Impact of helmets on injuries to riders of all-terrain vehicles.

BACKGROUND: Although helmet use has been shown to be effective in reducing traumatic brain injuries (TBIs) due to motorcycle and bicycle crashes, it is unknown whether helmet use is associated with different injury patterns and severity for users of all-terrain vehicles (ATVs). OBJECTIVES: To compare likelihood of injury and death between helmeted and unhelmeted riders of ATVs. METHODS: The National Trauma Data Bank for years 2002-2006 was used to examine the records of 11 589 patients hospitalized for injuries resulting from ATV use. The likelihood of receiving a TBI diagnosis or a significant injury to other body regions and differences in injury severity and in-hospital mortality between helmeted and unhelmeted ATV riders were compared. RESULTS: After multivariable adjustment, compared with helmeted riders, unhelmeted riders were significantly more likely to sustain any TBI (OR 1.62, 95% CI 1.49 to 1.76, p<0.001) and major/severe TBI (OR 3.19, 95% CI 2.39 to 4.25, p<0.001). Unhelmeted riders were significantly more likely to die while in hospital than were helmeted riders (OR 2.58, 95% CI 1.79 to 3.71, p<0.001). Significant injuries to the neck and face regions were also significantly more likely in unhelmeted riders (OR 3.53, 95% CI 1.28 to 9.71, p = 0.015, and OR 1.94, 95% CI 1.32 to 2.84, p = 0.001, respectively). CONCLUSIONS: ATV riders who do not wear helmets are more likely to receive significant injuries to the head, face, and neck. Prevention strategies and enforceable policy interventions to increase helmet use among ATV riders appear warranted.

Evaluation of autologous chondrocyte transplantation via a collagen membrane in equine articular defects: results at 12 and 18 months.

OBJECTIVE: To evaluate a technique of autologous chondrocyte implantation (ACI) similar to the other techniques using cell-seeded resorbable collagen membranes in large articular defects. METHODS: Autologous cartilage was harvested arthroscopically from the lateral trochlear ridge of the femur in fifteen 3-year-old horses. After culture and expansion of chondrocytes the newly created ACI construct (autologous chondrocytes cultured expanded, seeded on a collagen membrane, porcine small intestine submucosa) was implanted into 15mm defects on the medial trochlear ridge of the femur in the opposite femoropatellar joint. Using two defects in each horse, the ACI technique was compared to collagen membrane alone (CMA) and empty cartilage defects (ECDs). RESULTS: Arthroscopic evaluations at 4, 8, 12 and 18 months demonstrated that CMA was significantly worse compared to ACI or ECD treatments, with ACI having the best overall subjective grade. Overall raw histological scores demonstrated a significant improvement with ACI compared to either CMA or ECD treated defects and ACI defects had significantly more immunohistochemical staining for aggrecan than CMA or ECD treated defects (with significantly more type II collagen in ACI and ECD compared to CMA defects) at 12 and 18 months. CONCLUSIONS: Histologic and immunohistochemistry results from this long-term randomized study are particularly encouraging and demonstrate superiority with the ACI technique. Although there is no comparable study published with the traditional ACI technique in the horse (or with such a large defect size in another animal model), the use of a solid autologous cell-seeded-constructed implant would appear to offer considerable clinical advantages.

Results from demineralized bone creep tests suggest that collagen is responsible for the creep behavior of bone.

Cortical and trabecular bone have similar creep behaviors that have been described by power-law relationships, with increases in temperature resulting in faster creep damage accumulation according to the usual Arrhenius (damage rate approximately exp (-Temp.-1)) relationship. In an attempt to determine the phase (collagen or hydroxyapatite) responsible for these similar creep behaviors, we investigated the creep behavior of demineralized cortical bone, recognizing that the organic (i.e., demineralized) matrix of both cortical and trabecular bone is composed primarily of type I collagen. We prepared waisted specimens of bovine cortical bone and demineralized them according to an established protocol. Creep tests were conducted on 18 specimens at various normalized stresses sigma/E0 and temperatures using a noninvasive optical technique to measure strain. Denaturation tests were also conducted to investigate the effect of temperature on the structure of demineralized bone. The creep behavior was characterized by the three classical stages of decreasing, constant, and increasing creep rates at all applied normalized stresses and temperatures. Strong (r2 > 0.79) and significant (p < 0.01) power-law relationships were found between the damage accumulation parameters (steady-state creep rate d epsilon/dt and time-to-failure tf) and the applied normalized stress sigma/E0. The creep behavior was also a function of temperature, following an Arrhenius creep relationship with an activation energy Q = 113 kJ/mole, within the range of activation energies for cortical (44 kJ/mole) and trabecular (136 kJ/mole) bone. The denaturation behavior was characterized by axial shrinkage at temperatures greater than approximately 56 degrees C. Lastly an analysis of covariance (ANCOVA) of our demineralized cortical bone regressions with those found in the literature for cortical and trabecular bone indicates than all three tissues creep with the same power-law exponents. These similar creep activation energies and exponents suggest that collagen is the phase responsible for creep in bone.

Creep contributes to the fatigue behavior of bovine trabecular bone.

Repetitive, low-intensity loading from normal daily activities can generate fatigue damage in trabecular bone, a potential cause of spontaneous fractures of the hip and spine. Finite element models of trabecular bone (Guo et al., 1994) suggest that both creep and slow crack growth contribute to fatigue failure. In an effort to characterize these damage mechanisms experimentally, we conducted fatigue and creep tests on 85 waisted specimens of trabecular bone obtained from 76 bovine proximal tibiae. All applied stresses were normalized by the previously measured specimen modulus. Fatigue tests were conducted at room temperature; creep tests were conducted at 4, 15, 25, 37, 45, and 53 degrees C in a custom-designed apparatus. The fatigue behavior was characterized by decreasing modulus and increasing hysteresis prior to failure. The hysteresis loops progressively displaced along the strain axis, indicating that creep was also involved in the fatigue process. The creep behavior was characterized by the three classical stages of decreasing, constant, and increasing creep rates. Strong and highly significant power-law relationships were found between cycles-to-failure, time-to-failure, steady-state creep rate, and the applied loads. Creep analyses of the fatigue hysteresis loops also generated strong and highly significant power law relationships for time-to-failure and steady-state creep rate. Lastly, the products of creep rate and time-to-failure were constant for both the fatigue and creep tests and were equal to the measured failure strains, suggesting that creep plays a fundamental role in the fatigue behavior of trabecular bone. Additional analysis of the fatigue strain data suggests that creep and slow crack growth are not separate processes that dominate at high and low loads, respectively, but are present throughout all stages of fatigue.

The tensile behavior of demineralized bovine cortical bone.

Bone is frequently modeled as a two-phase composite of hydroxyapatite mineral crystals dispersed throughout an organic collagen matrix. However, because of the numerous limitations (e.g. small sample size, poor strain measuring techniques, rapid demineralization with acids) of previous mechanical tests of bone with its hydroxyapatite chemically removed, we have determined new, accurate data on the material properties of the demineralized bone matrix for use in these composite models. We performed tensile tests on waisted specimens of demineralized bovine cortical bone from six humeral diaphyses. Specimens were demineralized over 14 days with a 0.5 M disodium EDTA solution that was replaced daily. Atomic absorption spectrophotometry was used to track the demineralization process and to determine the effectiveness of our demineralization protocol. Mechanical tests were performed at room temperature under displacement control at an approximate strain rate of 0.5% per s. We imposed nine preconditioning cycles before a final ramp to failure, and measured gauge length displacements using a non-invasive optical technique. The resulting stress-strain curves were similar to the tensile behavior observed in mechanical tests of other collagenous tissues, exhibiting an initial non-linear 'toe' region, followed by a linear region and subsequent failure without evidence of yielding. We found an average modulus, ultimate stress, and ultimate strain of 613 MPa (S.D. = 113 MPa), 61.5 MPa (S.D. = 13.1 MPa), and 12.3% (S.D. = 0.5%), respectively. Our average modulus is approximately half the value frequently used in current composite bone analyses. These data should also have clinical relevance because the early strength of healing fractured bone depends largely on the material properties of the collagen matrix.

Compressive creep behavior of bovine trabecular bone.

There are almost no published data that describe the creep behavior of trabecular bone (at the specimen level), even though the creep behavior of cortical bone has been well documented. In an effort to characterize the creep behavior of trabecular bone and to compare it with that of cortical bone, we performed uniaxial compressive creep tests on 24 cylindrical specimens of trabecular bone taken from 19 bovine proximal tibiae. Six different load levels were used, with the applied stress normalized by the specimen modulus measured prior to creep loading. We found that trabecular bone exhibits the three creep regimens (primary, secondary, and tertiary) associated with metals, ceramics, and cortical bone. All specimens eventually fractured at strains less than 3.8%. In addition, the general shape of the creep curve was independent of apparent density. Strong and highly significant power law relationships (r2 > 0.82, p < 0.001) were found between the normalized stress sigma/E0 and both time-to-failure tf and steady-state creep rate d epsilon/dt: tf = 9.66 x 10(-33) (sigma/E0)-16.18; d epsilon/dt = 2.21 x 10(33) (sigma/E0)17.65. These data indicate that the creep behaviors of trabecular and cortical bone are qualitatively similar. In addition, the strength of trabecular bone can be reduced substantially if relatively large stresses (i.e. stresses approximately half the ultimate strength) are applied for 5 h. Such strength reductions may play a role in the etiology of progressive, age-related spine fractures if adaptive bone remodeling does not arrest creep deformations.