Design parameters and the material coupling are decisive for the micromotion magnitude at the stem–neck interface of bi-modular hip implants

Several bi-modular hip prostheses exhibit an elevated number of fretting-related postoperative complications most probably caused by excessive micromotions at taper connections. This study investigated micromotions at the stem–neck interface of two different designs: one design (Metha, Aesculap AG) has demonstrated a substantial number of in vivo neck fractures for Ti–Ti couplings, but there are no documented fractures for Ti–CoCr couplings. Conversely, for a comparable design (H-Max M, Limacorporate) with a Ti–Ti coupling only one clinical failure has been reported. Prostheses were mechanically tested and the micromotions were recorded using a contactless measurement system.For Ti–Ti couplings, the Metha prosthesis showed a trend towards higher micromotions compared to the H-Max M (6.5 ± 1.6 μm vs. 3.6 ± 1.5 μm, p = 0.08). Independent of the design, prostheses with Ti neck adapter caused significantly higher interface micromotions than those with CoCr ones (5.1 ± 2.1 μm vs. 0.8 ± 1.6 μm, p = 0.001). No differences in micromotions between the Metha prosthesis with CoCr neck and the H-Max M with Ti neck were observed (2.6 ± 2.0 μm, p = 0.25).The material coupling and the design are both crucial for the micromotions magnitude. The extent of micromotions seems to correspond to the number of clinically observed fractures and confirm the relationship between those and the occurrence of fretting corrosion.     

四种螺纹截面形态对种植体初期稳定性影响的三维有限元研究

目的 利用即刻负载有限元模型研究种植体不同螺纹截面类型因素对初期稳定性的影响.方法 利用Pro/E软件、Hypermesh 7.0软件及ABAQUS 6.5有限元软件,建立4类种植体即刻负载的三维有限元模型,比较4种螺纹截面形态(V型,支撑型, 矩型和反支撑型)在分别垂直和水平加载时对种植体初期稳定性的影响.结果 对不同螺纹截面形态种植体的微动程,垂直加载时支撑型螺纹种植体微动最小,反支撑型螺纹种植体微动最大,水平加载时反支撑型螺纹种植体微动最小,支撑型螺纹种植体微动最大.结论 螺纹截面的形态和强度对垂直相对位移有影响, 对颊舌相对位移影响不大,螺纹顶边和底边与种植体体部的角度越大,抵抗垂直向的能力越强,但强度差,因此在螺纹种植体设计时,要兼顾螺纹截面角度和强度.     

Comparison of the stability of press-fit hip prosthesis femoral stems using a synthetic model femur

The motion of three different press-fit hip prosthesis femoral components was compared with that of a cemented stem and an Austin Moore noncemented hemiarthroplasty. Synthetic composite femurs were used as an experimental model to reduce the variations in shape and quality typical among cadaver femurs. Motion was measured under axial and rotational loading approximating a walking load. On the initial application of load, axial subsidence was as much as several millimeters for the noncemented stems. Considerable tightening occurred during the 5,000 walking cycles, such that the motion of the noncemented stems in some directions eventually was as small as that of the cemented stem, on the order of tens of microns.     

非骨水泥人工股骨头柄近端的轴向、旋转微动与移位实验研究

目的探讨人工股骨头微动实验的模型与研究方法。实验测试人工股骨头的轴向、旋转微动和移位，评价其生物力学性能。方法6对新鲜冷冻人股骨，分别随机置入有股骨距托假体人工股骨头、无股骨距托假体人工股骨头。试件装置于MTS万能材料试验机。2个可变磁阻式传感器（DVRT）分别用骨蜡固定至股骨近端与人工股骨头柄，经导线与计算机系统连接。采用轴向、旋转复合加载，轴向负荷为680N，旋转负荷为22N-M，加载频率5Hz，循环1000次。计算机采集数据并分析。结果二种假体轴向移动，下沉无明显差别，有股骨距托假体下沉55μm，无股骨距托假体下沉38μm；旋转微动，移位差别显著。旋转微动：有股骨距托型45μm，无股骨距托型25μm；旋转移位：有股骨距托型200μm，无股骨距托型50μm。结论两种假体都具有防止假体轴向微动和下沉的作用。无股骨距托假体柄特殊的多平面楔形设计。具有良好的防止旋转微动和移位的性能。     

In vitro long-term fatigue endurance of the secondary "Cement Injection Stem" hip prosthesis

A secondary cementation hip stem (Cement Injection Stem; Aesculap, Tuttlingen, Germany) was designed to reduce the risk of fat embolism, and achieve precise implant position and high-quality cement mantle. A validated long-term in vitro simulation was carried out that replicated 24 years of activity of a very demanding patient. Inducible and permanent micromotions were monitored. The cement mantle was sectioned and inspected for signs of fatigue damage. The stem-cement interface was inspected for fretting. Results were compared against previously published results for a conventionally implanted stem with comparable design (Centrament; Aesculap) from which this project was derived. Comparable micromotions were found (slightly larger proximally, in correspondence to the precured centralizer). No sign of fretting was observed. All fatigue damage indicators were comparable or significantly better than those for the conventionally implanted stem. The few cement cracks found were mainly localized in proximity of a proximal drainage hole. It is foreseen that when this detail is optimized, long-term endurance will further improve.     

Biomechanical comparison of stemless humeral components in total shoulder arthroplasty

BackgroundThe purpose of this study was to compare initial fixation strength between various stemless and stemmed humeral components and to correlate implant fixation strength with bone mineral density (BMD).MethodsFive humeral stem designs were investigated: Stemless-A (four hollow fins), Stemless-B (central body, three solid fins), Stemless-C (central screw, peripheral rim-fit), Short stem (50 mm), and Standard stem (130 mm). Fifty cadaveric human humerii were obtained and divided into five groups. BMD within the humeral head was determined for all samples. The mean BMD was similar between groups. The 25 samples with the lowest and highest BMDs were categorized as “Low” and “High,” respectively, with a BMD threshold of 0.35 g/cm², creating BMD subgroups. After implantation, each sample underwent a standardized biomechanical testing protocol, with axial loading followed by torsional loading. Sensors attached to the specimen recorded micromotion throughout testing. Axial loading consisted of cyclic loading for 100 cycles at 3 peak forces (220, 520, and 820 N). Torsional loading consisted of 100 cycles of internal/external rotation at 0.1 Hz at 6 peak torques, or until failure (±2.5, 5, 7.5, 10, 12.5, and 15 Nm). Failure was defined as the torque at which any bone fracture, implant detachment from anchor/stem, or an excess of 50° internal/external rotation occurred. Groups and BMD subgroups were compared.ResultsAt maximal axial loading, Stemless-B demonstrated greater micromotion (540 μm) than Stemless-C (192 μm) (P = .003). Stemless-B and Stemless-A (387 μm) also had greater micromotion than Short stem (118 μm, P < .001, P = .03) and Standard stem (85 μm, P < .001, P = .01). When comparing low-BMD samples at maximal axial loading, these differences were accentuated, but comparison of high-BMD samples showed no significant differences between groups. Torsional testing demonstrated that Standard stem failed at greater torque (7.2 Nm) than Stemless-B (2.3 Nm, P < .001), Stemless-A (1.9 Nm, P < .001), and Stemless-C (3.9 Nm, P = .01). When comparing torsional testing results of low-BMD samples, both Standard stem and Short stem failed at greater torque than Stemless-B (P = .02, P = .003) and Stemless-A (P = .03, P = .004) but failed at a similar torque to Stemless-C. Torsional testing of high-BMD samples showed that Standard stem failed at a greater torque than all stemless designs.ConclusionStemless humeral implants should be used with caution in low-BMD settings (<0.35 g/cm²). A central screw and peripheral rim-fit stemless anchor design demonstrated greater fixation strength at low BMD when compared with other designs, while all stemless designs performed similarly at high BMD.Level of evidenceBasic Science Study; Cadaveric Study     

Cementless fixation of radial head implants is affected by implant stem geometry: An in vitro study

Background

Metal radial head implants are widely used for the management of unreconstructible fractures. However, the effect of implant stem design on initial fixation and stability is unknown. This study determined the effects of radial head stem geometry on the initial stability of the uncemented implant. It was hypothesized that cortical contact and hence increased fixation would be achieved with an increased stem diameter, stem length and a tapered shape.

Methods

Eleven radii received five implants with differing stem geometries: short and long undersized, short and long optimally sized and short tapered. Inferiorly-directed compressive loads were applied successively to the anterior, posterior, medial and lateral edges of the implants. Maximum contralateral radial head “lift-off” was quantified by the distance between bone and implant surface markers.

Findings

The short and long undersized implants experienced greater micromotion with approximately 100 μm and 160 μm more lift-off respectively (P < 0.02). The remaining optimally sized and tapered implants averaged 50 μm. There was greater motion for the undersized implants loaded at the lateral position (P ? 0.005).

Interpretation

This study shows that filling the diameter of the medullary canal is more important than filling the length of the canal. The taper did not offer any additional advantages compared to the straight stem, suggesting that reaming of bone to accept the taper did not produce enhanced initial purchase. If rigid initial implant fixation is desired with an uncemented radial head implant, a canal-filling stem reduces initial implant micromotion to less than 50 μm which may enhance bone ingrowth.     

微动引起人工关节无菌性松动的实验研究

目的　研究一定参数的微动对骨组织生长分化的影响。方法　用特制的骨微动室模型 ,通过动物实验分别计算在无微动组 (A组 )、微动组 (B组 )、微动后停止微动组 (C组 )长入骨微动室模型中的骨组织占所有新生组织的比例。结果　在A组及C组中可见大量不成熟新生骨形成 ,B组可见少量或无新生骨组织可见 ,但可见大量的纤维结缔组织。A组中 ,骨组织占新生组织的比例为 4 0 65 %± 8 2 5 % ,B组为 6 65 %± 2 3 4 % ,C组为 3 5 5 5 %± 6 96%。结论　一定参数的微动可以显著抑制骨组织的形成而有利于纤维组织的形成。当微动停止后 ,原有的纤维组织又可重新由骨组织替代。     

Tissue ingrowth into titanium and hydroxyapatite-coated implants during stable and unstable mechanical conditions.

Lack of initial mechanical stability of cementless prostheses may be responsible for fibrous tissue fixation of prosthetic components to bone. To study the influence of micromovements on bony ingrowth into titanium alloy (Ti) and hydroxyapatite (HA)-coated implants, a loaded unstable device producing movements of 500 microns during each gait cycle was developed. Mechanically stable implants served as controls. The implants were inserted into the weight-bearing regions of all four femoral condyles in each of seven mature dogs. Histological analysis after 4 weeks of implantation showed a fibrous tissue membrane surrounding both Ti and HA-coated implants subjected to micromovements, whereas variable amounts of bony ingrowth were obtained in mechanically stable implants. The pushout test showed that the shear strength of unstable Ti and HA implants was significantly reduced as compared with the corresponding mechanically stable implants (p less than 0.01). However, shear strength values of unstable HA-coated implants were significantly greater than those of unstable Ti implants (p less than 0.01) and comparable to those of stable Ti implants. The greatest shear strength was obtained with stable HA-coated implants, which was threefold stronger as compared with the stable Ti implants (p less than 0.001). Quantitative determination of bony ingrowth agreed with the mechanical test except for the stronger anchorage of unstable HA implants as compared with unstable Ti implants, where no difference in bony ingrowth was found. Unstable HA-coated implants were surrounded by a fibrous membrane containing islands of fibrocartilage with higher collagen concentration, whereas fibrous connective tissue with lower collagen concentration was predominant around unstable Ti implants. In conclusion, micromovements between bone and implant inhibited bony ingrowth and led to the development of a fibrous membrane. The presence of fibrocartilage and a higher collagen concentration in the fibrous membrane may be responsible for the increased shear strength of unstable HA implants. Mechanically stable implants with HA coating had the strongest anchorage and the greatest amount of bony ingrowth.     

Influences of microgap and micromotion of implant–abutment interface on marginal bone loss around implant neck

ObjectiveTo review the influences and clinical implications of micro-gap and micro-motion of implant-abutment interface on marginal bone loss around the neck of implant.DesignLiteratures were searched based on the following Keywords: implant-abutment interface/implant-abutment connection/implant-abutment conjunction, microgap, micromotion/micromovement, microleakage, and current control methods available. The papers were then screened through titles, abstracts, and full texts.ResultsA total of 83 studies were included in the literature review. Two-piece implant systems are widely used in clinics. However, the production error and masticatory load result in the presence of microgap and micromotion between the implant and the abutment, which directly or indirectly causes microleakage and mechanical damage. Consequently, the degrees of microgap and micromotion further increase, and marginal bone absorption finally occurs. We summarize the influences of microgap and micromotion at the implant-abutment interface on marginal bone loss around the neck of the implant. We also recommend some feasible methods to reduce their effect.ConclusionsClinicians and patients should pay more attention to the mechanisms as well as the control methods of microgap and micromotion. To reduce the corresponding detriment to the implant marginal bone, suitable Morse taper or hybrid connection implants and platform switching abutments should be selected, as well as other potential methods.