The strain distribution in the upper tibia after insertion of two different unicompartmental prostheses.

The strain distribution in the proximal tibia in seven human autopsy specimens was investigated with strain-gauge rosettes attached on the medial proximal aspect of the tibia. The strains measured were about the same on the proximal bone and on the bone in the metaphysis. The direction of the minimal principal strain (compression) was about 45 degrees counterclockwise (left knee). After insertion of a unicompartmental prosthesis medially, a non-constrained prosthesis with a loose meniscus-bearing and a constrained prosthesis, the tensile strain was about four times higher in the most anteromedial gauge. No significant differences were found between prostheses. Tests were also performed with the two prostheses inserted into three plastic models. The constrained prosthesis was more sensitive to outward rotation of tibia versus femur, which made the femoral component climb up the slope of the tibial component and caused a marked change in the strain distribution compared to loading in the neutral position. With the other prosthesis, a malpositioning of the tibial component medially caused the meniscus bearing to lie close to the medial rim of the tibial component. An external rotation of tibia then made the system constrained and dramatically changes the strain distribution.     

Strain distribution in the proximal human femur. An in vitro comparison in the intact femur and after insertion of reference and experimental femoral stems

Six pairs of human cadaver femora were divided equally into two groups one of which received a non-cemented reference implant and the other a very short non-dependent experimental implant. Thirteen strain-gauge rosettes were attached to the external surface of each specimen and, during application of combined axial and torsional loads to the femoral head, the strains in both groups were measured. After the insertion of a non-cemented femoral component, the normal pattern of a progressive proximal-to-distal increase in strains was similar to that in the intact femur and the strain was maximum near the tip of the prosthesis. On the medial and lateral aspects of the proximal femur, the strains were greatly reduced after implantation of both types of implant. The pattern and magnitude of the strains, however, were closer to those in the intact femur after insertion of the experimental stem than in the reference stem. On the anterior and posterior aspects of the femur, implantation of both types of stem led to increased principal strains E1, E2 and E3. This was most pronounced for the experimental stem. Our findings suggest that the experimental stem, which has a more anatomical proximal fit without having a distal stem and cortex contact, can provide immediate postoperative stability. Pure proximal loading by the experimental stem in the metaphysis, reduction of excessive bending stiffness of the stem by tapering and the absence of contact between the stem and the distal cortex may reduce stress shielding, bone resorption and thigh pain.     

聚醚醚酮髋股骨头假体置换术后股骨近段的力学分析

[目的]探讨复合材料住全髋股骨头假体中的应用前景，旨在寻找能与股骨紧密结合、增加股骨近端应力传递的新型假体，期望进一步提高全髋关节置换术的远期疗效。[方法]5对人体新鲜尸体股骨平均分成左右2组，1组行钴铬钼合金（CoCrMo）股骨头假体置换术，另1组行碳纤维增强聚醚醚酮（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。首先记录正常股骨产生的应变分布，然后行2种假体的股骨头置换术，再记录2组标本所产生的应变分布。[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。2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CoCrMo合金假体组更接近正常股骨。[结论]CF／PEEK复合材料股骨头假体能提供术后即刻稳定性和良好的近端载荷传递，因此能进一步减少应力遮挡、骨吸收、骨萎缩，最终避免假体松动失败。     

The trapezio-metacarpal joint: the strain of the ligaments as a function of the thumb position. Study on an enlarged model

INTRODUCTION: The aim of this paper was to develop an enlarged anatomical model of the trapezio-metacarpal joint in order to measure the strains on the ligaments when this joint was passively moved in several directions under constant loading. MATERIAL AND METHOD: A model of the two first rays of the hand was made in polystyrene, at a X3 enlargement, and the ligaments substituted by rubber bands with well characterized mechanical properties so as to reproduce the actual ratio of stiffness (approximately = 10) of the different tissues (bones and ligaments) found in real life. The first metacarpal was moved in 6 directions as described by Ebskov (1970) and Pieron (1973, 1980) using a small spring exerting a constant force (1.5 N) tilted at 30 degrees with respect to the transverse plane. The strain was measured between two white marks for each model ligament and each direction respectively, and the percentage of lengthening was calculated. A statistical study was performed using the non-parametrical Test of Wilcoxon in order to compare the ligament strains obtained in the different directions of loading. RESULTS: The largest strains were observed in the intermetacarpal ligament and in the anterior oblique ligament reaching 26 to 39% in direction J (posteromedial) and in direction L (posterolateral). Deformations of the two parts of the dorsoradial ligament and of the posterior oblique ligament were equal or inferior to 12% and were observed in the other 4 directions: D, F, K, I (Anterolateral, maximal anteposition, anteromedial, medial) and their combinations. CONCLUSION:. These data may be useful for helping the understanding of the biomechanics of the basal joint of the thumb. Nevertheless, we are dealing here with a simplified model, which must be considered with caution if the results are to be applied to the living joint.     

Effects of coronally slotted femoral prostheses on cortical bone strain

The photoelastic method was used to assess the effects on femoral cortical strain of total hip arthroplasty cementless femoral prostheses containing distal coronal slots. Eight cadaveric femurs were tested, although three were eliminated secondary to fractures. Loaded and unloaded cortical strains were determined at 72 points on the implanted femoral cortex and compared with the values obtained in the intact femur. Three different prostheses were sequentially implanted, in a random order, into each femur. The prostheses consisted of a standard solid stem, an identical stem with a coronal slot in its distal one fourth, and an identical stem with a coronal slot in its distal one half. The slotted stems did not enhance axial load transfer to the proximal medial femur but did result in increased proximal medial assembly strains and statistically significant (P < .05) decreased anterior and posterior assembly strains. The increased proximal medial assembly strains are hypothesized to enhance proximal medial femoral loading, while the decreased anterior and posterior assembly strains may minimize operative implantation fractures.     

Alterations in femoral and acetabular bone strains immediately following cementless total hip arthroplasty: an in vitro canine study

Alterations in strain patterns in both the femur and acetabulum of the dog, due to cementless total hip arthroplasty (THA), were evaluated by monitoring cortical bone strains before and immediately following implantation of components, using an accurate in vitro jig, simulating the loading of the canine hip in vivo. Femoral arthroplasty with straight stem, canal-filling, femoral components, made of titanium-based alloy, led to a statistically significant reduction of proximal femoral bone strains, with proximal medical compressive strains reduced, on average, to only 22% of intact strain magnitude (p less than 0.01), even though the prostheses contained collars and were not fixed with cement. Strain alterations on the acetabulum, following cementless THA, were more complex and variable. Laterally, a significant decrease (p less than 0.01) in periacetabular strains to between 20 and 60% of the intact state was observed, while medial wall strains varied inconsistently after arthroplasty. These strain alterations reflect major changes in mechanical loading of both the femur and acetabulum immediately following cementless THA, using components similar to those used clinically in humans. Major reductions in strains on the proximal femur occurred with cementless femoral components, while the change in strains on the acetabulum were variable, and could be a result of variations in fit and placement of components inherent even in well-controlled implantation techniques. The physiologic in vitro loading of the canine femur with the pelvis, used to obtain these data, can be applied to future investigations exploring stress transfer around uncemented components and the relationship of stress transfer to morphological changes of bone, in the canine model.     

Closeness of fit of uncemented stems improves the strain distribution in the femur

Differences in the cross-sectional shapes of intramedullary stems are expected to affect the strains in the femur, a close proximal fit being proposed as particularly advantageous. We compared uncemented femoral stems of different designs, as well as a cemented stem, using the intact femur as a control. The collarless stems were custom-made, standard asymmetric, symmetric (i.e., neutral), and cemented symmetric. These different types of stem were inserted in sequence into eight cadaver femurs. A photoelastic coating technique was used to give a measure of the shear strains over the bone surface. The mean reductions of maximum shear strains in the upper 30 mm of the proximal-medial regions for the custom, asymmetric, symmetric, and cemented stems were 11, 24, 6, and 75%, respectively. The strains for the symmetric stem were concentrated in the proximal-medial region just below the neck resection line and then decreased rapidly below that level. For the asymmetric stems, the relative strains in the proximal-anterior and proximal-medial regions were dependent on the tightness of fit in the anterior-posterior and medial-lateral directions. The custom stems consistently produced a pattern of strain distribution that, overall, was closer to normal than that of the other stems; extremes at the anterior and medial regions were avoided. The strains down the entire medial side of the cemented stem were significantly less than for the intact femur or for the uncemented stems.     

Loosening of sacral screw fixation under in vitro fatigue loading.

Sacral screw fixation is frequently used for fusion of the lower lumbar spine, but sacral screws appear to offer less secure fixation than lumbar pedicle screws, and failure due to loosening under fatigue loading is common. The aim of this study was to examine in vitro the stability of medial and lateral bicortical and unicortical sacral screw fixation under a physiologically relevant fatigue-loading pattern. Bone mineral density, screw insertion torque, and screw-fixation stiffness were measured prior to cyclic loading between 40 and 400 N compression at 2 Hz for 20,000 cycles. The screw-fixation stiffness was measured every 500 cycles, and the axial pullout strength of the screws was recorded following loading. All of the lateral insertions loosened under the applied loading, but some of the medial insertions remained stable. Medial insertions proved stiffer and stronger than lateral insertions, and bicortical fixations were stronger than unicortical fixations. Bone mineral density and insertion torque were correlated with screw stiffness and pullout strength, although better correlation was found for insertion torque than bone mineral density. Bone mineral density is a good preoperative indicator of sacral screw-fixation strength, and insertion torque is a good intraoperative indicator. An insertion torque greater than 1.5 Nm is suggested as an indicative value for a stable medial unicortical insertion, whereas an insertion torque greater than 2 Nm suggests a stable medial bicortical insertion. It appears that, apart from the choice of technique (screw orientation and depth), minimizing the load on the screws during the initial part of the fusion process is also critical to maintain stability of the fused section and to obtain a solid fusion mass.     

Effect of press-fit femoral stems on strains in the femur. A photoelastic coating study

A photoelastic coating method was used to study the strain patterns on the surface of the femur, before and after insertion of femoral stems. The anatomically shaped stems were press-fit without a collar, press-fit with a collar, and press-fit with proximal cementing to approximate a bone ingrowth situation. The strain patterns of the intact femurs were consistent with bending. The pattern changed considerably after stem insertion, probably due to the stiffening effect of the stems. Nevertheless, the collarless press-fit stem preserved 64% of the magnitude of the proximal medial strain. The collar restored the average to normal, but the surface strains varied because of the localized regions of contact between the collar and the bone. Proximal cementing gave results similar to those of the collarless press-fit. The press-fit stems often showed local patches of high strain, probably reflecting local endosteal contact points. Local high stresses at the level of the distal tip were seen in only a few instances.     

Proximal strain distribution in the loaded femur. An in vitro comparison of the distributions in the intact femur and after insertion of different hip-replacement femoral components

The distribution of strain in the proximal part of loaded cadaver femora was measured in vitro using strain gauges applied to the cortex. The loading conditions simulated single-limb stance and the strains were recorded first with the femora intact and then with the femoral components of six different designs inserted. Each femur served as its own control. After insertion of a femoral component, the pattern of strain in the proximal part of the femur was reversed compared with that in the intact femur, in that the maximum strain occurred around the tip of the prosthesis rather than at the calcar femorale. A massive decrease in stress in the region of the calcar femorale was found when the implants were in place, and it was concluded that this decrease could contribute substantially to the calcar femorale resorption sometimes observed in patients after total hip replacement. Transfer of load directly to the calcar femorale through a larger collar in direct contact with the cortical bone restored 30 to 40 per cent of the normal strain to the calcar femorale and shifted the strain pattern toward normal. Compared with the less stiff stems tested, the larger, stiffer stems, which provide more protection against fatigue failure, did not affect the strain pattern adversely.     

In situ calibration of miniature sensors implanted into the anterior cruciate ligament. Part I: Strain measurements

The goals of this study were to (a) evaluate the differential variable reluctance transducer as an instrument for measuring tissue strain in the anteromedial band of the anterior crudciate ligament, (b) develop a series of calibration curves (for simple states of knee loading) from which resultant force in the ligament could be estimated from measured strain levels in the anteromedial band of the ligament, and (c) study the effects of knee flexion angle and mode of applied loading on ouput from the transducer. Thirteen fresh-frozen cadaveric knee specimens underwent mechanical isolation of a bone cap containing the tibial insertion of the anterior cruciate ligament and attachment of a load cell to measure resultant force in the ligament. The transducer (with barbed prongs) was inserted into the anteromedial band of the anterior cruciate ligament to record local elongation of the instrumented fibers as resultant force was generated in the ligament. A series of calibration curves (anteromedial bundle strain versus resultant force in the anterior cruciate ligament) were determined at selected knee flexion angles as external loads were applied to the knee. During passive knee extension, strain readings did not always follow the pattern of resultant force in the ligament; erratic strain readings were often measured beyond 20° of flexion, where the anteromedial band was slack. For anterior tibial loading, the anteromedial band was a more active contributor to resultant ligament force beyond 45° of flexion and was less active near full extension; mean resultant forces in the range of 150–200 N produced strain levels on the order of 3–4%. The anteromedial band was also active during application of internal tibial torque; mean resultant forces on the order of 180–220 N produced strains on the order of 2%. Resultant forces generated by varus moment were relatively low, and the anteromedial band was not always strained. Mean coefficients of variation for resultant force in the ligament (five repeated measurements) ranged between 0.038 and 0.111. Mean coefficients of variation for five repeated placements of the strain transducer in the same site ranged from 0.209 to 0.342. Insertion and removal of this transducer at the anteromedial band produced observable damage to the ligament. In our study, repeatable measurements were possible only if both prongs of the transducer were sutured to the ligament fibers.     

Effect of cementless bowed stem distal surface contour and coronal slot on femoral bone strains and torsional stability

Six pairs of unembalmed cadaver femurs were instrumented with strain gauges and prepared with flexible reamers for insertion of long bowed cementless femoral stems. A fully porous-coated cobalt chrome stem was inserted into each left femur, and a distally fluted, slotted, stem of the same implant geometry and implant material was inserted into each right femur. Bone strains were measured during stem insertion and torsional stability tested after the stems were fully seated. Distal strains were significantly higher (p < 0.05) for the fully porous compared to the distally fluted, slotted stem. Three fractures occurred in femurs with fully porous-coated stems. There was no difference in torsional stability between the two stem geometries. Our data demonstrate that a long bowed cementless stem with a distal coronal slot and flutes is associated with decreased bone strains and fracture risk during stem insertion compared to a fully porous-coated stem.     

Strain in the human medial collateral ligament during valgus loading of the knee

The medial collateral ligament is one of the most frequently injured ligaments in the knee. Although the medial collateral ligament is known to provide a primary restraint to valgus and external rotations, details regarding its precise mechanical function are unknown. In this study, strain in the medial collateral ligament of eight knees from male cadavers was measured during valgus loading. A material testing machine was used to apply 10 cycles of varus and valgus rotation to limits of +/- 10.0 N-m at flexion angles of 0 degrees, 30 degrees, 60 degrees, and 90 degrees. A three-dimensional motion analysis system measured local tissue strain on the medial collateral ligament surface within 12 regions encompassing nearly the entire medial collateral ligament surface. Results indicated that strain is significantly different in different regions over the surface of the medial collateral ligament and that this distribution of strain changes with flexion angle and with the application of a valgus torque. Strain in the posterior and central portions of the medial collateral ligament generally decreased with increasing flexion angle, whereas strain in the anterior fibers remained relatively constant with changes in flexion angle. The highest strains in the medial collateral ligament were found at full extension on the posterior side of the medial collateral ligament near the femoral insertion. These data support clinical findings that suggest the femoral insertion is the most common location for medial collateral ligament injuries.     

Initial effect of collarless stem stiffness on femoral bone strain

Stress shielding resulting from a stiffness mismatch between bone and femoral prosthesis stems (leading to bone resorption in the proximal femur) is believed to contribute to failure in total hip arthroplasty. In this study, strains were measured under compressive femoral head loads both in the intact femur and after implanting first a collarless steel stem and then a geometrically identical fiber-reinforced polymer composite stem 64% less stiff. Decreasing stem stiffness would be expected increase load transfer from the stem to the proximal medial femur, decreasing the degree of stress shielding. The authors found that proximal medial bone strains were significantly lower with either the steel or composite stem implanted than in the intact case. However, there were no significant differences in strain patterns between the steel and composite stem cases. This apparent insensitivity to prosthesis stiffness may result from factors related to implant geometry and fit.     

A biomechanical evaluation of the Gamma nail.

We examined the effect of the Gamma nail on strain distribution in the proximal femur, using ten cadaver femora instrumented with six unidirectional strain gauges along the medial and lateral cortices. The femora were loaded to 1800 N and strains were determined with or without distal interlocking screws before and after experimentally created two-part and four-part fractures. Motion of the sliding screw and the nail was also determined. Strain patterns and screw motion were compared with previously obtained values for a sliding hip screw device (SHS). The Gamma nail was shown to transmit decreasing load to the calcar with decreasing fracture stability, such that virtually no strain on the bone was seen in four-part fractures with the posteromedial fragment removed; increasing compression was noted, however, at the proximal lateral cortex. Conversely, the SHS showed increased calcar compression with decreasing fracture stability. The insertion of distal interlocking screws did not change the pattern of proximal femoral strain. The Gamma nail imparts non-physiological strains to the proximal femur, probably because of its inherent stiffness. These strains may alter bone remodelling and interfere with healing. Distal interlocking screws may not be necessary for stable intertrochanteric fractures.     

Biomechanical analysis and quantitative analysis of bone scintigraphy on thrust plate hip prosthesis

Introduction To study the load distribution of the thrust plate hip prosthesis (TPP, Sulzer Medica, Baar, Switzerland) on femoral bone, a TPP insertion group and intramedullary cemented stem insertion group were prepared with the use of embalmed femoral anatomic specimens, and strain on the femoral bone induced by loading was measured.Materials and methods Bone scintigraphy was performed in 26 joints which underwent total hip arthroplasty with the use of TPP, to enable quantitative evaluation of postoperative remodeling of the bone.Results In the TPP insertion group, the strain was almost equivalent to that in the untreated femoral control group. On the other hand, compared with the control group and TPP insertion group, strain in the proximal portion of the femoral bone (medial side of the femoral neck) was significantly smaller in the cemented stem group (control group versus cemented stem insertion group, p=0.0062; cemented stem insertion group versus TPP 130° insertion group, p=0.0005; cemented stem insertion group versus TPP 140° insertion group, p=0.0031). Bone scintigrams revealed decreased uptake at 12 weeks after operation compared with 6 weeks after operation, indicating that TPP allows earlier implant stabilization than the conventional stem.Conclusion Compared with the conventional stem, TPP is a prosthesis that can perform physiological load transmission.     

Changes in proximal femoral strain after insertion of uncemented standard and customised femoral stems. An experimental study in human femora

We have compared the changes in the pattern of the principal strains in the proximal femur after insertion of eight uncemented anatomical stems and eight customised stems in human cadaver femora. During testing we aimed to reproduce the physiological loads on the proximal femur and to simulate single-leg stance and stair-climbing. The strains in the intact femora were measured and there were no significant differences in principal tensile and compressive strains in the left and right femora of each pair. The two types of femoral stem were then inserted randomly into the left or right femora and the cortical strains were again measured. Both induced significant stress shielding in the proximal part of the metaphysis, but the deviation from the physiological strains was most pronounced after insertion of the anatomical stems. The principal compressive strain at the calcar was reduced by 90% for the anatomical stems and 67% for the customised stems. Medially, at the level of the lesser trochanter, the corresponding figures were 59% and 21%. The anatomical stems induced more stress concentration on the anterior aspect of the femur than did the customised stems. They also increased the hoop strains in the proximomedial femur. Our study shows a consistently more physiological pattern of strain in the proximal femur after insertion of customised stems compared with standard, anatomical stems.     

Effect of femoral stem length on stress raisers associated with revision hip arthroplasty

The objective of this study was to experimentally determine the optimal length of a femoral component in revision total hip arthroplasty (THA). Embalmed cadaveric femurs were loaded in a physiologic manner, and strains on the lateral cortex were measured. Two kinds of defects were tested to simulate THA after removal of a nail plate and after removal of a loose femoral stem. A drill hole was made in the lateral cortex of the femur to simulate the removal of a nail plate. A reaming defect was made, using flexible reamers to thin the cortex from the lesser trochanter distally to a site corresponding to the tip of a standard femoral component, to simulate THA after removal of a previously inserted femoral stem. Femurs were tested intact, with the defects, and after insertion of femoral components with stem lengths of 100 to 250 mm. The strain increased with the creation of a defect and decreased with the insertion of an implant. For a femur with a defect, the strain was minimized when the stem length extended 1.5 femoral diameters past the defect.     

In vivo measurements show tensile axial strain in the proximal lateral aspect of the human femur

Two conflicting theories exist concerning the stress pattern for the proximal lateral aspect of the human femur. According to the classic theory of Pauwels, a bending moment on the femur leads to compression medially and to tension laterally. The alternative theory is that muscle forces contribute to a moment-free loading of the femur, with both the medial and lateral cortices subjected to compression. To examine these theories, we measured the strain at the external surface of the proximal lateral aspect of the femur of two female patients undergoing surgery for “snapping hip syndrome.” During the surgical procedure, a strain-gauge rosette was bonded to the lateral aspect of the femur and the cortical strains were monitored while the patient performed a series of activities. In both patients, principle tensile strain increased significantly during one-legged stance, walking, and stair climbing as compared with that during two-legged stance. During each loading situation, the principal tensile strain was aligned within 22° to the longitudinal femoral axis. Dynamic strain measurements consistently revealed tensile axial strain at the lateral aspect of the femur during each activity. The present studv supports the classic bending theory of Pauwels and demonstrates that the proximal lateral aspect of the femur is subjected to tension during the stance phase of gait.     

Medial collateral ligament insertion site and contact forces in the ACL-deficient knee.

The objectives of this research were to determine the effects of anterior cruciate ligament (ACL) deficiency on medial collateral ligament (MCL) insertion site and contact forces during anterior tibial loading and valgus loading using a combined experimental-finite element (FE) approach. Our hypothesis was that ACL deficiency would increase MCL insertion site forces at the attachments to the tibia and femur and increase contact forces between the MCL and these bones. Six male knees were subjected to varus-valgus and anterior-posterior loading at flexion angles of 0 degrees and 30 degrees. Three-dimensional joint kinematics and MCL strains were recorded during kinematic testing. Following testing, the MCL of each knee was removed to establish a stress-free reference configuration. An FE model of the femur-MCL-tibia complex was constructed for each knee to simulate valgus rotation and anterior translation at 0 degrees and 30 degrees, using subject-specific bone and ligament geometry and joint kinematics. A transversely isotropic hyperelastic material model with average material coefficients taken from a previous study was used to represent the MCL. Subject-specific MCL in situ strain distributions were used in each model. Insertion site and contact forces were determined from the FE analyses. FE predictions were validated by comparing MCL fiber strains to experimental measurements. The subject-specific FE predictions of MCL fiber stretch correlated well with the experimentally measured values (R2 = 0.95). ACL deficiency caused a significant increase in MCL insertion site and contact forces in response to anterior tibial loading. In contrast, ACL deficiency did not significantly increase MCL insertion site and contact forces in response to valgus loading, demonstrating that the ACL is not a restraint to valgus rotation in knees that have an intact MCL. When evaluating valgus laxity in the ACL-deficient knee, increased valgus laxity indicates a compromised MCL.