Finite element analysis of the initial stability of ankle arthrodesis with internal fixation: flat cut versus intact joint contours

Comparative study of two retrograde locked intramedullary nail designs for ankle arthrodesis: A finite element analysis

Ankle arthrodesis is the gold standard for treatment of end-stage arthritis. The goal of ankle arthrodesis is to obtain bony union between the tibia and the talus. Retrograde intramedullary nailing is typically reserved for ankle and subtalar joints arthrodesis. The purpose of this study is to evaluate the effect of two different materials, two locking pin configurations and two nail designs of a retrograde locked intramedullary nail used for ankle arthrodesis. Using the finite element analysis, a numerical study of ankle arthrodesis was developed to evaluate the effect of materials: TI-6Al-4V and stainless steel AISI 316 LVM; two locking pin configurations: five and six pins, on two intramedullary nails: Ø10 × 180 mm and Ø11 × 200 mm. A model of a healthy foot was created from tomographic scans. It was found that the mechanical stimulus required to achieve bone fusion were higher for Ø10 × 180 nails (6.868 ± 0.047) than the Ø11 × 200 nails (5.918 ± 0.047; p < 0.001; mean ± SEM). We also found that six-pin configuration had a higher mechanical stimulus (6.470 ± 0.047) than the five-pin configuration (6.316 ± 0.046; p = 0.020). Similarly, it was higher for titanium (6.802 ± 0.047) than those for stainless steel (5.984 ± 0.046; p < 0.001). Finally, the subtalar zone presented higher values (7.132 ± 0.043) than the tibiotalar zone (5.653 ± 0.050; p < 0.001). The highest mechanical stimulus around the vicinity of tibiotalar and subtalar joint was obtained by Ø10 × 180 nails, made of titanium alloy, with 6P.

Clinically useful finite element models of the natural ankle - A review

BACKGROUND

Biomechanical simulation of the foot and ankle complex is a growing research area but compared to simulation of joints such as hip and knee, it has been under investigated and lacks consistency in research methodology. The methodology is variable, data is heterogenous and there are no clear output criteria. Therefore, it is very difficult to correlate clinically and draw meaningful inferences.

METHODS

The focus of this review is finite element simulation of the native ankle joint and we will explore: the different research questions asked, the model designs used, ways the model rigour has been ensured, the different output parameters of interest and the clinical impact and relevance of these studies.

FINDINGS

The 72 published studies explored in this review demonstrate wide variability in approach. Many studies demonstrated a preference for simplicity when representing different tissues, with the majority using linear isotropic material properties to represent the bone, cartilage and ligaments; this allows the models to be complex in another way such as to include more bones or complex loading. Most studies were validated against experimental or in vivo data, but a large proportion (40%) of studies were not validated at all, which is an area of concern.

INTERPRETATION

Finite element simulation of the ankle shows promise as a clinical tool for improving outcomes. Standardisation of model creation and standardisation of reporting would increase trust, and enable independent validation, through which successful clinical application of the research could be realised.

Disease-Specific Finite element Analysis of the Foot and Ankle

Finite-element analysis is a computational modeling technique that can be used to quantify parameters that are difficult or impossible to measure externally in a geometrically complex structure such as the foot and ankle. It has been used to improve our understanding of pathomechanics and to evaluate proposed treatments for several disorders, including progressive collapsing foot deformity, ankle arthritis, syndesmotic injury, ankle fracture, plantar fasciitis, diabetic foot ulceration, hallux valgus, and lesser toe deformities. Parameters calculated from finite-element models have been widely used to make predictions about their biomechanical correlates.

EXPERIMENTAL SUBSTANTIATION OF THE BIOMECHANICAL EFFICIENCY OF THE ANTERIOR PLATE COMBINED WITH TWO SPONGIOUS SCREWS FOR ANKLE ARTHRODESIS

Актуальность. Несмотря на существующее значительное количество разнообразных методик, применяемых для артродеза голеностопного сустава, ряд авторов указывают на определенные технические сложности указанных операций, потерю коррекции заданного положения таранной и большеберцовой костей в процессе анкилозирования, несращения. Проблема совершенствования методики фиксации при артродезе голеностопного сустава требует новых решений. Цель провести сравнительный биомеханический анализ стабильности фиксации при артродезе голеностопного сустава тремя спонгиозными винтами и разработанной пластиной, комбинируемой с двумя спонгиозными винтами, методом конечных элементов. Материалы и методы. Методом конечных элементов выполнена оценка биомеханических характеристик трех вариантов систем фиксации голеностопного сустава при артродезе: три спонгиозных винта, разработанная пластина, комбинируемая с двумя спонгиозными винтами, проксимальный винт в пластине кортикальный, а также разработанная пластина, комбинируемая с двумя спонгиозными винтами, проксимальный винт в пластине с угловой стабильностью. Изучены напряжения и деформации при приложении различных видов нагрузок. Результаты. В модели фиксации голеностопного сустава передней пластиной, комбинируемой с двумя спонгиозными винтами и проксимальным кортикальным винтом, имплантаты и таранная кость испытывали наименьшие напряжения по сравнению с двумя другими моделями. Так, максимальное эквивалентное напряжение в имплантатах при втором варианте составило 68-124 МПа, при первом варианте 92-147 МПа, при третьем 130-331 МПа. Эквивалентное напряжение в таранной кости во втором варианте фиксации составило от 20 до 46 МПа, в первом и третьем вариантах 28-58 МПа и 47-65 МПа, соответственно. Показатели максимального контактного давления на границе большеберцовой и таранной костей оказались наибольшими в первом варианте по сравнению с двумя другими моделями (34 МПа, 31 МПа и 31 МПа соответственно). Заключение. Среди изученных систем фиксации голеностопного сустава для артродеза, применение передней пластины, комбинируемой с двумя спонгиозными винтами и проксимальным кортикальным винтом, является наиболее предпочтительным с точки зрения биомеханики.

Finite element analysis of the initial stability of arthroscopic ankle arthrodesis with three-screw fixation: posteromedial versus posterolateral home-run screw

Objective

Arthroscopic ankle arthrodesis (AAA) is a standard surgical method for the treatment of advanced traumatic ankle arthritis and has become more popular due to its advantages. To fix the tibiotalar joint, the use of three percutaneous screws is considered to have better mechanical stability than the use of two screws. However, it is sometimes difficult to insert three screws because they might block each other due to the small area of the tibiotalar joint surface and the large diameter of the screws; few articles illustrate how to insert three screws without the screws disturbing each other. The purpose of this study is to explore possible screw configurations of tripod fixation in arthroscopic ankle arthrodesis that avoid the collision of screws and yield better biomechanical performance.

Methods

We used the finite element method to examine the impact of different screw positions and orientations on the biomechanical characteristics of a three-dimensional (3D) ankle model. Maximum and average micromotion, pressure on the articular surface, and von Mises stress values of the tibia and the talus were used to evaluate the initial stability of the ankle.

Results

Five kinds of three-screw configurations were identified, and finite element analysis results suggested that configurations with the posteromedial home-run screw presented lower micromotion (maximum, 17.96 ± 7.49 μm versus 22.52 ± 12.8 μm; mean, 4.88 ± 1.89 μm versus 5.19 ± 1.92 μm) (especially configuration 3) and better screw distributions on the articular surface than those with the posterolateral home-run screw.

Conclusion

Screw configurations with the posteromedial home-run screw avoid collision and are more biomechanically stable than those with the posterolateral home-run screw. Thus, inserting the home-run screw through the posteromedial approach is recommended for clinical practice.

Total ankle arthroplasty and ankle arthrodesis affect the biomechanics of the inner foot differently

Ankle arthrodesis and total ankle arthroplasty are the two primary surgeries for treatment of end-stage degenerative ankle arthritis. The biomechanical effects of them on the inner foot are insufficient to identify which is superior. This study compared biomechanical parameters among a foot treated by ankle arthrodesis, a foot treated by total ankle arthroplasty, and an intact foot using computational analysis. Validated finite element models of the three feet were developed and used to simulate the stance phase of gait. The results showed total ankle arthroplasty provides a more stable plantar pressure distribution than ankle arthrodesis. The highest contact pressure, 3.17 MPa, occurred in the medial cuneonavicular joint in the total ankle arthroplasty foot. Neither of the surgeries resulted in contact pressure increase in the subtalar joint. The peak stress in the metatarsal bones was increased in both surgical models, especially the second and third metatarsals. This study enables us to get visual to the biomechanics inside of an intact foot, and feet treated by total ankle arthroplasty and ankle arthrodesis during walking.

Use of three-dimensional finite element models of the lateral ankle ligaments to evaluate three surgical techniques

Objective To compare three surgical techniques for lateral ankle ligament reconstruction using finite element (FE) models. Methods A three-dimensional FE model of the left foot of a healthy volunteer and lateral collateral ligament injury models were developed. Three tendons [one-half of the autologous peroneus longus tendon (PLT), one-half of the peroneus brevis tendon (PBT), and an allogeneic tendon] were used for lateral collateral ligament reconstruction. The ankle varus stress and anterior drawer tests were performed to compare the three surgical techniques. Results The ankle varus stress test showed that the equivalent stresses of the anterior talofibular ligament (ATFL) (84.00 MPa) and calcaneofibular ligament (CFL) (27.01 MPa) were lower in allogeneic tendon reconstruction than in the other two techniques but similar to those of normal individuals (138.48 and 25.90 MPa, respectively). The anterior drawer test showed that the equivalent stresses of the ATFL and CFL in autologous PLT reconstruction (31.31 and 28.60 MPa, respectively) and PBT reconstruction (31.47 and 29.07 MPa, respectively) were lower than those in allogeneic tendon reconstruction (57.32 and 52.20 MPa, respectively). Conclusions The allogeneic tendon reconstruction outcome was similar to normal individuals. Allogeneic tendon reconstruction may be superior for lateral ankle ligament reconstruction without considering its complications.

Transfibular ankle arthrodesis using burring, curettage, multiple drilling, and fixation with two retrograde screws through a single lateral incision

PURPOSE

To review the outcome of 23 ankle arthrodeses using burring, curettage, multiple drilling, and fixation with 2 retrograde screws through a single lateral incision.

METHODS

Records of 22 consecutive patients aged 39 to 79 (mean, 62.4) years who underwent 23 ankle arthrodeses for end-stage ankle arthritis were reviewed. Through a single lateral incision, articular cartilage was removed using burring and curettage, and multiple holes were drilled using a Kirschner wire, followed by fixation with 2 retrograde screws. The resected distal fibula was fixed to the distal part of the talus and tibia. The position of the ankle and subtalar joint arthrosis was assessed by 2 orthopaedic specialists. Pre- and post-operative American Orthopaedic Foot and Ankle Society (AOFAS) scores were evaluated.

RESULTS

The mean operating time was 122 minutes. The mean follow-up period was 41 months. The mean postoperative ankle alignment was suboptimal: 2.7º varus, 6.7º plantar flexion, and 2.9º internal rotation. The mean AOFAS score improved from 30 to 71 (p<0.01). The postoperative varus ankle alignment was not associated with the AOFAS score (r= -0.13, p=0.569). Of the 23 cases, one was nonunion and 22 achieved bone union after a mean of 5.4 (range, 2-16) months; 3 of them were delayed union. Despite bone union, 7 patients complained of persistent pain; 4 of them had progressive arthrosis of the adjacent subtalar joints (n=2) or subtalar and talonavicular joints (n=2).

CONCLUSION

Ankle arthrodesis using burring, curettage, multiple drilling, and fixation with 2 retrograde screws achieved a high union rate and acceptable functional score without serious complications.

Midterm results of posterior arthroscopic ankle fusion

PURPOSE

The presented study was performed to evaluate the midterm clinical and radiological results and complication rates of the first 40 patients with an ankle fusion through a posterior arthroscopic approach.

METHODS

Forty consecutive patients with end-stage post-traumatic ankle osteoarthritis were treated with posterior arthroscopic ankle fusion. All patients were assessed clinically as well as radiologically with a minimum follow-up of 2 years. The Foot and Ankle Ability Measure (FAAM) and Foot Function Index (FFI) were used to assess clinical improvement.

RESULTS

Clinical fusion was achieved in 40 patients within 3 months (100%), and radiological fusion was achieved in 40 patients at 12 months. Two screw mal-placements occurred. Both complications were solved following revision surgery. A significant improvement was noted for both the FAAM [median 38 (range 17-56) to 63 (range 9-84)] and FFI scores [median 66 (range 31-89) to 32 (range 11-98)] for all 40 patients.

CONCLUSION

The posterior arthroscopic ankle fusion is an effective and safe treatment option for end-stage post-traumatic ankle osteoarthritis at midterm follow-up.

LEVEL OF EVIDENCE

Prospective cohort study, Level IV.

Safety and efficiency of posterior arthroscopic ankle arthrodesis

PURPOSE

To study the safety and efficiency of posterior arthroscopic ankle arthrodesis.

METHODS

Ten fresh-frozen human lower leg specimens without evidence of previous surgery to the foot and ankle were selected. Arthroscopic debridement of the tibiotalar joint was performed in all specimens using a standardized protocol. Anatomical dissections were regarded the gold standard for safety analysis. To evaluate the efficacy of the posterior ankle arthroscopic arthrodesis, the debrided articular tibiotalar parts were resected and subsequently analysed using Image-analysis software, Image J (Wayne Rasband, National Institutes of Health, Bethesda, MD).

RESULTS

In none of the specimens, iatrogenic damage was detected during the anatomical dissection following the posterior ankle arthroscopic procedure. A total talar joint surface area of 95 % and total tibial joint surface area of 96 % was addressed during the arthroscopic debridement.

CONCLUSIONS

The posterior ankle arthroscopic arthrodesis can be regarded safe and is also highly effective in the debridement of cartilage, resulting in optimal biology for fusion to occur. Therefore, this new arthroscopic technique potentially will diminish existing non-union rates for the fusion of the ankle joint.

Effects of Ankle Arthrodesis on Biomechanical Performance of the Entire Foot

Background/Methodology

Ankle arthrodesis is one popular surgical treatment for ankle arthritis, chronic instability, and degenerative deformity. However, complications such as foot pain, joint arthritis, and bone fracture may cause patients to suffer other problems. Understanding the internal biomechanics of the foot is critical for assessing the effectiveness of ankle arthrodesis and provides a baseline for the surgical plan. This study aimed to understand the biomechanical effects of ankle arthrodesis on the entire foot and ankle using finite element analyses. A three-dimensional finite element model of the foot and ankle, involving 28 bones, 103 ligaments, the plantar fascia, major muscle groups, and encapsulated soft tissue, was developed and validated. The biomechanical performances of a normal foot and a foot with ankle arthrodesis were compared at three gait instants, first-peak, mid-stance, and second-peak.

Principal Findings/Conclusions

Changes in plantar pressure distribution, joint contact pressure and forces, von Mises stress on bone and foot deformation were predicted. Compared with those in the normal foot, the peak plantar pressure was increased and the center of pressure moved anteriorly in the foot with ankle arthrodesis. The talonavicular joint and joints of the first to third rays in the hind- and mid-foot bore the majority of the loading and sustained substantially increased loading after ankle arthrodesis. An average contact pressure of 2.14 MPa was predicted at the talonavicular joint after surgery and the maximum variation was shown to be 80% in joints of the first ray. The contact force and pressure of the subtalar joint decreased after surgery, indicating that arthritis at this joint was not necessarily a consequence of ankle arthrodesis but rather a progression of pre-existing degenerative changes. Von Mises stress in the second and third metatarsal bones at the second-peak instant increased to 52 MPa and 34 MPa, respectively, after surgery. These variations can provide indications for outcome assessment of ankle arthrodesis surgery.

Three-Dimensional Modelling and Finite Element Analysis of an Ankle External Fixator

The use of ankle external fixator to treat pilon fracture Type III is popular amongst surgeons as it can reduce complications such as non-union and mal-union. Even though configurations of the connecting bars are important, the material also plays a major factor for a successful outcome. In this paper, the Delta external fixator with simulated ankle pilon fractures Type III were modelled and analysed under two different materials; titanium alloy and stainless steel. The finite element model includes tibia, fibula, talus, calcaneus, cuboid, navicular, three cuneiforms and five metatarsals bone. To simulate the pilon fractures Type III, a cutting segment was utilised. The ligaments were assigned with linear spring properties and cartilages were modelled using Mooney-Rivlin hyper-elastic behaviour. The Delta external fixator was designed using a three-dimensional software with two different material properties - titanium alloy and stainless steel. High von Mises stress concentrated at the pin-bone interface with the highest value observed for the titanium fixation. The results also showed less deformation for the stainless steel compared to titanium.

A biomechanical analysis of two anterior ankle arthrodesis systems

BACKGROUND

An increasingly popular ankle fusion method uses plates and screws positioned on the anterior aspect of the tibiotalar joint. This study compared the mechanical properties of 2 contemporary plate systems for ankle arthrodesis, one based on a single anterior plate and the other comprising paired anteromedial and anterolateral plates.

METHODS

Ten matched pairs of fresh-frozen cadaver ankle joints underwent arthrodesis with a single anatomically contoured anterior plate or 2 anatomic plates applied anteromedially and anterolaterally. Each arthrodesed specimen was subjected to controlled sagittal and coronal plane bending and internal and external rotation. Tibiotalar joint bending stiffness, bending angulation, torsional stiffness, and joint rotation were documented.

RESULTS

Bending stiffness of the 2-plate system was 1.5 to 5 times greater than that of the single-plate system in plantarflexion, dorsiflexion, eversion, and inversion (P = .005-.050). Angulation in each bending direction was several-fold greater than for the single plate (P = .005-.014) at the peak applied moment. Torsional stiffness of the 2-plate system nearly doubled that of the single plate in both rotation directions (P = .014, P = .005). Approximately half as much arthrodesis site rotation occurred with 2-plate fixation at the peak applied torque (P = .005, P = .007).

CONCLUSION

The tested 2-plate arthrodesis system has the potential to optimize arthrodesis site stiffness compared with fixation with a contemporary single plate.

CLINICAL RELEVANCE

Use of the stiffer 2-plate system could improve the clinical fusion rate, especially in patients with suboptimal bone quality.

Optimal double screw configuration for subtalar arthrodesis: a finite element analysis

PURPOSE

The subtalar arthrodesis using screws has been performed to manage traumatic subtalar arthritis. Even though clinically there might not have been big difference between using single screw or double screws for subtalar arthrodesis, a double screw fixation is expected to bring a better initial stability in a mechanical view. This study aimed to assess the optimal configuration of double screw fixation for subtalar arthrodesis.

METHODS

From the CT-scanned images of an ankle of a Korean male (21 year old), polygon models of the talus and calcaneus were reconstructed. The polygon models were converted to tetrahedron finite elements. Young's modulus was assigned locally to each element based on the Hounsfield unit, and a Poisson's ratio of 0.4 was commonly. Four fixation configurations of double screw subtalar arthrodesis were modeled by combination of a same placement of a neck screw and one of four different placements of a dome screw, i.e., anterolateral (AL), anteromedial (AM), posterolateral (PL), and posteromedial (PM) placements. External and internal rotation torques of 4 N-m were applied when evaluating the stability of each fixation configuration.

RESULTS

Among the four fixation configurations, the fixation configuration of a neck screw plus a PM dome screw had the least translation of 0.9 and 0.8 mm for external and internal rotational torques of 4 N-m, respectively. The fixation configuration of a neck screw plus a PM dome screw showed the least rotation of 5.0° and 4.8° for external and internal rotational torques of 4 N-m, respectively. The divergence angle or the contact length did not solely match well to the better stability. However, the integration of both the divergence angle of 2 screws and the contact length between screw and bones were proportionally related to the better rotational stability.

CONCLUSION

A posteromedial dome screw combined with a neck screw can be the best surgical choice, which will bring out excellent union rate of the subtalar arthrodesis as well as the best mechanical stability.

Comparison of initial compression of the medial, lateral, and posterior screws in an ankle fusion construct

BACKGROUND

One of the requirements for successful ankle arthrodesis is adequate compression by the fixation across the fusion surfaces. A common screw construct for ankle fusion is three crossed screws from proximal-to-distal. Because the screws are inserted nearly orthogonal to each other, it is possible minimal additional compression is obtainable once the first screw is inserted. The aim of this study was to determine which of the three screws gave the greatest initial compression and theoretically should be inserted first.

MATERIALS AND METHODS

Seventeen cadaver limbs were dissected to expose the anterior and posterior aspects of the tibiotalar joint. Three Fuji film templates were created for each ankle joint with a hole to accommodate a 7.0-mm cannulated screw. Each film was tested with a single medial, lateral, or posterior screw. The Fuji films were then analyzed for contact area, percent contact area, and pressure.

RESULTS

There was no difference in the total contact area, percent contact area, or pressure generated between the three screws. The mean contact area for all screws was 11% of the joint surface. All three screws had greater contact area and percent contact area over the anterior half of the ankle joint.

CONCLUSION

The medial, lateral, and posterior screws were equivalent with respect to contact area, percent contact area, and pressure generated across the tibiotalar joint. All three screws had greater contact area over the anterior half of the joint. Only 11% of the tibiotalar joint surface came in contact following the insertion of a single partially threaded screw.

CLINICAL RELEVANCE

In a neutrally aligned ankle arthrodesis the order of screw insertion does not affect the amount of compression ultimately achieved at the fusion site.

Surgery on the rheumatoid ankle joint: efficacy versus effectiveness

This article examines synovectomy and ankle arthrodesis for the rheumatoid ankle joint. Reviews of osteoimmunology and gait analyses specific to rheumatoid arthritis are included. Comparison studies including ankle arthrodesis and total ankle arthroplasty are reviewed.

External ring fixation versus screw fixation for ankle arthrodesis: a biomechanical comparison

BACKGROUND

The use of external ring fixation for ankle arthrodesis offers an alternative to internal screw fixation when bone quality is suboptimal, when complex ankle pathology is present, and as a salvage procedure for complicated cases. Additionally, it permits earlier weightbearing and postoperative adjustment of bone alignment and joint compression. This study was designed to determine whether there are differences in initial ankle joint fixation integrity between these two techniques.

MATERIALS AND METHODS

Ten fresh-frozen cadaveric lower extremities underwent ankle arthrodesis with a hybrid ring fixator, and the bending and torsional stiffness of each arthrodesed joint was quantified. Stiffness was calculated from angulation measured at the tibiotalar joint using electronic clinometers attached to those bones. The external fixators were then removed and ankle arthrodesis performed using 6.5-mm cannulated screws: three across the ankle joint, one fixing the fibula to the tibia, and one fixing the fibula to the talus. Stiffness testing was then repeated.

RESULTS

No statistically significant difference in bending stiffness, torsional stiffness, or joint rotation under torsional load was demonstrable between the two techniques.

CONCLUSION

The similarity in ankle fusion site stability achieved with external ring fixation and an optimized screw fixation technique under the described testing conditions indicates that, in cases in which suboptimal bone quality or complex ankle pathology preclude the use of internal fixation, an external fixator is a viable option.

CLINICAL RELEVANCE

Establishing that the primary stability of external ring fixation is comparable to that of screw fixation provides a basis for its use in cases in which screw fixation may be problematic.

An accurate estimation of bone density improves the accuracy of subject-specific finite element models

An experimental-numerical study was performed to investigate the relationships between computed tomography (CT)-density and ash density, and between ash density and apparent density for bone tissue, to evaluate their influence on the accuracy of subject-specific FE models of human bones. Sixty cylindrical bone specimens were examined. CT-densities were computed from CT images while apparent and ash densities were measured experimentally. The CT/ash-density and ash/apparent-density relationships were calculated. Finite element models of eight human femurs were generated considering these relationships to assess their effect on strain prediction accuracy. CT and ash density were linearly correlated (R(2)=0.997) over the whole density range but not equivalent (intercep t <0, slope >1). A constant ash/apparent-density ratio (0.598+/-0.004) was found for cortical bone. A lower ratio, with a larger dispersion, was found for trabecular bone (0.459+/-0.100), but it became less dispersed, and equal to that of cortical tissue, when testing smaller trabecular specimens (0.598+/-0.036). This suggests that an experimental error occurred in apparent-density measurements for large trabecular specimens and a constant ratio can be assumed valid for the whole density range. Introducing the obtained relationships in the FE modelling procedure improved strain prediction accuracy (R(2)=0.95, RMSE=7%). The results suggest that: (i) a correction of the densitometric calibration should be used when evaluating bone ash-density from clinical CT scans, to avoid ash-density underestimation and overestimation for low- and high-density bone tissue, respectively; (ii) the ash/apparent-density ratio can be assumed constant in human femurs and (iii) the correction improves significantly the model accuracy and should be considered in subject-specific bone modelling.

Ankle fractures in diabetic neuropathic arthropathy

Between 2000 and 2006 we performed salvage tibiotalar arthrodesis in 17 diabetic patients (17 ankles) with grossly unstable ankles caused by bimalleolar fractures complicated by Charcot neuro-arthropathy. There were ten women and seven men with a mean age of 61.6 years (57 to 69). A crossed-screw technique was used. Two screws were used in eight patients and three screws in nine. Additional graft from the malleoli was used in all patients. The mean follow-up was 26 months (12 to 48) and the mean time to union was 5.8 months (4 to 8). A stable ankle was achieved in 14 patients (82.4%), nine of whom had bony fusion and five had a stiff fibrous union. The results were significantly better in underweight patients, in those in whom surgery had been performed three to six months after the onset of acute Charcot arthropathy, in those who had received anti-resorptive medication during the acute stage, in those without extensive peripheral neuropathy, and in those with adequate peripheral oxygen saturation (> 95%). The arthrodesis failed because of avascular necrosis of the talus in only three patients (17.6%), who developed grossly unstable, ulcerated hindfeet, and required below-knee amputation.

Subject-specific finite element models can accurately predict strain levels in long bones

The prediction of the stress-state and fracture risk induced in bones by various loading conditions in individual patients using subject-specific finite element models still represents a challenge in orthopaedic biomechanics. The accuracy of the strain predictions reported in the literature is variable and generally not satisfactory. The aim of the present study was to evaluate if a proper choice of the density-elasticity relationship can lead to accurate strain predictions in the frame of an automatic subject-specific model generation strategy. To this aim, a combined numerical-experimental study was performed comparing finite element predicted strains with strain-gauges measurements obtained on eight cadaver proximal femurs, each instrumented with 15 rosettes mostly concentrated in the bone metaphyses, tested non-destructively in vitro under six different loading scenarios. Three different density-elasticity power relationships were selected from the literature and implemented in the finite element models derived from computed tomography data. The results of the present study confirm the great influence of the density-elasticity relationship used on the accuracy of numerical predictions. One of the tested constitutive laws provided a very good agreement (R(2)=0.91, RMSE lower than 10% of the maximum measured value) between numerical calculations and experimental measurements. The presented results show, in addition, that the adoption of a single density-elasticity relationship over the whole bone density range is adequate to obtain an accuracy that is already suitable for many applications.

The effect of bone quality on the stability of ankle arthrodesis. A finite element study

BACKGROUND

Despite uniform operating techniques, lack of fusion still occurs after ankle arthrodesis. Differences in the biological healing potential may be a factor but the mechanical performance of the arthrodesis construct because of varying bone quality also may be important. Internal compression techniques are preferred because of higher union rates, shorter fusion times, and fewer complications. A three-screw configuration has been shown to be more stable than a two-screw configuration, but it is not obvious when it should be used.

METHODS

Three-dimensional finite element models of intact and flat-cut ankle arthrodeses were built, using two and three screws in different configurations. Poor bone quality was simulated by decreasing Young's modulus of the bone. The constructs were loaded in torsion and dorsiflexion, and micromotions at the fusion site were measured.

RESULTS

Bone quality had a marked effect on the stability at the arthrodesis site. Inserting two screws at 30 degrees relative to the longitudinal axis of the tibia in an intact arthrodesis seemed the best option, especially as bone quality worsened. The addition of a third screw increased the stability at the arthrodesis site.

CONCLUSIONS

Overall, intact joint surfaces and three-screw fixation, with the lateral and medial screws inserted produced the most stable arthrodesis constructs when bone quality was poor. CLINICAL RELEVANCE. Ankle arthrodeses are technically demanding because of the shape and small size of the talus. Preoperative planning is an absolute necessity to determine placement and number of screws. This study shows that poor bone quality decreases the stability of the arthrodesis constructs, suggesting that an attempt should be made to create the most stable three-screw configuration. Finite element models can be used as an effective preoperative tool for planning screw number and placement.

Initial stability of ankle arthrodesis with three-screw fixation. A finite element analysis

OBJECTIVE

Compare the initial stability at the fusion site of ankle arthrodesis fixed with two and three screws.

DESIGN

Finite element models of ankle arthrodesis were developed from computed tomography images. Two-screw constructs were augmented with a third screw in different orientations and subjected to loads likely to affect the ankle postoperatively.

BACKGROUND

More stable fixation seems to increase the chance of fusion, as it minimises the motion between the tibiotalar interfaces.

METHODS

Non-linear elastic finite element analyses were performed in external torsion and dorsiflexion. The micromotions at the tibiotalar interface were computed to compare the two- and three-screw fixation in intact and flat-cut arthrodesis.

RESULTS

Adding a third screw reduced the micromotions at the fusion site. Inserting the third screw anteriorly predicted lower peak micromotions than inserting the screw posteriorly, except for the intact arthrodesis tested in dorsiflexion. Three-screw intact arthrodesis predicted lower peak micromotions than flat-cut arthrodesis.

CONCLUSIONS

Better stability was predicted for three-screw ankle arthrodesis. In flat-cut arthrodesis, a third screw inserted anteriorly performed better than a posterior screw. In intact arthrodesis, a posterior screw seemed a better option when flexion stability was the main concern. Even with three-screw fixation, the configuration of the first two-crossed screws may still be important to improve the stability at the fusion site.