Cervical Vertebral Lesions in Equine Stenotic Myelopathy.

Skeletal lesions in the articular processes of cervical vertebrae C2 to C7 were compared between Thoroughbred horses with cervical stenotic myelopathy (17 males, 2 females; age, 6-50 months) and controls (6 males, 3 females; age, 9-67 months). Lesions identified by magnetic resonance imaging occurred with an increased frequency and severity in diseased horses and were not limited to sites of spinal cord compression. Lesions involved both the articular cartilage and trabecular bone and were further characterized using micro-computed tomography and histopathology. The most common histologic lesions included osteochondrosis, osseous cyst-like structures, fibrous tissue replacement of trabecular bone, retained cartilage matrix spicules, and osteosclerosis. Osseous cyst-like structures were interpreted to be true bone cysts given they were a closed cavity with a cellular lining that separated the cyst from surrounding bone. This is the first report of bone cysts in the cervical articular processes of horses with cervical stenotic myelopathy. The morphology and distribution of the lesions provide additional support for the previously proposed pathogenesis that developmental abnormalities with likely secondary biomechanical influences on the cervical spine contribute to equine cervical stenotic myelopathy.

Halo pin loosening: a biomechanical comparison of experimental and conventional designs.

Loosening of the pins is the most common complication associated with use of the halo orthosis. The purpose of this study was to test the hypothesis that a new cylindrical cutting pin tip design which minimizes damage to adjacent bone and does not rely on high axial forces to maintain fixation would perform better mechanically than conventional conical tip pins. Conventional and experimental halo pins were tested for mechanical stability in human cadaveric skull bone using a servohydraulic load frame (Model 858 Bionix, MTS Corp., Minneapolis, MN). A cyclic transverse load of +/-300 N was applied through the pins for 10,000 cycles in a sinusoidal wave form in both fully tightened and reduced axial load situations. Load-to-failure testing was also performed to determine the strength and stiffness of each configuration. Photomicrographs of thin decalcified sections through a hole formed by each pin tip were compared for gross evidence of bony damage. With the pins fully tightened, there was no statistically significant difference in the motion between the experimental design (mean +/- 95% confidence interval: 0.41+/-0.027 mm) and the conventional halo pin (0.38+/-0.075 mm). After the axial pin force was intentionally decreased, there was no significant increase in the motion of the experimental pins (0.43+/-0.032 mm), however, there was a significant increase in the motion of the conventional pins (3.15+/-2.403 mm)(p < 0.05). The failure strength of the experimental pins (2010+/-366.4 N) was significantly greater than the conventional pins (1128+/-94.5 N)(p < 0.005). The pin bone interface stiffness of the experimental pins (1728+/-144.4N/mm) was also significantly greater than that of the conventional pins (1393+/-202.6 N/mm)(p < 0.03) (Fig. 5). Qualitatively, the photomicrographs demonstrated considerably more particulate debris on the boundary of the hole formed by the conventional pin compared to the experimental pin. The data obtained herein support our hypothesis and indicate that the experimental pin design possesses biomechanical characteristics superior to current designs. These characteristics may translate into fewer complications in the clinical setting.

Stress analysis of halo pin insertion by non-linear finite element modeling.

Halo fixation is associated with a high complication rate. The most common complications are loose pins and pin site infections believed to be exacerbated by loose pins. Although pin designs and the technique of pin insertion have changed little in over 30 years, the pin/skull mechanics are poorly understood. Halo pin insertion was modeled using nonlinear finite element analyses to determine the stress distribution in the human skull underlying and surrounding the point of pin fixation. Model validity was established by comparing pin insertion depth and the profile of the hole generated in the bone to the results of experimental mechanical tests. The region surrounding the pin tip within 1 mm was found to undergo plastic deformation and compressive loading in excess of the compressive yield strength of cortical bone. The implication is that damaged bone in this region is responsible for the high incidence of halo pin loosening. Resorption or migration of bone particles with periodic relief of compression in this region due to daily cyclic forces might result in an enlarged pin site and eventually, a loose pin.

Mechanical properties of compacted morselized cancellous bone graft using one-dimensional consolidation testing.

Failures of orthopaedic procedures that use morselized cancellous bone (MCB) graft for load bearing are often due to gross displacement within the graft material. For this reason the mechanical behavior of MCB must be better understood. Our purpose is to present a detailed testing methodology for the mechanical characterization of MCB, and to illustrate how this methodology can be used to study the influence of water and fat content. Complete one-dimensional consolidation testing was performed on bovine cancellous bone processed to represent MCB typically used in surgery (52% water, 31% fat). The one-dimensional consolidation strain under a stress of 1.09MPa was 30.9% and the confined modulus was 8.0MPa. The coefficient of consolidation (rate of consolidation) was 2. 2x10(-5)cm(2)/s and the coefficient of secondary strain (steady-state creep rate) was 1.9%. While reducing the water content alone had some influence on properties, reducing the fat content improved both the static and dynamic behavior. A sample of MCB which had fat intentionally minimized and a lower overall moisture content (56% water, 5% fat) demonstrated 23.1% strain, a confined modulus of 9.6MPa, a coefficient of consolidation of 3.4x10(-3)cm(2)/s, and a coefficient of secondary strain of 0.9%. The test methods described in this technical note can be used to evaluate the influence of fluid content on the mechanical behavior of MCB.

Impacted morselized cancellous bone: mechanical effects of defatting and augmentation with fine hydroxyapatite particles.

Geotechnical engineering testing techniques were used to study the mechanical properties of morselized cancellous bone (MCB) and the effects of defatting and augmentation with fine hydroxyapatite (HA) particles. Bovine and human cancellous bone was morselized, rinsed, and manually squeezed to remove excess fluid, producing a standard surgical MCB sample that was also used as a control. Some of the MCB was defatted with heat and detergent and mixed with HA particles in ratios ranging from 0% to 100% HA. Compaction tests were used to determine the effects of moisture content and the amount of MCB that can be packed into a confined space. One-dimensional consolidation tests were used to determine the uniaxial strain behavior, confined modulus, and steady-state creep rate. The compaction tests demonstrated that defatting and adding HA particles significantly increased density. The one-dimensional consolidation tests showed that strain was decreased, modulus was increased and the creep rate was decreased by defatting and adding HA.

A human cadaver model for determination of pathologic fracture threshold resulting from tumorous destruction of the vertebral body.

STUDY DESIGN: Thoracic vertebrae were subjected to compressive loads after drilling of the centrum to simulate destruction from metastatic tumorous involvement. OBJECTIVE: To determine whether a threshold exists that is predictive of fractures to establish a correlation between significant variables and vertebral strength. SUMMARY OF BACKGROUND DATA: The mechanical effects of metastatic destruction of thoracic vertebral bodies and their correlation to pathologic fractures has been analyzed in few studies. In additional studies on intact vertebral strength, investigators have determined that bone mineral density and geometric factors are important. METHOD: Fifty-four cadaveric thoracic vertebrae were studied. All were examined by quantitative computed tomography. T4 and T10 served as mechanical controls to predict the intact strength of T7. The test vertebrae were drilled from the anterior cortex through to the posterior cortex before they were loaded. RESULTS: Linear correlation between the strength of T4 and T10 in each spine supported the predicted strengths of T7. Because of variation from other factors, no threshold defect size was noted beyond which failure consistently occurred. Results of linear correlation analyses showed that the best combination of parameters for predicting vertebral strength was the product of bone mineral density and the remaining intact vertebral body cross-sectional area. This vertebral strength index correlated linearly with the strength of intact and compromised T7 vertebrae (r2 = 0.52). CONCLUSIONS: The vertebral strength index can be used to predict the strength of any thoracic vertebra. When compared with an idealized vertebral strength index based on the intact vertebral cross-sectional area and normal bone mineral density, a patient's actual vertebral strength index can be used as one of the criteria for prophylactic stabilization.

Femoral component revision using impacted morsellised cancellous graft. A biomechanical study of implant stability.

We have tested the axial and torsional stability of femoral components after revision arthroplasty in a cadaver model, using impacted morsellised cancellous graft and cement. Each one of six matched pairs of fresh frozen human femora had either a primary or a revision prosthesis cemented in place. For the 'revision' experiments, all cancellous bone was removed from the proximal femur which was then over-reamed to create a smooth-walled cortical shell. An MTS servohydraulic test frame was used to apply axial and torsional loads to each specimen through the prosthetic femoral heads with the femur submerged in isotonic saline solution at 37 degrees C. The mean subsidence was 0.27 +/- 0.17 mm for the primary and 0.52 +/- 0.30 mm for the revision groups. The difference was statistically significant (p < 0.025), but the mean subsidence was <1 mm in both groups. The mean maximum torque before failure was 42.9 +/- 26.9 N-m for the primary and 34.8 +/- 20.7 N-m for the revision groups. This difference was not statistically significant (p > 0.015). Based on our results we suggest that revision of the femoral component using morsellised cancellous graft followed by cementing with a collarless prosthesis with a polished tapered stem restores the integrity of the proximal femur and provides immediate stability of the implant.

Arthroplasty in the goat hip.

The goat is a reliable and practical animal model for study of the hip. The authors describe a safe and effective operative approach to the goat hip.

Is hydroxyapatite cement an alternative for allograft bone chips in bone grafting procedures? A mechanical and histological study in a rabbit cancellous bone defect model.

To evaluate in vivo performance of hydroxyapatite cement (HAC) as a porous bone graft substitute, HAC was mixed (1:1 ratio) with either porous calcium-phosphate granules (80% tricalcium phosphate, 20% hydroxyapatite) or defatted morsellized cancellous bone (MCB) allograft and implanted bilaterally in cylindrical drill holes in distal femurs of rabbits. Groups with empty defects and impacted MCB were used for reference. After 8 weeks, one femur from each pair was examined histologically. All contralateral specimens and Time-0 specimens were used for mechanical indentation tests. Histology showed that some empty defects were filled with newly formed osteopenic bone after 8 weeks. The impacted MCB showed remodeling into new vital bone. Incorporation of the HAC/MCB composite was incomplete, whereas minimal new bone ingrowth was found in the HAC/granule composites. Though not different from each other, both composites were significantly stronger than empty defects, incorporated impacted MCB, and intact cancellous bone. At Time 0, the mechanical behavior of impacted MCB was similar to both HAC composites. In conclusion, composites of HAC and porous biomaterials can maintain relatively high strength over 8 weeks in vivo, but their incorporation into a new bony structure is slower than impacted MCB. The HAC/MCB composite showed favorable incorporation behavior.

Fracture site motion with Ilizarov and "hybrid" external fixation.

OBJECTIVES: To evaluate differences in fracture site motion by using different external fixators. DESIGN: A wooden dowel was used to simulate a long bone with a transverse diaphyseal fracture. Ilizarov, "hybrid," and strutaugmented "hybrid" external fixation was used to stabilize the "fracture." The wooden dowel was subjected to separate axial, four-point bending, and torsional loads. Fracture site motion in the axial plane, off-axis motion (shear and bending), and rotation were measured. SETTING: All mechanical testing was performed with a sevohydraulic test frame (MTS Systems, Minneapolis, MN, U.S.A.). Fracture site motion was measured with an interfragment motion device developed in this laboratory. INTERVENTION: Comparison was made between a traditional fourring Ilizarov fixator, a "hybrid" fixator using rings and threaded pins attached by a unilateral aluminum bar, and a "hybrid" fixator augmented with a V-shaped strut. MAIN OUTCOME MEASUREMENT: Load-deformation behavior in axial displacement, shear displacement, and bending displacement were compared between the different configurations under identical conditions of axial loading, torsional loading, and four-point bending. In torsional loading, rotational displacement was also measured. RESULTS: The Ilizarov configuration allowed significantly less off-axis fracture site motion in all loading modes than either "hybrid" configuration while still allowing axial compression of the fracture ends. CONCLUSIONS: In a completely unstable fracture with poor bone apposition, the mechanical behavior of a four-ring Ilizarov external fixator is superior to the mechanical behavior of a unilateral "hybrid" frame.

Should distal interlocking of tibial nails be performed from a medial or a lateral direction? anatomical and biomechanical considerations.

OBJECTIVE: To compare the relative risks of neurovascular injury from perforation during distal interlocking and the biomechanical stability of two approaches to distal interlocking of tibial nails. DESIGN: In vitro anatomical and biomechanical study. SETTING: All mechanical testing was performed in a servohydraulic test frame with a customized motion transducer. INTERVENTION: Tibial nails were interlocked distally with a medial-to-lateral (ML) or a lateral-to-medial (LM) approach. MAIN OUTCOME MEASURE: The distances from the nearest end of each distal locking screw to four neurovascular structures were measured manually with calipers, and two-dimensional motion under simulated stance load across the fracture site was recorded. RESULTS: There were greater distances from the posterior tibial neurovascular bundle and the superficial peroneal nerve with distal targeting from the LM direction compared with targeting from the ML direction. Biomechanically, the ML nail configuration demonstrated slightly greater resistance to bending than the LM configuration. CONCLUSIONS: Distal tibial interlocking from the LM direction appears to be safer than interlocking from the ML direction with regard to relative distances from the neurovascular structures. This small anatomical advantage must be considered in light of slightly greater resistance to bending of the ML interlocking configuration compared with the LM interlocking configuration.

Pullout strengths of self-reinforced poly-L-lactide (SR-PLLA) rods versus Kirschner wires in bovine femur.

OBJECTIVE: To determine the relative amount of fixation of self-reinforced poly-L-lactide (SR-PLLA) rods and Kirschner wires in bovine cancellous bone by comparing their pullout strength DESIGN: An in vitro laboratory study was performed using bovine femurs. Ten two-millimeter-diameter pins of each type were inserted into cancellous bone and then pulled out, using a material testing machine. The maximum force (pullout strength) was selected over other measurements to compare the amount of fixation of the two types of pins. All of the pins were retrieved for microscopic analysis. A paired t test was performed to analyze the differences between the pullout strength of the two types of pins. SETTING: Orthopaedic Bioengineering Laboratory, University of Louisville School of Medicine, Louisville, Kentucky, U.S.A. SPECIMENS: Two young fresh bovine distal femurs, ten two-millimeter-diameter Kirschner wires, ten two-millimeter-diameter bioabsorbable SR-PLLA rods MAIN OUTCOME MEASUREMENTS: Pullout strength in Newtons, and microscopic pin surface aspect after insertion. RESULTS: Significant differences were noted between the maximum force required to remove the two types of pins (p < 0.01) The K-wire mean pullout force was 37.7 N (SD 13.6), and the SR-PLLA rod mean pullout force was 53.6 N (SD 19.3). Microscopic analysis indicated surface modification only on the SR PLLA rods. DISCUSSION: SR-PLLA composites have shown comparable clinical results to their metallic counterparts. In this study, the pullout strength of SR-PLLA rods was compared with that of conventional K-wires. A significant difference (p < 0.01) favoring bioabsorbable pullout strength was noted. The bioabsorbable pin surface modification during insertion is an interesting finding that warrants further investigation as a potential source of improved fixation. CONCLUSION: SR-PLLA rods retain their hold in bovine cancellous bone better than K-wires. This finding offers to the orthopaedic surgeon more information about new pin fixation methods.

Fatigue properties of a twelve-hole versus a five-hole intramedullary supracondylar nail.

OBJECTIVES: To evaluate a design change intended to improve fatigue properties of Intramedullary Supracondylar nails (IMSC). DESIGN: Fatigue testing was performed on 12-hole and 5-hole IMSC nails in 18 plastic-fiber composite femurs. Fractures were simulated by a 1 cm wide osteotomy. Nails were cycled until failure with femoral loads ranging from 133 N to 1869 N (30 to 420 lb.) for the 12 mm nails and 133 N to 1335 N (30 to 300 lb.) for the 11 mm nails. A simple finite element computer model (FEM) was also created to determine the difference in stress distribution between the two designs. SETTING: All mechanical testing was performed using a servohydraulic test fram (MTS Systems, Minneapolis, MN). The FEM was performed using I-DEAS software (SDRC, Milford, OH) running on an Apollo 9000/735 workstation (Hewlett-Packard, Palo Alto, CA). INTERVENTION: The nails were locked proximal and distally. MAIN OUTCOME MEASUREMENT: The number of cycles to failure and the maximum tensile stress (FEM) was measured and compared between the two designs. RESULTS: The 11 mm and 12 mm 12-hole nails failed at an average of 53,514 cycles and 102,433 cycles respectively. No nail failures were seen in the 5-hole design. The FEM showed a 400% increase in maximum tensile stress due to the transverse screw hole in the cannulated nail. CONCLUSIONS: Elimination of screw holes at the level of a supracondylar metaphyseal defect significantly improves the fatigue life of the IMSC nail.

Cross-sectional geometry of the sacral ala for safe insertion of iliosacral lag screws: a computed tomography model.

OBJECTIVE: To measure the dimensions of the narrowest portion of the sacral ala for safe insertion of iliosacral lag screws. DESIGN: Computed tomography (CT) model. SETTING: Level One trauma center. PATIENTS: Thirteen adult patients underwent pelvic CT imaging. MAIN OUTCOME MEASURE: Axial CT scans of intact pelves were reformatted in the sagittal plane at three-millimeter intervals from the first sacral body (S1 body) to the sacroiliac (SI) joint. Computer analysis and measurements of sacral geometry were used to determine the narrowest portion of the bony sacral ala. The maximum height, maximum width, and slope of the sacral ala through its geometric center in cross-section were measured. RESULTS: The narrowest portion of the sacral ala in all patients was consistently located at the junction between the sacral body and the alar wings, termed the sacral pedicle, directly cephalad to the first sacral foramen. The average slope of the sacral ala at the sacral pedicle was 45.08 degrees (range 25 to 65 degrees). The average maximum height at the geometric center in cross-section was 27.76 millimeters, and the average width was 28.05 millimeters. However, outside the geometric center there was a sharp decrease in height and width of the sacral ala that was in large part determined by its relative slope. CONCLUSION: Although the cross-sectional geometry of the sacral ala is highly variable among patients, there is ample space for iliosacral screws. To ensure safe insertion, iliosacral lag screws must be positioned in the geometric center of the sacral ala to avoid extraosseous placement.

Intervertebral measurement of lumbar segmental motion with a new measuring device.

A new Intervertebral Motion Device (IMD) was developed in this study. Depending on its configuration, the IMD was used to measure motion in the sagittal, frontal, and transverse planes. Calibration results showed that the root-mean-square (RMS) error of the IMD was 0.092 mm in axial translation, 0.065 mm in shear translation, and 0.091 in rotation. Using the IMD, nine intact human lumbosacral spine specimens (L3-S1) were tested under a simulated physiological load on an MTS (Model 858 Bionix, MTS System Corporation Minneapolis, MN). The ranges of motion (ROMs) of intact and instrumented specimens were measured in terms of angular motion (main motion) and coupled translation in the sagittal plane, and angular motion in the transverse plane. The results demonstrated that simulated fusion with CS instrumentation at the level of L4-L5 significantly decreased the ROMs of L4-L5 for all main and coupled motions (P < 0.03). The application of CD rods had less influence on the angular ROM in L/R axial rotation compared to the angular ROMs in Flex/Ext and L/R lateral bending.

Traditional versus alternative olecranon osteotomy. Historical review and biomechanical analysis of several techniques.

Olecranon osteotomy for visualization of the distal humerus has a high complication rate. Two basic osteotomies, intra-articular and extra-articular, are in current use. Each is examined using the engineering technique of free-body analysis. The osteotomy fragment mechanics are found to influence both the nature of the fixation and the clinical outcome. In addition to the advantages gained from not disrupting the joint surface, oblique extra-articular osteotomy may have mechanical advantages over transverse intra-articular osteotomy because of decreased osteotomy surface distraction force with triceps contraction.

Spinal ligament loading during axial distraction: a biomechanical model.

The objective of this study was to develop a method of measuring spinal ligament forces during axial distraction to understand the load-bearing contributions of the individual ligamentous structures in the lumbar spine. A sequential ligament-cutting technique and the arthroscopically implantable force probe (AIFP, MicroStrain, Burlington, VT) were used to determine loading of the anterior longitudinal ligament (ALL), the posterior longitudinal ligament (PLL), and the remaining posterolateral complex (PLC) in an in vitro corpectomy model. During axial spinal distraction, the relative percentages of the total axial load in the individual structures were as follows: ALL, 37.5%; PLL, 17.2%; PLC, 45.3%.

Posterior lumbar interbody cages do not augment segmental biomechanical stability.

The effect on stiffness of installing posterior threaded interbody cages at LA-L5 was evaluated using fresh human cadaveric spine specimens. The cages did not increase spine stiffness significantly in any tested range of motion. Supplemental posterior pedicular screw/rod instrumentation, however, significantly increased stiffness. The assertion that use of cages as isolated posterior implants improves stability may be invalid.

Bone healing using a bi-phasic ceramic bone substitute demonstrated in human vertebroplasty and with histology in a rabbit cancellous bone defect model.

A non-exothermic material that demonstrates clinical pain relief comparable to polymethylacrylate (PMMA) for vertebroplasty and promotes bone healing is desirable. The purpose of this investigation is to demonstrate clinical pain score improvement and bone healing following vertebroplasty with a novel bi-phasic ceramic cement. Twenty patients were prospectively treated for compression fractures in a single center in the USA with the injectable bi-phasic ceramic bone substitute. Statistical comparison of pain scores was made during a 12 month follow-up retrospectively against a matched cohort of patients treated with PMMA vertebroplasty by the same neuroradiologist (HPH) in the same setting. The bone remodeling material was also evaluated with histology in a New Zealand white rabbit model. The bi-phasic material demonstrated a pre-operative mean VAS score of 8.5 (± 1.6) with a significant post-operative pain relief mean VAS score of 1.8 (± 2.5) after one week, which was maintained throughout the 12 month follow-up period. These data are in line with the pain scores for the PMMA treated cohort. CT scans six and 12 months after surgery with the bi-phasic cement showed healing of the osteoporotic fractures. In the rabbit model, histology with the study material showed evidence of incorporation, new bone growth and bone healing in a cancellous bone defect. Both the clinical results and the histologic evidence of bone healing and new bone growth support the application of this new bioinjectable material as an alternative to the use of PMMA for vertebroplasty.

Increased peak contact stress after incongruent reduction of transverse acetabular fractures: a cadaveric model.

BACKGROUND: The objective of this study was to determine the peak contact pressure with varying degrees of articular cartilage step-off in a transtectal acetabular fracture model. METHODS: Five fresh frozen cadaveric hip joints were potted in a custom loading fixture. The five specimens were then tested at loads of 445 N (newton) (100 lb) and 1,335 N (300 lb) intact and after a transverse osteotomy at step-off levels from 0 to 5 mm in 1-mm increments. RESULTS: Articular cartilage step-off of greater than 1 mm led to significantly increased contact stress at the loaded acetabular articular surface. Mean peak pressure measured at 1,335 N of loading in all intact specimens before the osteotomy was approximately 10 MPa. Peak pressure after a transverse acetabular fracture did not change when the fracture was perfectly reduced. At 1 mm of step-off, the peak pressure increased by approximately 20% but was not statistically significant. With step-off of > 2 mm or greater, the peak pressure increase was approximately 50% and was statistically significant. CONCLUSION: On the basis of our study, transverse acetabular fractures with greater than 1 mm of displacement can lead to significant increase in peak pressure at the articular surface.