Stephen D. Cook

Effects of bone mineral density on pedicle screw fixation

Study Design In an attempt to evaluate the effects of bone mineral density on the quality of fixation of pedicle screws in the lumbar spine, the axial pullout force was determined and compared in normal and osteoporotic human lumbar spines. Objectives Four techniques of screw hole preparation were evaluated. Two pedicle screw/offset laminar hook constructs also were evaluated to determine whether the adjunct fixation of the laminar hooks would improve quality of fixation to a level sufficient to allow their use in the osteoporotic lumbar spine. Methods Pedicle screws were inserted by one of the listed techniques into fresh frozen cadaveric human spines. The fixation strength then was evaluated by pullout on a uniaxial testing frame. Results Bone mineral density was a strong influence on axial pullout force. In normal bone, the method of screw hole preparation did not significantly affect the quality of fixation. However, in the osteoporotic spine, either an untapped screw hole or the tapping of a screw hole with a 5.5 mm tap improved the pullout force a statistically significant amount (P < 0.003). Also, a pedicle screw with offset hooks at two adjacent levels improved the fixation significantly, increasing the pullout force to twice the expected value. Conclusion Pedicle screw pullout strength was highly correlated with bone mineral density. A 5.5 mm tap or preparation with a ganglion knife improved pullout strength. Use of pedicle screws in conjunction with laminar hooks at two levels improved pullout strength.

Age-related decline in proprioception.

Joint-position sense of the knee was measured in 29 subjects with normal knee joints ranging in age from 20 to 82 years. Joint-position sense was determined by two common techniques that measure the threshold to detection of motion and the ability to reproduce passive knee positioning. Joint-position sense was found to deteriorate with increasing age as measured by both tests, with a correlation coefficient that was significant at the p < 0.001 level for each test. The two tests were found to correlate at the p < 0.025 level, indicating that the same biologic parameter was being measured by both tests. Deterioration of proprioception, or joint-position sense, as measured may be a sensitive indicator of subclinical degenerative joint disease of the knee, as well as a means of quantitating proprioception in suspected neuropathic joints.

Recombinant human bone morphogenetic protein-7 induces healing in a canine long-bone segmental defect model.

An ulnar segmental defect model was used in adult male dogs to examine the effect of recombinant human bone morphogenetic protein-7 (recombinant human Osteogenic Protein-1 [rhOP-1]; Creative Biomolecules, Hopkinton, Massachusetts) on new bone induction and healing, and to test the mechanical strength of healed 2.5-cm segmental bone defects. The rhOP-1 composites consisted of a carrier of 500 mg of demineralized, guanidine-extracted, insoluble bovine bone matrix (collagen carrier), reconstituted with rhOP-1. Six animals received 1200 micrograms rhOP-1 unilaterally and were killed at 12 weeks for torsional load-to-failure testing using the contralateral side as a control. Two further animals received varying amounts of rhOP-1 bilaterally and were studied histologically. All defect sites receiving rhOP-1 were completely bridged radiographically by eight weeks. A control composite, containing no rhOP-1, failed to induce new bone formation at any time. Histologically, rhOP-1-treated sites examined at 16 weeks had formation of new cortical and cancellous bone, with normal appearing marrow elements in the reconstituted medullary canal. The torsional strength of the rhOP-1-implanted ulnae averaged 72% of control (range, 30-99%). The angular deformation to failure averaged 92% of control (range, 39-122%). The energy absorption to failure averaged 67% of control (range, 27-111%). This study demonstrates the efficacy of rhOP-1 in healing segmental osteoperiosteal defects in a canine model.

Hydroxyapatite-coated titanium for orthopedic implant applications.

The interface mechanical characteristics and histology of commercially pure (CP) titanium- and hydroxyapatite- (HA) coated Ti-6Al-4V alloy were investigated. Interface shear strength was determined using a transcortical push-out model in dogs after periods of three, five, six, ten, and 32 weeks. Undecalcified histologic techniques with implants in situ were used to interpret differences in mechanical response. The HA-coated titanium alloy implants developed five to seven times the mean interface strength of the uncoated, beadblasted CP titanium implants. The mean values for interface shear strength increased up to 7.27 megaPascals (MPa) for the HA-coated implants after ten weeks of implantation, and the maximum mean value of interface shear strength for the uncoated CP titanium implants was 1.54 MPa. For both implant types there was a slight decrease in mean shear strength from the maximum value to that obtained after the longest implantation period (32 weeks). Histologic evaluations in all cases revealed mineralization of interface bone directly onto the HA-coated implant surface, with no fibrous tissue layer interposed between the bone and HA visible at the light microscopic level. The uncoated titanium implants had projections of bone to the implant surface with apparent direct bone-implant apposition observed in some locations. Measurements of the HA coating material made from histologic sections showed no evidence of significant HA resorption in vivo after periods of up to 32 weeks.

In vivo evaluation of recombinant human osteogenic protein (rhOP-1) implants as a bone graft substitute for spinal fusions

Study Design. Posterior, spinal fusion segments were evaluated in adult mongrel dogs at 6, 12, and 26 weeks post-implantation. Four sites on each animal received implants consisting of recombinant human osteogenic protein-1 on a bone collagen carrier, bone collagen carrier alone, autogenous iliac crest bone, or no implant material. Objective. To determine the efficacy of recombinant human osteogenic protein-1 as a bone graft substitute in achieving posterior spinal fusion and compare the results to those obtained using autogenous bone graft. Summary of Background Data. Posterior spinal fusion generally includes onlay grafting of autogenous or allogeneic bone after decortication of bony surfaces of the vertebral elements. The search for an acceptable bone graft substitute material has in recent years centered upon proteins capable of inducing bone in vivo. Recombinant human osteogenic protein-1 has demonstrated efficacy in healing large segmental osteoperiosteal defects in rabbits, dogs, and monkeys and appears ideally suited as a bone graft substitute for spinal fusions. Methods. The quality of fusion and new bone formation was evaluated using plain films, computed tomography, and magnetic resonance imaging. Results. Radiographic and histologic studies demonstrated that recombinant human osteogenic protein-1-treated fusion segments attained a stable fusion by 6 weeks post-implantation and were completely fused by 12 weeks. The autograft sites demonstrated fusion at 26 weeks post-implantation. Conclusions. The results indicated that recombinant human osteogenic protein-1 is an effective bone graft substitute for achieving stable posterior spinal fusions in a significantly more rapid fashion than can be achieved with autogenous bone graft.

Hydroxyapatite-coated porous titanium for use as an orthopedic biologic attachment system.

The biologic attachment characteristics of hydroxyapatite (HA)-coated porous titanium and uncoated porous titanium implants were investigated. The implants were placed transcortically in the femora of adult mongrel dogs and evaluated after periods of three, six, and 12 weeks. The HA coating was applied using a modified plasma spray process to samples with pore volume and pore size of the porous coating expanded to equal the pore morphology of uncoated porous specimens. Mechanical push-out testing revealed that the bone-porous material interface shear strength increased with time in situ for both the uncoated and HA-coated implants. The use of the HA coating on porous titanium, however, did not significantly increase attachment strength. Histologic and microradiographic sections yielded similar qualitative results in the amount of bone grown into each system. After three weeks, both systems displayed primarily woven bone occupying approximately 50% of the available porous structure. Six and 12 weeks postimplantation, each system displayed more extensive bone ingrowth, organization, and mineralization, with only limited areas of immature bone. Histologically, differences were noted at the ingrown bone-porous material interface between the two implant types. The HA coating supported mineralization directly onto its surface, and a thin osseous layer was found lining all HA-coated surfaces. An extremely thin fibrous layer was observed separating the uncoated titanium particle surface from ingrown bone. There was no extensive direct apposition or lining of the ingrown bone to the uncoated porous titanium particle surfaces.

Acceleration of tibia and distal radius fracture healing in patients who smoke

A low intensity ultrasound device was investigated as an accelerator of cortical and cancellous bone fracture healing in smokers and non-smokers. Statistically significant reductions in healing time for smokers and nonsmokers were observed for tibial and distal radius fractures treated with an active ultrasound device compared with a placebo control device. The healing time for a tibial fracture was reduced 41% in smokers and 26% in nonsmokers with the active ultrasound device. Similarly, distal radius fracture healing time was reduced by 51% in smokers and 34% in nonsmokers with the active device. Treatment with the active ultrasound device also substantially reduced the incidence of tibial delayed unions in smokers and nonsmokers. The use of the active ultrasound device accelerates cortical and cancellous bone fracture healing, substantially mitigates the delayed healing effects of smoking, speeds the return to normal activity, and reduces the long-term complication of delayed union.

Osteogenic protein-1: biology and applications.

Osteogenic proteins, also referred to as BMPs, are a family of bone matrix polypeptides isolated from a variety of mammalian species. These proteins are members of the transforming growth factor-beta superfamily of molecules that contain a highly conserved 7 cysteine transforming growth factor-beta domain in their C-termini. Use of recombinantly produced human osteogenic protein-1, also referred to as BMP-7, implanted in conjunction with bovine bone derived Type 1 collagen or various non-proteinaceous biodegradable carriers into surgically created large diaphyseal segmental defects in animals leads to the regeneration of new bone that is fully functional biologically and biomechanically. Further study has shown that osteogenic protein-1 can be used as a bone graft substitute to promote spinal fusion and to aid in the incorporation of metal implants. Finally, osteogenic protein-1 shows promise as an agent for repair of osteochondral defects.

The effect of osteogenic protein-1 on the healing of segmental bone defects treated with autograft or allograft bone.

Background: Large amounts of bone graft are frequently used to elicit the healing of bone defects resulting from reconstructive procedures. Autograft and allograft bone are often used, but each has its limitations. Bone morphogenetic proteins (BMPs) improve the healing of segmental bone defects treated with autograft or allograft. The objective of the present study was to determine the effect of implantation of a recombinant osteogenic protein-1 (OP-1) in combination with bone graft on the healing of a critical-sized (2.5-cm) segmental defect in canine ulnae. Methods: Either autograft bone, allograft bone, osteogenic protein-1 (OP-1) mixed with type-1 bovine collagen, or various combinations of OP-1 and collagen (OP-1 device) mixed with allograft or autograft were implanted in the segmental bone defects. The combinations included 67% bone graft with 33% OP-1 device and 33% bone graft with 67% OP-1 device. The healing of the defects was assessed with radiographic, biomechanical, and histological studies. The animals were killed at twelve weeks postoperatively. Results: The use of the OP-1 device alone or any combination of autograft or allograft bone and the OP-1 device demonstrated improved healing on radiographic, mechanical, and histological studies compared with that demonstrated after use of autograft or allograft bone alone. The highest radiographic and histological grades and the greatest mechanical strength were achieved with the use of 33% allograft and 67% OP-1 device, although no significant differences were observed among the different groups containing the OP-1 device. At twelve weeks postoperatively, the defects treated with any amount of the OP-1 device obtained greater mechanical strength than that obtained by autograft bone alone. Conclusions: Major bone defects may be treated with allograft bone combined with the OP-1 device, instead of autograft alone, to avoid complications associated with the use of autograft. The combination of allograft bone and the OP-1 device resulted in optimum healing of the defect, according to the radiographic, mechanical, and histological parameters measured in this study. Clinical Relevance: The combination of freeze-dried allograft bone with the OP-1 device is an attractive graft material for the treatment of large bone defects. Although similar results were observed when autogenous bone graft was used in combination with the OP-1 device, the results of the present study suggest that allograft, because of its relatively unlimited supply, can be substituted without reduced efficacy. In addition, avoiding the need to harvest autogenous bone eliminates the additional operative time and risk associated with a second surgical procedure.

Quantitative analysis of tissue growth into human porous total hip components.

Histologic and radiographic analysis was performed on 36 porous-coated total hip components (22 femoral and 14 acetabular) retrieved from 30 patients. The average patient age was 53.0 years (range, 28-78 years) for femoral components and 56.9 years (range, 28-78 years) for acetabular components. The average time in situ was 13.6 months (range, 2-36 months) for femoral components and 5.5 months (range, 1-18 months) for acetabular components. Fourteen implants were retrieved for instability or dislocation, 12 for late infection, and 8 for persistent pain, and 2 were retrieved after the patient died. Radiographs were reviewed for Singh index, heterotopic bone, implant fit, subsidence, and presence of lucent and sclerotic zones. Histologic and microradiographic sections of the implant and adherent tissue were examined for type, amount, and anatomic distribution of tissue ingrowth. Four femoral stems had no bone ingrowth, 10 had minimal bone growth into the available pore volume (less than 2%), 3 had moderate bone ingrowth (2-5%), and 5 had more extensive bone ingrowth (5-10%). Bone ingrowth tended to occur where the implant made direct contact with the endosteal cortical surface and was rarely seen in proximal metaphyseal bone. Bone ingrowth was completely absent in eight acetabular components, minimal (less than 2%) in three, moderate (2-5%) in two, and more extensive (5-10%) in one and tended to occur adjacent to fixation pegs, spikes, or screws. Radiographic or clinical findings were unreliable in predicting the presence or extent of bone ingrowth in either femoral or acetabular components.