Michael W. Hast

Biomechanical effects of total knee arthroplasty component malrotation: a computational simulation.

Modern total knee arthroplasty (TKA) is an effective procedure to treat pain and disability due to osteoarthritis, but some patients experience quadriceps weakness after surgery and have difficulty performing important activities of daily living. The success of TKA depends on many factors, but malalignment of the prosthetic components is a major cause of postoperative complications. Significant variability is associated with femoral and tibial component rotational alignment, but how this variability translates into functional outcome remains unknown. We used a forward‐dynamic computer model of a simulated squatting motion to perform a parametric study of the effects of variations in component rotational alignment in TKA. A cruciate‐retaining and posterior‐stabilized version of the same TKA implant were compared. We found that femoral rotation had a greater effect on quadriceps forces, collateral ligament forces, and varus/valgus kinematics, while tibial rotation had a greater effect on anteroposterior translations. Our findings support the tendency for orthopedic surgeons to bias the femoral component into external rotation and avoid malrotation of the tibial component.

The role of animal models in tendon research

Tendinopathy is a debilitating musculoskeletal condition which can cause significant pain and lead to complete rupture of the tendon, which often requires surgical repair. Due in part to the large spectrum of tendon pathologies, these disorders continue to be a clinical challenge. Animal models are often used in this field of research as they offer an attractive framework to examine the cascade of processes that occur throughout both tendon pathology and repair. This review discusses the structural, mechanical, and biological changes that occur throughout tendon pathology in animal models, as well as strategies for the improvement of tendon healing. Cite this article: Bone Joint Res 2014;3:193–202.

Nonsurgical treatment and early return to activity leads to improved Achilles tendon fatigue mechanics and functional outcomes during early healing in an animal model

Achilles tendon ruptures are common and devastating injuries; however, an optimized treatment and rehabilitation protocol has yet to be defined. Therefore, the objective of this study was to investigate the effects of surgical repair and return to activity on joint function and Achilles tendon properties after 3 weeks of healing. Sprague–Dawley rats (N = 100) received unilateral blunt transection of their Achilles tendon. Animals were then randomized into repaired or non‐repaired treatments, and further randomized into groups that returned to activity after 1 week (RTA1) or after 3 weeks (RTA3) of limb casting in plantarflexion. Limb function, passive joint mechanics, and tendon properties (mechanical, organizational using high frequency ultrasound, histological, and compositional) were evaluated. Results showed that both treatment and return to activity collectively affected limb function, passive joint mechanics, and tendon properties. Functionally, RTA1 animals had increased dorsiflexion ROM and weight bearing of the injured limb compared to RTA3 animals 3‐weeks post‐injury. Such functional improvements in RTA1 tendons were evidenced in their mechanical fatigue properties and increased cross sectional area compared to RTA3 tendons. When RTA1 was coupled with nonsurgical treatment, superior fatigue properties were achieved compared to repaired tendons. No differences in cell shape, cellularity, GAG, collagen type I, or TGF‐β staining were identified between groups, but collagen type III was elevated in RTA3 repaired tendons. The larger tissue area and increased fatigue resistance created in RTA1 tendons may prove critical for optimized outcomes in early Achilles tendon healing following complete rupture.

Pediatric laryngotracheal reconstruction with tissue-engineered cartilage in a rabbit model

To develop an effective rabbit model of in vitro‐ and in vivo‐derived tissue‐engineered cartilage for laryngotracheal reconstruction (LTR).

Tendon mineralization is progressive and associated with deterioration of tendon biomechanical properties, and requires BMP-Smad signaling in the mouse Achilles tendon injury model.

Ectopic tendon mineralization can develop following tendon rupture or trauma surgery. The pathogenesis of ectopic tendon mineralization and its clinical impact have not been fully elucidated yet. In this study, we utilized a mouse Achilles tendon injury model to determine whether ectopic tendon mineralization alters the biomechanical properties of the tendon and whether BMP signaling is involved in this condition. A complete transverse incision was made at the midpoint of the right Achilles tendon in 8-week-old CD1 mice and the gap was left open. Ectopic cartilaginous mass formation was found in the injured tendon by 4weeks post-surgery and ectopic mineralization was detected at 8 to 10weeks post-surgery. Ectopic mineralization grew over time and volume of the mineralized materials of 25-weeks samples was about 2.5 fold bigger than that of 10-weeks samples, indicating that injury-induced ectopic tendon mineralization is progressive. In vitro mechanical testing showed that max force, max stress and mid-substance modulus in the 25-weeks samples were significantly lower than the 10-weeks samples. We observed substantial increases in expression of bone morphogenetic protein family genes in injured tendons 1week post-surgery. Immunohistochemical analysis showed that phosphorylation of both Smad1 and Smad3 was highly increased in injured tendons as early as 1week post-injury and remained high in ectopic chondrogenic lesions 4-weeks post-injury. Treatment with the BMP receptor kinase inhibitor (LDN193189) significantly inhibited injury-induced tendon mineralization. These findings indicate that injury-induced ectopic tendon mineralization is progressive, involves BMP signaling and associated with deterioration of tendon biomechanical properties.

Anteroinferior 2.7-mm versus 3.5-mm plating of the clavicle: A biomechanical study.

INTRODUCTION Lower patient satisfaction and high rates of plate prominence has led to the use of lower profile, smaller plates in the treatment of midshaft clavicle fractures. Specifically regarding the use of 2.7mm reconstruction plates, there lacks biomechanical comparison to its more robust 3.5mm counterpart. This study was designed to compare the mechanical properties of anteroinferior plate fixation on a clavicle fracture model using either 2.7mm or 3.5mm reconstruction plates. METHODS Forty-eight synthetic left clavicles were divided into two groups based on the type of fixation: 3.5mm or 2.7mm pelvic reconstruction plate fixed in the anteroinferior position. Fixation was tested on AO/OTA 15B1.3 transverse midshaft fractures. Each specimen underwent the following three mechanical tests: axial compression, torsion, and four-point bending. RESULTS Significant differences were observed in axial (p=0.016) and torsional (p=0.00097) stiffness between the two groups. The average bending rigidity (EI) was found to be significantly lower for the 2.7-mm plates as compared to the 3.5-mm plates (p=0.03). The loading scenarios performed in the mechanical tests did not lead to failure of any implants. CONCLUSION While our results show clear mechanical superiority of 3.5-mm reconstruction plates over 2.7-mm plates, superior results in the clinical setting may not necessarily translate. With exceptional mechanical strength also noted for the 2.7mm plate, well above the biomechanical properties of an intact clavicle, these results may obviate the need for robust plates in general.

Accuracy of MRI-based finite element assessment of distal tibia compared to mechanical testing

High-resolution MRI-derived finite element analysis (FEA) has been used in translational research to estimate the mechanical competence of human bone. However, this method has yet to be validated adequately under in vivo imaging spatial resolution or signal-to-noise conditions. We therefore compared MRI-based metrics of bone strength to those obtained from direct, mechanical testing. The study was conducted on tibiae from 17 human donors (12 males and five females, aged 33 to 88years) with no medical history of conditions affecting bone mineral homeostasis. A 25mm segment from each distal tibia underwent MR imaging in a clinical 3-Tesla scanner using a fast large-angle spin-echo (FLASE) sequence at 0.137mm×0.137mm×0.410mm voxel size, in accordance with in vivo scanning protocol. The resulting high-resolution MR images were processed and used to generate bone volume fraction maps, which served as input for the micro-level FEA model. Simulated compression was applied to compute stiffness, yield strength, ultimate strength, modulus of resilience, and toughness, which were then compared to metrics obtained from mechanical testing. Moderate to strong positive correlations were found between computationally and experimentally derived values of stiffness (R2=0.77, p<0.0001), yield strength (R2=0.38, p=0.0082), ultimate strength (R2=0.40, p=0.0067), and resilience (R2=0.46, p=0.0026), but only a weak, albeit significant, correlation was found for toughness (R2=0.26, p=0.036). Furthermore, experimentally derived yield strength and ultimate strength were moderately correlated with MRI-derived stiffness (R2=0.48, p=0.0022 and R2=0.58, p=0.0004, respectively). These results suggest that high-resolution MRI-based finite element (FE) models are effective in assessing mechanical parameters of distal skeletal extremities.

Plantarflexor metabolics are sensitive to resting ankle angle and optimal fiber length in computational simulations of gait

BACKGROUND Plantarflexor structure is an important predictor of function in healthy, athletic, and some patient populations. Computational simulations are powerful tools capable of testing the isolated effects of muscle-tendon structure on gait function. RESEARCH QUESTION The purpose of this study was to characterize the sensitivity of plantarflexor muscle function based on muscle-tendon unit (MTU) parameters. We hypothesized that plantarflexor metabolics and shortening dynamics would be sensitive to MTU parameters. METHODS Stance phase of gait was simulated using a musculoskeletal model and computed muscle control algorithm. Optimal muscle fiber length, resting ankle angle, and tendon stiffness parameters were systematically changed to test these effects on plantarflexor metabolics, activation, and power. Dorsiflexor metabolics were also measured to determine the impact of the action of the antagonist muscle group. RESULTS AND SIGNIFICANCE Plantarflexor metabolic demands were 1.5 and 2.7 times more sensitive to optimal fiber length and resting ankle angle, respectively, compared to the effect of tendon stiffness. Increased resting ankle plantarflexion induced a large passive plantarflexion moment during early stance, which required non-physiologic dorsiflexor contractions. Conversely, longer optimal fiber and more neutral resting ankle angles increased the shortening demands of the plantarflexors. These findings highlight the importance of carefully selecting MTU parameters when modeling gait with musculoskeletal models, especially in pathologic or high-performance athlete populations.

Simulating Contact Using the Elastic Foundation Algorithm in OpenSim

Modeling joint contact is necessary to test many questions using simulation paradigms, but this portion of OpenSim is not well understood. The purpose of this study was to provide a guide for implementing a validated elastic foundation contact model in OpenSim. First, the load-displacement properties of a stainless steel ball bearing and ultra high molecular weight polyethylene (UHMWPE) slab were recorded during a controlled physical experiment. These geometries were imported and into OpenSim and contact mechanics were modeled with the on-board elastic foundation algorithm. Particle swarm optimization was performed to determine the elastic foundation model stiffness (2.14×1011 ± 6.81×109 N/m) and dissipation constants (0.999 ± 0.003). Estimations of contact forces compared favorably with blinded experimental data (root mean square error: 87.58 ± 1.57 N). Last, total knee replacement geometry was used to perform a sensitivity analysis of material stiffness and mesh density with regard to penetration depth and computational time. These simulations demonstrated that material stiffnesses between 1011 and 1012 N/m resulted in realistic penetrations (< 0.15mm) when subjected to 981N loads. Material stiffnesses between 1013 and 1015 N/m increased computation time by factors of 12–23. This study shows the utility of performing a simple physical experiment to tune model parameters when physical components of orthopaedic implants are not available to the researcher. It also demonstrates the efficacy of employing the on-board elastic foundation algorithm to create realistic simulations of contact between orthopaedic implants.

Tendon slack length is the primary determinant of plantarflexor muscle-tendon function in computational simulations of gait

Background: Locomotion is partly dictated by plantarflexor function and structure. Computational simulations are powerful tools capable of testing the isolated effects of muscle-tendon structure on gait function. Research Question: The purpose of this study was to characterize the sensitivity of plantarflexor muscle function based on muscle-tendon unit (MTU) parameters. We hypothesized that plantarflexor metabolics and shortening dynamics would be sensitive to MTU parameters. Methods: Stance phase of gait was simulated using a musculoskeletal model and computed muscle control algorithm. Optimal muscle fiber length, tendon slack length, and tendon stiffness parameters were systematically changed to test the effects on plantarflexor metabolics and shortening dynamics. Results and Significance: Plantarflexor metabolic demands were 8 and 28 times more sensitive to muscle fiber and tendon slack lengths, respectively, compared to the effect of tendon stiffness. Shortened tendon slack lengths induced a large passive plantarflexion moment during early stance, which required non-physiologic dorsiflexor contractions. Conversely, longer muscle fiber and tendon slack lengths increased the shortening demands of the plantarflexors to account for the added length of the MTU. These findings highlight the importance of carefully selecting MTU parameters when modeling gait with musculoskeletal models, especially in pathologic or high-performance athlete populations.