation levels and visible material distortion for representative gamma-sterilized conventional and Hylamer devices suggest oxidative embrittlement as a driving force for crack inception and subsequent fracture. Brittle fracture of theHXL glenoid resulted from a combination of elevated contact stress due to a nonconforming surface, an internal flaw, and reduced resistance to fatigue crack growth. This demonstrates that glenoid fracture associated with oxidation has not been eliminated with the advent of modern materials (HXL) in the shoulder domain. LEVEL OF EVIDENCE Basic Science Study; Implant Retrieval Study.

Designing for Crosslinked UHMWPE Implants: Clinical Consequences of Stress Concentrations

Ultrahigh molecular weight polyethylene (UHMWPE) remains the polymer bearing of choice for total joint replacements (TJR) [1]. However, the long-term performance of this polymer has been limited by in vivo wear: UHMWPE wear debris generated in the joint space can travel into the periprosthetic bone, initiating osteolysis and implant loosening [2]. Crosslinked UHMWPE (through ionizing radiation) has demonstrated increased wear resistance [3], but at the cost of reduced fatigue crack propagation and fracture resistance [4]. Additionally, radiation processes can release free radicals which, when not eliminated through thermal treatment, can increase UHMWPE susceptibility to oxidation and mechanical embrittlement [5]. Such tradeoffs present clinical concerns when implant designs incorporate stress concentrations that experience elevated stresses under loading. These compromises are evaluated through the failure analysis of several crosslinked UHMWPE retrievals that fractured in vivo.Copyright

The Effects of Sliding, Rolling, and Rotation on Cross-Shear and Wear of UHMWPE in Knee Replacements

Wear debris from the ultra-high molecular weight polyethylene (UHMWPE) tibial bearing surface leading to loosening is still the main cause for revisions after 5 years for total knee replacements (TKR) [1]. Wear of UHMWPE is greatly increased under cross-shear motion, such as in the hip joint, due to how the molecules and crystalline regions can align in one direction and make the material weaker in the other [2]. The motion in the knee joint is mainly linear rolling and sliding, but there is rotation and medial-lateral sliding that introduce cross-shear [3]. Wear tests are typically performed with basic motion parameters and simplified geometry (pin-on-disk tests) or under gait simulation to test specific designs, but little is known about the effects of the different motions in the knee on UHMWPE wear. There is also disagreement over how to best quantify cross-shear and model how much wear it will cause [3–5]. This study investigates the effect on wear of the different individual and combined motions in TKR: sliding, rolling, and rotation, for a total of eight separate kinematic conditions. The different measures of cross-shear present in each case were calculated in a computer model and compared to the wear test results.Copyright

Macro- and Microscale Analysis of Bearing Surface Damage on Total Shoulder Replacements

Damage to bearing surfaces of total joint replacements (TJR) can have clinical consequences: wear debris generated from ultra-high molecular weight polyethylene (UHMWPE) surfaces can cause osteolysis and subsequent implant loosening [1]. Counterbearing metallic damage may significantly increase UHMWPE wear [2]. Documenting the morphology, frequency and location of bearing surface damage may provide insight into wear initiation and prevention. While scoring methodologies have been available and validated for total hip replacements (THR) and total knee replacements (TKR) [3–4], there is a paucity of validated scoring protocols for total shoulder replacements (TSR) [5]. Our previous work presented a damage scoring methodology to evaluate the severity and coverage of six damage modes on retrieved cobalt chrome (CoCr) humeral heads [6]. In this study, we adapt that protocol to include bearing surface damage on the counter-bearings (UHMWPE glenoid components). Additionally, we incorporate the results of 3D profilometry analysis of scratches in the Co-Cr humeral heads [6]. Ultimately, this macroscale and microscale analysis, combined with clinical data, for coupled TSR retrievals will provide insight on the origin, evolution and consequences of bearing damage in vivo.Copyright

Profilometry Analysis and Improvements of a Novel Damage Scoring Method for Metal Bearing Surfaces of Shoulder Replacements

Characterizing the type and extent of in vivo damage to total joint replacements (TJR) is important for improving the success of arthroplasty outcomes, modeling damage modalities, and validating simulator studies. A method for quantifying the damage present on Cobalt Chrome (CoCr) humeral heads was developed in our lab to fulfill a much-needed gap in clinical knowledge regarding total shoulder replacements as well as metallic bearing surfaces [1,2]. A lack of inter-observer consistency with regard to severity classifications from our initial protocol [1] prompted several modifications to the method, which are tested and described here in this study. Also, since sub-micron scale ultra-high molecular weight polyethylene (UHMWPE) wear debris is linked to osteolysis and implant loosening, additional analysis with high magnification 3D optical profilometry was performed on a subset of damage modes with a long-term goal of correlating surface damage with propensity for osteolysis in TJR [3,4].Copyright