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.

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.