The Trident acetabular system is the second most common cementless cup implanted in the UK. Recent studies have shown that malseating of the liner can be as high as 16.4%. We felt this was very high and were prompted to review our series and early clinical outcomes.

We reviewed 118 hips in 110 patients, implanted between from 2005-2007. We reviewed initial post operative X-rays using the technique described by Howcroft to identify malseating. The posterior approach was used in all cases. All cups were Trident PSL and all 85 Patients had OA, 10 RA, 8 AVN, 5 DDH, 3 OA post trauma, 2 Perthes, 2 Psoriatic Arthritis, 3 other. We only identified 3 malseated cups in 118 hips. 2 were in patients with OA secondary to trauma and 1 in primary OA. The rate of malseating for trainees operating was 5 % and only 1% when consultants were operating. There were no adverse events in these patients. No-one required revision. Oxford Hip Score (OHS) improved from 47 pre-op to 20 post op. This was compared to 47 and 22 in the correctly seated group (115 cases). Surprisingly the subgroup with the poorest OHS at 1 year had surgery for DDH, with a mean OHS of 31. The reasons for this are unclear.

Contrary to other studies our malseating rate is very low. We do not feel that malseating is a problem with Trident if adequate exposure is obtained. In those patients with sclerotic bone, we suggest over reaming the rim of the acetabulum by 1mm to avoid excess deformation of the shell which may lead to difficulty with seating the liner. We suggest trainees are supervised closely when using Trident.

Mismatch of bearing component centres and tension of soft tissues surrounding the hip joint can lead to component separation during gait cycle and cause the femoral head to contact the rim of an acetabular liner, which could increase wear and shorten lifespan of an implant. This study aims to investigate the contact and wear mechanics of a metal-on-polyethylene hip joint under dynamic separation by using Finite Element Analysis (FEA). A Pinnacle® cup with a Marathon neutral liner 36×56mm with a 45° inclination was constrained by a spring element in the medial-lateral axis. The spring was pre-compressed by 4mm to represent the corresponding translational mismatch of a simulator testing. Archard's law was used to predict wear over one ISO 14242-1 gait cycle. Contact pressure is proportional to the load input during the stance phase, associated with concentric contact condition; it increases threefold just before the swing phase (time C), reaching 46.2MPa, where edge loading occurs. Consequently, separation climbs to 3.54mm, which is comparable to the mathematical prediction (3.34mm) and dynamic FEA (3.2mm). The predicted volumetric wear after this gait cycle is 1.22 × 10–5 mm3. Dynamic separation between femoral head and acetabular liner can result in edge loading, consequently high contact pressure on the edge of a liner. In combination with cyclic loading, fatigue damage could take place and may be worth investigating in the future

Summary Statement. This study assesses oxidation, mechanical behavior and revision reasons of 2. nd. generation HXLPE used in total hip and knee arthroplasty. While oxidation was low for both X3 and E1 HXLPEs, oxidative regional variations were detected in the sequentially annealed cohort. Introduction. First generation highly crosslinked polyethylenes (HXPLEs) have proven successful in lowering both penetration and osteolysis rates. However, 1. st. generation annealing and remelting thermal stabilization have been associated with in vivo oxidation or reduced mechanical properties. Thus, 2. nd. generation HXLPEs were developed to improve oxidative stability while still maintaining material properties. Little is known about the in vivo clinical failure modes of these 2. nd. generation HLXPEs. The purpose of this study was to assess the revision reasons, wear, oxidative stability, and mechanical behavior of retrieved sequentially annealed Vitamin E diffused HXLPE in THA and TKA. Methods. 251 2. nd. Generation HXLPE hip and knee components were consecutively retrieved during revision surgeries and continuously analyzed in a prospective, IRB approved, multicenter study. 123 acetabular liners (Implanted 1.2y; Range 0–5.0y) and 117 tibial inserts (Implanted 1.6y; Range 0–5.8y) were highly crosslinked and annealed in 3 sequential steps (X3). Five acetabular liners (Implanted 0.6y; Range 0–2.0y) and six tibial inserts (Implanted 1.3y; Range 0.5–1.8y) were diffused with Vitamin E (E1). Patient information was collected from medical records. Linear penetration of liners was measured using a calibrated digital micrometer (accuracy: 0.001 mm). Surface damage of tibial components was assessed using the Hood method. Thin sections were taken from the acetabular liners (along the superior/inferior axis) and the tibial components (along the medial condyle and central spine) for oxidation analysis and analyzed according to ASTM 2102. Mechanical behavior was assessed via the small punch test (ASTM 2183). Results. The liners and tibial components fabricated from both HXLPEs were revised predominantly for loosening, instability, and infection. The average penetration rate for the Sequentially Annealed group was low (PR=0.045mm/yr). Pitting, scratching and burnishing were the predominant damage mechanisms of the tibial inserts within both material groups, with no evidence of delamination. Oxidation indices were low (Mean OI≤0.3) and similar between liners and inserts of the Sequentially Annealed components at the bearing and backside surface (p≥0.15). Oxidation was positively correlated with implantation time at the bearing surface of the Sequentially Annealed groups (Rho>0.29, p<0.005). The Ultimate Load of the Sequentially Annealed acetabular liners was statistically higher than the tibial components (p<0.001), however the mean difference was minimal (∼6N). Discussion. This study evaluated the properties of 2. nd. generation HXLPEs used in THA and TKA. Sequentially Annealed liners had penetration rates comparable with 1. st. generation HXLPEs. While oxidation was low for both sequentially annealed and Vitamin E HXLPEs, we were able to detect regional variations in the oxidative in the sequentially annealed cohort. Longer-term retrievals are necessary to fully assess the oxidative stability of Vitamin E diffused HXLPE used in TKA and THA

Non-optimal clinical alignment of components in total hip replacements (THRs) may lead to edge loading of the acetabular cup liner. This has the potential to cause changes to the liner rim not accounted for in standard wear models. A greater understanding of the material behaviours could be beneficial to design and surgical guidance for THR devices. The aim of this research was to combine finite element (FE) modelling and experimental simulation with microstructural assessment to examine material behaviour changes during edge loading. A dynamic deformable FE model, matching the experimental conditions, was created to simulate the stress strain environment within liners. Five liners were tested for 4Mc (million cycles) of standard loading (ISO14242:1) followed by 3Mc of edge loading with dynamic separation (ISO14242:4) in a hip simulator. Microstructural measurements by Raman spectroscopy were taken at unloaded and highly loaded rim locations informed by FE results. Gravimetric and geometric measurements were taken every 1Mc cycles. Under edge loading, peak Mises stress and plastic deformation occur below the surface of the rim during heel strike. After 7Mc, microstructural analysis determined edge loaded regions had an increased crystalline mass fraction compared to unloaded regions (p<0.05). Gravimetric wear rates of 12.5mm. 3. /Mc and 22.3mm. 3. /Mc were measured for standard and edge loading respectively. A liner penetration of 0.37mm was measured after 7Mc. Edge loading led to an increase in gravimetric wear rate indicating a different wear mechanism is occurring. FE and Raman results suggest that changes to material behaviour at the rim could be possible. These methods will now be used to assess more liners and over a larger number of cycles. They have potential to explore the impact of edge loading on different surgical and patient variables

Summary. Sequentially irradiated and annealed UHMWPE hip and knee retrievals showed subsurface in vivo oxidation in both the articular surface and unloaded surfaces, while three of four never-implanted shelf stored liners had oxidation in the bulk. Introduction. Highly cross-linked polyethylene was developed to improve the wear resistance of UHMWPE bearing surfaces in total hip arthroplasty. First generation irradiated and annealed polyethylene showed high oxidation in vivo, largely attributed to only the partial-quenching of free radicals, along with additional radicals generated during terminal gamma sterilization. A second generation, three-step sequential irradiation and annealing method was advanced with the promise of better oxidative stability and improved mechanical properties. We hypothesised that without the complete elimination of free radicals combined with gas plasma sterilization requiring oxygen-permeable packaging, that this second generation material would be prone to shelf-oxidation in addition to in vivo oxidation. Patients & Methods. Fifty surgically-retrieved sequentially irradiated and annealed, gas plasma-sterilised UHWMPE acetabular liners and tibial bearings (X3™, Stryker, Mahwah, NJ), with in vivo durations of 0.5–73 months, were analyzed at their articular surface and an unloaded surface, along with four never implanted acetabular liners. Infrared microscopy was used to evaluate lipid absorption, oxidation (per ASTM F2102-01ε1) and hydroperoxide levels after nitric oxide staining. Gravimetric swelling analysis assessed cross-link density (per ASTM F2214), and crystallinity measurements were performed using differential scanning calorimetry. Results. There was detectable oxidation (OI > 0.1) in 37 of the 50 components with as little as 2 weeks of in vivo service. Maximum oxidation values averaged OI = 0.30 ± 0.30 (range = 0.03–1.59). Oxidation profiles were predominantly characterised by subsurface oxidation peaks approximately 1–2 mm below the surface, in both the articular surface and rim, along with a pattern of embrittlement induced white banding in four and six year liners. Three short in vivo duration liners (0.1–15.5 month) showed oxidation and degradation of material properties throughout the bulk. Three of four never-implanted liners, with up to five years shelf storage, also showed bulk oxidation (Max OI ≤ 1.5), loss of cross-link density and increased crystallinity. Discussion/Conclusion. High levels of detectable oxidation, subsurface oxidation peaks, and white banding were all identified in sequentially irradiated and annealed UHMWPE retrievals with short in vivo durations. These results raise concerns about the long-term clinical performance of these materials. Oxidation measured in shelf-stored, never implanted liners also raises concerns that liners may already be oxidatively compromised before being implanted into patients. Due to gas plasma sterilization methods, these free-radical containing liners are packaged and stored in air, likely resulting in a pre-implantation oxidation effect similar to that historically reported in gamma-in-air sterilised UHMWPE. Longer-term retrievals are needed to better understand the progress of these in vivo changes and whether or not it will compromise the longevity of the implants