J. Michael Kabo

The Effect of Regional Gene Therapy with Bone Morphogenetic Protein-2-Producing Bone-Marrow Cells on the Repair of Segmental Femoral Defects in Rats*

BACKGROUND Recombinant human bone morphogenetic proteins (rhBMPs) can induce bone formation, but the inability to identify an ideal delivery system limits their clinical application. We used ex vivo adenoviral gene transfer to create BMP-2-producing bone-marrow cells, which allow delivery of the BMP-2 to a specific anatomical site. The autologous BMP-2-producing bone-marrow cells then were used to heal a critical-sized femoral segmental defect in syngeneic rats. METHODS Femoral defects in five groups of rats were filled with 5 x 10(6) BMP-2-producing bone-marrow cells, created through adenoviral gene transfer (twenty-four femora, Group I); twenty micrograms of rhBMP-2 (sixteen femora, Group II); 5 x 10(6) beta-galactosidase-producing rat-bone-marrow cells, created through adenoviral gene transfer of the lacZ gene (twelve femora, Group III); 5 x 10(6) uninfected rat-bone-marrow cells (ten femora, Group IV); or guanidine hydrochloride-extracted demineralized bone matrix only (ten femora, Group V). Guanidine hydrochloride-extracted demineralized bone matrix served as a substrate in all experimental groups. Specimens that were removed two months postoperatively underwent histological and histomorphometric analysis as well as biomechanical testing. RESULTS Twenty-two of the twenty-four defects in Group I (BMP-2-producing bone-marrow cells) and all sixteen defects in Group II (rhBMP-2) had healed radiographically at two months postoperatively compared with only one of the thirty-two defects in the three control groups (beta-galactosidase-producing rat-bone-marrow cells, uninfected rat-bone-marrow cells, and guanidine hydrochloride-extracted demineralized bone matrix alone). Histological analysis of the specimens revealed that defects that had received BMP-2-producing bone-marrow cells (Group I) were filled with coarse trabecular bone at two months postoperatively, whereas in those that had received rhBMP-2 (Group II) the bone was thin and lace-like. Defects that had been treated with bone-marrow cells producing beta-galactosidase (Group III), uninfected bone-marrow cells (Group IV), or guanidine hydrochloride-extracted demineralized bone matrix only (Group V) demonstrated little or no bone formation. Histomorphometric analysis revealed a significantly greater total area of bone formation in the defects treated with the BMP-2-producing bone-marrow cells than in those treated with the rhBMP-2 (p = 0.036). Biomechanical testing demonstrated no significant differences, with the numbers available, between the healed femora that had received BMP-2-producing bone-marrow cells and the untreated (control) femora with respect to ultimate torque to failure or energy to failure. CONCLUSIONS This study demonstrated that BMP-2-producing bone-marrow cells created by means of adenoviral gene transfer produce sufficient protein to heal a segmental femoral defect. We also established the feasibility of ex vivo gene transfer with the use of biologically acute autologous short-term cultures of bone-marrow cells.

Ankle sprain prophylaxis: an analysis of the stabilizing effects of braces and tape

Five cadaveric ankles were used to determine the ef fects of prophylactic bracing and tape on resisting an inversion moment applied to the ankle. The ankles were tested in neutral flexion and 30° of plantar flexion and with both low- and high-top shoes. Eight different strap-on braces were studied. High-top sneakers sig nificantly increased the passive resistance to inversion afforded by all braces and tape. Many of the braces functioned to resist inversion at a level that was com parable with or exceeded the capability of freshly ap plied tape. This finding was independent of the type of footwear. Braces that were not as effective as freshly applied tape nevertheless retained the advantage over tape in that they could be easily readjusted and their effectiveness restored, whereas the quality of the sup port provided with tape deteriorated with usage.

Genetic loci influencing natural variations in femoral bone morphometry in mice

This study identifies genetic loci affecting femoral bone length and width measures in mature mice. Sixteen month old female F2 progeny of a C57BL/6J and DBA/2J intercross were examined for femur length and width of the femoral head, intertrochanteric region and three locations of the diaphysis using digitized images of femur radiographs obtained in the anterior‐posterior and lateral projections. A genome wide linkage map was constructed using microsatellite markers at an average density of 20 cM, and quantitative trait locus analysis used to identify regions of the genome showing linkage with the traits measured. Femur length showed significant linkage with loci on proximal chromosome 3 (lod 6.1), and suggestive linkage with a locus on chromosome 14. A major locus on mid‐chromosome 7 controlled width of the diaphysis (lod 6.8). Other loci were identified on chromosomes 2 and 4. Width at the intertrochanteric region had suggestive linkage with loci on chromosomes 6 and 19. No loci were found with linkage for width of the femoral head. Candidate genes related to bone development or metabolism are present at most of these loci. These findings show that genetic regulation of femoral bone morphology is complex, and are consistent with the distinct biologic processes that control longitudinal and lateral growth of the femur.

Efficacy of soft splints in reducing severe knee-flexion contractures.

This study evaluated the effectiveness of soft splints made from polyurethane foam in reducing severe knee‐flexion contractures of patients with cerebral palsy. The splints were applied nightly over a period of 10 months. Knee‐flexion contractures were reduced by an average of 24° in all patients. Younger patients generally presented with less severe initial contractures and had the greatest percentage of improvement. When used on a single limb when bilateral contractures were present, sympathetic reduction of the contracture occurred to varying degrees in the unsplinted limb. It is recommended that the splints be replaced regularly to maximize their effectiveness. The benefit‐to‐cost ratio is extremely high and their low cost makes soft splints feasible for use in developing countries and by low‐income families.

Effects of polyaxial Pedicle screws on lumbar construct rigidity

Pedicle screw constructs have been shown to increase fusion rates in the lumbar spine. Manufacturers have created pedicle screws with one or two degrees of freedom built into the screw head to allow for easier incorporation of the interlocking rod, but the effects of these screws on construct stiffness has not been tested. The purpose of this study is to compare and contrast the stiffness of lumbar pedicle screw constructs with and without the use of polyaxial pedicle screws. Nontapered, self-taping pedicle screws (6.0-mm diameter × 30-mm length, titanium) were used in the fixation of porcine spines from L3–L5. Group 1 (n = 5) contained six standard pedicle screws from one manufacturer. Group 2 (n = 5) contained six standard pedicle screws from a second manufacturer. Group 3 (n = 5) contained four standard pedicle screws placed at L3 and L5, as well as two polyaxial screws placed at L4. Group 4 (n = 5) contained six polyaxial pedicle screws. A rotational variable differential transformer was used to record angular displacement between vertebrae in the construct as it is loaded in flexion, extension, right bend, left bend, clockwise torque, and counterclockwise torque. Stiffness curves were linear throughout the range of applied force. The average r2 value for the generated stiffness graphs was 0.94 (SD = 0.06). No construct failure occurred during any of the testing. There were no significant differences (p < 0.05, two-way analysis of variance) in moment versus angle noted in any of the four groups tested. For torque tests, the all-polyaxial screw constructs showed significantly increased stiffness compared with the other groups. The current study has shown that the incorporation of polyaxial screws in pedicle screw constructs did not significantly decrease the construct stiffness. There is a suggestion that the use of all polyaxial screws may increase the resistance to torque by allowing better purchase of intervertebral rods.

Ultra-high-molecular weight polyethylene wear: An in vitro comparison of acetabular metal types and polished surfaces

The generation of debris from the wear of ultra-high-molecular weight polyethylene (UHMWPE) is a well-recognized factor in the development of osteolysis and the long-term failure of total joint arthroplasties. Wear between the articulation of the femoral head and the polyethylene has been recognized for many years, but more recently, both retrieval and in vitro studies have demonstrated that convex surface wear or backside wear also occurs and may be of significance. Currently, modular acetabular components are being designed with polished surfaces, fewer screw holes, various polyethylene locking mechanisms, and stiffer metal alloys in an attempt to reduce backside wear. The purpose of this study was to determine if differences existed in UHMWPE wear based on the metal alloy used and the surface finish in modular acetabular components. Sixteen components in 4 groups were subjected to 10 million gait cycles using an in vitro joint simulator. All components used 28-mm cobalt chrome femoral heads on cobalt chrome tapered stems. The 4 groups differed only in the type of metal backing and type of interior finished surface: polished cobalt chrome, unpolished cobalt chrome, polished titanium, and unpolished titanium. UHMWPE changes were examined in terms of articular (concave) surface wear, backside (convex) surface wear, and frictional torque. The overall linear and volumetric wear rates were 1.05 mm/10 million cycles and 325 mm3/10 million cycles. No significant differences in linear and volumetric wear rates were detected between the cobalt chrome and titanium acetabular components. Surface finish did not influence wear rates. In terms of backside wear, all specimens in the 4 groups demonstrated total loss of all sputtered gold with the exception of those areas extruded through the screw holes. Extrusion through the screw holes was on the order of 0.0004 inch for all groups, and no significant difference was seen among the groups for this parameter. The measurements of articular frictional torque demonstrated a significant difference among the polished and unpolished cobalt chrome components (17.3 N x m vs 11.5 N x m; P = .0039, 2-way analysis of variance, Students Newman Keuls method). Some designs in modular acetabular components have favored stiffer alloys, such as cobalt chrome, with polished concave surfaces to decrease wear on both the concave and the convex surfaces. In this study, there was no significant difference in wear rates noted between cobalt chrome and titanium acetabular components, and polishing of the components had no appreciable affect in reducing backside wear.

Effects of distal femoral centralizers on bone—cement in total hip arthroplasty: An experimental analysis of cement—centralizer bonding, cement void formation, and crack propagation

Distal femoral centralizers of five different designs were inserted into model femoral stems and cemented into closed-ended tubes simulating a proximal femoral canal. Specimens underwent cyclic loading from 50 to 500 lb. for 0, 1, 2, 5, and 10 million cycles. Each specimen was then sectioned transversely at multiple levels to obtain serial cross-sections, beginning at the femoral stem tip and proceeding distally so as to include the full extent of the centralizer. The area of each section occupied by a centralizer and the total amount of porosity present in the cement surrounding the centralizers were measured using an image analyzer. A dye penetrant was then applied to each section to visualize cement cracks and areas of incomplete bonding between cement and centralizers. The number, length, and location of cement cracks were catalogued for each section. No cement cracks or lack of bonding was observed at the interface between cement and centralizers. There was greater porosity in the specimens containing centralizers than in controls without centralizers (P < .05). The cement surrounding two of the centralizer designs had a significantly smaller amount of porosity than the cement surrounding the other three designs (P < .05). The number of cracks did not depend on whether a centralizer was used, the type of centralizer, or the cycling duration. In the control specimens, failure to adequately plug the centralizer receptacle hole in the stem tip resulted in very large cement voids.

Enhancement of pedicle screw fixation through washers.

This study examined the effect of washer usage on initial pedicle screw fixation and on the salvage of replaced pedicle screws, and the effect of minor adjustments of pedicle screws on insertional torque. Titanium, nontapered pedicle screws (6.5-mm in diameter and 35-mm in length) from one manufacturer and custom-made 5-mm washers were used in the fixation of porcine lumbar spines. Insertional torque was measured with an electronic torque screwdriver and failure strength was determined by straight pullout of the screws using an MTS machine. Initial insertional torque values were significantly greater in pedicle screws placed with washers compared with screws placed without washers. When the screw placed without a washer was salvaged with the addition of a washer, a significant increase in insertional torque resulted. Pullout testing failed to show a significant difference between the screws that were placed with washers and the screws that were placed without washers. In the second part of the experiment, there was a significant decrease in insertional torque after backing out the screw as little as 90°. This current study showed that (1) washers significantly increase the insertional torque of pedicle screws; (2) screws placed without a washer can be salvaged and replaced with a washer, which results in significantly increased insertional torque; and (3) backing out a pedicle screw 90° significantly decreases its insertional torque. Washers can be used with pedicle screws to enhance the initial stability of the screw constructs, and to maximize insertional torque when screws need to be replaced, revised, or adjusted (backed out).

Effects of antihistamines on joint stiffness and bone healing after periarticular fracture

In an effort to blunt the inflammatory response following injury, we studied the effects of antihistamines on joint stiffness and bone healing after periarticular fracture. The hind limbs of 37 New Zealand white rabbits were instrumented with intramedullary Steinmann pins to allow immobilization after creating a distal tibia fracture. After periarticular ankle fracture, the rabbits were divided into groups to receive chlorpheniramine, low-dose terfenadine, high-dose terfenadine, or no antihistamine. Antihistamine was administered for the first 5 days after fracture. At 3 weeks after fracture, the rabbits treated with antihistamines showed a significant reduction in joint stiffness compared to control animals. To determine if antihistamine also retarded bone healing, an additional 12 New Zealand white rabbits underwent bilateral drilling of the midshaft of the femur with a 2.5 mm Steinmann pin. Six animals received oral terfenadine, and six served as controls. After 5 weeks of cage activity, the animals treated with the antihistamine showed a significant decrease in energy to failure of the femurs on torsional testing. Qualitative microscopic examination revealed less mature callus at the site of the femoral cortical defect in those animals treated with the terfenadine. Antihistamine may have heretofore unconsidered benefits in preventing post-traumatic joint stiffness but may slow healing of associated bony injuries.

Determination of joint stiffness of the human proximal interphalangeal joints: Development and clinical evaluation of a new device

A moment angle plotter has been designed and constructed to measure the stiffness of the human proximal interphalangeal joints of the index, middle and ring fingers. Variability in the measurements for seventeen normal subjects are presented.