Talus Implants Research Articles

Lessons from Revision Total Ankle Replacement: Tibias Fail Early, and Taluses Fail Late (And Fail Again)

Introduction/Purpose: With the last decade’s surge in total ankle replacement (TAR), there is an anticipated commensurate increase in the number of revisions that orthopaedic foot and ankle surgeons will encounter. The salvage and implant options to deal with a failed primary TAR are expanding. However, the literature on survivorship and outcomes after revision TAR in the modern era is relatively limited. What’s more, little is known about the risk factors for further revision or failure of revision TAR. This study aimed to describe the timing to and survivorship after revision TAR. We hypothesized that tibial-sided failures would occur earlier after the index surgery, and secondary revisions after failure of revision TAR would occur more due to talar-sided failures than tibial-sided failures. Methods: This is a single-institution retrospective cohort study of TAR patients (2012-2022) with minimum 2-year follow-up. Revision TARs (defined as exchange of tibial and/or talar components) with any implant (Cadence, Inbone, Invision, Infinity, Salto, STAR, Vantage, Zimmer; or custom total talus replacement [TTR]) were included. Five surgeons contributed patients. Demographics, primary and revision surgical data, and postoperative complications were recorded. Etiology of failure necessitating revision (tibial failure, talus failure, combined failure) and ultimate outcomes after revision (revision TAR survived, additional revisions, conversion to fusion, below-knee-amputation [BKA]) were recorded. Revisions for periprosthetic joint infection (PJI) and conversions to fusion were excluded. There were 59 ankles that underwent revision for any cause. Excluding 9 2-stage revisions for PJI and 3 conversions to ankle or tibiotalocalcaneal fusion, there were 47 ankles that underwent revision TAR that were included for analysis. Chi-square and ANOVA tests were used to compare risk factors and timing for failure. Results: There were 47 revision TARs, with mean age 60.6 (range: 31-77) years, mean BMI 29.5 kg/m2, 19 (40.4%) females, and mean 3.5 years follow-up. Revisions for tibial failure (n=22) occurred significantly earlier (1.3 ± 0.5 years) than those for talus failure (n=19, 2.3 ± 1.7 years) or combined tibial/talus failure (n=6, 2.9 ± 3.3 years) (P=0.048). Revisions for tibial-only failure had significantly better survivorship (95.5%) than revisions for talus or combined tibia/talus failures: 26% of talus failures and 33% of combined tibia/talus failures underwent at least one more revision (P=0.033). Of the 7 failures after revision talus, 2 ultimately underwent BKA, 2 were converted to TTR, 2 were revised to modular stemmed talus implants, and 1 was treated with explant and cement spacer for PJI. Conclusion: This study demonstrates that TAR tibial failures occur earlier than talus failures or combined tibial/talus failures. When patients with isolated tibial failure undergo revision of both tibial and talar components, they usually do well with good survivorship post-revision. However, revisions for talar failures and combined tibial/talar failures occur later but are more devastating: nearly 1/3 go on to a second revision. This is important given the consequences of talar implant subsidence, bone necrosis, loss of bone stock, and limited salvage options. As TAR utilization expands, it is imperative to develop implants and surgical strategies to maximize success for revision surgery. Flowchart of failures after revision TAR. PJI = periprosthetic joint infection. TTC = tibiotalocalcaneal. BKA = below-knee amputation. Tibial implant geometry included: Low-pro = lowprofile tibial implant. Stem = stemmed tibial implant. Keel = keeled tibial implant. Talus implant geometry included: chamfer/round and flat-cuttalus.

Design of a lightweight universal talus implant using topology optimization.

Total talus replacement is a promising alternative treatment for talus fractures complicated by avascular necrosis and collapse. This surgical option replaces the human talus bone with a customized talus implant and can maintain ankle joint functionality compared to traditional treatment (e.g., ankle fusion). However, the customized implant is costly and time-consuming due to its customized nature. To circumvent these drawbacks, universal talus implants were proposed. While they showed clinically satisfactory results, existing talus implants are heavier than biological talus bones as they are solid inside. This can lead to unequal weight between the implant and biological talus bone, and therefore leading to other complications. The reduction of the implants' weight without compromising its performance and congruency with surrounding bones is a potential solution. Therefore, this study aims to design a lightweight universal talus implant using topology optimization. This is done through establishing the loading and boundary conditions for three common foot postures: neutral, dorsi- and plantar-flexion. The optimized implant performance in terms of mass, contact characteristics with surrounding joint cartilage and stress distributions is studied using a 3D Finite Element (FE) model of the ankle joint. The mass of the optimized implant is reduced by approximately 66.6% and its maximum stresses do not exceed 70MPa, resulting in a safety factor of 15.7. Moreover, the optimized and solid implants show similar contact characteristics. Both implants produced peak contact pressures that were approximately 19.0%-196% higher than those produced by the biological talus. While further mechanical testing under in-vivo loading conditions is required to determine clinical feasibility, preliminarily, the use of a lightweight universal implant is expected to provide the patient with a more natural feel, and a reduced waiting period until surgery.

Electron beam manufacturing 3D printed titanium with titanium nitride coating shows favorable overall performance when compared to selective laser sintering cobalt chrome in patient specific talus implants

Background: Selective Laser Sintering (SLS) Cobalt Chrome (CoCr) has been utilized extensively in medical implants. However, Electron Beam Manufacturing (EBM) 3D Printed Titanium-6 Aluminum-4 Vanadium Extra Low Interstitial (Ti6Al4V ELI) is increasingly being utilized due to its higher strength to weight ratio, biocompatibility, resistance to corrosion, and capacity for osseointegration. More recently, Titanium Nitride (TiN) coatings have been applied to Ti6Al4V ELI to improve its long-term durability. While durability and tribology are a critical measure of an implant’s overall performance, no studies comparatively examine these properties in Ti6Al4V ELI TiN and CoCr talar implants. Methods: As a result, two independent laboratories were engaged to test these properties using: ASTM F1978-18 “Standard Test Method for Measuring Abrasion Resistance of Metallic Thermal Spray Coatings by Using the Taber Abraser”, and ISO 22622:2019 “Implants for surgery-Wear of total ankle-joint prostheses”. Results: Testing using method ASTM F1978-18, Ti6Al4V ELI TiN performed 34x more favorably than mirror polished CoCr. Using method ISO 22622:2019, Ti6Al4V ELI TiN and SLS performed similarly, showing only slight wear when tested against Ultra High Molecular Weight Polyethylene (UHMWPE) over 5 million cycles. Conclusion: With density, durability, tribology and ingrowth potential as a combined measure of overall performance, results support Ti6Al4V ELI TiN being favorable to CoCr in certain talar implants.Clinical Relevance: In addition to offering superior abrasion performance and slightly better wear resistance, 3D printed Ti6Al4V ELI TiN coated implants offer physicians and patients a highly customizable implant with a lower strength to weight ratio than existing CoCr SLS implants.Level of Evidence: V.

Short-Term Efficacy and Safety of Combined Total Talus and Total Ankle Replacement

Category:Ankle; Ankle ArthritisIntroduction/Purpose:The indications for both total talus replacement (TTR) and total ankle replacement (TAR) are expanding. Combined total ankle-total talus (combined TAR) is a novel treatment option for patients with end-stage ankle arthritis and talar avascular necrosis (AVN) and patients with a prior TAR and talar component collapse. End-stage talar AVN with subchondral collapse is a challenging entity to treat. Historically, an alternative treatment option was tibiotalocalcaneal arthrodesis with structural allograft which results in fair outcomes including nonunion rates up to 40%. Combined TAR is a treatment option that theoretically maintains joint range of motion and restores anatomic alignment. The purpose of this study is to evaluate the short-term outcomes for combined TAR including pain, functional outcomes, radiographic outcomes, and complications.Methods:Consecutive patients who underwent combined TAR from 2016-2020 were retrospectively reviewed. All surgeries were performed by one of four fellowship-trained foot and ankle orthopaedic surgeons at a single academic institution. All talus implants were custom 3D printed total tali (Additive Orthopaedics, Little Silver, NJ), composed of an alloy primarily made of cobalt chrome. The implants were sized based on computed tomography scans of the contralateral talus and created to articulate with multiple TAR systems. Patient demographics, comorbidities, and surgical data were collected. Outcomes included the Visual Analog Scale (VAS) scores, radiographic alignment, range of motion, and complications. Data analysis was performed with paired t- tests and a significance level of p<0.05.Results:A total of 66 patients (67 ankles) were included with an average 12-month follow-up. There were 35 (52.2%) men, and the average age was 56.4 years old. The majority of patients (n=42, 62.7%) underwent combined TAR for talar AVN and tibiotalar arthritis while 21 (31.3%) patients were converted from an isolated TAR and 4 patients (6.0%) from an isolated TTR to combined TAR. A total of 23 (34.3%) patients had a previous talus fracture. Significant postoperative improvements compared to preoperative included: VAS (2.8 vs. 8.2; p<0.0001), ankle dorsiflexion (11.0° vs. 4.7°; p=0.0007), ankle plantarflexion (31.9° vs. 23.7°; p<0.0001), talar declination angle (20.7° vs. 11.6°; p=0.0007), Meary’s angle (2.2° vs. 10.4°; p=0.0043), and talocalcaneal height (79.6mm vs. 74.2mm; p <0.0001). There was a total of 10 (14.9%) complications, 7 (10.4%) of which required repeat surgery. There were 3 (4.5%) failures requiring explant, revision, or amputation (Table 1).Conclusion:Combined TAR is an efficacious and safe procedure. Patients experienced improvement in pain, ankle range of motion, and radiographic parameters postoperatively. This technique provides an anatomic treatment with preservation of ankle motion for patients with severely deficient bone stock due to talar AVN with ankle arthritis or failed TAR. To confirm these preliminary positive results, further studies are required including continued longer-term follow-up, prospective cohorts, and comparative analyses to other treatment options.

Total Talus Replacement with Subtalar Arthrodesis for End-Stage Talar Osteonecrosis and Adjacent Joint Arthritis

Category:Ankle; Hindfoot; Trauma; OtherIntroduction/Purpose:Late stage talar avascular necrosis (AVN) results in devascularization of the talus with osteonecrosis, subchondral collapse, talus fracturing, loss of talar height and declination, and adjacent joint arthritis. This results in significant disability for patients with limited treatment options. Tibiotalocalcaneal arthrodesis with structural allograft has been the historic treatment option for this challenging clinical scenario, however, the outcomes are fair with reported nonunion rate up to 40%. Total talus replacement (TTR) is an emerging treatment option for patients with talar AVN. TTR with subtalar arthrodesis may be utilized for end-stage talar AVN with adjacent joint arthritis. The purpose of this study is to evaluate the short-term outcomes for TTR with subtalar arthrodesis including pain, ankle range of motion, union rates, and complications.Methods:Consecutive patients who underwent TTR with a subtalar arthrodesis from 2016-2020 were retrospectively reviewed. All surgeries were performed by one of two fellowship-trained foot and ankle orthopaedic surgeons at a single academic institution. All talus implants were custom 3D printed total tali (Additive Orthopaedics, Little Silver, NJ), composed of a cobalt chrome alloy. The implants were sized based on computed tomography scans of the contralateral talus. The plantar surface of the tali were prepared with an ingrowth surface and drill holes to accept screws and facilitate subtalar arthrodesis. Patient demographics, comorbidities, and surgical data were collected. Outcomes included the Visual Analog Scale (VAS) scores, range of motion, union rates, and complications. Data analysis was performed with paired t-tests and a significance level of p<0.05.Results:A total of 19 patients were included with an average of 11.4 months follow-up. There were 11 (57.9%) men, and the average age was 52.3 (range, 18-75) years-old. About half (n=9, 47.4%) of the cohort had a previous talus fracture, and 17 (89.5%) patients had previous ipsilateral ankle surgery. There were 5 (26.3%) patients who underwent concomitant talonavicular arthrodesis at the time of TTR and subtalar arthrodesis. Significant postoperative improvements were observed in VAS scores (2.4 vs. 8.6; p<0.0001), ankle dorsiflexion (10.7° vs. 1.8°; p=0.0131), and ankle plantarflexion (32.9° vs. 21.3°; p<0.0068). All patients achieved a successful union of their subtalar arthrodesis (n=19/19, 100%) and talonavicular arthrodesis (n=5/5, 100%). There were 2 (10.5%) complications including 1 (5.3%) delayed wound healing successfully treated nonoperatively and 1 (5.3%) reoperation in a patient with persistent varus ankle deformity requiring distal tibia and fibula osteotomies. There were no postoperative infections, explants, or amputations required.Conclusion:TTR with subtalar arthrodesis should be considered as a treatment option for patients with end-stage talar AVN and adjacent joint arthritis. Patients experienced significant improvement in pain and ankle range of motion. The complication rate was 10.5% with only 1 patient requiring a reoperation. A union rate of 100% was observed with no postoperative infections, explants, or amputations. This technique perves ankle motion while eliminating hindfoot pain in patients with loss of talar bony integrity and adjacent joint arthritis. Further studies are required including longer-term follow-up and prospective cohorts to ensure continued safety and efficacy.

Polycarbonate-urethane coating can significantly improve talus implant contact characteristics

Talus implants can be utilized in cases of talus avascular necrosis and has been regarded as a promising treatment method. However, existing implants are made of stiff materials that directly oppose natural cartilage. The risk of long-term cartilage wear and bone fracture from the interaction between the cartilage and stiff implant surfaces has been documented in post-hemiarthroplasty of the hip, knee and ankle joints.The aim is to explore the effects of adding a layer of compliant material (polycarbonate-urethane; PCU) over a stiff material (cobalt chromium) in talus implants. To do so, we obtained initial ankle geometry from four cadaveric subjects in neutral standing to create the finite element models. We simulated seven models for each subject: three different types of talus implants, each coated with and without PCU, and a biological model. In total, we constructed 28 finite element models. By comparing the contact characteristics of the implant models with their respective biological model counterparts, our results showed that PCU coated implants have comparable contact area and contact pressure to the biological models, whereas stiff material implants without the PCU coating all have relatively higher contact pressure and smaller contact areas. These results confirmed that adding a layer of compliant material coating reduces the contact pressure and increases the contact area which in turn reduces the risk of cartilage wear and bone fracture. The results also suggest that there can be clinical benefits of adding a layer of compliant material coating on existing stiff material implants, and can provide valuable information towards the design of more biofidelic implants in the future.

Investigation of the Average Shape and Principal Variations of the Human Talus Bone Using Statistic Shape Model.

Due to the complexity of articular interconnections and tenuous blood supply to the talus, talus fractures are often associated with complications (e.g., avascular necrosis). Currently, surgically fusing the talus to adjacent bones is widely used as treatment to talus fractures, but this procedure can greatly reduce mobility in the ankle and hindfoot. Alternatively, customized talus implants have shown an overall satisfactory patient feedback but with the limitation of high expenses and time-consuming manufacturing process. In order to circumvent these disadvantages, universal talus implants have been proposed as a potential solution. In our study, we aimed to develop a methodology using Statistical Shape Model (SSM) to simulate the talus, and then evaluate the feasibility of the model to obtain the mean shape needed for universal implant design. In order to achieve this, we registered 98 tali (41 females and 57 males) and used the registered dataset to train our SSM. We used the mean shape derived from the SSM as the basis for our talus implant template, and compared our template with that of previous works. We found that our SSM mean shape talus implant was geometrically similar to implants from other works, which used a different method for the mean shape. This suggests the feasibility of SSM as a method of finding mean shape information for the development of universal implants. A second aim of our study was to investigate if one scalable talus implant can accommodate all patients. In our study, we focused on addressing this from a geometric perspective as there are multiple factors impacting this (e.g., articular surface contact characteristics, implant material properties). Our initial findings are that the first two principal components should be afforded consideration for the geometrical accuracy of talus implant design. Additional factors would need to be further evaluated for their role in informing universal talus implant design.

Treatment Outcomes of Total Ankle Arthroplasty Using an Artificial Talus

Category:Ankle ArthritisIntroduction/Purpose:Problems associated with mid- to long-term total ankle arthroplasty (TAA) include loosening and sinking of the talus implant. These problems arise due to a lack of stability compared with artificial joints such as knees and hips. In Japan, aluminum ceramic talus implants have been developed and are reported to result in good treatment outcomes. Here we investigated and report the treatment outcomes for combined TAA (cTAA), in which TAA is combined with a talus implant during the initial procedure.Methods:We assessed 15 cases involving 16 feet that underwent cTAA for which follow-up was available for at least 6 months. These cases comprised five feet with stage IIIb osteoarthritis and nine feet with stage IV osteoarthritis according to the Takakura classification as well as two feet with talus necrosis. The mean patient age was 73.5 years, and the mean follow-up period was 9.1 months. The talus implant was completely customized for all cases, and TNK Ankle (Kyocera, Japan) tibial components were used. Assessments were performed before and after surgery using the Japanese Society for Surgery of the Foot scale (JSSF scale) for physician-led objective evaluation and the Japanese Orthopaedic Association/Japanese Society for Surgery of the Foot and Self- Administered Foot Evaluation Questionnaire (SAFE-Q) subscales for patient-reported subjective evaluation. The validity and reliability of the JSSF scale and SAFE-Q have been verified by metering psychological verification. Scores were compared using paired t-tests.Results:The mean JSSF score improved significantly from 46.7 points preoperatively to 85.3 points postoperatively. For the SAFE-Q, pain and pain-related scores improved significantly from 40.5 points preoperatively to 75.8 points postoperatively (p < 0.001), physical function and daily lifestyle status improved significantly from 41.1 points preoperatively to 72.9 points postoperatively (p < 0.001), social lifestyle function improved significantly from 37.8 points preoperatively to 76.4 points postoperatively (p < 0.001), and overall sense of health improved significantly from 51.7 preoperatively to 80.7 postoperatively (p < 0.001). Shoe-related results improved from 50.0 points preoperatively to 69.4 points postoperatively (p = 0.038). Only one postoperative complication was noted, which was a case of deep wound infection.Conclusion:Short-term outcomes for cTAA were mostly good in our study population. The findings suggest that talus implants could be used as a treatment option for cases involving severe deformities, crushed talus injuries, and bone defects in which the use of TAA had previously been difficult, which can be handled with cTAA. These findings reveal new possibilities for expanded indications going forward. However, as these were short-term treatment outcomes, further follow-up is required in future studies.

Contact of Rough Surfaces in Ankle Implants Under Combined Normal and Twist Loading

The medical application of implant replacements to remedy the pain in joints has necessitated a comprehensive study of wear due to contact of implant surfaces. Excessive wear can lead to toxicity and other implant associated medical issues such as patient discomfort and decreased mobility. Since implant wear is the result of contact between surfaces of tibia and talus implant, it is important to establish a model that can address implant surface contact mechanics with roughness effects. In this research, a statistical contact model is developed for the interaction of tibia and talus including normal and lateral contact in which surface roughness effects are included. The model accounts for the elastic–plastic interaction of the implant surface with roughness. For this purpose, tibia and talus implants are considered as macroscopic surfaces containing micron-scale roughness. Approximate equations are obtained that relate the contact force to the mean surface separation explicitly. Closed-form equations are obtained for hysteretic energy loss in implant using the approximate equations. Such a function can serve as a very useful tool for implant designers and manufacturers. Natural frequencies of both adduction-abduction and planter-dorsiflexion rotations are obtained using nonlinear vibration analyses.

Neural Network as an Assisting Tool in Designing Talus Implant

The design of current talus implant are focusing too much on mechanical simplicity and usually based on certain population which tends to ignore the anatomically difference between populations. An anatomically talus implant design is known can reduce the contact pressure but one of the constraints for designing implant anatomically is to get bone parameters. This is due to the difficulty to get enough volunteers in getting bone parameters using hazardous method (X-ray or CT scan) .Thus, the talus implant (TI) for particular population was developed based on artificial neural network (ANN) prediction. By using Finite Element Method (FEM), numerical models that include mainly the talus bone and the talus implants are created to compare the contact pressure distribution of the newly develop talus implant with the three different kind of current talus implant designs (BOX, STAR &amp; TNK). For FEM results, only BOX and the newly develop talus implant exceeded the contact stress recommended for the superior articular surface compared to the others. The results also showed that the stress increased near the resected surface. Thus, it is agreed that excessive bone resection may not support the force at the ankle which consequently may contribute to early loosening and subsidence of the talus implant. It is concluded that the excessive bone resection can be avoided by perfectly match talus implant which only can be achieved by designing talus implant for a particular population.

Morphometry of the articular facets on the superior, medial and lateral surfaces of the body of talus and its clinical relevance

Background: In the formation of Ankle joint, tibio-fibular mortice receives superior, medial and lateral articular surfaces of body of Talus. Because of very limited availability of the data on the Morphometry of the articular facets on the Body of the dry human tali, this study was undertaken. Aims: To prepare the database on the articular facets on the superior, medial and lateral surfaces of body of talus, to find if there is statistically significant difference between both the sides of measurements and to compare the results with the previous studies. Methods and Material: 40 Dry Human Tali (20 Right and 20 Left) were measured with Digital vernier caliper for the following Measurements: On the Trochlear surface: Medial length, Central length, Lateral length, Anterior width, Central width, Posterior width. On the lateral triangular articular facet: Central height, Central width. On the coma shaped medial articular facet: Central height, Central width. Results: Mean values of Medial, Central and Lateral lengths were 31.02, 30.39 and 29.63mm on Right side and 31.79, 30.65 and 29.45mm on Left side. Mean Anterior, Central and Posterior widths were 28.87, 28.16 and 21.59mm on right side and 29.08, 27.54 and 21.78mm on left side. On the medial articular surface, mean central height was 11.93mm on the right side and 11.29mm on the left side, Mean central width was 27.94mm on the right side and 28.29mm on the left side. On the lateral articular surface, mean central height was 22.14mm on the right side and 22.63mm on the left side. Mean central width was 18.93mm on the right side and 18.99mm on the left side. There is no significant difference between right and left sides of measurements. Conclusion: The trochlear articular surface is wider in front, measurements of opposite talus bone can be used as a control during talus bone replacement surgery, it may help surgeons to plan pre-operatively the complex talar fracture surgeries, to design accurate talus bone prosthesis and talus implants.

Total Ankle Arthroplasty Accuracy and Reproducibility Using Preoperative CT Scan-Derived, Patient-Specific Guides

Preoperative navigation has provided many potential benefits for total knee arthroplasty, including patient-specific alignment, repeatable implant placement, and decreased operative time. For the first time, this technology was applied to total ankle arthroplasty (TAA). This study evaluated repeatability of tibia and talus patient-specific guide placement and deviation between the preoperative plan and actual implant placement. Routine ankle CT scans were acquired of 15 cadaveric lower extremity limbs, converted into 3D solid models, and imported into a computer-assisted design assembly. Anatomic landmarks defining tibia/talus alignment were established and used to perform a virtual TAA. Commercially available implant components were placed to mimic traditional cases. An operative guide referencing the cadaver-specific anatomy was engineered to define the resection planes necessary to re-create virtual placement of traditional tibia and talus implants in the postoperative position. Board-certified TAA orthopaedic surgeons with no prior preoperative navigation experience placed the operative guides onto the bones based on tactile and visual feedback. Guide placement was repeated 4 times to determine variability. Final implant position was recorded with an infrared probe, confirmed with CT scans, and compared to the preoperative plan. Average deviations between planned and actual guide placement were determined for all rotational and translational degrees of freedom (DOF). In addition, implant component location was measured radiographically. Intraobserver tibia and talus guide variation between all trials was 0.26 ± 0.18 degrees and 0.36 ± 0.25 degrees in flexion/extension, 0.61 ± 0.58 and 0.53 ± 0.53 in varus/valgus, and 0.79 ± 0.38 degrees and 1.15 ± 0.77 degrees in internal/external rotation, respectively. Average variation between preoperative and postoperative implant placement was less than 2 degrees and 1.4 mm in all specimens tested. Preliminary data suggest that preoperative navigation and custom operative guides result in reliable and reproducible placement of TAA implants and patient-specific ankle alignment. Deviation of final implant placement from the preoperative plan was less than 2 degrees in all angular DOF, providing greater accuracy than the ±3 degrees determined in other implant system studies using traditional instrumentation and computer navigation. We have further demonstrated that final implant position is successfully guided by these patient-specific guides, with reproducibility of tibial component placement falling within 2 degrees of the intended target. This level of reproducibility suggests a promise for this technology, and it is hoped this level of accuracy will become the benchmark for the next generation of total ankle arthroplasty.

Three-dimensional geometric analysis of the talus for designing talar prosthetics

Proper understanding of the complex geometric shape of the talus bone is important for the design of generic talar body prosthetics and restoration of the proper ankle joint function after surgery. To date, all talus implants have been patient-specific with the limitation that complex computer modeling is required to produce a mirrored image from the unaffected opposite side followed by machining a patient-specific prosthesis. To develop an "off-the-shelf" non-custom talar prosthesis, it is important to perform a thorough investigation of the geometric shape of the talus bone. This article addresses the applicability of a scaling approach for investigating the geometric shape and similarity of talus bones. This study used computed tomography scan images of the ankle joints of 27 different subjects to perform the analysis. Results of the deviation analyses showed that the deviation in the articulating surfaces of the talus bones was not excessive in terms of talus size. These results suggest that a proposed range of five implant sizes is possible. Finally, it is concluded that the talus bones of the ankle joints are geometrically similar, and a proposed range of five implant sizes will fit a wide range of subjects. This information may help to develop generic talus implants that might be applicable to patients with a severe talus injury.

A contact mechanics model for ankle implants with inclusion of surface roughness effects

Total ankle replacement is recognized as one of the best procedures to treat painful arthritic ankles. Even though this method can relieve patients from pain and reproduce the physiological functions of the ankle, an improper design can cause an excessive amount of metal debris due to wear, causing toxicity in implant recipient. This paper develops a contact model to treat the interaction of tibia and talus implants in an ankle joint. The contact model describes the interaction of implant rough surfaces including both elastic and plastic deformations. In the model, the tibia and the talus surfaces are viewed as macroscopically conforming cylinders or conforming multi-cylinders containing micrometre-scale roughness. The derived equations relate contact force on the implant and the minimum mean surface separation of the rough surfaces. The force is expressed as a statistical integral function of asperity heights over the possible region of interaction of the roughness of the tibia and the talus implant surfaces. A closed-form approximate equation relating contact force and minimum separation is used to obtain energy loss per cycle in a load–unload sequence applied to the implant. In this way implant surface statistics are related to energy loss in the implant that is responsible for internal void formation and subsequent wear and its harmful toxicity to the implant recipient.