Philippe Favre

Misalignment of total ankle components can induce high joint contact pressures

BACKGROUND A major cause of the limited longevity of total ankle replacements is premature polyethylene component wear, which can be induced by high joint contact pressures. We implemented a computational model to parametrically explore the hypothesis that intercomponent positioning deviating from the manufacturers recommendations can result in pressure distributions that may predispose to wear of the polyethylene insert. We also investigated the hypothesis that a modern mobile-bearing design may be able to better compensate for imposed misalignments compared with an early two-component design. METHODS Two finite element models of total ankle replacement prostheses were built to quantify peak and average contact pressures on the polyethylene insert surfaces. Models were validated by biomechanical testing of the two implant designs with use of pressure-sensitive film. The validated models were configured to replicate three potential misalignments with the most CLINICAL RELEVANCE version of the tibial component, version of the talar component, and relative component rotation of the two-component design. The misalignments were simulated with use of the computer model with physiologically relevant boundary loads. RESULTS With use of the manufacturers guidelines for positioning of the two-component design, the predicted average joint contact pressures exceeded the yield stress of polyethylene (18 to 20 MPa). Pressure magnitudes increased as implant alignment was systematically deviated from this reference position. The three-component design showed lower-magnitude contact pressures in the standard position (<10 MPa) and was generally less sensitive to misalignment. Both implant systems were sensitive to version misalignment. CONCLUSIONS In the tested implants, a highly congruent mobile-bearing total ankle replacement design yields more evenly distributed and lower-magnitude joint contact pressures than a less congruent design. Although the mobile-bearing implant reduced susceptibility to aberrant joint contact characteristics that were induced by misalignment, predicted average contact stresses reached the yield stress of polyethylene for imposed version misalignments of >5 degrees.

The effect of component positioning on intrinsic stability of the reverse shoulder arthroplasty

HYPOTHESIS Anterior instability is one of the most common complications in reverse shoulder replacement. This study hypothesized that intrinsic stability of a reverse prosthesis varies with the degree of version of the humerus and glenoid components. This should provide guidelines on how to best position the implant components to decrease the rate of dislocation. MATERIALS AND METHODS Resistance to anterior dislocation of a reverse implant was measured in a mechanical testing machine by means of the stability ratio (ratio of peak dislocation/axial compressive forces). Versions of the humeral and glenoid components were modified in 10 degrees steps in the 90 degrees abducted and resting positions. RESULTS In both tested positions, the effect of humeral component version was highly significant. Only a glenoid component retroversion of 20 degrees led to a statistically significant drop in stability ratio for the 20 degrees abducted position. Intrinsic stability in the typical component positioning (neutral glenoid version and 20 degrees humeral retroversion) yielded comparably low intrinsic stability, which could only be improved by increasing anteversion of the humeral component. DISCUSSION Version of the humeral component is the critical factor for intrinsic stability. Version of the glenoid component is less important for intrinsic stability, but special care should be given to avoid retroversions of more than 10 degrees . Within this range, the surgeon can concentrate primarily on other parameters critical for long-term outcome (range of motion, secure fixation) when choosing the appropriate glenoid version. CONCLUSION Anterior stability can be improved by implanting the humeral component in neutral or with some anteversion.

Latissimus dorsi transfer to restore external rotation with reverse shoulder arthroplasty: A biomechanical study

In patients with pseudoparesis of the shoulder resulting from irreparable rotator cuff tears, reverse shoulder arthroplasty (RSA) can restore active elevation, but external rotation remains less predictable. Latissimus dorsi transfer (LDT) has been shown to be effective in restoring external rotation in patients with posterosuperior tears of the rotator cuff. The aim of this study is to determine the capacity of the LDT to restore external rotation in combination with RSA and to investigate the mechanical advantage produced by 3 different insertion sites. A biomechanical model was created using a reverse total shoulder prosthesis with 3 different transfer insertions. Moment arms were measured for 2 static positions and 1 motion of the humerus. The moment arm analysis showed that LDT can improve active external rotation in the setting of a reverse prosthesis. An insertion site on the posterior side of the greater tuberosity (adjacent to the teres minor insertion) produced a greater external rotation moment arm.

Medial support by fibula bone graft in angular stable plate fixation of proximal humeral fractures: an in vitro study with synthetic bone

BACKGROUND Failure to achieve stable fixation with medial support in proximal humeral fractures can result in varus malalignment and cut-through of the proximal screws. The purpose of this study was to investigate the influence of an intramedullary fibula bone graft on the biomechanical properties of proximal humeral fractures stabilized by angular stable plate fixation in a bone model under cyclic loading. METHODS Two fixation techniques were tested in 20 composite analog humeri models. In group F- (n = 10), fractures were fixed by an anatomically formed locking plate system. In group F+ (n = 10), the same fixation system was used with an additional fibular graft model with a length of 6 cm inserted in an intramedullary manner. Active abduction was simulated for 400 cycles by use of a recently established testing setup. Fragment gap distance was measured, and thereby, intercyclic motion, fragment migration, and residual plastic deformation were determined. RESULTS The addition of a fibular graft to the fixation plate led to 5 times lower intercyclic motion, 2 times lower fragment migration, and 2 times less residual plastic deformation. Neither screw pullout, cut-through, nor implant failure was observed. CONCLUSION Medial support with an intramedullary fibular graft in an angular stable fixation of the proximal humerus in vitro increases overall stiffness of the bone-implant construct and reduces migration of the humeral head fragment. This technique might provide a useful tool in the treatment of displaced proximal humeral fractures, especially when there is medial comminution.

Superior versus anteroinferior plating of the clavicle: a finite element study.

Objectives: Fixation plate positioning remains controversial in clavicle fracture reconstruction. Biomechanical studies favor a superior plate placement and clinical series report very low mechanical complications for anteroinferior plate placement. To clarify this apparent discrepancy, a biomechanical finite element analysis of the deformation mode, stress patterns, and peak stresses involved with superior and anteroinferior clavicle plate fixation was performed. Methods: Finite element models of the superior and anteroinferior reconstructions were built and the results were compared with those of the intact clavicle when loaded in axial compression and cantilever bending. Results: Superior plate placement was less likely to fail in axial compression but the anteroinferior plate placement was less likely to fail in cantilever bending. For all placements and loading modes, the region near the fracture gap experienced the highest stresses and was consequently critical for the behavior of the whole construct. The anteroinferior placement led to a deformation mode similar to the intact clavicle in both loading configurations, whereas the deformation mode with the superior placement was non-physiological. Conclusions: Anterorinferior plating is generally preferable, because it induces deformation modes similar to the intact clavicle and is less likely to fail during normal physiological loading (cantilever bending). Superior placement of the reconstruction plate may be recommended for a patient with a high risk of shoulder impacts (axial compression). Design improvements in the bridging area of the plate and special attention to obtain a good fixation around the fracture could reduce plate failures and provide a stiffer construct.

Numerical modelling of the shoulder for clinical applications

Research activity involving numerical models of the shoulder is dramatically increasing, driven by growing rates of injury and the need to better understand shoulder joint pathologies to develop therapeutic strategies. Based on the type of clinical question they can address, existing models can be broadly categorized into three groups: (i) rigid body models that can simulate kinematics, collisions between entities or wrapping of the muscles over the bones, and which have been used to investigate joint kinematics and ergonomics, and are often coupled with (ii) muscle force estimation techniques, consisting mainly of optimization methods and electromyography-driven models, to simulate muscular action and joint reaction forces to address issues in joint stability, muscular rehabilitation or muscle transfer, and (iii) deformable models that account for stress–strain distributions in the component structures to study articular degeneration, implant failure or muscle/tendon/bone integrity. The state of the art in numerical modelling of the shoulder is reviewed, and the advantages, limitations and potential clinical applications of these modelling approaches are critically discussed. This review concentrates primarily on muscle force estimation modelling, with emphasis on a novel muscle recruitment paradigm, compared with traditionally applied optimization methods. Finally, the necessary benchmarks for validating shoulder models, the emerging technologies that will enable further advances and the future challenges in the field are described.

Primary Isolated Subtalar Arthrodesis: Outcome after 2 to 5 Years Followup:

Background: Favorable to excellent clinical results have been reported for isolated subtalar joint arthrodesis. Pedobarography after subtalar bone-block distraction arthrodesis have demonstrated a more laterally shifted gait line. However pedobarographic measurements after primary in-situ isolated subtalar arthrodesis have not been reported. This is the first study considering this. Materials and Methods: Physical examination, AOFAS Hindfoot score, full weightbearing anterior/posterior and lateral radiographs were assessed in 15 feet. Peak pressures, ground reaction force and force distribution at foot-flat and push-off were measured. Results: Average AOFAS-Score significantly improved. Subjective satisfaction was high. Non-union was found in 1 foot (7%), screws were removed in 4 of the 15 feet (27%). One new asymptomatic arthritic talonavicular joint was found. The pressure and force distributions under the operated and contralateral foot showed a different pattern compared to a normal foot. Ground reaction force under both the operated and contralateral feet were lower than a normal foot. Discussion: This study found good clinical, subjective and radiographic results matching that of the reported literature. However, pedobarographic assessment suggests that great functional differences still remain when compared to a normal foot. Subtalar arthrodesis may induce an abnormal gait pattern by preventing compensation of axial rotation of the tibia. This is also reflected in the unaffected side, which may indicate an effort in the general locomotor control to keep a symmetrical gait pattern. This finally alters the pressure and force distribution under both feet. Nevertheless, subtalar arthrodesis is considered a valuable treatment for various isolated subtalar disorders.

An integrated model of active glenohumeral stability

We present the first model of the glenohumeral joint implementing active muscle driven humeral positioning and stabilization without a priori constraints on glenohumeral kinematics. Previously established methods were used to predetermine the path, activation timing and resultant force contribution of 27 individual muscle segments at any given joint position. Artificial boundary conditions were applied in a three-dimensional finite element model of the joint and progressively released until the humeral head was completely free to rotate and translate within the fixed glenoid according to the compressive component of the predetermined resultant force. The shear component was then added such that no boundary conditions other than muscular force were applied. The framework was exploited to simulate elevation as a composite of instantaneous positions and theoretically demonstrate that joint stability can be achieved exclusively through muscular activity. Predicted muscle moment arms, muscle activation timing, humeral head translations, joint contact forces and stability ratio were comparable with existing experimental and in vivo data. This framework could be valuable for subject specific modeling and may be used to address clinical hypotheses related to shoulder joint stability that cannot be pursued using simplified modeling approaches.

Tensile and Shear Loading Stability of All-Inside Meniscal Repairs An In Vitro Biomechanical Evaluation

Background: Most biomechanical studies for evaluation of the structural properties of meniscal repairs have been performed in tensile loading scenarios perpendicular to the circumferential meniscal fibers. However, meniscal repair constructs are also exposed to shear forces parallel to the circumferential meniscal fibers during healing, particularly in the midportion of the meniscus. Hypothesis: Material properties of meniscal repair devices cannot be extrapolated from tensile to shear load scenarios. Study Design: Controlled laboratory study. Methods: In 84 harvested and isolated bovine lateral menisci following removal of adjacent soft tissue, a standardized vertical lesion was set followed by repair using all-inside flexible (FasT-Fix, FasT-Fix AB, RapidLoc) and rigid (Meniscus Screw, Meniscus Arrow) meniscal repair devices. Vertical and horizontal 2.0 Ethibond sutures were used as controls. The repaired meniscal construct was tested in a tensile (parallel to the axis of the tested repair device) and shear load scenario (perpendicular to the axis of the tested repair device) at 5 mm/min and 37°C environmental temperature. Maximum load to failure, stiffness, and failure mode were recorded. Results: The absolute load to failure values of each repair device in the shear scenario were only marginally different from the tensile load scenario. However, the stiffness of several tested devices was markedly reduced in the shear scenario. In both scenarios, large differences of the load to failure and the stiffness between the implant types up to 5-fold were found (P < .05). The failure mode of several all-inside flexible repair devices was different in the shear versus tensile load scenario, while the failure mode of the rigid systems was similar in both scenarios. Conclusion: All-inside meniscal repair devices exposed to shear load scenarios have comparable maximum loads to failures as tensile load scenarios. However, the stiffness of the majority of the flexible meniscal repair implants in a shear load scenario is markedly reduced. The applied scenario also affects the failure mode in several flexible meniscal repair devices. Clinical Relevance: Meniscal repair devices with sufficient stiffness and stability against shear loads may be favored for meniscal repair, especially within the midportion of the meniscus where shear loads occur during healing.

Metatarsophalangeal joint arthrodesis after failed Keller-Brandes procedure.

Background: Keller-Brandes resection arthroplasty for correction of symptomatic hallux valgus deformity can obtain early good results, but late complications, such as recurrence of the deformity and instability of the first ray, have been described. Arthrodesis of the first metatarsophalangeal, (MTP) joint can be done as a salvage procedure. The aim of this prospective study was to evaluate the clinical outcome of the arthrodesis and its effect on the biomechanics of the first ray. Methods: Between October, 1999, and December, 2002, arthrodesis of the MTP joint was done after a failed Keller-Brandes procedure in 28 feet of 26 consecutive patients. Twenty patients (22 feet) with a minimum of 24 months followup were available for clinical and radiographic assessment. Pedobarographic measurements were obtained at latest followup in 16 patients (17 feet). Results: Sixteen feet (72%) were pain-free and six feet (28%) had mild, occasional pain. The American Orthopaedic Foot and Ankle Society (AOFAS) forefoot score increased from a preoperative 44 (range 29 to 67) points to 85 (range 73 to 90) points at longest clinical followup (average 34 months, range 23 to 48, p < 0.001). The average hallux valgus angle was corrected from 24.0 (range 7 to 47) degrees preoperatively to 16.0 (range 0 to 40) degrees postoperatively (p < 0.001). Two feet had pseudoarthroses. Biomechanically, the MTP joint arthrodesis could not fully restore the function of the hallux but produced a significant improvement, allowing a more physiologic loading pattern under the hallux and the metatarsal heads. Conclusions: First MTP joint arthrodesis after a failed Keller-Brandes procedure is a technically safe and reliable technique. It resulted in a marked reduction of pain and gain of function that produced high patient satisfaction.