Construct damage and loosening around glenoid implants: A longitudinal micro‐CT study of five cadaver specimens

The evolution of failure of bone and cement leading to loosening of glenoid components following shoulder arthroplasty is not well understood. The purpose of this study was to identify and visualize potential mechanisms of mechanical failure within cadavers, cemented with two types of components, and subject to cyclic loading. Five glenoid cadaver bones were implanted with either a three‐pegged polyethylene component, or prototype posteriorly augmented component which addresses posterior bone loss. Specimens were loaded by constant glenohumeral compression combined with cyclic anterior–posterior displacement of the humeral head relative to the glenoid. At six time points across 100,000 cycles, implant loosening micromotions were optically measured, and specimens were imaged by micro‐computed tomography. Scans were 3D registered and inspected for crack initiation and progression, and micro‐CT based time‐lapse movies were created. Cement cracking initiated at stress concentrations and progressed with additional cyclic loading. Failure planes within trabecular bone and the bone–cement interface were identified in four of the five specimens. Implant subsidence increased to greater than 1.0 mm in two specimens. Cemented glenoid structural failure can occur within the cement, along planes of trabecular bone, or at the bone cement interface. © 2015 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 34:1053–1060, 2016.     

In vivo loss of cement–bone interlock reduces fixation strength in total knee arthroplasties

Prevention of aseptic loosening of total knee arthroplasties (TKAs) remains a clinical challenge. Understanding how changes in morphology at the implant–bone interface with in vivo service affect implant stability and strength could lead to new approaches to mitigate loosening. Enbloc TKA retrievals and freshly‐cemented TKA tibial components were used to determine if the mechanical strength of the interface depended on the amount of cement–bone interlock and the morphology of the supporting bone under the cement layer. Implants were sectioned into small specimens of the cement–interface–bone from under the tibial tray. Micro‐CT scans were used to document interlock morphology and architecture of the supporting trabecular bone. Axial compression tests were used to assess mechanical behavior. Postmortem retrievals had lower contact fraction (42 ± 55%) compared to freshly‐cemented constructs (121 ± 61%) (p = 0.0008). Supporting bone architecture parameters were not different for the two groups. Increased interface contact fraction and supporting bone volume fraction (BV/TV) were positive predictors of interface strength (r² = 0.72, p = 0.0001). For the same supporting bone BV/TV, postmortem specimens had weaker interfaces; they were also more compliant. Cemented TKAs with in vivo service experience a loss of fixation strength and increased micro‐motion due to the loss of cement–bone interlock. © 2014 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 32:1052–1060, 2014.     

Quantification of regional variations in glenoid trabecular bone architecture and mineralization using clinical computed tomography images

The purpose of this study was to demonstrate feasibility of a clinical CT imaging and analysis technique to quantify regional variations in trabecular bone architecture and mineralization of glenoid bones. Specifically, our objective was to determine to what extent clinical CT imaging of intact upper extremities can describe variations of trabecular bone architectures at anatomic and peri‐implant regions by comparing trabecular bone architectures as measured by high‐resolution, micro CT imaging of same excised glenoid bones. Bone volume fraction (BVF), trabecular bone thickness (TbTh), number of trabecular bone (TbN), spacing (TbS), pattern factor (TbPf), bone surface area (BSA), and skeletal connectivity (Conn.), in addition to bone mineral content (BMC) and bone mineral density (BMD), were quantified from both clinical and micro CT images using whole bone, anatomic, and peri‐implant bone masks. Strong correlations of BVF, TbTh, TbSp, BMC, and BMD were found between clinical CT and micro CT imaging methods. The variations in BVF, TbTh, TbSp, TbN, BMC, and BMD at anatomical and peri‐implant regions were larger than those at whole bone regions. In this study, we have demonstrated that this clinical CT imaging methodology can be used to quantify variations of a patient's glenoid bone at anatomic and peri‐implant levels. Statement of Clinical Significance. An in vivo quantitative assessment of glenoid trabecular bone architecture in the anatomic and peri‐implant regions may improve our understanding on the role of bone quality on glenoid component loosening following total shoulder arthroplasty. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:85–96, 2018.     

Micromechanics of postmortem‐retrieved cement–bone interfaces

The cement–bone interface plays an important role in load transfer between cemented implant systems and adjacent bone, but little is known about the micromechanical behavior of this interface following in vivo service. Small samples of postmortem‐retrieved cement–bone specimens from cemented total hip replacements were prepared and mechanically loaded to determine the response to tensile and compressive loading. The morphology of the cement–bone interface was quantified using a CT‐based stereology approach. Laboratory‐prepared specimens were used to represent immediate postoperative conditions for comparison. The stiffness and strength of the cement–bone interface from postmortem retrievals was much lower than that measured from laboratory‐prepared specimens. The cement–bone interfaces from postmortem retrievals were very compliant (under tension and compression) and had a very low tensile strength (0.21 ± 0.32 MPa). A linear regression model, including interface contact fraction and intersection fraction between cement and bone, could explain 71% (p < 0.0001) of the variability in experimental response. Bony remodeling following an arthroplasty procedure may contribute to reduced contact between cement and bone, resulting in weaker, more compliant interfaces. © 2009 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 28:170–177, 2010     

Endosseous implant anchorage is critically dependent on mechanostructural determinants of peri‐implant bone trabeculae

Low bone mass is highly prevalent among patients receiving endosseous implants. In turn, the implantation prognosis in low‐density skeletal sites is poor. However, little is known about the mechanostructural determinants of implant anchorage. Using metabolic manipulations that lead to low bone density and to its rescue, we show here that anchorage is critically dependent on the peri‐implant bone (PIB). Titanium implants were inserted horizontally into the proximal tibial metaphysis of adult rats 6 weeks after orchiectomy (ORX) or sham ORX. Systemic intermittent administration of human parathyroid hormone (1–34) [iahPTH(1–34)] or vehicle commenced immediately thereafter for 6 weeks. The bone‐implant apparatus was then subjected to image‐guided failure assessment, which assesses biomechanical properties and microstructural deformation concomitantly. Anchorage failure occurred mainly in PIB trabeculae, 0.5 to 1.0 mm away from the implant. Mechanically, the anchorage performed poorly in ORX‐induced low‐density bone, attributable mainly to decreased trabecular number. iahPTH(1–34) rescued the PIB density and implant mechanical function by augmenting trabecular thickness (Tb.Th). However, implant biomechanical properties in low‐density bone were relatively insensitive to implant surface treatment that affected only the osseointegration (%bone‐implant contact). These results support a model wherein anchorage failure involves buckling of the weakest trabecular struts followed by sequential failure of the stronger trabeculae. Treatment with iahPTH(1–34) induced thicker struts, which were able to delay and even prevent failure of individual elements, thus implicating trabecular thickness as a prime target for enhancing implant anchorage by systemic bone anabolic therapy. © 2010 American Society for Bone and Mineral Research.     

Cadaveric study validating in vitro monitoring techniques to measure the failure mechanism of glenoid implants against clinical CT

Definite glenoid implant loosening is identifiable on radiographs, however, identifying early loosening still eludes clinicians. Methods to monitor glenoid loosening in vitro have not been validated to clinical imaging. This study investigates the correlation between in vitro measures and CT images. Ten cadaveric scapulae were implanted with a pegged glenoid implant and fatigue tested to failure. Each scapulae were cyclically loaded superiorly and CT scanned every 20,000 cycles until failure to monitor progressive radiolucent lines. Superior and inferior rim displacements were also measured. A finite element (FE) model of one scapula was used to analyze the interfacial stresses at the implant/cement and cement/bone interfaces. All ten implants failed inferiorly at the implant‐cement interface, two also failed at the cement‐bone interface inferiorly, and three showed superior failure. Failure occurred at of 80,966 ± 53,729 (mean ± SD) cycles. CT scans confirmed failure of the fixation, and in most cases, was observed either before or with visual failure. Significant correlations were found between inferior rim displacement, vertical head displacement and failure of the glenoid implant. The FE model showed peak tensile stresses inferiorly and high compressive stresses superiorly, corroborating experimental findings. In vitro monitoring methods correlated to failure progression in clinical CT images possibly indicating its capacity to detect loosening earlier for earlier clinical intervention if needed. Its use in detecting failure non‐destructively for implant development and testing is also valuable. The study highlights failure at the implant‐cement interface and early signs of failure are identifiable in CT images. © 2018 The Authors. Journal of Orthopaedic Research^® Published by Wiley Periodicals, Inc. on behalf of the Orthopaedic Research Society. J Orthop Res 36:2524–2532, 2018.     

Covalently‐linked hyaluronan promotes bone formation around Ti implants in a rabbit model

The goal of this study was the in vivo evaluation of nanoporous titanium (Ti) implants bearing a covalently linked surface hyaluronan (HA) layer. Implant surface topography and surface chemistry were previously evaluated by scanning electron micorscopy and X‐ray photoelectron spectroscopy. Results showed that the surface modification process did not affect surface topography, yielding a homogeneously HA‐coated nanotextured implant surface. In vivo evaluation of implants in both cortical and trabecular bone of rabbit femurs showed a significant improvement of both bone‐to‐implant contact and bone ingrowth at HA‐bearing implant interfaces at 4 weeks. The improvement in osteointegration rate was particularly evident in the marrow‐rich trabecular bone (bone‐to‐implant contact: control 22.5%; HA‐coated 69.0%, p < 0.01). Mechanical testing (push‐out test) and evaluation of interfacial bone microhardness confirmed a faster bone maturation around HA‐coated implants (Bone Maturation Index: control 79.1%; HA‐coated 90.6%, p < 0.05). Suggestions based on the biochemical role of HA are presented to account for the observed behavior. Published by Wiley Periodicals, Inc. J Orthop Res 27: 657–663, 2009     

Influence of trabecular bone quality and implantation direction on press‐fit mechanics

Achieving primary stability of uncemented press‐fit prostheses in patients with poor quality bone can involve axial implantation forces large enough to cause bone fracture. Radial implantation eliminates intraoperative impaction forces and could prevent this damage. Platens of two commercial implant surfaces (“Beaded” and “Flaked”) were implanted onto trabecular bone specimens of varying quality in a press‐fit simulator. Samples were implanted with varying interference, either axially (shear) or radially (normal). Push‐in and pull‐out forces were measured to assess stability. Microstructural changes in the bone were determined from μCT analysis. For force‐defined implantation analysis, push‐in and pull‐out forces both increased proportionally with increasing radial force, independent of implantation direction, bone quality or implant surface. For position‐defined implantation analysis, pull‐out forces were generally found to increase with interference and to be greater for radial than axial implantation direction, and to be lower for poor quality bone. Bone density increased locally at the tested interface due to implantation, in particular for the Beaded surface under axial implantation. If a safe radial stress can be determined for cortical bone in a particular patient, the associated implantation force, and pull‐out force which represents primary stability, can be directly derived, regardless of implantation direction, bone quality or implant surface. Radial implantation delivers primary stability that is no worse than that for axial implantation and may eliminate potentially damaging impaction forces. Development of implant designs based on this principal might improve implant fixation. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:224–233, 2017.

     

Trabecular resorption patterns of cement–bone interlock regions in total knee replacements

With in vivo service, there is loss of mechanical interlock between trabeculae and PMMA cement in total knee replacements. The mechanisms responsible for the loss of interlock are not known, but loss of interlock results in weaker cement–bone interfaces. The goal of this study was to determine the pattern of resorption of interdigitated bone using a series of 20 postmortem retrieved knee replacements with a wide range of time in service (3–22 years). MicroCT scans were obtained of a segment of the cement–bone interface below the tibial tray for each implant. Image processing methods were used to determine interface morphology and to identify supporting, interdigitated, resorbed, and isolated bone as a function of axial position. Overall, the amount of remaining interdigitated bone decreased with time in service (p = 0.0114). The distance from the cement border (at the extent of cement penetration into the bone bed) to 50% of the interdigitated volume decreased with time in service (p = 0.039). Isolated bone, when present, was located deep in the cement layer. Overall, resorption appears to start at the cement border and progresses into the cement layer. Initiation of trabecular resorption near the cement border may be a consequence of proximity to osteoclastic cells in the adjacent marrow space. Clinical Significance: Aseptic loosening of joint replacements remains an important clinical problem. This work explores the process and pattern of trabecular bone resorption responsible for loss of interface fixation. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:2773–2780, 2017.

     

Glenoid loosening after total shoulder arthroplasty: An in vitro CT‐scan study

Glenoid fixation failure has only been grossly characterized. This lack of information hinders attempts to improve fixation because of a lack of methodologies for detecting and monitoring fixation failure. Our goal was twofold: to collect detailed data of glenoid fixation fracture, and to investigate computed tomography (CT)‐scanning as a tool for investigations of fixation failure. Six cadaver scapulas and six bone‐substitute specimens were cyclically loaded and CT‐scanned at clinical settings after 0, 1,000, 5,000, 10,000, 30,000, 50,000 and 70,000 load cycles. The fixation status was evaluated by inspection of the scans. After 70,000 cycles, the specimens were sectioned, and the fixation inspected by microscopy. The results of the microscopy analysis were compared to the CT‐scan analysis. Fracture of the glenoid fixation initiated at the edge of the glenoid rim and propagated towards and around the keel of the implant. The entire process from initiation to complete fracture took place at the polyethylene implant–cement interface, while the cement, the adjacent bone, and the cement–bone interface remained intact. Thus, strengthening the polyethylene–cement interface should improve glenoid fixation. Microscopy results validated the CT methodology, suggesting that the CT technique is reliable. © 2009 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 27:1589–1595, 2009     

Healing in peri‐implant gap with BMP‐2 and systemic bisphosphonate is dependent on BMP‐2 dose—A canine study

The bone–implant interface of cementless orthopedic implants can be described as a series of uneven sized gaps with discontinuous areas of direct bone–implant contact. Bridging these voids and crevices by addition of an anabolic stimulus to increase new bone formation can potentially improve osseointegration of implants. Bone morphogenetic protein 2 (BMP‐2) stimulates osteoblast formation to increase new bone formation but also indirectly stimulates osteoclast activity. In this experiment, we investigate the hypothesis that osseointegration, defined as mechanical push‐out and histomorphometry, depends on the dose of BMP‐2 when delivered as an anabolic agent with systemic administration of the anti‐resorptive agent zoledronate to curb an increase in osteoclast activity. Four porous coated titanium implants (one with each of three doses of surface‐applied BMP‐2 (15 µg; 60 µg; 240 µg) and untreated) surrounded by a 0.75 mm empty gap, were inserted into the distal femurs of each of twelve canines. Zoledronate IV (0.1 mg/kg) was administered 10 days into the observation period of 4 weeks. Bone–implant specimens were evaluated by mechanical push‐out test and histomorphometry. The 15 µg implants had the best fixation on all mechanical parameters and largest surface area covered with new bone compared to the untreated, 60 and 240 µg implants, as well as the highest volume of new bone in the implant gap compared to 60 and 240 µg implants. The results in a canine implant model demonstrated that a narrow range of BMP‐2 doses have opposite effects in bridging an empty peri‐implant gap with bone, when combined with systemic zoledronate. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:1406–1414, 2018.

     

Experimental micromechanics of the cement–bone interface

Despite the widespread use of cement as a means of fixation of implants to bone, surprisingly little is known about the micromechanical behavior in terms of the local interfacial motion. In this work, we utilized digital image correlation techniques to quantify the micromechanics of the cement–bone interface of laboratory‐prepared cemented total hip replacements subjected to nondestructive, quasistatic tensile and compressive loading. Upon loading, the majority of the displacement response localized at the contact interface region between cement and bone. The contact interface was more compliant (p = 0.0001) in tension (0.0067 ± 0.0039 mm/MPa) than compression (0.0051 ± 0.0031 mm/MPa), and substantial hysteresis occurred due to sliding contact between cement and bone. The tensile strength of the cement–bone interface was inversely proportional to the compliance of the interface and proportional to the cement/bone contact area. When loaded beyond the ultimate strength, the strain localization process continued at the contact interface between cement and bone with microcracking (damage) to both. More overall damage occurred to the cement than to the bone. The opening and closing at the contact interface from loading could serve as a conduit for submicron size particles. In addition, the cement mantle is not mechanically supported by surrounding bone as optimally as is commonly assumed. Both effects may influence the longevity of the reconstruction and could be considered in preclinical tests. © 2008 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 26:872–879, 2008     

Effect of cavity preparation and bone mineral density on bone‐interface densification and bone‐implant contact during press‐fit implantation of hip stems

Implant loosening and periprosthetic fracture are two major revision causes for uncemented hip stems. The chosen method of cavity preparation could play a key role for both failure mechanisms. The aim of this study was to determine the dependence of the broach type as well as patient bone mineral density (BMD) on densification and contact conditions at the bone‐implant interface. Hip stems were implanted into cadaveric femora using compaction, blunt extraction or sharp extraction broaches with computed tomography scans performed prior to broaching, after broaching and after stem implantation. Proximal periprosthetic bone densification as well as press‐fit, contact area and stem seating relative to the last broach were determined. Median bone densification was higher with the compaction and blunt extraction broaches compared to sharp extraction broaches (181% and 177%, respectively, p = 0.002). The bone densification of femora prepared with compaction broaching increased with higher BMD (R² = 0.183, p = 0.037), while stem seating decreased with higher BMD for all broach types (R² = 0.259, p = 0.001). Incomplete seated prostheses were associated with smaller press‐fit and bone‐implant contact area (R² = 0.249, p = 0.001; R² = 0.287, p < 0.001). Clinical Significance: The results suggest that compaction broaching maximizes bone densification in patients with higher bone density. However, there appears to be an increased risk of insufficient stem seating in high‐density bone that could limit the benefits for primary stability. For lower quality bone, the broach type appears to play a lesser role, but care must be taken to limit extensive stem seating which might increase periprosthetic fracture risk. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 37:1580–1589, 2019.     

Implant placement increases bone remodeling transiently in a rat model

To examine bone remodeling following implant placement, 88 female Sprague–Dawley rats underwent either sham ovariectomy (sham‐ovx) or ovariectomy (ovx) at 4.5 months. At 11 months, 17 baseline control animals were euthanized, while 71 rats received bilateral intramedullary femoral implants. Implanted rats were randomized to 4‐, 8‐, or 12‐week follow‐up times. Microcomputed tomography was used to assess cortical area and trabecular architecture in all rats. Dynamic and static histomorphometry were performed in a subset to examine the trabecular and endocortical bone in the distal femoral metaphysis adjacent to the implant and the periosteal surface at the midshaft superior to the implant (n = 59). Implantation did not affect bone volume in either sham‐ovx or ovx rats compared to baseline controls. Implant placement significantly increased mineralizing surface, mineral apposition rate, and bone formation rate in both sham‐ovx and ovx rats at the trabecular and endocortical surfaces at four and sometimes 8 weeks, with a return to baseline values by 12 weeks. At the periosteal surface, implant placement increased bone formation at 4 weeks with a return to baseline levels by 8 weeks. Thus, implant placement increases bone remodeling transiently without affecting bone volume in sham‐ovx and ovx rats. © 2012 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 31: 800–806, 2013     

Cement augmentation of implants—no general cure in osteoporotic fracture treatment. A biomechanical study on non‐displaced femoral neck fractures

Femoral neck fractures in the elderly are a common problem in orthopedics. Augmentation of screw fixation with bone cement can provide better stability of implants and lower the risk of secondary displacement. This study aimed to investigate whether cement augmentation of three cannulated screws in non‐displaced femoral neck fractures could increase implant fixation. A femoral neck fracture was simulated in six paired human cadaveric femora and stabilized with three 7.3 mm cannulated screws. Pairs were divided into two groups: conventional instrumentation versus additional cement augmentation of screw tips with 2 ml TraumacemV+ each. Biomechanical testing was performed by applying cyclic axial load until failure. Failure cycles, axial head displacement, screw angle changes, telescoping and screw cut‐out were evaluated. Failure (15 mm actuator displacement) occurred in the augmented group at 12,500 cycles (± 2,480) compared to 15,625 cycles (± 4,215) in the non‐augmented group (p = 0.041). When comparing 3 mm vertical displacement of the head no significant difference (p = 0.72) was detected between the survival curves of the two groups. At 8,500 load‐cycles (early onset failure) the augmented group demonstrated a change in screw angle of 2.85° (± 0.84) compared to 1.15° (± 0.93) in the non‐augmented group (p = 0.013). The results showed no biomechanical advantage with respect to secondary displacement following augmentation of three cannulated screws in a non‐displaced femoral neck fracture. Consequently, the indication for cement augmentation to enhance implant anchorage in osteoporotic bone has to be considered carefully taking into account fracture type, implant selection and biomechanical surrounding. © 2015 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 34:314–319, 2016.     

Effects of material properties of femoral hip components on bone remodeling.

Bone loss around femoral hip stems is one of the problems threatening the long-term fixation of uncemented stems. Many believe that this phenomenon is caused by reduced stresses in the bone (stress shielding). In the present study the mechanical consequences of different femoral stem materials were investigated using adaptive bone remodeling theory in combination with the finite element method. Bone-remodeling in the femur around the implant and interface stresses between bone and implant were investigated for fully bonded femoral stems. Cemented stems (cobalt-chrome or titanium alloy) caused less bone resorption and lower interface stresses than uncemented stems made from the same materials. The range of the bone resorption predicted in the simulation models was from 23% in the proximal medial cortex surrounding the cemented titanium alloy stem to 76% in the proximal medial cortex around the uncemented cobalt-chrome stem. Very little bone resorption was predicted around a flexible, uncemented "iso-elastic" stem, but the proximal interface stresses increased drastically relative to the stiffer uncemented stems composed of cobalt-chrome or titanium alloy. However, the proximal interface stress peak was reduced and shifted during the adaptive remodeling process. The latter was found particularly in the stiffer uncemented cobalt-chrome-molybdenum implant and less for the flexible iso-elastic implant.     

Trabecular bone strength is not an independent predictive factor for dynamic hip screw migration—A prospective multicenter cohort study

This study assessed whether mechanically measured trabecular bone strength is an independent predictor of dynamic hip screw (DHS) stability, i.e., DHS migration (DHSM) after the fixation of proximal femoral fractures. One‐hundred and seven patients older than 50 years with proximal femoral fractures were included. During fracture fixation, a mechanical probe (DensiProbe? Hip) was inserted at the site where the DHS tip would ultimately be positioned. Peak torque to breakaway the trabecular bone was measured. Fracture reduction, primary implant position and postoperative DHSM were assessed by radiographs taken postoperatively, at 6 and 12 weeks after surgery. Univariate regression analysis revealed no association between peak torque and DHSM (R² = 0.025, p = 0.135). DHSM correlated with the primary DHS position, i.e., the distance between the DHS and (i) the central femoral neck axis (CNFAD, R² = 0.230; p < 0.0001) and (ii) the apex of the femoral head (R² = 0.110; p = 0.001). DHSM did not correlate with areal BMD of the contralateral proximal femur. Multivariable regression modeling revealed the CFNAD as predictive factor for screw migration. The primary implant position measured by the CFNAD, rather than DensiProbe? Hip measured bone strength, is an independent predictor of DHSM. © 2015 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 33:1680–1686, 2015.

     

Finite element analysis of the effect of cementing concepts on implant stability and cement fatigue failure

Background and purpose Two contradictory cementing techniques (using an undersized stem versus a canal-filling stem) can both lead to excellent survival rates, a phenomenon known as the “French paradox”. Furthermore, previous studies have indicated that the type of bone supporting the cement mantle may affect implant survival. To further evaluate the mechanical consequences of variations in cementing technique, we studied the effect of implant size and type of bone supporting the cement mantle on the mechanical performance of cemented total hip arthroplasty, using finite element analysis.Methods In a generic 2-dimensional plane-strain finite element model of a transverse section of a cemented total hip arthroplasty with a Charnley-Kerboull stem, we varied implant size and type of bone supporting the cement mantle. The models were subjected to 2 × 10⁶ cycles of an alternating loading pattern of torque and a transverse load. During this loading history, we simulated cement fatigue crack formation and tracked rotational stability of the implant.Results Canal-filling stems produced fewer cement cracks and less rotation than undersized stems. Cement mantles surrounded by trabecular bone produced more cement cracks and implant rotation than cement mantles surrounded by cortical bone.Interpretation Our investigation provides a possible explanation for the good clinical results obtained with canal-filling Charnley-Kerboull implants. Our findings also indicate that inferior mechanical properties are obtained with these implants if the cement is supported by trabecular bone, which may be minimized by an optimal cementing technique.     

Local delivery of zoledronate from a poly (D,L‐lactide)—Coating increases fixation of press‐fit implants

Early secure fixation of total joint replacements is crucial for long‐term survival. Antiresorptive agents such as bisphosphonates have been shown to increase implant fixation. We investigated whether local delivery of zoledronate from poly‐D, L‐lactide (PDLLA)‐coated implants could improve implant fixation and osseointegration. Experimental titanium implants were bilaterally inserted press‐fit into the proximal tibiae of 10 dogs. On one side the implant was coated with PDLLA containing zoledronate. The contralateral implant was uncoated and used as control. Observation period was 12 weeks. Implant fixation was evaluated with histomorphometry and biomechanical push‐out test. We found an approximately twofold increase in all biomechanical parameters when comparing data from the zoledronate group with their respective controls. Histomorphometry showed increased amount of preserved bone and increased bone formation around the zoledronate implants. This study indicates that local delivery of zoledronate from a PDDLA coating has the potential to increase implant fixation. © 2015 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 34:65–71, 2016.     

Parathyroid hormone (1–34) increases attachment of PMMA cement to bone

The attachment of an implant material to bone is related to the surface of the implanted material and the ability of the bone to form around the implant. Intermittent parathyroid (PTH) administration increases bone formation by stimulating osteoblastic activity. Little is known about the effect of PTH administration on orthopedic implant incorporation. The present study determined how PTH (1–34) administration influenced bone bonding, i.e., the bone-cement interfacial tensile strength, of vacuum-mixed polymethylmethacrylate (PMMA) bone cement (surface roughness; Ra, 4.8 μm). Bone bonding was evaluated by a detachment test. We used unloaded cement surfaces, which could be detached from the bone. Titanium plates were developed such that a cement fill was contained within a plate that was contained within a titanium holder. Thus, a flat cement surface came into contact with traumatized bone only, and the rest of the plate had no contact with tissue. After implantation of the plate in the left tibia, 20 adult male rats were injected daily with human PTH (1–34) at 60 μg/kg per injection (n = 10) or vehicle (n = 10); the animals were killed after 4 weeks. The plates were detached from the bone by a perpendicular force. PTH treatment increased the median pull-away strength (0.21 MPa), compared with that in the vehicle-treated rats, (0.04 MPa) (P = 0.02). The results suggest that PTH treatment may have the potential to enhance the incorporation of cemented orthopedic implants. Received: September 18, 2000 / Accepted: June 6, 2001