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Due to their high stiffness, metal femoral implants in total knee arthroplasty may cause stress shielding of the peri-prosthetic bone, which can lead to loss of bone stock. Using a polymer (PEEK) femoral implant reduces the stiffness mismatch between implant and bone, and therefore has the potential to decrease strain shielding. The goal of the current study was to evaluate this potential benefit of PEEK femoral components in cadaveric experiments. Cadaveric femurs were loaded in a materials testing device, while a 3-D digital image correlation set-up captured strains on the surface of the intact femurs and femurs implanted with PEEK and CoCr components. These experimental results were used to validate specimen-specific finite element models, which subsequently were used to assess the effect of metal and PEEK femoral components on the bone strain energy density. The finite element models showed strain maps that were highly comparable to the experimental measurements. The PEEK implant increased strain energy density, relative to the preoperative bone and compared to CoCr. This was most pronounced in the regions directly under the implant and near load contact sites. These data confirm the hypothesis that a PEEK femoral implant can reduce peri-prosthetic stress shielding.

Contains fulltext : 173922.pdf (Publisher’s version ) (Closed access) Post-operative changes in trabecular bone morphology at the cement-bone interface can vary depending on time in service. This study aims to investigate how micromotion and bone strains change at the tibial bone-cement interface before and after cementation. This work discusses whether the morphology of the post-mortem interface can be explained by studying changes in these mechanical quantities. Three post-mortem cement-bone interface specimens showing varying levels of bone resorption (minimal, extensive and intermediate) were selected for this study Using image segmentation techniques, masks of the post-mortem bone were dilated to fill up the mould spaces in the cement to obtain the immediately post-operative situation. Finite element (FE) models of the post-mortem and post-operative situation were created from these segmentation masks. Subsequent removal of the cement layer resulted in the pre-operative situation. FE micromotion and bone strains were analyzed for the interdigitated trabecular bone. For all specimens micromotion increased from the post-operative to the post-mortem models (distally, in specimen 1: 0.1 to 0.5microm; specimen 2: 0.2 to 0.8microm; specimen 3: 0.27 to 1.62microm). Similarly bone strains were shown to increase from post-operative to post-mortem (distally, in specimen 1: -185 to -389microepsilon; specimen 2: -170 to -824microepsilon; specimen 3: -216 to -1024microepsilon). Post-mortem interdigitated bone was found to be strain shielded in comparison with supporting bone indicating that failure of bone would occur distal to the interface. These results indicate that stress shielding of interdigitated trabeculae is a plausible explanation for resorption patterns observed in post-mortem specimens.

The success of a total hip arthroplasty is strongly related to the initial stability of the femoral component and to the stress shielding effect. In fact, for cementless stems, initial stability is essential to promote bone ingrowth into the stem coating. An inefficient primary stability is also a cause of thigh pain. In addition, the bone adaptation after the surgery can lead to an excessive bone loss and, consequently, can compromise the success of the implant. These factors depend on prosthesis design, namely on material, interface conditions and shape. Although, surgeons use stems with very different geometries, new computational tools using structural optimization methods have been used to achieve a better design in order to improve initial stability and therefore, the implant durability. In this work, a multi-criteria shape optimization process is developed to study the relationship between implants performance and geometry. The multi-criteria objective function takes into account the initial stability of the femoral stem and the effect of stress shielding on bone adaptation after the surgery. Then, the optimized stems are tested using a concurrent model for bone remodeling and osseointegration to evaluate long-term performance. Additionally, the sensitivity to misalignments is analyzed, since femoral stems are often placed in varus or valgus position. Results show that the different criteria are contradictory resulting in different characteristics for the hip stem. However, the multi-criteria formulation leads to compromise solutions, with a combination of the geometric characteristics obtained for each criterion separately.

This bibliographic review is focused on ligament tissue rehabilitation, its anatomy-physiology, and, mainly, on the dimensioning considerations of a composite material solution. The suture strength is problematic during the tissue recovering, implying reduction of mobility for several months. However, early postoperative active mobilization may enable a faster and more effective recovering of tissue biomechanical functions. As the risk of tendon rupture becomes a significant concern, a repair technique must be used to withstand the tensile forces generated by active mobilization. However, to avoid stress shielding effect on ligament tissue, an augmentation device must be designed on stiffness basis, that preferably will decrease. Absorbable biocomposite reinforcements have been used to allow early postoperative active mobilization and avoid the shortcomings of current repair solutions. Tensile strength decrease of the repair, during the initial inflammatory phase, is expected, derived from oedema and tendon degradation. In the fibroblastic phase, stiffness and strength will increase, which will stabilize during the remodeling phase. The reinforcement should be able to carry the dynamic load due to locomotion with a mechanical behavior similar to the undamaged natural tissue, during all rehabilitation process. Moreover, the degradation rate must also be compatible with the ligament tissue recovering. The selection and combination of different biodegradable materials, in order to make the biocomposite reinforcement functionally compatible to the damaged sutured tissue, in terms of mechanical properties and degradation rate, is a major step on the design process. Modelling techniques allow pre-clinical evaluation of the reinforcement functional compatibility, and the optimization by comparison of different composite solutions in terms of biomechanical behavior.

Central third of patellar tendon (PT) is used as an autograft for anterior cruciate ligament (ACL) reconstruction. Previous studies investigated temporal changes in material properties of healing tissues in PT after resection of the central third. However, no study has been performed on effects of stress shielding (SS) and restressing (RS) on the properties of healing tissues. The present study hypothesised that SS adversely affects the mechanical integrity of healing tissues, which is recovered by subsequent RS. An entire rectangular defect was created in the central third of rabbit PT. Operated PTs were subjected to either SS or no stress shielding (NSS). A subgroup of stress-shielded PTs was followed by the resumption of normal loading, namely RS. Tensile properties of tissues regenerated in the defect and residual tendons were evaluated. Regenerated tissues of SS for 3 weeks resulted in significantly lower strength than NSS, which was recovered to NSS level by 3 weeks of RS. Strength of residual tissues in RS reversed SS effects, leading to the strength at NSS level after 12 weeks. However, tangent modulus of residual tissues in RS was still significantly lower than that of NSS at 12 weeks. Therefore, SS induces detrimental effects on the mechanical integrity of healing PTs, and the response to RS was different between regenerate and residual tissues, the latter of which took longer period to reach NSS level.

This communication reports important preliminary results of a parametric analysis into the stress shielding effects of loading conditions and material properties of a total knee replacement (TKR) prosthesis. A previously developed finite element (FE) model of the proximal tibia that incorporated orthotropic and heterogeneous bone properties was used. Tibiofemoral joint compression and soft tissue (ligament and muscle) forces were also included to better represent the loading condition in the tibia. Stress shielding effects were studied for a prosthesis similar to a commercially available model. Results from the model show that the hypothesis of relatively higher Young's modulus of implant compared to bone as the primary cause of stress shielding is not sufficiently descriptive. Loading conditions as a result of altered bone or implant condylar surface geometry, load placement on the condylar surface, and load pattern created by the TKR are at least as important or, in some cases, more important factors in observed stress shielding immediately post-operation. This finding can be used to focus new implant design on altered loading conditions as well as material selection.

Previous studies have shown that, in the stress-shielded patellar tendon, the mechanical properties of the tendon were dramatically reduced and TGF-beta was over-expressed in tendon fibroblasts. In the present study, therefore, we tested two supportive hypotheses using 40 rabbits: One was that an application of TGF-beta1 might significantly increase the tensile strength and the tangent modulus of the stress-shielded patellar tendon. The other one was that an administration of anti-TGF-beta1 antibody might significantly reduce the mechanical properties of the stress-shielded patellar tendon. In the results, an application of 4-ng TGF-beta1 significantly increased the tangent modulus of the stress-shielded patellar tendon at 3 weeks ( p = 0.019 ), compared with the sham treatment. Concerning the tensile strength, the 4-ng TGF-beta1 application increased the average value, but a statistical significance was not reached. An application of 50-μg anti-TGF-beta1 antibody significantly reduced the tangent modulus and the tensile strength of the stress-shielded patellar tendon at 3 weeks ( p = 0.0068 and p = 0.0355 ), compared with the sham treatment. Because the stress-shielding treatment used in this study dramatically reduces the tangent modulus and the tensile strength of the patellar tendon, the present study suggested that an administration of TGF-beta1 weakly but significantly inhibited the reduction of the mechanical properties of the stress-shielded patellar tendon, and that inactivation of TGF-beta1 with its antibody significantly enhanced the reduction of the mechanical properties that occurs in the stress-shielded patellar tendon. These results suggested that TGF-beta1 plays an important role in remodeling of the stress-shielded patellar tendon.

This communication reports the results of a three-dimensional finite element (FE) model of stresses in a surgically altered femur and tibia. The model incorporated a novel approach in implementing orthotropic and inhomogeneous bone properties and non-uniform distributed loading. Cortical, cancellous, and subchondral bone of the femur and the tibia were modeled. Mechanical properties for the cortical and cancellous bone were mapped from published data characterizing the anisotropy and inhomogeneity of the bone properties. Mesh adequacy was determined using stress convergence and strain energy error convergence. Qualitatively, the results of the study compare well with experimental principal compressive strains from the literature. With respect to tunnel placement in anterior cruciate ligament reconstruction, the model predicted stress-shielding at the postero-lateral region of the tunnel wall, and increased stress at the postero-medial region of the tunnel wall. The stresses in the cancellous bone beneath the tunnel were, in general, lower than those above the tunnel. Prolonged stress shielding leads to bone resorption of the posterior tunnel wall leading to tunnel enlargement, and possible compromise of the ACL reconstruction. The stresses on the femoral cortex produced from a button-type fixation were noticeable for low levels of loading; the stress levels were very similar in models incorporating bone properties of patients aged 45 and 65. Repeated compression of the femoral cortex at these stress levels may cause microdamage to the cortex eventually resulting in fatigue failure.

All hip replacement prostheses alter the load transfer from the hip joint into the femur by changing the mechanical loading of the proximal femur from an externally to an internally loaded system. This alteration of the load transfer causes stress shielding and might lead to severe bone loss, especially with uncemented prostheses. To minimize these effects, load transfer to the femur should occur as proximal as possible. In order to support sufficient primary stability, however, directly post operative (PO) distal stabilization is reasonable. Consequently, the prostheses have to alter its mechanical characteristics after implantation. This concept is referred to as load-shift concept. Primary stability during the early PO state is provided by a prosthesis shaft, which is widened at the tip by a biodegradable pin. After resorption of the pin load transfer occurs no longer distally. The objective of this study was the numerical evaluation of the load-shift concept. The analysis was performed with a finite element model. Three-dimensional non-linear dynamic gait analyses data were used to evaluate whether the load transfer during walking can be altered effectively by insertion and resorption of a distal pin. Directly PO 38% of the transverse load is transferred through the prosthesis shaft and micromotion of the proximal prostheses tip is below 55 μm. After resorption of the pin, no transverse loads are transferred through the prosthesis shaft. Therefore, the loading of the proximal bone tissue is far more pronounced than in the case of a standard prosthesis, demonstrating the feasibility of the load-shift concept. A balanced degradation of the pin simultaneously with the ingrowth of the prosthesis is expected to reduce hip replacement complications.

Implant fixation implies a strong and durable mechanical bond between the prosthetic component and host skeleton. Assuming the short-term impediments to implant fixation are successfully addressed and that longer-term issues such as late infection and mechanical failure of the components are avoided, the biological response of the host tissue to the presence of the implant is critical to long-term success. In particular, maintenance of adequate peri-prosthetic bone stock is a key factor. Two major causes of bone loss in the supporting bone are adverse bone remodeling in response to debris shed from the implant and stress-shielding. Here, I review some of the major lessons learned from studying stress-shielding-induced bone loss. It is well known that stress-shielding can be manipulated by altering implant design, but less well appreciated that the development of bone anabolic agents may make it possible to reduce the severity of stress-shielding and the associated bone loss by augmenting the host skeleton through the use of locally or systemically delivered agents. In most cases, mechanical, material and biological factors do not act in isolation, emphasizing that it is often not possible to optimize all boundary conditions.

Cementless surface replacement arthroplasty (CSRA) of the shoulder was designed to preserve the individual anatomy and humeral bone stock. A matter of concern in resurfacing implants remains the stress shielding and bone remodeling processes. The bone remodeling processes of two different CSRA fixation designs, conical-crown (Epoca RH) and central-stem (Copeland), were studied by three-dimensional (3-D) finite element analysis (FEA) as well as evaluation of contact radiographs from human CSRA retrievals. FEA included one native humerus model with a normal and one with a reduced bone stock quality. Compressive strains were evaluated before and after virtual CSRA implantation and the results were then compared to the bone remodeling and stress-shielding pattern of eight human CSRA retrievals (Epoca RH n =4 and Copeland n =4). FEA revealed for both bone stock models increased compressive strains at the stem and outer implant rim for both CSRA designs indicating an increased bone formation at those locations. Unloading of the bone was seen for both designs under the central implant shell (conical-crown 50–85%, central-stem 31–93%) indicating high bone resorption. Those effects appeared more pronounced for the reduced than for the normal bone stock model. The assumptions of the FEA were confirmed in the CSRA retrieval analysis which showed bone apposition at the outer implant rim and stems with highly reduced bone stock below the central implant shell. Overall, clear signs of stress shielding were observed for both CSRAs designs in the in vitro FEA and human retrieval analysis. Especially in the central part of both implant designs the bone stock was highly resorbed. The impact of these bone remodeling processes on the clinical outcome as well as long-term stability requires further evaluation.

The natural stress distribution in the femur is significantly altered after total hip arthroplasty (THA). When an implant is introduced, it will carry a portion of the load, causing a reduction of stress in some regions of the remaining bone. This phenomenon is commonly known as stress shielding. In response to the changed mechanical environment the shielded bone will remodel according to Wolff's law, resulting in a loss of bone mass through the biological process called resorption. Resorption can, in turn, cause or contribute to loosening of the prosthesis. The problem is particularly common among younger THA recipients. This study explores the hypothesis that through redesign, a total hip prosthesis can be developed to substantially reduce stress shielding. First, we describe the development of a new femoral hip prosthesis designed to alleviate this problem through a new geometry and system of proximal fixation. A numerical comparison with a conventional intramedullary prosthesis as well as another proximally fixed prosthesis, recently developed by Munting and Verhelpen (1995. Journal of Biomechanics 28(8), 949-961) is presented. The results show that the new design produces a more physiological stress state in the proximal femur.

Living tissues and organs are dynamic and change their mechanical properties and structure in response to stress alteration as a phenomenon of functional adaptation and optimal operation. This phenomenon is called 'Tissue Remodeling', and Wolff's law on bone remodeling is widely known. Several recent studies have shown that fibrous connective tissues such as tendons and ligaments also have the ability of remodeling. However, relatively little is known about the stress and motion effects on tissue homeostasis in biological soft tissues. This article primarily deals with changes of the biomechanical properties of knee joint tendons and ligaments through a wide variety of treatment modalities, including stress deprivation, recovery after stress deprivation, and stress enhancement. The experimental results indicate that tendons and ligaments have an ability to adapt in response to the change of stress if the extent of stress alteration is within allowable ranges.

Given the relatively short survival time of total hip replacement in young patients, a different concept of femoral implant was designed and evaluated biomechanically prior to clinical application. The femoral component has no intramedullary stem and fits into an angular resection of the femoral neck. A trans-trochanteric screw provides immediate strong fixation. A comparative biomechanical study with cadaver femora and a dynamic hip simulator was performed. The study had three objectives. (1) The strains generated under loading in the intact proximal femur were compared to those generated in the presence of the experimental implant and a classic cemented implant. (2) The short-term stability of the experimental prosthesis under loading conditions encountered during fast walking was evaluated. (3) Most importantly, the implant design was improved in the light of the experimental observations prior to any clinical trial. Indeed, the prototype underwent several important modifications in the course of the study. With the modified design, later used in a clinical trial, it was demonstrated that if accurate fit and position of the prosthesis were provided, fixation remained sound for over one million loading cycles with peak loads of three times the body weight. The measured interface micromotion was compatible with direct bone apposition and ingrowth. No statistical difference was found between the strain measurements obtained before and after implantation of the experimental prosthesis whereas significant stress shielding was observed with a stemmed, cemented prosthesis.

This work is focused on the in vitro simulation of the loads occurring in the femur during early stance in gait, for hip prostheses stress shielding test purposes. Ten thigh muscles (the three gluteal muscles, the three vasti, rectus femoris, adductor longus and magnus, biceps femoris), simulated by nylon straps, were tested in order to establish their influence on the strains in the proximal femur. Axial and hoop strains were recorded from 16 strain gauges for the effect of each muscle and compared to the strains recorded as a result of the hip joint reaction force only (i.e. without muscle simulation). It appears that the three glutei are the principal muscles in determining the vertical strains, however the rectus femoris, biceps femoris and the adductors were also seen to significantly affect the strain pattern. The inadequacy of increasing the adduction angle and applying the resultant force at the hip joint to simulate the abductors was also confirmed.

A major problem threatening the long-term integrity of total hip replacement is the loss of proximal bone often found around noncemented stems in the long term. It is generally accepted that 'stress shielding' is the cause for this problem: after implantation of the prosthesis the surrounding bone is partially 'shielding' from load carrying and starts to resorb. One of the proposed answers to this problem is the application of press-fitted stems. These smooth-surfaced implants are thought to provoke higher proximal bone loading, and, hence, less stress shielding than bonded implants, because they are wedged into the femur every time when loaded. However, in a two-year experiment in dogs, similar amounts of resorption of the proximal cortex were found around press-fitted and bonded implants. The question arises how similar resorption patte! rns can develop under completely different stress conditions, and whether this phenomenon can be explained by adaptive bone remodeling theories based on Wolff's law. In the present study an answer was sought for this question. An advanced iterative computer simulation model was used to analyze the remodeling process in the animal experiment. Three-dimensional finite element models were constructed from the animal experimental configuration, in which smooth, press-fitted stems were applied unilaterally in the canine. The FE model was integrated with iterative remodeling procedures, validated in earlier studies. In the model an appropriate non-linear representation of the loose bone-implant interface was realized, also capable of simulating the proximal interface gap that was found around the uncoated implants. The simulation models predicted similar amounts of proximal bone loss and distal bone densification as found in the animal model. Hence, the cortical bone loss could i! ndeed be predicted by the strain-adaptive bone remodeling theory. By unraveling the simulation process, the question stated above could be answered. Densification of the distal bone bed during the initial remodeling process was found to cause reduced axial stem displacement (elastic subsidence), decreasing the wedging effect of the stem and, hence, decreasing the loading of the proximal bone, resulting in proximal bone loss. Hence, whereas in the case of bonded stems the proximal resorption process develops monotonously to a new equilibrium, the process around smooth, press-fitted stems develops nonmonotonously. This is due primarily to the unbonded interface conditions and the development of a proximal fibrous membrane. The remodeling process then gradually causes the stem to be jammed in the distal diaphyses (proximal 'stress bypass').

Since the late 1980s, computational analysis of total hip arthroplasty (THA) prosthesis components has been completed using macro-level bone remodeling algorithms. The utilization of macro-sized elements requires apparent bone densities to predict cancellous bone strength, thereby, preventing visualization and analysis of realistic trabecular architecture. In this study, we utilized a recently developed structural optimization algorithm, design space optimization (DSO), to perform a micro-level three-dimensional finite element bone remodeling simulation on the human proximal femur pre- and post-THA. The computational simulation facilitated direct performance comparison between two commercially available prosthetic implant stems from Zimmer Inc.: the Alloclassic and the Mayo conservative. The novel micro-level approach allowed the unique ability to visualize the trabecular bone adaption post-operation and to quantify the changes in bone mineral content by region. Stress-shielding and strain energy distribution were also quantified for the immediate post-operation and the stably fixated, post-remodeling conditions. Stress-shielding was highest in the proximal region and remained unchanged post-remodeling; conversely, the mid and distal portions show large increases in stress, suggesting a distal shift in the loadpath. The Mayo design conserves bone mass, while simultaneously reducing the incidence of stress-shielding compared to the Alloclassic, revealing a key benefit of the distinctive geometry. Several important factors for stable fixation, determined in clinical evaluations from the literature, were evident in both designs: high levels of proximal bone loss and distal bone densification. The results suggest this novel computational framework can be utilized for comparative hip prosthesis shape, uniquely considering the post-operation bone remodeling as a design criterion.

The presence of a femoral prosthesis superior to a shaft fracture severely complicates fixation and treatment. This study uses two-dimensional, multithickness, plane stress finite-element models of a femur with prosthesis to investigate the stresses developed with the application of three popular fixation techniques: revision to a long stem prosthesis, lateral plating with a cortical bone allograft strut and cerclage wires, and custom plate application with proximal Parham band fixation with distal cortical screws (Ogden plate). The plate and bone contact as well as the fracture site contact were modelled by using orthotropic elements with custom-fit moduli so that only the normal stress to the interface was significant. A thermal analogy was used to model the cerclage and Parham band preloads so that representative preloads in the proximal fixation of the two types of plate treatments could be modelled. A parametric study was performed with the long-prosthesis model to show variations in stem lengths of one, two and three femoral diameters distal to the fracture site. The Ogden plate model showed a transfer of tensile stress near the proximal-most band, with the highest tensile stress being at the fracture site with evidence of stress shielding of the proximal lateral cortex. The cortical bone strut model showed a transfer of tensile stress to the bone strut but showed less shielding of the proximal cortex. The cerclage wires at the base of the bone strut showed the highest changes in load with the distalmost wire increasing to almost four times its original preload.(ABSTRACT TRUNCATED AT 250 WORDS)

Understanding the load transfer within a resurfaced femur is necessary to determine the influence of mechanical factors on potential failure mechanisms such as early femoral neck fractures and stress shielding. In this study, an attempt has been made to measure the stem-bone micromotion and implant cup-bone relative displacements (along medial-lateral and anterior-posterior direction), in addition to surface strains at different locations and orientations on the proximal femur and to compare these measurements with those predicted by equivalent FE models. The loading and the support conditions of the experiment were closely replicated in the FE models. A new experimental set-up has been developed, with specially designed fixtures and load application mechanism, which can effectively impose bending and deflection of the tested femurs, almost in any direction. High correlation coefficient (0.92-0.95), low standard error of the estimate (170-379 muepsilon) and low percentage error in regression slope (12.8-17.5%), suggested good agreement between the numerical and measured strains. The effect of strain shielding was observed in two (out of eight) strain gauges located on the posterior side. A pronounced strain increase occurred in strain gauges located on the anterior head and neck regions after implantation. Experimentally measured stem-bone micromotion and implant cup-bone relative displacements (0-13.7 microm) were small and similar in trends predicted by the FE models (0-25 microm). Despite quantitative deviations in the measured and numerical results, it appears that the FE model can be used as a valid predictor of the actual strain and stem-bone micromotion.

Theoretical concerns about the use of cemented or press-fit stems in revision total knee arthroplasty (TKA) include stress shielding with adverse effects on prosthesis fixation. Revision TKA components are commonly stemmed to protect the limited autogenous bone stock remaining. Revision procedures with the use of stems can place abnormal stresses through even normal bone by their constrained design, type of materials and fixation method and may contribute for bone loss. Experimental quantification of strain shielding in the proximal synthetic tibia following TKA is the main purpose of the present study. In this study, cortical bone strains were measured experimentally with tri-axial strain gauges in synthetic tibias before and after in vitro knee surgery. Three tibias were implanted with cemented and press-fit stem augments and solely with a tibial tray (short monobloc stem) of the P.F.C. Sigma Modular Knee System. The difference between principal strains of the implanted and the intact tibia was calculated for each strain gauge position. The results demonstrated a pronounced strain-shielding effect in the proximal level, close to tibial tray with the cemented stem augment. The press-fit stem presented a minor effect of strain shielding but was more extensively throughout the stem. An increase of strains closely to the distal tip of the cemented and the press-fit stem augment was observed. This suggests for a physiological condition, a potential effect of bone resorption at the proximal region for the cemented stem augment. The localized increase of strains in stems tip can be related with the clinical finding of the pain, at the end of stem after revision TKA.

We previously found that interleukin (IL)-1beta is over-expressed in the fibroblasts of the stress-shielded patellar tendon using a stress-shielding model [Uchida, H., Tohyama, H., Nagashima, K., Ohba, Y., Matsumoto, H., Toyama, Y., Yasuda, K., 2005. Stress deprivation simultaneously induces over-expression of interleukin-1beta, tumor necrosis factor-alpha, and transforming growth factor-beta in fibroblasts and mechanical deterioration of the tissue in the patellar tendon. Journal of Biomechanics 38(4), 791–798.]. Therefore, IL-1beta may play a role in tendon deterioration in response to stress deprivation. This study was conducted to clarify the effects of local administration of interleukin-1 receptor antagonist (IL-1ra) on the mechanical properties of the stress-shielded patellar tendon as well as the tendon fascicles harvested from it. Twenty-six mature rabbits were equally divided into Groups IL-1ra and PBS after the right patellar tendon underwent the stress-shielding treatment, which completely released the patellar tendon from tension by stretching the flexible wire installed between the patella and the tibial tubercle. In Group IL-1ra, IL-1ra was injected between the patellar tendon and the infra-patellar fat pad. In Group PBS, phosphate-buffered saline was injected in the same manner as IL-1ra. All rabbits were evaluated at 3 weeks after the stress-shielding procedure. The tangent modulus and the tensile strength of the patellar tendons were significantly greater in Group IL-1ra than in Group PBS, while there was no significant difference in the strain at failure between Groups IL-1ra and PBS. Concerning the mechanical properties of the fascicles harvested from the patellar tendon, however, we could not detect any significant differences in the tangent modulus, tensile strength, or strain at failure between Groups IL-1ra and PBS. The present study suggested that IL-1 plays an important role in the deterioration of the mechanical properties of the patellar tendon in response to stress shielding and that IL-1 does not affect the fascicles themselves.

To test the hypothesis that stress deprivation induces over-expression of cytokines in the patellar tendon, 40 rats were divided into the following two groups. In the stress-shielded group, we slackened the patellar tendon in the right knee by drawing the patella toward the tibial tubercle with flexible wires. In the control group, we performed a sham operation on the right knee. Animals were killed at 2 or 6 weeks for immunohistological evaluation and biomechanical examination. For IL-1beta, TNF-alpha and TGF-beta, the ratio of positively stained specimens to total specimens was significantly higher in the stress-shielded tendons than in the control tendons. The elastic modulus of the stress-shielded tendon was significantly lower than that of the control tendon, while the cross-sectional area of the stress-shielded tendon was significantly greater than that of the control tendon. Therefore, the present study indicated that stress shielding induced the over-expression of IL-1beta, TNF-alpha and TGF-beta in patellar tendon fibroblasts with mechanical deterioration of the tendon. Regarding clinical relevance, the present study suggests a possible application of an anti-IL-1beta or anti-TNF-alpha strategy for reducing the mechanical deterioration of tendons and ligaments in response to stress deprivation, although this study did not directly show that over-expression of IL-1beta or TNF-alpha in response to stress deprivation was the causation of mechanical deterioration of tendons.

Item does not contain fulltext The long-term fixation of cemented femoral components may be jeopardised by the presence of a fibrous tissue layer at the bone-cement interface. This study used both experimental and finite element (FE) methods to investigate the load transfer characteristics of two types of cemented hip replacements (Lubinus SPII and Muller-Curved) with a fibrous tissue layer.The experimental part investigated six stems of each type, where these were implanted in composite femurs with a specially selected silicone elastomer modelling the soft interfacial layer. Two fibrous tissue conditions were examined: a layer covering the full cement mantle, representing a revision condition; and a layer covering the proximal portion of the cement mantle, representing a non-revised implant with partial debonding and fibrous tissue formation. The FE method was used to model the full fibrous tissue layer condition, for both implants. The layer was modelled as a homogeneous, linearly isotropic material. A cross-comparison was performed of the experimental and FE findings.Agreement between experimental and FE models was verified to be within 15%. Varying the stiffness parameter of the FE soft tissue layer had little influence on the cortical bone strains, though had considerable effect on the cement strains. Stress shielding occurred for both stems under both fibrous tissue conditions, with the greatest reduction around the calcar. However, the cortical bone strains were generally larger than those for the equivalent well-fixed stems. The fibrous tissue layer was not found to increase the general strain pattern of the cement mantle, though localised regions of high stress were detected.

The present work reports the pre-clinical validation of an innovative partially cemented femoral prosthesis called cement-locked uncemented (CLU) prosthesis. The inventors of the device under investigation claimed that, when compared to a comparable fully cemented stem, the new stem would present various advantages. Two previous experimental studies confirmed that primary stability and stress shielding were comparable to those of cemented stems. Aim of the present study was to investigate if the remaining claims were confirmed as well. A complete finite element model of the bone-implant complex was created from CT data. The model was validated against in vitro measurements of bone surface strains as well as against primary stability measurements. The peak stresses predicted in the CLU cement mantle were not found significantly lower than those reported in other studies on fully cemented stems. However, once the cement inlet geometry is optimised and the associated stress risers are eliminated, the CLU cement mantle should be subjected to much lower stresses. The stress induced in the stems by both load cases was well below the fatigue limit of the Ti6Al4 V alloy. Finite element models predicted for all load cases relative motion between cement and metal lower than 60 μm. This amplitude may be fully accommodated by elastic deformations of the cement micro-ridges. The experimental and numerical results showed the validity of the new fixation concept, although a further optimisation of the geometry of the cement pockets is needed in order to further reduce the stresses in the cement.

Stress shielding of the femur is known to be a principal factor in aseptic loosening of hip replacements. This paper considers the use of a hollow stemmed hip implant for reducing the effects of stress shielding, while maintaining acceptably low levels of stress in the cement. Using finite element modelling, the stresses in the proximal femur using different shapes of hollow stem were compared with those produced using comparable sizes of solid stem with different values of elastic modulus. A reduction in stress shielding could be achieved with a hollow stem. A cylindrical hollow stem design was then optimised in order to control the maximum allowable stress in the cement, the minimum allowable stresses in the bone, and a combination of the two. The resulting stems achieved an increase in proximal bone stress of about 15% for the first case and 32% for a model using high strength cement, compared with solid stems of the same nominal outside diameter. The gains of these theoretically optimised designs dropped off rapidly further down the stem. Linearly tapered hollow stems reached a 22% gain, which could be a good compromise between acceptable cement stresses and ease of manufacture.

The purpose of the present study was to test the hypothesis that cortical bone loss, trabecular bone density and the amount of bone ingrowth vary as a function of stem stiffness in a canine cementless hip replacement model. The study was motivated by the problem of cortical bone atrophy in the proximal femur following cementless total hip replacement. Two stem stiffnesses were used and both designs were identical in external geometry and porous coating placement. The high stiffness stem caused approximately 26% cortical bone stress-shielding and the low stiffness stem caused approximately 7.5% stress-shielding, as assessed by beam theory. Each group included nine adult, male canines who received unilateral arthroplasties for a period of six months. The animals with the low stiffness stems tended to lose less proximal cortical bone than the animals with high stiffness stems (4% +/- 9 as opposed to 11% +/- 14), but the difference was not statistically significant (p = 0.251). However, the patterns of bone ingrowth into the implant and change in medullary bone density adjacent to the implant were fundamentally different as a function of stem stiffness (p < 0.01). Most importantly, while the high stiffness group had peaks in these variables at the distal end of the stem, the low stiffness group had peak values proximally. These different patterns of functional adaptation are consistent with the idea that reduced stem stiffness enhances proximal load transfer.

The subject of this article is the development and application of computer-simulation methods to predict stress-related adaptive bone remodeling, in accordance with ‘Wolff's Law’. These models are based on the Finite Element Method (FEM) in combination with numerical formulations of adaptive bone-remodeling theories. In the adaptive remodeling models presented, the Strain Energy Density (SED) is used as a feed-back control variable to determine shape or bone density adaptations to alternative functional requirements, whereby homeostatic SED distribution is assumed as the remodeling objective. These models are applied to investigate the relation between ‘stress shielding’ and bone resorption in the femoral cortex around intramedullary prostheses, such as used in Total Hip Arthroplasty (THA). It is shown that the amount of bone resorption depends mainly on the rigidity and the bonding characteristics of the implant. Homeostatic SED can be obtained when the resorption process occurs at the periosteal surface, rather than inside the cortex, provided that the stem is adequately flexible.