Biomechanics of short hip endoprostheses--the risk of bone failure increases with decreasing implant size

Influence of stem design parameters on periprosthetic femoral fractures examined by subject specific finite element analyses

Due to the increasing number of periprosthetic femoral fractures (PFF), the optimisation of implant design gains importance. For the presented research a validated, subject specific finite element model of a human femur with an inlying total hip stem was used to compare the influence of different geometrical implant parameters on the development of PFF. The heterogeneous bone tissue was modelled on the basis of computed tomography scans. A ductile damage model with element deletion was applied to simulate bone fracture in a load case re-enacting a stumbling scenario. The results were compared in terms of fracture load, subsidence and fracture pattern to analyse the influence of friction at the implant-bone interface, implant size and stem length. The results showed that higher friction coefficients lead to an increase of fracture load. Also, the usage of an oversized implant has a negligible effect while an undersized implant reduces the fracture load by 48.9% for the investigated femur. Lastly, a higher fracture load was reached with an elongated stem, but the bending and change in fracture path indicate a more distal force transmission and subsequent stress shielding in the proximal femur.

Stress shielding effect after total hip arthroplasty varies between combinations of stem design and stiffness-a comparing biomechanical finite element analysis

PURPOSE

Total hip arthroplasty (THA) has become a highly frequent orthopaedic procedure. Multiple approaches have been made to design the femoral component for THA with a mechanical behaviour as close as possible to a natural femur. The aim of this study was to compare different combinations of design and biomechanical properties of THA prostheses and their impact on stress shielding of the periprosthetic bone.

METHODS

Virtual implantation of different stem designs (straight standard stem, straight short stem, anatomical short stem) by finite element analysis based on in vivo data from computer tomography was performed. For each stem, three grades of stiffness were generated, followed by a strain analysis.

RESULTS

Reduction of stem stiffness led to less stress shielding. Implantation of an anatomical short-stem prosthesis with low stiffness provided the most physiological strain-loading effect (p < 0.001).

CONCLUSION

A combination of a short and an anatomically designed stem with a low stiffness might provide a more physiological strain transfer during THA. Biomechanical properties of the femoral component for THA should be considered as a multifactorial function of dimensions, design, and stiffness.

Short-Term Functional Outcomes of Short Femoral Neck Stems Are the Same as Those of Conventional Stems in Primary Total Hip Arthroplasty

(1) Background: In this study, two types of implants were compared-a conventional hip stem and a femoral neck prosthesis. (2) Methods: The femoral neck prosthesis study group included 21 patients, while the conventional hip stem control group was 40 patients. The first examination was the pre-op check, while the next ones were performed 6 weeks, 1 year, and 3 years after surgery. The Harris Hip Score (HHS), Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC), Oxford Hip Score (OHS), University of California at Los Angeles Activity Score (UCLA), and Visual Analog Scale EQ (VAS EQ) forms were completed at each clinical study visit. (3) Results: The HHS in the femoral neck prosthesis group and the conventional hip stem group 6 weeks after surgery was 68.8 ± 16.47 and 67.6 ± 8.92, respectively, and 1 year after surgery, this was 93 ± 5.58 vs. 90.6 ± 5.17, respectively. The OHS of the femoral neck prosthesis group was 34.8 points after 6 weeks, 45.5 points after 1 year, and 43.9 points after 3 years. The respective values in the conventional hip stem group were 35.5, 41.55, and 42.13 points. The WOMAC values for the femoral neck prosthesis group were 70.6, 92.7, and 86 points, respectively, while for the conventional hip stem group, they were 74, 88.1, and 86.1 points. The UCLA scores recorded in the conventional hip stem group ranged from 3.15 to 5.05 points, but a higher mean value of 5.33 points was obtained in the femoral neck prosthesis group. VAS EQ was equal to 84 points three years after the operation. (4) Conclusions: The study showed no significant differences in the functional scores of both groups, and the new type of cervical femoral stem could be the first choice in younger patients.

Effect of stem position and length on bone-stem constructs after cementless hip arthroplasty

Aims

There are concerns regarding initial stability and early periprosthetic fractures in cementless hip arthroplasty using short stems. This study aimed to investigate stress on the cortical bone around the stem and micromotions between the stem and cortical bone according to femoral stem length and positioning.

Methods

In total, 12 femoral finite element models (FEMs) were constructed and tested in walking and stair-climbing. Femoral stems of three different lengths and two different positions were simulated, assuming press-fit fixation within each FEM. Stress on the cortical bone and micromotions between the stem and bone were measured in each condition.

Results

Stress concentration was observed on the medial and lateral interfaces between the cortical bone and stem. With neutral stem insertion, mean stress over a region of interest was greater at the medial than lateral interface regardless of stem length, which increased as the stem shortened. Mean stress increased in the varus-inserted stems compared to the stems inserted neutrally, especially at the lateral interface in contact with the stem tip. The maximum stress was observed at the lateral interface in a varus-inserted short stem. All mean stresses were greater in stair-climbing condition than walking. Each micromotion was also greater in shorter stems and varus-inserted stems, and in stair-climbing condition.

Conclusion

The stem should be inserted neutrally and stair-climbing movement should be avoided in the early postoperative period, in order to preserve early stability and reduce the possibility of thigh pain, especially when using a shorter stem.

Cite this article: Bone Joint Res 2021;10(4):250–258.

Comparison of 5-year postoperative results between standard-length stems and short stems in one-stage bilateral total hip arthroplasty: a randomized controlled trial

PURPOSE

Short stems have recently become popular in total hip arthroplasty. Previous studies aimed at elucidating the efficacy of short stems did not eliminate the influence of other factors aside from stem length. This study aimed to evaluate the usefulness of short stems compared with that of standard-length stems that have the same proximal morphology, surface coating, and material.

METHODS

This was a prospective randomized study comparing 5-year midterm outcomes in 29 patients who underwent one-stage bilateral total hip arthroplasty with short and standard-length stems inserted in each of the two femurs. Clinical, radiographical, and dual-energy X-ray absorptiometry outcomes were compared.

RESULTS

No significant differences were found in perioperative and radiographic characteristics (femoral neck anteversion, flare index, operation sequence, operation side, operation time, stem anteversion, and stem alignment). The number of joints with complications, appearance of radiopaque lines around the stems, or bone mineral density changed in stem regions 5 years postoperatively. However, greater micromotion of the stem was seen on the side of the short stem. Satisfactory improvement in hip function was seen on both sides.

CONCLUSION

Based on the 5-year midterm outcomes, both stems obtained satisfactory clinical outcomes despite the greater micromotion with short stems. Both stems attained bone ingrowth fixation. Moreover, the stems were not significantly different in terms of stress shielding; however, further long-term studies (> 5 years) are required to validate our findings related to stress shielding.

Effects of condensation and compressive strain on implant primary stability: A longitudinal, in vivo, multiscale study in mice

Aims

Surgeons and most engineers believe that bone compaction improves implant primary stability without causing undue damage to the bone itself. In this study, we developed a murine distal femoral implant model and tested this dogma.

Methods

Each mouse received two femoral implants, one placed into a site prepared by drilling and the other into the contralateral site prepared by drilling followed by stepwise condensation.

Results

Condensation significantly increased peri-implant bone density but it also produced higher strains at the interface between the bone and implant, which led to significantly more bone microdamage. Despite increased peri-implant bone density, condensation did not improve implant primary stability as measured by an in vivo lateral stability test. Ultimately, the condensed bone underwent resorption, which delayed the onset of new bone formation around the implant.

Conclusion

Collectively, these multiscale analyses demonstrate that condensation does not positively contribute to implant stability or to new peri-implant bone formation.

Cite this article:Bone Joint Res. 2020;9(2):60–70.

Reduced periprosthetic fracture rate when changing from a tapered polished stem to an anatomical stem for cemented hip arthroplasty: an observational prospective cohort study with a follow-up of 2 years

Background and purpose - Straight collarless polished tapered stems have been linked to an increased risk for periprosthetic femur fractures in comparison with anatomically shaped stems, especially in elderly patients. Therefore, we evaluated the effect of an orthopedic department's full transition from the use of a cemented collarless, polished, tapered stem to a cemented anatomic stem on the cumulative incidence of postoperative periprosthetic fracture (PPF). Patients and methods - This prospective single-center cohort study comprises a consecutive series of 1,077 patients who underwent a cemented hip arthroplasty using either a collarless polished tapered stem (PTS group, n = 543) or an anatomic stem (AS group, n = 534). We assessed the incidence of PPF 2 years postoperatively and used a Cox regression model adjusted for age, sex, ASA class, cognitive impairment, BMI, diagnosis, and surgical approach for outcome analysis. Results - Mean age at primary surgery was 82 years (49-102), 73% of the patients were female, and 75% underwent surgery for a femoral neck fracture. The PPF rate was lowered from 3.3% (n = 18) in the PTS group to 0.4% (n = 2) in the AS group. The overall complication rate was also lowered from 8.8% in the PTS group to 4.5% in the AS group. In the regression model only cognitive dysfunction (HR 3.8, 95% CI 1.4-10) and the type of stem (PTS vs AS, HR 0.1, CI 0.0-0.5) were correlated with outcome. Interpretation - For elderly patients with poor bone quality use of cemented anatomic stems leads to a substantial reduction in periprosthetic fracture rate without increasing other complications.

Femoral neck prostheses: A systematic analysis of the literature

Primary total hip arthroplasty (THA) is one of the most successful surgical procedures. Considering the demographic change the use of new ultra-short femoral implants has gained importance especially when treating young patients. Main features are bone conservation, metaphyseal anchoring and thus reducing stress shielding by proximal load transferring. The objective of this study is to give an overview over the subject of femoral neck prostheses. A systematic review was conducted. A total number of 27 publications were taken into this systematic review. Over all, just a few follow-up, biomechanical and radiostereometric studies have been conducted in the past. Still no long-term results (>10 years of follow-up) are available. The available mid-term results indicate unsatisfactory survival rates. Aseptic loosening was the most common reason for revision. Valgus angle and good bone mineral density were considered to be crucial for primary stability of femoral neck prostheses. Register data report a very low percentage of femoral neck prostheses in THA with even more diminishing implantation rates. To conclude, further studies are necessary in order to provide evidence-based recommendations. Currently, due to the inhomogeneous and poor data a reasonable and legitimate recommendation cannot be given.

Biomechanical behaviours of the bone-implant interface: a review

In recent decades, cementless implants have been widely used in clinical practice to replace missing organs, to replace damaged or missing bone tissue or to restore joint functionality. However, there remain risks of failure which may have dramatic consequences. The success of an implant depends on its stability, which is determined by the biomechanical properties of the bone-implant interface (BII). The aim of this review article is to provide more insight on the current state of the art concerning the evolution of the biomechanical properties of the BII as a function of the implant's environment. The main characteristics of the BII and the determinants of implant stability are first introduced. Then, the different mechanical methods that have been employed to derive the macroscopic properties of the BII will be described. The experimental multi-modality approaches used to determine the microscopic biomechanical properties of periprosthetic newly formed bone tissue are also reviewed. Eventually, the influence of the implant's properties, in terms of both surface properties and biomaterials, is investigated. A better understanding of the phenomena occurring at the BII will lead to (i) medical devices that help surgeons to determine an implant's stability and (ii) an improvement in the quality of implants.

Fourteen-year experience with short cemented stems in total hip replacement

PURPOSE

The age of the population requiring total hip replacement (THR) is increasing and this may lead to a return of cemented stems. Advantages of a short cemented femoral device include preservation of metaphyseal bone, easier insertion, and easier cement removal in case of revision. The purpose of this study is to describe the rationale and assess midterm results of unique innovative short cemented double-tapered polished stem applied with contemporary cementing techniques.

METHODS

Our experience with this short cemented stem includes two different groups of elderly patients. Group 1 (prototype version of the short stem) from January 2005 to January 2008 counts 43 THR. Group 2 (final commercial version of the short stem) from January 2013 to January 2015 counts 54 THR. The average age in groups 1 and 2 was 79 and 75 respectively. Patients underwent clinical follow-up with the Harris Hip Score (HHS) and completed radiographic evaluation.

RESULTS

Thirty-one patients of group 1 had died for reasons unrelated to their THR. The surviving 9 hips have a follow-up of 11.2 years. In group 2, eight  patients died for reasons unrelated to their THR. Follow-up for the surviving 40 patients is 4.6 years. HHS improved in both groups. In 34/43 hips of group 1 and in 41/54 of group 2 we observed a Barrack grade A cement mantle. Survival with revision of the stem for aseptic loosening as the endpoint was 100%.

CONCLUSIONS

This study confirms the effectiveness of a short, polished, collarless, tapered cemented stem implanted with contemporary cementing techniques which appears as successful as the standard sized components.

Head Taper Corrosion Causing Head Bottoming Out and Consecutive Gross Stem Taper Failure in Total Hip Arthroplasty

BACKGROUND

Taper corrosion in total hip arthroplasty for bearings with metal heads against polyethylene has developed from an anecdotal observation to a clinical problem. Increased taper wear and even gross taper failure have been reported for one particular design. It is hypothesized that corrosion of the female head taper results in taper widening, allowing the cobalt-chromium head to turn on the stem and wear down the softer titanium alloy by abrasive wear, ultimately causing failure. The purpose of this study is to investigate the time course of this process and the general role of taper dimensions and material in this problem.

METHODS

Retrieved cobalt-chromium alloy heads (n = 30, LFIT; Stryker, Mahwah, NJ) and Ti-12Mo-6Zr-2Fe (TMZF) stems (n = 10, Accolade I; Stryker) were available for analysis. Taper material loss was determined using three-dimensional coordinate measurements and scanning. The pristine tip clearance between head and stem was analytically determined. The influence of taper material and taper size on taper deformation and micromotion was investigated using a finite element model.

RESULTS

Material loss at the head taper increased with time in situ up to a volume of 20.8 mm³ (P < .001). A mean linear material loss above 76 μm at the head taper was analytically confirmed to result in bottoming out, which was observed in 12 heads. The finite element calculations showed significantly larger deformations and micromotions for a small 11/13 TMZF taper combined with a distinctly different micromotion pattern compared to other materials and taper designs.

CONCLUSION

A 11/13 TMZF taper design with 36-mm head diameters bears a higher risk for corrosion than larger tapers made from stiffer materials. Failures of this combination are not restricted to the head sizes included in the recall. Patients with this implant combination should be closely monitored.

Periprosthetic femoral fracture as cause of early revision after short stem hip arthroplasty-a multicentric analysis

BACKGROUND

The objective of this study was to analyze the prevalence and causes of early re-operation after hip replacement surgery using short bone-preserving stems in a large multicentre series. Specifically, we evaluated the clinical features of periprosthetic fractures occurring around short stems.

METHODS

A total of 897 patients (1089 hips) who underwent primary total hip arthroplasty or bipolar hemiarthroplasty from January 2011 to February 2015 using short bone-preserving femoral stems were recruited. Mean patient age was 57.4 years (range, 18-97 years), with a male ratio of 49.7% (541/1089). Re-operation for any reason within two years was used as an endpoint. The incidence and clinical characteristics of the periprosthetic femoral fractures were also recorded. Mean follow-up period was 5.1 years (range, 2-7.9 years).

RESULTS

Early re-operation for any reason was identified in 16 (1.5%) of 1089 hips. The main reason for re-operation was periprosthetic femoral fracture, which accounted for eight (50%) of the 16 re-operations. The overall incidence of periprosthetic femoral fracture at two years was 1.1% (12/1089). According to the Vancouver classification, two fractures were AG type and the other ten were B1 type. Advanced age, higher American Society of Anesthesiologist grade, femur morphology of Dorr type C, and the use of a calcar-loading stem increased the risk for periprosthetic femoral fracture.

CONCLUSION

Periprosthetic femoral fracture was the major reason for re-operation after hip replacement surgery using short bone-preserving stems accounting for 50% (8/16) of re-operations two years post-operatively, but did not seem to deteriorate survivorship of implanted prostheses.

Bone preserving level of osteotomy in short-stem total hip arthroplasty does not influence stress shielding dimensions - a comparing finite elements analysis

Background

The main objective of every new development in total hip arthroplasty (THA) is the longest possible survival of the implant. Periprosthetic stress shielding is a scientifically proven phenomenon which leads to inadvertent bone loss. So far, many studies have analysed whether implanting different hip stem prostheses result in significant preservation of bone stock. The aim of this preclinical study was to investigate design-depended differences of the stress shielding effect after implantation of a selection of short-stem THA-prostheses that are currently available.

Methods

Based on computerised tomography (CT), a finite elements (FE) model was generated and a virtual THA was performed with different stem designs of the implant. Stems were chosen by osteotomy level at the femoral neck (collum, partial collum, trochanter sparing, trochanter harming). Analyses were performed with previously validated FE models to identify changes in the strain energy density (SED).

Results

In the trochanteric region, only the collum-type stem demonstrated a biomechanical behaviour similar to the native femur. In contrast, no difference in biomechanical behaviour was found between partial collum, trochanter harming and trochanter sparing models. All of the short stem-prostheses showed lower stress-shielding than a standard stem.

Conclusion

Based on the results of this study, we cannot confirm that the design of current short stem THA-implants leads to a different stress shielding effect with regard to the level of osteotomy. Somehow unexpected, we found a bone stock protection in metadiaphyseal bone by simulating a more distal approach for osteotomy. Further clinical and biomechanical research including long-term results is needed to understand the influence of short-stem THA on bone remodelling and to find the optimal stem-design for a reduction of the stress shielding effect.

Electronic supplementary material

The online version of this article (doi:10.1186/s12891-017-1702-2) contains supplementary material, which is available to authorized users.

Influence of trabecular bone quality and implantation direction on press-fit mechanics

Achieving primary stability of uncemented press-fit prostheses in patients with poor quality bone can involve axial implantation forces large enough to cause bone fracture. Radial implantation eliminates intraoperative impaction forces and could prevent this damage. Platens of two commercial implant surfaces ("Beaded" and "Flaked") were implanted onto trabecular bone specimens of varying quality in a press-fit simulator. Samples were implanted with varying interference, either axially (shear) or radially (normal). Push-in and pull-out forces were measured to assess stability. Microstructural changes in the bone were determined from μCT analysis. For force-defined implantation analysis, push-in and pull-out forces both increased proportionally with increasing radial force, independent of implantation direction, bone quality or implant surface. For position-defined implantation analysis, pull-out forces were generally found to increase with interference and to be greater for radial than axial implantation direction, and to be lower for poor quality bone. Bone density increased locally at the tested interface due to implantation, in particular for the Beaded surface under axial implantation. If a safe radial stress can be determined for cortical bone in a particular patient, the associated implantation force, and pull-out force which represents primary stability, can be directly derived, regardless of implantation direction, bone quality or implant surface. Radial implantation delivers primary stability that is no worse than that for axial implantation and may eliminate potentially damaging impaction forces. Development of implant designs based on this principal might improve implant fixation. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:224-233, 2017.

Substantially higher prevalence of postoperative peri-prosthetic fractures in octogenarians with hip fractures operated with a cemented, polished tapered stem rather than an anatomic stem

Background and purpose - Recent studies have demonstrated a high incidence of postoperative periprosthetic femoral fracture (PPF) in elderly patients treated with 2 commonly used cemented, polished tapered stems. We compared the prevalence and incidence rate of PPF in a consecutive cohort of octagenerians with femoral neck fractures (FNFs) treated with either a collarless, polished tapered (CPT) stem or an anatomic matte stem (Lubinus SP2). Patients and methods - In a multicenter, prospective cohort study, we included 979 hips in patients aged 80 years and above (72% females, median age 86 (80-102) years) with a femoral neck fracture as indication for surgery. 69% of the patients were classified as ASA class 3 or 4. Hip-related complications and repeat surgery were assessed at a median follow-up of 20 (0-24) months postoperatively. Results - 22 hips (2.2%) sustained a PPF at a median of 7 (0-22) months postoperatively; 14 (64%) were Vancouver B2 fractures. 7 of the 22 surgically treated fractures required revision surgery, mainly due to deep infection. The cumulative incidence of PPFs was 3.8% in the CPT group, as compared with 0.2% in the SP2 group (p < 0.001). The risk ratio (RR) was 16 (95% CI: 2-120) using the SP2 group as denominator. Interpretation - The CPT stem was associated with a higher risk of PPF than the SP2 stem. We suggest that the tapered CPT stem should not be used for the treatment of femoral neck fractures in patients over 80 years.

Trade-off between stress shielding and initial stability on an anatomical cementless stem shortening: in-vitro biomechanical study

Shortened cementless femoral stems have become popular with the advent of minimally invasive total hip arthroplasty (THA). Successful THA requires initial stem stability and prevention of stress shielding-mediated bone loss, although the effect of stem shortening is controversial. Here we experimentally examined whether stem shortening affects stress shielding and initial stability. Anatomical stems (length, 120 mm) were cut to an 80 mm or 50 mm length. Ten tri-axial strain gauges measured the cortical strain on each stem-implanted femur to evaluate stress shielding. Two transducers measured axial relative displacement and rotation under single-leg stance loading. The 50 mm stem increased the equivalent strains with respect to the original stem in the proximal calcar region (31.0% relative to intact strain), proximal medial region (63.1%), and proximal lateral region (53.9%). In contrast, axial displacement and rotation increased with a decreasing stem length. However, the axial displacement of the 50 mm stem was below a critical value of 150 µm for bone ingrowth. Our findings indicate that, with regard to a reduction in stem length, there is a tradeoff between stress shielding and initial stability. Shortening the stem up to 50 mm can promote proximal load transfer, but bone loss would be inevitable, even with sufficient initial stability for long-term fixation.

Periprosthetic fractures: concepts of biomechanical in vitro investigations

PURPOSE

Experimental in vitro studies investigating periprosthetic fractures after joint replacement are used increasingly. The purpose of this review was to deliver a condensed survey of studies in order to provide researchers with an overview of relevant scientific results and their clinical relevance.

METHODS

A literature search was conducted to obtain all available papers dealing with periprosthetic fractures, with particular attention being paid to articles with an experimental research design. Study goals, scientific methods and results, their interpretation and clinical relevance were assessed and compared. The main focus was on comparability with clinical fracture patterns and physiological joint loads.

RESULTS

Excluding duplicates, 24 studies with regard to artificial hip, knee and shoulder joints were found dating back to August 2000. Almost all studies were performed quasi-statically and without consideration of muscle forces and thus reflect selected loading conditions and no dynamic situation during activities of daily living (ADL). Various experimental protocols were used, differing in the choice of experimental material, implant and fixation system and load application.

CONCLUSIONS

In vitro studies regarding periprosthetic fracture research allow controlling for disturbances, such as clinically occurring risk factors like reduced bone mineral density (BMD) or greater patient age. Notwithstanding, due to methodological differences, comparisons between studies were possible to a limited degree only. For this reason, and because of quasi-static loading typically applied, results can only be partially applied to clinical practice.

Friction coefficient and effective interference at the implant-bone interface

Although the contact pressure increases during implantation of a wedge-shaped implant, friction coefficients tend to be measured under constant contact pressure, as endorsed in standard procedures. Abrasion and plastic deformation of the bone during implantation are rarely reported, although they define the effective interference, by reducing the nominal interference between implant and bone cavity. In this study radial forces were analysed during simulated implantation and explantation of angled porous and polished implant surfaces against trabecular bone specimens, to determine the corresponding friction coefficients. Permanent deformation was also analysed to determine the effective interference after implantation. For the most porous surface tested, the friction coefficient initially increased with increasing normal contact stress during implantation and then decreased at higher contact stresses. For a less porous surface, the friction coefficient increased continually with normal contact stress during implantation but did not reach the peak magnitude measured for the rougher surface. Friction coefficients for the polished surface were independent of normal contact stress and much lower than for the porous surfaces. Friction coefficients were slightly lower for pull-out than for push-in for the porous surfaces but not for the polished surface. The effective interference was as little as 30% of the nominal interference for the porous surfaces. The determined variation in friction coefficient with radial contact force, as well as the loss of interference during implantation will enable a more accurate representation of implant press-fitting for simulations.

High risk of early periprosthetic fractures after primary hip arthroplasty in elderly patients using a cemented, tapered, polished stem

BACKGROUND AND PURPOSE

Postoperative periprosthetic femoral fracture (PPF) after hip arthroplasty is associated with considerable morbidity and mortality. We assessed the incidence and characteristics of periprosthetic fractures in a consecutive cohort of elderly patients treated with a cemented, collarless, polished and tapered femoral stem (CPT).

PATIENTS AND METHODS

In this single-center prospective cohort study, we included 1,403 hips in 1,357 patients (mean age 82 (range 52-102) years, 72% women) with primary osteoarthritis (OA) or a femoral neck fracture (FNF) as indication for surgery (367 hips and 1,036 hips, respectively). 64% of patients were ASA class 3 or 4. Hip-related complications and need for repeat surgery were assessed at a mean follow-up time of 4 (1-7) years. A Cox regression analysis was used to evaluate risk factors associated with PPF.

RESULTS

47 hips (3.3%) sustained a periprosthetic fracture at median 7 (2-79) months postoperatively; 41 were comminute Vancouver B2 or complex C-type fractures. The fracture rate was 3.8% for FNF patients and 2.2% for OA patients (hazard ratio (HR) = 4; 95% CI: 1.3-12). Patients > 80 years of age also had a higher risk of fracture (HR = 2; 95% CI: 1.1-4.5).

INTERPRETATION

We found a high incidence of early PPF associated with the CPT stem in this old and frail patient group. A possible explanation may be that the polished tapered stem acts as a wedge, splitting the femur after a direct hip contusion. Our results should be confirmed in larger, registry-based studies, but we advise caution when using this stem for this particular patient group.

Femoral head diameter considerations for primary total hip arthroplasty

The configuration of total hip arthroplasty (THA) implants has constantly evolved since they were first introduced. One of the key components of THA design is the diameter of the prosthetic femoral head. It has been well established that the risk of dislocation is lower as the head diameter increases. But head diameter impacts other variables beyond joint stability: wear, cam-type impingement, range of motion, restoration of biomechanics, proprioception and groin pain. The introduction of highly cross-linked polyethylene and hard-on-hard bearings has allowed surgeons to implant large-diameter heads that almost completely eliminate the risk of dislocation. But as a result, cup liners have become thinner. With femoral head diameters up to 36 mm, the improvement in joint range of motion, delay in cam-type impingement and reduction in dislocation risk have been clearly demonstrated. Conversely, large-diameter heads do not provide any additional improvements. If an "ecologically sound" approach to hip replacement is embraced (e.g. keeping the native femoral head diameter), hip resurfacing with a metal-on-metal bearing must be carried out. The reliability of large-diameter femoral heads in the longer term is questionable. Large-diameter ceramic-on-ceramic bearings may be plagued by the same problems as metal-on-metal bearings: groin pain, squeaking, increased stiffness, irregular lubrication, acetabular loosening and notable friction at the Morse taper. These possibilities require us to be extra careful when using femoral heads with a diameter greater than 36 mm.

Results of total hip arthroplasty using a bionic hip stem

PURPOSE

The trabecular-orientated bionic hip stem was designed to mimic the natural force transmission through the femur in total hip arthroplasty, resulting in supposedly longer prosthesis survivability. The aim of this study was to compare the second-generation bionic hip stem to a standard uncemented hip stem.

METHODS

A group of 18 patients (21 hips) who underwent total hip arthroplasty with a bionic stem (bionic group) was compared with a historic group of 12 patients (12 hips) treated with standard anatomic hip stem (control group). During the first year after the procedure, the densitometric measurements of the bone around the prosthesis were taken. Radiographic and clinical assessments were additionally performed preoperatively and at the three month, six month, one year and three year follow-ups in the bionic group.

RESULTS

In the bionic group, one patient was revised for aseptic loosening and 16 patients (19 hips) were available to the final follow-up. A significant decrease of bone mineral density was found in Gruen zones 3, 4 and 5 in the bionic group, and in zone 7 in both groups. The bionic group had a significantly higher bone mineral density in Gruen zone 1 at the one year follow-up. At the final follow-up, all prostheses were radiologically stable in both groups.

CONCLUSIONS

Provided that a good implant position is achieved, comparable short-term results can be obtained using a bionic stem. Still, a decrease of bone mineral density in Gruen zone 7 occurred in both groups. Further studies are required to determine survivability of the bionic stem.

Is the length of the femoral component important in primary total hip replacement?

Many different lengths of stem are available for use in primary total hip replacement, and the morphology of the proximal femur varies greatly. The more recently developed shortened stems provide a distribution of stress which closely mimics that of the native femur. Shortening the femoral component potentially comes at the cost of decreased initial stability. Clinical studies on the performance of shortened cemented and cementless stems are promising, although long-term follow-up studies are lacking. We provide an overview of the current literature on the anatomical features of the proximal femur and the biomechanical aspects and clinical outcomes associated with the length of the femoral component in primary hip replacement, and suggest a classification system for the length of femoral stems.

Periprosthetic fracture torque for short versus standard cemented hip stems: an experimental in vitro study

In an attempt to preserve proximal femoral bone stock and achieve a better fit in smaller femora, especially in the Asian population, several new shorter stem designs have become available. We investigated the torque to periprosthetic femoral fracture of the Exeter short stem compared with the conventional length Exeter stem in a Sawbone model. Forty-two stems; 21 shorter and 21 conventional stems both with three different offsets were cemented in a composite Sawbone model and torqued to fracture. Results showed that Sawbone femurs break at a statistically significantly lower torque to failure with a shorter compared to conventional-length Exeter stem of the same offset. Both standard and short-stem designs are safe to use as the torque to failure is 7-10 times that seen in activities of daily living.

[Endoprosthetic treatment of osteoporosis-related coxarthrosis : aspects of safe patient treatment]

BACKGROUND

With increasing life expectancy the prevalence of osteoarthritis is also substantially rising. Patients aged between 65 and 75 years scheduled for total joint arthroplasty suffer from undetected osteoporosis in 20-25% of cases.

OBJECTIVES

How to determine osteoporosis during preoperative workup? Which conclusions can be drawn for the operation treatment and the postoperative course?

METHODS

The literature dealing with the prevalence of osteoporosis, perioperative complications of total hip arthroplasty, selected register informations, guidelines for diagnostics and treatment of osteoporosis and for the postoperative treatment are summarized and discussed.

RESULTS

The fracture risk is determined according to the guidelines of the Dachverband Osteologie (DVO, Governing Body on Osteology). The implant and the anchorage are selected based on the risk of suffering from osteoporosis. An intraoperative fracture and early aseptic loosening are the main operative risk factors. For the postoperative course in addition to education about arthroplasty, adequate support for prevention of falls is mandatory. Continuous physiotherapy with muscular strengthening is advisable. The long-term medication should be checked for risks in the PRISCUS list of potentially inappropriate medication in the elderly and non-steroidal anti-inflammatory drugs (NSAIDs) should not be prescribed in patients with cardiac comorbidities. Patients with confirmed osteoporosis should be treated with antiresorptive agents.

The influence of bone damage on press-fit mechanics

Press-fitting is used to anchor uncemented implants in bone. It relies in part on friction resistance to relative motion at the implant-bone interface to allow bone ingrowth and long-term stability. Frictional shear capacity is related to the interference fit of the implant and the roughness of its surface. It was hypothesised here that a rough implant could generate trabecular bone damage during implantation, which would reduce its stability. A device was constructed to simulate implantation by displacement of angled platens with varying surface finishes (polished, beaded and flaked) onto the surface of an embedded trabecular bone cube, to different nominal interferences. Push-in (implantation) and Pull-out forces were measured and micro-CT scans were made before and after testing to assess permanent bone deformation. Depth of permanent trabecular bone deformation ('damage'), Pull-out force and Radial force all increased with implantation displacement and with implantation force, for all surface roughnesses. The proposed hypothesis was rejected, since primary stability did not decrease with trabecular bone damage. In fact, Pull-out force linearly increased with push-in force, independently of trabecular bone damage or implant surface. This similar behaviour for the different surfaces might be explained by the compaction of bone into the surfaces during push-in so that Pull-out resistance is governed by bone-on-bone, rather than implant surface-on-bone friction. The data suggest that maximum stability is achieved for the maximum implantation force possible (regardless of trabecular bone damage or surface roughness), but this must be limited to prevent periprosthetic cortical bone fracture, patient damage and component malpositioning.

Biomechanics of a short stem: In vitro primary stability and stress shielding of a conservative cementless hip stem

Short stem prostheses provide conservative surgery and favorable metaphyseal load transmission. However, clinical long-term results are lacking. Therefore, in vitro trials can be used to predict bone-implant performance. In this in vitro study, primary stability and stress shielding of a new cementless short stem implant was evaluated in comparison to a straight stem using nine pairs of human cadaver femurs. Primary stability, including reversible micromotion and irreversible migration, was assessed in a hip simulator. Furthermore, changes in the pattern of cortical strain were evaluated. The short stem was more resistant to reversible micromotion and irreversible migration into retroversion. Axial stability was similar, with mean reversible micromotions of 9 µm for the short stem and 7 µm for the straight stem. Proximal load transmission was more physiological with the short stem, though both implants could not avoid stress shielding in Gruen zones 1 and 7. Primary stability of the short stem prosthesis was not negatively influenced compared to the straight shaft. Furthermore, proximal femoral strain pattern was more physiological after insertion of the short stem prosthesis.

Wear patterns of taper connections in retrieved large diameter metal-on-metal bearings

Wear of the modular taper between head and shaft has been related to clinical failure resulting from adverse reactions to metallic debris. The problem has become pronounced in large metal-on-metal bearings, but the mechanism has not yet been fully understood. We analyzed retrieved components from five patients revised with various diagnoses. Two distinct wear patterns were observed for the head tapers. Three samples demonstrated "asymmetric" wear towards the inner end of the head taper. The other two showed "axisymmetric" radial wear (up to 65 µm) presenting the largest wear volumes (up to 20 mm(3)). Stem tapers demonstrated relatively little wear, and the fine thread on the stem taper surface was observed to be imprinted on the taper inside of the head. Our findings demonstrate that the cobalt-chrome head wears preferentially to the titanium stem taper. "asymmetric" wear suggests toggling due to the offset of the joint force vector from the taper. In contrast, samples with "axisymmetric" radial wear and a threaded imprint suggested that corrosion led to head subsidence onto the stem taper with gradual rotation.

High friction moments in large hard-on-hard hip replacement bearings in conditions of poor lubrication

Disappointing clinical results for large diameter metal replacement bearings for the hip are related to compromised lubrication due to poor cup placement, which increases wear as well as friction moments. The latter can cause overload of the implant-bone interfaces and the taper junction between head and stem. We investigated the influence of lubrication conditions on friction moments in modern hip bearings. Friction moments for large diameter metal and ceramic bearings were measured in a hip simulator with cup angles varying from 0° to 60°. Two diameters were tested for each bearing material, and measurements were made in serum and in dry conditions, representing severely compromised lubrication. Moments were lower for the ceramic bearings than for the metal bearings in lubricated conditions, but approached those for metal bearings at high cup inclination. In dry conditions, friction moments increased twofold to 12 Nm for metal bearings. For ceramic bearings, the increase was more than fivefold to over 25 Nm. Although large diameter ceramic bearings demonstrate an improvement in friction characteristics in the lubricated condition, they could potentially replicate problems currently experienced due to high friction moments in metal bearings once lubrication is compromised.