William L. Carson

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° 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° < P-P angle < 60°) are structures. However, as P-P angle approaches 0°, the structure becomes more flexible (unstable) and some internal force and moment components exponentially increase (starting at approximately a 30° 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.

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 ware studled. 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. Single level instrumentation adjacent to a comminuted segment resisted the entire axial load which produced a max bending moment of 11.4 Nm per 445 N axial lumbar load at each corresponding screw-longitudinal element junction as compared to less than 2 Nm when load sharing by a healthy motion segment existed. Thus finite implant fatigue life was predicted when instrumenting comminuted lumbar segments, with low probability of fatigue whan load sharing equivalent to a healthy motion segment existed. Instrumenting two levels adjacent to a comminuted human lumbar segment was predicted to reduce the flexion moment at screw-longitudinal element interconnections by 16% when using 4.76 mm rods, 32% for 6.36 mm rods, 36% for VSP plates, end 50% when ki = kv. These results Illustrate the clinical need to create load sharing when possible, to select an implant creating the desired ki/kv ratio depending upon the trade off between potential iatrogenic effects of stress shielding relative to the fatigue strength/life of the selected implant, and to control the patients type of activity and number of cycles until fusion has satisfactorily been achieved.

A modular spinal rod linkage system to provide rotational stability

The effect of cross linkage on the In vitro stability of paired Harrington distraction rods was studied in an unstable fracture model using calf spine segments. Cross linkage used in conjunction with sublaminar wires significantly improved torsional stability, improved lateral bending stability, and had no adverse affect on stability for axial, forward flexion, or extension loading compared to rods alone, rods with bridges, and wired rods.

Biomechanical analysis of pedicle screws in osteoporotic bone with bioactive cement augmentation using simulated in vivo multicomponent loading.

Study Design. Biomechanical analysis of bioactive cements augmenting pedicle screw resistance to loosening in osteoporotic synthetic bone. Objective. To simulate in vivo loading-loosening of pedicle screws in osteoporotic vertebrae; and to compare biomechanical efficacy of the following bioactive cements: calcium phosphate (CP), calcium sulfate (CS), and proprietary mixture (M). Summary of Background Data. Pedicle screw instrumentation in osteoporotic spines is limited by poor bone-screw interface strength, resulting in screw loosening fixation failure. Previous in vivo studies evaluated augmented pedicle screw resistance to pure pullout, not simulating in vivo loading/failure. Methods. A pedicle screw—instrumented osteoporotic thoracic vertebra subjected to combined pullout, transverse, moment loading was simulated. Unconstrained 3-dimensional screw motion relative to vertebra was optically measured during quasi-static, and dynamic loading. Results. Augmented groups (CP, CS, M) produced (P < 8.0E-07) higher quasi-static failure initiation force (61.2,45.6, 40.3 N) than those by the nonaugmented group (21.0 N), with no significant difference in small screw displacement up to these loads. Nonaugmented screw motion after failure initiation was primarily rotation (toggle-migration) with minimal pullout until the screw tip contacted the superior endplate, followed by more prominent screw pullout. Augmented screw motion (with cement remaining intact on screw) was similar, but with eventual bone fracture anterior to the pedicle region. Dynamic loading produced similar failure initiation force and screw motion. Conclusion. We believe our test protocol produced screw loosening failure similar to that observed clinically, and that it has the ability to detect differences in failure initiation force and failure modes to compare short-term efficacy of screw augmentation techniques. All cements improved screw resistance to failure. The CP > CS > M failure initiation force (P < 0.006) was because of differences in cement distribution. Animal studies may be required to characterize the remodeling activity of bioactive cements and their longer term efficacies.

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.

Role of genetic background in determining phenotypic severity throughout postnatal development and at peak bone mass in Col1a2 deficient mice (oim)

Osteogenesis imperfecta (OI) is a genetically and clinically heterogeneous disease characterized by extreme bone fragility. Although fracture numbers tend to decrease post-puberty, OI patients can exhibit significant variation in clinical outcome, even among related individuals harboring the same mutation. OI most frequently results from mutations in type I collagen genes, yet how genetic background impacts phenotypic outcome remains unclear. Therefore, we analyzed the phenotypic severity of a known proalpha2(I) collagen gene defect (oim) on two genetic backgrounds (congenic C57BL/6J and outbred B6C3Fe) throughout postnatal development to discern the phenotypic contributions of the Col1a2 locus relative to the contribution of the genetic background. To this end, femora and tibiae were isolated from wildtype (Wt) and homozygous (oim/oim) mice of each strain at 1, 2 and 4 months of age. Femoral geometry was determined via muCT prior to torsional loading to failure to assess bone structural and material biomechanical properties. Changes in mineral composition, collagen content and bone turnover were determined using neutron activation analyses, hydroxyproline content and serum pyridinoline crosslinks. muCT analysis demonstrated genotype-, strain- and age-associated changes in femoral geometry as well as a marked decrease in the amount of bone in oim/oim mice of both strains. Oim/oim mice of both strains, as well as C57BL/6J (B6) mice of all genotypes, had reduced femoral biomechanical strength properties compared to Wt at all ages, although they improved with age. Mineral levels of fluoride, magnesium and sodium were associated with biomechanical strength properties in both strains and all genotypes at all ages. Oim/oim animals also had reduced collagen content as compared to Wt at all ages. Serum pyridinoline crosslinks were highest at two months of age, regardless of strain or genotype. Strain differences in bone parameters exist throughout development, implicating a role for genetic background in determining biomechanical strength. Age-associated improvements indicate that oim/oim animals partially compensate for their weaker bone material, but never attain Wt levels. These studies indicate the importance of genetic background in determining phenotypic severity, but the presence of the proalpha2(I) collagen gene defect and age of the animal are the primary determinants of phenotypic severity.

Enhanced Fracture and Soft-Tissue Healing by Means of Anabolic Dietary Supplementation

BACKGROUND Malnutrition is common in hospitalized injured patients. It contributes to delayed fracture-healing and increased morbidity. However, relatively little attention has been directed toward nutritional strategies for augmenting musculoskeletal recovery after a fracture. This animal study was designed to examine the effects of dietary protein intake and the role of conditionally essential amino acids in muscle and bone-healing after a fracture. METHODS One hundred adult male rats were used. Ten rats served as controls and received a 15% protein diet throughout the study. The remaining ninety rats received a 6% protein diet for five weeks to induce protein malnutrition. The rats underwent intramedullary nailing and closed midshaft fracture of one femur. After the fracture, they were separated into three isocaloric dietary groups. Group P6 received a diet with 6% protein; Group P15, a diet with 15% protein; and group P30, a diet with 30% protein with conditionally essential amino acids. At two, four, and six weeks after surgery, ten animals from each group were killed and the femora were evaluated with dual x-ray absorptiometry, histomorphometric assessment of callus, and torsional testing. The quadriceps muscles were analyzed for total mass, total protein content, and for mRNA expression of insulin-like growth factor-1 (IGF-1), IGF-2, IGF receptors, actin, myosin, and vascular endothelial growth factor (VEGF). RESULTS The P30 group demonstrated elevations in albumin, body mass, muscle mass, total protein content of muscle, and bone mineral density in the fracture callus compared with the P6 diet group at six weeks (p < 0.05). Molecular analysis of muscle revealed that IGF-1, IGF-2, IGF receptors, myosin, actin, and VEGF gene expression were significantly (p < 0.001) higher in the P6 group compared with the P30 group. Biomechanical testing of the femora, however, showed no significant differences. CONCLUSIONS Dietary supplementation with conditionally essential amino acids in malnourished animals had anabolic effects on bone mineralization, body mass, and muscle mass.

The effect of arthrodesis, implant stiffness, and time on the canine lumbar spine.

Study Design Canine posterior lumbar instrumentation and fusion. Objectives To study effects of implant rod size and time on the stiffness of related spine construct elements. Summary of Background Data The ideal stiffness of posterior spinal implants to successfully treat clinical instability or deformity with minimal side effects is unknown. Methods Twenty-six canines were divided into 7 groups: control, and 6 or 12-month survival after sham or lumbar L3-5 arthrodesis (facet, posterior, and posterolateral) with either 4.76 or 6.35 mm diameter rod-pedicle screw instrumentation. Axial flexion-compression stiffness of the L3-5 segment components and axial compression stiffness of the bypassed and adjacent anterior column elements were measured. Results Posterior instrumentation initially increased flexion-compression stiffness of the L3-5 segment more than the intrinsic stiffness of the implant due to control of spinal column flexion buckling. Sham operation did likewise, apparently by posterior scar tissue tethering. The percent contribution of the implant construct to instrumented segment stiffness was significantly less at 6 months without further change from 6 to 12 months; 14% and 22% for 4.76 and 6.35 mm rod constructs, respectively. Spinal column as well as posterior column stiffness after fusion was independent of rod size at 6 months and increased at 12 months in only the 4.76 mm rod group. Bypassed L4 vertebral body stiffness decreased significantly at 6 months, was not rod size dependent and changed little between 6 and 12 months. Bypassed disk stiffness responded in a biphasic manner, apparently increasing at 6 months with significant decrease from 6 to 12 months. Adjacent disk compression stiffness progressively decreased over time independent of rod size, also decreasing after sham operation. Conclusions Both rod sizes were associated with 100% fusion and produced similar changes in bypassed bone and disks, and adjacent disks. There was delayed fusion stress shielding by 6.35 mm rod constructs.

An in vitro biomechanical comparison of dynamic condylar screw plate combined with a dorsal plate and double plate fixation of distal diaphyseal radial osteotomies in adult horses.

OBJECTIVE To compare stiffness and strength of a dynamic condylar screw plate combined with dorsal broad dynamic compression plate (DCS-bDCP) fixation with double broad dynamic compression plate (dbDCP) fixation used to repair oblique distal fractures of adult equine radii. STUDY DESIGN Experimental. SAMPLE POPULATION Adult equine radii (n=10 pair). METHODS An unconstrained three-dimensional loading-measurement system was used to determine stiffness of a 50 mm long intact, and then DCS-bDCP or dbDCP-plated osteotomized/ostectomized segment of radii when subjected to a nondestructive sequence of compression, torsion, and lateral-to-medial (LM), medial-to-lateral (ML), cranial-to-caudal (CrCa), and caudal-to-cranial (CaCr) bending. Uniform load over the entire length of construct identified its weakest characteristics during torsion and LM and CrCa bending to failure. RESULTS No difference was observed between osteotomized/ostectomized DCS-bDCP and dbDCP construct stiffness for all 6 loading modes, and strength for all 3 failure loads. Ostectomized DCS-bDCP and dbDCP construct stiffness was significantly lower than osteotomized radii, the latter approaching intact for axial, LM, and CrCa bending. Most frequent failure was bone fracture through exit site of a screw located adjacent to osteotomy/ostectomy. CONCLUSIONS DCS-DCP and dbDCP constructs had comparable strength and stiffness when repairing osteotomies/ostectomies in equine adult radius bone. Fracture reduction increased stiffness that approached intact bone for loads that placed the unplated side in compression. CLINICAL RELEVANCE DCS-bDCP and dbDCP constructs are comparable in stiffness and strength when applied to oblique distal diaphyseal osteotomies/ostectomies in equine radius bone. However, the DCSs localized effect on distal epiphyseal structure because of additional bone removal remains to be investigated under in vivo articular loading conditions.

Biomechanical evaluation of location and mode of failure in three screw fixations for a comminuted transforaminal sacral fracture model

Background Pelvic ring–comminuted transforaminal sacral fracture injuries are rotationally and vertically unstable and have a high rate of failure. Objective Our study purpose was to use three-dimensional (3D) optical tracking to detect onset location of bone–implant interface failure and measure the distances and angles between screws and line of applied force for correlation to strength of pelvic fracture fixation techniques. Methods 3D relative motion across sacral–rami fractures and screws relative to bone was measured with an optical tracking system. Synthetic pelves were used. Comminuted transforaminal sacral–rami fractures were modelled. Each pelvis was stabilised by either (1) two iliosacral screws in S1, (2) one transsacral screw in S1 and one iliosacral screw in S1 and (3) one trans-alar screw in S1 and one iliosacral screw in S1; groups 4–6 consisted of fixation groups with addition of anterior inferior iliac pelvic external fixator. Eighteen-instrumented pelvic models with right ilium fixed simulate single-leg stance. Load was applied to centre of S1 superior endplate. Five cycles of torque was initially applied, sequentially increased until permanent deformation occurred. Five cycles of axial load compression was next applied, sequentially increased until permanent deformation occurred, followed by axial loading to catastrophic failure. A Student t test was used to determine significance (p < 0.05). Results The model, protocol and 3D optical system have the ability to locate how sub-catastrophic failures initiate. Our results indicate failure of all screw-based constructs is due to localised bone failure (screw pull-in push-out at the ipsilateral ilium–screw interface, not in sacrum); thus, no difference was observed when not supplemented with external fixation. Conclusion Inclusion of external fixation improved resistance only to torsional loading. Translational Potential of this Article Patients with comminuted transforaminal sacral–ipsilateral rami fractures benefit from this fixation.