Troels T. Andreassen

Biomechanical properties of keratoconus and normal corneas

The uniaxial tensile strength was determined for six keratoconus and seven normal corneas. Strip specimens were stretched at a constant speed in a materials testing machine while recording load and deformation. Load-deformation curves were transformed into load-strain and stress-strain curves. The following parameters were derived from the load-strain and stress-strain curves: the maximum load and stress values and the corresponding strain value, the values for maximum stiffness, and the relative energy absorption of the specimens during testing. With the exception of the strain value at maximum load or stress, these parameters were smaller in the keratoconus group than in the normal group. The load and stress values at corresponding strain values were less in keratoconus corneas than in normal corneas both for the initial and for the linear parts of the curves. Biochemically, no difference was found in the concentration of uronic acid in the two groups. Similarly no difference was found in the concentration of hydroxyproline in the two groups. The solubility of pepsin treated collagen in the keratoconus corneas was greater than that in the normal corneas, however, the solubility was less than that in tendons.

Intermittent Parathyroid Hormone (1–34) Treatment Increases Callus Formation and Mechanical Strength of Healing Rat Fractures

The influence of intermittent parathyroid hormone (PTH(1–34)) administration on callus formation and mechanical strength of tibial fractures in rats was investigated after 20 and 40 days of healing. A dose of 60 μg of PTH(1–34)/kg/day and 200 μg of PTH(1–34)/kg/day, respectively, was administered during the entire periods of healing, and control animals with fractures were given vehicle. The dose of 200 μg of PTH(1–34)/kg/day increased the ultimate load and the external callus volume of the fractures by 75% and 99%, respectively, after 20 days of healing and by 175% and 72%, respectively, after 40 days of healing. The dose of 60 μg of PTH(1–34)/kg/day did not influence either ultimate load or external callus volume of the fractures after 20 days of healing, but the ultimate load was increased by 132% and the external callus volume was increased by 42% after 40 days of healing. During the healing period, the callus bone mineral content (BMC) increased in all groups. After 40 days of healing, the callus BMC was increased by 108% in the 200 μg of PTH(1–34)/kg/day group and by 76% in the 60 μg of PTH(1–34)/kg/day group. Both doses of PTH(1–34) steadily augmented the contralateral intact tibia BMC (20 days and 40 days: 60 μg of PTH (1–34)/kg/day 9% and 19%, respectively; 200 μg of PTH (1–34)/kg/day 12% and 27%, respectively) and bone mineral density (20 days and 40 days: 60 μg of PTH(1–34)/kg/day 11% and 12%, respectively; 200 μg of PTH(1–34)/kg/day 11% and 15%, respectively).

Reduced concentrations of collagen cross-links are associated with reduced strength of bone

The known cross-links of bone collagen are derived from lysine and hydroxylysine. The first step in the enzymatic cross-linking process is a deamination by lysyl oxidase producing an aldehyde which then may condense with a lysyl or hydroxylysyl residue of a neighbouring collagen molecule. Some of the resulting divalent aldimine and oxo-imine cross-links may later on be incorporated in trivalent hydroxylysyl-pyridinoline and lysyl-pyridinoline cross-links. In bone collagen prepared from the cancellous bone of vertebral bodies of osteoporotic individuals we found a reduced stability towards acetic acid and pepsin, and a substantial reduction in the concentration of the divalent collagen cross-links compared with sex- and age-matched controls. To what extent do the collagen cross-links influence the mechanical properties of bone? beta-amino-propionitrile (BAPN) irreversibly inhibits the enzyme lysyl oxidase and therefore, the formation of cross-links between the collagen molecules. In the present study female rats, 70 days old, injected subcutaneously two times daily with BAPN (333 mg/kg/day) for 1 month and saline injected control rats were studied. The concentration of the hydroxypyridinium cross-links of femoral mid-diaphyseal cortical bone was determined by HPLC with fluorescence detection and the mechanical properties of the rat femoral diaphyses were analyzed by a materials testing machine. The BAPN injections resulted in a 45% reduction in the concentration of the hydroxypyridinium cross-links and a 31% decrease in the stability of the bone collagen towards acetic acid and pepsin compared with the control rats. No changes were found in ash or collagen concentrations of the cortical bone.(ABSTRACT TRUNCATED AT 250 WORDS)

Local injection of TGF-β increases the strength of tibial fractures in the rat

The effect of Transforming Growth Factor beta (TGF-beta) administered locally around the fracture line of healing rat tibial fractures was investigated after 40 days of healing. TGF-beta in a dose of 4 ng or 40 ng was injected every second day during the healing period. The strength, stiffness, energy absorption and deflection of the fractures were measured in a materials-testing machine. Compared with placebo-treated animals, the ultimate load of the fractures increased in the group injected with 40 ng of TGF-beta, but not in those injected with 4 ng. TGF-beta induced a dose-dependent increase in the cross-sectional area of the callus and bone at the fracture line. Consequently, local treatment of fractures with TGF-beta increases the callus formation and strength. The energy absorption and deflection capacities of the healing fractures are preserved.

Hypocalcemia and osteopathy in mice with kidney-specific megalin gene defect

Megalin is an endocytic receptor highly expressed in the proximal tubules of the kidney. Recently, we demonstrated that this receptor is essential for the renal uptake and conversion of 25‐OH vitamin D3 to 1,25‐(OH)2 vitamin D3, a central step in vitamin D and bone metabolism. Unfortunately, the perinatal lethality of the conventional megalin knockout mouse model precluded the detailed analysis of the significance of megalin for calcium homeostasis and bone turnover in vivo. Here, we have generated a new mouse model with conditional inactivation of the megalin gene in the kidney by using Cre recombinase. Animals with a renal‐specific receptor gene defect were viable and fertile. However, lack of receptor expression in the kidney results in plasma vitamin D deficiency, in hypocalcemia and in severe bone disease, characterized by a decrease in bone mineral content, an increase in osteoid surfaces, and a lack of mineralizing activity. These features are consistent with osteomalacia (softening of the bones) as a consequence of hypovitaminosis D and demonstrate the crucial importance of the megalin pathway for systemic calcium homeostasis and bone metabolism.

Increases in callus formation and mechanical strength of healing fractures in old rats treated with parathyroid hormone

We studied the effects of intermittent administration of parathyroid hormone (PTH(1-34)) on callus formation and mechanical strength of tibial fractures in 27-month-old rats after 3 and 8 weeks of healing. 200 w g PTH(1-34)/kg was administered daily during both periods of healing, and control animals with fractures were given vehicle. At 3 weeks, PTH treatment increased maximum load and external callus volume by 160% and 208%; at 8 weeks, by 270% and 135%. It also enhanced callus bone mineral content (BMC) by 190% and 388% (3 and 8 weeks). From week 3 to week 8, callus BMC increased by 60% in the vehicle-injected animals, and by 169% in the PTH-treated animals. In the contralateral intact tibia, PTH treatment increased BMC by 18% and 21% (3 and 8 weeks). No differences in body weight were found between the vehicle-injected and the PTH-treated animals during the experiment. In conclusion, PTH treatment enhances fracture strength, callus volume and callus BMC after 3 and 8 weeks of healing.

Mechanical properties of the human lens capsule

The human lens capsule has recently been the subject of much attention in an attempt to understand its physiological function in relation to the accommodative function, its functional reserve in the elderly population, and its potential in relation to cataract surgery. This overview presents our current knowledge of the mechanical properties of the human lens capsule, discussed on basis of its structure and its role in accommodation and cataract surgery.

Mechanical properties of rat tail tendon in relation to proximal-distal sampling position and age

The mechanical properties of 3, 15 and 25 month-old rat tail tendons were investigated in relation to proximal-distal sampling location along the fibre length. For the 15 and 25 month-old tendons maximum load as well as collagen content per mm fibre length (unit collagen) increased markedly from the proximal to the distal location. A linear regression analysis of the collagen content and mechanical parameters (maximum load, maximum slope of the load-strain curve and energy absorption) showed that these parameters were linearly correlated to the collagen content. However, normalization of the mechanical parameters with regard to the collagen content did not cancel the dependency of the parameters on proximal-distal sampling location. Normalized load and energy values for the 3 month-old tendons and normalized slope values for the 15 and 25 month-old tendons were found to decrease from proximal to distal location. These findings showed that tail tendons are heterogeneous along their length in respect to mechanical strength. The regression analysis also indicated the existence of an inverse relationship between unit collagen and mechanical quality of the collagen. Alternatively, the mechanical properties of tendon fibres might be influenced by other components than collagen.

Bone repair inhibited by indomethacin: Effects on bone metabolism and strength of rabbit osteotomies

We measured mineral content, maximum bending strength, and regional blood flow after tibial osteotomy fixed with a small metal plate in 38 rabbits. Half of the animals were treated with indomethacin (10 mg/kg/day) while the other half served as controls. After 2 and 6 weeks, the bone mineral content and maximum bending strength were lower in the indomethacin group when compared with the controls. Compared with the controls, the blood flow at the osteotomy site was decreased after 2 weeks and increased after 6 weeks in the indomethacin-treated animals. Inhibition of blood flow increase by indomethacin medication in the early period following osteotomy, as well as retarded bone healing, are probably caused by inhibition of the inflammatory reaction.

The effect of aging on fracture healing in the rat

SummaryThe effect of age on the biomechanical properties of healing tibial fractures was studied by comparing the fracture healing in 2-year-old male Wistar rats with the fracture healing in 3-month-old male Wistar rats after 40 and 80 days of healing. There were no significant differences in the mechanical parameters after 40 days of healing, but after 80 days, a considerable delay in the fracture healing process was noted in the old rats compared with the young adult rats when evaluated by maximum load, maximum stress, stiffness, and energy absorption in a three-point bending procedure. In the contralateral, nonfractured bones, the tibiae from the old animals sustained higher loads and had higher stiffness than the bones from the young adult animals, but stress values, elastic modulus, and capacity for energy absorption was much lower in the old animals.