High-order simulation scheme for active particles driven by stress boundary conditions.

We study the dynamics and interactions of elliptic active particles in a two dimensional solvent. The particles are self-propelled through prescribing a fluid stress at one half of the fluid-particle boundary. The fluid is treated explicitly solving the Stokes equation through a discontinuous Galerkin scheme, which allows to simulate strictly incompressible fluids. We present numerical results for a single particle and give an outlook on how to treat suspensions of interacting active particles.

Mechanical effects of defect closure following BPTB graft harvest for ACL reconstruction.

Anterior cruciate ligament injury affects roughly 120,000 athletes in the United States every year. One of the most common techniques is the use of a bone-patellar tendon-bone graft. Graft harvest creates a sizeable defect in the remaining patellar tendon. Closure of this defect is based on surgeon preference. To date there has been no study on the effects of defect closure on the mechanical properties of remaining donor patellar tendon. The goal of this study was to investigate the effect of closure on both the strength and stiffness of the remaining patellar tendon. 7 pairs of fresh frozen cadaver patellar tendons were matched by tendon dimensions. Bone-patellar tendon-bone grafts were harvested from all of the specimens and then half of the paired tendons underwent defect closure. All of the donor tendons were then tested in a servohydraulic load frame to failure at a constant displacement rate at room temperature. This study found no differences in the load at failure, the engineering failure stress, stiffness or in the engineering modulus between the donor tendons that underwent defect closure versus those that did not.

Surface damage to an Oxinium femoral head prosthesis after dislocation.

During open reduction of an irreducible anterior dislocation of a total hip replacement with an Oxinium femoral head, it was observed that the head had been significantly damaged. Gross and scanning electron microscopic examination revealed cracking, gouging, and delamination of the surface. Because of the risk which this poses for damaging the polyethylene acetabular liner, it is strongly recommended that patients with this type of prosthetic head be carefully monitored after a dislocation.

The effects of fluid penetration and interfacial porosity on the fixation of cemented femoral components.

We have investigated the role of the penetration of saline on the shear strength of the cement-stem interface for stems inserted at room temperature and those preheated to 37 degrees C using a variety of commercial bone cements. Immersion in saline for two weeks at 37 degrees C reduced interfacial strength by 56% to 88% after insertion at room temperature and by 28% to 49% after preheating of the stem. The reduction in porosity as a result of preheating ranged from 71% to 100%. Increased porosity correlated with a reduction in shear strength after immersion in saline (r = 0.839, p < 0.01) indicating that interfacial porosity may act as a fluid conduit.

Effects of the initial temperature of acrylic bone cement liquid monomer on the properties of the stem-cement interface and cement polymerization.

It has been shown that preheating the femoral stem prior to insertion minimizes interfacial porosity at the stem-cement interface. In this study, the effects of methylmethacrylate monomer temperature prior to mixing on the properties of stem-cement interface and cement polymerization were evaluated for 4 degrees C, room temperature, and 37 degrees C using a test model and cementing techniques that simulated a clinical situation. The nature and extent of interfacial porosity of stem-cement interface was quantified, the static shear strength of the stem-cement interface determined, and the time and temperature of polymerization at the cement-bone interface were measured. Compared to RT monomer, preheating monomer to 37 degrees C produced higher polymerization temperatures and greater initial interfacial shear strength with an unchanged amount of interfacial porosity. Precooling monomer to 4 degrees C produced lower polymerization temperatures and decreased initial interfacial shear strength, with the amount of interfacial porosity unchanged compared to the RT group. Although clinical techniques of preheating or precooling bone cement have some effects on the properties of the stem-cement interface and cement polymerization, they do not appear to enhance implant fixation.

Augmentation of anterior vertebral body screw fixation by an injectable, biodegradable calcium phosphate bone substitute.

STUDY DESIGN: A biomechanical study to evaluate the effects of a biodegradable calcium phosphate (Ca-P) bone substitute on the fixation strength and bending rigidity of vertebral body screws. OBJECTIVES: To determine if an injectable, biodegradable Ca-P bone substitute provides significant augmentation of anterior vertebral screw fixation in the osteoporotic spine. SUMMARY OF BACKGROUND DATA: Polymethylmethacrylate (PMMA) augmented screws have been used clinically; however, there is concern about thermal damage to the neural elements during polymerization of the PMMA as well as its negative effects on bone remodeling. Injectable, biodegradable Ca-P bone substitutes have shown enhanced fixation of pedicle screws. METHODS: Sixteen fresh cadaveric thoracolumbar vertebrae were randomly divided into two groups: control (no augmentation) (n = 8) and Ca-P bone substitute augmentation (n = 8) groups. Bone-screw fixation rigidity in bending was determined initially and after 10(5) cycles, followed by pullout testing of the screw to failure to determine pullout strength and stiffness. RESULTS: The bone-screw bending rigidity for the Ca-P bone substitute group was significantly greater than the control group, initially (58%) and after cyclic loading (125%). The pullout strength for Ca-P bone substitute group (1848 +/- 166 N) was significantly greater than the control group (665 +/- 92 N) (P < 0.01). Stiffness in pullout for the Ca-P bone substitute groups (399 +/- 69 N/mm) was significantly higher than the control group (210 +/- 51 N/mm) (P < 0.01). CONCLUSION: This study demonstrated that augmentation of anterior vertebral body screw fixation with a biodegradable Ca-P bone substitute is a potential alternative to the use of PMMA cement.

The effect of stem modularity and mode of fixation on tibial component stability in revision total knee arthroplasty.

The effect of stem length, diameter, and mode of fixation on the motion and stress transfer of a cemented tibial tray were evaluated for in 12 cadaver knees. There was a significant decrease in motion of the tibial tray with increasing press-fit stem length (75-150 mm) and increasing stem diameter (10-14 mm). Cemented tibial stems showed significantly less tray motion than uncemented stems. The short cemented stems produced tray stability equivalent to long press-fit stems. Although there was a trend for increased proximal tibial stress shielding with the use of cement and longer, wider stems, the trend was not statistically significant. Modular, press-fit stems can achieve tray stability similar to a smaller cemented stem and can avoid the potential problems with cement.

Effect of the angle of the screw on the stability of the mandibular sagittal split ramus osteotomy: a study in sheep mandibles.

We compared the biomechanical stability of 60 degrees and 90 degrees angles for insertion of screws for fixation of sagittal split ramus osteotomies (SSROs) in 10 sheep mandibles in vitro. After 5 mm advancements, the osteotomies were fixed with screws 2 mm in diameter inserted with a torque control screwdriver at either 60 degrees or 90 degrees to the long axis of bone, in a matched pair experimental design. All specimens were then loaded in a physiological manner and the load-displacement of the osteotomies was recorded. Data for resistance to movement were compared by a paired t -test. The mean (SD) resistance to movement for the 60 degrees and 90 degrees screw insertions was 63.4 (27.7) N/mm and 59 (22.3) N/mm respectively. There was no significant difference between the groups. This correlates with clinical findings.

Effect of articular step-off and meniscectomy on joint alignment and contact pressures for fractures of the lateral tibial plateau.

OBJECTIVES: To determine the effects of intraarticular step-off and lateral meniscectomy on the alignment of the articular axis, contact area, and pressures for lateral tibial plateau fractures. DESIGN: Biomechanical cadaver study. INTERVENTION: Six fresh cadaveric knees were used. A simulated split fracture of the lateral tibial plateau was reproducibly created by osteotomies, and articular step-offs of zero, one, two, four, and six millimeters were achieved by using support shims. The knee was loaded with 500 newtons in 0 degrees and 350 newtons in 30 degrees of flexion. A digital camera determined changes in the alignment of the articular axis, and F-Scan sensors were inserted into the medial and lateral joint compartments to determine the pressures and pressure distributions. MAIN OUTCOME MEASUREMENT: Each specimen was tested at step-offs of zero, one, two, four, and six millimeters, with the presence or absence of the lateral meniscus. The changes in alignment of the articular axis, the contact area, and the average and maximum contact pressures for each condyle were obtained. RESULTS: Increased articular step-off heights progressively increased valgus angulation and average and maximum contact pressures and progressively decreased contact areas in lateral compartment. At a six-millimeter step-off with 0 degrees of flexion, the valgus angle increased an average of 7.6 degrees, and average contact pressures and maximum contact pressures increased an average of 208 percent and 97 percent, respectively, and contact area decreased an average of 33 percent (p < 0.05). Meniscectomy increased valgus angles by an average of 38 percent and contact pressures by an average of 45 percent and decreased contact areas by 26 percent in the lateral compartment at the same articular step-off heights (p < 0.05). CONCLUSION: The results of this study show the importance of decreasing articular step-off heights in treating lateral tibial plateau split fractures, particularly if a meniscectomy is performed.

Fixation of periprosthetic femoral shaft fractures: a biomechanical comparison of two techniques.

OBJECTIVE: To determine which of two currently used techniques for the treatment of periprosthetic femoral shaft fractures provides the greater fixation rigidity and strength. DESIGN: A laboratory study using six matched pairs of femurs. METHODS: Embalmed femur prosthesis constructs had a simulated periprosthetic fracture created and were fixed with a plate with proximal cables and distal bicortical screws (Ogden concept) or two allograft struts and cables. Fixation stability was compared in various loading modalities before and after cycling. They were then tested to failure. OUTCOME MEASUREMENTS: Fixation rigidity was defined as the ratio of applied load to the amount of displacement at the fracture. RESULTS: In all loading modalities, the Ogden construct was more rigid than the allograft strut fixation. The Ogden construct required 1,295 newtons for failure and the allograft strut fixation required 950 newtons (p < 0.05). CONCLUSION: The Ogden construct provided a more rigid and stronger initial fixation of a periprosthetic fracture than did the allograft construct.

Effect of piriformis versus trochanteric starting point on fixation stability of short intramedullary reconstruction nails.

Recently, a new, shorter IM nail using two 6 mm reconstruction screws for proximal fixation was introduced in two versions for femoral insertion: piriformis fossa (FAN) and greater trochanter (TAN). These nails were compared experimentally for their fixation stability, proximal load transmission, and failure strength in an unstable intertrochanteric fracture model in cadaveric femurs. Vertical and axial loads were first applied to the intact femurs. Fractures were created, subsequent fixation applied, and the femurs underwent a series of both vertical and axial loading tests. There was no significant difference in strain readings between the nails for either axial loading or cyclical loading. There was no statistically significant difference between the loads to failure for the trochanteric nails and the standard antegrade nails. The average ultimate loadfor the FAN and TAN nails were 3010 N and 2830 N respectively. These two nails performed very similarly throughout our testing.

The effect of screw type on the fixation of depressed fragments in tibial plateau fractures.

The ability of various screw types to stabilize depressed tibial plateau fractures was determined in a biomechanical study using a Sawbones model. Two sizes of both cancellous and cortical screws were evaluated for both supportfrom below and through the depressed fragment. As a general trend, cancellous bone screws provided a greater resistance to fragment displacement than cortical bone screws, and screws with a smaller thread diameter provided greater resistance to displacement than screws of the same thread type with a larger diameter. These results agree with the accepted standard that cancellous screws provide better fixation for tibial plateau fractures, but also are counterintuitive in that smaller screws provided greater fixation than larger screws of the same type.

The effect of distal screw orientation on the intrinsic stability of a tibial intramedullary nail.

To compare the intrinsic stability of two distal interlocking screw orientations for tibial nailing of distal third tibial diaphyseal fractures without isthmal support, six Depuy (Warsaw, Indiana) tibial intramedullary nails were implanted in simulated distal tibiae. The constructs received both two parallel (medial to lateral) and two perpendicular (one medial to lateral, one anterior to posterior) distal interlocking screws in a random order Angular, translational, and torsional displacements of the nails were measured in response to 70 N proximal applications of anterior, posterior, medial, and lateral loads, and a 7.7 Newton-meter torsional load. There were no differences in medial or lateral angulations between the screw orientations (average: 2.5 degrees, p > 0.8). Angulation in the sagittal plane (anterior and posterior) was slightly less for parallel screw fixation (1.6 degrees versus 2.4 degrees), but this was not statistically significant (p > 0.1). Rotational angulation was higher in the parallel (average: 9.9 degrees) versus the perpendicular (average: 8.1 degrees) screw orientation, but these results were not statistically significant (p > 0.1). Pure translation did not occur in either the parallel or perpendicular screw orientations. These results indicate that fixation stability of these tibial intramedullary nails is not significantly influenced by distal interlocking screw orientation in response to sagittal, coronal, or rotational forces.

Dislocations of the sternoclavicular joint.

The effects of the anterior and posterior sternoclavicular joint (SCJ) soft tissue structures on joint dislocation strength by sequential sectioning the ligaments and capsule of twenty-eight SCJs were evaluated. The medial clavicle of each specimen was initially loaded in the anterior and posterior directions to provide control values for joint laxity. The anterior or posterior ligaments and capsular structures of the SCJs were then selectively cut and the specimens retested for laxity and then loaded to failure simulating either anterior or posterior dislocation. Testing of intact specimens showed that the posterior ligaments were stiffer than other structures in that it was significantly more difficult to posteriorly displace the SCJ than in any other direction and that the capsule was the important anterior structure affecting joint laxity. Load-to-failure testing showed that it required 50% more force to create a failure by posterior dislocation than by anterior dislocation. The results of this study explain the clinical rarity of posterior sternoclavicular joint dislocations.

Tension wire position for hybrid external fixation of the proximal tibia.

OBJECTIVE: To compare a new configuration of proximal wires for hybrid external fixation with the standard configuration. DESIGN: Biomechanical testing of five matched pairs of fresh cadaveric tibia. INTERVENTION: The authors compared the standard tension wire configuration of the three proximal wires with a more sagittal orientation of the oblique wires. A second study compared the new configuration with two wires and an offset half-pin. A two-centimeter segmental defect was created just distal to the tibial tubercle and the tibias fixed in a Montecelli Spinelli (Howmedica, NJ, U.S.A.) hybrid frame. The constructs were biomechanically tested using an Instron servohydraulic biaxial testing machine. RESULTS: There was a significant 67 percent decrease in displacement during anterior posterior bending and a significant 40 percent decrease in displacement in posterior bending with the new configuration compared with the standard configuration (p < 0.05). The differences in stability in all other testing modes were not significant. There was no significant difference between the new configuration and the two wire and off-set half-pin configuration. CONCLUSION: We recommend anterior placement of the oblique tension wires in the proximal tibia to more effectively resist bending in the sagittal plane, which is the most common deforming force on proximal metaphyseal fractures.