A statistical analysis of human talar shape and bone density distribution.

The shape of the talus, its internal structure, and its mechanical properties are important in determining talar behavior during loading, which may be significant for the design of surgical tools and implants. Although recent studies using statistical shape modeling have described quantitative talar external shape variation, no similar quantitative study exists to describe the density distribution of internal talar structure. The goal of this study is to quantify statistical variation in talar shape and density to benefit the design of talar implants. To this end, weight-bearing computed tomography (CT) scans of the ankle were collected in neutral, bilateral standing posture, and three-dimensional models were generated for each talus. Local density derived from the Hounsfield unit of each CT voxel was extracted. A weighted spherical harmonic analysis was performed to quantify the talar external shape. One hundred and seventy-nine volumes of interest were placed in the same relative position within each talus to quantify the talar density. Additionally, a finite element analysis (FEA) was conducted on a talus with both heterogeneous and homogeneous material properties to observe the effect of these properties on the stress and strain response. Significant differences were found in the talar density in sex and age, as well as in the stress and strain response between homogeneous and heterogeneous FEA. These differences show the importance of considering heterogeneity when examining the load response of tarsal bones.

Fixation strength of swelling copolymeric anchors in artificial bone.

Fixation with suture anchors and metallic hardware for osteosynthesis is common in orthopedic surgeries. Most metallic commercial bone anchors achieve their fixation to bone through shear of the bone located between the threads. They have several deficiencies, including stress-shielding due to mechanical properties mismatch, generation of acidic by-products, poor osteointegration, low mechanical strength and catastrophic failure often associated with large bone defects that may be difficult to repair. To overcome these deficiencies, a swelling porous copolymeric material, to be used as bone anchors with osteointegration potential, was introduced. The purpose of this study was to investigate the fixation strength of these porous, swelling copolymeric bone anchors in artificial bone of various densities. The pull-out and subsidence studies indicate an effective fixation mechanism based on friction including re-fixation capabilities, and minimization of damage following complete failure. The study suggests that this swelling porous structure may provide an effective alternative to conventional bone anchors, particularly in low-density bone.

Biomechanical Comparison of Fixation of Metacarpal Shaft Fractures With Intramedullary Threaded Nail and Dorsal Plate.

PURPOSE: The purpose of this study was to investigate the mechanical properties of metacarpal long oblique and transverse shaft fractures stabilized by threaded intramedullary nails and dorsal plates and screws. METHODS: Transverse and oblique midshaft osteotomies were created in 28 paired left and right metacarpal bones from four fresh frozen cadavers. Each matched pair was fixed with one 4.5-mm threaded intramedullary nail and one 2.0-mm dorsal stainless-steel plate and a screw construct. The bones were secured at the proximal end, by a 3-D-printed customized jig, to a tensile testing machine and were loaded through a Kevlar wire tensioned over the metacarpal head, simulating muscle/tendon loading during grip. Loading to failure was performed, and the applied force and failure mode were recorded. RESULTS: Oblique fractures fixed with nails failed at a greater force than those fixed with dorsal plates (183 ± 50 N vs 130 ± 40 N). Transverse fractures showed comparable strength between the nail (215 ± 33 N) and the plate (183 ± 64 N). Plate failure modes included three diverse types of bone fracture and yielding deformation of the plate. Intramedullary nail failure modes included yielding of the nail, bone fracture without nail yielding, and relative rotation between the two sections of the bone. CONCLUSIONS: Fixation of oblique and transverse metacarpal shaft fractures using a 4.5-mm threaded intramedullary nail in a simulated grip test model showed similar or greater strength than a 2.0-mm dorsal plate and screw construct. CLINICAL RELEVANCE: Threaded intramedullary nail fixation of metacarpal shaft fractures may be an appropriate alternative to plate fixation in these fracture patterns.

Comparative Bending Strength of Metacarpal Neck Fractures Fixed with Two Types of Intramedullary Screws.

Objectives: Intramedullary (IM) screw fixation of metacarpal fractures is a technique, which has gained in popularity owing to its simplicity, speedy rehabilitation, and good functional outcomes. A new, larger diameter, non-compression screw designed specifically for IM metacarpal fixation was recently introduced which could provide better fracture stability and reduce the risk of hardware failure. Our goal was to evaluate the strength of this screw compared to a first-generation screw. Methods: This mechanical study was designed to compare a 4.5 mm metacarpal headless screw (MCHS) to data from our prior research evaluating a 3.0 mm headless screw (HS). Accordingly, we used identical bone models, testing constructs, equipment, and protocols. A metacarpal neck osteotomy was created in 10 Sawbones models. A 4.5 mm x 50 mm MCHS was inserted retrograde to stabilize the fracture. Flexion bending strength was measured through a cable tension construct on a materials testing machine. Failure mechanism and strength was recorded and compared to data with a 3.0 mm screw construct. Results: Eight models failed by bending of the intramedullary screw. Two models failed by rotation of the metacarpal head. Failure occurred at an average of 539 N (Range 315 - 735 N). The MCHS demonstrated a significantly greater load to failure compared to the previously studied 3.0 mm HS at 215 N (P<0.05). Conclusion: A larger, 4.5 mm metacarpal-specific headless screw is more than twice as strong as a 3.0 mm diameter screw in a metacarpal neck fracture model.

Numerical analysis of the mechanical response of novel swelling bone implants in polyurethane foams.

In this study, a numerical framework was developed in order to analyze the swelling properties, mechanical response and fixation strength of swelling bone anchors. Using this framework, fully porous and solid implants, along with a novel hybrid design (consisting of a solid core and a porous sleeve), were modeled and studied. Free swelling experiments were conducted to investigate their swelling characteristics. The finite element model of swelling was validated using the conducted free swelling. Compared with the experimental data, results obtained from the finite element analysis proved the reliability of this frame-work. Afterwards, the swelling bone anchors were studied embedded in artificial bones with different densities with two different interface properties: considering frictional interface between the bone anchors and artificial bones (simulating the stages prior to osteointegration, when the bone and implant are not fully bonded and the surface of the implant can slide along the interface), and perfectly bonded (simulating the stages subsequent to osteointegration, when the bone and implant are fully bonded). It was observed that the swelling considerably decreases while the average radial stress on the lateral surface of the swelling bone anchor surges in the denser artificial bones. Ultimately, the pull-out experiments and simulations of the swelling bone anchors from the artificial bones were conducted to look into the fixation strength of the swelling bone anchors. It was found that the hybrid swelling bone anchor exhibits mechanical and swelling properties close to those of solid bone anchors, while also bone in-growth is expected to happen, which is an integral factor to these bone anchors.

A porous swelling copolymeric material for improved implant fixation to bone.

Most metallic commercial bone anchors, such as screws and suture anchors achieve their fixation to bone through shear of the bone located between the threads. They have several deficiencies, potentially leading to failure, which are particularly evident in low-density bone. These include stress-shielding resulting from mechanical properties mismatch; lack of mechanically induced remodeling and osteointegration; and when the pullout force on the anchor, during functional activities, exceeds their pullout strength, catastrophic failure occurs leaving behind large bone defects that may be hard to repair. To overcome these deficiencies, we introduced in this study a porous swelling co-polymeric material and studied its swelling and compressive mechanical characteristics as bone anchor under different configurations. Porosity was achieved by adding a non-dissolvable agent (NaCl) during the process of polymerization, which was later dissolved in water, leaving behind a porous structure with adequate porosity for osteointegration. Three different groups of cylindrical samples of the swelling co-polymer were investigated. Solid, fully porous, and partially porous with a solid core and a porous outer layer. The results of the swelling and simple compression study show that the partially porous swelling co-polymer maintains excellent mechanical properties matching those of cancellous bone, quick swelling response, and an adequate porous outer layer for mechanically induced osteointegration. These suggest that this material may present an effective alternative to conventional bone anchors particularly in low-density bone.

Definitions and Measurements of Hindfoot Alignment and Their Biomechanical and Clinical Implications.

This article presents a critical review of the past and the current state of the art in defining and measuring hindfoot, ankle, and subtalar alignment. It describes the transition occurring at present from two-dimensional to three-dimensional (3D) alignment measurements, which accompany the emergence of new, functional, high-resolution imaging modalities such as the weight-bearing cone-beam computerized tomography (CT) imaging. To ease and enhance the transition and acceptability of 3D alignment measurements, new acceptable standards for different clinical application are highly desirable.

Three-dimensional ankle, subtalar, and hindfoot alignment of the normal, weightbearing hindfoot, in bilateral posture.

The first goal of this study was to develop reliable three-dimensional definitions of alignment for the ankle, subtalar, and hindfoot joints. These alignments are based on three-dimensional morphological features derived from renderings of the bones obtained from weightbearing computer tomography. The second goal was to establish a database quantifying the alignment of the ankle, subtalar, and hindfoot joints in a healthy population during weightbearing bilateral standing. This level 1 study was performed on 95 normal subjects in which random subjects were recruited into a control group. Weightbearing computed tomography scans of the leg were collected in neutral, bilateral, standing posture. In 30 of the subjects, both the left and right leg was scanned. Six alignment parameters for each joint were calculated from morphological measurements conducted on three-dimensional renderings of the bones. Intra- and intertester reliability was assessed from repeated measurements by several testers. Analysis of variance statistics of the alignment parameters showed no statistical differences due to age, gender, or foot side. Intraclass correlation coefficient analysis showed excellent inter- and intratester reliability. It was concluded that the alignment process is comprehensive and reliable. Therefore, without classification by gender or age, it may be used as a foundation for quantifying abnormal alignment associated with various ankle deformities. Clinical significance: The alignment methodology and control database may be used to diagnose ankle, subtalar, and hindfoot misalignment. It can also serve as basis for surgical planning designed to restore normal alignment in various hindfoot pathologies, such as ankle realignment in total ankle replacement.

Level of Sagittal Plane Fit of Plates on the Radial Shaft: A Cadaveric Study Comparing Precontoured and Straight Titanium Plates.

BACKGROUND: During radial shaft fracture fixation, it is important to contour the plate appropriately to restore the radial bow in order to maintain normal forearm mechanics and motion. The aim of this study was to investigate the fit of precontoured radial shaft plates versus surgeon-contoured plates. METHODS: Six 10-hole Acumed® precontoured volar and dorsolateral radius plates and twelve 10-hole Synthes straight titanium 3.5 mm LC-DCP plates were drilled with arrays of 1.5 mm diameter holes to permit measurement of the plate distance off bone. Plates were applied to 6 cadaver radii and secured with a screw on each end. Three plate conditions were tested: precontoured plates, precontoured plates with further surgeon contouring, and straight plates with surgeon contouring. Surgeon contouring time for each plate was recorded. Each plate was divided into 3 equal regions, and the average distance gaps for each region and the entire plate were calculated. RESULTS: For the volar side, precontoured plates had a larger total gap compared to that plate with additional surgeon contouring (1.4 mm difference) and the straight surgeon-contoured plates (1.2 mm difference). On the dorsal side, there was no difference in fit between the 3 plate conditions at any location. No differences were found in plate contouring times. CONCLUSIONS: The precontoured dorsal plate fit was as good as the surgeon-contoured plates indicating this plate could potentially be used in fracture surgery without further bending. The precontoured volar plate was under-contoured, on average, and would likely require further bending to restore the radial bow.

Weightbearing CT assessment of foot and ankle joints in Pes Planovalgus using distance mapping.

INTRODUCTION: The goal of this study was to describe the abnormal joint surface interaction at the ankle, hindfoot and midfoot joints in patients presenting with Pes Planovalgus (PPV) using three-dimensional (3D) distance mapping on weightbearing computed tomography (WBCT) images by comparing a series of PPVs to a series of normally-aligned feet. We hypothesized that in PPVs joint interactions would reveal significantly increased spaces in the medial side of the ankle, hindfoot and midfoot joints. METHODS: In this case-control study, ten feet (10 patients) with asymptomatic PPV were compared to 10 matched-paired (by age, gender and body mass index) normally-aligned feet (10 patients). Three-dimensional models were produced from the images and distance maps representing joint surface configuration were generated for the ankle, hindfoot and midfoot joints. The distance maps for each joint were then compared between the two groups and between regions in the same group. RESULTS: In PPV patients there was a significantly increased surface-to-surface distance anteromedially at the ankle joint (+46.3%, p < 0.001) along with an increased distance on the anterior halves of both the medial (+21.3%, p = 0.098) and lateral malleoli (+22.7%, p = 0.038). At the posterolateral corner of the posterior facet of the subtalar joint we found an increased surface-to-surface distance (by 57.1%, p < 0.001), while at the talonavicular joint there was a reduction of the distance at the superomedial corner (-20%, p = 0.097) along with a significant increase in the upper central (+20%, p = 0.039) and lateral (+30.7%, p = 0.015) zones. A reduction of the surface-to-surface distance was also observed in three of the four zones of the calcaneocuboid joint. Finally, a statistically significant increase in the mean distance was observed at the naviculocuneiform and tarsometatarsal joints in a range between 38% and 93.4% (p < 0.001 in all cases). CONCLUSION: We found significant differences in surface-to-surface interaction at the foot and ankle joints between Pes Planovalgus and normally-aligned controls. Distance mapping on WBCT images could be used in clinical practice as a diagnostic support to gauge the morphological changes of articular spaces occurring in Pes Planovalgus. LEVEL OF EVIDENCE: Level III, case-control study.

Effect of artificial surface shapes and their malpositioning on the mechanics of the replaced ankle joint for possible better prosthesis designs.

BACKGROUND: The clinical outcomes of total ankle replacement are limited by prosthesis component malpositioning during surgery. The goal of this study is to assess the mechanical impact of this malpositioning in a validated computer model. METHODS: In a previously developed multi-body dynamic model of the human ankle complex three different artificial implants were designed, each one presenting a different approximation of the natural articular surfaces of the corresponding specimen. The most common implant translational and rotational malpositionings were defined and mimicked. Dynamic simulations of joint motion were run for the various surfaces and malpositionings. The same input loading conditions derived from a previous in-vitro experiment on the corresponding natural specimen were applied. FINDINGS: From load-displacement graphs it was observed that all three artificial surfaces reproduced well physiological motion between the calcaneus and the tibia/fibula, with a maximum difference of 2°. It was found that antero-posterior translation of either the tibial or the talar component and inclination of the tibial component in the sagittal plane led to considerable increases in the range of motion. Antero-posterior and dorsiflexion of the tibial component resulted in an increased internal-external rotation by up to 3.5° and 4.0°, respectively. The corresponding increase of inversion-eversion was 5.0° and 6.5°. INTERPRETATION: This study showed that relatively small surgical errors have great consequences in replaced joint mechanics. The present model can be used in future studies to analyse the effect of malpositioning with any specific current total ankle prosthesis.

Distance mapping of the foot and ankle joints using weightbearing CT: The cavovarus configuration.

INTRODUCTION: The goal of this study was to characterize the abnormal joint surface interaction at the ankle, hindfoot and midfoot joints of the cavovarus foot using distance mapping on weightbearing computed tomography (WBCT) images by comparing a series of cavovarus feet to a series of normally-aligned feet. METHODS: In this case-control study, ten feet (10 patients) with asymptomatic cavovarus shape (cases; N = 10) were compared to 10 matched-paired (by age, gender and body mass index) normally-aligned feet (10 patients) (controls; N = 10). Three-dimensional models were produced from the images and distance maps representing joint surface configuration were generated for the ankle, hindfoot and midfoot joints. The distance maps for each joint were then compared between the two groups and between regions in the same group. RESULTS: In the cavovarus group there was a significant increase in surface-to-surface distance at the posterior tibiotalar joint and a reduced distance at the anterior part, together with a greater distance at the posterior half of the medial gutter. Also, a decrease in surface-to-surface distance on the anterior half of the anterior facet and an increased distance on the posterior quadrants of the posterior facet of the subtalar joint were found. At the sinus tarsi, the lateral aspect of the talonavicular joint, the naviculocuneiform and the tarsometatarsal joints there was a statistically significant increase in surface-to-surface distance in cavovarus patients as compared to controls. CONCLUSION: Distance mapping analysis on WBCT images identified significant differences in surface-to-surface interaction at the foot and ankle joints between cavovarus and normally-aligned feet. LEVEL OF EVIDENCE: Level III, case-control study.

A novel Cervical Spine Protection device for reducing neck injuries in contact sports: design concepts and preliminary in vivo testing.

Head and neck injuries are common in contact sports such as American football. Different mechanisms can produce such injuries, including compressive impact forces on the crown of the helmet with the neck in a flexed chin-down position. The aim of this paper was developing and testing a novel Cervical Spine Protection Device (CSPD) designed to keep the neck within its safe physiological range. The cervical spine range of motion (ROM) of ten participants was measured under four conditions: free; wearing a football gear; wearing the CSPD; and wearing the CSPD underneath the gear. The CSPD was tested in terms of passive and active restraint of head motion, and for its capability to improve endurance time of the neck extensor muscles. Wearing the CSPD resulted in a significant 40-60% reduction in ROM across the three anatomical planes, and in increased endurance of the neck extensor muscles (FREE: 114 ± 57 s; CSPD: 214 ± 95 s; p = 0.004). In quasi-static loading conditions the CSPD was capable of keeping the neck within its physiological range, thus it may be used to decrease the risk of severe injuries due to dangerous chin-down positions.

Estimating the stabilizing function of ankle and subtalar ligaments via a morphology-specific three-dimensional dynamic model.

Knowledge of the stabilizing role of the ankle and subtalar ligaments is important for improving clinical techniques such as ligament repair and reconstruction. However, this knowledge is incomplete. The goal of this study was to expand this knowledge by investigating the stabilizing function of the ligaments using multiple morphologically subject-specific computational models. Nine models were created from the lower extremities of nine donors. Each model consisted of the articulating bones, articular cartilage, and ligaments. Simulations were conducted in ADAMS™ - a dynamic simulation program. During simulation, tibia and fibula were fixed while cyclic moments in all three anatomical planes were applied to the calcaneus one-at-a-time. The resulting displacements between the bones and the forces in each ligament were computed. Simulations were conducted with all ligaments intact and after simulated ligament serial sectioning. Each model was validated by comparing the simulation results to experimental data obtained from the specimen used to construct the model. From the results the stabilizing role of each ligament was established and the effect of ligament sectioning on Range of Motion and Overall Laxity was identified. On the lateral side, ATFL provided stabilization in supination, CFL restrained inversion, external rotation and dorsiflexion and PTFL limited dorsiflexion and external rotation. On the medial side, PTTL restrained dorsiflexion and internal rotation, ATTL limited plantarflexion and external rotation, and TCL limited dorsiflexion, eversion and external rotation. At the subtalar joint, ITCL limited plantarflexion and its posterior-lateral bundle restrained subtalar inversion. CL restrained plantarflexion/dorsiflexion, and internal and external rotation. The large inter-model variability observed in the results indicate the importance of using multiple subject-specific models rather than relying on one "representative" model.

Subluxation of the Middle Facet of the Subtalar Joint as a Marker of Peritalar Subluxation in Adult Acquired Flatfoot Deformity: A Case-Control Study.

BACKGROUND: Progressive peritalar subluxation (PTS) is part of adult acquired flatfoot deformity (AAFD). We investigated the use of the middle facet as an indicator of PTS using standing, weight-bearing computed tomography (CT) images. We hypothesized that weight-bearing CT would be an accurate method of measuring increased subluxation ("uncoverage") and incongruence of the middle-facet among patients with AAFD. METHODS: We included 30 patients with stage-II AAFD (20 female and 10 male; mean age, 57.4 years [range, 24 to 78 years]) and 30 matched controls (20 female and 10 male; mean age, 51.8 years [range, 19 to 81 years]) who underwent standing, weight-bearing CT. Two independent and blinded fellowship-trained foot and ankle surgeons measured the amount of subluxation (percentage of uncoverage) and the incongruence angle of the middle facet at the midpoint of its longitudinal length, using coronal-plane, weight-bearing, cone-beam CT images. Intraobserver and interobserver reliabilities were assessed using intraclass correlation coefficients (ICCs). Comparisons were performed using independent t tests or Wilcoxon tests. P values of <0.05 were considered significant. RESULTS: Substantial to almost perfect intraobserver and interobserver reliability was observed for both measurements. We found that the middle facet demonstrated significantly increased PTS in patients with AAFD, with a mean value for joint uncoverage of 45.3% (95% confidence interval [CI], 38.5% to 52.1%) compared with 4.8% (95% CI, 3.2% to 6.4%) in controls (p < 0.0001). A significant difference was also found for the incongruence angle, with a mean value of 17.3° (95% CI, 14.7° to 19.9°) in the AAFD group and 0.3° (95% CI, 0.1° to 0.5°) in controls (p < 0.0001). A joint incongruence angle of >8.4° was found to be diagnostic for symptomatic stage-II AAFD. CONCLUSIONS: We investigated the use of the middle facet of the subtalar joint as a marker for PTS in patients with AAFD. We confirmed that standing, weight-bearing CT images allowed accurate measurements and that significant differences were found in the percentage of joint uncoverage and the incongruence angle compared with controls. CLINICAL RELEVANCE: The assessment of the amount of subluxation and incongruence of the middle facet of the subtalar joint represents an accurate diagnostic tool for symptomatic adult acquired flatfoot deformity.

Comparison of cartilage and bone morphological models of the ankle joint derived from different medical imaging technologies.

BACKGROUND: Accurate geometrical models of bones and cartilage are necessary in biomechanical modelling of human joints, and in planning and designing of joint replacements. Image-based subject-specific model development requires image segmentation, spatial filtering and 3-dimensional rendering. This is usually based on computed tomography (CT) for bone models, on magnetic resonance imaging (MRI) for cartilage models. This process has been reported extensively in the past, but no studies have ever compared the accuracy and quality of these models when obtained also by merging different imaging modalities. The scope of the present work is to provide this comparative analysis in order to identify optimal imaging modality and registration techniques for producing 3-dimensional bone and cartilage models of the ankle joint. METHODS: One cadaveric leg was instrumented with multimodal markers and scanned using five different imaging modalities: a standard, a dual-energy and a cone-beam CT (CBCT) device, and a 1.5 and 3.0 Tesla MRI devices. Bone, cartilage, and combined bone and cartilage models were produced from each of these imaging modalities, and registered in space according to matching model surfaces or to corresponding marker centres. To assess the quality in overall model reconstruction, distance map analyses were performed and the difference between model surfaces obtained from the different imaging modalities and registration techniques was measured. RESULTS: The registration between models worked better with model surface matching than corresponding marker positions, particularly with MRI. The best bone models were obtained with the CBCT. Models with cartilage were defined better with the 3.0 Tesla than the 1.5 Tesla. For the combined bone and cartilage models, the colour maps and the numerical results from distance map analysis (DMA) showed that the smallest distances and the largest homogeneity were obtained from the CBCT and the 3.0 T MRI via model surface registration. CONCLUSIONS: These observations are important in producing accurate bone and cartilage models from medical imaging and relevant for applications such as designing of custom-made ankle replacements or, more in general, of implants for total as well as focal joint replacements.

Headless Screw Fixation of Metacarpal Neck Fractures: A Mechanical Comparative Analysis.

BACKGROUND: The purpose of this study was to compare the mechanical properties of metacarpal neck fracture fixation by headless compression screw (HCS) with that of Kirschner wire (KW) cross-pinning and locking plate (LP) fixation. METHODS: A metacarpal neck fracture was created in 30 fourth-generation composite Sawbones metacarpal models. A volar-based wedge was removed using a custom jig to simulate a typical apex dorsal fracture, unstable in flexion. The models were divided into 3 equal groups based on the method of fixation: retrograde cross-pinning with two 1.2-mm KWs, 2.0-mm dorsal T-plate with six 2.0-mm locking screws (LP), and a 3.0-mm retrograde HCS. Models were fixed at the proximal end, mounted in a material testing machine, and loaded through a cable tensioned over the metacarpal head, simulating grip loading. Cyclic loading from 0 to 40 N was performed, followed by loading to failure. Load, displacement, and failure mode were recorded. RESULTS: Stiffness of the HCS (7.3 ± 0.7 N/m) was significantly greater than the KW (5.8 ± 0.5 N/m) but significantly less than the LP (9.5 ± 1.9 N/m). With cyclic loading to 40 N, the LP exhibited significantly less displacement (0.2 ± 1.3 mm) compared with the HCS (2.5 ± 2.3 mm) and KW (2.8 ± 1.0 mm). Load to failure for the HCS (215.5 ±3 9.0 N) was lower than that of the KW (279.7 ± 100.3 N) and of the LP (267.9 ± 44.1 N), but these differences were not statistically significant. CONCLUSIONS: The HCS provided mechanical fracture fixation properties comparable with KW fixation. The LP construct allowed significantly less displacement and had the highest strength of the 3 fixation methods.

New comprehensive procedure for custom-made total ankle replacements: Medical imaging, joint modeling, prosthesis design, and 3D printing.

Many failures in total joint replacement are associated to prosthesis-to-bone mismatch. With recent additive-manufacturing, that is, 3D-printing, custom-made prosthesis can be created by laser-melting metal powders layer-by-layer. Ankle replacement is particularly suitable for this progress because of the limited number of sizes and the poor bone stock. In this study a novel procedure is presented for subject-specific ankle replacements, including medical-imaging, joint modelling, prosthesis design, and 3D-printing. Three shank-foot specimens were CT-scanned, and corresponding 3D bone models of the tibia, fibula, talus, and calcaneus were obtained. From these models, specimen-specific implant sets were designed according to three different concepts, and 3D-printed from cobalt-chromium-molybdenum powder. Accuracy of the overall procedure was assessed via distance map comparisons between original anatomical and final metal implants. Restoration of natural ankle joint mechanics was check after implantation of each of the three sets. In a special rig, a manually-driven dorsi/plantar-flexion was applied throughout the passive arc. Additionally, at three different joint positions, joint torques were imposed in the frontal and axial anatomical planes. Mean manufacturing errors were found to be smaller than 0.08 mm. Consistent motion patterns were observed over repetitions, with the mean standard deviation smaller than 1.0 degree. In each ankle specimen, mobility, and stability at the replaced joints compared well with the original natural condition. For the first time, custom-made implants for total ankle replacements were designed, manufactured with additive technology and tested. This procedure is a first fundamental step toward the development of completely personalized prostheses. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res.

Analysis of surface-to-surface distance mapping during three-dimensional motion at the ankle and subtalar joints.

Joint surface interaction and ligament constraints determine the kinematic characteristics of the ankle and subtalar joints. Joint surface interaction is characterized by joint contact mechanics and by relative joint surface position potentially characterized by distance mapping. While ankle contact mechanics was investigated, limited information is available on joint distance mapping and its changes during motion. The purpose of this study was to use image-based distance mapping to quantify this interaction at the ankle and subtalar joints during tri-planar rotations of the ankle complex. Five cadaveric legs were scanned using Computed Tomography and the images were processed to produce 3D bone models of the tibia, fibula, talus and calcaneus. Each leg was tested on a special linkage through which the ankle complex was loaded in dorsiflexion/plantarflexion, inversion/eversion, and internal/external rotation and the resulting bone movements were recorded. Fiduciary bone markers data and 3D bone models were combined to generate color-coded distance maps for the ankle and subtalar joints. The results were processed focusing on the changes in surface-to-surface distance maps between the extremes of the range of motion and neutral. The results provided detailed insight into the three-dimensional highly coupled nature of these joints showing significant and unique changes in distance mapping from neutral to extremes of the range of motion. The non-invasive nature of the image-based distance mapping technique could result, after proper modifications, in an effective diagnostic and clinical evaluation technique for application such as ligament injuries and quantifying the effect of arthrodesis or total ankle replacement surgery.

Experimental evaluation of current and novel approximations of articular surfaces of the ankle joint.

Kinematics and flexibility properties of both natural and replaced ankle joints are affected by the geometry of the articulating surfaces. Recent studies proposed an original saddle-shaped, skewed, truncated cone with laterally oriented apex, as tibiotalar contact surfaces for ankle prosthesis. The goal of this study was to compare in vitro this novel design with traditional cylindrical or medially centered conic geometries in terms of their ability to replicate the natural ankle joint mechanics. Ten lower limb cadaver specimens underwent a validated process of custom design for the replacement of the natural ankle joint. The process included medical imaging, 3D modeling and printing of implantable sets of artificial articular surfaces based on these three geometries. Kinematics and flexibility of the overall ankle complex, along with the separate ankle and subtalar joints, were measured under cyclic loading. In the neutral and in maximum plantarflexion positions, the range of motion under torques in the three anatomical planes of the three custom artificial surfaces was not significantly different from that of the natural surfaces. In maximum dorsiflexion the difference was significant for all three artificial surfaces at the ankle complex, and only for the cylindrical and medially centered conic geometries at the tibiotalar joint. Natural joint flexibility was restored by the artificial surfaces nearly in all positions. The present study provides experimental support for designing articular surfaces matching the specific morphology of the ankle to be replace, and lays the foundations of the overall process for designing and manufacturing patient-specific total ankle replacements.