Parathyroid Hormone Treatment Increases Fixation of Orthopedic Implants with Gap Healing: A Biomechanical and Histomorphometric Canine Study of Porous Coated Titanium Alloy Implants in Cancellous Bone

Parathyroid hormone (PTH) administered intermittently is a bone-building peptide. In joint replacements, implants are unavoidably surrounded by gaps despite meticulous surgical technique and osseointegration is challenging. We examined the effect of human PTH(1–34) on implant fixation in an experimental gap model. We inserted cylindrical (10 × 6 mm) porous coated titanium alloy implants in a concentric 1-mm gap in normal cancellous bone of proximal tibia in 20 canines. Animals were randomized to treatment with PTH(1–34) 5 μg/kg daily. After 4 weeks, fixation was evaluated by histomorphometry and push-out test. Bone volume was increased significantly in the gap. In the outer gap (500 μm), the bone volume fraction median (interquartile range) was 27% (20–37%) for PTH and 10% (6–14%) for control. In the inner gap, the bone volume fraction was 33% (26–36%) for PTH and 13% (11–18%) for control. At the implant interface, the bone fraction improved with 16% (11–20%) for PTH and 10% (7–12%) (P = 0.07) for control. Mechanical implant fixation was improved for implants exposed to PTH. For PTH, median (interquartile range) shear stiffness was significantly higher (PTH 17.4 [12.7–39.7] MPa/mm and control 8.8 [3.3–12.4] MPa/mm) (P < 0.05). Energy absorption was significantly enhanced for PTH (PTH 781 [595–1,198.5] J/m2 and control 470 [189–596] J/m2). Increased shear strength was observed but was not significant (PTH 3.0 [2.6–4.9] and control 2.0 [0.9–3.0] MPa) (P = 0.08). Results show that PTH has a positive effect on implant fixation in regions where gaps exist in the surrounding bone. With further studies, PTH may potentially be used clinically to enhance tissue integration in these challenging environments

Mechanical properties of the human posterior lens capsule. Invest Ophthalmol Vis Sci.

PURPOSE. To investigate mechanical properties of the human posterior lens capsule. METHODS. Twenty-five human donor eyes were obtained from an eye bank. The age of the donors ranged from 1 to 94 years. Test specimens were prepared as tissue rings from posterior lens capsules by means of excimer laser. Capsular thickness was measured microscopically as the difference in focus between microspherules placed on the outer and inner surfaces of the capsule. The capsular rings were slipped over two pins connected to a motorized micropositioner and a force transducer and stretched at a constant rate with continuous recording of load and deformation. Data for the posterior lens capsule were compared with previously published data for the anterior lens capsule. RESULTS. The thickness of the posterior lens capsule ranged from 4 to 9 m and showed no significant changes with age. Ultimate mechanical strength of the posterior lens capsule decreased significantly with age. Ultimate strain ranged from 101% to 34%, ultimate load ranged from 15.9 to 1.1 mN, ultimate stress ranged from 16.1 to 1.1 N/mm 2 , ultimate elastic stiffness ranged from 52.1 to 5.7 mN, and ultimate elastic modulus ranged from 27.4 to 3.3 N/mm 2 . The load-strain and the stress-strain relationships in the posterior lens capsule were nonlinear, and therefore elastic stiffness and elastic modulus varied as a function of strain. In the low-strain region (0%-10% strain), elastic stiffness and elastic modulus ranged between 0.3 to 2.4 mN and 0.3 to 2.3 N/mm 2 , respectively, and seemed to increase during the first part of life until middle age. CONCLUSIONS. Mechanical strength of the posterior lens capsule was found to decrease markedly with age. The age-related loss of mechanical strength seemed to begin earlier in the posterior lens capsule than in the anterior lens capsule. In accommodative function range (low strains), the mechanical quality of the posterior lens capsule was similar to the anterior lens capsule, which indicates that the mechanical effectiveness of the lens capsule in situ varies proportionally with capsular thickness. (Invest Ophthalmol Vis Sci. 2003;44:691-696) DOI:10.1167/ iovs.02-0096 K nowledge of the mechanical properties of the human lens capsule is essential for the understanding of its physiological function in relation to the accommodative function, its functional reserve in the elderly population, and its potential in relation to cataract surgery. Mechanical properties of the anterior human lens capsule have been investigated, 1,2 whereas mechanical properties of the posterior lens capsule have not yet been described quantitatively. The anterior and posterior lens capsule differ in several aspects. The posterior lens capsule is substantially thinner than the anterior capsule. 3-5 It loses its epithelial cells in fetal life 2,9 MATERIALS AND METHODS Twenty-five human posterior lens capsules were obtained from an eye bank. Donor age ranged from 1 to 94 years. Mean postmortem time was 44 Ϯ 18 hours. Excluded from the study were donors with diabetes mellitus and eyes with cortical and subcapsular lenticular opacities or severe nuclear sclerosis. The posterior capsule was dissected from the lens and stored at Ϫ80°C until mechanical testing. The investigation was approved by the institutional ethics committee and adhered the tenets of the Declaration of Helsinki. Test specimens were prepared as tissue rings from the central part of the posterior lens capsule by an excimer laser technique. 2,10 A metal ring was placed on the central part of the posterior lens capsule to shape the laser output. The outer diameter of the metal ring was 3.2 mm and the width was 100 m. Capsular thickness was measured optically as the difference in focus between latex spherules placed on the upper and the lower surfaces of the capsular rings (precision: 0.3 m). Thickness measurements were repeated twice at eight points of the capsular rings, and the mean values were used as the average thickness of the capsular rings. The width of the capsular rings was measured with a micrometer eyepiece (precision: 1.3 m). For mechanical testing, the capsular rings were slipped over two pins connected to a motorized micropositioner and a force transducer and stretched at a constant rate, with continuous recording of load (resolution: 0.01 mN) and elongation (resolution: 0.1 m). 2,10 Strain values were calculated as the elongation expressed in percent of the initial length of the test specimen. Load-strain data showed the mechanical response of the capsular rings and reflected the mechanical effectiveness of the lens capsule in situ. The following parameters were calculated from the load-strain curves: ultimate strain (strain at failure), ultimate load (load at failure), elastic stiffness (0%-10% strain) determined as the slope of the loadstrain curves from 0 -10% strain, and ultimate elastic stiffness determined as the slope of the linear steepest part of the load-strain curves up to the point of failure. Stress values were calculated by normalizing the load values to the cross-sectional area of the capsular rings. Stress-strain data reflect the From th