Effect of freezing on histologic and biomechanical failure patterns in the rabbit capital femoral growth plate.

Ten pairs of proximal femora were harvested from 8-week-old New Zealand white rabbits. Experimental shear fractures were then created in vitro, and load-displacement curves were recorded. One specimen from each animal was tested within 4 h of death, whereas the contralateral specimen was frozen for 10 to 20 days and thawed prior to testing. Histologic sections of the fractured specimens were studied. No statistically significant differences could be demonstrated between the fresh and the frozen/thawed specimens in either the histologic patterns or the shear load or stress to failure.

Effects of short-term cyclosporin-A on biomechanical properties of intact and fractured bone in the rat.

Cyclosporin-A is a new immunosuppressant drug being used widely for solid organ transplantation. The systemic effects of Cyclosporin-A on the skeletal system have not previously been defined. This study examined the effects of Cyclosporin-A on biomechanical properties of intact bone and fracture repair in a rat model. Closed midshaft femoral fractures were produced over a preplaced intramedullary pin. Cyclosporin-A, dissolved in olive oil and administered by gavage, was begun the day following surgery and continued for 14 consecutive days at a dose known to prolong solid organ allograft survival in rats (7 mg/kg/day), and a second group of control animals received comparable volumes of plain olive oil under identical circumstances. Groups of both experimental and control animals were killed at 4, 8, 12, or 16 weeks following fracture, after which both intact and fractured femurs were tested to failure in torsion. As judged by torque, angular deformation, stiffness, and energy absorption, Cyclosporin-A did not significantly alter the biomechanical properties of fracture repair or intact bone turnover. Based on current biomechanical data, we can find no reason to discourage the short-term clinical use of Cyclosporin-A in patients with or without fractures. Histomorphometric studies are required to accurately assess the biologic effects of this drug on bone biology and to complete the analysis of skeletal toxicity.

Biomechanics of the rabbit cervical spine as a function of component transection.

The rabbit cervical spine in the form of the functional spinal unit was tested in a manner analogous to that used to study human functional spinal units. Anatomies and biomechanical similarities and differences with the human were demonstrated. The rabbit cervical spine failed abruptly after a well-defined series of anterior or posterior segmental injuries. However, up to the failure point, the rabbit cervical spine exhibited an increased range of motion after each segmental injury. The results of this study were used to develop an in vivo model for the study of spinal injury in subsequent projects.

Temporal biomechanics of posterior cervical spine injuries in vivo in a rabbit model.

There are numerous clinical and biomechanical criteria for determination of acute spinal instability. No data, however, exist on the biomechanical behavior of the injured, untreated spine during the healing period. Using a rabbit model, this study was designed to quantify changes in the mechanical properties of the injured spine over time. Sixteen rabbits were assigned to four injury groups: Injury 1 (C4-5 supra/intraspinous ligament transection), Injury 2 (C4 laminectomy), or Injury 3 (C4 laminectomy, plus bilateral facet capsular ligament transection). The fourth injury group was Sham (posterior exposure of C4-5, no spinal injury). Preinjury range of motion was determined by standardized flexion-extension radiographs on all animals. Postinjury flexion-extension radiographs were taken at 4, 8, and 12 weeks. At the end of 12 weeks, animals were killed. Mean range of motion at C4-5 for all injury groups decreased from preinjury values. The greatest decrease was for the most severe injury. At 8 weeks, the range of motion for this injury was less than half of the preinjury level and increased to 62% of the preinjury level at 12 weeks.

The effect of radiation on the fracture repair process. A biomechanical evaluation of a closed fracture in a rat model.

The effects of a single dose of irradiation on the biomechanical parameters of the fracture healing process were studied in a rat model. Intramedullary pinning was performed before production of a closed femoral midshaft fracture. The experimental group was exposed to 900 rad 3 days after fracture and was compared with a control group with a similar fracture that received no irradiation. Animals were killed at intervals ranging from 2-16 weeks after surgery and the bones were tested until failure in torsion. In the irradiated groups, a delay of 4 weeks was noted in the biomechanical parameters associated with fracture healing (torque to failure, torsional stiffness, angle to failure, and biomechanical stage). Despite this delay in the normal temporal progression, the staging and stiffness approached normal controls within an 8-week period. However, the torque to failure remained below normal levels at the conclusion of this study. These results differ from a previous study using an open fracture model.

Effects of freezing and freeze-drying on the biomechanical properties of rat bone.

The effects of various preservation techniques on the biomechanical properties of bone have been investigated in a rat model. Freezing of the specimens to -20 degrees C, -70 degrees C, and -196 degrees C did not adversely affect the strength of long bones tested in torsion or of vertebral bodies tested in compression. Freeze-drying did not markedly affect the compression properties of the vertebral specimens; however, it did produce a significant deleterious reduction in the torsional strength of the long bones.

Radiation-induced alterations of fracture healing biomechanics.

The effects of irradiation on the normal temporal progression of the physical properties of healing fractures were studied in a rat model. Fractures were surgically produced in the femur, stabilized with an intramedullary pin, and irradiated. One group of rats was exposed to 2,500 rads in divided doses over 2 weeks, beginning 3 days after fracture, and compared to a control group with fractures which were not irradiated. Animals were sacrificed at periodic intervals and the bones were tested to failure in torsion. The torque, stiffness, and energy increased and the angle decreased for the nonirradiated specimens in the expected fashion. This progression was deleteriously altered in the irradiated femurs.

Postfracture irradiation effects on the biomechanical and histologic parameters of fracture healing.

The effects of single-dose local irradiation on the biomechanical properties of closed femoral fractures were studied in 75 mature Sprague-Dawley rats. Ten days after fracture, the rats were irradiated with 900 rads at 250 kV to the entire fractured femur. At 2, 3, 4, 8, and 16 weeks after fracture, both fractured and contralateral intact femurs were recovered and evaluated biomechanically by testing to failure in torsion. Results were compared with those from a similar study involving fractures irradiated 3 days after fracture as well as nonirradiated control fractures. Fracture healing progressed faster when irradiation was delayed 10 days than when delayed 3 days, and control fractures healed more rapidly than after either delay. In the 10-day delay group, fractures showed greater strength than did those in the 3-day delay group at 8 weeks, but the strength of irradiated fractures in both groups was similarly depressed at 16 weeks, with a maximum torque well below that of control fractures. These results suggest that delaying radiation exposure of a fracture may mitigate short-term deleterious effects on fracture repair, but that long-term results may be similar to those associated with expeditious irradiation.

The influence of ibuprofen on fracture repair: biomechanical, biochemical, histologic, and histomorphometric parameters in rats.

Ibuprofen is a widely used cyclo-oxygenase inhibitor in clinical practice. It has been demonstrated by others to have an inhibitory effect on fracture repair in animals. In the present study, we were unable to demonstrate any significant alterations in fracture biomechanics as measured by torsion testing and fracture stage in mature Sprague-Dawley rats treated with 30 mg/kg/day oral dose of ibuprofen, starting 3 days following fracture, over a 12-week time interval. Fracture histology and serum osteocalcin levels were no different in treated animals than control animals. Furthermore, histomorphometric parameters of bone remodeling, including bone volume and bone formation rate in the intact tail vertebrae of these animals with unilateral femur fractures, were no different between treated and control animals.

Effects of prefracture irradiation on the biomechanical parameters of fracture healing.

This study examined the effects on the biomechanical parameters of fracture healing of a single dose of 900 rad (the approximate single-dose equivalent of 2,500 rad in 10 divided doses), given 1 day prior to closed fracture of the femur. The femurs were recovered at 2, 3, 4, 8, and 16 weeks after fracture and were mounted and tested to failure in torsion; the results were compared with those in nonirradiated controls from a previously published study. Prefracture irradiation delayed the progressive increase in biomechanical parameters of fracture healing. The delay was statistically significant up to 8 weeks after fracture. At 4 weeks, the normalized torque was 44% that of intact bone in the treated group compared with 75% for the control group. Sixteen weeks after fracture, the biomechanical and histological parameters of fracture healing of the irradiated femurs were no different from those of the nonirradiated controls. Within the treated group, the irradiated fractures remained significantly weaker than their contralateral intact bone at all time intervals, with a torque of only 79% that of intact bone at 16 weeks. Thus, femoral fractures in rats healed (or regained substantial strength) following palliative doses of radiation delivered 1 day prior to injury, but the repair process was delayed compared with that of nonirradiated controls.

Allograft incorporation: a biomechanical evaluation in a rat model.

Allogeneic bone from Sprague-Dawley rat femurs was subjected to levels of freezing and/or irradiation that are known to have produced changes in the associated immune responses to these grafts. This bone was transplanted into an experimentally created gap in the femur of Lewis rats. The subsequent healing of the transplants in the Lewis rats was studied at 2, 4, 8, and 16 weeks after transplantation using torsion testing to failure. There was no clear advantageous biologic response in the union of the grafted material accompanying the alterations in immunologic response as measured by biomechanical testing of the proximal osteosynthesis site in torsion. The torsional strength of all of these groups remained lower than that of intact bone. Furthermore, none of the frozen and/or irradiated allografts exceeded the strength of the fresh allograft at a statistically significant level.

Chemotherapy-induced alterations in the biomechanics of rat bone.

The effects of methotrexate and doxorubicin (Adriamycin) on intact and fractured bone were studied in a rat model. Methotrexate, 0.1 mg/kg i.p., was administered five times a week, or doxorubicin, 1 mg/kg i.v., was given once a week. Animals were killed and evaluated at 4, 8, 12, and 16 weeks after surgical production of a unilateral femoral osteotomy and initiation of the chemotherapeutic program. Both intact and fractured bones were tested to failure in rapid torsion, and the torque, angular deformation, stiffness, and energy were measured. There was a significant alteration in the strength of the healing osteotomies in doxorubicin- and methotrexate-treated animals compared with the control group. The torsional strength of intact bone was not significantly altered by either drug protocol over the period of this observation.

Three-dimensional load-displacement curves due to forces on the cervical spine.

Load-displacement curves were measured for six types of pure force loading of the cervical spine specimens obtained from fresh human cadavers. A new measuring and mounting technique was developed that yielded data for all of the functional spinal units for each specimen tested. All five of the coupled, as well as the main, load-displacement curves were studied. For anterior and posterior shear loadings, the main resulting motions were translation in that direction (1.6 +/- 0.3 and 1.9 +/- 0.3 mm), and the major coupled motions were flexion and extension (3.6 degrees +/- 1.2 degrees and 6.3 degrees +/- 1.2 degrees). The main motions with right and left lateral shear loadings were translations laterally (1.4 +/- 0.3 and 1.6 +/- 0.3 mm), and the major coupled motions were axial rotations (1.5 degrees +/- 0.6 degrees and 2.3 degrees +/- 0.6 degrees) and not lateral bending. For compression and distraction loadings, the main motions were translations in that direction (0.7 +/- 0.3 and 1.1 +/- 0.3 mm), and the major coupled motions were flexion and extension (2.0 degrees +/- 1.0 degrees and 2.8 degrees +/- 1.0 degrees) and lateral bending (1.4 degrees +/- 0.3 degrees and 1.9 degrees +/- 0.3 degrees). The neutral zones for anterior and posterior shear forces were 1.6 +/- 0.2 mm of translation and 5.8 degrees +/- 1.3 degrees of rotation, for lateral shear force 1.4 +/- 0.3 mm and 2.0 degrees +/- 0.5 degrees, and for compression/distraction 0.6 +/- 0.1 mm and 2.8 degrees +/- 0.9 degrees.

Wave propagation across a bony discontinuity simulating a healing fracture.

Stress wave propagation in a long bone with a progressively increasing defect in the bony cortex, simulating a healing fracture, was studied by recording the outputs of bonded semiconductor strain gages, proximal and distal to the defect. Statistically significant relations were found between the relative size of the discontinuity and the transmission coefficient, dispersion and transit time across the fracture. We also showed that the stress wave in a bone could be monitored from the vibration response of a traction pin, placed in a magnetic field. The results may be helpful for the development of new methods to measure the rate of fracture healing, as well as aiding our understanding of the dynamic loading of bone.

Stress wave propagation in bone.

The traveling wave characteristics for a single compressive pulse were studied in fresh and embalmed human long bones. The stress wave was generated by the longitudinal impact of a steel ball on one end of a bone and was monitored by bonded strain gages. The dynamic properties, namely velocity, attenuation coefficient and dispersion were correlated with the mineral density, porosity, and cross-sectional area of the specimens. Statistically significant relationships were found between several of these parameters. These stress wave propagation characteristics are important for our understanding of the dynamic loading of bone and they may also provide a basis for the development of noninvasive techniques for studies of diseased or fractured bone.

Cervical spine stabilization. A three-dimensional, biomechanical evaluation of rotational stability, strength, and failure mechanisms.

The three-dimensional rotational biomechanical properties of several different types of posterior stabilizing procedures are reported. A severe ligamentous and bony injury was simulated with three vertebral body human cervical spine segments. Good stabilization was noted for all of the repairs in flexion loading. Without polymethylmethacrylate supplementation, none of the repairs was stable in extension. All of the repairs provided reasonable stabilization for lateral bending except for the posterior wiring without methacrylate, and all but the posterior wiring and facet fusion provided reasonable stabilization against axial rotation loading. The supplementation of all of these repairs with polymethylmethacrylate added considerably to the stability of all the constraints. These findings may be useful in clinical decision-making for determining the kind of repairs and postoperative brace protection to use.

The injured canine cervical spine after six months of healing. An in vitro three-dimensional study.

The cervical spine is a common site of spinal injuries. The stability of an injured cervical spine is not only dependent on injury severity, but also on the degree of healing time. Using a canine model, three injuries of varying degrees of severity were surgically produced at the C4-C5 level and allowed to heal for 6 months. No internal or external support was provided. The harvested cervical spines (C2-C7) were subjected to three-dimensional biomechanical testing by applying individually six pure moments. The resulting three-dimensional displacements were recorded using stereophotogrammetry, and the intervertebral motions were calculated. The results are compared with the in vivo behavior of the same specimens and with an in vitro control group. At 1 N-m, the average flexion-extension range of motion (ROM) for the intact C4-C5 level was 24.5 degrees (standard deviation [SD], 6.6 degrees). A facetectomy at this level significantly increased the in vitro ROM to 51.1 degrees (SD, 4.4 degrees). The in vitro ROM decreased to 19.8 degrees (SD, 7.3 degrees) in the facetectomized group of this study, which were allowed to heal for 6 months before death. Similar results were obtained in axial rotation and lateral bending. The findings show that after 6 months of healing, the injured canine spine, although acutely hypermobile, exhibited biomechanical characteristics that were not different from those of the normal intact specimens.

Functional stability of the canine cervical spine after injury. A three-month in vivo study.

Although clinical instability is an in vivo problem, most spinal instability criteria are either subjective or are based on in vitro experiments. The authors performed an in vivo experiment using a canine model to study the natural history of spinal instability as a function of healing time up to 12 weeks. Three injuries were produced surgically: sham; laminectomy at C4; and bilateral facetectomy at C4-C5. Three 1.5-mm steel balls were implanted into C3 to C6 vertebrae at the time of surgery. Standardized functional flexion-extension stereoradiographs of the cervical spine were obtained before injury, immediately after injury and at 0.5, 1, 1.5, 2, 3, 4, 5, 6, 7.5, 9, and 12 weeks postinjury and immediately after killing the animals. In general, the authors found decreased ranges of motion (ROM) at the C4-C5 level, compared with the pre-injury values, for all injuries, but most significantly for the facetectomy. The maximum decrease occurred between 0 and 0.5 weeks postinjury. Between 2 and 12 weeks, there was recovery in the ROM, especially for the two less severe injuries. The changes in the ROM at each spinal level were explained by simultaneous presence of a destabilizing factor, caused by the three different injuries with the sham as the least and the facetectomy as the most destabilizing injury, and a stabilizing mechanism of muscle spasm in the beginning and of healing and other adaptive responses in the late phase after injury. Because of the significant differences between the canine model and the human cervical spine, the present findings should be extrapolated to the human situation with caution.

Biomechanical aspects of bone autografts and allografts.

The structural requirements of skeletal reconstruction should be included in the consideration of an appropriate type of bone graft, whether autogenous or allogeneic tissues are chosen. This analysis should include the nature of any fixation devices to be used, as well as characteristics of the host and donor bone. The mechanical properties and biomechanical response of the graft must be balanced against the type and magnitude of the load to which the graft will be subject. For example, the information presented here suggests that, from a biomechanical perspective, frozen bone would be better suited than freeze-dried bone when the graft is subject to large torsional loads or else the graft must be appropriately protected during incorporation by adequate internal fixation or external bracing. In a situation that is primarily subjected to compressive loads, however, freeze-dried grafts would be just as biomechanically sound. Thus, an understanding of the normal biomechanics of the anatomic region to be reconstructed is crucial. The mechanical properties of the graft are affected by preservation, storage, and sterilization. Incorporation and remodeling of the graft further alter its properties. These properties are, in turn, influenced by the host immune response as well as the local biomechanical environment. The influence of each of these factors is predictable. Obviously, there are numerous considerations in choosing approaches to skeletal reconstruction other than the mechanical issues discussed here. However, an understanding of the mechanical properties involved will help in optimizing the clinical success of these choices.

Dimensions of the rabbit femur during growth.

In the rabbit, the femur grew rapidly in length during the first 4 weeks of life of the animal. Growth continued at a slower rate up to 8 to 10 weeks of age and then reached a plateau. A similar growth pattern (rapid increase followed by a plateau) was seen for the dimensions of the femoral head, neck, and shaft. Significant growth differences between the left and right femurs did not occur.