Total Ankle Replacement Implants Research Articles

Printable functionally graded tibial implant for TAR: FE study comparing implant materials, FGM properties, and implant designs

Tibial implants with functionally graded material (FGM) for total ankle replacement (TAR) can provide stiffness similar to the host tibia bone. The FGM implants with low stiffness reduce stress shielding but may increase implant-bone micromotion. A trade-off between stress shielding and implant-bone micromotion is required if FGMs are to substitute traditionally used Ti and CoCr metal implants. The FGM properties such as material gradation law and volume fraction index may influence the performance of FGM implants. Along with the FGM properties, the design of FGM implants may also have a role to play. The objective of this study was to examine FGM tibial implants for TAR, by comparing implant materials, FGM properties, and implant designs. For this purpose, finite element analysis (FEA) was conducted on 3D FE models of the intact and the implanted tibia bone. The tibial implants were composed of CoCr and Ti, besides them, the FGM of Ti and HA was developed. The FGM implants were modelled using exponential, power, and sigmoid laws. Additionally, for power and sigmoid laws, different volume fraction indices were taken. The effect of implant design was observed by using keel type and stem type TAR fixation designs. The results indicated that FGM implants are better than traditional metal implants. The power law is most suitable for developing FGM implants because it reduces stress shielding. For both power law and sigmoid law, low values of the volume fraction index are preferrable. Therefore, FGM implant developed using power law with 0.1 vol fraction index is ideal with the lowest stress shielding and marginally increased implant-bone micromotion. FGM implants are more useful for keel type fixation design than stem type design. To conclude, with FGMs the major complication of stress shielding can be solved and the longevity and durability of TAR implants can be enhanced.

A numerical investigation for the development of functionally graded Ti/HA tibial implant for total ankle replacement: Influence of material gradation law and volume fraction index.

Stress shielding is one of the major concerns for total ankle replacement implants nowadays, because it is responsible for implant-induced bone resorption. The bone resorption contributes to the aseptic loosening and failure of ankle implants in later stages. To reduce the stress shielding, improvements can be made in the implant material by decreasing the elastic mismatch between the implant and the tibia bone. This study proposes a new functionally graded material (FGM) based tibial implant for minimizing the problem of stress shielding. Three-dimensional finite element (FE) models of the intact tibia and the implanted tibiae were created to study the influence of material gradation law and volume fraction index on stress shielding and implant-bone micromotion. Different implant materials were considered that is, cobalt-chromium, titanium (Ti), and FGM with Ti at the bottom and hydroxyapatite (HA) at the top. The FE models of FGM implants were generated by using different volume fractions and the rule of mixtures. The rule of mixtures was used to calculate the FGM properties based on the local volume fraction. The volume fraction was defined by using exponential, power, and sigmoid laws. For the power and sigmoid law varying volume fraction indices (0.1, 0.2, 0.5, 1, 2, and 5) were considered. The geometry resembling STAR® ankle system tibial implant was considered for the present study. The results indicate that FGMs lower stress shielding but also marginally increase implant-bone micromotion; however, the values were within the acceptable limit for bone ingrowth. It is observed that the material gradation law and volume fraction index influence the performance of FGM tibial implants. The tibial implant composed of FGM using power law with a volume fraction index of 0.1 was the preferred option because it showed the least stress shielding.

Dependence of Total Ankle Tibial Baseplate Stability Upon Bone Density

Category: Ankle Arthritis Introduction/Purpose: The success of total ankle replacement (TAR) in improving patient outcomes is linked to its initial stability after implantation. Limiting early micromotion between implant and bone improves longer-term stability. Tibial component design fixation features play a critical role in determining early stability, with retention of medial and lateral bone sidewalls and interference press-fit both used to supplement fixation. However, tibial component stability is likely also influenced by regional bone density characteristics, and the exact relationship between the two is not adequately understood. The goal of this study was to investigate how bone density affects tibial component stability and bone-implant interface micromotion between the tibial component of a specific TAR and the distal tibia, by using finite element analysis (FEA) during simulated gait. Methods: A commercially available TAR tibial component baseplate was virtually inserted into a computer model of the distal tibia from two patients with end-stage ankle arthritis. These patients were selected based on having different bone density profiles in the affected ankle. The tibia models were generated from patient CT scans, having a CT density-based inhomogeneous material distribution (Fig. 1a) that allowed for bone compaction after press-fit. The tibial component was modeled as titanium alloy material. Two different fixation cases were simulated using FEA: (1) retained medial/lateral sidewalls + line-to-line fit, and (2) retained sidewalls + 50µm interference press-fit. FEA was performed using body weight-scaled kinetic (forces/moments) profiles representing the stance phase of gait, applied to the distal implant surface, while the proximal tibia was held fixed. Press-fit was simulated prior to gait simulation (interference press-fit cases). Micromotions were defined as displacement differences between bone-implant closest nodal pairs, computed from FEA output. Results: For the case with sidewalls+line-to-line fit, micromotions were largest early and late in the stance phase of gait (Fig. 1b), with the largest micromotions observed at heel strike (0% stance). Dorsiflexion and inversion moments dominate in early stance, stressing the anterior edge of the tibia in contact with the implant, leading to relatively large posterior/lateral gapping (Fig. 1b, inset). The observed differences in micromotion between the two patients correlated with differences in bone quality at the tibia contact surface, particularly around the implant pegs (Fig. 1a). However, when interference press-fit was modeled, the differences in micromotion between the two subjects largely disappeared (Fig. 1c), as adequate bone compaction was generated around the interference regions with sufficient bone quality to resist micromotion (Fig. 1d). Conclusion: This study presents novel insight into the effect of TAR fixation features and the associated micromotion at the bone- implant interface in patients with varying distal tibia bone density. We found the stability of the tibial implant to be considerably affected by surrounding bone quality, but the influence of bone quality on implant stability was minimized when interference press- fit was introduced. While a more comprehensive understanding of TAR implant features and their performance is needed, we believe the results of this study clearly demonstrate the importance of bone quality and particularly implant interference press-fit in the stability of TAR implants.

The ceramic coated implant (CCI). Evolution total ankle replacements: a retrospective analysis of 40 ankles with 8 years follow-up.

Diminutive data is available on the outcome of several previously used total ankle replacement implants. The purpose of this study was to investigate the medium-term functional and radiological outcome and implant survival of the CCI Evolution implant. Consecutive series of 40 ankles operated in our hospital with primary TAR using the CCI Evolution implant in 2010-2013 were available for follow-up. The prospective clinical and radiographic data including the Kofoed score, subjective satisfaction and standard radiographs were collected preoperatively and at fixed time-points postoperatively. A CT was obtained in cases where osteolysis or loosening were suspected. The improvement of the Kofoed score and subjective satisfaction were statistically significant (p<0.0001). The implant survival was 97% (95% confidence interval (CI) 81%-100 %) at 5 years, and 81 % (95% confidence interval (CI) 60 %-92%) at 8 years. There were altogether 25 (64%) complications. Overall revision rate was 28% and failure rate 13%. The CCI implant outcome was not acceptable. The malposition of prosthetic components, subsidence, and peri-implant osteolysis were recorded often. Although the patient reported outcome measures improved, mostly due to positive changes in pain severity, overall revision and failure rates were high and comparable with previous findings of the CCI implant.

Finite element stress analysis of the bearing component and bone resected surfaces for total ankle replacement with different implant material combinations

BackgroundA proper combination of implant materials for Total Ankle Replacement (TAR) may reduce stress at the bearing component and the resected surfaces of the tibia and talus, thus avoiding implant failure of the bearing component or aseptic loosening at the bone-implant interface.MethodsA comprehensive finite element foot model implanted with the INBONE II implant system was created and the loading at the second peak of ground reaction force was simulated. Twelve material combinations including four materials for tibial and talar components (Ceramic, CoCrMo, Ti6Al4V, CFR-PEEK) and three materials for bearing components (CFR-PEEK, PEEK, and UHMWPE) were analyzed. Von Mises stress at the top and articular surfaces of the bearing component and the resected surfaces of the tibia and talus were recorded.ResultsThe stress at both the top and articular surfaces of the bearing component could be greatly reduced with more compliant bearing materials (44.76 to 72.77% difference of peak stress value), and to a lesser extent with more compliant materials for the tibial and talar components (0.94 to 28.09% difference of peak stress value). Peak stresses at both the tibial and talar bone-implant interface could be reduced more strongly by using tibial and talar component materials with smaller material stiffness (7.31 to 66.95% difference of peak stress value) compared with bearing materials with smaller material stiffness (1.11 to 24.77% difference of peak stress value).ConclusionsImplant components with smaller material stiffness provided a stress reduction at the bearing component and resected surfaces of the tibia and talus. The selection of CFR-PEEK as the material of tibial and talar components and UHMWPE as the material of the bearing component seemed to be a promising material combination for TAR implants. Wear testing and long-term failure analysis of TAR implants with these materials should be included in future studies.

Evaluation of True Ankle Motion Following Total Ankle Replacement Utilizing XROMM technology

Category:Ankle ArthritisIntroduction/Purpose:Total ankle replacement (TAR) is common tool used by the foot and ankle specialist to treat end stage ankle arthritis. Current data about ankle motion following TAR is derived from gait analysis utilizing external markers. Utilizing Xray Reconstruction of Moving Morphology (XROMM), which combines 3-D mapping technology with biplanar fluoroscopy in vivo to visualize true skeletal motion, we can evaluate true motion of TAR implants. Current TAR replacement systems are either mobile bearing or fixed bearing. We hypothesized that subjects implanted with a fixed bearing prosthesis would exhibit less tibiotalar rotation and translation than subjects implanted with a mobile bearing prosthesis.Methods:Six subjects with total ankle replacement at least one-year post implantation gave informed consent before participating (IRB #H16496). Three subjects with a mobile bearing prosthesis with an average age 63.3+-11.1 yrs were compared to three matched subjects with a fixed bearing prosthesis with an average age of 64.7+-1.5 yrs. Utilizing 3D slicer software, lower body CT scans for each subject were evaluated to create 3D models of the foot and ankle bones and implant components. All subjects walked for several trials at a self-selected pace along a walkway while their foot and ankle motions were captured by a high-speed biplanar fluoroscopic x-ray motion analysis (XMA) system. The 3D models were combined with the x-ray images within a 3D animation platform and rotoscoped to resolve accurate kinematic motions at the tibiotalar joint during stance phase of gait. We examined for differences between the two groups using a two-sample t-test (p<0.05).Results:Subjects with a mobile-bearing prosthesis demonstrated mean ROM’s of 7.4+-1.1°, 5.3+-2.3° and 7.1+-4.3° for dorsiflexion/plantarflexion, inversion/eversion, and internal/external rotation, respectively. Subjects with a fixed bearing ankle prosthesis did not exhibit significantly different mean ROM’s for dorsiflexion/plantarflexion (9.1+-4.0°, p=0.35), inversion/eversion (4.4+-2.1°, p=0.42), and internal/external rotation (9.0+-3.4°, p=0.35), respectively. Subjects with a fixed bearing prosthesis displayed significantly more translation along the anteroposterior (3.6+-1.2mm, p<0.01) and mediolateral (2.2+-0.7mm, p<0.01) axes compared to the mobile bearing prosthesis (1.8+-1.2mm and 1.3+-0.8mm, respectively).Conclusion:Our preliminary results indicate that mobile and fixed bearing prosthesis provides similar angular motion at the tibiotalar joint, however, the fixed bearing prosthesis exhibits greater translational motion during walking. Further, there is the same amount of internal and external translation with both component designs. The implications of this work on success or failure of current implant designs is beyond the scope of this study but this work will provide the basis for future studies to help determine optimal future total ankle replacement designs.

Total Ankle Arthroplasty Survivorship: A Meta-analysis

The gold standard for management of end-stage ankle arthritis was previously ankle arthrodesis; however, improvements in total ankle replacements are making this a more viable treatment option. The primary aim of this meta-analysis was to evaluate the survivorship of total ankle replacement implants currently in use.An extensive search strategy initially captured 20,842 citations that were evaluated for relevance.Abstract screening produced 97 articles to be read in entirety, of which 10 articles studying 1963 implants met all prospective inclusion criteria for analysis.Overall survivorship of all implants was93.0% (95% confidence interval, 85.2-96.9) using a random effect model. There was significant heterogeneity between the studies (Q = 131.504).Meta-regression identified an inverse relationship between survivorship and study follow-up duration (p < .0001).Furthermore, age (p = .36) and implant type (fixed-bearing [95.6%, 95% confidence interval, 85.9-98.7] versus mobile-bearing ]89.4%, 95% confidence interval, 79.6%-94.8%]) did not have a statistically significant impact on survivorship, p = .213.However, patients with higher preoperative functional scores had improved survivorship (p = .001). Complications were inconsistently reported with varied definitions.In order of reported frequency, complications were classified into technical error (28.15%), subsidence (16.89%), implant failure (13.28%), aseptic loosening (6.3%), intraoperative fracture (5.67%), wound problems (4.3%), deep infection (1%), and postoperative fracture (0.0001%).Overall study quality was low, with only 10% being prospective and 90% from nonregistry data.The results from this meta-analysis revealed a promising overall survivorship of current implants in use for total ankle replacement; however higher quality studies with standardized outcomes measures are needed.

Comparing Sports Activity Following Total Ankle Replacement Versus Ankle Arthrodesis

Category: Ankle, Ankle Arthritis, Sports Introduction/Purpose: Total ankle replacement (TAR) and ankle arthrodesis (AA) are two common surgical treatment modalities for end stage tibiotalar arthritis. A key deciding point between the two is anticipated functional outcome postoperatively, especially in regards to sports related activities. However, there is a paucity of data available to help advise patients in their decision making. While TAR provides a theoretical benefit of improved functionality, the outcomes of several European studies have shown mixed results. These studies are limited by small sample size, obsolete TAR implants not used in the United States, and nonspecific outcome measures. The purpose of this study was to compare postoperative sports activity levels following modern TAR and AA in a U.S. population, which may benefit surgical decision making and guide patient expectations. Methods: We conducted a retrospective comparative study that consisted of patients who underwent a TAR (N=62) or AA (N=51) between 2009-2015. The mean age of the arthrodesis group was 57.7 years ± 12.12 (28.84-85.26). There were 27 male participants and 24 female participants. The TAR group had 31 male and 31 female participants with a mean age of 64.9 years ± 8.57 (45-79.6). Exclusion criteria included paralysis, rheumatoid arthritis, revision surgery, incomplete pre- and post-operative scores, and follow up less than 2 years. General health and foot-ankle function were assessed using the SF-12 Health Survey and the revised Foot Function Index (FFI-R) preoperatively and at final follow-up. In addition, activity levels were assessed using a Return to Activities Following Surgery questionnaire that was administered at final follow up. This form included a Visual Analog Scale for Pain, satisfaction questions, and a list of 25 activities. Patients were asked to record their current level of activity, ability to participate pre- and post-surgery, and whether their desired level was met. All three measurements tools were compared between both treatment groups. Results: The SF-12 physical score both groups significantly increased postoperatively from 33.18 ± 10.37 to 43 ± 10.32 for AA’s and from 32.88 ± 9.44 to 45.81 ± 12.94 (p &lt; 0.001) for TAR’s. The FFI scores showed a significant increase in both groups (p &lt; 0.001). In the AA group, 88% of patients returned to work and would repeat the surgery, compared to 92% of patients in the TAR group. In terms of satisfaction and pain, the TAR group was more satisfied (1.78 vs. 1.44) and had less postoperative pain (1.32 vs. 2.56 p &lt; 0.05). The AA group reported a significant increase in six activities including: golf (p &lt; 0.05), weight lifting, and walking (p&lt;0.001), while the TAR group reported significant increase in 15 activities, including hiking, tennis, and yoga (p&lt;0.001). Conclusion: Our study revealed a significant increase in general physical function, foot function, and activity level in both groups. The TAR group was able to perform a wider range of activity and sports compared to the AA group. Overall, TAR patients were significantly more satisfied with their procedure compared to AA patients.

Total ankle replacement design and positioning affect implant-bone micromotion and bone strains

Implant loosening – commonly linked with elevated initial micromotion – is the primary indication for total ankle replacement (TAR) revision. Finite element modelling has not been used to assess micromotion of TAR implants; additionally, the biomechanical consequences of TAR malpositioning – previously linked with higher failure rates – remain unexplored. The aim of this study was to estimate implant-bone micromotion and peri-implant bone strains for optimally positioned and malpositioned TAR prostheses, and thereby identify fixation features and malpositioning scenarios increasing the risk of loosening. Finite element models simulating three of the most commonly used TAR devices (BOX®, Mobility® and Salto®) implanted into the tibia/talus and subjected to physiological loads were developed. Mobility and Salto demonstrated the largest micromotion of all tibial and talar components, respectively. Any malpositioning of the implant creating a gap between it and the bone resulted in a considerable increase in micromotion and bone strains. It was concluded that better primary stability can be achieved through fixation nearer to the joint line and/or while relying on more than a single peg. Incomplete seating on the bone may result in considerably elevated implant-bone micromotion and bone strains, thereby increasing the risk for TAR failure.

Wear study of Total Ankle Replacement explants by microstructural analysis

The implantation of Total Ankle Replacement (TAR) prostheses generally gives satisfactory results. However, a high revision rate is associated with the Ankle Evolutive System (AES) implant, due to periprosthetic osteolysis that generates significant cortical lesions and bone cysts in the periprosthetic region. Radioclinical and histological analyses of peri-implant tissues show the presence of numerous foreign particles that may come from the implant. It is known that a precocious wear of materials may lead to an important rate of foreign body in tissues and may generate osteolysis lesions and inflammatory reactions. Thus the objectives of this retrospective study of 10 AES TAR implants (recovered after revision surgeries) are to understand how the prostheses wear out, which part is the most stressed and to determine the nature and size of foreign body particles. A better understanding of friction mechanisms between the three parts of the implant and of the nature and morphology of foreign particles generated was needed to explain the in vivo behavior of the implant. This was achieved using microstuctural and tomographic analysis of both implants parts and periprosthetic tissues.

Autoinflammation Around AES Total Ankle Replacement Implants.

Failure of total ankle replacement (TAR) can be characterized by early peri-implant osteolysis even in the presence of very modest numbers of wear particles. The hypothesis of the study was that this reaction is in part mediated by autoinflammatory responses mediated via damage-associated molecular patterns (DAMPs, danger signals) and pattern-recognizing danger signal receptors (PRRs). Peri-implant tissue and control samples from 10 patients with AES implants were immunostained for hypoxia inducible factor-1α (HIF-1α), activated caspase-3, high-mobility group box 1 (HMGB1), receptor for advanced glycation end product (RAGE), and toll-like receptors TLR2 and TLR4. Results were evaluated on a 0 to 4 scale (from 0% to >50% stained area). Peri-implant tissue around failed TAR implants had a relatively high mean HIF-1α score of 3 on a scale, which however was similar in control samples. HMGB1 (a DAMP) was seen to be mobilized from nuclei to cellular cytoplasm, and the active caspase-3(+) cells were increased. All PRRs were increased in revision samples. Increased expression of HMGB1 and other danger signals together with increased PRR-dependent responsiveness could contribute to autoinflammatory peri-implantitis, multilocular cyst formation, and osteolysis in failed TAR implants. Level IV, case series.

AKILE™ total ankle arthroplasty: Clinical and CT scan analysis of periprosthetic cysts

IntroductionDespite good clinical results following total ankle replacement (TAR), the development of large periprosthetic cysts (>400 mm2) in the medium-term is a source of concern. ObjectiveThe primary objective of this study was to detect any large periprosthetic cysts in a cohort of AKILE™ patients using radiographs and CT scans, and then to compare these findings to published ones. Material and methodsA total of 127 TAR procedures were performed between June 1995 and January 2012. We retrospectively reviewed 68 cases with the newest AKILE™ implant design that had a minimum follow-up of 36 months. The average follow-up was 81±33 months; eight patients were lost to follow-up. The outcomes consisted of analyzing radiographs (A/P and lateral weight bearing views, Meary view and lateral views of flexion/extension) and helical CT scans, performing clinical evaluations (range of motion, AOFAS score, Foot Function Index, pain levels) and determining the survivorship of TAR implants. ResultsTAR survival at 5 years was 79% for in situ implants and 62% for revision-free implants. The AOFAS score improved from 33.7±14.7 to 77.1±15.1 (out of 100) and the pain sub-score was 30.2±9.7 (out of 40) at the last follow-up. The average ankle range of motion was 32.3°±12.7° on the radiographs. CT scan revealed Type A cysts (<200 mm2) under the talar implant in 52% of cases and in the tibia in 50% of cases; these cysts were smaller than 100 mm2 in 80% of cases and had no effect on the implants. No periprosthetic cysts larger than 400 mm2 in size were identified. DiscussionThe medium-term functional results and survivorship are comparable to those reported for other TAR designs. The incidence of cysts was low overall and there were no large-diameter cysts, which should improve long-term survival. The implant's design and materials likely played a role in preserving the periprosthetic bone stock. The AKILE™ TAR has distinctive features related to the low rate of large periprosthetic cysts in the medium-term. Level of evidenceIV (retrospective case series).

Advances in Ankle Replacement: A Review

Advanced ankle arthritis is a disabling chronic condition associated with severe pain, deformity, disruption of physical function and quality of life. Ankle arthrodesis has long been used as a successful treatment for advanced ankle arthritis. Total ankle replacement is a fast growing alternative to arthrodesis as it preserves joint mobility and function with good pain relief. Longer term outcome studies showing better survival rates with improved implant and instrumentation designs further fuel this revival of interest in total ankle replacement. This article reviews the evolution of total ankle replacement implants and instrumentation in addressing the various challenges in ankle replacement.

Histologic study of periprosthetic osteolytic lesions after AES total ankle replacement. A 22 case series

Medium-term results for total ankle replacement (TAR) are in general satisfactory, but there is a high redo rate for periprosthetic osteolysis associated with the AES implant. Comparing radioclinical findings and histologic analysis of implant revision procedure specimens can account for the elevated rate of osteolysis associated with the AES TAR implant. In a prospective series of 84 AES TAR implants (2003-2008), 25 underwent revision for osteolysis (including three undergoing revision twice) at a mean 59.8 months. Eight patients had hydroxyapatite (HA) coated models and the others had titanium-hydroxyapatite (Ti-HA) coatings. Radiographs were systematically analyzed on Besse's protocol and evolution was monitored on AOFAS scores. The 94 specimens taken for histologic analysis during revision were re-examined, focusing specifically on foreign bodies. Macroscopically, no metallosis or polyethylene wear was found at revision. AOFAS global and pain scores fell respectively from 89.7/100 at 1 year postoperatively to 72.9 before revision and from 32.5/40 to 20.6/40, although global scores were unchanged in 25% of patients. Radiologically, all patients showed tibial and talar osteolytic lesions, 45% showed cortical lysis and in 25% the implant had collapsed into the cysts. All specimens showed macrophagic granulomatous inflammatory reactions in contact with a foreign body; the cysts showed necrotic remodeling. Some of the foreign bodies could be identified on optical histologic examination with polyethylene in 95% of the specimens and metal in 60% (100% of HA-coated models and 33.3% of Ti-HA-coated models). Unidentifiable material was associated: a brownish pigment in Ti-HA-coated models (33.3%) and flakey bodies in 44.4% of the HA-coated models and 18.2% of the Ti-HA-coated models. The phenomenon of periprosthetic osteolysis is still poorly understood, although implant wear debris seems to be implicated. All the patients with HA-coated implants with modular tibial stem had metal particles in the tissue around the implant, although their exact nature could not be determined. The double-layer Ti-HA coating may induce delamination by fretting while the biological bone anchorage is forming. Prospective cohort study - Level IV.

Tri-Component, Mobile Bearing, Total Ankle Replacement: Mid-term Functional Outcome and Survival

Tri-component, mobile bearing, uncemented, total ankle replacements were introduced after the high failure rates of cemented, highly constrained, first-generation, total ankle replacement implants. A total of 30 primary total ankle replacements in 29 patients (20 males and 9 females) were followed up in the present retrospective study for up to 13 (mean 5.1 ± 4) years. The postoperative functional and radiographic outcomes were measured. Failure was defined as revision of either of the components for any reason or conversion of the total ankle replacement to arthrodesis because of debilitating pain that did not resolve after surgery. Of the 29 patients, 2 underwent revision and 1 underwent arthrodesis. All 3 patients had the malpositioned talar implant revised. The mean American Orthopaedic Foot and Ankle Society score was 81 at 1 year postoperatively. Revision of the tibial or talar component for any reason or conversion of the ankle replacement to arthrodesis was considered failure for the survival analysis. Kaplan-Meier analysis showed a 5-year survival rate of 87.6%. The last failure occurred 23.3 months after surgery.

RANKL in the osteolysis of AES total ankle replacement implants

Peri-implant tissue reactions in failed total ankle replacement (TAR) are characterized by early developing peri-implant osteolysis. The hypothesis of the study was that this reaction is mediated by receptor activator of nuclear factor kappa B ligand (RANKL).Samples of peri-prosthetic tissues from failed TAR implants were stained for macrophages, RANKL, its receptor RANK and osteoprotegerin (OPG), and compared to control samples.The failed TAR implants were surrounded by implant capsule, synovial lining-like interface membrane or necrotic tissues. Infiltrating scavenger receptor I positive CD163+ macrophages were frequent, in particular around necrotic soft tissues or bone sequestrate, and possibly in part formed due to ischemia and mechanical factors. In contrast, implant-derived wear debris was scanty. Still many RANK+ macrophages were often seen in close contact with RANKL+ mesenchymal cells, whereas OPG was mostly located at a distance in vascular endothelial cells. Foreign body giant cells were frequent.RANKL seems to stimulate locally accumulated CD163+ RANK-expressing cells to fusion, which leads to the local formation of multinuclear foreign body giant cells (and probably of osteoclasts). Therefore, peri-implant osteolysis in early TAR implant failure seems to be caused by the RANKL-driven chronic foreign body inflammation directed against, not implant-derived particles, but against necrotic autologous tissues.

The effect of specimen size and geometry on the mechanical behaviour of trabecular bone specimens

Implant loosening – commonly linked with elevated initial micromotion – is the primary indication for total ankle replacement (TAR) revision. Finite element modelling has not been used to assess micromotion of TAR implants; additionally, the biomechanical consequences of TAR malpositioning – previously linked with higher failure rates – remain unexplored. The aim of this study was to estimate implant-bone micromotion and peri-implant bone strains for optimally positioned and malpositioned TAR prostheses, and thereby identify fixation features and malpositioning scenarios increasing the risk of loosening. Finite element models simulating three of the most commonly used TAR devices (BOX®, Mobility® and Salto®) implanted into the tibia/talus and subjected to physiological loads were developed. Mobility and Salto demonstrated the largest micromotion of all tibial and talar components, respectively. Any malpositioning of the implant creating a gap between it and the bone resulted in a considerable increase in micromotion and bone strains. It was concluded that better primary stability can be achieved through fixation nearer to the joint line and/or while relying on more than a single peg. Incomplete seating on the bone may result in considerably elevated implant-bone micromotion and bone strains, thereby increasing the risk for TAR failure.