Transgenic over-expression of plasminogen activator inhibitor-1 results in age-dependent and gender-specific increases in bone strength and mineralization.

The plasminogen activation system (PAS) and its principal inhibitor, plasminogen activator inhibitor-1 (PAI-1), are recognized modulators of matrix. In addition, the PAS has previously been implicated in the regulation of bone homeostasis. Our objective was to study the influence of active PAI-1 on geometric, biomechanical, and mineral characteristics of bone using transgenic mice that over-express a variant of human PAI-1 that exhibits enhanced functional stability. Femora were isolated from male and female, wildtype (WT) and transgenic (PAI-1.stab) mice at 16 and 32 weeks of age (n=10). Femora were imaged via DEXA for BMD and muCT for cortical mid-slice geometry. Torsional testing was employed for biomechanical properties. Mineral composition was analyzed via instrumental neutron activation analysis. Female femora were further analyzed for trabecular bone histomorphometry (n=11). Whole animal DEXA scans were performed on PAI-1.stab females and additional transgenic lines in which the functional domains of the PAI-1 protein were specifically disrupted. Thirty-two week female PAI-1.stab femora exhibited decreased mid-slice diameters and reduced polar moment of area compared to WT, while maintaining similar cortical bone width. Greater biomechanical strength and stiffness were demonstrated by 32 week PAI-1.stab female femora in addition to a 52% increase in BMD. PAI-1.stab trabecular bone architecture was comparable to WT. Osteoid area was decreased in PAI-1.stab mice while mineral apposition rate increased by 78% over WT. Transgenic mice expressing a reactive-site mutant form of PAI-1 showed an increase in BMD similar to PAI-1.stab, whereas transgenic mice expressing a PAI-1 with reduced affinity for vitronectin were comparable to WT. Over-expression of PAI-1 resulted in increased mineralization and biomechanical properties of mouse femora in an age-dependent and gender-specific manner. Changes in mineral preceded increases in strength/stiffness and deterred normal cross-sectional expansion of cortical bone in females. Trabecular bone was not altered in PAI-1.stab mice whereas MAR increased significantly, further supporting mineral changes as the underlying factor in strength differences. The primary influence of PAI-1 occurred during a period of basal bone remodeling, attributing a role for this system in remodeling as opposed to development. Comparison of transgenic lines indicates that PAI-1's influence on bone is dependent on its ability to bind vitronectin, and not on its proteolytic activity. The impact of PAI-1 on mouse femora supports a regulatory role of the plasminogen activation system in bone homeostasis, potentially elucidating novel targets for the treatment of bone disease.

An in vitro biomechanical comparison of two fixation methods for transverse osteotomies of the medial proximal forelimb sesamoid bones in horses.

OBJECTIVE: This study compared the mechanical properties of the normal intact suspensory apparatus and two methods of fixation for repair of transverse, midbody fractures of the proximal sesamoid bones of adult horses: transfixation wiring (TW) and screws placed in lag fashion (LS). STUDY DESIGN: An in vitro, paired study using equine cadaver limbs mounted in a loading apparatus was used to test the mechanical properties of TW and LS. ANIMAL OR SAMPLE POPULATION: Seventeen paired (13 repaired, 4 normal) equine cadaver limbs consisting of the suspensory apparatus third metacarpal bone, and first and second phalanges. METHOD: The two methods of repair and normal intact specimens were evaluated in single cycle-to-failure loading. Yield failure was defined to occur at the first notable discontinuity (>50 N) in the load-displacement curve, the first visible failure as evident on the videotape, or a change in the slope of the moment-fetlock angle curve. Ultimate failure was defined to occur at the highest load resisted by the specimen. Corresponding resultant force and force per kg of body weight on the suspensory apparatus, fetlock joint moment, and angle of fetlock dorsiflexion were calculated by use of specimen dimensions and applied load. These were compared along with specimen stiffness, and ram displacement. RESULTS: Load on the suspensory apparatus, load on the suspensory apparatus per kg of body weight, moment, applied load, and angle of fetlock dorsiflexion at yield failure were significantly greater for the TW-repaired than for the LS-repaired specimens. A 3 to 5 mm gap was observed before yield failure in most TW-repaired osteotomies. CONCLUSIONS: Transfixation wiring provided greater strength to yield failure than screws placed in lag fashion in single cycle load-to-failure mechanical testing of repaired transverse osteotomized specimens of the medial proximal forelimb sesamoid bone.

Biomechanical evaluation of a crimp clamp system for loop fixation of monofilament nylon leader material used for stabilization of the canine stifle joint.

OBJECTIVE: To test a crimp clamp system designed to secure monofilament nylon leader (MNL) material commonly used as lateral fabellotibial sutures (LFS) in extra-articular stabilization of the canine stifle joint. STUDY DESIGN: In vitro biomechanical tests of MNL loops secured with either the crimp clamp system or knots were performed. Suture loops (n = 94) were created from 27.3 kg tensile strength MNL and fastened with knots or crimp clamps. Tests were conducted on steam-sterilized, ethylene-oxide-sterilized, and nonsterilized MNL sutures. Loops were evaluated in single load tests and cycled tests. Values for load to failure, initial loop tension, loop elongation, mode of failure, and point of failure were determined. RESULTS: Crimp-clamped loops were superior to knotted loops in all parameters tested in both cycled and noncycled tests. Loop failure generally occurred by breaking within 3 mm of the fixation in both clamped and knotted tests. Loop elongation after cycling was greater in the knotted loops compared with clamped loops (P < .001). Load to failure was greater in clamped tests than in knotted tests (P < .001), regardless of sterilization technique used. Significantly higher initial loop tension could be achieved with the clamp system compared with knot fixation (P < .001). CONCLUSIONS: The crimp clamp system provides superior in vitro loop fixation characteristics compared with knot fixation in 27.3 kg test MNL. CLINICAL RELEVANCE: Based on the results of biomechanical testing and the known biocompatibility of the system's implant components, clinical trials using the crimp clamp system are warranted.

Comparison of the degree of abdominal adhesion formation associated with chromic catgut and polypropylene suture materials.

OBJECTIVE: To evaluate the histologic pattern and biomechanical properties of adhesions caused by chromic catgut and polypropylene sutures, using an enteropexy model. DESIGN: Enteropexies were created in dogs, using chromic catgut and polypropylene suture. The adhesions associated with the enteropexies were examined histologically and mechanically. ANIMALS: 6 mixed-breed dogs weighing 16 to 20 kg. PROCEDURE: 72 enteropexies were created between the jejunum and abdominal wall. 36 sites were sutured with chromic catgut and 36 were sutured with polypropylene. 3 dogs were euthanatized after 1 week. The remaining dogs were euthanatized after 1 month. Samples of the enteropexy sites were obtained for histologic examination. The remaining sites were mechanically distracted until failure of the enteropexy site or adjacent tissue occurred. RESULTS: Histologic examination of the enteropexy sites did not reveal substantial differences in the degree of inflammation between the 2 suture types at 1 week or 1 month. The degree of inflammation decreased and the maturity of fibrous tissue formed at the enteropexy sites increased for all specimens over time. No statistically significant difference in breaking strength was observed between suture types at 1 week or 1 month. CONCLUSION: In dogs, the formation and strength of intentionally created abdominal adhesions are not increased by use of chromic catgut. CLINICAL RELEVANCE: Selection of chromic catgut suture for use in surgical procedures where adhesions are desired is unwarranted.

The effects of implant stiffness on the bypassed bone mineral density and facet fusion stiffness of the canine spine.

STUDY DESIGN: Effects of spinal implant stiffness and removal/retention on bypassed bone mineral density and column/fusion stiffness were studied in dogs. METHODS: After facet fusion and bicortical peripedicle screw placement, one group of eight dogs received 6.35 mm and another 4.76 mm rod instrumentation at L3-L5. At 12 weeks, four in each group had implants removed. Bone mineral density was analyzed by dual energy x-ray absorptiometry at 1 to 24 weeks. Axial compressive stiffness of the L3-L5 construct, spinal column, fused facets, and instrumentation were measured. Percent load through the vertebral column was predicted. RESULTS: Five observations were made for this canine model. First, stiffer implants resulted in more bypassed bone mineral loss at 6 and 12 weeks, plateauing and not different at 24 weeks. Second, after implant removal, a significant and similar rebound in bone mineral density occurred. Third, 4.76 mm rod instrumentation (initially 71% load through column) resulted in stiffer posterior fusions and vertebral columns than 6.35 mm rod instrumentation (initially 57% load through column). Fourth, marked stiffening of the anterior-middle columns (apparently disks) occurred. Fifth, percent load borne by the vertebral column increased with time. CONCLUSIONS: There appears to be a range of percent load through the vertebral column that creates optimum fusion/column stiffening while limiting bone stress shielding effects. The 6.35 mm rod constructs were predicted to allow greater than 70% axial load through the adult human thoracic/lumbar spine, implying biologic responses similar to 4.76 mm rods in dogs.

Longitudinal element size effect on load sharing, internal loads, and fatigue life of tri-level spinal implant constructs.

The effects of implant stiffness on load sharing and stress shielding, of vertebral column load sharing on implant fatigue life, and of instrumenting two versus one level adjacent to a comminuted segment on implant internal loads were studied. Finite element models of six screw constructs with 4.76 mm rod; 6.35 mm rod, and VSP plate tri-level instrumentation of two motion segments (healthy vertebra case and comminuted) and an adjacent healthy motion segment with dimensions representative of the human lumbar spine were used. Also a simplified model was developed to predict the percent of axial load passing through the column, which is a function of ki/kv the ratio of implant axial stiffness to instrumented vertebral column axial stiffness. For constructs with dimensions typical of the human lumbar spine, 77 to 80% of the axial load was predicted to pass through one or two healthy motion segments when instrumented with either 6.35 mm rod or VSP plates, compared to 90% when instrumented with 4.76 mm rods. When instrumenting smaller motion segments (in dogs) for comparison, 60% of the axial load was predicted to pass through the column for 4.76 mm rod and 33% for 6.35 mm rod constructs due to increased implant stiffness ki as a result of decreased AP and longitudinal construct dimensions, and lower canine motion segment stiffness kv.(ABSTRACT TRUNCATED AT 250 WORDS)

Experimental evaluation of seven different spinal fracture internal fixation devices using nonfailure stability testing. The load-sharing and unstable-mechanism concepts.

Fracture site immobilization capabilities of seven internal spine fixation systems were experimentally evaluated: Harrington double distraction (plain, supplemented with Edwards sleeves, supplemented with sublaminar wires), Luque rectangle, Kaneda device, transfixed Kaneda device, and Steffee plates. Stability evaluation involved comparing the three-dimensional motion that occurred across an experimentally created L1 slice fracture, and the load resistance of the implants when subjected to axial, flexion, extension, lateral bending, and torsional loads. Each implant was tested on 15 different vertebral segments from 200-250-lb calves. All implants load-shared with the fractured vertebral column to varying degrees. All except the Steffee plate system showed an obvious biomechanical weakness in one or more load modes. The unstable 4R-4bar mechanism configuration of bilevel spinal implants was identified. Mechanism configurations allow free movement with little or no resistance to the applied load until load sharing by the spinal column stops the collapse and resists the applied load.

The effect of a stiff spinal implant on the bone-mineral content of the lumbar spine in dogs.

The response of canine lumbar vertebral bone to the application of a stiff transpedicular screw-plate implant from the third to the fifth lumbar vertebra, without arthrodesis, was investigated. Five groups of six dogs each were studied: dogs that had not had an operation (control group); dogs that had had a sham operation, with survival periods of three and six months; and dogs that had received an implant, with survival periods of three and six months. The results were the same in specimens from the control group and the group that had had a sham operation. In the groups that had received an implant, dual-photon absorptiometry revealed an insignificant decrease in bone-mineral content at the bypassed fourth lumbar segment (17 per cent at three months and 12 per cent at six months). When the data for the three and six-month intervals were pooled, the mean decrease in bone-mineral content of 14 per cent was significant. Histomorphic study yielded similar results; the mean decrease in bone-mineral content for the pooled three and six-month specimens was significant (16 per cent). Losses were similar for the ventral and dorsal columns. Histomorphometric analysis also showed a significant (13 per cent) loss of bone-mineral content at the adjacent caudad (sixth lumbar) vertebra for the pooled three and six-month groups. The results of the methods of analysis of loss of bone-mineral content correlated strongly. Under these experimental conditions, a stiff spinal implant caused loss of bone-mineral content of the bypassed vertebral segment, although the loss was less than anticipated and did not increase between three and six months.

Internal forces and moments in transpedicular spine instrumentation. The effect of pedicle screw angle and transfixation--the 4R-4bar linkage concept.

The three-dimensional components of force and moment within the plates and screws of a bilevel transpedicular spine implant construct subjected to different physiological loads were determined by experimental and finite element methods. The effect of pedicle screw angle and transfixation were studied. Untransfixed 0 degrees pedicle-to-pedicle (P-P) angle constructs with limited screw-bone torsional resistance are unstable 4R-4bar linkages. They will not resist lateral load or (when not in a rectangular position) axial load until the spinal column load shares. Untransfixed constructs with (0 degrees less than P-P angle less than 60 degrees) are structures. However, as P-P angle approaches 0 degrees, the structure becomes more flexible (unstable) and some internal force and moment components exponentially increase (starting at approximately a 30 degrees P-P angle). Transfixation eliminated the linkage instability and associated exponential increase in internal loads. These observations apply to all bilevel systems that allow no relative joint motion between pedicle screw and longitudinal member. If relative motion does exist, other types of linkage instability can occur.

Recording and measuring the interior features of intracranial aneurysms removed at autopsy: method and initial findings.

Among the factors that determine the behavior of an intracranial aneurysm is the relationship between its volume and the size of the orifice. The investigative method described herein is the means being used to define that relationship in humans. It is a postmortem study that focuses on unruptured aneurysms. Central to the protocol was a synthetic rubber cast of the aneurysm's interior. The cast was made under normal arterial pressure so that unruptured aneurysms were reexpanded to lifelike size and shape. After the cast was removed intact from the specimen, the lumenal features recorded upon it were verified by comparison with the opened aneurysm. Working now only with the cast, the chamber was cut from the artery through its neck. The orifice area was determined by dividing it, mathematically, into many smaller, measurable forms. Chamber volume was ascertained by a fluid displacement technique. Both measurements were made with magnification and engineering instruments. Casts of ruptured or thrombosed aneurysms gave helpful morphological information, but were of limited value for measurement. The techniques are described, and examples of the initial results are presented.

Curvature of the femur and the proximal entry point for an intramedullary rod.

The degree of anterior curvature of 14 human femurs and four currently marketed intramedullary rods was analyzed with the use of an interactive graphic computer program. The radius of curvature of the femurs ranged from 188.5 to 68.9 cm (average, 114.4 cm). The radius of curvature of three of the four rods analyzed fell beyond the spectrum of radii found in the human femurs. A concomitant investigation of the most appropriate proximal entry site for an intramedullary rod was done based on the patterns of exit sites for both flexible guide wires and two intramedullary rods introduced in a retrograde fashion from the intercondylar notch in 14 matched pairs of human femora. The most appropriate area for proximal access into the medullary canal for these devices would appear to be at the junction of the femoral neck and the greater trochanter slightly anterior to or in the pyriformis recess.