Cruciate injury patterns in knee hyperextension: a cadaveric model.

We created an experimental model to evaluate the effects of strain rate on the mechanism of combined cruciate ligament injuries in knee hyperextension. Using straight knee hyperextension to rupture the anterior and posterior cruciates, two strain rates (approximately 100% per second and 5400% per second) were applied to reproduce two clinical injury patterns of the knee: low energy (sporting) and high energy (pedestrian-motor vehicle accident). Ten pairs of fresh-frozen cadaveric knees were injured to 45 degrees of hyperextension. Strain rate sensitivity of the posterior cruciate ligament was shown in this model, with midsubstance tears occuring in specimens tested at a low rate and avulsion "stripping" injuries from the femoral side occuring at a high rate. A variable pattern of anterior cruciate ligament tears at both high and low rates suggests that the specific injury mechanism may also involve other factors including notch morphology. We present a simplified mathematic model used to estimate posterior cruciate ligament strain during knee hyperextension.

Laser assisted soldering: microdroplet accumulation with a microjet device.

BACKGROUND AND OBJECTIVE: We investigated the feasibility of a microjet to dispense protein solder for laser assisted soldering. STUDY DESIGN: Successive micro solder droplets were deposited on rat dermis and bovine intima specimens. Fixed laser exposure was synchronized with the jetting of each droplet. After photocoagulation, each specimen was cut into two halves at the center of solder coagulum. One half was fixed immediately, while the other half was soaked in phosphate-buffered saline for a designated hydration period before fixation (1 hour, 1, 2, and 7 days). After each hydration period, all tissue specimens were prepared for scanning electron microscopy (SEM). RESULTS: Stable solder coagulum was created by successive photocoagulation of microdroplets even after the soldered tissue exposed to 1 week of hydration. CONCLUSIONS: This preliminary study suggested that tissue soldering with successive microdroplets is feasible even with fixed laser parameters without active feedback control.

Laser assisted soldering: effects of hydration on solder-tissue adhesion.

Wound stabilization is critical in early wound healing. Other than superficial skin wounds, most tissue repair is exposed to a hydrated environment postoperatively. To simulate the stability of laser-soldered tissue in a wet environment, we studied the effects of hydration on laser soldered rat dermis and baboon articular cartilage. In this in vitro study, we used a solder composed of human serum albumin, sodium hyaluronate, and Indocyanine Green. A 2 µL solder droplet was deposited on each tissue specimen and then the solder was irradiated with a scanning laser beam (808 nm and 27 W/cm2). After photocoagulation, each tissue specimen was cut into two halves dividing the solder. One half was reserved as control while the other half was soaked in saline for a designated period before fixation (1 h, 1, 2, and 7 days). All tissue specimens were prepared for scanning electron microscopy (SEM). SEM examinations revealed nonuniform coagulation across the solder thickness for most of the specimens, likely a result of the temperature gradient generated by laser heating. Closer to the laser beam, the uppermost region of the solder formed a dense coagulum. The solder aggregated into small globules in the region anterior to the solder-tissue interface. All cartilage specimens soaked in saline suffered coagulum detachment from tissue surface. We noted a high concentration of the protein globules in the detached coagulum. These globules were likely responsible for solder detachment from the cartilage surface. Solder adhered better to the dermis than to cartilage. The dermal layer of the skin, composed of collagen matrix, provided a better entrapment of the solder than the smooth surface of articular cartilage. Insufficient laser heating of solder formed protein globules. Unstable solder-tissue fusion was likely a result of these globules being detached from tissue substrate when the specimen was submerged in a hydrated environment. The solder-tissue bonding was compromised as a result of this phenomenon. © 1998 Society of Photo-Optical Instrumentation Engineers.

Small-angle HeNe laser light scatter and the compressive modulus of articular cartilage.

Light scattering is a widely used technique for probing the microarchitecture and interactions of biological materials and solutions. In this paper, we describe the use of this method in the study of articular cartilage. The experiments presented utilize small-angle static scattering of HeNe laser light (632.8 nm) from 40 microns thick samples of cartilage taken from the superficial zone of baboon femoral condyle. The specimens were taken from a total of 26 sites in eight animals of various ages. In addition to measuring the dependence of the intensity of scattered light on scatter angle, we performed mechanical testing at the test sites using creep-indentation techniques. The results from the optical and mechanical experiments were compared, and a significant correlation was noted between the average scatter angle and the compressive aggregate modulus. In addition, it was noted that the cartilage of skeletally immature animals had a smaller aggregate modulus and scattered to a higher average angle than the cartilage of skeletally mature animals. A quantitative theory was developed to explain the relation, between mechanical and optical properties in terms of the degree of order in the spatial arrangement of the collagen fibers in cartilage.

A molecular theory of cartilage viscoelasticity.

Recent work on the subject of cartilage mechanics has begun to focus on the relationship between the microscopic structure of cartilage and its macroscopic mechanical properties (Bader et al., Biochem. Biophys. Acta, 1116 (1992) 147-154; Buschmann, PhD Thesis, Massachusetts Institute of Technology, 1992; Kovach, Biophys. Chem., 53 (1995) 181-187; Lai et al., J. Biochem. Eng., 113 (1991) 245-248; Armstrong and Mow, J. Bone Jt. Surg., 64A (1982) 88; Jackson and James, Biorheology, 19 (1982) 317-330). This paper reviews recent theoretical developments and presents a comprehensive explanation of the viscoelastic properties of cartilage in terms of molecular structure. In doing this, a closed form hybrid solution to the non-linear, cylindrical Poisson-Boltzmann equation is developed to describe the charge-dependent component of the equilibrium elasticity arising from polysaccharide charge (Benham, J. Chem. Phys., 79 (4) (1983) 1969-1973; Einevoll and Hemmer, J. Phys. Chem., 89 (1) (1988) 474-484; Fixman, J. Chem. Phys., 70 (11) (1979) 4995-5001; Ramanathan and Woodburg, J. Chem. Phys., 82 (3) (1985) 1482-1491; Wennerstrom et al., J. Chem. Phys., 76 (9) (1982) 4665-4670). This solution agrees with numerical solutions found in the literature (Buschmann, PhD Thesis, Massachusetts Institute of Technology, 1992). The charge-independent, entropic contribution to the equilibrium elasticity is explained in a manner similar to that recently presented for concentrated proteoglycan solution (Kovach, Biophys. Chem., 53 (1995) 181-187). This approach exploits a lattice model of the solution, subject to a Bragg-Williams type approximation to derive the volume dependence of polysaccharide configuration entropy (Flory, Principles of Polymer Chemistry, Cornell University Press, Ithaca, NY, 1953; Huggins, Some properties of Solutions of Long-chain Compounds, 1941, pp. 151-157; Stanley, Introduction to Phase Transitions and Critical Phenomena, Oxford University Press, Oxford, 1971). Together, these two contributions accurately reproduce the experimentally determined osmotic pressure of cartilage as previously determined by Maroudas (Maroudas and Bannon, Biorheology, 18 (1981) 619-632). The time-dependent, or creep, phenomena which cartilage exhibits when subject to mechanical load is explained in terms of frictional drag on the polysaccharide chain monomers in terms of a Kirkwood-Riseman type model (Kirkwood and Riseman, J. Chem. Phys., 16 (6) (1948) 573-579). This approach is shown to accurately predict the hydraulic permeability of cartilage as previously determined by Maroudas (Madouras, Ann. Rheum. Dis., 34 (suppl. 3) (1975) 77). By use of a quasi-static approximation (neglecting inertial effects) the time-dependent response to a uniform compressive force is determined and also found to be in good agreement with experimental values from the literature.

The importance of polysaccharide configurational entropy in determining the osmotic swelling pressure of concentrated proteoglycan solution and the bulk compressive modulus of articular cartilage.

One important contribution to the osmotic swelling pressure of concentrated proteoglycan and hence the elasticity of articular cartilage arises from the configurational entropy of the polysaccharide chains in the extracellular matrix. The work presented here provides a theoretical determination of this entropy and an analysis of its effect on the equilibrium osmotic swelling pressure of concentrated proteoglycan solutions. This effect is calculated in a manner similar to the Flory-Huggins technique where the solution is treated as a lattice (P. Flory, Principles of polymer chemistry (Cornell Univ. Press, Ithaca, 1953); J. Chem. Phys. 12 (1944) 425). In addition, the charge-related contribution to the elasticity of these materials is reviewed in the form of a Donnan equilibrium model (T. Hill, Faraday Soc. Discussions 21 (1956) 31; A.G. Ogston and J.D. Wells, Biochem. J. 119 (1970) 67; C. Tanford, Physical Chemistry of Macromolecules (Wiley, New York, 1961)). It is found that the configurational entropy of the glycosaminoglycan (GAG) chain polysaccharides together with the charge effects reproduce the equilibrium swelling pressure of concentrated proteoglycan solutions as experimentally determined by J.P.G. Urban et al., Biorheol. 16 (1979) 447. In addition this theoretical model is manifestly independent of the proteoglycan molecular weight, consistent with prior experimental findings (J.P.G. Urban et al., Biorheol. 16 (1979) 447). The model is also extended to include polydispersity of proteoglycan size and to predict the equilibrium bulk compressive modulus of articular cartilage. This work represents the first comprehensive theoretical description of the equilibrium elastic properties of proteoglycan solutions.(ABSTRACT TRUNCATED AT 250 WORDS)