The effects of polyethylene cup thickness on wear of total hip prostheses

In vivo creep and wear performance of vitamin-E-diffused highly crosslinked polyethylene in total hip arthroplasty

INTRODUCTION

An acetabular liner thickness of around 6 mm remains the "gold standard" in total hip arthroplasty. Some surgeons have been recommending the use of the thickest possible liner because contact stress and strain in articulating surfaces decrease with increasing the wall thickness. The purpose of this study was to determine whether in vivo creep and wear performance could be enhanced using a thicker liner over the standard thickness in vitamin-E-diffused highly crosslinked polyethylene (HXLPE).

MATERIALS AND METHODS

One hundred and twenty-two hips were allocated to age-matched, sex-matched, and body mass index-matched two subgroups implanted either with a 6.8- or 8.9-mm-thick vitamin-E-diffused HXLPE liner against 28-mm cobalt-chrome femoral head, and followed-up for 7 years. Linear and volumetric penetration of femoral head into the liners attributed to creep and wear were analyzed for each group.

RESULTS

Compressive creep strain generated at the initial 6 months was significantly larger in the 6.8-mm group (2.6%) than in the 8.9-mm group (2.2%). The linear steady-state wear observed after 2 years was 0.0019 and 0.0015 mm/year, whereas the volumetric steady-state wear was 0.54 and 0.45 mm3/years in the 6.8- and 8.9-mm-thick groups, respectively. Although less strain in the thicker group resulted in a slightly less wear, it did not reach significant differences in the steady-state wear rates between the groups.

CONCLUSION

No clinical significance for using a thicker liner over the standard thickness (6.8 mm → 8.9 mm) was confirmed in the vitamin-E-diffused HXLPE according to the 7-year follow-up. The wear rates for both thicknesses were very low enough to prevent osteolysis, and no mechanical failure was observed at any follow-up interval. Nevertheless, since the significantly higher strain was seen in the thinner liner, further follow-up is needed to compare the longer term wear and the incidence of osteolysis and component fracture.

The effect of structural parameters of total hip arthroplasty on polyethylene liner wear behavior: A theoretical model analysis

Using large femoral heads in total hip arthroplasty (THA) has been widely advocated to improve the function and longevity of the components. However, increasing the head size has been shown to accelerate polyethylene liner wear. Few studies have investigated the effect of other important structural parameters (such as polyethylene liner thickness, metal cup size, head-liner conformity, loading conditions, etc.) on the biomechanical functions of the THAs. In this study, an analytical model was used to evaluate the polyethylene liner wear characteristics of the THAs (defined using a biomechanical wear factor) with various structural parameters of the THAs and loading conditions. For all the THA systems examined in this study, under the same loading conditions, a larger head leads to increasing contact areas, lower contact stresses, and higher biomechanical wear factors. When the head size is fixed, a decrease in the polyethylene liner thickness or a decrease in the head-liner conformity leads to higher peak contact stresses and smaller contact areas and consequently, lower biomechanical wear factors. This study provides a parametric analysis tool for the optimal design/selection of the THA systems and for prediction of early effects of various structural parameters on the biomechanical function (such as contact stresses) and longevity (such as polyethylene liner wear) of the THA systems.

Femoral Head Penetration Rates of Second-Generation Sequentially Annealed Highly Cross-Linked Polyethylene at Minimum Five Years

BACKGROUND

Highly cross-linked polyethylene (HXLPE) liners in total hip arthroplasty (THA) have demonstrated decreased wear rates, resilience to cup orientation, and reduced osteolysis compared to conventional polyethylene. Sequential irradiation and annealing below the melting temperature is unique compared to most HXLPE which is irradiated and remelted. This study purpose is to provide minimum 5-year femoral head penetration rates of sequentially annealed HXLPE in primary THA.

METHODS

A retrospective review of a prospectively collected database identified 198 consecutive, cementless primary THAs utilizing sequentially annealed HXLPE (X3; Stryker, Mahwah, NJ). Operative technique was standardized. Radiographs were analyzed utilizing the Martell method with minimum 5-year and 1-year radiographs as baseline to minimize the initial bedding-in period.

RESULTS

Seventy-seven hips with minimum 5-year follow-up were analyzed. Mean steady state linear and volumetric head penetration rates were 0.095 mm/y and 76 mm³/y, respectively. Volumetric head penetration was significantly less for 32-mm compared to 36-mm (P = .028). In addition, less head penetration was observed for ceramic 32-mm heads at nearly half the rate compared to cobalt-chromium 36-mm heads (P ≥ .092). No correlations existed between penetration rates and age, body mass index, University of California Los Angeles Activity Level, polyethylene thickness, cup inclination, or anteversion (P ≥ .10). No radiographic osteolysis was observed.

CONCLUSION

Surprisingly, linear head penetration rates of sequentially annealed HXLPE were nearly identical to the osteolysis threshold for conventional polyethylene and greater than reports of irradiated and remelted HXLPE. Furthermore, these data corroborate reports that HXLPE is resilient to cup orientation and demographic variables. Longer term follow-up is recommended.

Contemporary Dual Mobility Head Penetration at Five Years: Concern for the Additional Convex Bearing Surface?

BACKGROUND

Dual mobility (DM) bearings are increasingly popular and second-generation designs contain highly cross-linked polyethylene. The purpose of this study is to report head penetration rates in modern DM bearings.

METHODS

A review of 63 consecutive DM bearings was performed. Radiographs were analyzed for head penetration using Martell methodology at regular postoperative intervals.

RESULTS

Thirty-four DM bearings were analyzed. Mean linear head penetration was 1.59 mm/y at 1 year, 1.07 mm/y at 2 years, and 0.27 mm/y at 5 years following an exponential regression model (R² = 0.999). Mean volumetric wear was 783 mm³/y at 1 year, 555 mm³/y at 2 years, and 104 mm³/y at 5 years following an exponential regression model (R² = 0.986).

CONCLUSION

Initial head penetration of DM bearings is larger than contemporary cross-linked polyethylene bearings; however, rates approach steady state after 2 years, analogous to traditional bearings. The larger "bedding-in" head penetration may be due to the additional convex bearing surface, creating 2 surfaces for deformation/wear.

Vitamin E-stabilized highly crosslinked polyethylenes: The role and effectiveness in total hip arthroplasty

Morphology and design of ultra-high molecular weight polyethylene (UHMWPE or simply PE) acetabular components used in total hip arthroplasty (THA) have been evolving for more than half a century. Since the late-1990s, there were two major technological innovations in PE emerged from necessity to overcome the wear-induced periprosthetic osteolysis, i.e., the development of highly crosslinked PEs (HXLPEs). There are many literature reporting that radiation crosslinked and remelted/annealed (first-generation) HXLPEs markedly reduced the incidence of osteolysis and aseptic loosening. Regardless of such clinical success in the first-generation technologies, there were some recent shifts in Japan toward the use of new second-generation HXLPEs subjected to sequential irradiation/annealing or antioxidant vitamin E (α-tocopherol) incorporation. Although the selection rate of first-generation liners still account for more than half of all the PE THAs (∼58% in 2015), the use of vitamin E-stabilized liners has been steadily growing each year since their clinical introduction in 2010. In these contexts, it is of great importance to evaluate and understand the real clinical benefits of using the new second-generation liners as compared to the first generation. This article first summarizes structural evolution and characteristic features of first-generation HXLPEs, and then provides a detailed description of second-generation antioxidant HXLPEs in regard to the role of vitamin E incorporation on their chemical and mechanical performances in THA.

Long-term results with the Atlas IIIp elastic cementless acetabular component in total hip replacement

PURPOSE

Modular cementless elastic acetabular systems have advantages over cemented and hard shell cementless acetabular systems. There are few reports on the medium-term and long-term follow up of this particular type of implant. This study describes our experience with the Atlas IIIp modular acetabular system, which is a thin shell cementless elastic acetabular implant for total hip replacement commercialized under this name in many countries.

METHODS

We prospectively followed 244 patients treated with Atlas IIIp acetabular system between 2001 and 2004. Minimum ten year follow up was available for 148 hips (139 patients) from the original cohort of 263 hips (244 patients). One hundred five patients had died from unrelated causes and were excluded from the results. Post-operative and follow up radiographs of patients were assessed; and Harris hip scores were used as clinical outcome. Revision for any reason was defined as the end point for survivorship analysis.

RESULTS

The mean pre-operative Harris hip score was 48 (S.D. 16) and the average post-operative score was 82 (S.D. 12). The mean follow up in our series was 11.5 years, ranging from ten to 13.5 years. Thirteen hips required further surgery in our cohort; of which ten cases required cup revision. The 13-years cumulative implant survival was 91.2 % and the risk of implant revision was 8.8 % at 13 years in 148 hips (139 patients). Kaplan-Meier analysis showed the implant survival rate of 95.2 % at ten years for revision for any reason and 99.4 % for aseptic loosening.

CONCLUSIONS

Our clinical experience with this acetabular cup suggests good long-term survival rates that are similar to other cups on the market. The clinical experience in this study shows long-term survival rates that are consistent, acceptable and good results achieved with a low revision rate.

LEVEL OF EVIDENCE

Therapeutic III; therapeutic study.

The increase in cobalt release in metal-on-polyethylene hip bearings in tests with third body abrasives

Hypersensitivity reactions in patients receiving metal-on-metal hip replacements have been attributed to corrosion products as observed by elevated cobalt and chromium ions in the blood. Although the majority of cases are reported in metal-on-metal, incidences of these reactions have been reported in the metal-on-polyethylene patient population. To date, no in vitro study has considered cobalt release for this bearing combination. This study considered four 28 mm and seven 52 mm diameter metal-on-polyethylene bearings tested following ISO standard hip simulator conditions as well as under established abrasive conditions. These tests showed measurable cobalt in all bearings under standard conditions. Cobalt release, as well as polyethylene wear, increased with diameter, increasing from 52 to 255 ppb. The introduction of bone cement particles into the articulation doubled polyethylene wear and cobalt release while alumina particles produced significant damage on the heads demonstrated by cobalt levels of 70,700 ppb and an increased polyethylene wear from a mean value of 9–160 mm³/mc. Cobalt release was indicative of head damage and correlated with polyethylene wear at the next gravimetric interval. The removal of third body particles resulted in continued elevated cobalt levels in the 52 mm diameter bearings tested with alumina compared to standard conditions but the bearings tested with bone cement particles returned to standard levels. The polyethylene wear in the bone cement tested bearings also recovered to standard levels, although the alumina tested bearings continued to wear at a higher rate of 475 mm³/mc. Cobalt release was shown to occur in metal-on-polyethylene bearings indicating damage to the metal head resulting in increased polyethylene wear. While large diameter metal-on-polyethylene bearings may provide an increased range of motion and a reduced dislocation risk, increased levels of cobalt are likely to be released and this needs to be fully considered before being widely adopted.

Is the use of thin, highly cross-linked polyethylene liners safe in total hip arthroplasty?

PURPOSE

Due to the high number of total hip arthroplasties (THA) revised due to instability, the use of large femoral heads to reduce instability is justifiable. It is critical to determine whether or not large femoral heads used in conjunction with thin polyethylene liners lead to increased wear rates, which can lead to osteolysis. Therefore, by using validated wear-analysis software, we evaluated linear wear rates in a consecutive cohort of patients who underwent primary THA with thin polyethylene liners.

METHODS

All patients were selected from a consecutive, prospectively collected database of 241 THAs performed at a single institution by two fellowship-trained joint-reconstruction surgeons between July 2007 and June 2011. These patients were 1:1 matched to a cohort of patients who had conventional-thickness polyethylene liners.

RESULTS

No significant differences were observed between linear wear rates of thin or conventional-thickness liners. The Kaplan-Meier survivorship for both cohorts was 100 %, and no cases of polyethylene fracture were observed in either cohort.

CONCLUSIONS

Our results suggest that according to a mean follow-up of 4 years, the use of thin liners in THA is promising. Longer follow-up is required to assess whether these outcomes are observed later.

Importance of polyethylene thickness in total shoulder arthroplasty: a finite element analysis

BACKGROUND

Articular surfaces reconstruction is essential in total shoulder arthroplasty. Because of the limited glenoid bone support, thin glenoid component could improve anatomical reconstruction, but adverse mechanical effects might appear.

METHODS

With a numerical musculoskeletal shoulder model, we analysed and compared three values of thickness of a typical all-polyethylene glenoid component: 2, 4 (reference) and 6mm. A loaded movement of abduction in the scapular plane was simulated. We evaluated the humeral head translation, the muscle moment arms, the joint force, the articular contact pattern, and the polyethylene and cement stress. Findings Decreasing polyethylene thickness from 6 to 2mm slightly increased humeral head translation and muscle moment arms. This induced a small decreased of the joint reaction force, but important increase of stress within the polyethylene and the cement mantel. Interpretation The reference thickness of 4mm seems a good compromise to avoid stress concentration and joint stuffing.

Influence of demographic, surgical and implant variables on wear rate and osteolysis in ABG I hip arthroplasty

Periprosthetic osteolysis is associated with accelerated wear rates. The goal of this study was to investigate the influence of demographic and technical variables on wear rates and size of osteolytic lesions. Eighty retrieved ABG I prostheses were analyzed according to prospectively established criteria. There were 22 men and 58 women with an average age of 52 years (34-65) at the time of revision. The average time from index surgery to revision was 67 months (26 to 106). Polyethylene wear measurements were performed using a Universal-type measuring microscope. The average linear wear and volumetric wear rate was 0.363 mm per year (0-0.939, SD 0.241) and 161 mm(3) per year (0-467, SD 118.2), respectively. The wear rates were significantly higher (a) in patients with primary osteoarthritis in comparison with postdysplastic hips, (b) in hips where zirconia prosthetic heads articulated against the polyethylene liner, and (c) in cups placed laterally to Kohler's line. Risk that linear wear rate could be more than 0.2 mm per year was three times higher in patients who were operated in 1997 and later (OR 3.0, 95 % CI 1.126-7.993, p = 0.03). A strong association was revealed between magnitude of wear and size of femoral osteolysis.

Activity level in young patients with primary total hip arthroplasty: a 5-year minimum follow-up

Increased activity level after total hip arthroplasty (THA) is considered a risk factor for early prosthetic failure in young patients. Forty-one primary total hip arthroplasties in 34 patients were evaluated. Walking activity was measured using a pedometer to record gait cycles. Patients completed a University of California, Los Angeles (UCLA) activity questionnaire. Linear wear rates were measured. Mean ages at surgery and final follow-up were 42 and 50.3 years, respectively (mean gait cycles per year, 1.2 million; mean UCLA score, 6; mean linear wear, 0.16 mm/y). Increased body mass index and age correlated with decreased gait cycles per year. Patients with systemic disease were less active than patients with localized hip conditions. Femoral head diameter was a predictor of linear wear. The average gait cycles per year and wear rate for this population do not appear accelerated relative to average values reported in older populations.

Wear of highly cross-linked polyethylene acetabular cup in Japan

The wear characteristics of ultra-high-molecular-weight polyethylene acetabular cups irradiated with 6.0 to 7.5 Gy of gamma-irradiation were studied in a hip simulator and in vivo in 70 hips for 6 to 39 months. The linear wear was 2.5% to 4.5% of the wear seen in conventional polyethylene cups. A biphasic wear pattern was observed over time with a relatively large wear in the first year and a subsequent leveling of the wear rate curve. The linear wear rate in the second phase (steady state) was 0.006 mm/y. The extent of decrease in clinical wear of 6.0-Mrad polyethylene (Aeonian, Kyocera Corp, Kyoto, Japan) cups is comparable with the decrease of the in vitro wear measured by the hip simulation test.

Femoral head diameter affects the revision rate in total hip arthroplasty: an analysis of 1,720 hip replacements with 9-21 years of follow-up

BACKGROUND

In a previous study concerning 1,660 ScanHip THAs that were followed for up to 12 years, the cumulative revision rate was not found to be dependent on whether a 22-mm or a 32-mm head size had been used. We have re-examined these patients to see whether a longer follow-up time (9-21 years) would disclose an effect of head size on the revision rate.

PATIENTS AND METHODS

We analyzed the cumulative revision rate for 1,720 Scan Hip arthroplasties with either 22-mm or 32-mm femoral heads. The patients were followed for 9-21 years.

RESULTS

Arthroplasties with 32-mm head had 2.8-times higher cumulative revision rate than those with a 22-mm head. Older age reduced the risk of revision while male sex increased the risk.

INTERPRETATION

We found that head size affects revision risk, but that even in a reasonably large material a long follow-up time is required to disclose the effects of head size--and thus wear--on survival.

The effect of socket design, materials and liner thickness on the wear of the porous coated anatomic total hip replacement

The wear of joint replacement prostheses represents the greatest challenge to their continued development. Parameters such as polyethylene quality, liner thickness and metal backing have all been implicated as potential detractors in the search for the lowest-wearing socket. This study examined the effect of these parameters through an extensive study of the two versions of the porous coated anatomic (PCA) hip prosthesis (one-piece socket and snaplock socket). For the whole cohort the wear rate was found to be 88 (SE 10) mm3/year and the clinical wear factor was 2.00 (SE 0.28) x 10(-6) mm3/N m. When the two socket types were investigated individually, the wear factors found were 2.39 (SE 0.44) x 10(-6)mm3/N m and 0.99 (SE 0.25) x 10(-6) mm3/N m for the one-piece and snaplock, respectively. This illustrates that the metal backing per se does not predispose these sockets to rapid wear. The good wear performance of the snaplock liner may be attributed to the high quality of the ultra-high molecular weight polyethylene (UHMWPE) used and the shorter implantation period compared to that for the one-piece design. No correlation was found between the thickness of the liner and the clinical wear factor. Within the range of thicknesses tested here, UHMWPE thickness is not an influential parameter for the hip prosthesis and this is confirmed

Clinical experience with ceramics in total hip replacement

As part of a search for better articulation in total hip prostheses, the decrease in the thickness of the socket in different total hip prostheses was measured in vivo. The wear rates of (1) RCH 1000 (molecular weight, 10(6)) socket gamma-irradiated with 100 Mrad articulating with a crude COP (stainless steel containing 20% cobalt and 0.01% phosphorous) metal femoral head; (2) RCH 1000 socket nonirradiated articulating with a crude COP femoral head; (3) RCH 1000 socket irradiated with 100 Mrad articulating with an alumina femoral head; (4) ultra high molecular weight polyethylene (molecular weight, 5-6 x 10(6)) socket articulating with an alumina femoral head; and (5) ultrahigh molecular weight polyethylene socket articulating with a stainless steel femoral head (T-28) were 0.06, 0.30, 0.06, 0.1 and 0.25 mm/year, respectively, in the authors' clinical cases. Alumina femoral heads were effective in decreasing wear of the polyethylene socket. However, the wear rates of gamma-irradiated sockets articulating with alumina and with metal femoral heads wear very low and were not different from each other. Regarding the relationship between wear rate and the thickness of the ultra high molecular weight polyethylene socket articulating with a 28 mm alumina femoral head, on radiographs, average wear rates of socket thicknesses of 7, 8, 9, 10 and 11 mm were 0.14, 0.15, 0.12, 0.06, and 0.08 mm/year, respectively. On measuring retrieved prostheses, average wear rates of 7, 8, 9 and 11 mm thickness sockets were 0.2, 0.19, 0.14, and 0.1 mm/year, respectively. The wear of sockets has been proven to be minimal in alumina femoral heads articulating with ultrahigh molecular weight polyethylene sockets thicker than 10 mm.