Neck Taper Research Articles

Trunnionosis: Does Head Size Affect Fretting and Corrosion in Total Hip Arthroplasty?

BackgroundWear and tribocorrosion at the modular head–neck taper interface may be a cause of failure in metal-on-polyethylene total hip arthroplasty (THA). The present investigation endeavored to elucidate the effect of femoral head diameter on fretting and corrosion in retrieved head–neck tapers. MethodsA retrieval analysis of THA prostheses in vivo for a minimum of 1 year was performed. Twenty-three femoral heads of 32-mm diameter were matched with 28-mm heads based on time in vivo and head length (−3 mm to +8 mm). All included implants featured a single taper design from a single manufacturer. Fretting and corrosion damage scoring was performed for each implant under stereomicroscopic visualization. ResultsHead diameter was observed to affect fretting (P = .01), with 32-mm femoral heads exhibiting greater total fretting scores than 28-mm heads. Fretting damage was greatest (P = .01) in the central concentric zone of the femoral head bore tapers, regardless of head diameter, length, or stem offset. No significant effect on total corrosion scores was observed for any head or stem variable. Retrieved implant total corrosion scores were positively correlated (ρ = 0.51, P < .001) with implantation time. ConclusionIncreased femoral head diameter in THA may produce greater fretting damage owing to and increased head–neck moment arm. There is no associated increase in corrosion with 28-mm and 32-mm heads of this taper design. The longer a THA prosthesis is implanted, the greater the risk of damage due to corrosion.

Influence of geometry and materials on the axial and torsional strength of the head–neck taper junction in modular hip replacements: A finite element study

The assembly force is important in establishing the mechanical environment at the head–neck taper junction of modular hip replacements. Previous experimental results of the assembled taper junctions with different material combinations (Co–28Cr–6Mo and Ti–6Al–4V) reported similar axial strengths (pull-off loads), but lower torsional strengths (twist-off moments) for the CoCr/CoCr junction. However, mechanics of the junction and the strength behaviour have not been understood yet. A three dimensional finite element model of an isolated femoral head–neck junction was developed to explore the assembly and disassembly procedures, particularly the axial and torsional strengths for different material combinations and geometries. Under the same assembly load, the contacting length between the CoCr head and titanium neck was greater than that of in CoCr/CoCr. The contact length in the titanium neck was more sensitive to the assembly force when compared to the CoCr neck. For instance, with increasing the assembly force from 1890 to 3700N, the contact length increased by 88% for CoCr/Ti and 59% for CoCr/CoCr junctions. The torsional strength of the junction was related to the lateral deformation of the neck material due to the applied moment. The angular mismatch existing between the head and neck components was found to play the main role in the torsional strength of the junction. The smaller mismatch angle the higher torsional strength. It is suggested to consider reducing the mismatch angle, particularly in CoCr/CoCr junctions, and ensure a sufficiently high assembly force is applied by impaction for this combination.

Effect of impact assembly on the Interface deformation and fretting corrosion of modular hip tapers: an in vitro study

Event Abstract Back to Event Effect of impact assembly on the Interface deformation and fretting corrosion of modular hip tapers: an in vitro study Anna Panagiotidou1, 2*, Timothy Cobb1*, Jayantilal Meswania1*, Alister Hart1, 3*, Fares Haddad2, John Skinner1, 3* and Gordon Blunn1* 1 UCL, Institute of Orthopaedics and Musculoskeletal Science, United Kingdom 2 University College Hospital, United Kingdom 3 Royal National Orthopaedic Hospital, United Kingdom Introduction: Early failure of large head diameter metal-on-metal hip replacements (LH-MoM-HR) has been attributed to wear and corrosion of modular head-neck tapers[1]. Previous studies have also shown that taper surface finish and contact area affect the amount of mechanically assisted corrosion (MAC) at this interface[2]. The industry standard is to manufacture the neck taper with parallel horizontal ridges. The degree of deformation is likely to influence the way that the taper behaves the greater the impaction, the greater the deformation and, in theory, the greater the strength of the surface interlock. As a result, micromotion and fretting are minimised, reducing the potential for MAC and production of wear debris. This study aims to investigate the effect of varying impact assembly load on the amount of fretting corrosion at the head-neck taper junction. Materials and Methods: All samples were made to a single specification. Manufactured neck components, representing short tapers with high surface roughness (average Rz=16.58µm, Ra=4.14µm) or long tapers with low surface roughness (average Rz=3.82µm, Ra=0.81µm), were assembled under controlled conditions with 2kN, 4kN or 8kN of impaction force. All heads were CoCrMo alloy and had a smooth finish similar to the industry standard. Heads had neck offset to induce a high bending moment at the taper interface. Half of the neck tapers used were CoCrMo and the other half were Ti-6Al-4V alloy. For each material, the neck tapers were either a smooth or rough finish. Light microscopy and scanning electron microscopy (SEM) of the component surfaces allowed assessment of surface deformation pre and post in-vitro simulation, while measurement of fretting current allowed evaluation of mechanically assisted corrosion at the taper interface during cyclic loading (5 million cycles). Assessment of corrosion was carried out by high power light microscopy, scanning electron microscopy and optical microscopy. Results: Fretting and amplitude current during the cyclic loading test showed that both smooth-long and rough-short necks, assembled using 8kN of force were significantly reduced compared with either 2kN or 4kN assembly loads. Similarly for both Ti-6Al-4V surface finishes and the CoCrMo rough-short taper, the least fretting current was observed in samples with the greatest force of impaction (p = <0.001) (Fig. 1). Simulator testing over 5 million cycles demonstrated a reduction in fretting corrosion for samples impacted with 4kN and 8kN of force. However, in the case of those impacted at 8kN the fretting current seen after the first one million cycles was considerably lower than those samples impacted at 4kN. For 4kN, there was a significant (p=?) drop in maximum fretting current and amplitude between 1 million and 2 million cycles (p = <0.001), but no such change between 2 million and 3 million cycles. The downward reduction in fretting current continued at 4 and 5 million cycles (Fig.1). At 8kN impaction force, there was no statistically significant difference in current between 1 million and 2 million cycles, and very little change up to 5 million cycles. High-powered light microscopy demonstrated evidence of deformation of the surface of the tapers, which increased with assembly load and was more pronounced on Ti-6Al-4V neck tapers (Fig. 2). Upon inspection using SEM, showed evidence of MAC which was greater in the 4kN assemblies compared with 8kN assemblies and for both impaction loads corrosion occurred in areas associated with large bending moment. Discussion: 1. The impaction force used to assemble modular heads of hip implants should be more than 4kN. 2. When a light impaction force of only 2kN is used, the quality of fixation and resistance to fretting corrosion is inferior. 3. With an impaction force of 4 - 8kN, there is a progressive improvement in fixation and resistance to MAC corrosion over 5 million cycles. 4. With an impaction force of 8kN the resistance to fretting corrosion was immediately effective for Ti-6Al-4V rough-short neck tapers coupled with CoCrMo heads. The clinical implication of this study is that surgeons should apply a greater impact assembly force than is used at present and this is particularly important for titanium stems and for short tapers.

Taperosis: Does head length affect fretting and corrosion in total hip arthroplasty?

Tribocorrosion at the head-neck taper interface - so-called 'taperosis' - may be a source of metal ions and particulate debris in metal-on-polyethylene total hip arthroplasty (THA). We examined the effect of femoral head length on fretting and corrosion in retrieved head-neck tapers in vivo for a minimum of two years (mean 8.7 years; 2.6 to 15.9). A total of 56 femoral heads ranging from 28 mm to 3 mm to 28 mm + 8 mm, and 17 femoral stems featuring a single taper design were included in the study. Fretting and corrosion were scored in three horizontally oriented concentric zones of each taper by stereomicroscopy. Head length was observed to affect fretting (p = 0.03), with 28 mm + 8 mm femoral heads showing greater total fretting scores than all other head lengths. The central zone of the femoral head bore taper was subject to increased fretting damage (p = 0.01), regardless of head length or stem offset. High-offset femoral stems were associated with greater total fretting of the bore taper (p = 0.04). Increased fretting damage is seen with longer head lengths and high-offset femoral stems, and occurs within a central concentric zone of the femoral head bore taper. Further investigation is required to determine the effect of increased head size, and variations in head-neck taper design.

A High Prevalence of Corrosion at the Head–Neck Taper with Contemporary Zimmer Non-Cemented Femoral Hip Components

Mechanically assisted crevice corrosion (MACC) occurs at metal/metal modular junctions in which at least one of the components is fabricated from cobalt–chromium alloy and may lead to adverse local tissue reaction (ALTR) in patients with metal-on-polyethylene (MoP) total hip arthroplasty. This type of reaction has been previously described in hips with head/neck modularity, but the prevalence is unknown. We found a prevalence of 1.1 percent in a consecutive series of 1356 contemporary Zimmer non-cemented femoral hip components followed for a minimum of 2years. The average time to presentation was 3.7years (range, 9–105months); delay in treatment led to irreversible soft tissue damage in three patients. We recommend usage of ceramic heads until this problem is further understood.

Finite element analysis of the head–neck taper interface of modular hip prostheses

The purpose of this paper was to identify which parameters influence the micromotion at the head–neck taper interface of modular hip prostheses. Finite element analysis was performed where 3D models of the head–neck taper interface were subjected to an assembly force, 3300N of compression, and 100N of tension. The micromotion increased as the head size, assembly force, and taper size increased. The micromotion also increased when a mixed alloy material combination (CoCr head and Ti6Al4V neck) was used instead of all CoCr alloy prosthesis and when the center of the femoral head was in a more superior position relative to the center of the neck taper.

Risk Factors for Proximal Neck Complications After Endovascular Aneurysm Repair Using the Endurant Stentgraft

To assess the incidence and risk factors for proximal aneurysm neck related complications with a late generation device for endovascular abdominal aneurysm repair (EVAR). Data were retrieved from a prospective registry (Endurant Stent Graft Natural Selection Global Postmarket Registry) involving 79 institutions worldwide. The risk factors tested were age, gender, surgical risk profile, proximal neck length (<10mm), diameter (>30mm), supra- and infrarenal angulation (>60° and 75°), mural thrombus/calcification (>50%) and taper (>10%), and AAA diameter (>65mm). Two neck related composite endpoints were used, for intra-operative (type-1a endoleak, conversion, deployment/retrieval complication or unintentional renal coverage) and post-operative (type-1a endoleak or migration) adverse events. Independent risk factors were identified using multivariable backwards modeling. The study included 1263 patients (mean age 73, 10.3% female) from March 2009 to May 2011. Twenty three (1.8%) intra-operative adverse events occurred. Neck length <10mm (OR 4.9, 95% CI 1.1-22.6) and neck thrombus/calcification >50% (OR 4.8, 95% CI 1.7-13.5) were risk factors for intra-operative events. The planned 1 year follow up visit was reached for the entire cohort, and the 2 year visit for 431 patients. During this time, 99 (7.8%) events occurred. Female gender (HR 1.9, 95% CI 1.1-3.2), aneurysm diameter >65mm (HR 2.8, 95% CI 1.9-4.2), and neck length <10mm (HR 2.8, 95% CI 1.1-6.9) were significant post-operative risk factors. Neck angulation, neck taper, large diameter neck, and presence of thrombus/calcification were not predictors of adverse outcome in this study. These results support the adequacy of this device in the face of adverse neck anatomy, and confirm neck length as the most relevant anatomical limitation for EVAR. Additionally, the study confirms the decline in early to mid-term intervention rates with a newer generation device in a large patient sample. Lastly, it suggests that neck related risk factors affect outcome and impact on prognosis in varying degrees.

Partial two-stage exchange of the infected total hip replacement using disposable spacer moulds.

The common recommended treatment for infected total hip replacement is two-staged exchange including removal of all components. However, removal of well-fixed femoral stems can result in structural bone damage. We recently reported on an alternative treatment of partial two-stage exchange used in selected cases, in which a well-fixed femoral stem was left and only the acetabular component removed, the joint space was debrided thoroughly, an antibiotic-laden polymethylmethacrylate spacer was moulded using a bulb-type syringe and placed in the acetabulum, intravenous antibiotics were administered during the interval, and delayed re-implantation was performed. In 19 patients treated with this technique from January 2000 to January 2011, 89% were free of infection at a mean follow-up of four years (2 to 11). Since then, disposable silicone moulds have become available to fabricate spacers in separate femoral and head units. The head spacer mould, which incorporates various neck taper adapter options, greatly facilitates the technique of partial two-stage exchange. We report our early experience using disposable silicone head spacer moulds for partial two-stage exchange in seven patients with infected primary hip replacements.

Do You Have to Remove a Corroded Femoral Stem?

Corrosion at the head–neck taper has been recently identified as a cause of adverse local tissue reaction. There are no guidelines concerning removal of fixed femoral components when corrosion is present. The objective of this study is to report the survivorship when a new metal ball is placed on a corroded stem. We examined 86 retrieved femoral heads from metal-on-polyethylene THAs that underwent head and liner exchanges after a minimum 10years in-vivo and evaluated the subsequent survivorship. There were 7 rerevisions (8.1%) but none were for corrosion-related diagnoses and there was no difference in the survivorship between the 32 THAs with high-grade head taper corrosion and the 54 THAs with low-grade corrosion. We do not currently recommend removing well-fixed femoral stems with corrosion.

Total hip arthroplasty head–neck contact mechanics: A stochastic investigation of key parameters

A variety of design and patient parameters have been implicated in recent reports of fretting corrosion at modular connections in total hip arthroplasty. We sought to identify the relative sensitivity of mechanical fretting to a comprehensive set of parameters such that attention may be focused on key variables.Stochastic finite element simulation of the head–neck taper–trunnion junction was performed. Four-hundred parameter sets were simulated using realistic variations of design variables, material properties and loading parameters to predict contact pressures (P), micromotions (M) and fretting work (coefficient of friction×P×M) over cycles of gait.Results indicated that fretting work was correlated with only three parameters: angular mismatch, center offset and body weight (r=0.47, 0.53 and 0.43, p<0.001). Maximum contact pressure increased by 85MPa for every 0.1° of angular mismatch. Maximum micromotion increased by 5µm per 10mm additional head offset and 1µm per 10kg increased body weight. Uncorrelated parameters included trunnion diameter, trunnion length and impaction forces.It was concluded that appropriate limiting of angular mismatch and center offset could minimize fretting, and hence its contribution to corrosion, at modular connections.

Corrosion in modular total hip replacements: An analysis of the head–neck and stem–sleeve taper connections

In this retrieval study, modular junctions of retrieved S-ROM® implants were examined to determine the extent of corrosion at the head–neck and stem–sleeve junctions. Corrosion severity was graded in relation to the bearing surface material over time. It was found that the corrosion at the head–neck taper is greater for cobalt–chrome femoral heads compared to ceramic femoral heads. The stem–sleeve junction had significantly more corrosion damage (p < 0.05) in implants that had hard-on-hard bearing surfaces compared to hard-on-soft bearings. This study suggests that bearing surface materials and head size affect the amount of corrosion that is present at the modular junctions.

Micromotions at the taper interface between stem and neck adapter of a bimodular hip prosthesis during activities of daily living

The stem-neck taper interface of bimodular hip endoprostheses bears the risk of micromotions that can result in ongoing corrosion due to removal of the passive layer and ultimately cause implant fracture. We investigated the extent of micromotions at the stem-neck interface and the seating behavior of necks of one design made from different alloys during daily activities. Modular hip prostheses (n = 36, Metha®, Aesculap AG, Germany) with neck adapters (CoCr29Mo6 or Ti6Al4V) were embedded in PMMA (ISO 7206-4) and exposed to cyclic loading with peak loads ranging from walking (Fmax = 2.3 kN) to stumbling (Fmax = 5.3 kN). Translational and rotational micromotions at the taper interface and seating characteristics during assembly and loading were determined using four eddy-current sensors. Seating during loading after implant assembly was dependent on load magnitude but not on material coupling. Micromotions in the stem-neck interface correlated positively with load levels (CoCr: 2.6-6.3 µm, Ti: 4.6-13.8 µm; p < 0.001) with Ti neck adapters exhibiting significantly larger micromotions than CoCr (p < 0.001). These findings explain why high body weights and activities related to higher loads could increase the risk of fretting-induced implant failures in clinical application, especially for Ti-Ti combinations. Still, the role of taper seating is not clearly understood.

Patterns of stress distribution at the proximal femur after implantation of a modular neck prosthesis. A biomechanical study

BackgroundModular total hip arthroplasty incorporating a double taper design is an evolution offering potential advantages compared to single head–neck taper or monolithic designs. Changes in femoral offset, neck length or femoral anteversion are expected to alter the strain distribution. MethodsWe therefore analyzed the strain patterns after usage of all types of necks of a modular neck prosthesis, implanted in composite femurs. FindingsThe load distribution presented a repeatable pattern. Anteverted neck combinations resulted in higher stress at the anterior surface, whereas the retroverted ones at the posterior (e.g. at the middle frontal site, stress is 13.63% higher when we shifted from the long neutral neck to the long 15° anteverted neck and at the middle back site 19.73% higher when we shifted from the long neutral to the long 15° retroverted neck). Compressive stress was larger at the calcar region and exacerbated by the use of the varus neck (e.g. at the frontal 1 site stress increased by 44.01% when we used the long 8° varus neck in comparison to the long neutral neck). Anteverted neck combinations resulted in higher strain at the anterior cortex around the tip of the prosthesis. Short necks exhibited lower stress at the femoral shaft and higher at the trans-trochanteric area. InterpretationAnteverted neck combinations could be more prone to anterior thigh pain. Because of the possible risk of adaptive hypertrophy and early mechanical failure due to increased stress, the surgeon should be cautious when using necks with combined characteristics or short necks.

Mixing and matching causing taper wear

We report a case of a male patient presenting with bilateral painful but apparently well-positioned and -fixed large-diameter metal-on-metal hip replacements four years post-operatively. Multiple imaging modes revealed a thick-walled, cystic expansile mass in communication with the hip joint (a pseudotumour). Implant retrieval analysis and tissue culture eliminated high bearing wear or infection as causes for the soft-tissue reaction, but noted marked corrosion of the modular neck taper adaptor and corrosion products in the tissues. Therefore, we believe corrosion products from the taper caused by mismatch of the implant components led to pseudotumour formation requiring revision.

Ion release during fretting at taper joint interface of hip joint prosthesis

Metallic implants can be subjected to surface damage resulting in wear debris generation and ion release. Large amounts of metal ions released into body fluids may cause various harmful phenomena, from minor allergic reactions to more dangerous tissue necrosis and implant loosening. In hip joint prostheses, ions can be released from the bearing surfaces as a result of sliding at the head/cup interface and from non-bearing surfaces as a result of fretting or corrosion phenomena. In this paper, the focus is on the head–neck taper joint of the modular hip joint prosthesis subjected to high contact pressure fretting corrosion. Tests were performed on a Cr–Co–Mo tribocouple at 37°C in 0·9%NaCl solution using a ball on disc fretting tester. The free corrosion potential measurements were carried out during the friction process to assess how the sliding action at the interface affects the passivity of the material and how the passivity is re-established once the fretting wear ceases. Loading conditions were selected in order to simulate 60 and 80 kg patient body weight. Inductively coupled plasma measurements were carried out for solution samples collected after fretting experiments to detect metallic ions released during tribological contact. The wear tracks were analysed, and wear volumes were assessed using a white light profilometer. For all tests, the fretting loops characterising sliding behaviour of the interface and open circuit potential curves characterising electrochemical reactions were obtained. The average ion concentrations as a function of experiment duration were plotted and compared with volumetric wear. From the fretting loops and potential curves, the correlation between sliding conditions and depassivation/repassivation rate is discussed. The origin of metal loss for mechanical (fretting) and electrochemical (corrosion) processes is discussed. Analysis of the head/neck interface indicates the need for a long term fretting degradation model to consider various patient weights and forms of activity.

Influence of material coupling and assembly condition on the magnitude of micromotion at the stem–neck interface of a modular hip endoprosthesis

Hip prostheses with a modular neck exhibit, compared to monobloc prostheses, an additional interface which bears the risk of fretting as well as corrosion. Failures at the neck adapter of modular prostheses have been observed for a number of different designs. It has been speculated that micromotions at the stem–neck interface were responsible for these implant failures. The purpose of this study was to investigate the influence of material combinations and assembly conditions on the magnitude of micromotions at the stem–neck interface during cyclic loading.Modular (n=24) and monobloc (n=3) hip prostheses of a similar design (Metha, Aesculap AG, Tuttlingen, Germany) were subjected to mechanical testing according to ISO 7206-4 (Fmin=230N, Fmax=2300N, f=1Hz, n=10,000 cycles). The neck adapters (Ti–6Al–4V or Co–Cr29–Mo alloy) were assembled with a clean or contaminated interface. The micromotion between stem and neck adapter was calculated at five reference points based on the measurements of the three eddy current sensors.The largest micromotions were observed at the lateral edge of the stem–neck taper connection, which is in accordance with the crack location of clinically failed prostheses. Titanium neck adapters showed significantly larger micromotions than cobalt–chromium neck adapters (p=0.005). Contaminated interfaces also exhibited significantly larger micromotions (p<0.001). Since excessive micromotions at the stem–neck interface might be involved in the process of implant failure, special care should be taken to clean the interface prior to assembly and titanium neck adapters with titanium stems should generally be used with caution.

Early Failure of a Modular Femoral Neck Total Hip Arthroplasty Component

The PROFEMUR Z modular total hip arthroplasty component (Wright Medical Technology, Arlington, Tennessee) is a primary total hip replacement system that offers surgeons the ability to alter the length and version of the femoral neck after the femoral stem has been implanted. The modularity is achieved through taper interfaces at the distal and proximal ends of the femoral neck. The proximal femoral neck taper engages with the femoral head, and the distal taper engages with the femoral stem. In this report, we present the case of a patient who twice underwent total hip arthroplasty revision surgery. The first revision was due to a fractured ceramic femoral head. The second revision was due to a complete fracture of the modular femoral neck. This report will focus on the second revision, and, specifically, the failure of the modular femoral neck at the junction of the neck to the femoral stem. The patient was informed that data concerning the case would be submitted for publication, and he consented. A sixty-two-year-old man (height, 181 cm; weight, 84 kg; body mass index, 25.6 kg/m2) presented with severe primary osteoarthritis of the left hip. He underwent uneventful primary total hip replacement surgery in November 2006. The components implanted were a 58-mm Lineage cup fixed with two screws; a size-5 PROFEMUR Z modular stem with a modular, long, 8° retroverted neck; a 32-mm ceramic femoral head with a +3.5-mm offset; and a ceramic Transcend liner (Wright Medical Technology). The patient recovered well with no postoperative complications. In February 2007, the patient presented to the emergency department with a painful left hip. Radiographs revealed superior migration of the femoral head, and a fracture of the ceramic head was suspected. The patient was hospitalized, and revision total hip arthroplasty was performed. During the procedure, the head was found …

Corrosion of a Hip Stem With a Modular Neck Taper Junction: A Retrieval Study of 16 Cases

Since the early 1990s, there has been a dramatic increase in modular total hip designs ranging from a stem with a proximal taper and modular head, to a distal stem, double taper proximal neck, and modular head. Clinical advantages of the modular neck include intraoperative adjustment of leg length via the neck-head taper and femoral anteversion via the neck-stem taper. Sixteen cases of a double tapered cone, Margron hip prosthesis, were presented for retrieval analysis. Macroscopic inspection, corrosion testing, light microscopy, and scanning electron microscopy were conducted to elucidate mechanisms of failure. In this regard, 6 neck components showed significant fretting, and crevice corrosion of the neck-stem taper with an average implantation time of 39 months compared with the remaining retrievals, which showed no corrosion with and average time in situ of 2.7 months. This retrieval study demonstrates that even with a modern taper design and corrosion-resistant materials, increased modularity can lead to fretting and crevice corrosion, metal ion generation, and particulate debris that may contribute to periprosthetic osteolysis and loosening.

Selections of the cylinder implant neck taper and implant end fillet for optimal biomechanical properties: A three-dimensional finite element analysis

In this paper, effects of the implant neck taper and the implant-end fillet on the maximum Von Mises stresses were evaluated in jaw bones, and maximum displacements examined in an implant–abutment complex by a finite element method (FEM). The implant-neck tapers ( T) ranged from 45° to 70°, and fillets of implant ends ( R) ranged from 0.5 to 1.5 mm. Results suggested that under axial load by the maximum Von Mises stresses in cortical and cancellous bones decreased by 71.6% and 14.8%, respectively, and under 45° buccolingual load by 68.2% and 11.0%, respectively. The maximum displacement of implant–abutment complex decreased by 9.1% and 22.8% under axial and 45° buccolingual load, respectively. When T ranged from 64° to 73° and R exceeded 0.8 mm, minimum stress/displacement was obtained and the evaluating targets were more sensitive to T than to R. Data indicated that the taper of implant neck favored stress distribution in cortical bones more than the fillet of implant end did; taper of implant neck affected implant stability more than the fillet of implant end did; and the taper of implant neck of 64–73° and fillet of implant end exceeding 0.8 mm were optimal selections for the type B/2 bone in a cylinder implant by biomechanical consideration.

Application of Low Plasticity Burnishing (LPB) to Improve the Fatigue Performance of Ti-6Al-4V Femoral Hip Stems

Abstract Low plasticity burnishing (LPB) is a surface enhancement method that produces a deep layer of compressive residual stress with minimal cold working and an improved surface finish. Extensive fatigue testing, performed on numerous metal alloys in simulated environmental conditions, demonstrates that LPB significantly improves fatigue strength of highly stressed components. LPB is a flexible process, capable of being implemented on a wide variety of CNC machine tools. A product-specific LPB process was developed and applied to the modular neck taper junction of a Ti-6Al-4V total hip prosthesis (THP). LPB produced a compressive residual stress field with an improved surface finish, which enhanced component fatigue strength and resistance to fretting damage. X-ray diffraction (XRD) residual stress measurements, made before and after LPB application, are shown. High cycle fatigue (HCF) results obtained on LPB-processed hip stems are shown along with baseline data for unprocessed hip stems. HCF tests demonstrate complete elimination of fretting fatigue failures in the LPB processed area of the taper junction and a substantial increase in overall THP fatigue strength.