Modern Total Hip Arthroplasty Research Articles

Motor Nerve Palsy After Direct Anterior Versus Posterior Total Hip Arthroplasty: Incidence, Risk Factors, and Recovery

BackgroundThere is limited literature on motor nerve palsy in modern total hip arthroplasty (THA). The purpose of this study was to establish the incidence of nerve palsy following THA using the direct anterior (DA) and postero-lateral (PL) approaches, identify risk factors, and describe the extent of recovery. MethodsUsing our institutional database, we examined 10,047 primary THAs performed between 2009 and 2021 using the DA (6,592; 65.6%) or PL (3,455; 34.4%) approach. Postoperative femoral (FNP) and sciatic/peroneal nerve palsies (PNP) were identified. Incidence and time to recovery was calculated, and association between surgical and patient risk factors and nerve palsy were evaluated using Chi-square tests. ResultsThe overall rate of nerve palsy was 0.34% (34/10,047) and was lower with the DA approach (0.24%) than the PL approach (0.52%), P = .02. The rate of FNPs in the DA group (0.20%) was 4.3 times more than the rate of PNPs (0.05%), while in the PL group the rate of PNPs (0.46%) was 8 times more than that of FNPs (0.06%). Higher rates of nerve palsy were observed with women, shorter patients, and nonosteoarthritis preoperative diagnoses. Full recovery of motor strength occurred in 60% of cases with FNP and 58% of cases with PNP. ConclusionNerve palsy is rare after contemporary THA through the PL and DA approaches. The PL approach was associated with a higher rate of PNP, whereas the DA approach was associated with a higher rate of FNP. Femoral and sciatic/peroneal palsies had similar rates of complete recovery.

Heterotopic Ossification After Modern Total Hip Arthroplasty: Predisposing Factors, Prophylaxis, and Surgical Treatment.

Heterotopic ossification (HO) is a common radiographic finding and potentially serious complication after modern total hip arthroplasty. Although historically associated with the posterolateral approach, HO has been noted in 10% to 40% of patients having direct anterior or anterior-based muscle sparing approaches. The available data are uncertain whether robotic arm-assisted procedures are associated with this complication. Current prophylaxis for patients considered high risk of this complication includes postoperative, nonsteroidal, anti-inflammatory medication for several weeks or low-dose perioperative irradiation. The surgical treatment of symptomatic HO associated with severely restricted motion or ankylosis of the hip should be individualized but may include wide excision of bone, acetabular revision to prevent instability, and prophylaxis to prevent recurrence.

Highly Crosslinked Polyethylene Liner Thickness Does Not Influence Survival in Primary Total Hip Arthroplasty After Mean Follow-Up of 13 Years: A Study of 2,565 Hips With a Single Design Liner

BackgroundConcerns remain that thinner highly crosslinked polyethylene (HXLPE) liners in modern total hip arthroplasty (THA) may lead to premature liner-related failures or revision. The aim of this study was to evaluate the effect of liner thickness on survival and revision rates of HXLPE in primary THA after more than 10 years. MethodsWe retrospectively identified 2,565 primary THAs using HXLPE with a mean follow-up of 13 years (range, 11 to 19). Patients were grouped for each mm of polyethylene thickness. Liner thickness varied from 4.9 to 12.7 mm, with one third measuring less than 6 mm. Primary outcomes were reoperation, all-cause revision, and liner-related revision. ResultsThe reoperation rate was 7.3%, the all-cause revision rate was 5.6%, and the liner-related revision rate was 0.04%. There was no significant difference in all-cause revision when stratified by liner thickness (P = .286) and liner thickness was not associated with liner-related revision (n = 1). There was a statistically, but not clinically significant difference in mean liner thickness for the cohort that underwent reoperation (7.09 versus 6.89 mm, P = .01) and all-cause revision (7.16 versus 6.89 mm, P = .031). ConclusionIn our cohort, liner thickness was not associated with all-cause revision-free survival, and there was no clinically significant difference in liner thickness between those patients who did require a reoperation or all-cause revision, and those who did not. There was only 1 liner-related failure in the entire cohort. Our results indicate that using thinner HXLPE liners to maximize femoral head size in THA is a safe practice that does not lead to increased revision rates or liner failure at a mean 13-year follow-up.

Managing Native Hip Protrusio: Simplified Classification and Surgical Recommendations

Protrusio acetabuli is a unique osteological condition that has been long described in the literature and is known to potentially increase both the surgical complexity and the risk of complications when performing total hip arthroplasty. Although grading systems for native hip protrusio have been described in the past, there has not yet been a widely adopted classification system that categorizes the condition into separate classes in order to guide management. We propose a novel classification system with the goal of simplifying and standardizing the management of protrusio acetabuli in the context of modern total hip arthroplasty. This classification system describes protrusio based on the relationship of the femoral head to the ilioischial and iliopectineal lines, allowing for a more reproducible and consistent categorization of pathology. We also discuss general recommendations and technical pearls for total hip arthroplasty in the protrusio patient population, including the use of strategic soft tissue releases, fluoroscopy, navigation, bone graft, and augments.

Cementation in total hip arthroplasty: history, principles, and technique.

Total hip arthroplasty (THA) is a remarkably successful operation that has grown rapidly its utilization. Early modern THA constructs as developed by Sir John Charnley featured cemented femoral stems and acetabular components. The technique of cementing components for THA has evolved over time. Modern acetabular preparation requires exposure of the subchondral bone with appropriate cement penetration into the trabecular bone, whereas femoral preparation requires cleaning of the canal, cement restrictor placement, retrograde filling, and pressurization of the cement. When used appropriately, these techniques result in excellent long-term survivorship of implants and are also widely considered to be the ideal method of fixation for hip fractures. The purpose of this article to review the history, properties, techniques, and outcomes of bone cement utilization in THA.

Direct anterior approach in total hip arthroplasty: more indications and advantages than we found

Mounting attention has been oriented to the direct anterior approach (DAA) in total hip arthroplasty (THA) because of alleged advantages in terms of tissue-sparing and minimal invasiveness, as well as fast recovery after surgery in the past decades. Doubt has also been raised by critics regarding learning curve, indications, technical feasibility, intraoperative risks and complications, and unconfirmed long-term consequences of the approach. The controversies were elaborately reviewed and discussed in a recent publication in Arthroplasty by Realyvasquez et al. from the perspective of American surgeons. This inclusive review covered the current status, technical issues, outcome evaluation, and developmental concerns of DAA in modern THA. As one of the pioneers of DAA in hip surgery communities in China, Prof. Y. Z., the corresponding author of the paper, has his own understanding and manipulation of DAA on the basis of thousands of DAA procedures he has performed. The purpose of this article was to respond to the pivotal issues discussed in the article by Realyvasquez et al., and to present our own view of points about the indications/contraindications and advantages as different from existing ideas. In particular, we proposed a Xinqiao Predictive Algorithm to quantitatively assess the indications for and feasibility of DAA for the first time. The algorithm was constructed on multiple factors derived from Chinese patients. Our study concluded that the potential advantages of DAA could be achieved by personalizing the pelvic functional position and placing the component into the real safe zone, by means of adapting to the spaciotemporal change of the hip-spine coordination.

Evolution of total hip arthroplasty in patients younger than 30 years of age: A systematic review and meta-analysis.

While surgical technique and implant technology for total hip arthroplasty (THA) has improved over the years, it is unclear whether recent progress has translated to improved clinical outcomes for young patients. The goal of this study is to determine trends in (1) indications, (2) surgical technique (3) clinical and radiographic outcomes, and (4) survivorship for THA in patients younger than 30years of age. MedLine, Cochrane, EMBASE, and Google Scholar were searched using several key phrases for articles focusing on THA performed on patients younger than 30years of age between 1971 and 2020. A total of 34 qualifying articles were identified and stratified into three groups according to operative years and compared to one another on the basis of (1) indications; (2) fixation technique; (3) implant design; (4) clinical and radiographic outcomes; and (7) survivorship. The mean patient age at index THA were 20.5 (9-30), 22.1 (11-30) and 21.5 (10-30) years, respectively, for each study group. Over time, patients underwent fewer THAs for JRA (Juvenile Rheumatoid Arthritis) (p < 0.001) but more for post-treatment and iatrogenic avascular necrosis (p < 0.001; p < 0.001). Early THAs primarily used metal on UHMWPE (Ultra high molecular weight polyethylene) (71.7%, p < 0.001), modern THA predominantly use ceramic on HXLPE (Highly cross-linked polyethylene) (42.5%, p < 0.001). Early fixation methods used cement (60.4%, p < 0.001), and modern fixation primarily use press fit technology (95.9%, p < 0.001). Prevalence of radiographic loosening decreased significantly (p < 0.001) over time. There was no significant difference in clinical improvement on HHS. Lastly, fewer patients required THA revision in recent decades (p < 0.001). Advances in surgical technique and technology have served to improve implant longevity. Surprisingly, subjective clinical scores showed no significant improvement over time, suggesting that early iterations of THA were extremely successful.

Severe osteolysis and periprosthetic femoral fracture 45\u2009years after acrylic hemiarthroplasty of the hip: a case report

BackgroundHip arthroplasty using acrylic prosthesis was once conducted; however, it has now been abandoned because of early breakages and wear of material. Therefore, complications or revision surgeries due to the use of acrylic prostheses are becoming rare.Case presentationA 76-year-old woman presented with a sudden onset of severe pain in her left femur while walking. Radiographs revealed severe osteolysis and periprosthetic femoral fracture 45 years after the initial hemiarthroplasty using an acrylic prosthesis. We performed a Girdlestone resection arthroplasty by removing the prosthesis and fixing the fracture site using an intramedullary nail and metal plate. The patient was pain-free in her hip and leg 2 years and 9 months after the surgery. Although she walked with a cane or crutches, no postoperative complications were observed.ConclusionsThe combined use of an intramedullary nail and plate fixation with resection arthroplasty could offer acceptable results for patients with severe osteolysis and periprosthetic fracture after femoral head replacement using an acrylic prosthesis. Our report seems relevant because it not only reminds us of the significant steps made in the development of modern total hip arthroplasty; it also highlights one of the surgical options for severe osteolysis and periprosthetic fracture of the hip.

Variability in stem taper surface topography affects the degree of corrosion and fretting in total hip arthroplasty

Degradation at the modular head-neck interface in total hip arthroplasty (THA) is predominately expressed in the form of corrosion and fretting, potentially causing peri-prosthetic failure by adverse reactions to metal debris. This retrieval study aimed to quantify variations in stem taper surface topographies and to assess the influence on the formation of corrosion and/or fretting in titanium alloy stem tapers combined with metal and ceramic heads. Four hip stem designs (Alloclassic, CLS, Bicontact and SL-Plus) were characterized using high-resolution 3D microscopy, and corrosion and fretting were rated using the Goldberg scoring scheme. Quantification of the taper surface topographies revealed a high variability in surface characteristics between threaded stem tapers: Alloclassic and CLS tapers feature deeply threaded trapezoid-shaped profiles with thread heights over 65 µm. The sawtooth-shaped Bicontact and triangular SL-Plus taper are characterized by low thread heights below 14 µm. Significantly lower corrosion and fretting scores were observed in lightly threaded compared to deeply threaded tapers in ceramic head combinations. No significant differences in corrosion or fretting scores with thread height were found in pairings with metal heads. Understanding the relationship between stem taper surface topography and the formation of corrosion and fretting could help to improve the performance of modern THAs and lead to longer-lasting clinical results.

Femoral Stem Fracture in Hip Revision Arthroplasty: A Systematic Literature Review of the Real-World Evidence.

Background Total hip arthroplasty (THA) presents as an excellent treatment for the osteoarthritic hip, demonstrating good survival rates. However, aseptic loosening and infection are the main causes of operative revision. The methods used in revision surgery are non-modular or modular THA implants. In addition to the abovementioned revision reasons for THA, this treatment could be associated with the possibility of femoral stem fracture, especially in the modular system. The topic of material failure has been focused on in the public media. The question arises as to how such media reports correlate with the published literature. The observed mentioned number of cases concerning a femoral stem fracture vary between one single case and up to 18.5% within a clinical study, thus presenting an inhomogeneous data situation with a large span. The specific aim of this systematic review is to establish facts and clarify the number of unforeseen events of a femoral stem fracture based on peer review articles and registry data. This clarification is important to us, as these media reports have led to uncertainty among patients. Methods A systematic review was performed in accordance with the PRISMA statement. Peer review articles in English and German, presenting original articles, meta-analyses, or case reports, were searched from the turn of the millennium up to December 2019. Only articles that reported a femoral stem component fracture, with content of clinical data as well as register data, were included. Relevant papers published after the defined research time frame were taken into account within the discussion. Results In total, 218 fractures of a femoral stem (141 primary and 77 revision THA) component could be identified within the selected literature. Most cases of a femoral stem fracture occurred in the modular THA implants compared to the non-modular stems. Regarding revision THA, in summary, 77 implants, presenting 23 non-modular and 54 modular implants, failed by means of femoral stem fracture. A review of 11 National Joint Registries shows a revision rate between 0.04 and 0.05% in only 2 registers according to the specific subject of a femoral stem fracture. For the remaining 9 registers, however, detailed information is lacking and only nonspecific information such as a generic “implant failure” or “other reason” (which can cover a multitude of causes) is supplied. Conclusion A femoral stem fracture presents a devastating complication for the patient, the surgeon as well as for the manufacturer of the implant. Modular THA implants play an increasingly valuable role concerning restoration of individual anatomy in modern THA revision surgery, especially within complex cases. Regarding revision procedures, data suggests a lower risk of femoral stem fracture for modular implants compared to primary procedures, while the risk of fracture for non-modular implants seems to increase during revision. Ultimately, it cannot be proven whether this is actually applicable, since the absolute number of implanted prosthesis systems is not known. Various implant-, patient-, and surgeon-related factors may lead to these reported femoral stem fractures. However, this systematic review suggests that this is, in general, a rare complication.

Hallmarks in the History and Development of Total Hip Arthroplasty.

Total hip replacement remains one of the most successful and popular operative techniques in orthopedic surgery. Over several decades, several patients have been benefited by this mechanical and surgical achievement. It is an innovation that stands among the most significant surgical and technological achievements of the 20th century. The purpose of this review was to summarize the main landmarks on the evolution of hip surgery and the resulting modern total hip arthroplasty technique.

Surgical implications of the hip-spine relationship in total hip arthroplasty.

Total hip arthroplasty (THA) is considered the most successful orthopedic surgical procedure of the last century with excellent survivorship up to 20-years. However, instability remains a major issue representing the most common reason for revision after THA. Hip-spine relationship has gained progressive interest between arthroplasty surgeons and its understanding is crucial in order to identify high-risk patients for postoperative dislocation. Spinal deformity and abnormal spinopelvic mobility have been associated with increased risk for instability, dislocation and revision THA. Preoperative workup begins with standing anteroposterior pelvis x-ray and lateral spinopelvic radiographs in the standing and sitting position. Hip-spine stiffness needs to be addressed before THA in consideration of adapting the preoperative planning to the patient’s characteristics. Acetabular component should be implanted with different anteversion and inclination angles according to the pattern of hip-spine motion in order to reduce the risk of impingement and consequent dislocation. Different algorithmic approaches have been proposed in case of concomitant hip-spine disease and in case of altered sagittal balance and pelvic mobility. The aim of this review is to investigate and clarify the hip-spine relationships and evaluate the impact on modern total hip arthroplasty.

Impact of alignment and kinematic variation on resistive moment and dislocation propensity for THA with lipped and neutral liners.

Instability and dislocation remain leading indications for revision of total hip arthroplasty (THA). Many studies have addressed the links between implant design and dislocation; however, an understanding of the impact of alignment and kinematic variability on constraint of modern THA constructs to provide joint stability is needed. The objective of this study is to provide objective data to be considered in the treatment algorithm to protect against joint instability. Joint contact and muscle forces were evaluated using musculoskeletal models of THA patients performing activities consistent with posterior and anterior dislocation. Position and joint loads were transferred to a finite element simulation with an experimentally calibrated hip capsule representation, where they were kinematically extrapolated until impingement and eventual dislocation. Cup anteversion and inclination were varied according to clinical measurements, and variation in imposed kinematics was included. The resistive moment provided by the contact force and joint capsule, and overall dislocation rate (dislocations/total simulations) were determined with neutral and lipped acetabular liners. Use of a lipped liner did increase the resistive moment in posterior dislocation, by an average of 5.2 Nm, and the flexion angle at dislocation by 1.4° compared to a neutral liner. There was a reduction in similar magnitude in resistance to anterior dislocation. Increased cup anteversion and inclination, hip abduction and internal rotation all reduced the occurrence of posterior dislocation but increased anterior dislocation. A quantitative understanding of tradeoffs in the dislocation risk inherent to THA construct options is valuable in supporting surgical decision making.

Primary Total Hip Arthroplasty in Patients 20 Years Old and Younger

Historically, total hip arthroplasty (THA) performed in patients ≤20 years old has been associated with poor survivorship because of bearing-surface wear with conventional polyethylene, acetabular loosening with cemented sockets, and liner fracture in ceramic-on-ceramic (CoC) THA. For this population, there is a paucity of data regarding outcomes of THAs performed with use of modern implants and bearing surfaces. The purpose of the present study was to examine the mid- to long-term outcomes of modern THA in patients ≤20 years old. Utilizing a single-institution, prospectively collected total joint registry, we retrospectively identified 91 primary THAs performed in 78 patients ≤20 years old from 1998 to 2016. The average patient age was 17 years (range, 11 to 20 years), and the average body mass index was 26 kg/m (range, 16 to 49 kg/m). Forty-eight THAs (53%) were performed in male patients, and 47 (52%) were performed on the right hip. Bearing surfaces included CoC (53 THAs, 58%), metal-on-highly cross-linked polyethylene (MoP; 28 THAs, 31%), and ceramic-on-highly cross-linked polyethylene (CoP; 10 THAs, 11%). Outcome measures included reoperations, revisions, complications, clinical outcome scores, and bearing-surface wear. At an average follow-up of 8 years (range, 2 to 18 years), the average modified Harris hip score was 92 (range, 54 to 100), and 95% of patients reported feeling "much better" following the surgical procedure. Survivorship at 2, 5, and 10 years postoperatively was 96.7%, 96.7%, and 95.0% for reoperation; 98.9%, 98.9%, and 97.2% for revision; and 91.2%, 91.2%, and 89.5% for complications, respectively. The most common complications were instability (3 THAs, 3%), aseptic acetabular loosening (2 THAs, 2%), and postoperative foot drop (2 THAs, 2%). Linear articular wear averaged 0.019 mm/yr. There were no correlations between age, sex, body mass index, bearing surface, femoral head size, use of cement, or operative time and survivorship from complications, reoperations, or revisions. There were no differences in linear wear among CoC, CoP, and MoP bearing surfaces. In patients ≤20 years old, THAs performed with use of modern implants exhibit excellent clinical outcome scores and survivorship at mid- to long-term follow-up. CoC, CoP, and MoP bearing surfaces have similar survivorship, clinical outcomes, and bearing-surface wear in this population. Therapeutic Level IV. See Instructions for Authors for a complete description of levels of evidence.

Total Hip Arthroplasty in Adolescents and Young Adults

Similar to hip arthroplasty for primary osteoarthritis, the goal for total hip arthroplasty in the adolescent and young adult is to restore function and relieve pain. However, total hip arthroplasty in the young patient poses unique challenges given the anatomical deformities, higher physical demands, and increased likelihood for revision over the lifespan of these patients. Extensive preoperative planning must be undertaken to ensure proper surgical approach and implant selection. Outcomes are overall favorable with the use of modern implants and total hip arthroplasty should be regarded as a viable primary treatment option and not solely a salvage procedure.

Diagnosis and Management of Adverse Reactions to Metal Debris

Modern total hip arthroplasty implants have incorporated modularity into their designs, providing the benefits of intraoperative flexibility and the ability to exchange the femoral heads in the future if necessary. However, this feature has unfortunately predisposed patients to the effects of corrosion, potentially resulting in adverse local tissue reactions (ALTR) and even systemic effects. A thorough understanding of the science of corrosion is important for the treating surgeon so that they can understand the underlying pathology, quickly diagnose the condition of ALTR, and risk stratify their patients to determine the best method of treatment. Revision surgery is not always necessary in cases of trunnionosis or ALTR, but the results of revision surgery are generally favorable.

Modern trunnion designs do not affect clinically significant patient-reported outcomes.

Trunnion geometry is known to vary between hip systems. Trunnionosis and the impact of trunnion design on total hip arthroplasty (THA) survival, has gained attention as a failure mechanism. We sought to report the differences in patient-reported outcome measures (PROMs) between the most commonly utilised modern THA trunnions. We reviewed primary unilateral THA patients from May 2007 to October 2011. The most frequently used stems were included. LEAS, HOOS subdomains, and SF-12 were obtained pre and post operatively while satisfaction was measured at 2 years after THA. Trunnions were grouped by taper geometry and manufacturer. The 2-year change in PROMs for each trunnion was compared to the pooled 2-year change in HOOS for all other trunnions. 3950 THA patients were studied. 6 trunnion designs were evaluated from 5 manufacturers. The range in differences between the 2-year change in individual PROMs were as follows: HOOS pain (0.6-2.4), HOOS symptoms (0-3.8), HOOS ADL (0.4-4), and HOOS QOL (0.5-3.6). None of the differences in the 2-year change in PROMs reached a minimal clinically important change (MCIC), which we previously determined to be a minimum of 9 points for all HOOS domains. All of the trunnions designs utilised in our study cohort demonstrated excellent clinical results. Small differences were well below the known MCIC; and were not clinically relevant. The findings of this study should prompt further investigations into the long-term impact of trunnion design on clinical patient-reported outcomes.

Health Policy: Ethics, Regulatory, and Financial Aspects of Innovation in Orthopedics

The field of orthopedic surgery is laden with many examples of how progress in technology can greatly enhance patient care. The story of the modern total hip arthroplasty, one of the most successful surgical interventions of the century, is an example of how technology can evolve and provide significant benefits. There is a need for innovation and surgeons willing to challenge the status quo in order for the field of orthopedic surgery to continue to improve patient care. Thus, it is important for providers to understand the process of introducing new technology, ethical concerns with new technology, and the cost of developing and implementing new technology. In this article, we review current literature and highlight contemporary concerns with the introduction of new technologies in orthopedic care. Examples of successes and failures are abundant. The foot and ankle subspecialty in particular has a multitude of innovation and new technology that offer potential improvements to prior standards of care. New findings suggesting total ankle replacements as an alternative to ankle arthrodesis are an example of this progress. Continued research and innovation is crucial for improving orthopedic care, and the orthopedic community must learn from these experiences and remain vigilant of potential pitfalls by maintaining a balance between regulatory oversight with a climate suitable for ongoing medical exploration and innovation. Device companies, regulators, and health care systems and providers need to work synergistically with the common goal of improving patient care.

Preoperative digital planning versus postoperative outcomes in total hip arthroplasty using a calcar-guided short stem: frequent valgization can be avoided.

Modern total hip arthroplasty is largely dependent on the successful preservation of hip geometry. Thus, a successful implementation of the preoperative planning is of great importance. The present study evaluates the accuracy of anatomic hip reconstruction predicted by 2D digital planning using a calcar-guided short stem of the newest generation. A calcar-guided short stem was implanted in 109 patients in combination with a cementless cup using the modified anterolateral approach. Preoperative digital planning was performed including implant size, caput-collum-diaphyseal angle, offset, and leg length using mediCAD II software. A coordinate system and individual scale factors were implemented. Postoperative outcome was evaluated accordingly and was compared to the planning. Intraoperatively used stem sizes were within one unit of the planned stem sizes. The postoperative stem alignment showed a minor and insignificant (p=0.159) mean valgization of 0.5° (SD 3.79°) compared to the planned caput-collum-diaphyseal angles. Compared to the planning, mean femoral offset gained 2.18 (SD 4.24) mm, while acetabular offset was reduced by 0.78 (SD 4.36) mm during implantation resulting in an increased global offset of 1.40 (SD 5.51) mm (p=0.0094). Postoperative femoroacetabular height increased by a mean of 5.00 (SD 5.98) mm (p<0.0001) compared to preoperative measures. Two-dimensional digital preoperative planning in calcar-guided short-stem total hip arthroplasty assures a satisfying implementation of the intended anatomy. Valgization, which has been frequently observed in previous short-stem designs, negatively affecting offset, can be avoided. However, surgeons have to be aware of a possible leg lengthening.

Friction in modern total hip arthroplasty bearings: Effect of material, design, and test methodology.

The purpose of this study was to characterize the effect of a group of variables on frictional torque generated by acetabular components as well as to understand the influence of test model. Three separate test models, which had been previously used in the literature, were used to understand the effect of polyethylene material, bearing design, head size, and material combinations. Each test model differed by the way it simulated rotation of the head, the type of frictional torque value it reported (static vs. dynamic), and the type of motion simulated (oscillating motion vs. continuous motion). It was determined that not only test model may impact product ranking of fictional torque generated but also static frictional torque may be significantly larger than a dynamic frictional torque. In addition to test model differences, it was discovered that the frictional torque values for conventional and highly cross-linked polyethylenes were not statistically significantly different in the more physiologically relevant test models. With respect to bearing design, the frictional torque values for mobile bearing designs were similar to the 28-mm diameter inner bearing rather than the large diameter outer liner. Testing with a more physiologically relevant rotation showed that frictional torque increased with bearing diameter for the metal on polyethylene and ceramic on polyethylene bearings but remained constant for ceramic on ceramic bearings. Finally, ceramic on ceramic bearings produced smaller frictional torque values when compared to metal on polyethylene and ceramic on polyethylene groups.