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Thomas D Brown

Publications and source records attributed to Thomas D Brown.

At least 55 records · Page 3Linked to original sources
PubMed2003

Orthopaedic crossfire--Larger femoral heads: a triumph of hope over reason! In the affirmative.

The authors' wear studies of total hip arthroplasty cohorts have shown that less polyethylene wear and less deleterious effects of third body debris were found when smaller femoral head sizes were used. The authors' sliding-distance-coupled finite element model findings were corroborated by these clinical wear studies. Thus, with polyethylene on metal bearing surfaces, less wear should occur when smaller head sizes are used. Careful, precise component positioning is important to prevent dislocation.

Finite Element Analysis↗
PubMed2003

Gait cycle finite element comparison of rotating-platform total knee designs.

Functional load transmission and kinematic performance were compared for standard versus posterior-stabilized versions of a rotating-platform total knee implant, over a standardized loading cycle, using three-dimensional contact finite element analysis. These two design variants differ primarily in terms of the latter's polyethylene insert having a cam that engages with the femoral component during appreciable flexion, thereby inducing femoral component rollback. The finite element model, previously validated experimentally, afforded direct comparisons of anterior lift-off of the insert from the tibial tray, of bearing mobility (insert rotation about the pivot post), of femoral rollback, and of metal-on-polyethylene contact stresses at the bearing and backside surfaces of the insert. Both design variants generally performed comparably, exhibiting an internal and external rotation range of approximately 5 degrees, approximately 1.5 mm peak lift-off at the anterior aspect of the insert, and approximately 15 mm of posterior rollback, the respective maxima for both designs occurring at approximately the same instants in the gait cycle. However, the posterior-stabilized design had slightly more rollback, and slightly less anterior lift-off and rotation, than did the standard rotating-platform design. Peak polyethylene stresses occurred on the backside of the insert near the posterior edge of the medial compartment, the magnitude being approximately 18% higher for the posterior-stabilized design (21 MPa) than for the standard design.

Finite Element Analysis↗
PubMed2003

A biomechanical analysis of polyethylene liner cementation into a fixed metal acetabular shell.

BACKGROUND: A common clinical scenario encountered by an orthopaedic surgeon is a patient with a secure cementless acetabular shell and a failed polyethylene liner. One treatment option is to cement a new liner into the fixed shell. The purpose of this study was to evaluate technical variables to improve the mechanical strength of such cemented liner constructs. METHODS: The contributions of shell texturing, liner texturing, and cement mantle thickness (between the liner and the shell) were evaluated by comparing torsional strength (among nine groups of constructs) and lever-out strength (among eight groups of constructs). RESULTS: Failure almost always occurred at the cement-liner interface. The two exceptions (failure at the shell-cement interface) occurred with a polished, untextured shell with no screw-holes. This finding indicates that if a shell has existing texturing (such as holes), further intraoperative scoring of the shell is unnecessary, but some sort of texturing is necessary to avoid construct failure at the shell-cement interface. Textured liners had significantly (a = 0.05) greater torsional and lever-out strength than untextured liners. The greatest construct strength occurred when liner grooves were oriented so as to oppose the applied loading. A 4-mm-thick cement mantle resulted in slightly greater torsional strength than a 2-mm-thick cement mantle, and a 2-mm-thick cement mantle resulted in considerably greater lever-out strength than a 4-mm-thick cement mantle, but these differences were not significant. CONCLUSIONS: When cementing a liner into a well-fixed shell, a surgeon should ensure that both the shell and the liner are textured, as interdigitation of the cement with the shell and the liner is crucial to the mechanical strength of this construct.

Arthroplasty, Replacement, Hip↗
PubMed2003

Clinical biomechanics of wear in total hip arthroplasty.

Complementary clinical and laboratory studies were performed to identify variables associated with polyethylene wear following total hip replacement, and to elucidate the mechanisms responsible for accelerated wear in the total hip arthroplasty construct. Observational cohort studies were performed using a prospective clinical database of more than 4000 consecutive primary total hip arthroplasties performed by a single surgeon, to identify wear-related variables. These variables included head size, acetabular/femoral component impingement, and third body debris. Novel digital edge detection techniques were developed and employed to accurately measure wear, and to determine the relationships of head size and third body debris to acceleration of wear. A novel sliding-distance-coupled finite element model was formulated and employed to examine the mechanisms responsible for wear. The long-term cohort studies demonstrated smaller head sizes to be associated with less wear. Third body debris generated from cable fretting was associated with an increase in wear, osteolysis, and acetabular loosening, especially with larger head sizes. The sliding-distance-coupled finite element model replicated the wear rates occurring in vitro and in vivo, demonstrating the importance of sliding distance on polyethylene wear following total hip arthroplasty. It also demonstrated substantial increases in wear associated with femoral head scratching from third body debris. Further extension of the finite element formulation demonstrated the potential for acetabular component rim damage from impingement wear, and the enhanced potential for third body ingress to the bearing surface with larger head sizes. Edge detection wear measurement techniques demonstrated that early wear rates were predictive of long-term wear rates. These complementary clinical and laboratory investigations have provided insight into 1) the significance of sliding distance and physiologic loci of motion as contributing factors in minimizing wear, 2) the deleterious effects of third body particulates in accelerating wear, 3) the potential for, and factors related to, impingement wear, and 4) the potential advantages and compromises related to the use of larger head sizes in the bearing surface construct.

Arthroplasty, Replacement, Hip↗
PubMed2002

Interfragmentary surface area as an index of comminution energy: proof of concept in a bone fracture surrogate.

Fracture mechanics theory postulates a monotonic relationship between energy absorption and fracture surface generation. We hypothesized that this relationship was demonstrable to the point that, on a continuous scale, comminuted fractures created with disparate levels of energy delivery could be discriminated. Using a bone fracture surrogate in conjunction with digital image analysis of CT fracture data, we measured the surface area freed by controlled, discrete fracture simulations. Prior to these simulations, the reproducibility of the digital image analysis algorithm was validated with repeated measurements by two different operators. The parametric fracture series results showed a statistically significant difference in measured de novo surface area between four specimen groups, over a range of input energies from 1.4 x 10(10)-9.1 x 10(10)J/m(3) (or 12.5-80.2J/specimen). The results of this study provide confirmation that comminution severity can indeed be measured on a continuous scale, based on energy absorption (another clinically meaningful index).

Algorithms↗
PubMed2002

Spatial distribution of hip capsule structural and material properties.

Contemporary computational models potentially allow the practical incorporation of the effects of a joint capsule on both motion and the loads transmitted to the other parts of the joint. However, the required material properties have not been available for this purpose. To determine these properties we took both hip joints from five fresh-frozen, nondiseased cadavers. Following dissection and potting of the hemi-pelvis, distraction of the intact joint was conducted to measure the structural tangent stiffness of the joint capsule. Anatomical insertion points of the hip capsule were then recorded, and a complete capsulectomy was performed. Once excised, the capsule was sectioned into eight, approximately even sectors, and initial geometrical measurements were recorded for material property calculations. Material properties (i.e., structural tangent stiffness, failure load, ultimate strength, tangent modulus) were calculated using the load-displacement and geometric data collected for each of the sectors. This specimen-to-specimen thickness variability reveals significantly lower (p<0.01) average tangent structural stiffness values in the posterior-inferior portion of the capsule. Explorations of hip stability using numerical models can now be enhanced by incorporation of these experimental capsule data.

Aged↗
PubMed2002

Effects of acetabular component orientation on dislocation propensity for small-head-size total hip arthroplasty.

OBJECTIVE: Examine the role of surgical orientation of the acetabular cup on posterior dislocation propensity for small-head-size total hip arthroplasty. DESIGN: A finite element model of a widely used total hip arthroplasty system was examined for peak resisting moment and range-of-motion prior to impingement, as well as prior to onset of posterior dislocation. Acetabular component surgical orientation was varied. BACKGROUND: Dislocation is a leading cause of total hip replacement failure, with an incidence between 2% and 11%. Clinical registries imply acetabular component orientation to be a leading predictor of dislocation. The finite element method permits this complex kinetic behavior to be addressed systematically. METHODS: Twenty-five combinations of cup abduction (five angles) and anteversion (five angles) were studied, with the resultant resisting moment about the cup center being tracked in each case. Key events were benchmarked, and a novel dislocation resistance index was developed for multi-factor comparison. RESULTS: Increasing tilt and/or anteversion resulted in a monotonically increasing range-of-motion prior to impingement, as well as increased peak resisting moment. Range of motion was more sensitive to tilt, while peak resisting moment was more sensitive to anteversion. Peak resisting moment for 22-mm head size was nearly 25% less than that for a 26-mm head. CONCLUSIONS: Increased cup tilt and anteversion discourage posterior dislocations of small-head-size components. RELEVANCE: Pre-existing soft tissue compromise and untoward patient motions/postures are largely beyond surgeon control. However, other factors being equal, especially for small-head-size components, many posterior dislocations that would otherwise occur might be prevented by suitable tilt and anteversion of the acetabular component.

Acetabulum↗
PubMed2002

A new animal model of femoral head osteonecrosis: one that progresses to human-like mechanical failure.

Existing animal models of femoral head osteonecrosis, while displaying varying levels of concordance with early histopathologic features of the human disorder, generally fail to progress to end-stage mechanical collapse. A new animal model of osteonecrosis is here introduced, utilizing the emu (Dromaius novaehollandie). These animals' bipedality and their high activity level represent a much more challenging biomechanical environment to the hip than seen in quadrupedal models of this disorder. Femoral head osteonecrosis was induced surgically, using a combination of ischemic (vessel ligation) and cryogenic (liquid nitrogen) insults. Of nineteen emus allowed free-roaming pen activity to study the natural history of such lesions, eighteen progressed to an osseous structural failure, sixteen of them developing incapacitating lameness at an average time point 11.75 weeks after the surgical insult. Histologically, the animals showed close concordance with both the early- and late-stage human pathology, in six cases even to the point of developing a crescent sign. Because of its large physical size and its consistent progression to mechanical collapse, the emu appears to offer a unique opportunity for the near-human-scale study of surgical interventions to forestall femoral head collapse. Toward this end, various directions for model refinement are outlined.

Animals↗
PubMed2002

Reliability of detecting prosthesis/cement interface radiolucencies in total hip arthroplasty.

A laboratory study assessed the reliability of detecting radiolucencies at the prosthesis/cement interface in femoral components for total hip replacement. Radiolucencies (thicknesses = 0.1, 0.3, 0.5 and 0.7 mm) were created by randomly masking non-tip Gruen zones (six per stem) in a group of 72 matte-finish femoral components. Only half of all Gruen zones were masked. The femoral components were reproducibly implanted in composite fiberglass replicate femurs. Anteroposterior and lateral radiographs taken using conventional techniques were then presented to 10 experienced readers, who scored each of 432 non-tip Gruen zones for radiolucency presence or absence. The series-average radiolucency detection rate was 47.1%, with a breakdown of 9.8%, 20.7%, 69.8% and 88.0, for radiolucency thicknesses of 0.1, 0.3, 0.5 and 0.7 mm, respectively. The false positive rate was 7.5%. The findings argue strongly that in many or most instances, initial cement-prosthesis debonding is not reliably detected from conventional plain film radiographs. Radiolucencies of at least 0.7 mm width are necessary to be accurately detectable by most viewers.

Arthroplasty, Replacement, Hip↗
PubMed2002

Polyethylene liner cementation into fixed acetabular shells.

A patient presenting with a secure cementless acetabular component and with femoral head penetration through the polyethylene liner is a common clinical problem. Cementing a new liner into the fixed shell is one option. We evaluated this option in a clinical series of 17 cases and with a preliminary mechanical study. In the 1 clinical failure (5.9%), the failure occurred at the cement-liner interface. The most important variable in optimizing the mechanical strength of the cemented liner construct was adequate preparation of the cement-liner interface. This approach to treating the patient with a fixed cementless shell and a worn polyethylene liner can provide a durable construct with minimal morbidity.

Aged↗
PubMed2002

A voxel-based formulation for contact finite element analysis.

To date, voxel-based finite element models have not been feasible for contact problems, owing to the inherent stair-step boundary discontinuities. New preprocessing techniques are reported herein to mesh these boundaries smoothly, for purposes of contact stress analysis. Further, new techniques are reported to concentrate the mesh resolution automatically near the articular surface, thus reducing the problem size to levels compatible with executing nonlinear problems on contemporary engineering workstations. Close approximations to Hertzian analytical solutions were obtained for spherical and cylindrical geometries meshed in this manner, and an illustrative anatomical contact problem of the human hip joint is presented.

Computer Simulation↗
PubMed2002

Choices and compromises in the use of small head sizes in total hip arthroplasty.

Observation cohort studies, a sliding-distance-coupled finite element model, and an expanded finite element model that simulates dislocation were used to evaluate the benefits and compromises associated with the use of smaller femoral heads in the total hip arthroplasty construct. Wear studies of total hip arthroplasty cohorts showed less polyethylene wear and less deleterious effects of third body debris when smaller femoral head sizes were used. The sliding-distance-coupled finite element model findings were corroborated by these clinical wear studies. The dislocation model predicted the increased propensity for dislocation when smaller modular head femoral components were used in the cohort studies. Impingement is not the only contributor to frank dislocation. The dislocation model explicitly defined the range of motion changes from impingement to dislocation, and the resisting moment changes between construct designs.

Adult↗
PubMed2002

Fractures of the olecranon: an in vitro study of elbow joint stresses after tension-band wire fixation versus proximal fracture fragment excision.

BACKGROUND: Displaced fractures of the olecranon usually require operative treatment, by either open reduction with internal fixation (ORIF) or excision of the proximal fragment. However, the relative merits of these treatment options have not been fully delineated. One treatment outcome measure of joint function is residual intra-articular stress. The purpose of this study was to evaluate the effect of these two types of olecranon fracture treatment on humeroulnar joint stress. METHODS: Eight matched pairs of fresh frozen cadaveric upper extremities were thawed; stripped of skin, muscular, and neurovascular tissue; and potted in polymethylmethacrylate. The intra-articular humeroulnar joint peak pressures were measured at 90 degrees of elbow flexion using pressure-sensitive film after application of a 0.15 kg-m torque through the remaining triceps muscle attachment. First, pretreatment (normal) pressures were obtained from the major contact regions of the humeroulnar joint. A 50% olecranon osteotomy was then performed simulating a fracture, and the elbows from each of the paired specimens were randomly assigned to one or the other of two treatment groups: ORIF (using a tension-band wiring technique) and proximal fragment excision. Joint pressures were remeasured. A two-tailed paired t test was used for statistical analysis. RESULTS: After osteotomy, the peak pressures were higher, overall, in the excision group. Comparing each posttreatment experimental group to its pretreatment (normal) counterpart revealed that the peak pressures in the distal medial and distal lateral articular subzones were significantly higher for the fragment excision group (p = 0.005 and p= 0.0008, respectively), but were not significantly different in the ORIF group (p = 0.545 and p= 0.153, respectively). CONCLUSION: The findings of this study indicate that ORIF restores the normal biomechanics of the elbow joint and proximal fragment excision results in abnormally elevated joint stresses. These elevated joint stresses may, over time, contribute to the development of elbow pain and osteoarthrosis. Therefore, ORIF should continue to be regarded as the treatment of choice for displaced fractures of the olecranon involving large proximal fracture fragments similar in size to those in this study.

Biomechanical Phenomena↗
PubMed2002

Local head roughening as a factor contributing to variability of total hip wear: a finite element analysis.

Large inter-patient variability in wear rate and wear direction have been a ubiquitous attribute of total hip arthroplasty (THA) cohorts. Since patients at the high end of the wear spectrum are of particular concern for osteolysis and loosening, it is important to understand why some individuals experience wear at a rate far in excess of their cohort average. An established computational model of polyethylene wear was used to test the hypothesis that, other factors being equal, clinically typical variability in regions of localized femoral head roughening could account for much of the variability observed clinically in both wear magnitude and wear direction. The model implemented the Archard abrasive/adhesive wear relationship, which incorporates contact stress, sliding distance, and (implicitly) bearing surface tribology. Systematic trials were conducted to explore the influences of head roughening severity, roughened area size, and roughened area location. The results showed that, given the postulated wear factor elevations, head roughening variability (conservatively) typical of retrieval specimens led to approximately a 30 degrees variation in wear direction, and approximately a 7-fold variation in volumetric wear rate. Since these data show that randomness in head scratching can account for otherwise-difficult-to-explain variations in wear direction and wear rate, third-body debris may be a key factor causing excessive wear in the most problematic subset of the THA population.

Acetabulum↗
PubMedDate not supplied

Kinematic and contact stress analysis of posterior malleolus fractures of the ankle.

OBJECTIVE: To determine if there are measurable dynamic contact stress aberrations and kinematic abnormalities (instability) that have not been observed in conventional static loading studies of posterior malleolar ankle fractures. DESIGN: Cadaveric fracture model. SETTING: Biomechanics laboratory. INTERVENTION: Seven fresh cadaveric specimens were fixed in an unconstrained testing apparatus and loaded to one body weight. The ankle was moved from 25 degrees of plantarflexion to 15 degrees of dorsiflexion. The model included the intact ankle and four fracture simulations (50% fracture without internal fixation, 2 mm gap and step malreductions, and anatomically fixed). MAIN OUTCOME MEASURE: Motion at the ankle was monitored with an electromagnetic tracking device, and intra-articular contact stresses were measured using a real-time stress sensor. RESULTS: There were no kinematic abnormalities suggestive of tibiotalar subluxation in any of the fracture simulations. There was no increase in peak contact stress in any of the fracture models compared with the unfractured model. However, there was a shift in the location of the contact stresses to a more anterior and medial location following the fracture. When summed over the range of motion, these areas of cartilage bore significantly higher cumulative contact stresses relative to the nonfracture situation. CONCLUSIONS: We found no talar subluxation and no increase in contact stresses near the articular incongruity, making it unlikely that these factors explain the increased incidence of arthrosis after trimalleolar fractures (OTA/AO classification 44 B3 fractures). Rather, we found that the joint remaining bears increased stress and that the center of stress shifts anteriorly, loading cartilage that normally sees little load.

Ankle Joint↗
PubMedDate not supplied

Incongruity-dependent changes of contact stress rates in human cadaveric ankles.

Cartilage biosynthetic transduction and injury characteristics have been shown to be particularly sensitive to changes in contact stress rates. This study investigated incongruity-associated changes in contact stress rates that resulted from an articular surface stepoff of the distal tibia in human cadaveric ankles. Ten human cadaveric ankles were subjected to quasi-physiologic stance-phase motion and loading and instantaneous contact stresses were captured at 132 Hz over the entire articular surface using a custom-fabricated stress transducer. An osteoarticular fragment consisting of the anterolateral 25% of the distal tibia was osteotomized. Testing was repeated after displacing the fragment proximally between 0.0 mm to 4.0 mm in 1.0 mm increments. Transient contact stress measurements were used to calculate contact stress rates. Compared to intact ankles, the anatomic configuration had modest increases in global and peak postitive and negative contact stress rates throughout the motion cycle. In contrast, stepoff specimens had significant increases in global and complete motion cycle peak positive and negative contact stress rates, as high as 3.1X intact values in specimens with a 4.0 mm stepoff. Contour plots of contact stress rates also demonstrated an instability event during motion. An anterolateral stepoff of the distal tibia caused significant changes in positive and negative contact stress rates in cadaveric ankles. Incongruity-associated changes in contact stress rates and incongruity-associated instability events may be important pathomechanical determinants of post-traumatic arthritis.

Ankle Injuries↗