Initial stability of fully and partially cemented femoral stems

Objective. To test the initial stability of a newly designed partially cemented femoral stem in comparison with a fully cemented conventional stem.

Design. An in vitro study to determine the interface motion between femoral stem and bone as a response to loading.

Background. The aim of the new prosthesis design is a proximal load transfer by a defined partial cement fixation in the proximal femur region and a slim prosthesis stem in the distal region. Before a clinical study can be started, the new stem has to show an initial stability comparable to that of fully cemented prostheses.

Method. Six paired fresh cadaveric femora were used for the testing of the new partially cemented stem (Option 3000, Mathys Orthopaedics, Bettlach, Switzerland) and a fully cemented stem (Weber Shaft, AlloPro, Baar, Swizerland). Under cyclic loading up to 1600 N hip joint forces, the interface motion between implants and bone was measured at six locations.

Results. Both stems showed uncritical interface motions below 43 μm. However, the Option 3000 stem exhibited significantly smaller motions in the proximal region and slightly larger movements in the distal regions than the Weber prosthesis.

Conclusions. The new type of partially cemented stem provided a comparable initial stability to the fully cemented Weber prosthesis.Relevance

The high initial stability of the Option 3000 stem justified the clinical use of the new implant. More than 100 implantations in the last three years, with very good preliminary clinical results, support the preclinical findings.     

聚醚醚酮髋股骨头假体置换术后股骨近段载荷传递的力学性能

背景:人工关节无菌性松动的力学因素是传统金属假体的弹性模量远远超过骨的弹性模量,组合后产生应力遮挡,引起骨吸收、骨萎缩.目的:体外测试自制的碳纤维增强聚醚醚酮(CF/PEEK)假体置换后股骨的应变,并与国产AML假体进行比较,探讨复合材料在全髋股骨头假体中的应用前景.设计、时间及地点:对比分析实验,于2002-09/2004-03在苏州大学第一附属医院骨科实验室和上海大学生物力学工程研究所完成.材料:CF/PEEK假体为自行设计,内层采用长CF/PEEK复合材料,模量为100 GPa;外层采用短CF/PEEK复合材料,模量为 20 GPa;在应力集中区,适当加强铺层密度;几何形态与相关尺寸同国产仿AML型钴合金假体.AML假体为北京华杰豪公司生产.标本为5对新鲜成人尸体股骨10具.方法:5 对人体新鲜尸体股骨平均分成左右两组,一组行钴铬钼合金股骨头假体置换术,另一组行CF/PEEK假体置换术.在假体和近端股骨表面粘贴应变片,模拟单肢站立施加载荷.主要观察指标:①正常股骨表面和两种假体置换后的股骨表面的应变分布.②两种假体置换后股骨-假体界面剪切应变分布. 结果:股骨应变在假体植入后,从近端到远端逐渐增加,变化形式与完整股骨的应变形式相似,并且在假体远端最大.两种假体植入后,股骨内外侧表面的应变皆减少;但CF/PEEK假体组产生的应变形式和大小比钴铬钼合金假体组更接近正常股骨. 结论:CF/PEEK复合材料假体的力学性能与骨组织相近,加载时两者的应变量一致,因此能提供术后即刻稳定性和良好的近端载荷传递.     

Influence of preoperative mechanical bone quality and bone mineral density on aseptic loosening of total hip arthroplasty after seven years

Objective. To test mechanical bone quality and bone mineral density of the femoral head at the day of implantation as indicators for femoral prosthesis loosening.

Methods. Mechanical bone quality of a femoral head slice was assessed by destructive compression testing combined with bone mineral density measurements using peripheral quantitative computed tomography. Fourteen patients with walking pains were attainable for a radiographical follow-up mean 7.1 years after implantation.

Results. Radiolucent lines along the stem were evident in 11 of 14 femurs, most of them seen in Gruen zones 7, 6, 1, 3, 14, and showed strong correlations to preoperative bone strength (r=−0.80; P<0.001) and axial stiffness (r=−0.75; P=0.002), yet not to bone mineral density (r=−0.67; P=0.009). Slight varus deviations <3° were noted in six femurs. Preoperative strength was reduced in this femurs to 54% (P=0.006), and stiffness to 61% (P=0.038), while bone mineral density did not differ significantly.

Conclusions. Femoral prosthesis loosening after seven years can be predicted by mechanical bone quality of the femoral head at the time of implantation. Bone mineral density measurements may also indicate future stem loosening but have to interpreted carefully, keeping in mind a poorer predictive value.

Relevance Indications and choice of type of hip arthroplasty should be balanced in osteoporotic bones in particular. While preoperative bone mineral density measurement allows the prediction of mechanical bone quality, its relevance in predicting failure in arthroplasty treatment remains unclear.     

Finite element analysis of the cervico-trochanteric stemless femoral prosthesis

OBJECTIVE: To investigate the biomechanical performance of a newly designed cervico-trochanteric stemless prosthesis by comparing the stress distribution with that of the traditional stem-type porous-coated anatomic prosthesis. DESIGN: Three-dimensional finite element models were created for the intact femur, cervico-trochanteric implanted femur and porous-coated anatomic implanted femur. The stress distributions on the femur and the implant were compared. The effects of using two or three screws fixation for the cervico-trochanteric implanted femur were also investigated. BACKGROUND: Local bone loss after implantation of traditional stem-type prostheses remains an unsolved problem during the long-term application of total hip replacement. The stress shielding effect and osteolysis were thought to be the two main factors that result in local bone loss after prosthesis implantation. In order to eliminate the mechanical and the biological causes of bone loss after total hip arthroplasty, a newly designed stemless femoral prosthesis was investigated. METHODS: Three-dimensional finite element models were created for the intact, cervico-trochanteric (with two or three fixation screws), and porous-coated anatomic implanted femora with the geometry of a standardized composite femur. Analysis was performed for a loading condition simulating the single-legged stance. The von Mises stress distributions of each model were analyzed and compared. RESULTS: The results can be summarized as follows: (1) Von Mises stress in the proximal, medial femur for the cervico-trochanteric implanted model was higher than that of the intact model and the porous-coated anatomic implanted model; (2) stress-shielding effect of the cervico-trochanteric models (with two or three fixation screws) were eliminated as compared with the porous-coated anatomic model; (3) no obvious difference in von Mises stress distribution for the cervico-trochanteric implanted model with two or three fixation screws. CONCLUSIONS: The cervico-trochanteric femoral prosthesis may reduce the stress-shielding effect of the proximal femur and achieve a more physiological stress distribution on the proximal femur than that of the porous-coated anatomic prosthesis. RELEVANCE: The new concept of cervico-trochanteric stemless prosthesis has proven to possess several advantages based on the current results, and may be an alternative for traditional stem-type prostheses in future clinical applications.     

Assessment of femoral neck fracture risk for a novel proximal epiphyseal hip prosthesis

Background

This study addresses the risk of femoral neck fracture associated with resurfacing hip prostheses. A novel cemented Proximal Epiphyseal Replacement (PER) featuring a short curved stem was investigated.

Methods

Seven pairs of femurs were in vitro tested. One femur of each pair was randomly assigned for PER implantation. The contralateral femur was tested intact. All femurs were loaded to failure in a validated, physiological configuration. High-speed videos (10,000–12,000 frames/s) were acquired to identify the location of fracture initiation. For comparison, data were included from Birmingham Hip Resurfacing previously tested in an identical fashion (N = 3).

Findings

Relative to the contralateral intact femurs, the failure load of the PER and Birmingham implants was 15.4% higher and 10.0% lower, respectively. In six of the seven PER implants, fracture initiation (neck or inter-trochanteric) was similar to the contralateral intact femurs, suggesting comparable stress distribution. Conversely, fracture initiation in the Birmingham implants occurred at the lateral prosthesis rim, which differed substantially from the intact femurs. No correlation existed between bone quality and strengthening/weakening effect of the PER (failure load of implant as a percentage of intact: R^2 = 0.067). Conversely, Birmingham implantation weakened the femurs with lower density (R^2 = 0.92). Therefore, unlike most resurfacing prostheses, the PER seems suitable also for osteoporotic subjects.

Interpretation

This study seems to confirm that resurfacing with a Birmingham Hip tends to reduce the strength of the proximal femur. The opposite seemed to happen with the PER, which slightly reduced the risk of neck fracture, also in low-quality bones.     

Experimental investigation of bone remodelling using composite femurs

OBJECTIVE: To determine the load transfer patterns of femurs in the intact, immediate post-operative and long-term (remodelled) post-operative implanted conditions for Lubinus SPII and Müller-Curved cemented hip prostheses, and to examine to what extent remodelling may influence the long-term outcome. DESIGN: Experimental and finite element (FE) methods were applied to composite femurs under loads representing the heel-strike phase of gait, determining cortical bone and cement strains for the different femur conditions. BACKGROUND: The authors previously developed protocols to measure bone and cement strains, and to produce remodelled femur specimens, yet these have not been applied together to compare strain patterns of different femur conditions. The Lubinus SPII is clinically more successful than the Müller-Curved stem, with failure mainly due to aseptic loosening. METHODS: Cortical bone strains were determined in intact femurs. Six femurs each were implanted with the two stem types and cortical bone and cement strains were measured. Bone remodelling was recreated using a validated CAD-CAM procedure to remove a layer of proximal cortical bone, replicating a typical scenario found in stable clinical retrievals. Strains were remeasured. FE methods were used to compliment the experiments. RESULTS: Stress shielding was reduced with remodelling, though bone strains did not return to their intact values, particularly around the calcar. Cement strains increased with remodelling. Differences occurred between the two stems; the Müller-Curved produced a more severe strain transition. CONCLUSIONS: Procedures were successfully combined together to investigate in vitro the performance of two cemented stems, in immediate and long-term post-operative conditions. The increase of cement strains with remodelling is a potential indicator for in vivo cement failure. RELEVANCE: The consequences of femoral bone remodelling on the long-term success of joint replacements are not well understood, where remodelling may lead to increased bone and cement stresses.     

Evaluation of femoral strains with cementless proximal-fill femoral implants of varied stem length

Background

The design intent of proximally-filling lateral flare femoral stems is to load the endosteal surface of the proximal femur both laterally and medially, to achieve normal bone strains. However, the long stem can contact the femoral cortex and may offload the proximal region to some extent. Therefore, in this study, we sought to determine if reducing the stem length, would result in physiologic strain patterns.

Methods

Using the PhotoStress® method we analyzed 13 femurs intact and with three different stem length implants: stemless, ultra-short and short. The test rig loaded the femoral head by simulating the mid-stance single leg support phase of gait with the ilio-tibial band and the hip abductor forces. The strain distribution with each stem length implant was then compared to the intact strain distribution to determine which was most similar.

Findings

As the stem length increased the femurs exhibited a typical pattern of reduced proximal strain and increased distal strain. However, there was some variation in this pattern indicating that the exact stem position and the location of its interaction with the endosteal surface of bone was not the same in each femur.

Interpretation

The stemless design provided the best match compared to the native femur and therefore has the greatest potential to address the shortcomings of a stemmed femoral implant. However, the ultra-short implant also exhibited a strain distribution that closely emulated the intact femur, and may represent the best option as there are still several questions pertaining to stability and alignment of a stemless implant.     

Three-dimensional morphometry of the femoral condyles

Objective. The present study describes the geometry of the three-dimensional articular surfaces of the human femoral condyles based on measurements of surface coordinates.

Design. The purpose was not to obtain a complex representation of one single condyle, but to describe the femoral condyles using simple geometric parameters based on measurements using a number of specimens.

Background. In joint modeling, a representative knee joint geometry is often desired which requires an approximation of the irregular joint geometry while taking into account interspecimen variations.

Methods. An optical device was used to measure the condylar articular surfaces of 12 human femurs in the femorotibial contact region. The sagittal profiles were reconstructed by means of two circular arcs and the radial profiles by means of one circular arc.

Results. The results provide the geometric parameters necessary for the three-dimensional reconstruction of the articular surfaces of the femoral condyles. The results indicate that the medial and lateral condyles of the distal femur are significantly asymmetric in a number of morphological features.

Conclusion. The primary application of the results is expected to be in the formulation of finite element models of the knee joint for static contact problems.

Relevance Numerical models of the knee joint are being widely used to study the mechanics of the joint. However, formulation of such models demands a prior knowledge of the complex three-dimensional geometry of the articular surfaces of the natural joint to establish the input parameters of the model.     

New concepts — biomechanical studies of a newly designed femoral prosthesis (cervicotrochanter prosthesis)

Objective. A newly designed stemless (cervico-trochanter) prosthesis was developed for the purpose of reducing the incidence of the stress-shielding effect caused by the traditional stem-type prosthesis.

Design. Both mechanical test and three-dimensional finite element analysis were performed for comparing the differences of strain and stress distributions between the intact, C-T and PCA prosthetic femora.

Background. The stress-shielding effect and polyethylene (PE) wear debris were thought to be the main factors that resulted in local bone loss after the implantation of stem-type prostheses. In this study, we developed the new C-T prosthesis, which aimed to resolve the above-mentioned problems.

Methods. Six pairs of femora taken from human male cadavers were used to compare the strain magnitudes of intact (n = 12), C-T (n = 12) and PCA (n = 4) prosthetic femora in specific positions. Failure load tests of C-T (n = 8) and PCA (n = 12) prosthetic femora were also carried out from the load-displacement curve. The analysis of variance ( ) test was used for statistical analysis. In addition, three-dimensional finite element stress analyses were performed using a commercial package, , on a Convex 3810 computer.

Results. Both mechanical test and finite element results showed that the C-T prosthetic femora has a lower stress-shielding tendency than the PCA prosthetic femora (P < 0.001). The C-T prosthetic femora also withstood an average bearing load of 6312 N, which is greater than that of the PCA prosthesis at 5358 N (P < 0.01).

Conclusions. The C-T prosthetic femur could withstand a higher failure load than the PCA prosthesis, which effectively reduced the incidence of the stress-shielding effect. Moreover, the particular design of the C-T prosthesis also reduced localized osteolysis because of the overall coverage of the neck-trochanteric area.     

Changes in strain distribution of loaded proximal femora caused by different types of cementless femoral stems

BACKGROUND: A number of clinically used total hip femoral implants are claiming a more or less physiologic load transfer, mostly without providing experimental data. To compare three clinically cementless total hip stems of fundamentally different design, the strain distribution before and after insertion in human cadaver femora was measured in vitro. METHODS: A conventional straight stem based on a distal anchorage concept, a so-called "anatomic" stem designed to have a proximal force transmission and a "stemless" femoral neck prosthesis were evaluated under similar loading conditions. Strain distribution was measured with tri-axial strain-gauge rosettes before and after implantation of the stems. The same bending moment was used in all femora tested to compare magnitude and direction of the resulting strains. FINDINGS: The straight and the "anatomic" stem both led to a decrease of the longitudinal strains in the proximal femur, while the femoral neck implant mainly led to an increase of measured strains on the lateral side of the greater trochanter. The observed medial strains were closer to physiological values in the "stemless" prosthesis than those of the two full-stem prosthesis. INTERPRETATION: The decrease in strains seen in the proximal region of the femora with implanted conventional hip prosthesis corresponds well to the decrease of bone density in this region noted in clinical follow-up studies. The more physiological strain at the inferior base of the neck seen in "stemless implant" may induce a remodelling process that better retains bone stock in that area. However, the increase of strains noted after implantation of this prosthesis require further investigation to assess the risk they may pose to bone failure.     

Measurement of the migration of a cemented hip prosthesis in an in vitro test

OBJECTIVE: To develop a method to measure the migration of a cemented hip prosthesis in an in vitro experimental test. DESIGN: A device to measure prosthesis movement relative to bone was designed and fabricated. It was tested using a Lubinus prosthesis (W. Link, Germany) implanted in a composite femur. BACKGROUND: Clinical studies using radiostereophotogrammetry have shown that those cemented hip prosthesis that migrate rapidly in the first two post-operative years are the ones that require early revision. If migration be used as a basis for a pre-clinical test, then it should be possible to screen-out inferior designs before implantation in animal or clinical trials. METHODS: The micromotion measurement device consisted of a 'target' of three spheres arranged in a cruciform structure. Six linear variable displacement transducers were aligned with the spheres so that motion of the prosthesis relative to the bone could be measured. RESULTS: The displacement and rotation of the prosthesis relative to the composite femur was recorded for two million cycles. Relative rapid initial migration was found, followed by a period of steady-state migration. Distal migration (called 'subsidence' in this paper) of up to 0.1 mm was measured; however the variability in absolute prosthesis migration was very high despite efforts to ensure that all extraneous factors were minimised. In the majority of cases, the prostheses migrated medially, distally and anteriorly. The absolute subsidence, and its variability, were similar to that recorded clinically. CONCLUSIONS: A method has been designed and tested which measures prosthesis migration in an experimental test. It provides a basis for a pre-clinical testing standard. Relevance. Hip prostheses need to be tested experimentally before implantation. However, no reliable test exists for such experimental tests. Rapid migration of a cemented prosthesis relative to bone has been shown in vivo to correlate with early failure, and in this paper a method to make such migration measurements in vitro is described and tested.     

Biomechanical evaluation of two types of short-stemmed hip prostheses compared to the trust plate prosthesis by three-dimensional measurement of micromotions

Background

Stemless and short-stemmed hip prostheses have been developed to preserve femoral bone stock. While all these prostheses claim a more or less physiological load transfer, clinical long-term results are only available for the stemless thrust plate prosthesis. In this study, the in vitro primary stability of the thrust plate prosthesis was compared to two types of short-stemmed prostheses. In addition to the well-established Mayo prosthesis, the modular Metha prosthesis was tested using cone adapters with 130° and 140° neck-shaft-angles.

Methods

The prostheses were implanted in composite femurs and loaded dynamically (300–1700 N). Three-dimensional micromotions at the bone-prosthesis interface were measured. In addition, the three-dimensional deformations at the surface of the composite femur were measured to gain data on the strain distribution.

Findings

For all tested prostheses, the micromotions did not exceed 150 μm, the critical value for osteointegration. The thrust plate prosthesis revealed similar motions as the short-stemmed prostheses. The short-stemmed prosthesis with the 130° cone tended to have the highest micromotions of all tested short-stemmed prostheses. The thrust plate prosthesis revealed the lowest alteration of bone surface deformation after implantation.

Interpretation

The comparably low micromotions of the thrust plate prosthesis and the short-stemmed prostheses should be conducive to osseous integration. The higher alteration of load transmission after implantation reveals a higher risk of stress shielding for the short-stemmed prostheses.     

Influence of changes in stem positioning on femoral loading after THR using a short-stemmed hip implant

BACKGROUND: Short-stemmed hip implants were introduced to conserve proximal bone mass and may facilitate the use of minimally invasive surgery, in which smaller incisions limit access to the joint. This limited access may increase the risk of surgical mal-positioning of the implant, however the sensitivity of femoral loading to such mal-positioning of a short-stemmed implant has not been studied. METHODS: Finite element models were developed of a femur and a short-stemmed implant positioned to reproduce the intact hip centre, as well as with the implant placed in increased anteversion or offset. The effect of these surgical variables on femoral loading was examined for walking and stair climbing using loads from a validated musculoskeletal model. Results of the implanted models were compared with an intact model to evaluate stress shielding. FINDINGS: Implant position had little influence on cortical strains along the length of the diaphysis, although strains decreased by up to 95% at the neck resection level compared to the intact femur. In the proximal Gruen zones I and VII strain energy density among the implanted models varied by up to 0.4 kJ/m(3) (28%) and 0.6 kJ/m(3) (24%) under walking and stair climbing, respectively. All implanted models showed characteristic proximal stress shielding, indicated by a decrease in strain energy density of up to 5.4 kJ/m(3) (69%) compared to the intact femur. INTERPRETATION: Small changes in stem placement would likely have little influence on the internal loading of the femur after bone ingrowth has been achieved, however a reduction in strain energy density and therefore stress shielding was seen even for a short-stemmed implant, which may have consequences for longer-term bone remodelling.     

Ipsilateral shoulder and elbow replacements: on the risk of periprosthetic fracture

Background. Ipsilateral shoulder and elbow replacements may leave only a short segment of bone bridging the two implants in the humerus. The potential for high stress concentrations as a result of this geometry has been a concern with regard to periprosthetic fracture, especially with osteoporotic bone. The study aims to determine the optimum length of the bone-bridge between shoulder and elbow humeral implants, and to assess the effect of filling the canal with cement.

Methods. A three-dimensional finite element model was used to compare the stresses between a humerus with a solitary prosthesis and a humerus with both proximal and distal cemented prostheses. The length of the bone-bridge and the effect of filling the canal with cement were studied under bending and torsion.

Findings. Gradual load transfer from prosthesis to bone was observed for all cases, and no stress concentration was evident. The length of the bone-bridge had no deleterious effect on stresses in the humerus, and filling the canal with cement did not appreciably decrease the loads carried by the humerus.

Interpretation. The length of the bone-bridge between stem tips has little effect on the resultant stresses in the humerus. Filling the canal with cement adds little benefit to the structural integrity of the humerus. Ipsilateral shoulder and elbow prostheses may be considered independent of one another in terms of risk of periprosthetic fracture.     

骨水泥外套开裂对股骨假体柄微动和松动的影响:附18例骨水泥型假体尸体标本观察

背景:无菌性松动是骨水泥型人工髋关节置换后失败的主要原因,从骨水泥型假体患者尸体标本研究中探讨其发病机制对于预防松动具有重要意义.目的:通过对骨水泥型假体患者尸体标本研究,旨在验证作者的推测:骨水泥的开脱和假体柄的微动与骨水泥固定失败相关,骨水泥外套的开裂预示着骨水泥型假体无菌性松动.设计、时间及地点:观察性实验,于2003-05/11在美国贝勒医学院骨科研究所完成.对象:18例带有骨水泥假体柄股骨标本是从18例原先行髋关节置换的遗体捐献者体内获得.这些遗体来源于美国贝勒医学院解剖系.包括男7例,女11例.方法:18例带有骨水泥假体柄的股骨标本在假体柄下3 cm处切断股骨,将标本浸泡在含有茜蓝素染色剂的溶液中,用真空泵抽吸股骨24 h染色.电锯将每个股骨标本沿矢状面两侧对称剖开,首先在显微镜下测量假体柄与骨水泥间隙,取出假体柄后,显微镜下观测骨水泥内面的开裂和磨损情况.用立体光学显微镜在3个等距离位置测量骨水泥开裂的宽度.同时记录假体柄磨损情况,按Gruen分区记录.主要观察指标:假体柄与骨水泥间隙的宽度,骨水泥外套内面开裂的位置和宽度,骨水泥外套内面磨损的区域和假体柄磨损的区域.结果:几乎所有假体柄都从骨水泥外套中开脱,16例标本骨水泥内面都有轻微磨损,15例标本骨水泥内面有开裂,而开裂的位置大多在假体柄与骨水泥的转角处(87%,P=0.002).无假体柄下沉超过0.5 mm,假体柄与骨水泥外套间裂隙平均宽度(0.33±0.06)mm,每一标本的骨水泥外套纵向开裂的平均宽度和假体柄与骨水泥外套间的裂隙平均宽度呈正相关(r~2=0.691,P=0.001).结论:在临床成功的骨水泥型人工关节置换患者中,骨水泥开裂是较常见的一种现象.骨水泥开裂一旦发生,假体柄就会出现微动.通常骨水泥开裂并不一定会导致假体无菌性松动,但随着微动产生的磨损微粒泵入骨床中而产生骨溶解最终会导致假体的松动.     

Primary stability and strain distribution of cementless hip stems as a function of implant design

Background

Short stem prostheses have been developed to preserve the femoral bone stock. The purpose of this study was to evaluate the stress-shielding effect in the proximal femur as well as the micromotion between bone and implant as a measure of primary stability for a new short stem in comparison to a clinically successful short stem and a straight stem.

Methods

Using paired fresh human femurs, stress shielding was examined by using tri-axial strain gage rosettes. The strain distribution of the proximal femur was measured before and after implantation of three cementless prostheses of different design concepts and stem lengths. Furthermore, interface motion and rotational stability were investigated under dynamic loading (100–1600 N) after 100,000 load cycles using inductive miniature displacement transducers.

Findings

A reduction of longitudinal cortical strains in the proximal femur was displayed for all three implants. The reduction was less pronounced for the shorter stem implants, however.Interface motion was below the critical threshold of 150 μm at almost all measuring points for all three stems, with a tendency for greater rotational stability in the shorter stem implants.

Interpretation

The new short stem prosthesis displayed reduced stress shielding and comparable primary stability to an established short stem and a conventional shaft design. Shortening the stem did not negatively influence primary stability. The clinical implications of these findings remain to be proven.     

不同骨质密度下生物型及骨水泥型股骨假体置入后的三维有限元分析

背景:患者自身骨质情况往往决定髋关节置换手术中假体的选择,目前对假体的选择尚无金标准,而通过计算机二维有限元分析、模拟对比力学实验,对不同骨密度下股骨假体类型进行选择,将更具科学性.目的:通过计算机三维有限元分析,选择不同骨密度下的股骨假体类型,并行模拟对比力学实验,从而确定临床全髋关节置换选择股骨假体的年龄标准.设计、时间及地点:开放性实验,于2006-09/2007-05 在吉林大学第一临床医院骨科研究所和吉林大学生物力学研究所完成.材料:成人新鲜股骨尸体骨.方法:利用Solidworks计算机软件建立人体股骨三维有限元模型.利用已有公式转化计算不同年龄段下骨质的密度及弹性模量,分别输入计算机.模拟人体单足站立状态,对重建的股骨模型进行力的加载与约束.主要观察指标:测量生物型和骨水泥型股骨假体在不同股骨密度下的应力分布及初始微动等生物力学特性.结果:生物型股骨假体上的整体应力在男性30～60岁年龄段分布相对较均匀,初始微动变化较小;在女性40～55岁年龄段分布相对较均匀,仞始微动变化较小.骨水泥型股骨假体上的整体应力在男性60～70岁年龄段分布相对较均匀,初始微动变化较小;在女性55岁后分布相对较均匀,初始微动变化较小.结论:计算机三维有限元分析及模拟对比力学实验结果提示,男性小于60岁,女性小于55岁使用生物型人上股骨假体优于骨水泥型人工股骨假体.     

Three-dimensional measurement of cemented femoral stem stability: an in vitro cadaver study

OBJECTIVE: To compare the in vitro stability of two cemented hip stem designs: Stem I was a collarless, double-tapered, highly polished implant; Stem II had a collar and matt finish. BACKGROUND: Stability of the femoral component of a hip implant is important for its long-term clinical success. Excessive migration or cyclic motion can increase the risk of early implant failure. METHODS: The stems were implanted in paired human cadaver femurs, and custom-designed micromotion sensors were used to measure three-dimensional motions of the stems at proximal, middle and distal locations during simulated in vivo loading cycles. RESULTS: This study found that despite 'rigid' fixation, cemented stems exhibit detectable motions under a limited number of cycles of simulated physiologic loads. At four times the donor body weight, Stem I showed a subsidence of 90 microm, compared to 25 microm of Stem II (P<0.05). In contrast, the proximal end of Stem II exhibited greater cyclic motions in the medial-lateral direction (P<0.05). CONCLUSIONS: The different motion patterns could be due to the design differences, such as surface finish and geometry.RelevanceImplant design is an important factor related to the behavior of the cement/bone interface and the overall success of the implant. This study compares in vitro micromotion of two cemented femoral prostheses with differing proximal designs.     

股骨远端恶性骨肿瘤切除并人工全膝关节置换术20例护理配合

目的:探讨股骨远端恶性骨肿瘤切除并人工全膝关节置换术患者的护理配合方法.方法:对20例股骨远端恶性骨肿瘤切除并人工全膝关节置换患者做好术前准备、术中配合、术后指导.结果:本组随访5～27个月,无一例关节松动、折断及感染等并发症发生.结论:认真做好股骨远端恶性骨肿瘤切除行人工全膝关节置换术患者的护理配合,可确保手术顺利进行,减少术后并发症的发生.     

Strain shielding for cemented hip implants

BackgroundLong-term survival of hip implants is of increasing relevance due to the rising life expectancy. The biomechanical effect of strain shielding as a result of implant insertion may lead to bone resorption, thus increasing risk for implant loosening and periprosthetic fractures. Patient-specific quantification of strain shielding could assist orthopedic surgeons in choosing the biomechanically most appropriate prosthesis.MethodsValidated quantitative CT-based finite element models of five femurs in intact and implanted states were considered to propose a systematic algorithm for strain shielding quantification. Three different strain measures were investigated and the most appropriate measure for strain shielding quantification is recommended. It is used to demonstrate a practical femur-specific implant selection among three common designs.FindingsStrain shielding measures demonstrated similar trends in all Gruen zones except zone 1, where the volumetric strain measure differed from von-Mises and maximum principal strains. The volumetric strain measure is in better agreement with clinical bone resorption records. It is also consistent with the biological mechanism of bone remodeling so it is recommended for strain shielding quantification. Applying the strain shielding algorithm on three different implants for a specific femur suggests that the collared design is preferable. Such quantitative biomechanical input is valuable for practical patient specific implant selection.InterpretationVolumetric strain should be considered for strain shielding examination. The presented methodology may potentially enable patient-specific pre-operative strain shielding evaluation so to minimize strain shielding. It should be further used in a longitudinal study so to correlate between strain shielding predictions and clinical bone resorption.