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（英文）

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100GPa;外层采用短CF/PEEK复合材料,模量为20GPa;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。     

让昨天告诉今天：人工髋关节研究的学术与技术进展

1890年,德国医师Gluck第一个将象牙制作的股骨头假体植入人体。1923年,美国医师Smith Petersen在波士顿设计了玻璃杯关节假体。1938年,英国医师PhilipWiles采用不锈钢的臼杯与股骨假体,实施全球第1例全髋关节置换。1938年,美国医师Smih Petersen用牙科医生使用的钴铬钼合金材料做成钟状开口的金属杯,用于髋关节置换。1940年,Judet兄弟在法国用固定牙的丙烯酸合成树脂制造人工髋关节。1941年,美国的Moore和F．R．Thompson分别研发出完整的股骨球头置换术。1950年,英格兰医师Charnley进行了关节摩擦和润滑机制研究的设想。1953年,美国医师Haboush第一次将丙烯酸骨水泥应用于全髋关节假体固定。1958年,英格兰医师Charnhey根据重体环境滑润理论,用聚四氟乙烯髋臼和金属股骨头制成低摩擦的人工关节。1962年,经过大量的生物材料摩擦试验,英格兰医师Charnley设计出直径22．5mm的金属股骨头和超高分子聚乙烯髋臼组合的假体,用聚甲基丙烯酸甲酯（骨水泥）固定,从而创建了低摩擦的人工关节置换术。1966年,英格兰医师Charnley首先使用了空气层流净化手术间、个人空气隔离系统、预防性抗生素,使置换后感染率显著降低。由于Charnley对人工关节作出的重大贡献,被公认为现代人工关节之父。1971年,表面微孔型钴铬钼股骨头假体在巴黎出现,其特点为假体柄全长都布满了空隙。由于远端孔隙内骨长入牢靠而产生应力遮挡效应,使股骨近端骨质萎缩,因此被迫放弃了,目前将假体柄近端设计为微孔,孔径直径为100-500μm的假体已被广泛应用。1978年,美国的Miller和Harris医师改进了股骨柄骨水泥的使用方法,使骨水泥技术由第一代发展到第三代（现代）骨水泥技术,在长达20年的病例随访中,股骨柄假体松动率仅为3％。1994年,出现了混合型固定的全髋关节置换术,很多专家都把混合型人工关节置换作为首选模式。美国发展与共识会议正式提出了骨水泥型假体柄和非骨水泥臼的混合型全髋关节假体。     

全髋置换影响应力遮挡效应的相关因素

近年来仝髋关节置换得到了迅速发展,对一些终末性髋关节疾病进行了关节功能的重建,并取得了满意的疗效.但是全髋关节置换后股骨近端应力负荷会发生改变,导致股骨近端的所谓"适应性骨重建",其特点是皮质骨变薄,骨密度下降,引起股骨近端假体周围,尤其是股骨距区骨丢失.文章从骨水泥和非骨水泥全髋假体、假体涂层、全髋关节置换术式改变三个方面具体分析了对股骨近端应力遮挡效应的影响,通过改进假体设计,使用低弹性模量的材料、减少假体截面积、使用骨水泥型假体、使用适当的涂层材料,甚至改进全髋关节置换方式来减少应力遮挡效应.     

人工关节置换治疗脑卒中偏瘫侧股骨颈骨折20例

选择1997-02/2007-08桂林市人民医院及桂林市博爱医院骨科采用人工关节置换治疗脑卒中后偏瘫侧股骨颈骨折患者20例.应用的人工假体为北京普鲁斯医疗器械有限公司生产的全髋、半髋关节,钴铬钼合金;美国捷迈公司生产的全髋、半髋关节,钛合金:北京百慕航材高科医疗器械股份有限公司生产的京航全髋、半髋关节,钴铬钼合金.13例行人工股骨头置换,7例行全髋置换.全组病例平均随访46个月.术后人工关节脱位3例,假体松动1例,髋痛3例,下肢深静脉血栓形成1例,无一例发生感染.按Harris评分标准评定,优7例,良9例,中3例,差1例,优良率80.0%.     

全髋股骨侧假体置入前倾角对股骨上段应力分布的影响

背景股骨侧假体置换时按照原前倾角置入假体是公认的做法,但目前并无严格的生物力学实验证明其合理性.目的验证以不同前倾角置入股骨侧假体对股骨上段应力分布的影响.设计随机分组、相互对照的实验研究.地点和对象吉林大学机械学院力学系;15件新鲜成人股骨标本(自愿捐献).干预将15件标本随机分为3组,每组5件标本,对3组分别以原前倾角,+15°(在原前倾角的基础上再前倾15°),+30°前倾角置入非骨水泥固定假体,对置换前后的试件分别施以轴向载荷及复合载荷(轴向载荷+扭转载荷).主要观察指标置换前后各试件各点的应变值.结果轴向载荷下的应变分布规律,在小转子上缘以下105 mm外方3组间差异有显著性意义,以原前倾角组应变最大(平均166.08%),+30°组次之(平均121.24%),+15°组最小(平均105.40%),以较原前倾角大约15°的角度置入全髋股骨侧假体与以原前倾角置入假体相比会使更多的应力经股骨近端传导,其股骨表面的应变分布最接近于未处置股骨.结论以+15°前倾角置入假体会获得最佳的近端匹配,使置换后股骨获得更多的近端承载,有效降低应力遮挡,有利于骨长入及远期稳定.     

远端固定生物假体结合异体颗粒骨植骨在股骨假体翻修术中的应用

目的 探讨远端固定生物型股骨假体结合异体颗粒骨植骨在近端骨缺损型股骨翻修术中的应用.方法 2001年4月至2008年10月对23例非感染性股骨假体松动患者进行翻修手术,其中2例为再次翻修,均采用远端固定生物型股骨假体结合异体颗粒骨植骨.男9例,女14例,年龄57-77岁,平均64岁.翻修前人工股骨头置换7例,全髋关节置换16例,从初次关节置换到翻修手术的间隔时间最短6年,最长16年,平均12.6年.股骨骨缺损根据Paprosky分型,I型8髋,Ⅱ型11髋,ⅢA型4髋.术前Harris评分(39.3 ±14.6)分.翻修用假体:MP(Link)2髋,AML(DePuy)13髋,Full-coated(Zimmer)6髋,Enchelon(Smith-Nephew)股骨距替代型假体2髋.结果 本组术后切口均I期愈合,无手术相关并发症发生.患者均获随访,平均随访时间38个月(22-56个月).术后Harris评分为(87.6 ±9.4)分,与术前比较差异有统计学意义(P<0.05).术后X线摄片显示植入骨愈合良好,假体部位骨皮质密度和厚度明显增加.结论 远端固定生物型股骨假体结合异体颗粒骨植骨应用于伴有近端骨缺损的股骨翻修术中,中短期疗效满意,远期疗效尚待进一步随访.     

人工股骨头置换与内固定治疗高龄股骨转子间骨折疗效的比较

目的:比较人工股骨头置换与内固定治疗高龄股骨转子间骨折并探讨其临床适应证.方法:以"人工髋关节,转子间骨折,内固定"为中文关键词;"Hip replacement,Intertrochanteric Fractures, Internal Fixations"为英文关键词.采用计算机检索1969-01/2009-10相关文章.纳入与有关生物材料与组织工程材料相关的文章;排除重复研究或Meta分析类文章.以10篇文献为重点进行了讨论.临床验证选择大连医科大学第二附属医院骨外科收治的168例75岁以上不稳定型股骨转子间骨折的患者,分别采用人工股骨头置换和内固定方法治疗.分别对两组患者的手术时间、失血量、术后并发症、卧床时间、髋关节功能恢复结果、住院时间进行比较.结果:高龄股骨转子间骨折临床上多以采用内固定为主,对是否行人工股骨头置换仍存在争议.目前,人工髋关节假体材料主要有钴合金和钛合金、聚乙烯、多孔聚砜类物、多缩醛类树脂、碳素等材料.主要采取的固定方式有:骨水泥型假体、生物型假体、骨水泥型与生物型假体互补及现代骨水泥技术.经临床验证,人工股骨头置换在手术时间、置换后并发症、卧床时间、住院时间均好于内固定方法(P < 0.05);置换后1,3,6个月髋关节功能恢复优于内固定方法(P < 0.05),置换后12个月两种方法差异无显著性意义(P > 0.05).结论:人工股骨头置换治疗高龄不稳定型股骨转子间骨折具有置换后恢复好、能早期负重、并发症少等近期疗效好的优点,其远期并发症有待观察和进一步研究解决.     

髋关节置换股骨偏心距和下肢长度重建方法

背景:人工髋关节置换股骨偏心距和下肢长度的重建直接影响患者术后功能的恢复和假体的使用寿命,目前多采用术前模板测量来预测股骨假体的型号及置入位置,估计股骨颈的截骨水平,但是测量比较繁琐,欠精确.目的:设计通过股骨头假体长度和股骨颈截骨平面的选择来重建股骨偏心距和下肢长度的计算方法.设计、时间及地点:探索性实验,于2008-08/2009-02在单县中心医院完成.对象:选择单县中心医院接受髋关节置换的22例股骨颈骨折和股骨头缺血坏死患者.方法:术前测算健侧股骨偏心距E和髋关节旋转中心至小转子最高点的垂直距离D(下肢内旋15°).设股骨假体偏心距为E,垂降为H.设股骨颈截骨平面最下缘距小转子最高点的垂直距离为h,标准股骨头假体+0,其余为+L.股骨假体颈干角的补角为A.通过数学方法推导髋关节置换股骨偏心距和下肢长度重建股骨头长度及股骨颈截骨平面选择的计算公式.主要观察指标:股骨偏心距、髋关节旋转中心至小转子最高点的垂直距离、股骨假体偏心距、股骨假体颈干角、股骨假体垂降.结果:①E=E'+LsinA.②D=h+H+LcosA.结论:成功推导出髋关节置换股骨偏心距和下肢长度重建股骨头长度及股骨颈截骨平面选择的计算公式,该公式简单准确.     

金属对金属髋关节表面置换后的有限元分析

背景:与全髋关节置换相比,髋关节表面置换术有着诸多优势,如保留股骨头、股骨颈,方便于日后翻修,保存了股骨近端的生理力学分布,已成为年轻或对髋关节活动度要求较高的髋关节骨关节炎终末期患者更具吸引力的手术治疗方式.目的:观察人工髋关节表面置换术后殷骨及假体的应力分布.设计、时间及地点:单一样本观察,于2007-09/2008-04在南方医科大学解剖及生物力学实验室完成.对象:选择南方医科大学珠江医院收治的行金属对金属髋关节表面置换术男性患者1例(自愿参与实验),年龄40岁,检查排除健侧髋关节疾患.方法:建立股骨和假体有限元模型,分析在步行峰值关节载荷作用下,表面置换术后股骨颈及假体的应力水平.主要观察指标:股骨侧与假体的应力.结果:①髋关节表面置换术后股骨受力总体模式不变,股骨颈存在应力遮挡.应力峰值存在于股骨颈基底部,与其他区应力值相比,差异具有显著性意义(P<0.001),特别是转子窝附近以及骨-假体界面.②假体应力分布:应力峰值存在于髋臼及股骨假体外上方、髋臼杯沿与股骨假体交界处以及骨-假体界面.结论:髋关节表面置换术后股骨受力总体模式不变,股骨颈存在应力遮挡.峰应力存在于股骨颈基底部及骨-假体界面,为了减少术后股骨颈骨折的发生率,术中应特别加以保护上述区域.     

人工髋关节股骨柄假体断裂的研究进展

<正>近年来,人工髋关节置换技术得到快速发展,在股骨头坏死、先天性髋关节发育不良和髋部骨折的治疗上取得了良好的效果^[1]。股骨柄假体断裂是人工髋关节置换术的一种并发症,随着现代制造技术和冶金技术的进步,该并发症的发生率逐渐降低。与不锈钢材料相较,高强度钛或钴铬钼合金制成的股骨柄假体很少发生断裂^[2-3]。尽管如此,由于部分患者植入假体时间较早,股骨柄设计工艺和假体植入技术存在差异,股骨柄假体断裂的翻修手术目前仍然是一大挑战。该文就股骨柄假体断裂的流行病学、风险因素及处理对策的研究进展作一综述。     

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.     

In vitro testing of a novel limb salvage prosthesis for the distal femur

Objective. The aim of this paper was to define strain pattern in the host bone following distal femoral resection and implantation of a massive prosthesis. Two methods of coupling the prosthesis to the bone were compared: the Compliant Pre-Stress device, and a standard cemented tumour prosthesis.

Design. The composite femur model was selected to minimize variables. Four femurs were tested before and after implantation. Both coaxial and cantilever loading were applied.

Background. Cemented distal femoral replacement following resection of malignant tumours has a high failure rate at 5 years and is associated with extensive bone resorption thought to be secondary to stress shielding.

Methods. Strain was measured in the medial and lateral sides at four levels with physiologic loads applied, in the intact, Compliant Pre-Stress, and cemented femurs. Repeated measurements were taken. Strains in the implanted femur were calculated as percentage of the intact, and statistically analyzed.

Results. The most reproducible results were noted in cantilever bending (variability <5%). The Compliant Pre-Stress device demonstrated a more physiologic strain pattern than the cemented stem. The most significant difference between the two implants was in the area adjacent to the interface.

Conclusion. The Compliant Pre-Stress device shows less stress shielding than a standard cemented implant. The protocol described and the use of composite femurs demonstrated reproducible results.     

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.     

Bone remodelling inside a cemented resurfaced femoral head

BACKGROUND: Although the short-term performance of modern resurfacing hip arthroplasty is impressive, the long-term performance is still unknown. It is hypothesised that bone remodelling and the resulting changes in stress/strain distribution within the resurfaced femur influence the risk of fixation failure. METHOD: Three-dimensional finite element models and adaptive bone remodelling algorithms have been used to predict long-term changes in bone density following cemented femoral head resurfacing. Applied loading conditions include normal walking and stair climbing. The remodelling simulation was validated by comparing the results of an analysis of a proximal femur implanted with a Charnley femoral component with known clinical data in terms of bone density adaptations. FINDINGS: Resurfacing caused a reduction of strain of 20-70% in the bone underlying the implant as compared to the intact femur, immediately post operative. Elevated strains, ranging between 0.50 and 0.80% strain, were generated post-operatively around the proximal femoral neck regions, indicating a potential risk of neck fracture. However, this strain concentration was considerably reduced after bone remodelling. After remodelling, bone resorption of 60-90% was observed in the bone underlying the implant. Reduction in bone density of 5-47% occurred in the lateral femoral head. Bone apposition was observed in the proximal-medial cortex, around the inferior edge of the implant. Hardly any changes in bone density occurred in the distal neck or the femoral diaphysis. INTERPRETATION: Although resurfacing has produced encouraging clinical results, bone remodelling within the femoral head might be a concern for long-term fixation. Regions of strain concentration at the head-neck junction, which may increase the initial risk of femoral neck fracture, are reduced with bone remodelling. In order to reduce this risk of femoral neck fracture, patients should avoid activities which induce high loading of the hip during the early rehabilitation period after surgery.     

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.     

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.     

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.     

Investigation into the effect of varus-valgus orientation on load transfer in the resurfaced femoral head: a multi-femur finite element analysis

BACKGROUND: Femoral head resurfacing is a popular procedure for younger active hip replacement patients. Whilst the current generation of metal-on-metal resurfacing arthroplasties appear to have cured the osteolysis problems that plagued earlier resurfacing implants, fracture of the femoral head and aseptic loosening are still factors of concern in its survivorship. Several studies have shown a tendency towards failure in resurfaced femurs where the implant has been set at a varus angle. This work aims to investigate the influence of varus-valgus orientation on load transfer within the resurfaced proximal femur. METHODS: This study uses Computer Tomography based finite element analysis to determine the effect of implant orientation on load transfer in the proximal femur with respect to the intact femur. A group of 16 femurs were studied to take into account inter-patient variation; four models were produced for each femur, one of the intact femur, one resurfaced with the implant set inline with the femoral neck and one each representing varus and valgus implant alignment. FINDINGS: Results showed the valgus aligned resurfaced femur to produce strain patterns more akin to the intact femur. As the implant's angle to the femoral shaft increases from varus to valgus the strains in the superior femoral neck are reduced while those in the inferior neck are increased. INTERPRETATION: The study concluded that valgus alignment of the resurfacing arthroplasty is preferential to varus alignment; as it induces a more physiological strain pattern and reduces the risk of femoral neck fracture. These findings are in line with clinical experience, which has shown an increase of failure with varus implanted prostheses.     

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.     

Load response of an osseointegrated implant used in the treatment of unilateral transfemoral amputation: An early implant loosening case study

BackgroundOsseointegrated implants for transfemoral amputees facilitate direct load transfer between the prosthetic limb and femur; however, implant loosening is a common complication, and the associated implant-bone loads remain poorly understood. This case study aimed to use patient-specific computational modeling to evaluate bone-implant interface loading during standing and walking in a transfemoral amputee with an osseointegrated implant prior to prosthesis loosening and revision surgery.MethodsOne male transfemoral amputee with an osseointegrated implant was recruited (age: 59-yrs, weight: 83 kg) and computed tomography (CT) performed on the residual limb approximately 3 months prior to implant failure. Gait analyses were performed, and the CT images used to develop a finite element model of the patient's implant and surrounding bone. Simulations of static weight bearing, and over-ground walking were then performed.FindingsDuring standing, maximum and minimum principal strains in trabecular bone adjacent to the implant were 0.26% and −0.30%, respectively. Strains generated at the instant of contralateral toe-off and contralateral heel strike during walking were substantially higher and resulted in local trabecular bone yielding. Specifically, the maximum and minimum principal strains in the thin layer of trabecular bone surrounding the distal end of the implant were 1.15% and −0.98%, respectively.InterpretationLocalised yielding of trabecular bone at the interface between the femur and implant in transfemoral amputee osseointegrated prosthesis recipients may present a risk of implant loosening due to periprosthetic bone fracture during walking. Rehabilitation exercises should aim to produce implant-bone loading that stimulates bone remodelling to provide effective bone conditioning prior to ambulation.