Biomedical subjects

S A Yerby

Publications and source records attributed to S A Yerby.

12 recordsLinked to original sources

PubMed1999

Measurement of strain distributions within vertebral body sections by texture correlation.

STUDY DESIGN: A high-resolution strain measurement technique was applied to axially loaded parasagittal sections from thoracic spinal segments. OBJECTIVES: To establish a new experimental technique, develop data analysis procedures, characterize intrasample shear strain distributions, and measure intersample variability within a group of morphologically diverse samples. SUMMARY OF BACKGROUND DATA: Compression of intact vertebral bodies yields structural stiffness and strength, but not strain patterns within the trabecular bone. Finite element models yield trabecular strains but require uncertain boundary conditions and material properties. METHODS: Six spinal segments (T8-T10) were sliced in parasagittal sections 6-mm thick. Axial compression was applied in 25-N increments up to sample failure, then the load was removed. Contact radiographs of the samples were made at each loading level. Strain distributions within the central vertebral body were measured from the contact radiographs by an image correlation procedure. RESULTS: Intrasample shear strain probability distributions were log-normal at all load levels. Shear strains were concentrated directly inferior to the superior end-plate and adjacent to the anterior cortex, in regions where fractures are commonly seen clinically. Load removal restored overall sample shape, but measurable residual strains remained. CONCLUSIONS: This experimental model is a suitable means of studying low-energy vertebral fractures. The methods of data interpretation are consistent and reliable, and strain patterns correlate with clinical fracture patterns. Quantification of intersample variability provides guidelines for the design of future experiments, and the strain patterns form a basis for validation of finite element models. The results imply that strain uniformity is an important criterion in assessing risk of vertebral failure.

Aged↗

PubMed1999

Characteristics of pedicle screw loading. Effect of surgical technique on intravertebral and intrapedicular bending moments.

STUDY DESIGN: A static nondestructive bending analysis of pedicle screws inserted into vertebral analogues was conducted. Pedicle screw load was studied as a function of variables in insertion technique. OBJECTIVES: To determine how the sagittal bending moment in pedicle screws is affected by changes in pedicle screw length, insertional depth, and sagittal placement. BACKGROUND DATA: An unexpectedly high rate of clinical failure has been observed in pedicle screws used in short-segment instrumentation for unstable burst fractures. The majority of screws fail in sagittal bending within the pedicle. Little is known of the insertion technical factors that affect in situ loads incurred by pedicle screws. METHODS: Synthetic vertebral analogues were fabricated. Pedicle screws internally instrumented with strain gauges were used as load transducers to determine screw bending moments within the pedicle and body of the analogue. Analogues were loaded in compression to simulate loading of an unstable burst fracture. RESULTS: Screw bending moments within the pedicle increased 33% and 52% when screws were left 3 mm and 5 mm short of full insertion. Intrapedicular moments increased 20% to 29% in screws inserted superiorly or inferiorly within the pedicle. Thirty-five-millimeter screws developed intrapedicular moments 16% higher than 40-mm and 45-mm screws. CONCLUSIONS: In situ pedicle screw loads increased significantly as a direct result of variations in surgical technique. Screws left short of full insertion, placed off center in the sagittal plane of the pedicle, or less than 40 mm long developed increased intrapedicular bending moments.

Bone Screws↗

PubMed1998

Revision of failed pedicle screws using hydroxyapatite cement. A biomechanical analysis.

STUDY DESIGN: The biomechanical influence of in situ setting hydroxyapatite cement was examined for use in pedicle screw revision surgery. Pull-out testing of control and pedicle screws augmented with hydroxyapatite cement was performed in human cadaver vertebrae. OBJECTIVES: To determine the immediate effect of using hydroxyapatite cement to augment revision pedicle screws after failure of the primary pedicle screw fixation. SUMMARY OF BACKGROUND DATA: The potential problems associated with using polymethylmethacrylate to augment revision pedicular instrumentation have prompted the search for other solutions. The introduction of resorbable hydroxyapatite pastes may have provided new biocompatible solutions for pedicle screw revision. METHODS: Ten human cadaver vertebrae were instrumented with 6.0-mm pedicle screws in each pedicle. The screws were loaded to failure in axial tension (pull-out). The failed pedicles then were instrumented with 7.0-mm pedicle screws, either augmented with hydroxyapatite cement or nonaugmented, which also were loaded to failure. Finally, the nonaugmented 7.0-mm screw hole was reinstrumented with a hydroxyapatite cement-augmented, 7.0-mm pedicle screw and loaded to failure. RESULTS: The pull-out strength of the 7.0-mm, hydroxyapatite cement-augmented screws was 325% (P = 2.9 x 10(-5)) of that of the 6.0-mm control screws, whereas the strength of the 7.0-mm nonaugmented screws was only 73% (P = 2.0 x 10(-2)) of that of the 6.0-mm control screws. The 7.0-mm screws augmented with hydroxyapatite cement also were able to salvage 7.0-mm pull-out sites to 384% (P = 6.9E-5) of the pull-out strength of the 7.0-mm nonaugmented screws. CONCLUSIONS: Hydroxyapatite cement may be a mechanically viable alternative to polymethyl methacrylate for augmenting revision pedicular instrumentation and should be considered for future experimental, animal, and clinical testing.

Biomechanical Phenomena↗

PubMed1998

Reinforcement of thoracolumbar burst fractures with calcium phosphate cement. A biomechanical study.

STUDY DESIGN: A biomechanical study on the stabilization of thoracolumbar burst fractures. OBJECTIVE: To demonstrate that the addition of a calcium phosphate cement into the fractured vertebral body through a transpedicular approach is a feasible technique that improves the stiffness of a transpedicular screw construct. SUMMARY OF BACKGROUND DATA: Short segment pedicle screw instrumentation is a commonly used method for reduction and stabilization of unstable burst fractures. Recent investigators, however, have reported a high rate of instrumentation failure and sagittal collapse when there is a loss of anterior column support. In this study, the ability of a new hydroxyapatite cement to augment anterior column support was investigated in a burst fracture model. METHODS: A cadaveric L1 burst fracture model was stabilized using short segment pedicle screw instrumentation. Specially instrumented-pedicle screws recorded screw-bending moments. The L1 vertebral body was reinforced with the hydroxyapatite cement through a transpedicular approach. Mechanical testing of the instrumented and instrumented-reinforced constructs were performed in flexion, extension, side bending, and torsion. Construct stiffness and screw-bending moments were recorded. RESULTS: Transpedicular vertebral body reconstruction with hydroxyapatite cement reduced pedicle screw-bending moments by 59% in flexion and 38% in extension. Mean initial stiffness in the flexion-extension plane was increased by 40% (P < 0.05). There were no statistically significant differences in these parameters with lateral bending or torsional movements. CONCLUSIONS: This hydroxyapatite cement compound augments anterior column stability in a burst fracture model. This technique may improve outcomes in burst fracture patients without the need for a secondary anterior approach.

Biocompatible Materials↗

PubMed1998

The effect of boundary conditions on experimentally measured trabecular strain in the thoracic spine.

Vertebral bodies are the primary structural entities of the spine, and trabecular bone is the dominant material from which vertebral bodies are composed. Understanding the mechanical characteristics of vertebral trabecular bone, therefore, is of critical importance in the many clinical conditions that affect the spine. Numerous studies have loaded vertebral bodies to investigate the influence of trabecular bone characteristics on deformation and failure patterns, but the methods of load application have been inconsistent. These differences in the method of load application are a potential confounding factor in the interpretation of the experimental results. We investigated this problem by measuring the distribution of minimum principal strain and maximum shear strain magnitude within 6.35 mm thick samples cut from thoracic spine segments (T8-T10) and loaded to simulate three common experimental configurations. Measurements were made using the texture correlation technique, which extracts deformation patterns from digitized contact radiographs of samples under load. The three loading configurations examined were a three-body construct, a single vertebral body loaded through sectioned intervertebral discs, and polymethylmethacrylate molded directly to the endplates. Results indicate that from both probability and spatial distribution standpoints the best simulation of in vivo loading generates the least uniform strains. Loading through disc remnants or through plastic molded to the endplates causes increasing degrees of strain homogenization. This result has implications not only for the design of experiments involving spinal loading, but also for theories concerning the adaptation of trabecular bone to functional loads.

Adult↗

PubMed1997

The effect of bone quality on pedicle screw loading in axial instability. A synthetic model.

STUDY DESIGN: In this biomechanical analysis of pedicle screw bending moments, custom-fabricated vertebral analogues were loaded in axial compression to produce sagittal bending forces. Moments were measured directly from internally instrumented pedicle screws. OBJECTIVES: To establish the role of cancellous vertebral modulus on pedicle screw bending moments within the vertebral body and the vertebral pedicle. SUMMARY OF BACKGROUND DATA: Pedicle screws are often used to manage axial instability of the spine. Clinical studies report a high incidence of screw bending failure, resulting in kyphosis and pain in some patients. Factors predisposing to bending failure are not well understood, although recent studies have shown that vertebral morphometry is important. METHODS: Axially canullated 7.0-mm pedicle screws, internally instrumented with paired strain gauges, were inserted into analogue vertebrae of uniform dimension. Cancellous modulus was varied from 25-100 MPa. Screws were rigidly mounted to a vertical testing frame, and axial loads were applied to the superior vertebral endplate, producing sagittal bending moments. Moments were recorded from gauges applied in the intrapedicular and intravertebral portions of the screw. Mean moments were compared using a Student's t test, with significance defined as P < 0.05. RESULTS: Cancellous modulus did not affect bending moments experienced in either the intrapedicular or intravertebral portions of the pedicle screws. Gauge accuracy was excellent, and with no gauge drift. CONCLUSIONS: Although small changes in pedicle morphometry can alter screw bending moments significantly, changes in cancellous modulus had no measurable impact on bending moments at these same loads. Bone density is likely to play a limited role in screw bending failure.

Biomechanical Phenomena↗

PubMed1997

Offset laminar hooks decrease bending moments of pedicle screws during in situ contouring.

STUDY DESIGN: A biomechanical study was conducted using cadaver spines to determine the influence of supplemental offset laminar hooks on pedicle screw bending moments and migration during in situ contouring of short-segment pedicle instrumentation. OBJECTIVES: To determine the effects of offset laminar hooks on short-segment pedicle instrumentation constructs during in situ contouring. It was hypothesized that the screw bending moments and screw migration would decrease when offset laminar hooks were used with short-segment pedicle instrumentation. SUMMARY OF BACKGROUND DATA: Clinical studies have implicated screw bending or breakage at the screw hub as failure mechanisms in short-segment pedicle instrumentation constructs used to stabilize thoracolumbar fractures, particularly when rods are contoured in situ. METHODS: Cadaver spines were instrumented using short-segment pedicle instrumentation or short-segment pedicle instrumentation with supplemental offset laminar hooks. The instrumentation was contoured in situ, and screw bending moments were measured at the hub of the screws. Screw migration was measured from lateral radiographs. Comparisons of screw bending moments and migration were made between the two instrumentation configurations. RESULTS: The addition of offset laminar hooks significantly reduced screw bending moments and screw migration during in situ contouring. The mean screw bending moments decreased approximately 30% at the maximum bending angle of 30 degrees (P < 0.05), and the mean screw migration during contouring decreased from 8 degrees to 2 degrees (P < 0.05). CONCLUSIONS: Addition of offset laminar hooks to short-segment pedicle instrumentation decreases screw bending moments and migration of the screws during in situ contouring of the rod. The authors speculate that decrease in loading of the screw will improve durability of the constructs clinically.

Biomechanical Phenomena↗

PubMed1997

The effect of pedicle morphometry on pedicle screw loading. A synthetic model.

STUDY DESIGN: Static nondestructive bending analysis of pedicle screws inserted into vertebral analogues was conducted. Pedicle screw bending load was studied as a function of pedicle morphometry. OBJECTIVES: To determine how sagittal bending moment in pedicle screws is affected by changes in pedicle height, length, and width. BACKGROUND DATA: An unexpectedly high rate of clinical failure has been observed in pedicle screws used in short-segment instrumentation for axially unstable fractures. The majority of screws fail in sagittal bending within the pedicle. To date, little is known of the exogenous factors that affect in situ loads incurred by pedicle screws. METHODS: Synthetic vertebral analogues were fabricated, varying pedicle height, length, or width independently. Pedicle screws internally instrumented with strain gages were used as load transducers to determine screw bending moments within the pedicle and body of the analogue. Analogues were loaded in compression to simulate loading of an unstable burst fracture. RESULTS: Screw bending moments within the pedicle increased incrementally with increasing pedicle length, rising 30% as length increased from 8.0 mm to 12.0 mm. Screw moment increased 20% when pedicle height dropped below 15.0 mm, consistent with a threshold effect. Changes in pedicle width did not affect screw loads within the pedicle. CONCLUSIONS: In situ pedicle screw loads increased significantly as pedicle length increased and as pedicle height decreased. Pedicle screws instrumented internally with strain gages are an effective research instrument allowing measurement of in situ loading along the axis of the screw.

Biomechanical Phenomena↗

PubMed1997

Loading of pedicle screws within the vertebra.

We studied cadaveric motion segments instrumented with unique pedicle screw transducers and loaded in a corpectomy model. We hypothesized that the pedicle screw bending moments could be characterized using a mathematical model. Previous studies have estimated the loading characteristics of pedicle screws either by finite element analysis or by experimentally measuring the screw bending strains external to the lamina and pedicle. In our study, the L4 vertebra was instrumented with modified pedicle screws and fixation rods, and loaded axially. The screws were instrumented to measure bending moments at three locations along the threaded shaft of the screw. The recorded bending moments were maximum near the screw hub and decreased in a non-linear manner toward the screw tip. A mathematical model was fit to the bending moment data and accurately described the loading response of a pedicle screw within a vertebra. This model validates previously unsubstantiated analytical models and provides a tool for predicting which construct design variables contribute to pedicle screw failure. In addition, this experimental model should prove useful in validating finite element models designed to investigate vertebral loading of pedicle screws.

Adult↗

PubMed1996

Short-segment pedicle instrumentation. Biomechanical analysis of supplemental hook fixation.

STUDY DESIGN: This biomechanical study of fractures in cadaver vertebrae used specially designed pedicle screws to determine screw strains during loading of two different fixation constructs. OBJECTIVES: The authors determined the relative benefit of adding offset sublaminar hooks to standard pedicle screw constructs to reduce screw bending moments and prevent fixation failure and sagittal collapse. SUMMARY OF BACKGROUND DATA: Clinical studies have demonstrated a high incidence of early screw failure in short-segment pedicle instrumentation constructs used to treat unstable burst fractures. Strategies to prevent early construct failure include longer constructs, anterior strut graft reconstruction, and use of offset sublaminar hooks at the ends of standard short-segment pedicle instrumentation constructs. METHODS: Human cadaver spines with an L1 burst fracture were instrumented with a standard short-segment pedicle instrumentation construct using specially instrumented pedicle screws. Mechanical testing was carried out in flexion, extension, side bending, and torsion, and stiffness and screw bending moments were recorded. Offset hooks were applied initially, then removed and testing repeated. Stiffness data were compared to intact and postfracture results, and between augmented and standard constructs. RESULTS: Addition of offset laminar hooks, supralaminar at T11 and infralaminar at L2, to standard short-segment pedicle instrumentation constructs increased stiffness in flexion by 268%, in extension by 223%, in side bending by 161%, and in torsion by 155% (all were significant except torsion). Sublaminar hooks also reduced pedicle screw bending moments to roughly 50% of standard in both flexion and extension (P < 0.05). CONCLUSIONS: Supplemental offset hooks significantly increase construct stiffness without sacrificing principles of short-segment pedicle instrumentation, and absorb some part of the construct strain, thereby reducing pedicle screw bending moments and the likelihood of postyield deformation and clinical failure.

Aged↗

PubMed1996

Corrosion at the interface. A possible solution to cobalt-chrome heads on titanium alloy stems.

The low-wear characteristics of cobalt-chrome femoral heads matched with the excellent biocompatibility and low modulus of titanium alloy femoral stems constitute the preferred combination used by many orthopaedic surgeons performing total hip arthroplasty. The combination of these materials in a synovial fluid environment, however, has proven to result in extensive crevice corrosion and metallosis of the surrounding tissues. This study investigates an alternative to the conventional mating of dissimilar metals at the head-neck junction. Five cobalt-chrome heads premated with titanium alloy sleeves were investigated by gross examination, dissecting microscopy, and scanning electron microscopy. Examination by both gross examination and dissecting microscope revealed no signs of corrosion. Scanning electron microscope examination revealed slight crevice corrosion in the only head with a +15-mm neck length.

Alloys↗

PubMed1996

A device for the measurement of pedicle screw moments by means of internal strain gauges.

Pedicle screws are commonly used in spinal reconstruction, and failure of pedicle screws due to bending is a significant clinical problem. To measure the moments typically placed on pedicle screws in situ we instrumented 7 mm Cotrel-Dubousset (CD) pedicle screws with internally mounted strain gauges. The screws were designed to measure flexion-extension moments at a single cross-section as dictated by strain gauge placement. It is possible to measure moments of up to 12 Nm at any location along the length of the screw by constructing transducers with varying strain gauge placements. These transducers are capable of measuring moments at points located within the vertebra including the pedicle, which is where failure usually occurs clinically. Transducer output was both linear and reproducible. These transducers are being used to investigate the load transfer characteristics between the pedicle screw and the vertebra. This technique could be applied to investigations of load sharing in reconstruction plates, lag-screws, and cross-locked intramedullary nails.

Biomechanical Phenomena↗